WO2006129080A1

WO2006129080A1 - Materials and methods for transducing cells with a viral vector

Info

Publication number: WO2006129080A1
Application number: PCT/GB2006/001972
Authority: WO
Inventors: Colin David Porter; Jeffrey Colin Bamber; Sarah Louise Taylor
Original assignee: The Institute Of Cancer Research: Royal Cancer Hospital
Priority date: 2005-05-31
Filing date: 2006-05-31
Publication date: 2006-12-07

Abstract

The invention relates to materials for transducing cells with a viral vector and in vitro and in vivo methods for transducing cells using said materials. In particular, the invention provides materials and methods for cell transduction providing targeted gene delivery using a retroviral vector and ultrasound exposure. In certain aspects the invention provides compositions and kits and in other aspects, their use in methods for transducing cells and medical uses.

Description

Materials and Methods for Transducing Cells with a Viral Vector

Field of the Invention

The present invention relates to materials for transducing cells with a viral vector and in vitro and in vivo methods for transducing cells using said materials.

Background of the Invention

Despite early hyperbole surrounding gene therapy, this approach has thus far failed to realise its therapeutic goals in the clinic, and only one treatment, Genedicine, has been licenced for use within China. Clinical successes in treating single-gene immunodeficiency disorders such as X-SCID (Hacein-Bey-Abina et al . , 2002) with viral vectors have been overshadowed by the incidence of leukaemia in some children enrolled in a French trial. Currently, most success in gene therapy has been achieved using ex vivo modification of cells, or direct injection into skeletal muscle or an accessible target organ. For example, precise and stable transgene expression can be achieved by direct injection of naked DNA into skeletal muscle. However, in many cases there is a need for therapeutic, non-invasive gene therapy for a range of less accessible tissues, which ultimately requires systemic administration of a vector which can deliver a transgene to achieve the desired therapeutic effect in the target area.

Currently there exist multiple obstacles to the accomplishment of this goal. Problems common to all genetic therapies regarding application for the treatment of disease are those of the delivery of sufficient material to the desired target cells with minimal delivery to "off-target" sites. Bioavailability of a systemically introduced vector is greatly reduced due to uptake by the liver or blood cells, by antibody-mediated immune response, or where there is a low blood supply to the target region. Transgene expression problems are encountered by non-viral vectors such as naked DNA; very few plasmids which enter a cell reach the nucleus and are transcribed, and those that do are degraded when the cell divides. Safety issues remain with regards to using viral vectors: an adenoviral vector caused a high-profile fatal inflammatory reaction in a clinical trial participant (Fox, 1999; Fox, 2000), while retroviral vectors^' have been implicated in leukaemia (including one death) following oncogenic insertional mutagenesis during bone marrow stem cell transduction (Hacein-Bey-Abina et al . , 2003). Physical methods of gene delivery such as electroporation, hydrodynamic delivery or the gene gun have failed so far to demonstrate efficient, safe delivery to inaccessible areas of the body in a clinically relevant experimental setting. Existing approaches to gene therapy tend to have focussed on vector modification and the use of genetic or biochemical techniques to prevent non-target transfection, immune recognition, oncogenic mutagenesis and clearance and to improve transgene delivery efficiency and intensity of expression. For example, there is a focus on vector modification to ease entry and intracellular passage.

Liposomal or lipid-based delivery of naked DNA increases uptake by reducing electrostatic repulsion and encouraging endocytosis at the cell membrane. However, uptake in the liver and spleen causes reduced bioavailability of vector and results in undesirable non- target gene expression. Efficiency is greater with adenoviral vectors, but as with liposomal methods, they suffer from transient gene expression, a high level of non-target delivery and accumulation in the liver. Retroviral vectors mediate stable gene delivery but are not easily produced in sufficient quantity. Retroviral gene delivery makes use of the natural ability of a retrovirus to infect a cell. A retrovirus obtains entry to a target cell via the interaction of a protein integral to its lipid envelope, known as the envelope protein, with a transmembrane protein acting as a receptor on the surface of the target cell. This process leads to membrane fusion and cytoplasmic entry of the viral core. Once inside the cell, a retrovirus is extremely efficient at delivering and integrating the transgene into the host genome for sustained, permanent gene expression. However, transduction of cells by retroviral vectors, if introduced systemically, lacks specificity. Some studies have modified the envelope protein on the retrovirus surface in an attempt at cell-targeting, but these remain in early stages of development. Viral particles that lack envelope proteins are incapable of normal cell entry by receptor-mediated membrane fusion and entry of the viral core; however, infectivity can be partially restored in the presence of cationic liposomes and, once inside the cell, the virus functions normally (Porter, 2002). In none of these cases is the use of a viral vector an efficient technique for targeted gene delivery. Ultrasound (US) has been used to modify gene delivery with non-viral vectors by facilitating uptake. Ultrasound assisted gene therapy uses the interaction between sound waves and gas-filled microbubbles in the circulation to modify cell membranes, enhancing DNA uptake in a target area. Although the mechanism of vector entry is unclear, it is known that microbubbles oscillate, or stably cavitate, in an US field and induce the formation of transient cell membrane pores (sonoporation) through which internalisation of macromolecules can occur (Bao et al . , 1997) . The gene delivery process may, additionally or alternatively, involve inertial cavitation. Inertial cavitation is the violent destruction of the microbubble in a high pressure US field, which may result in release of the DNA from the shell, sonoporation and perhaps the propulsion of DNA-coated shell fragments into surrounding cell membranes (Christiansen et al . , 2003) . Specific gene expression has been achieved in the heart

(Bekeredjian et al . , 2004), skeletal muscle (Christiansen et al., 2003) and kidney (Koike et al . , 2005) following systemic infusion of DNA-loaded microbubbles and US exposure focused on the target organ.

Gene expression using this technique, although specific, is transient and lacks high delivery efficiency, which limits the therapeutic applicability. In part this is due to inefficient nuclear trafficking of naked DNA and that it is not incorporated into the host genome; cell division results in concurrent loss of transgene expression. There are diseases, such as some cancers, for which brief expression of a cytotoxic gene resulting in the death of transduced cells might be of therapeutic benefit. However, in curing chronic conditions such as X-SCID or haemophilia, life-long gene expression from a genomically integrated transgene can offer a complete cure.

In addition to the potential to release DNA associated with a microbubble, phenomena associated with the interaction of a sound field with a microbubble, including the generation of localized intense heat and pressure, can be biologically destructive. Damage to the endothelial lining of blood vessels resulting from these phenomena has been used to facilitate subsequent access of gene vectors, including adenoviruses, beyond this barrier. Ultrasound has been used with retrovirus in an entirely different application in vitro, using radiation force as a localizing method to facilitate cell and virus encounter in a standing wave. In general there is a need for improved vectors for gene delivery to cells in gene therapy. In many cases, there is a need for a systemically administered vector which can deliver a transgene efficiently enough to achieve the desired therapeutic effect in a target area and specifically enough to minimise side effects.

Summary of the Invention

The present invention aims to address the need for vectors for transducing cells and effecting transgene expression in cells, in particular the need for improved vectors for gene delivery.

Generally, the invention lies in providing materials and methods for cell transduction with a viral vector. The invention generally relates to the provision of retroviral vectors and their use for gene delivery.

The invention in general lies in providing controllable and targetable cell transduction and gene delivery. Generally, the invention relates to gene delivery which is specific for a particular target or targets. Gene delivery may be temporally and spatially targeted.

The invention generally relates to activation of a viral vector for gene delivery. In particular, the invention is concerned with the targeted activation of a viral vector.

The invention is concerned with efficiency of gene delivery. In particular, the invention is concerned with efficient, targeted cell transduction and gene delivery. The invention relates to improving efficiency of gene delivery.

The invention is concerned with providing viral vectors for mediating gene delivery associated with stable expression of a transgene. The invention in general is concerned with the provision of viral vectors for gene therapy, in particular, viral vectors having desired characteristics for therapeutic applications . The invention particularly relates to efficient, targeted gene delivery for stable transgene expression. The invention further relates to efficient, targeted cell transduction suitable for therapeutic applications of gene delivery. More particularly, the invention relates to the provision and use of viral vectors capable of effecting targeted, efficient and/or stable gene delivery and associated expression of a transgene, particularly for use in therapeutic applications.

Accordingly, in a first aspect, the present invention provides a composition for use in transducing a target cell with a gene of interest, comprising a microbubble and a retroviral particle comprising the gene of interest. The present invention provides a composition for use in transducing a target cell with a gene of interest, comprising a microbubble and a retroviral particle comprising the gene of interest, wherein the composition is capable of transducing the target cell with the gene of interest when the target cell is exposed to the composition with the administration of ultrasound. In use, transduction of the target cell is effected by exposure of the target cell to the composition and ultrasound.

In one aspect, the present invention provides a kit for use in transducing a target cell with a gene of interest using ultrasound, the kit comprising a microbubble and a retroviral particle comprising the gene of interest, where the microbubble and the retroviral particle are for use in combination as a composition and the composition is capable of transducing the target cell with the gene of interest when used with the administration of ultrasound, wherein transduction of the target cell is effected by exposure of the target cell to the composition and ultrasound.

In a further aspect, the present invention provides a method of transducing a target cell with a gene of interest, the method comprising the steps of exposing the target cell to a composition of the invention and administering ultrasound to the target cell.

In a further aspect, the present invention provides a method of treating a subject having a disease or condition treatable by expression of a gene of interest in a target cell, the method comprising the steps of exposing the target cell to an effective amount of a composition of the invention and administering ultrasound to the target cell to effect expression of the gene of interest in the target cell. In further aspects, the present invention provides medical uses of compositions of the invention. The present invention provides a composition of the invention for use in therapy. The invention provides a composition of the invention for use in therapy by gene delivery to a target cell with the administration of ultrasound. The present invention provides use of a composition of the invention for the preparation of a medicament for use in the treatment by ultrasound administration of the gene to the target cell of a disease or condition treatable by expression of the gene of interest in a target cell. The present invention provides use of a composition of the invention for the preparation of a medicament for use in the treatment of a disease or condition treatable by expression of the gene of interest in a target cell, wherein the medicament is for administration by ultrasound. The medicament is capable of transducing a target cell with the gene of interest when used with the administration of ultrasound.

The present invention further provides medical uses of a microbubble and a retroviral particle comprising a gene of interest. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament for use in the treatment by ultrasound administration of the gene to the target cell of a disease or condition treatable by expression of the gene in a target cell. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament for use in the treatment of a disease or condition treatable by expression of the gene in a target cell by ultrasound administration of the gene to the target cell . The medicament is capable of transducing a target cell with the gene of interest when used with the administration of ultrasound. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament capable of transducing a target cell with the gene of interest when used with the administration of ultrasound for use in the treatment of a disease or condition treatable by expression of the gene in the target cell, wherein the medicament is for administration by ultrasound.

Features of any of the aspects of the invention may be any of the following, alone, or in combination with any of the other features. Preferably, a retroviral particle is incapable of independent cell entry. The retroviral particle may be non-internalizing such that it is non-infectious. The retroviral particle may comprise a modified envelope protein. The retroviral particle may lack an envelope protein.

Preferably, a retroviral particle is unstable in vivo. The retroviral particle may be modified by glycosylation. The retroviral particle may have α-galactosyl carbohydrate epitopes on its surface.

Preferably, a retroviral particle is murine leukaemia virus (MLV) , human immunodeficiency virus (HIV) or equine infectious anaemia (EIAV) . Preferably, a retroviral particle is recombinant. Preferably, a retroviral particle is associated with a microbubble. The retroviral particle may be associated with the microbubble by electrostatic interaction, affinity cross-linking or covalent attachment .

Preferably, the shell of a microbubble comprises lipid. The shell of a microbubble may comprise albumin or methacrylate . Preferably, the shell of the microbubble is cationic. Preferably, the gas in a microbubble is perfluorocarbon. The gas may be sulphur hexafluoride or air.

A microbubble or retroviral particle, or composition of the invention may further comprise a targeting moiety. The targeting moiety may be a ligand or an antibody. Where the targeting moiety is an antibody, the antibody may be an anti-ICAM-1 antibody, anti-E-selectin antibody, anti-VEGF receptor antibody or anti-α_vβ₃ antibody.

Preferably, a gene of interest is a therapeutic gene. A gene of interest may be one which is not functionally expressed in a target cell. A gene of interest may be one for which lack of functional expression in an individual is causative of disease in that individual. Lack of functional expression in an individual may be due to the presence of a mutated form of the gene, absence of the gene, or lack of functional gene product, in the individual. The gene of interest may not be functionally expressed in a target cell where it is not naturally expressed in the target cell. The gene of interest may be adenosine deaminase, VEGF, GM-CSF, factor VIII, factor IX, CFTR, p53 , TNFα, TIMP-3 or thymidine kinase.

A target cell used in the invention may be in vitro or ex vivo. A target cell may be exposed following systemic administration of a composition of the invention to a subject or direct administration of a composition of the invention to a subject in the vicinity of the target cell .

A composition of the invention may be for use in therapy, including for use in therapy by gene delivery to a target cell with the administration of ultrasound.

In the methods and medical uses of the invention, ultrasound may be administered to a target cell as pulsed ultrasound, with a frequency in the range of 1-3MHz; pulse repetition frequency of 1-1OkHz; pulse lengths of from 1 to 100 cycles; exposure time of up to 30 minutes; pressure amplitude of from 0. IMPa to 3MPa peak negative pressure.

Ultrasound may be administered to a target cell at IMHz, IKHz pulse repetition frequency; from 1 to 32 cycles per pulse; pressure amplitude of from 0.3MPa to 3Mpa peak negative pressure; an exposure time of from 1 to 8 seconds.

A disease or condition to be treated may be chronic. A disease or condition may result from a lack of functional expression of a gene of interest in the subject. Lack of functional expression may result from mutation in the gene of interest, absence of the gene of interest, or lack of functional gene product from the gene of interest, in the subject. The cause of the disease or condition may be unrelated to the gene of interest. The disease or condition may be haemophilia, cystic fibrosis, ADA-SCID, cancer, Lesch-Nyhan syndrome, restenosis, angina, ischaemia, atherosclerosis, or a wound.

A target cell for use in the invention may be one which lacks functional expression of a gene of interest. Lack of functional expression may result from mutation in the gene of interest, absence of the gene of interest, or lack of functional gene product from the gene of interest, in a target cell. A target cell may not naturally express the gene of interest. The target cell may be a vascular endothelial cell, tumour cell, bone marrow cell, muscle cell, or epithelial cell . Brief Description of the Figures

Embodiments of the invention will now be described in more detail, by^¬ way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows the results of a transduction assay to determine the effect of exposing ni-MLV (niv) in the presence or absence of microbubbles (MB) to ultrasound (US) . Error bars represent the standard deviation (n=2) .

Figure 2 shows a marked Opticell™. Each dot represents the position of one or more transduced cells in the monolayers following ultrasound-mediated transduction. The central window of the Opticell™ measures 75mm x 65mm.

Figure 3 shows western blot analysis of viral capsid protein content from various stages of vector production: input virus (Control, lane 4), first and second washes (lanes 1 and 2) following microbubble association and washed microbubbles (lane 3) .

Figure 4 shows example equipment used for ultrasound exposures, and measurements of the ultrasound field in the tank. a: Schematic representation of the exposure tank used in experiments. The cells and microbubbles are contained within the OptiCell™ unit at the ultrasound beam focus, for assisted transduction experiments. b: Lateral beam plots for the IMHz transducer produced with a 0.2mm diameter needle hydrophone, in the plane of the transducer focus, which is also the plane in which the cell monolayer is positioned in the tank during exposures . Plots are shown of the peak negative pressure amplitude as a function of radial distance from the acoustic beam axis in two orthogonal directions, i.e. the X and Y axes, where the sound wave travels along the Z axis. Black line=X axis; grey line=Y axis.

Figure 5 shows gene delivery using a virally-loaded microbubble vector, as a function of the peak negative acoustic pressure to which the cells were exposed. a: Western blot analysis of viral capsid protein content of the input suspension (input, lane 1) , showing washes obtained during the MLV-microbubble binding process and of the microbubble vector preparation used in transduction experiments. wl=first wash (lane 2) ; w2=second wash (lane 3) ; MB=microbubbles (lane 4) . b: Effect of altering the peak negative ultrasound pressure amplitude on transduction by virally-loaded microbubbles, as determined by counting the number of β-galactosidase-expressing cells in areas of the cell monolayer defined by pre-determined acoustic pressure boundaries. These boundaries were three concentric circles, centred on the acoustic axis, producing regions of radii 0.0-2.5mm (peak negative pressure >lMPa) , 2.5-7.5mm (peak negative pressure 0.5-1MPa), 7.5-13mm (peak negative pressure 0.2-0.5MPa) and >13mm (peak negative pressure <0.2MPa). In addition, in sham experiments the cells were entirely without ultrasound exposure (no US) . Data shown are the means for three experiments; the mean count within each spatial region was computed for each experiment prior to averaging the three, so that error bars represent the standard deviation of the corresponding three mean values about the global mean for each region. Numerical values for the "no US" datum are mean = 0.01%, standard deviation = 0.007%.

Figure 6 shows spatial distribution of retroviral gene delivery relative to the position of cells in the ultrasound beam. a: Photograph of an OptiCell™ following transduction and assay for β-galactosidase expression; marks represent the position of 1 or more blue cells as a crude demonstration that transduction was localised to the area closest to the beam focus . b: Detailed representation of percentage of transduced cells at fields of view within an area exposed to ultrasound of peak negative pressure 0.2-1.4MPa. The data represent the mean efficiency of each field of view for the three experiments described in Fig. 5. Circles indicate the boundaries where peak negative pressure equals IMPa, 0.5MPa and 0.2MPa (radii of 2.5, 7.5 and 13mm respectively). c-e: Representative fields of view at regions exposed to ultrasound of peak negative pressure :>lMPa, 0.2-0.5MPa and <0.2MPa, respectively.

Figure 7 shows the acoustic pressure-dependence of transduction. Cells exposed to virus-loaded microbubbles were insonated and scored for transduction as in Figure 5b, using an amplifier input amplitude of 8OmV (peak 1.2MPa) or 6OmV (1. OMPa). A: Radial variation of peak- negative acoustic pressure for each condition is shown as the mean ± standard deviation of four determinations (i.e. in each direction from the focus along the orthogonal axes depicted in Figure 4b) . The boundaries defining the regions within which transduced cells were scored are indicated by the broken lines . The spatial average peak- negative pressure for each region (indicated above/below for insonation at 8OmV/6OmV) was determined by circular integration of the varying peak-negative value. B: Plot of transduction efficiency within each region against the corresponding spatial average peak- negative pressure. The reduced tranduction efficiency for this experiment relative to the data in Figures 5 and 6 reflects prolonged storage of the microbubbles .

Detailed Description of the Invention The present invention provides materials and methods for cell transduction with a viral vector. In particular, the invention provides a composition for use in transducing a target cell with a gene of interest. The composition comprises a microbubble and a retroviral particle comprising the gene of interest. The composition is capable of transducing the target cell with the gene of interest when the target cell is exposed to the composition with the administration of ultrasound, such that in use transduction of the target cell is effected by exposure of the target cell to the composition and ultrasound.

The present invention further provides a kit for use in transducing a target cell with a gene of interest using ultrasound. The kit comprises a microbubble and a retroviral particle comprising the gene of interest. The microbubble and the retroviral particle are for use in combination as a composition. The composition is capable of transducing the target cell with the gene of interest when used with the administration of ultrasound, such that transduction of the target cell is effected by exposure of the target cell to the composition and ultrasound.

In other aspects, the present invention provides a method of transducing a target cell with a gene of interest. The method comprises the steps of exposing the target cell to a composition of the invention and administering ultrasound to the target cell. The present invention further provides a method of treating a subject having a disease or condition treatable by expression of a gene of interest in a target cell. The method comprises the steps of exposing the target cell to an effective amount of a composition of the invention and administering ultrasound to the target cell to effect expression of the gene of interest in the target cell.

In further aspects, the present invention provides medical uses of compositions of the invention. The present invention provides a composition of the invention for use in therapy, including therapy by gene delivery to a target cell with the administration of ultrasound. The present invention provides use of a composition of the invention for the preparation of a medicament for use in the treatment by ultrasound administration of the gene to the target cell of a disease or condition treatable by expression of the gene of interest in a target cell. The present invention provides use of a composition of the invention for the preparation of a medicament for use in the treatment of a disease or condition treatable by expression of the gene of interest in a target cell, wherein the medicament is for administration by ultrasound. The medicament is capable of transducing a target cell with the gene of interest when used with the administration of ultrasound.

The present invention further provides medical uses of a microbubble and a retroviral particle comprising a gene of interest. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament for use in the treatment by ultrasound administration of the gene to the target cell of a disease or condition treatable by expression of the gene in a target cell. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament for use in the treatment of a disease or condition treatable by expression of the gene in a target cell by ultrasound administration of the gene to the target cell. The medicament is capable of transducing a target cell with the gene of interest when used with the administration of ultrasound. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament capable of transducing a target cell with the gene of interest when used with the administration of ultrasound for use in the treatment of a disease or condition treatable by expression of the gene in the target cell, wherein the medicament is for administration by ultrasound. The present invention provides use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament for use in gene therapy by delivery of the gene of interest to a target cell for expression of the gene therein in by ultrasound administration to an individual suffering from a disease or condition treatable by expression of the gene in the target cell. The medicament is capable of transducing a target cell with the gene of interest when used with the administration of ultrasound.

Previously, either of two main distinct approaches has formed the basis for development of new vectors for gene delivery. One of the key requirements that must be met by a gene delivery vector in order to achieve cell transduction is the ability to deliver a gene of interest into a target cell. Generally, one or other of the alternative techniques of (i) viral gene delivery, making use of the inherent ability of a virus to independently enter a cell, or (ii) assisted gene delivery, providing means to facilitate entry where the vector is not viral, have been used. Both of these techniques achieve the same aim, namely delivery of a transgene into a target cell, and one or other is generally selected on the basis of suitability for a given situation.

Viral gene delivery makes use of the natural ability of a virus to infect a cell. A retrovirus obtains entry to a target cell via the interaction of a protein integral to its lipid envelope, known as the envelope protein, with a transmembrane protein acting as a receptor on the surface of the target cell. This process leads to membrane fusion and cytoplasmic entry of the viral core. Thus, a virus administered in vivo is capable of independent entry into a target cell to effect cell transduction. Conversely, naked DNA and non- viral vectors are not capable of independent entry into a target cell. Rather, entry must be facilitated by some means in order to effect cell transduction. Methods based on physical cell entry mechanisms have been used for facilitating entry into a cell, for example, the use of ultrasound and microbubbles carrying DNA.

In an attempt to provide an improved vector for gene delivery to cells, the present inventors, rather than following one of the usual approaches and focusing simply on modifying the vector, instead combined the alternative techniques of retroviral gene delivery and the use of ultrasound and microbutables for gene delivery. This novel approach of using a physical cell entry-based technique in combination with retroviral gene delivery has the surprising effect of providing means for improved gene delivery, in particular, of providing means of effecting targeted, efficient and/or stable gene delivery and associated expression of a transgene, suitable for use in therapeutic applications.

The mechanism underlying the cell transduction is not yet fully understood. Without being bound by any particular theory, it is believed that whilst the ultrasound interaction with the microbubbles most likely releases the retrovirus, insonation may additionally facilitate retrovirus entry into the target cell. Sonoporation, wherein the plasma membrane is transiently permeabilized, may be responsible for the phenomenon; the lack of gene delivery in the absence of microbubbles is consistent with previous work with FITC- dextran and naked DNA which has demonstrated their dependence on microbubbles for both sonoporation and gene delivery, respectively. Sonoporation may result under the influence of highly localized fluid streaming and pressure fluctuation due to the interaction of the sound field with gas bubbles, a process known as cavitation. It is possible that microstreaming effects created by stably cavitating microbubbles cause an increase in cell membrane fluidity and may enhance the likelihood of virus-cell fusion and subsequent release of the viral core into the cytoplasm of the target cell.

Alternatively or additionally, it may be that the microbubbles undergo inertial cavitation, in which their violent destruction in a high pressure ultrasound field creates transient pores in the cell membranes through which the whole virus particle can enter, being uncoated in the cytoplasm before nuclear entry.

Phenomena associated with the interaction of ultrasound with a microbubble, in particular, the generation of localized intense heat and pressure, are known to be biologically destructive. In spite of this, the combination of this technique with a retroviral particle that subsequently remains functional, capable of delivering and integrating into the genome of the target cell, as distinct from DNA and drugs that have been used previously, has been successful. The present invention thus surprisingly provides the ability to benefit from the distinct properties and characteristics of each of the alternative techniques.

The present approach provides improved gene delivery. In particular, the present invention has the advantage of providing means of effecting targeted gene expression. Another advantage associated with the approach is efficiency of gene delivery. The ability to achieve stable gene delivery and associated expression of a transgene is provided by the present invention. The approach benefits from its suitability for use in therapeutic applications.

The development of gene therapy strategies requiring systemic vector administration has been hampered by problems such as cell- or antibody-mediated clearance that limit efficient delivery, vector stability, toxicity and lack of target cell specificity. The present invention, using ultrasound and microbubble technology in concert with a retroviral vector, addresses these issues. The present invention provides cell transduction that is ultrasound-dependent, allowing spatial and temporal control over gene delivery. Gene delivery using the materials and methods of the invention has been shown to be restricted to cells at positions within the ultrasound beam, providing a proof-of-principle demonstration that viral gene delivery can be spatially and temporally controlled via dependence for activation upon ultrasound exposure. Thus, the present invention has the advantage of providing controllable and targetable cell transduction and gene delivery, allowing gene delivery which is specific for a particular target or targets and which is temporally and spatially targeted.

An effect of the present approach is to maximize bioavailability of retrovirus. Without being bound by any particular theory, association with microbubbles may render retrovirus unavailable for non-productive adsorption to other than the target cell as a result of retrovirus only becoming available for cell entry under ultrasound exposure, which is limited to the target cell. Microbubble- association may further render retrovirus unavailable for clearance. The microbubble in effect serves to ^λ inactivate' and protect the retrovirus whilst it is in association with it. The result is maintenance of high concentrations of the retrovirus in vivo, and maximising bioavailability at the target site for spatially- controlled ultrasound-mediated release. High concentrations of retrovirus in association with the microbubble may further increase the bioavailability of the retrovirus at the target site. Thus, in providing an approach in which ^vactivation' of the retrovirus is possible in a specific manner, by association of retrovirus with microbubbles and targeting of gene delivery with ultrasound, a further advantage of increased bioavailability of retrovirus at the target site is provided. Reduced non-specific gene delivery to non- target sites and reduction of in vivo clearance of retrovirus has the effect of increasing the efficiency of gene delivery. Thus, an advantage provided by the present approach is efficiency of gene delivery, particularly efficient, targeted gene delivery.

A benefit of combining an ultrasound-mediated technique with a retroviral rather than DNA-based vector is that once inside the cell, the retrovirus is extremely efficient at integrating its genetic material in the host genome for sustained and strong gene expression. Further, recent research has demonstrated that certain lentiviral vectors that are unable to integrate into the genome of the transduced cell, for example, due to a deficiency in the viral protein integrase, can nevertheless remain in the nucleus of non- dividing cells and express the transgene they carry. Such vectors not only still allow stable transgene delivery and expression, but also avoid any mutagenic potential due to disruption of endogenous genes. Thus, such vectors that are unable to integrate their genetic material into the transduced cell genome may be used in the invention and provide the benefit of stable transgene expression. The ability to effect long-term transgene expression expands the utility of the present approach. The approach is thus suitable not only for gene therapy treatment strategies requiring only short-term expression, such as those that require cell death as a therapeutic outcome, for example tumour ablation, but also for diseases that require chronic gene expression. Thus, the approach has applications in permanent compensation for disease-causing mutations in monogenic disease. The use of existing gene therapy techniques in the treatment of cancer is limited because it is not possible to achieve delivery to 100% tumour cells,- strategies to combat this based on delivery of immunomodulatory genes for example, in which transduced cells secrete factors that sensitise nearby cells to anti-tumour treatment or recruit immune cells, would likewise benefit from the advantages of stable transgene integration or presence in the nucleus and long-term expression provided by the present invention. The present approach is applicable for in vitro , ex vivo and in vivo use .

In vivo vector observation is also possible with the present approach. Microbubble-association offers a means to monitor retroviral vector biodistribution in real-time and correlate this to gene delivery. Microbubbles strongly reflect ultrasound, producing characteristic sub-harmonic signals which can either be observed on a diagnostic ultrasound scanner or analysed mathematically. Therefore the potential exists to determine quantitatively how much vector accumulates at the target site and monitor its subsequent release in the ultrasound field.

Features of any of the aspects of the invention may be any of the following, alone, or in combination with any of the other features.

The present invention may use retrovirus that is incapable of independent cell entry. Thus, the compositions, methods and medical uses of the invention may use a retroviral particle that is incapable of independent cell entry. As described herein, the exact nature of the mechanism underlying the successful cell transduction achieved with the present invention combining the alternative techniques of retroviral gene delivery and the use of microbubbles and ultrasound is not fully understood. However, the properties or characteristics, and the effects of each of the two separate techniques are known. Although prior to the present invention there was no incentive to combine two alternative techniques for achieving the same aim, a combination of the properties of each of the two techniques and the effects arising from them may be expected when the two techniques are used in combination. The present approach using retrovirus that is incapable of independent cell entry is, however, successful even though the retrovirus used lacks a key feature of the technique of retroviral gene delivery of ability of a retrovirus to enter a cell. The finding that the present approach works using retrovirus without this feature thus rather indicates that the alternative techniques may work differently, or the known properties of each may contribute to differing extents or produce different effects, when the two techniques are used in combination. Accordingly, in preferred embodiments, the retrovirus associated with the microbubble is incapable of independent cell entry. As described herein, when in association with a microbubble a retrovirus may be unavailable for cell entry. Thus, a retrovirus is unavailable for cell entry before the administration of ultrasound when it is in association with a microbubble. "Independent cell entry" refers to cell entry that is independent of the administration of ultrasound. Thus, a retrovirus "incapable of independent cell entry" refers to incapability to enter a cell without exposure to ultrasound. Such a retrovirus is only able to enter a target cell with the administration of ultrasound to the target cell exposed to the microbubble-associated retrovirus. Any retrovirus that does not enter the target cell as a result of the administration of ultrasound is unable to subsequently enter a cell, target cell or otherwise.

This incapability may arise as a result of the retrovirus being non- internalizing, for example as a result of having a non-functional envelope protein or lacking an envelope protein. Thus, such a retrovirus requires ultrasound to facilitate cell entry as it lacks such capability itself. As described herein and is well known in the art, the ability of a retrovirus to enter a cell or "internalize" requires a functional envelope protein. A retroviral envelope protein functions to allow the retrovirus to obtain entry into a target cell via the interaction of the envelope protein with a transmembrane protein acting as a receptor on the surface of the target cell. This process leads to membrane fusion and cytoplasmic entry of the viral core. Accordingly, a retrovirus may not be able to internalize where it has a non-functional envelope protein. Such a retrovirus may be referred to as non-infectious. An envelope protein may be non-functional and incapable of allowing internalization of the retrovirus as a result of modification of the protein. The mechanism of viral cell entry is well understood in the art and modification of an envelope protein to produce a nonfunctional protein is well within the capabilities of the skilled person. For example, modification may be by addition, substitution or deletion of one or more amino acid residues of the protein, or by covalent or other modification of the protein. A modified envelope protein may be obtained by standard methods known in the art, for example using standard recombinant techniques, and is not limited to a particular method. Further, a retrovirus may be non-internalizing due to absence of an envelope protein. In either case, whilst the ability of the retrovirus to internalize or infect a cell is removed, the retrovirus is otherwise functional as defined herein. Thus, the retrovirus retains its normal capacities once inside the cell, for example, the ability to lose the viral envelope or coat once in the cell cytoplasm prior to nuclear entry and integration of the viral genome into the host cell genome (Porter, 2002) .

The retrovirus may be incapable of independent cell entry as a result of in vivo instability. As described herein, when in association with a microbubble, a retrovirus may be protected from clearance in vivo. A retrovirus alone may generally be susceptible to in vivo clearance over time, having a degree of stability in vivo. In the present invention, a retrovirus which is unstable in vivo is one which is unstable when it is not in association with a microbubble or a target cell. Thus, after administration of ultrasound, such a retrovirus that has not entered a target cell as a result is effectively unable to enter a cell because it is rapidly degraded. A retrovirus may be unstable in vivo as a result of modification, to reduce its stability in vivo. For example, a retrovirus may be modified by glycosylation. For example, the retroviral particle may have α-galactosyl carbohydrate epitopes on its surface for interaction with serum natural antibody rendering it sensitive to serum-complement-mediated inactivation or degradation (Takeuchi, 1996) . Glycosylation of retrovirus may be carried out by any suitable technique as well known in the art. Any method suitable for rendering a retrovirus more sensitive to in vivo degradation or unstable in vivo may be used as known in the art.

The use of retrovirus which is incapable of independent cell entry has the advantage of further assisting in the specific targeting of gene delivery to the target cells because the retrovirus is only able to enter cells at the target site as a result of the targeted ultrasound exposure. Any retrovirus released following ultrasound exposure which does not immediately enter a target cell and escapes from the target site is unable to effect gene delivery outside of the target site. Further, use of retrovirus incapable of independent cell entry also further maximizes bioavailability at the target site as described herein. Prevention of non-specific gene delivery to non-target cells also has the advantage of addressing safety issues surrounding previous uses of viral vectors. As discussed herein, a retrovirus used in the invention may lack the capacity for proviral integration into the target cell genome. Preferably such a virus nevertheless has the capacity to remain in the target cell nucleus and express the transgene, providing stable transgene expression. In particular, certain lentiviral vectors, for example those that have a deficiency in the viral protein integrase, may be used. Whilst unable to integrate their genetic material into the genome of the transduced cell, these vectors are able to remain in the nucleus and express the transgene. In addition to lentiviruses, any other retroviruses having these features may be used.

A retrovirus used in the invention may be any retrovirus suitable for use in gene delivery methods. Such retroviruses are well known in the art. Preferably the choice of retrovirus is based on compatibility with the choice of target cell to be transduced. Such choices are well within the capability of the skilled person. Preferred retroviruses include, but are not limited to, murine leukaemia virus (MLV), or those derived from lentiviruses, such as ■ human immunodeficiency virus (HIV) or equine infectious anaemia virus (EIAV) . "Retrovirus" and "retroviral particle" are used interchangeably herein and refer to a retrovirus which is functional, optionally, other than in its ability internalize as discussed elsewhere herein, such that it is functional at least once inside a target cell. In particular, a retrovirus used in the invention is capable of integration of the viral genome into the target cell genome or is at least retained in the nucleus of the target cell.

A retrovirus used in the compositions, methods and medical uses of the invention is preferably recombinant. A recombinant virus may be obtained by any suitable recombinant techniques as are well known in the art. Recombinant techniques in general are described in, for example, Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N. Y. 1989. Thus for example, the gene of interest, the incapability for independent cell entry and the presence of a targeting moiety as described herein, are all features of a retrovirus used in the invention which may be introduced by manipulation of the retrovirus using standard recombinant techniques. Further characteristics of a retrovirus may be changed as appropriate, for example, to facilitate manipulation. A retroviral particle used in the compositions, methods and medical uses of the invention is preferably associated with a microbubble. The retroviral particle may be bound to the surface of the microbubble as described herein.

Microbubbles are shelled gas bubbles and are well known in the art. As will be appreciated by the skilled person, any microbubble suitable for use with ultrasound may be used with the invention, including microbubbles known in the art, and microbubbles generated by techniques known in the art. Microbubbles for use in the invention have a shell which preferably comprises lipid. The shell may comprise other material, such as albumin or methacrylate . Examples of commercially available microbubbles are Definity™ and Sonovue™ having a lipid shell, and Optison™ having a shell comprising albumin. Preferred commercially available microbubbles include Sonovue™. Additionally, methods are available for microbubble generation based on lipid (Unger, 1994) or albumin (Porter, 1996) .

The preferred retrovirus particle association with microbubbles can be achieved in a variety of ways, including electrostatic interaction, affinity cross-linking and covalent attachment. Incorporation of material into the microbubble shell such that it becomes cationic enables electrostatic association with anionic retroviral particles. For instance, lipid-shelled microbubbles can be formulated to include a cationic lipid component (Christiansen, 2003) . Such microbubbles are preferred for use in the invention. Retrovirus may be associated with such cationic-shelled microbubbles by electrostatic interaction. Modification of the microbubble shell and/or retroviral particle surface may be used to effect interaction, for example, via the high-affinity of avidin for biotin. Further suitable techniques are well known in the art for providing for association between two binding partners, and may be used with the present invention to effect association between retrovirus and microbubble.

Microbubbles for use in the invention are gas-filled. Preferably, the gas is perfluorocarbon but alternatives include other clinically acceptable gasses, such as sulphur hexafluoride or air. The gas may be any which is suitable in respect of the properties of in vivo stability, ultrasound resonance and transduction efficiency of the microbubble .

A microbubble or retrovirus, or composition of the invention, for use in the invention may comprise a targeting moiety. A targeting moiety may be a ligand or an antibody (Klibanov, 1999) . Examples of an antibody targeting moiety include, but are not limited to, anti-ICAM- 1, anti-E-selectin, anti-VEGF receptor or anti-α_vβ₃ antibodies for targeting vascular endothelium. Suitable targeting moieties for targeting particular cell types are well known in the art. For example, it has been demonstrated that DNA-loaded microbubbles can be targeted to a state of inflammation in mice by modifying the microbubble shell with antibodies against a vascular endothelial marker of inflammation (Champaneri et al . , unpublished) . An advantage of the presence of a targeting moiety is to further improve gene delivery to a target cell due to accumulation of the microbubble-associated retrovirus at the target site resulting from interaction of the targeting moiety with the target cell. For example, the use of antibodies against disease-specific vascular markers will lead to accumulation of the microbubble-associated retrovirus in the target area before activation by ultrasound. This may be particularly advantageous where there are sub-populations of different cell types in close proximity, only one of which is desired as the target. Incorporating a targeting moiety for the desired target cell type will allow preferential accumulation at that target, providing specificity and efficiency advantages. A targeting moiety may be provided by any suitable technique known in the art. For example, a recombinant retrovirus may express a targeting moiety on its envelope. A targeting moiety may be associated with the shell of a microbubble, for example using techniques suitable for association of retrovirus with microbubble, as described herein.

Examples of amounts of virus and microbubbles capable of effecting cell transduction with a gene of interest according to the invention and suitable for use in any aspects of the invention are described herein. However, as is apparent to those skilled in the art in light of the present disclosure, such parameters are variable whilst still achieving the desired cell transduction. Those skilled in the art also appreciate the benefit of optimizing such parameters for a given application, for example, to achieve a desired ^"level of transduction efficiency. Such optimization is well within the capabilities of those skilled in the art utilizing the disclosure provided herein.

Compositions, kits, methods and uses of the invention may also utilize a substance that is capable of modifying a cell membrane, in particular, of improving membrane permeability. One or more such substances may be used to further improve cell membrane permeability and further improve cell transduction. Compositions and kits of the invention may comprise such a membrane-modifying agent. Methods and medical uses of the invention may utilize compositions or medicaments comprising a membrane-modifying agent. Methods of the invention may include the step of exposing a target cell to a membrane modifying agent. This may be achieved by adding a membrane modifying agent to one or more of a target cell, or retrovirus, microbubble or composition employed. Medical uses of the invention may further comprise use of a membrane modifying agent in the preparation of a medicament. Medical uses of the invention may use a composition comprising a membrane modifying agent in the preparation of a medicament. Examples of membrane modifying agents suitable for use in the invention include lidocaine (Nozaki et al . , 2003) and others as are apparent to those skilled in the art.

Means for administering gene therapy vectors to a subject, including systemically, are well known in the art and may be used with the present invention. In the methods and medical uses of the invention, the target cell may be exposed following systemic administration of the composition or medicament of the invention to the subject. The target cell may be exposed following direct administration of the composition or medicament to the subject in the vicinity of the target cell. Administration may be by injection. Means for administering gene therapy vectors to a target cell in vitro or ex vivo are well known in the art and may be used with the present invention. A target cell may be exposed in vitro or ex vivo in a suitable cell chamber containing a composition or medicament of the invention.

Compositions or medicaments of the invention may be administered in methods and medical uses of the invention to individuals having or at risk of suffering from a condition or disease. Administration may be alone or in combination with one or more medicaments and/or treatments as are known in the art. Typically, compositions of the invention are formulated as pharmaceutical compositions. These compositions may additionally comprise a pharmaceutically acceptable excipient, carrier, buffer, stabiliser or other materials well known to those skilled in the art. Such materials should be non-toxic and should not interfere with the efficacy of the active ingredient. The precise nature of the carrier or other material may depend on the route of administration, e.g. oral, intravenous, cutaneous or subcutaneous, nasal, intramuscular, intraperitoneal routes. Pharmaceutical compositions for oral administration may be in tablet, capsule, powder or liquid form. A tablet may include a solid carrier such as gelatin or an adjuvant or an inert diluent. Liquid pharmaceutical compositions generally include a liquid carrier such as water, petroleum, animal or vegetable oils, mineral oil or synthetic oil. Physiological saline solution, dextrose or other saccharide solution or glycols such as ethylene glycol, propylene glycol or polyethylene glycol may be included. Such compositions and preparations generally contain at least 0.1% by weight of the active ingredient (s) . For intravenous, cutaneous or subcutaneous injection, or injection at the site of affliction, the active ingredient will be in the form of a parenterally acceptable aqueous solution which is pyrogen-free and has suitable pH, isotonicity and stability. Those of relevant skill in the art are well able to prepare suitable solutions using, for example, isotonic vehicles such as Sodium Chloride Injection,

Ringer's Injection, Lactated Ringer's Injection. Preservatives, stabilisers, buffers, antioxidants and/or other additives may be included, as required. Administration is preferably in a "prophylactically effective amount" or a "therapeutically effective amount" (as the case may be, although prophylaxis may be considered therapy), this being sufficient to show benefit to the individual. The actual amount administered, and rate and time-course of administration, will depend on the nature and severity of the condition being treated. Prescription of treatment, e.g. decisions on dosage etc, is within the responsibility of general practitioners and other medical doctors, and typically takes account of the disorder to be treated, the condition of the individual patient, the site of delivery, the method of administration and other factors known to practitioners. Examples of the techniques and protocols mentioned above can be found in Remington's Pharmaceutical Sciences, 16th edition, Oslo, A. (ed) , 1980. The medicaments and pharmaceutical compositions of the invention may be administered by any of several routes, e.g. oral, intravenous, cutaneous or subcutaneous, nasal, intramuscular, intraperitoneal routes. The dose of the medicament or pharmaceutical composition is based on well known pharmaceutically acceptable principles . General dosages are based on mg of the active ingredient per kg body weight, for example, 0.01 mg/kg to 100 mg/kg, more preferably 0.5 mg/kg to 10 mg/kg. The dose will depend on the route of administration in addition to the factors described above.

In the methods and medical uses of the invention, the target cell may be exposed in vitro, ex vivo or in vivo.

In the methods and medical uses of the invention the ultrasound may be administered to the target cell using parameters within the ranges known to be safe for clinical diagnostic use (Barnett, 2000). Preferably this is pulsed ultrasound with a frequency in the range of 1-3MHz, using a pulse repetition frequency of 1-1OkHz and pulse lengths of from 1 up to 100 cycles. The total exposure time may be up to 30 minutes, preferably up to 3 minutes. The pressure amplitude may be from 0. IMPa to 3MPa peak negative pressure. A standard ultrasound transducer known in the art may be used. Conditions for both in vitro and in vivo applications fall within such safe ranges. Preferably, the ultrasound is administered as pulsed ultrasound with a frequency of IMHz, using a pulse repetition frequency of IkHz and pulse lengths of from 1 to 32 cycles. Preferably, the pressure amplitude is from 0.3MPa to 3MPa peak negative pressure. Preferably, the overall exposure time is from 1 to 8 seconds .

Examples of ultrasound parameters suitable for effecting cell transduction with a gene of interest according to the invention and suitable for use according to the invention are described herein. However, as is apparent to those skilled in the art in light of the present disclosure, such parameters are variable whilst still achieving the desired cell transduction. Those skilled in the art also appreciate the benefit of optimizing such parameters for a given application, for example, to achieve a desired level of transduction efficiency. Such optimization is well within the capabilities of those skilled in the art utilizing the disclosure provided herein. As described herein, peak negative pressure is the parameter most relevant to achieving microbubble cavitation. Peak negative pressure may therefore preferentially be optimized for any given application of the invention, for example to achieve maximum transduction efficiency. Choice of source of ultrasound and method of administering the ultrasound may be selected for compatibility with the desired level of peak negative pressure,, or other parameters, as is apparent to those skilled in the art. Conveniently, clinical diagnostic ultrasound systems may be used to administer ultrasound in clinical applications of the invention.

Conveniently for most applications of the invention, the ultrasound administered is pulsed. Also, conveniently for most applications of the invention, the ultrasound used has a frequency of IMHz, although a frequency of 2 or 3MHz, for example, may be used. Pulses of ultrasound may consist of any number of cycles from 1 cycle up to 100 cycles, inclusive, preferably of any number of cycles from 1 cycle up to 32 cycles, inclusive; i.e. a pulse of ultrasound may consist of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc....32, etc.... or 100 cycles. Pulses may be generated with a repetition frequency of from IkHz up to

10kHz, inclusive, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10kHz, preferably with a pulse repetition frequency of IkHz . Conveniently, administration of ultrasound may be for a total time of any time up to and including 30 minutes, preferably any time up to and including 3 minutes. Longer exposures to ultrasound may be used where there is no detrimental effect on the viability of the target cell. For example, the total time of exposure may be any time up to and including 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 minutes. Conveniently, total time of exposure may be any time up to and including 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,

18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 etc 35, etc....40, etc....45, etc....50, etc....55, etc.... or 60 seconds. Preferably, overall exposure time may be any time up to and including 1 second to 8 seconds, for example, up to and including 5 seconds.

These exposure times are preferably for individual administrations of ultrasound, but may indicate the total time for multiple administrations of ultrasound. As is apparent to those skilled in the art, the amount of ultrasound ^λ on-time' during exposure will be less than the total time of exposure where ultrasound is pulsed; the actual amount of 'on-time' depends on the nature of the pulsed ultrasound. For example, for an exposure time of 5 s using pulsed ultrasound consisting of 10 cycles of IMHz frequency with a repetition frequency of IkHz, the ^l on-time' is 50 ms .

Administered ultrasound may achieve peak negative pressures of up to and including 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9 or 3MPa. Peak negative pressure may be in the range up to and including 0.2, 0.5, 1.0 or 1.4MPa, or in the range more than or equal to 0.2, 0.5, 1 or 1.4MPa. Preferably, the peak negative pressure is more than or equal to 0.5MPa or within the range 0.5 to IMPa, inclusive. As is apparent to the skilled person from the disclosure herein, peak negative pressures achieved will vary with distance from the transducer focus . The examples of peak negative pressure given here indicate the peak negative pressure to which the target cell is exposed.

Selection of individual parameters for ultrasound exposure as described herein may be selected independently of each other. A key consideration when selecting ultrasound parameters is the ability of ultrasound having such parameters to effect cell transduction when administered according to the invention, as is apparent to those skilled in the art. Suitable parameters for a given application can be selected according to the disclosure herein and optimization, if desired, is routine for those of skill in the art using the disclosure herein.

Administration of ultrasound may comprise a single administration of ultrasound or may comprise multiple exposures to ultrasound, for example, cycles of administration of ultrasound. Any ultrasound exposure may have any of the parameters as described herein. In in vivo applications of the invention, for example, ultrasound may be administered in multiple exposures, for example to coincide with multiple administration of compositions or medicaments of the invention. In in vivo applications, ultrasound may be administered in multiple exposures, for example to coincide with multiple exposures of a target cell to microbubbles and retrovirus. For example, repeat direct administration of a composition or medicament to a target cell- may be accompanied by repeat exposures to ultrasound. Systemic administration of a composition or medicament may be accompanied by repeat exposures to ultrasound, preferably to coincide with exposure of the target cell to the composition or medicament during circulation. Repeat exposure of a target cell to microbubbles and retrovirus comprising the gene of interest, for example as may occur following systemic administration, may be accompanied by repeat exposures to ultrasound. Where repeat administrations of a composition or medicament and/or repeat exposures of a target cell occurs, administration of ultrasound is timed to coincide with at least one exposure of a target cell, preferably with more than one exposure of target cell, to microbubbles and retrovirus. As described with respect to the parameters of ultrasound exposure, administration of ultrasound may be varied and optimized for a given application, as is apparent to those skilled in the art.

In in vitro or ex vivo applications of the invention, ultrasound may be administered to the target cell via the cell chamber containing the target cell and composition or medicament of the invention. In in vivo applications of the invention, ultrasound may be administered to the target cell via ex vivo administration of ultrasound directed to the target cell. Any technique for administering ultrasound to a subject in a clinical setting as known in the art may be used. For example, an ultrasound transducer may be placed on the skin of a subject, using contact gel for efficient propagation of the ultrasound, such that the ultrasound beam is targeted to the target cell.

The gene of interest present in the retrovirus may be any gene desired for expression in a target cell. Preferably, the gene of interest is a therapeutic gene. As used herein, a "therapeutic gene" may be one the expression of which in a target cell is advantageous to the target cell, and/or a cell other than the target cell. A "therapeutic gene" may be one the expression of which in a target cell is advantageous to an individual receiving and/or comprising the target cell. Thus, a therapeutic gene includes a gene the product of which is directly beneficial in the target cell in which it is expressed. It also includes a gene the product of which is beneficial to a cell other than the target cell following its transport from the target cell to that other cell. A therapeutic gene may be a gene the product of which is useful in ways unconnected to the target cell in which it is expressed. Preferably, the gene of interest is a therapeutic gene, for example encoding an enzyme, hormone, growth factor, transcription factor or inflammatory mediator. Examples include, but are not limited to, adenosine deaminase, VEGF, GM-CSF, factor VIII, factor IX, CFTR, p53, TNFα, TIMP-3 and thymidine kinase.

The gene of interest may be one which is naturally expressed in a target cell or one which is not naturally expressed in a target cell. The gene of interest may be one which is not functionally expressed in a target cell. A gene of interest which is not functionally expressed in a target cell may not do so as a result of lack of presence of a functional gene in the cell , for example due to absence or mutation of the gene. A gene of interest which is not functionally expressed in a target cell may not do so as a result of lack of production of a functional gene product from the gene (at the level of mRNA and/or polypeptide) , although the gene itself may encode a functional product. Thus, lack of a functional gene product may result through the processes of transcription and/or translation, or because a target cell does not naturally express that gene.

A gene of interest for use in the invention includes a gene that is causative of a condition or disease in an individual when it is not functionally expressed, as described herein, in that individual. A gene of interest of the invention includes a gene that is not causally linked to a condition or disease in an individual. A preferred gene of interest is one which is therapeutically useful and which may or may not be causally linked to disease.

Preferably, the gene of interest is a gene the presence of which in mutated form in an individual results in a disease state in the individual . Preferably, the gene of interest is a gene the absence of which in an individual results in a disease state in the individual. Preferably, the gene of interest is a gene the lack or reduction of a functional gene product of which in an individual results in a disease state in the individual. Preferably, the gene of interest is a gene the presence of which can be utilised for therapeutic effect that may be unrelated to the genetic background of the disease. Preferably, the gene of interest is a gene which is not causally linked with disease. Particularly for use with in vitro methods of the invention, the gene of interest may be one which the expression of which is desired to be studied. For example, the effect of a test compound on the expression of the gene in the target cell may be of interest. The effect of expression levels of the gene on the target cell may be of interest. The gene of interest may be one the gene product of which is desired.

The gene of interest may be accompanied by genetic control mechanisms, for example, a control element such as a promoter, terminator, and other regulatory control elements. Such control mechanisms may impart transcriptional specificity. Such a means for controlling the expression of a gene of interest in the target cell is operably linked to the gene of interest. A control element is "operably linked" to a gene of interest when it is placed in a functional relationship with it. For example, a promoter is operably linked to a gene of interest if it effects transcription of that gene in a target cell.

In the course of the development of gene therapy, a wide knowledge of the potential targets for gene therapy, including the types and range of conditions and diseases that potentially may benefit from gene therapy, has built up. Thus, the skilled person is aware of conditions and diseases, and genes associated with such conditions and diseases, that are suitable for treatment with gene therapy using those genes. The skilled person is aware of conditions and diseases in which there may not be an underlying genetic cause in the sense of a lack of a functional gene and/or lack of a functional gene product, but which nonetheless are suitable for treatment with gene therapy. In such cases, the skilled person is aware of genes, the products of which are suitable for treating such conditions and diseases . Also known in the art are conditions and diseases, which whilst having an underlying genetic cause, may be treatable with the products of genes unrelated to the genetic cause. Thus, suitable genes of interest and suitable conditions or diseases for gene therapy as are well known in the art are applicable to the methods and medical uses of the invention.

Suitable-diseases and conditions as known in the art may result from a lack of, or altered functional expression of a gene in an individual, due to lack of a functional gene and/or lack of production of a functional product of a gene. Such a disease or condition may be treatable using the invention with the gene associated with the disease or condition as the gene of interest. Such a disease or condition may be treatable using the invention with a gene of interest unrelated to the disease or condition. The cause of the disease or condition may or may not be related to the gene of interest, such that the presence of a disease or condition in an individual may be independent of the status of the gene of interest in that individual .

The condition or disease may be a chronic disease. The condition or disease may result from mutation in the gene of interest in the subject. The condition or disease may result from a lack of the gene of interest in the subject. The condition or disease may result from a lack or reduction of a functional gene product of the gene of interest in the subject, as described herein.

Examples of suitable genes of interest and diseases or conditions for use with the invention include, but are not limited to, factor VIII or IX for haemophilia, CFTR for cystic fibrosis, adenosine deaminase for ADA-SCID and p53 or thymidine kinase for cancer. For some situations, such as the example of ADA-SCID, ex vivo treatment of the target cells is most appropriate.

Examples of preferred applications of the invention include, but are not limited to use of factor VIII or IX for haemophilia, CFTR for cystic fibrosis, adenosine deaminase for ADA-SCID and p53 for cancer, as examples of genes of interest suitable for treating disease due to single gene defects. In such cases, target cells may be those in which the defective gene would normally be functional, such as the airway epithelial cells for expression of CFTR or cells other than those manifesting the defect in which expression of the gene of interest maybe therapeutic, such as muscle cells for expression of factor VIII for haemophilia. Other examples of preferred applications of the invention include, but are not limited to, the use of thymidine kinase, nitroreductase or cytosine deaminase for prodrug activation approaches for cytotoxicity, for example, in treating cancer or clearing atherosclerotic plagues. In such cases, target cells may be the cells causing disease, for example, tumour cells, or cells of their supportive stroma, including vascular endothelium. Further examples include, but are not limited to, genes such as TNFα or GM-CSF for immunomodulatory therapy. Yet further applications include, but are not limited to, ex vivo gene transfer of TIMP-3 or VEGF to aid vascular engraftment.

In particular, where the gene of interest is thymidine kinase, a target cell expressing this gene will be selectively killed by gancyclovir. Using this gene in methods of the invention with a target cell such as a tumour cell, or vascular endothelial cells in the vasculature supplying the tumour, will result in a reduction in tumour volume following exposure with gancyclovir (Mavria, 2005) .

The present invention is particularly useful for gene delivery targeting the vascular endothelium, as compositions and medicaments of the invention administered systemically may be constrained within the vascular compartment. Thus, the present invention is particularly useful for a number of conditions and disease states including, but not limited to the following. For example, the present invention is applicable where angiogenesis is required to be enhanced to help recovery, for example in angina, ischaemia, and for wound repair. For example, the present invention is applicable where angiogenesis needs to be prevented, such as in the early stages of tumour development or for the avoidance of restenosis after coronary artery angioplasty. The present invention can be used in cases where vascular structures require modification, for example, where there are artherosclerotic plaques, for wound repair, and for altering blood supply to established tumours.

The present invention is also particularly useful where a protein secreted into the circulation would provide therapeutic benefit, for example in metabolic diseases such as haemophilia and Lesch-Nyhan syndrome and for hormonal treatments.

A target cell for transduction with the compositions, methods and medical uses of the invention is one which is capable of functional expression of a gene introduced therein. Thus, a target cell is one which is capable of expression of the gene of interest introduced into it.

A suitable target cell for use in the invention will be apparent to the skilled person depending on the desired application of the invention. Suitable target cells for use with the invention are described herein, including preferred target cells for use with the invention in treatment of preferred conditions and diseases as described herein.

In methods and medical uses of the invention, the target cell may be one which lacks functional expression of the gene of interest, as described herein. For example, in the methods and medical uses of the invention, the target cell may be one which has the gene of interest in mutated form. The target cell may be one which lacks the gene of interest. The target cell may be one which lacks a functional gene product of the gene of interest as described herein. The target cell may be other than the above but such that expression of the gene of interest in the target cell is therapeutic. The target cell may be one which does not naturally express the gene of interest.

Examples of target cells that may be the subject of the invention include, but are not limited to the following.

Preferably, the target cell is a vascular endothelial cell. The target cell may be a tumour cell. The target cell may also be a bone marrow cell, particularly in the case of ex vivo gene delivery, a muscle cell, particularly in the case where secretion of therapeutic gene product, such as factor VIII, is desired, or an epithelial cell, for example for use with a gene of interest such as CFTR.

Examples and Experiments

The invention will now be further described by way of example and not limitation with reference to the following examples and accompanying drawings. It will be appreciated that what follows in this section is by way of example only and that modifications to detail may be made while still falling within the scope of the invention.

Materials and Methods Vector preparation

Cationic lipid-shelled, perfluorocarbon-filled microbubbles (Christiansen, 2003) were washed using centrifugation (500rpm, 5min) to remove excess lipids and resuspended in serum-free OptiMEM™ medium. Non-infectious murine leukaemia virus (ni-MLV) lacking envelope protein was prepared in serum-free OptiMEM™ medium from confluent cultures of TELCeBβ cells; these cells express the retroviral gag-pol gene products and package a recombinant retroviral vector transducing nuclear-localised β-galactosidase (Cosset, 1995) . The preparation was filtered (0.45μm) before use (Porter, 2002).

For attaching virus to microbubbles equal volumes of each were combined and incubated 30min before washing twice to remove unbound virus. All washes and microbubble samples were retained for western blot analysis of virus-microbubble association.

Similarly, Sonovue™ microbubbles were used.

Western blot analysis of virus-microbubble association

Samples were diluted 1:2 in SDS-loading buffer, heat-denatured and loaded onto a 10% SDS-PAGE electrophoresis gel. The gel was run at 3OmA to separate the protein content of the samples. The proteins were transferred to a nitrocellulose membrane at 20OmA and the membrane blocked in 5% milk in Tris-buffered saline (TBS) overnight. The membrane was then incubated lhr in TBS/0. l%Tween with 1:1000- diluted anti-p30 (goat-derived anti-capsid) antibody, followed by washing to remove unbound antibody. The membrane was then probed with a horse radish peroxidase-conjugated rabbit anti-goat 2° antibody, washed and developed by enhanced chemiluminescence and autoradiography.

Transduction protocol

TE671 human rhabdomyosarcoma cells were seeded in an Opticell™ (Biocrystal PIc) and incubated in DMEM/10%FCS to reach 80% confluence. The cell growth medium was aspirated and replaced with 10ml OptiMEM™, to which microbubbles and virus, or virally-loaded microbubbles, were added before injection into the cell chamber of the Opticell™. Exposures were performed by placing the Opticell™ unit on a stand in a custom-built water tank at 37°C. The unit was positioned either above or below the ultrasound transducer at its focal distance, dosimetry of the beam having been previously determined using a needle hydrophone. Alternatively cells were exposed within the near field of the transducer, to examine the difference in effect of different areas of the field. Exposures were performed at IMHz, IKHz pulse repetition frequency with various numbers of cycles per pulse (1-32) and at various pressure amplitude settings (0.3-3MPa peak negative pressure). Overall exposure times varied from 1-8 sec. After ultrasound exposure the cells were allowed to settle for 2 hrs at 37⁰C before replacing the medium with DMEM/10%FCS and allowing cells to grow for 48hrs.

Gene expression assay

Cells were fixed with 0.5% gluteraldehyde for 15min and histochemically stained with lmg/ml X-GaI solution to visualize expression of the β-galactosidase reporter. The Opticells™ were incubated at 37°C for 4hr to allow transfected cells to convert the colourless X-GaI to its blue product. Blue and uncoloured cells were counted using an inverted microscope to determine the percentage of viable cells which were transduced.

In vivo mouse model

Microbubbles and virus or virally-loaded microbubbles as described above are directly injected into a subcutaneous tumour or into a tail vein. Subsequently, to activate the vector, ex vivo exposure of the tumour to ultrasound is performed. Exposures of varying frequency, number of cycles, pressure amplitude settings and time of exposure are performed within ranges as described above. Transduction is analyzed as described above by visualizing β-galactosidase expression.

In other experiments, virus is used which comprises the therapeutic gene thymidine kinase in place of the β-galactosidase reporter gene, and a promoter specific for endothelial cells, such as that from the human pre-proendothelin-1 gene (Mavria, 2005) to direct expression in the tumour vasculature. Transduction is analyzed by administering gancyclovir which selectively kills cells expressing thymidine kinase. A reduction in tumour volume thus indicates gene expression in the tumour vasculature .

The mice are subjected to general anaesthesia prior to partial immersion in water, at a temperature chosen to maintain body heat and with the aid of a support for the animal, such that^" the tumour is placed at the focus of the ultrasound transducer and so that the ultrasound can be administered through the water. Alternatively, a tissue phantom is placed between the transducer and the tumour site without the need for immersion in water, or the transducer is placed directly on the skin, using contact gel for efficient propagation of the ultrasound.

Results

Ultrasound-mediated transduction by non-infectious MLV

To determine whether ultrasound could be used to enable cell entry by ni-MLV, a transduction assay was performed using virus particles carrying the β-galactosidase gene. The particles are structurally equivalent to infectious virions with the exception that they lack the virus-encoded envelope protein that is necessary to trigger fusion of the viral and cellular membranes. Ni-MLV was added to TE671 cell monolayers either alone, mixed with cationic lipid microbubbles just before transduction or pre-loaded onto the microbubbles (see below) . Each condition was investigated with or without US exposure. Very low levels of transduction were observed when virus alone was applied to cells, whether US exposure was performed or not (niv and niv+US) , in keeping with the lack of a means of triggering entry. Significant levels of transduction were only observed in experimental groups combining virus, US and microbubbles (niv+MB+US and nivMB+US) (Fig.l). When microbubbles were mixed with virus particles immediately before addition to cells (niv+MB+US) , an approximately 140-fold enhancement of transduction was observed over non-microbubble control groups. A further 17-fold enhancement was obtained by binding the niMLV particles to the microbubble surface before transduction (nivMB+US) .

Spatial localisation of transduction Despite uniform exposure of the cells within the Opticell™ to the vector, gene delivery using virally-loaded microbubbles was found primarily around the US beam focus, demonstrating spatial localisation of transduction according to US exposure. Using an inverted microscope, a mark was made on the surface of the Opticell™ where one or more β-galactosidase expressing cells were observed to give a visual representation of this localisation (Fig.2); gene delivery only occured in an area within approximately 1.5cm radius from the beam focus. This demonstrates that the vector was only capable of cell transduction in the presence of US exposure above a necessary dose threshold. Analysis of virus-microbubblβ association

The most efficient gene delivery was observed when microbubbles were incubated with virus prior to the transduction experiment. To assess the association, microbubbles were incubated with virus and then washed twice using centrifugation to remove unbound virus. Western blot analysis for viral capsid protein was performed on the wash supernatants and microbubble samples to evaluate the association with virus; while a great deal of virus was removed in the first wash (lane 1), more was present in the washed microbubble sample (lane 3) than in the second wash (lane 2) (Fig.3). This indicates that, despite vigorous washing to separate them, a significant proportion of the virus remained associated with the microbubbles.

Efficiency of transduction

As demonstrated above, the microbubble-associated vector gave the highest gene delivery levels, despite the loss of virus during washing stages in this experimental group due to incomplete binding to microbubbles. In these preliminary, proof-of-principle experiments, a maximum level of transduction of 0.35% was achieved using lOOμl virally-loaded microbubbles with US exposure. This compares favourably with the transduction efficiencies obtained in our previous experiments using a similar complex of ni-MLV with cationic liposomes at this dilution on TE671 target cells (Porter, 2002) .

In vivo mouse model

Tumour-targeted gene therapy is demonstrated in vivo in a mouse model demonstrating the clinical potential of the technique. The exposure of the tumour to ultrasound activates the retroviral vector in vivo in a targeted manner in the area of ultrasound exposure. In particular, the vector is activated in the tumour vasculature following exposure with ultrasound.

Discussion

Ultrasound-dependent gene delivery using a microbubblβ-associated viral vector

The development of gene therapy strategies requiring systemic vector administration is currently hampered by problems such as cell- or antibody-mediated clearance that limit efficient delivery, vector stability, toxicity and lack of target cell specificity. We have shown that US/microbubble technology in concert with viral vectors addresses these issues. Without being bound by any particular theory, association with microbubbles may render virus unavailable for non-productive adsorption or clearance and so maximise bioavailability at the target site following spatially-controlled US- mediated release. Vector instability (eg. upon serum exposure) could be a desirable feature in this system to limit gene delivery to the site of insonation if microbubble association effectively provides the stability prior to release. Thus, we have exemplified the interaction of recombinant retroviral vectors, lipid microbubbles and ultrasound.

Results demonstrate that US enables the transduction of TE671 cells in vitro by a microbubble-associated virus vector which is inherently incapable of cell entry; gene delivery is restricted to cells at positions within the US beam. Our experiments are a proof-of- principle demonstration that viral gene delivery can be spatially and temporally controlled via dependence for activation upon US exposure.

Further development of the method

The combination of a retroviral vector with a physical cell entry mechanism is novel. As with any new technique, further development will be beneficial. Whilst significant levels of transduction efficiency have been shown, further experimental work will further optimise the ultrasound parameters, virus dose-dependence and microbubble concentration in order to maximize efficiency of gene delivery and minimise toxicity. Concentrations of ni-MLV currently used have been relatively low and it is reasonable to believe that the efficiency can be further greatly improved; in fact, efficiency relative to the amount of ni-MLV used compares favourably to that achieved using cationic liposomes to mediate entry which is within an order of magnitude of the efficiency of infectious virus alone (Porter, 2002) . Assessing sonoporation under ultrasound conditions similar to those currently achieving transduction will help to elucidate the underlying mechanism further and assist in further optimisation of the technique.

An advantage of this new approach is that it enables both temporal and spatial control over gene delivery. The use of non-infectious MLV further enables gene delivery without the risk of infection at sites outside of the region of insonation. The observation that transduced cells occurred in colonies, as seen with infectious retroviral vectors, is indicative of stable transgene integration and its maintenance during cell division. Stable gene delivery offers a significant advantage over the transient expression possible with DNA-based vectors used in previous reports of ultrasound-mediated transfection .

Materials and Methods

Cell culture and non-enveloped virus production Human rhabdomyosarcoma TE671 cells (and TELCeBβ producer cells derived there from) were grown in Dulbecco's modifies Eagle's medium (DMEM) supplemented with 10% fetal calf serum (FCS) . For gene delivery experiments, TE671 cells were seeded in OptiCell™s (Biocrystal Pic) and incubated in DMEM (10% FCS) to reach 60% confluence. TΞLCeBβ cells, carrying the MFGnlsLacZ retroviral vector and expressing Moloney-MLV gag-pol proteins (Cossett et al . , 1995), produce viral particles that are non-infectious because they lack envelope proteins. Non-enveloped MLV was harvested in serum-free OptiMEM I (Invitrogen Ltd) from confluent cultures of TELCeBβ cells, and the supernatant passed through a 0.45μm filter (MiniSart) before use as described previously (Porter, 2002) .

Microbubble-retrovirus vector preparation

Cationic lipid-shelled, perfluorocarbon-filled microbubbles (Christiansen et al., 2003) were washed using centrifugation (5Og, 5min) to remove excess lipids and resuspended in PBS. The microbubble concentration was determined using a hemocytometer . To attach virus to microbubbles sufficient for three experimental replicates, approximately 6xlO⁷ microbubbles were added to 6ml virus suspension, and the mixture incubated at room temperature for 30min to allow an electrostatic attachment before washing the microbubbles twice to remove unbound virus . The virally loaded microbubble vector was then resuspended in 30ml OptiMEM I before application to cells (10ml per OptiCell™) .

Western blot analysis of virus-microbubble association Samples taken from the process of virally loaded microbubble production were diluted in SDS-loading buffer, heat-denatured and loaded onto a 12% acrylamide SDS-PAGE electrophoresis gel for analysis of viral capsid p30 protein content. The separated viral proteins were then transferred to a nitrocellulose membrane. The membrane was ^λ blocked' in 5% milk in Tris-buf fered saline overnight , followed by incubation with goat anti-Rauscher leukemia virus p30 1°

_* antibody (Quality Biotech Inc.). The membrane was washed to remove unbound antibody and then incubated with a horseradish peroxidase- conjugated rabbit anti-goat 2° antibody (Dako Ltd.) . The membrane was then washed and developed using enhanced chemiluminescence (Amersham International) and autoradiography.

Ultrasound exposure tank and apparatus A water tank was designed for reproducible cell exposures to ultrasound at the focal distance of the transducer (Pig. 4a) . The transducer was held within a positioning device at the base that enabled vertical alignment of the beam for a reproducible field position. An absorber placed at the top of the tank prevented repeated reflection of the ultrasound through the cells, which would increase their total exposure. To avoid reflection of the ultrasound field at a hard plastic tissue culture surface or the presence of a medium/air interface above the cells, which could create standing waves, we used the OptiCell™ unit for insonation of cells, supported at the height of the transducer focus. The OptiCell™ consists of two membranes, treated for cell growth, which have been demonstrated to be acoustically transparent and which therefore do not interfere with the ultrasound field in the tank; the membranes create a sterile chamber of 10ml, accessible through self-sealing ports. Ultrasound signals were produced using an arbitrary pulse generator system consisting of computer-controlled gated sinusoidal waveform signal generator, signal power amplifier and IMHz transducer. The transducer was a single piezoelectric element of 20mm diameter, spherically focused with a radius of curvature of 67mm (Imasonic) .

Ultrasound Dosimetry

The ultrasound field was measured using a calibrated 0.2mm diameter needle hydrophone (Precision Acoustics Ltd) . The transducer was fixed at the base of the tank, and the hydrophone placed at the position of the OptiCell™ membrane holding the cell monolayer (i.e. at the beam focus) . The hydrophone was moved in 0.5mm increments and the peak-negative pressures measured at each point to create a lateral beam plot of acoustic pressure. Transduction protocol

The growth medium was replaced with 10ml OptiMEM I containing ImI virus alone, ImI virus mixed with approximately 1x1O⁷ microbubbles, or (to test the effect of attaching the virus to the microbubble) ImI virally-loaded microbubbles. Note that to make the combined vector, 2ml virus per OptiCell™ was initially incubated with approximately 2xl0⁷microbubbles, to account for subsequent loss of approximately half of each during the association and washing steps. After injection of the microbubbles into the OptiCell™ chamber, each unit was allowed to stand for lOmin to allow the microbubbles to rise up against the cells. Insonations were performed by placing the OptiCell™ at the focal distance of the transducer in the custom- built water tank. The tank was filled with water at a temperature of 37°C, and prior to ultrasound exposure the cells were allowed to acclimatize for 60s. Exposure to IMHz pulsed ultrasound was performed at IkHz pulse repetition frequency, 10 identical sinusoidal cycles per pulse (pulse length) , with an amplifier input voltage amplitude (8OmV) chosen to produce a focal peak negative pressure of 1.4MPa. Overall exposure time was 5s. Controls were sham-exposed to provide a no ultrasound comparison. After ultrasound exposure the cells were allowed to recover for 4hrs at 37°C before replacing the medium with DMEM (10% FCS) and incubating cells for 48hrs for growth and transgene expression.

Gene expression assay- Cells were fixed with 0.5% gluteraldehyde (15min, room temperature) and histochemically stained with lmg/ml X-GaI at 37⁰C for 4h as described previously (Porter et al . , 1998) . Numbers of blue cells were counted by observation with a microscope within three boundaries with radii at 2.5, 7.5 and 13mm from the beam axis. The counts were then normalized, to account for varying cell number between experiments, by dividing the total number of blue cells by the total number of viable cells, and expressing the result as a percentage. Additionally, efficiency was determined for each 3.14mm² (2mm diameter) microscopic field in a 13x13 grid centered on the beam axis .

Results and Discussion

To determine whether ultrasound with microbubbles could be used to achieve specific gene delivery by means of effecting targeted entry of viral cores, transduction assays were carried out using an infection-deficient retroviral vector carrying the β-galactosidase marker gene. The effect of ultrasound exposure conditions on gene delivery and transduction efficiency was also determined.

An ultrasound system that could generate arbitrary pulses, and exposure apparatus, was assembled. Target cells adherent to the acoustically-transparent ^window' of an OptiCell™ cell culture unit were placed at the focal distance of a IMHz transducer, with an acoustic absorber placed behind to prevent the cells from being exposed to sound associated with reflections from the water surface as described (Fig. 4a) . Fig. 4a represents an example of apparatus suitable for carrying our ultrasound exposures. As is apparent to those skilled in the art, further apparatus and arrangements of equipment may be used to generate and deliver ultrasound as required to target cells in in vitro and in vivo settings. The transducer was driven by a gated sinusoidal signal whose voltage amplitude was chosen such that the acoustic pressure, which varies with distance from the transducer focus, would cover the range of values likely to be effective with reference to those used previously for DNA delivery. The peak-negative pressure, the parameter most relevant to achieving microbubble cavitation, varied equivalently along orthogonal axes through the focus (Fig. 4b) . Fig. 4b shows lateral beam plots for the IMHz transducer produced with a 0.2mm diameter needle hydrophone, in the plane of the transducer focus, which is also the plane in which the cell monolayer is positioned in the tank during exposures. Plots are shown of the peak negative pressure amplitude as a function of radial distance from the acoustic beam axis in two orthogonal directions, i.e. the X and Y axes, where the sound wave travels along the Z axis (black line=X axis; grey line=Y axis) . In these experiments, peak negative pressure values of ≥lMPa were reached over an area with 5mm diameter, with a maximum pressure of 1.4MPa at the center of the beam; pressures of >0.5MPa and ≥0.2MPa corresponded to diameters of 15mm and 26mm, respectively.

In a preliminary experiment, an infection-deficient retroviral vector, carrying the β-galactosidase marker gene was added to TE671 cells with cationic lipid microbubbles for exposure to ultrasound. Cationic microbubbles were chosen in anticipation of their electrostatic association with retroviral particles, comparable to the use of the former for binding plasmid DNA, and the association of retroviruses with cationic liposomes. Ultrasound exposure was 5s, during which pulses that consisted of 10 identical cycles of a sinusoidal wave of IMHz frequency were generated with a repetition frequency of IkHz, i.e. amounting to 50 ms of ultrasound "on time" during the 5s exposure. There was no transduction with virus alone, or when the microbubbles or ultrasound were omitted. In contrast, transduction was detected when cells were exposed to virus, microbubbles and ultrasound, indicating that ultrasound-induced microbubble cavitation is capable of effecting entry of the viral core. All conditions were evaluated in triplicate. Transduction was limited to the cells near the center of the area of exposure to ultrasound, consistent with a threshold acoustic pressure for microbubble cavitation-induced entry. The efficiency of gene delivery in this first experiment was, however, very low (% viable cells transduced - mean 0.01% +_ 0.0006s. d.).

To enhance their association, microbubbles and virus were pre- incubated before addition to the cell culture. Microbubbles were then washed by flotation under low-speed centrifugation, to separate microbubble-associated virus from free virus. Because of the geometry of the experiment (Fig. 4a) , association with the microbubbles would be expected to bring virus into close proximity to the cells during exposure, mainly via buoyancy of the microbubbles but perhaps also from the effect of acoustic radiation force in the direction of sound propagation (Rychak et al . , 2005). Microbubble- virus association was assessed by western blot analysis of viral capsid protein in the microbubble (MB) and wash fractions (wl and w2) : approximately 50% of virus was attached to the microbubbles (Fig. 5a) .

Transduction was greatly enhanced using microbubble-associated virus with the ultrasound exposure conditions used above (Fig. 5b) . Fig. 5b shows the effect of altering the peak negative ultrasound pressure amplitude on transduction by virally-loaded microbubbles, as determined by counting the number of β-galactosidase-expressing cells in areas of the cell monolayer defined by pre-determined acoustic pressure boundaries. These boundaries were three concentric circles, centred on the acoustic axis, producing regions of radii 0.0-2.5mm (peak negative pressure >lMPa) , 2.5-7.5mm (peak negative pressure 0.5-1MPa), 7.5-13mm (peak negative pressure 0.2-0.5MPa) and >13mm

(peak negative pressure <0.2MPa). In addition, in sham experiments the cells were entirely without ultrasound exposure (no US) . Data shown are the means for three experiments; the mean count within each spatial region was computed for each experiment prior to averaging the three, so that error bars represent the standard deviation of the corresponding three mean values about the global mean for each region. Numerical values for the "no US" datum are mean = 0.01%, standard deviation = 0.007%. An acoustic pressure threshold existed for entry of the infection-deficient virus, with little transduction occurring for peak negative pressures of <0.5MPa in these experiments. There was a background level of transduction with sham ultrasound exposure, presumably due to the cationic nature of the microbubble leading to enhanced cell-association of virus, perhaps also itself aiding entry, as has been observed for cationic liposomes (Porter, 2002) . However, the ultrasound-mediated delivery was >100- fold in excess of this background for pressures >0.5MPa.

As described above, the results presented in Fig. 5b provide averages and standard deviations, for three experiments, of mean transduction efficiency calculated within acoustic pressure bands defined by radial position of the cells in the sound field at the focal plane of the ultrasound source. Although the standard deviations help in assessing the significance of the difference between means in different bands, the averaging of cell counts within rings presupposes a degree of spatial homogeneity of transduction around each annulus, and in the background that was largely unexposed to ultrasound. To evaluate the spatial correlation of transduction with acoustic pressure more carefully, and particularly to confirm the expected symmetry of transduction about the acoustic axis, the efficiency was determined for each 2mm diameter microscopic field in a 13x13 grid centered on the beam axis (Fig. 6a-e) . The transducer had been designed such that the circularly-symmetrical fall in acoustic pressure within the ultrasound beam with increasing distance from the beam axis would be sufficiently gentle to provide for spatial registration of transduced cells with the local pressure: i.e. to enable the relationship between acoustic pressure and transduction to be determined by spatially registering the counts of transduced cells with the local pressure values. Fig. 6b shows a detailed representation of percentage of transduced cells at fields of view within an area exposed to ultrasound of peak negative pressure 0.2-1.4MPa. The data represent the mean efficiency of each field of view for the three experiments described in Fig. 5. Circles indicate the boundaries where peak negative pressure equals IMPa, 0.5MPa, and 0.2MPa (radii of 2.5, 7.5 and 13mm, respectively. As indicated by the darkest shading, transduction efficiencies of up to 2% were achieved. Fig. 6 c, d and e show representative fields of view at regions exposed to ultrasound of peak negative pressure ≥lMPa, 0.2-0.5MPa and <0.2MPa, respectively. The numbers of blue transduced cells reduce in number from c to e with the decreasing peak negative pressure. As seen in Fig. 6b, transduction was symmetrical about the beam axis, with significant efficiencies restricted to areas exposed to f>0.5MPa peak negative pressures in these experiments. Gene delivery was reduced at the very center of the beam, which was also associated with a small reduction (15%) in cell viability. This suggests that pressures as high as 1.4MPa may be detrimental at the pulse length, repetition rate and exposure time employed in these experiments, and that the optimal range of values for the peak-negative acoustic pressure under these conditions is 0.5-1MPa (giving transduction efficiencies of up to about 2%). Importantly for eventual clinical application, these ultrasound exposure parameters are within the range that can in principle be generated by clinical diagnostic ultrasound systems.

The maximum transduction efficiency achieved compares favourably to that obtained, without spatial localization, using cationic liposomes with this virus (Porter, 2002) . However, delivery was achieved with the application of ultrasound for 5 s only, in contrast to the 40 min exposure of the virus/lipid complex; although the microbubble- associated virus was present for a much greater period of time, delivery in the absence of ultrasound was negligible. The negligible delivery without the 5 s of ultrasound indicates not only that ultrasound is required for delivery, but suggests that the delivery seen when ultrasound was applied occurred within the 5 s window. In contrast, with the virus/lipid complex, delivery was on-going throughout exposure of the cells to this complex for 40 min. Therefore, the efficiencies reported here represent highly efficient usage of the virus, with delivery in 5 s compared to 40 min, with the additional benefit of localisation to the site of insonation. A treatment schedule involving multiple exposures to ultrasound and vector, mimicking the situation of in vivo administration and circulation of vector through the site of insonation, is expected to achieve cumulative increases in gene delivery sufficient for a therapeutic effect in vivo. In summary, gene therapy strategies to treat both chronic and acute diseases would be made more effective by the development of a vector which could achieve highly site-specific, long-term gene expression¹. Retroviral genome integration can achieve permanent gene expression; however, achieving specificity of delivery in vivo currently represents the major problem limiting the clinical application of retroviral vectors². We have addressed this problem with a novel retroviral delivery methodology capable of long-term, target-specific gene delivery. In vitro, ultrasound-targeted gene delivery to a predetermined area of a monolayer of cells was achieved using microbubbles loaded with infection-deficient murine leukemia virus (MLV) particles. In these experiments, gene delivery efficiencies of up to 2% were achieved near the beam focus, where ultrasound peak negative pressures of 0.5-1MPa were measured. In these experiments, significant transduction was restricted to areas exposed to :>0.5MPa acoustic pressure despite uniform application of the vector to all cells, demonstrating an ultrasound pressure threshold that can be exploited for targeted retroviral transduction. The methodology readily lends itself to implementation for controlling retroviral gene delivery in vivo following systemic administration.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

As discussed in detail above, prior to the present invention, neither viral nor non-viral approaches have satisfactorily addressed the requirement for an efficient, safe vector for transgene delivery into cells. Viral vectors are capable of highly efficient gene delivery, and retro- and lentiviral vectors integrate into the genome of the cell, achieving permanent gene expression for a sustained therapeutic outcome. Widespread use of these vectors, however, is compromised by the lack of control over gene delivery to target cells, and by safety issues raised after leukemia in clinical trial patients was attributed to insertional mutagenesis (Hacein-Bey-Abina et al . , 2003) . Whilst the latter point is addressable by vector modification, specificity of vector delivery remains a critical issue, especially for situations best suited to a systemic administration route. Gene delivery techniques tend to focus on modification of the vector itself in order to enhance delivery to cells. However, cells become more receptive to genetic material when exposed to ultrasound, due to temporary modification of cell membrane permeability. Ultrasound has been used in non-viral gene therapy to enhance the delivery of naked DNA into cells, described as 'sonication loading'. It has_φ subsequently been shown that transient pores form in the cell membranes during ultrasound exposure, through which DNA may enter the cytoplasm (Bao et al . , 1997) . Although the exact mechanism of DNA entry remains unclear, without being bound by any particular theory, it seems that cavitation (a process of gas bubble formation and oscillation which may include destruction) occurs in a solution exposed to an ultrasound field, and is likely to be responsible for the creation of shear forces in the cell growth medium which subsequently causes the formation of cell membrane pores .

Gene delivery efficiency can be increased significantly by the addition of cavitation 'nuclei', or microbubbles, to the transfection medium for DNA uptake . The technique was further developed by attachment of the genetic material to the microbubble. In vivo, this technique has been used to direct the delivery of therapeutic genes to muscle (Lu et al . , 2003) or tumor (Miller & Song, 2003) following direct injection of microbubbles and DNA, or by insonating the heart following intravenous administration of DNA-loaded microbubbles (Korpanty et al . , 2005). The further development of microbubbles as molecular imaging probes by incorporating targeting ligands promises to improve vector accumulation at the target site prior to insonation. While this technique effects specific gene delivery, naked DNA is lost when cell division occurs resulting in short term gene expression of around four days without repeat administration (Bekeredjian et al . , 2003). In many cases, long-term gene expression is important in achieving a therapeutic effect; for example in the use of anti-angiogenic or immunostimulatory approaches to treat tumors, or for expression of a secreted product such as factor VIII in treatment of hemophilia.

As discussed above, retroviral vectors provide for long-term expression, although restriction of gene delivery to target cells is problematic : approaches towards targeted binding and entry have generally not been successful and specificity of delivery represents a major problem limiting clinical application of retroviral vectors. Normally, retroviral entry is via receptor-mediated virus-cell membrane fusion, resulting in the cytoplasmic release of the uncoated core. Retroviral particles lacking the required envelope protein are infection-deficient, although the cores are still competent.

The present inventors have appreciated the need for an efficient, safe vector for specific transgene delivery into target cells achieving site-specific gene expression and the problems associated with meeting such a requirement. The inventors have addressed this problem with a novel retroviral delivery methodology. As described above, the inventors have demonstrated that ultrasound with microbubbles can be used to achieve specific gene delivery by means of effecting targeted entry of retroviral cores. Importantly for therapeutic use and clinical application, the ultrasound exposure parameters demonstrated to effect specific cell transduction as described herein are within the range that can be generated by clinical diagnostic ultrasound systems and are clinically acceptable.

The inventors have provided novel methods and shown that the use of ultrasound and microbubbles in concert with retroviral vectors addresses issues associated with development of gene therapy strategies requiring systemic vector administration, including efficiency of delivery, vector stability, toxicity and lack of target cell specificity. Without being bound by any particular theory, association with microbubbles may render virus unavailable for nonproductive adsorption or clearance and so maximise bioavailability at the target site following spatially-controlled ultrasound-mediated release. Vector instability (eg. upon serum exposure) could be a desirable feature in this system to limit gene delivery to the site of insonation if microbubble association effectively provides the stability prior to release.

The mechanism underlying cell entry is not yet fully understood. MLV particles are initially adsorbed by cells via a non-specific interaction, which is followed, in fully infectious particles, by the receptor-mediated fusion bete^seen the viral and cell membranes to allow entry of the viral core. Without being bound by any particular theory, sonoporation, the process of cell membrane pore formation during cavitation (in this case ultrasound excitation of microbubbles) , could be responsible for cell entry of the virus. Using electron microscopy, shards of microbubble shell have been observed embedded in cell membranes following ultrasound exposure, so it is possible that the virus is forced across the cell membrane attached to microbubble fragments. The possible fate of an intact viral particle after sonoporation-mediated entry is also not yet fully understood; for a fully infectious MLV particle the viral core enters the cell without the outer membrane envelope. Direct injection of virus particles into the cytoplasm of cells could help to assess whether MLV particles generate the necessary pre- integration complex when introduced without membrane fusion. An alternative mechanism could be that plasma membrane permeability changes in the ultrasound field lead to enhanced fusion between the viral and cell membranes. With such assisted virus entry, subsequent viral passage to the nucleus would be as for fully infectious MLV particles. Indeed, it has been shown that the defective MLV used in this study can transduce cells in the presence of DOTAP liposomes due to enhanced membrane fusogenicity (Porter, 2002).

Thus, the inventors have exemplified the interaction of recombinant retroviral vectors, lipid microbubbles and ultrasound. The results demonstrate that ultrasound enables the transduction of TE671 cells in vitro by a microbubble-associated virus vector which is inherently incapable of cell entry; gene delivery is restricted to cells at positions within the ultrasound beam. The experiments are a proof- of-principle demonstration that viral gene delivery can be spatially and temporally controlled via dependence for activation upon ultrasound exposure. An advantage of this new approach is that it enables both temporal and spatial control over gene delivery. The use of non-infectious MLV further enables gene delivery without the risk of infection at sites outside of the region of insonation. The observation that transduced cells occurred in colonies, as seen with infectious retroviral vectors, is indicative of stable transgene integration and its maintenance during cell division. Stable gene delivery offers a significant advantage over the transient expression possible with DNA-based vectors used in previous reports of ultrasound-mediated transfection .

As with any new technique, further development will be beneficial.

Whilst significant levels of transduction efficiency have been shown, further experimental work will further optimise the ultrasound parameters, virus dose-dependence and microbubble concentration in order to maximize efficiency of gene delivery and minimise toxicity. Given that the concentrations of viral vector used in these experiments have been relatively low, it is reasonable to expect that the efficiency can be further greatly improved by optimizing concentrations of virus and microbubbles . Assessing sonoporation under ultrasound conditions similar to those currently achieving transduction will help to elucidate the underlying mechanism further and assist in further optimisation of the technique. Since specific ultrasound conditions are required for microbubble cavitation, varying ultrasound parameters may greatly affect the transduction efficiency of the virus vector. For in vivo use, the particles need to be present at the target site in high concentrations to achieve therapeutic gene delivery. However, microbubbles exhibit similar behavior to red blood cells in circulation, moving at high speeds through the vasculature. As provided herein, it may therefore be beneficial to employ molecular targeting techniques to ensure adherence of the vector at the site to be transduced. For example, microbubbles carrying echistatin accumulate in angiogenic vessels of gliomas via attachment to the receptor α_vβ₃. Targeting microbubbles to vascular endothelial markers has application in the development of this vector towards clinical application in cancer and cardiovascular disease. As provided herein, further optimization of ultrasound exposure includes multiple exposures to ultrasound and vector to achieve cumulative increases in gene delivery allowing sufficient gene delivery for therapeutic effect in vivo.

As provided herein, further ways in which to optimize the technique include addition of substances to improve membrane permeability to increase gene delivery. Given that membrane permeabilization is likely to be involved in the mechanism underlying the cell transduction, addition of substances to further facilitate this is expected to increase the number of cells transduced. For example, lidocaine is capable of permeabilizing cell membranes and may be used in the context of ultrasound gene delivery (Nozaki et al . , 2003). In particular, lidocaine has been shown to increase membrane fluidity with a concomitant increase in gene delivery, with a GFP gene construct being delivered to more target cells. Moreover, lidocaine is compatible with in vivo use. The novelty of the inventors' approach is that spatial control at a distance is exerted upon viral delivery by subsequent exposure to ultrasound. For eventual clinical application, this process is noninvasive and the necessary acoustic pressures can be applied specifically to the target site. Attachment of targeting ligands to the microbubbles will facilitate accumulation of the vector at the target site. A further advantage for clinical application is that microbubble accumulation and destruction at the target site can be monitored using ultrasound imaging to assess vector delivery in real- time. The inventors' data demonstrate the development of a solution to the critical issue of specificity following systemic administration of retroviral vectors.

In summary, gene therapy strategies to treat both chronic and acute diseases would be made more effective by the development of a vector which could achieve highly site-specific, long-term gene expression. Retroviral genome integration can achieve permanent gene expression; however, achieving specificity of delivery in vivo currently represents the major problem limiting the clinical application of retroviral vectors. The present inventors have addressed this problem with a novel retroviral delivery methodology capable of long- term, target-specific gene delivery. The methodology readily lends itself to implementation for controlling retroviral gene delivery in vivo following systemic administration.

Further work

Acoustic pressure-dependence of transduction

As discussed above, the transducer had been designed such that the circularly-symmetrical fall in acoustic pressure within the ultrasound beam with increasing distance from the beam axis would be sufficiently gentle to provide for spatial registration of transduced cells with the local pressure: i.e. to enable the relationship between acoustic pressure and transduction to be determined by spatially registering the counts of transduced cells with the local pressure values. Whilst the data in Figure 6 verified acoustic pressure-dependence, the resolution of analysis (which was limited by the transduction efficiency) was insufficient to derive such a relationship. Transduction efficiency could, however, be related to the spatial average acoustic pressure for each of the annuli . Moreover, by varying the transducer input amplitude, and therefore the peak acoustic pressure produced and hence the spatial average acoustic pressures, it was possible to determine intermediate values for spatial average acoustic pressure and transduction efficiency (Figure 7) .

The transduction protocol was as previously described, using an amplifier input voltage amplitude of 6OmV producing a focal peak negative pressure of 1. OMPa in addition to the 8OmV used previously, which produces a focal peak negative pressure of 1.2MPa. Thus, cells exposed to virus-loaded microbubbles were insonated using an input amplitude of 8OmV (peak 1.2MPa) or 6OmV (peak 1. OMPa) and scored for transduction as in Figure 5b in regions defined by boundaries of radii at 2.5, 7.5 and 13mm centred on the acoustic axis corresponding to peak negative pressure >lMPa, 0.4-1MPa, 0.2-0.4MPa and <0.2MPa, respectively, for an amplifier input amplitude of 8OmV (peak 1.2MPa).

Figure 7A shows the radial variation of peak-negative acoustic pressure for each condition as the mean + standard deviation of four determinations (i.e. in each direction from the focus along the orthogonal axes depicted in Figure 4b) . The boundaries defining the regions within which transduced cells were scored are indicated by the broken lines. The results show again that peak negative pressure values of ≥lMPa were achieved over an area with 5mm diameter, with a maximum pressure of 1.2MPa at the centre of the beam, and pressures of >0.4MPa and >0.2MPa corresponded to diameters of 15mm and 26mm, respectively, using an amplifier input amplitude of 8OmV as previously demonstrated (Figure 4b) . Figure 7A also shows the peak negative pressure values produced using an amplifier input amplitude of 6OmV, with a peak negative pressure value of 1. OMPa at the centre of the beam. The spatial average peak-negative pressure for each region (indicated above/below for insonation at 80mV/60mV) was determined by circular integration of the varying peak-negative value. Figure 7B shows a plot of transduction efficiency within each region against the corresponding spatial average peak-negative pressure. The reduced tranduction efficiency for this experiment relative to the data in Figures 5 and 6 reflects prolonged storage of the microbubbles. Above a minimum threshold of 0.2MPa there was a monotonic increase in transduction efficiency with increasing spatial average peak-negative pressure. The standard deviations associated- with the efficiency measurements were larger for the higher values of pressure/efficiency due to the greater variation intrinsic to counting fewer cells within the smaller areas; i.e. the datapoints for higher efficiency nevertheless reflect fewer positive cells because the region area (and hence the total number of cells exposed to the respective pressure) is much less. For this reason although the relationship may have reached saturation by IMPa, a further increase in efficiency may result from greater acoustic pressures. Cell viability was unaffected in this experiment even for the band with the highest acoustic pressure.

Conclusions

We describe a novel means of targeting retroviral transduction, in which entry is determined by exposure to ultrasound subsequent to addition of virus associated with microbubbles . The importance for eventual clinical application is that specificity is determined by the localised application of ultrasound, separable from systemic vector administration; outside the region of ultrasound exposure there is negligible gene delivery, since the vector is inherently incapable of entry. Our data indicate a marked dependence on the acoustic pressure in the ultrasound field, resulting in transduction mainly in the area exposed to peak-negative pressures above 0.4MPa. Since the size of the region exposed to such pressures is a function of the transducer design, there is potential for adaptation of the transducer to the size of the intended target. Furthermore, the ultrasound exposure parameters used are within the range that can, in principle, be generated by clinical diagnostic ultrasound systems.

Clonal expansion of transduced cells indicates that, following ultrasound/microbubble-mediated entry, the processes of reverse transcription, integration and gene expression proceed as for infectious MLV. For the latter, receptor-mediated fusion between the viral and cell membranes enables entry of the viral core. Sonoporation, the process of cell membrane pore formation during ultrasound excitation of microbubbles, could be responsible for cell entry of the envelope-deficient virus. Using electron microscopy, shards of microbubble shell have been observed embedded in cell membranes following ultrasound exposure, so it is possible that the virus is forced across the cell membrane attached to microbubble fragments; however, an intact viral particle in the cytoplasm is unlikely to be competent for transduction. In experiments to explore this possibility, no gene expression followed scrape-loading of cells in the presence of the envelope-deficient virus, although 2% transfection was achieved in the presence of plasmid DNA, and cytoplasmic microinjection of virus particles similarly failed to transduce cells; this suggests that MLV particles are incapable of generating the necessary pre-integration complex when introduced without membrane fusion, although in neither case was particle delivery formally confirmed. Moreover, a mechanism other than sonoporation is implied in that the acoustic pressures required exceed those that are capable of effecting plasmid DNA delivery using the same experimental apparatus (Rahim et al . , submitted) . A more likely explanation is that membrane permeability and fluidity changes in the vicinity of microbubbles oscillating in the ultrasound field lead to enhanced fusion between the viral and cell membranes; if viral core entry was assisted in this way, its subsequent passage to the nucleus would be unhindered.

Critical to the mechanism of entry was the association of virus particles and microbubbles, with pre-incubation and washing of the microbubbles leading to a >100-fold enhancement of transduction over their simultaneous addition to the cells. Close apposition of the virus-loaded microbubbles to the target cell was also essential, consistent with a mechanism of entry involving localised membrane perturbation: exposure of cells growing on the lower membrane of the OptiCell^™ (by inversion of the unit) , 2mm below the accumulation of microbubbles at the upper membrane due to their buoyancy, abolished gene delivery (data not shown) .

For equivalent amounts of virus the maximum transduction efficiency achieved was comparable to that obtained using cationic liposomes with these target cells (Porter, 2002) . However, delivery followed ultrasound exposure for 5s only, in contrast to 40min for the virus/liposome complex. Therefore, the gene delivery reported represents highly efficient usage of the virus; rapid entry is likely to be important for clinical application. Moreover, spatial localization of transduction was controllable by means of ultrasound exposure. Background transduction in the absence of ultrasound was negligible, even though the vector was present for much longer than the period of ultrasound exposure; a low level of expression was, however, detectable upon extended {e.g. 24h) incubation, attributable to the cationicity of the microbubbles directly aiding virus entry (Porter, 2002). For eventual in vivo application, vector circulation will ensure both specificity and cumulative transduction at the site of insonation due to continual replenishment.

There are a number of potential ways in which to optimize ultrasound/microbubble-mediated transduction with envelope-deficient retrovirus. Since the efficiency of virus capture by the microbubbles was only -50% one possible means would be to achieve greater microbubble loading, either by altering the amount of charged lipid in the microbubble formulation or by using a ligand-based approach (e.g. avidin/biotin) , although avidity of binding may also be an issue affecting entry. Concentration of virus and microbubbles, or addition of substances to improve membrane permeability (Nozaki et al . , 2003), may increase transduction. Varying ultrasound exposure parameters will likely affect efficiency, since the optimal peak-negative acoustic pressure range of 0.4-1MPa relates specifically to the single parameter values chosen for pulse length, repetition rate and exposure time; indeed, from the relationship of Figure 4B it is not clear that the maximum effective pressure has been reached, although further increase may be offset by loss in cell viability. Combined with the targeting due to positioning of the ultrasound beam the potential for ligand-modified microbubbles to achieve proximity specifically to the desired target cells (which could be a molecularly-distinguishable subset of cells within the insonation zone) offers the means for highly-targeted transduction in vivo. Furthermore, since microbubbles exhibit similar behavior to red blood cells in the circulation, moving at high speeds through the vasculature, it may be advantageous to employ molecular targeting for concentration at the site to be transduced; e.g. microbubbles carrying echistatin accumulate in angiogenic vessels of gliomas via attachment to the receptor α_vβ₃. Targeting microbubbles to vascular endothelial markers would be an important step in the development of this vector towards clinical application in cancer and cardiovascular disease.

The novelty of our approach is that spatial control at a distance is exerted upon viral delivery by exposure to ultrasound. For eventual clinical application, this process is non-invasive and the necessary acoustic pressures can be applied specifically to the target site. A further advantage for clinical application is that microbubble accumulation and destruction at the target site can be monitored using ultrasound imaging to assess vector delivery in real-time. Our data delineate a solution to the critical issue of specificity following systemic administration of retroviral vectors by successfully combining the desirable attributes of viral and non- viral technologies .

References

The references cited herein are all expressly incorporated by reference .

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Claims

I. A composition for use in transducing a target cell with a gene of interest, comprising a microbubble and a retroviral particle comprising the gene of interest, wherein the composition is capable of transducing the target cell with the gene of interest when the target cell is exposed to the composition with the administration of ultrasound.

2. The composition according to claim 1, wherein the retroviral particle is incapable of independent cell entry.

3. The composition according to claim 2, wherein the retroviral particle is non-internalizing such that it is non-infectious.

4. The composition according to claim 3, wherein the retroviral particle comprises a modified envelope protein.

5. The composition according to claim 3, wherein the retroviral particle lacks an envelope protein.

6. The composition according to any one of the preceding claims wherein the retroviral particle is unstable in vivo.

7. The composition according to claim 6, wherein the retroviral particle is modified by glycosylation.

8. The composition according to claim 7, wherein the retroviral particle has α-galactosyl carbohydrate epitopes on its surface.

9. The composition according to any one of the preceding claims wherein the retroviral particle is murine leukaemia virus (MLV) , human immunodeficiency virus (HIV) or equine infectious anaemia (EIAV) .

10. The composition according to any one of the preceding claims, wherein the retroviral particle is recombinant.

II. The composition according to any one of the preceding claims, wherein the retroviral particle is associated with the microbubble.

12. The composition according to claim 11, wherein the retroviral particle is associated with the microbubble by electrostatic interaction, affinity cross-linking or covalent attachment.

13. The composition according to any one of the preceding claims, wherein the shell of the microbubble comprises lipid.

14. The composition according to any one of claims 1 to 12, wherein the shell of the microbubble comprises albumin or methacrylate.

15. The composition according to claim 13 or claim 14, wherein the shell is cationic.

16. The composition according to any one of the preceding claims, wherein the gas in the microbubble is perfluorocarbon.

17. The composition according to any one of the preceding claims, further comprising a targeting moiety.

18. The composition according to claim 17, wherein the targeting moiety is a ligand or an antibody.

19. The composition according to claim 18, wherein the antibody is an anti-ICAM-1 antibody, anti-E-selectin antibody, anti-VEGF receptor antibody or anti-α_vβ₃ antibody.

20. The composition according to any one of the preceding claims, wherein the gene of interest is a therapeutic gene.

21. The composition according to any one of the preceding claims, wherein the gene of interest is one which is not functionally expressed in the target cell.

22. The composition according to any one of the preceding claims, wherein the gene of interest is one which lack of functional expression in an individual is causative of disease in that individual .

23. The composition according to claim 22, wherein lack of functional expression in an individual is due to the presence of a mutated form of the gene, absence of the gene, or lack of functional gene product , in the individual .

24. The composition according to claim 21, wherein the gene of interest is not naturally expressed in the target cell.

25. The composition according to any one of the preceding claims, wherein the gene of interest is adenosine deaminase, VEGF, GM-CSF, factor VIII, factor IX, CFTR, p53, TNFα, TIMP-3 or thymidine kinase.

26. A kit for use in transducing a target cell with a gene of interest using ultrasound, the kit comprising a microbubble and a retroviral particle comprising the gene of interest, where the microbubble and the retroviral particle are for use in combination as a composition and the composition is capable of transducing the target cell with the gene of interest when used with the administration of ultrasound, wherein transduction of the target cell is effected by exposure of the target cell to the composition and ultrasound.

27. A method of transducing a target cell with a gene of interest, the method comprising the steps of exposing the target cell to the composition according to any one of claims 1 to 25 and administering ultrasound to the target cell.

28. The method according to claim 27, wherein the target cell is in vitro or ex vivo.

29. A method of treating a subject having a disease or condition treatable by expression of a gene of interest in a target cell, the method comprising the steps of exposing the target cell to an effective amount of a composition according to any one of claims 1 to 25 and administering ultrasound to the target cell to effect expression of the gene of interest in the target cell.

30. The method according to claim 29, wherein the target cell is exposed following systemic administration of the composition to the subject or direct administration of the composition to the subject in the vicinity of the target cell.

31. The composition of any one of claims 1 to 25 for use in therapy.

32. The composition of claim 31 for use in therapy by gene delivery to a target cell with the administration of ultrasound.

33. Use of a microbubble and a retroviral particle comprising a gene of interest for the preparation of a medicament for use in the treatment by ultrasound administration of the gene to the target cell of a disease or condition treatable by expression of the gene in a target cell.

34. The use according to claim 33, wherein the retroviral particle is incapable of independent cell entry.

35. The use according to claim 34, wherein the retroviral particle is non-internalizing such that it is non-infectious.

36. The use according to claim 35, wherein the retroviral particle comprises a modified envelope protein.

37. The use according to claim 35, wherein the retroviral particle lacks an envelope protein.

38. The use according to any one of claims 33 to 37, wherein the retroviral particle is unstable in vivo.

39. The use according to claim 38, wherein the retroviral particle is modified by glycosylation.

40. The use according to claim 39, wherein the retroviral particle has α-galactosyl carbohydrate epitopes on its surface.

41. The use according to any one of claims 33 to 40, wherein the retroviral particle is murine leukaemia virus (MLV) , human immunodeficiency virus (HIV) or equine infectious anaemia (EIAV) .

42. The use according to any one of claims 33 to 41, wherein the retroviral particle is recombinant.

43. The use according to any one of claims 33 to 42, wherein the retroviral particle is associated with the microbubble .

44. The use according to claim 43, wherein the retroviral particle is associated with the microbubble by electrostatic interaction, affinity cross-linking or covalent attachment.

45. The use according to any one of claims 33 to 44, wherein the shell of the microbubble comprises lipid.

46. The use according to any one of claims 33 to 44, wherein the shell of the microbubble comprises albumin or methacrylate .

47. The use according to claim 45 or claim 46, wherein the shell is cationic.

48. The use according to any one of claims 33 to 47, wherein the gas in the microbubble is perfluorocarbon.

49. The use according to any one of claims 33 to 48, wherein the microbubble or retroviral particle further comprises a targeting moiety.

50. The use according to claim 49, wherein the targeting moiety is a ligand or an antibody.

51. The use according to claim 50, wherein the antibody is an anti- ICAM-I antibody, anti-E-selectin antibody, anti-VEGF receptor antibody or anti-α_vβ₃ antibody.

52. The use according to any one of claims 33 to 51, wherein the gene of interest is a therapeutic gene.

53. The use according to any one of claims 33 to 52, wherein the gene of interest is one which is not functionally expressed in the target cell .

54. The use according to any one of claims 33 to 53, wherein the gene of interest is one which lack of functional expression in an individual is causative of disease in that individual.

55. The use according to claim 54, wherein lack of functional expression in an individual is due to the presence of a mutated form of the gene, absence of the gene, or lack of functional gene product, in the individual .

56. The use according to claim 53, wherein the gene of interest is not naturally expressed in the target cell.

57. The use according to any one of claims 33 to 56, wherein the gene of interest is adenosine deaminase, VEGF, GM-CSF, factor VIII, factor IX, CFTR, p53, TNFα, TIMP-3 or thymidine kinase.

58. The method according to claim 29 or the use according to any one of claims 33 to 57, wherein the target cell is ex vivo.

59. The method or use according to any one of claims 27 to 30 or claims 33 to 58, wherein the ultrasound is administered to the target cell as pulsed ultrasound, with a frequency in the range of 1-3MHz; pulse repetition frequency of 1-1OkHz; pulse lengths of from 1 to 100 cycles; exposure time of up to 30 minutes; pressure amplitude of from 0. IMPa to 3MPa peak negative pressure.

60. The method or use according to claim 59, wherein the ultrasound is administered to the target cell at IMHz, IKHz pulse repetition frequency; from 1 to 32 cycles per pulse; pressure amplitude of from 0.3MPa to 3Mpa peak negative pressure; an exposure time of from 1 to 8 seconds .

61. The method or use according to any one of claims 29 to 30 or claims 33 to 60, wherein the disease or condition is chronic.

62. The method or use according to any one of claims 29 to 30 or claims 33 to 61, wherein the disease or condition results from a lack of functional expression of the gene of interest in the subject.

63. The method or use according to claim 62, wherein lack of functional expression results from mutation in the gene of interest, absence of the gene of interest, or lack of functional gene product from the gene of interest, in the subject.

64. The method or use according to any one of claims 29 to 30 or claims 33 to 61, wherein the cause of the disease or condition is unrelated to the gene of interest.

65. The method or use according to any one of claims 29 to 30 or claims 33 to 64, wherein the disease or condition is haemophilia, cystic fibrosis, ADA-SCID, cancer, Lesch-Nyhan syndrome, restenosis, angina, ischaemia, atherosclerosis, or a wound.

66. The method or use according to any one of claims 27 to 30 or claims 33 to 65, wherein the target cell is one which lacks functional expression of the gene of interest.

67. The method or use according to claim 66, wherein lack of functional expression results from mutation in the gene of interest, absence of the gene of interest, or lack of functional gene product from the gene of interest, in the target cell.

68. The method or use according to claim 66, wherein the target cell does not naturally express the gene of interest.

69. The method or use according to any one of claims 27 to 30 or claims 33 to 68, wherein the target cell is a vascular endothelial cell, tumour cell, bone marrow cell, muscle cell, or epithelial cell.