GB2311138A - ESR-enhanced MRI using magnetic particles as contrast agents - Google Patents

Abstract

Superparamagnetic, ferrimagnetic, or ferromagnetic particles are used as contrast agents in ESR-enhanced MRI. Superparamagnetic particles having an ESR linewidth of less than 20G are preferred. The particles may comprise maghemite (gamma Fe 2 O 3 ), magnetite (Fe 3 O 4 ), or mixtures of such oxides. The particles may also comprise mixed metal oxides such as ferrites, hexagonal ferrites, or garnets. Other compositions are disclosed. The size of the particles may be less than 10 nm. The particles may be coated and suspended in an isotonic suspension.

Description

METHOD This invention relates to the use of particles with the magnetic properties of superparamagnetism, ferrimagnetism or ferromagnetism (hereinafter referred to as magnetic particles) in electron spin resonance enhanced magnetic resonance techniques, for example Overhauser MRI, and particularly to the use of superparamagnetic nanoparticles as electron spin resonance enhanced MRI contrast agents.

Magnetic resonance imaging (MRI) is a diagnostic technique that has become particularly attractive to physicians as it is non-invasive and does not involve exposing the patient under study to potentially harmful radiation such as is the case with X-rays.

Electron spin resonance enhanced MRI, referred to herein as OMRI (Overhauser MRI) but also referred to in earlier publications as ESREMRI or PEDRI, is a wellestablished form of MRI in which enhancement of the magnetic resonance signals from which images may be generated is achieved by virtue of dynamic nuclear polarization (the Overhauser effect) that occurs on VHF stimulation of an esr transition in a paramagnetic material in the subject under study. Magnetic resonance signal enhancement may be by a factor of a hundred or more thus allowing OMRI images to be generated rapidly and with relatively low primary magnetic fields.

OMRI techniques have been described by several authors, notably Leunbach, Lurie, Ettinger, Trucker, Ehnholm and Sepponen, for example in EP-A-296833, EP-A361551, WO-A-90/13047, J. Mag. Reson. 76:366-370(1988), EP-A-302742, SMRM 9:619(1990), SMRM 6:24(1987), SMRM 7:1094(1988), SMRM 8:329(1989), US-A-4719425, SMRM 8:816(1989), Mag. Reson. Med. 1i:140-147(1990), SMRM 9:617(1990), SMRM 9:612(1990), SMRM 9:121(1990), GB-A2227095, DE-A-4042212 and GB-A-2220269. One area of particular interest is the use of OMRI in determining oxygen concentrations in a sample (eg. a body) and this is the subject of co-pending US patent application Serial No. 08/540,146 of Leunbach.

In the basic OMRI technique, the imaging sequence involves initially irradiating a subject placed in a uniform magnetic field (the primary field Bo) with radiation, usually VHF radiation, of a frequency selected to excite a narrow linewidth esr transition in a paramagnetic material which is in or has been administered to the subject. Dynamic nuclear polarization results in an increase in the population difference between the excited and ground nuclear spin states of the imaging nuclei, i.e. those nuclei, generally protons, which are responsible for the magnetic resonance signals. Since MR signal intensity is proportional to this population difference, the subsequent stages of each imaging sequence, performed essentially as in conventional MRI techniques, result in larger amplitude MR signals being detected.

Paramagnetic materials which exhibit an ESR transition able to couple with an NMR transition of the imaging nuclei of interest may be naturally present within the sample (eg. oxygen) or may be administered as an OMRI contrast agent.

In WO-A-88/10419 (Hafslund Nycomed Innovation AB), for example, various OMRI contrast agents were proposed with particular emphasis on the use of stable nitroxide free radicals, although the chloranil semiquinone radical and Fremy's Salt were also specifically referred to.

In WO-A-90/00904 (Hafslund Nycomed Innovation AB) the use of deuterated free radicals, in particular of deuterated nitroxide free radicals, as OMRI contrast agents was proposed.

WO-A-91/12024 (Nycomed Innovation AB) refers generally to carbon free radicals i.e. radicals where the unpaired electron or electrons are primarily associated with carbon atoms and, of these, triarylmethyl radicals where the electron charge may be advantageously delocalised over a number of aromatic nuclei are stated to be preferred. More specifically, the use in OMRI of triarylmethyl radicals in which at least one aryl moiety is a sulphur-based heterocycle is the subject of International Patent Application No.

PCT/GB95/02151 (Nycomed Imaging AS) filed on 8 September 1995 with the title Free Radicals.

The use in OMRI of free radicals in which the electron charge is delocalised through a conjugated carbon-based H-system is referred to in WO-A-93/02711 (Hafslund Nycomed Innovation AB).

Particulate ferromagnetic, ferrimagnetic and superparamagnetic agents have been proposed for use as MR contrast agents in conventional MR imaging (ie.

without esr enhancement). In this regard such agents achieve their contrast effect by affecting the relaxation times of the imaging nuclei (generally water protons) from which the MR signal which is used for image generation derives. Oral formulations of such particulate agents, have become available commercially for imaging of the gastrointestinal tract, e.g. the CRT) product ABDOSCANkavailable from Nycomed Imaging.

However parenteral administration of such particulate agents has also been widely proposed for imaging of the liver and spleen as these organs act to remove foreign particulate matter from the blood relatively rapidly.

Thus, by way of example, liver and spleen imaging using such agents is proposed by Widder in US-A-4859210. The utility of such magnetic particles in Overhauser MRI has not previously been recognised.

It will be evident from the above precis of the prior art that the investigation of OMRI contrast agents has been limited primarily to paramagnetic organic free radicals. However, such materials frequently have properties which make their use as OMRI contrast agent problematic. Thus, for examples, organic free radicals generally only have a momentary existence (i.e. short half-life) which in in vivo use may lead to toxicity problems, and frequently their stability is reduced in physiological conditions. An added drawback is that the low relaxivity exhibited by many organic free radicals leads to poor electronic/nuclear spin coupling and a low Overhauser enhancement factor. Clearly, there is a need for OMRI contrast agents which are physiologically tolerable, electronically stable and provide high Overhauser enhancement.

Whilst work in this field continue to be generally directed towards finding more suitable organic free radicals the high inherent esr linewidth of available magnetic particles (typically > 200G) has detracted workers from pursuing non-organic based materials as potential OMRI contrast agents.

It has now been found that certain particles which exhibit co-operative magnetic behaviour for example superparamagnetic, ferromagnetic or ferrimagnetic behaviour are useful as OMRI contrast agents.

Thus viewed from one aspect the present invention provides the use of magnetic particles, preferably superparamagnetic particles (particularly preferably those having an esr linewidth of less than 20G) as OMRI contrast agents.

In a further aspect, the present invention provides a method of magnetic resonance investigation of a sample, preferably a human or non-human animal body (e.g. a mammalian, reptilian or avian body), said method comprising introducing into said sample magnetic particles, preferably superparamagnetic particles, exposing said sample to a first radiation of a frequency selected to excite electron spin transitions in said magnetic particles, exposing said sample to a second radiation of a frequency selected to excite nuclear spin transitions in selected nuclei in said sample, detecting free induction decay signals from said sample, and, optionally, generating an image or dynamic flow data from said detected signals. Still further, the invention provides the use of magnetic particles for the preparation of a contrast medium for use in a method as described hereinbefore.

The advantages which magnetic particles possess are many fold. Thus, their high inherent relaxivity has been found to lead to significantly higher Overhauser enhancement factors (even at a low degree of saturation) than those available from existing organic based contrast agents. In addition, magnetic particles are limited practically neither by a short inherent lifetime nor by short time-scale physiologically induced breakdown.

It is of course desirable to use magnetic particles whose esr linewidths are as low as possible, preferably those with a linewidth less than 20G, particularly preferably less than 10G, especially less than 1G, especially preferably 500 mG or less. By way of example, the esr linewidth of nano-particles of iron oxide is typically of the order of 180G and the relaxivity at least 100 times greater than conventional organic based free radicals (R - 0.2 mmol-ls-l in water at 37"C). The linewidth may be altered by making the particles more symmetrical or by varying the metal oxide compositions and crystal structure. Ideally particles are spherical in order to reduce the inhomogeneous broadening of the particle suspension (the demagnetisation field is zero).

Magnetic metal oxide compounds, including magnetic metal oxide hydroxide or mixed metal oxides for example those discussed in US-A-482794 (Groman), EP-A-525189 (Meito Sangyo), EP-A-580878 (BASF) and PCT/GB94/02097 (Nycomed) or in US-A-5160725 (Pilgrimm) and in UK Patent Application No. 9600427.0 entitled "Contrast Media" filed on 10 January 1996 in the name of NYCOMED IMAGING AS are a class of materials found to be particularly useful in the present invention. Particular mention may be made of magnetic iron oxide compounds of formula: (MIIO) n(MIII2o3) where MII and MIII are transition or lanthanide metals in the II or II valence state, at least one of which is Fe, and n is zero or a positive number, or more particularly of formula (MIIO)nFe2O3(MIII2O3)m where MII is a divalent metal such as Fe, Mg, Be, Mn, Zn, Co, Ba, Sr and Cu, MIII is a trivalent metal such as Al, Yb, Y, Mn, Cr or a lanthanide and n and m are each zero or a positive number.

Thus the magnetic particles may preferably be iron oxides of formula (FeO)nFe203 where n is in the range 0 to 1 e.g. maghemite (yFe2O3) and magnetite (Fe3O4) or mixtures of such oxides. Mixed metal iron oxides include but are not limited to ferrites e.g. MIIFe204, hexagonal ferrites e.g. BaFel2Ol9, garnets e.g. M3111Fe5O12 (wherein M is for example Y or a lanthanide metal, especially the compound Y3Fe5Ol2 YIG) and Fe3Al2Si3O12 or Ca3Fe2Si3Ol2, and compounds of formula M4IFeO4 or M2IIFeO4 (wherein MI is a monovalent metal for example an alkali metal and MII is a divalent metal for example an alkaline earth metal) and mixed alkali metal iron oxides such as Na5FeO4, K6[Fe2O6] or Nal4[Fe6Ol6].

Weissleder et al., Reviews of Magnetic Resonance in Medicine, i 1-20 (1992) provides a useful review of known pharmaceutical iron oxides which may be used in the method according to the invention.

Magnetic particles useful in in vivo OMRI will typically distribute into the vascular space. They will therefore in one embodiment of the invention be used as blood pool contrast agents (i.e. for imaging the vasculature). A contrast/noise radio about 10-103 higher than that which might be expected from conventional OMRI contrast agents is attainable.

For parenteral use, the size and size distribution of the magnetic particles (including coatings if present) and the chemical nature of the surface of the overall particle are of great importance in determining the contrast generation efficacy, the blood half-life, and the biodistribution and biodegradation of the contrast agent. Ideally the magnetic particle size (i.e. the crystal size of the magnetic material) is within the single domain size range (such that the particles are superparamagnetic and thus have no hysteresis and a reduced tendency to aggregate) and the overall particle size distribution is narrow so that the particles have uniform biodistribution, bioelimination and contrast effects.

The preparation of magnetic particles with the desired particle size, particle size distribution, crystal symmetry and without undue particle aggregation is achieved in some cases through known methods, for example the co-precipitation techniques described in US A-4452773 (Molday) and WO-A-89/03675 (Schrder) and particularly the methods described in UK Patent Application No. 9600427.0 entitled "Contrast Media" filed on 10 January 1996 in the name of NYCOMED IMAGING AS.

Mean crystal sizes (i.e. of the magnetic core material) of the magnetic materials useful in the invention preferably have the characteristics of monodomain particles e.g. for a useful Overhauser effect, particle sizes should preferably be of the order of less than 5000 nm, particularly less than 1000 nm, especially in the range 2 to 50 nm, especially preferably 3 to 20 nm and more especially preferably 5 to 12 nm, most preferably 6 to 10 nm, and for use as blood pool agents, the mean overall particle size including any coating material should preferably be below 100 nm, especially below 30 nm.

The magnetic core material may of course be provided in the form of composite particles with stabilisers such as dextran or surface coatings such as starch, albumin or polyethyleneglycol and other materials proposed by Pilgrimm in US-A-5160725 and WO-A94/21240, by Nycomed in PCT/GB94/02097, by Bracco in US A-5464696 and by Illum in US-A-4904479. The magnetic core material may also have attached target specific materials which permit the magnetic particle to be directed to a certain receptor or antigenic site or which alter the biodistribution e.g. by prolonging blood pool residence time as proposed by Pilgrimm in US-A5160725 and WO-A-94/21240. Typical targeting materials take the form of one or more coatings such as those described in UK Patent Application No. 9600427.0 entitled "Contrast Media" filed on 10 January 1996 in the name of NYCOMED IMAGING AS or proteins.

Thus the magnetic particles useful in the method according to the invention may be used alone, or in the form of composite particles as hereinbefore described and may be administered in any conventional pharmaceutical form, e.g. suspension, emulsion, powder etc. and may contain aqueous vehicles (such as water for injections) and/or ingredients to adjust osmolality, pH, viscosity, and stability. Ideally, the composition is in suspension form with the suspension being isotonic and isohydric with blood. For example, an isotonic suspension can be prepared by the addition of salts like sodium chloride, low-molecular weight sugars like glucose (dextrose), lactose, maltose, or mannitol or a soluble fraction of the coating agent or a mixture of these. Isohydricity can be achieved by the addition of acids like hydrochloric acid or bases like sodium hydroxide if only a minor adjustment of pH is required.

Buffers such as citrate, acetate, borate, tartrate, and gluconate may also be used. The chemical stability of the particle suspension can be modified by the addition of antioxidants like ascorbic acid or sodium pyrosulphite. Excipients may also be added to improve the physical stability of the preparation. Most frequently used excipients for parenteral suspensions are surfactants like polysorbates, lecithin or sorbitan esters, viscosity modifiers like glycerol, propyleneglycol and polyethylene glycols (macrogols), or cloud point modifiers, preferably non-ionic surfactants.

The compositions of the invention will advantageously contain the magnetic metal oxide at a diagnostically effective metal concentration, generally 0.1 to 250 mg metal/ml, preferably 0.5 to 100 mg metal/ml, and especially preferably 1 to 75 mg metal/ml.

For the method of the invention, the dosage used will be a contrast effective dosage. Generally this will lie in the region 0.05 to 30 mg metal/kg bodyweight, preferably 0.1 to 15 mg metal/kg and especially preferably 0.25 to 8 mg metal/kg.

The Examples given below are intended to illustrate the invention in a non-limiting fashion.

EXAMPLES Example 1 Various preparations of maghemite (yFe2O3) nanoparticles were investigated by esr. The size of the particles in 4 different samples was 8 nm, 7.5 nm, 6 nm and 5 nm.

The 8 nm particles (hereinafter referred to as MPP-FeOx are produced according to Example 6 of UK Patent Application No. 9600427.0 entitled "Contrast Media" filed on 10 January 1996 in the name of NYCOMED IMAGING AS and were stabilised by PEG in water. The particles of the remaining three samples were obtained from the Danish Technical University, Lyngby, Denmark and were also stabilised by PEG in water. The esr spectra measured at X-band at room temperature are presented in Figure 1. The narrowest linewidth is exhibited by MPP FeOx with a peak to peak linewidth of 50G.

Claims (6)

Claims

1. A method of magnetic resonance investigation of a sample comprising introducing magnetic particles into said sample, exposing said sample to a first radiation of a frequency selected to excite electron spin transitions in said magnetic particles, exposing said sample to a second radiation of a frequency selected to excite nuclear spin transitions in selected nuclei in said sample, detecting free induction decay signals from said sample and optionally generating an image or dynamic flow data from said detected signals.

2. A method as claimed in claim 1 wherein said magnetic particles are superparamagnetic particles.

3. A method as claimed in either of claims 1 or 2 wherein said magnetic particles are mixed metal oxides of formula: (M1ZO) n (M1112O3) (where MII and MIII are transition or lanthanide metals in the II or III valence state at least one of which is iron; and n is zero or a positive number).

4. A method as claimed in claim 3 wherein MII and MIII are iron(II) and iron(III) and n is 0 or 1.

5. A method as claimed in claim 2 wherein said superparamagnetic particles have an esr linewidth of less than 20G.

6. Use of magnetic particles as OMRI contrast agents.