NO320691B1 - New contrast release system. - Google Patents

Description

NEW CONTRAST MEDIUM RELEASE SYSTEM

The present invention relates to a contrast agent delivery system which is applicable in nuclear magnetic resonance and positron emission tomography, in which the delivery system comprises cross-linked alginate with a structure which is adjusted for controlled release of the contrast agent. More specifically, the invention relates to a delivery system for slow release which comprises alginate, with a content of o>gulopyranuronic acid of 40-80% and manganese as a cross-linking agent and image enhancing agent, and if desired, other bivalent, cross-linking ions.

Field of the invention

Nuclear Magnetic Resonance (MRI) is a well-known imaging technique and diagnostic tool used in medicine to produce high-quality images of the inside of a human or animal body, and images of compartments of selected cells or microorganisms.

MRI uses high-frequency waves and a powerful magnetic field to obtain clear and detailed images of internal organs and tissues. The technique is used for diagnostic or control purposes in various disease states, essentially in all parts of the human or animal body, including cancer, stroke, cardiovascular diseases, etc.

On the other hand, PET is a nuclear-based medical imaging technique where radioactive isotopes that emit a positron are used to obtain detailed images of organs and tissues. With PET, it is possible to examine areas in molecular biological detail by using radioactive isotopes with different uptake rates depending on the specific type of tissue involved. PET is used for the detection and control of several disorders, including cancer, neurological disorders and cardiovascular conditions.

For both techniques, the imaging of the desired organ or tissue in the body is facilitated by the injection of contrast agents. Contrast agents suitable for MRI or PET techniques may include or constitute image-enhancing compounds such as paramagnetic ions or isotopes, respectively. Manganese ions are taken up by living cells and can be used to distinguish between living cells and damaged cells, i.e. cells that have lost their ability to transport manganese across the cell membrane. However, manganese and other paramagnetic ions can be cytotoxic, and their use in living individuals is limited to certain dose levels by systemic administration, i.e. by intravenous injection.

To solve the toxicity problem associated with the common image enhancing ions, one approach has been to develop gelling compounds. By, for example, binding manganese to gelling compounds, a physiologically acceptable administration of manganese is proposed. Within the state of the art, several gelling compounds are described for use as contrast agents in MRI technology. For example, US 5,716,898 describes a contrast agent for MRI that uses an α-hydroxyketone-containing group as a manganese-releasing compound.

US Patent Application No. 20040101969 describes the use of a liposome that encapsulates a compound of interest and a contrast agent such as manganese. US patent application no. 20020090341 describes a method for early detection of myocardial ischemia. The method according to this patent application uses a physiologically acceptable manganese complex or salts thereof.

A clinically approved medium for the slow release of manganese that can be used in MRI is currently available for clinical use for intravenous administration and is produced by

. Amersham Health ASA (trademark: TESLASCAN, active

constituent/manganese-binding compound: Mangafodipir = manganese bound to the ligand dipyridoxyldiphosphate = MnDPDP).

In addition, US patent no. 6,337,064 B1 describes a manganese-releasing complex salt. The claimed advantages of this complex are said to be the high stability of the complex. In contrast to other gelation agents described in the prior art, for example Mn-DPDP (also called mangafodipir), the complex claimed in US 6,337,064 B1 will not dissociate and release manganese, cf. second column, lines 8-16.

US patent application US2003/0198599 Al relates to fluorinated and paramagnetic polyuronides and proteins useful as imaging probes, diagnostic agents and contrast agents. More specifically, this patent application relates to polyuronide polymers such as alginate spheres, directed at a receptor, in which the polyuronide is modified with a fluorine-containing group and/or a paramagnetic ion. The desired images of specific organs or tissues are then claimed to be obtained when the paramagnetic polyuronide polymer is taken up by the cells of the organ or tissue. Although this patent application lists manganese as one of several possible paramagnetic ions that can be used, the examples focus primarily on alginate beads comprising gadolinium. As will be apparent from the detailed description of the present invention below, it is further clear that a polyuronide polymer comprising manganese would not function according to the claimed purpose as claimed in US 2003/0198599 Al.

Summary of the invention

The present inventors have surprisingly found that alginate gels or

-spheres with a content of α-L-gulopyranuronic acid (G) of approximately 40-80% weight/volume are usable as a delivery system for the slow release of contrast-enhancing agents. The contrast-enhancing agent according to the present invention is, for example, manganese. According to the present invention, manganese is released after injection or implantation of

the delivery system by replacing suitable monovalent or divalent ions present in the injection site or implantation site (Na<+>, Ca<2+>, etc.).

The present invention thus provides a delivery system for contrast agents for nuclear magnetic resonance and positive emission tomography. In contrast to the vast majority of contrast agents described in the prior art, which aim to obtain stable and constant binding of a

contrast-enhancing agent such as a paramagnetic ion to one (gelating) compound, the present delivery system releases the contrast-enhancing agent in a controlled manner and thus provides detailed and specific images of biological structures, organs or tissues in microorganisms, cell cultures, fish, animals or human beings.

The present delivery system provides localized and stationary delivery of contrast enhancing agents, for example paramagnetic ions or isotopes, to specific extravascular body parts by injection or implantation where cellular uptake of manganese is desired in a certain dose and at a certain rate, to provide enhanced image contrast in MRI - (for example Mn<2+>) or PET imaging (for example <52m>Mn): The present invention is a useful tool for medical imaging (diagnosis, control of various diseases or disorders and treatment regimens, research, etc.). The invention is based on the association between a cross-linking and a contrast-enhancing agent, i.e. bivalent ions such as manganese with

alginates in the formation of gels and subsequent release of the contrast-enhancing agent from the gels when these are injected/implanted in physiological departments. These

the findings are supported by detailed in vitro and in vivo experiments described below.

More specifically, the invention provides a delivery system for slow

release of manganese comprising alginate with an α-gulopyranuronic acid content of 40-80% and manganese as a cross-linking and image-enhancing agent, and if desired other bivalent, cross-linking ions.

According to another embodiment of the invention, the present delivery system may comprise other cross-linking, bivalent ions in addition to the contrast-enhancing agent, for example Ba<2+>, Ca<2+> or Sr<2>"<1>".

According to a preferred embodiment of the present invention, alginate gels are provided with a controlled and selected release rate of the contrast enhancing agent. According to one embodiment of the invention, the release rate of the contrast enhancing agent is controlled by preparing alginate gels by manipulating the presence and concentration of other cross-linking, bivalent ions or by manipulating the G content and the alginate used.

According to another embodiment of the invention, the present supply system can have a size in the range of approximately 0.15-1.00 mm.

Detailed description of the invention

The present invention will now be described in more detail, with reference to figures and examples which constitute the preferred embodiments. However, the preferred embodiments should not be interpreted as limiting when it comes to the scope of the attached requirements.

Brief description of the figures

Figure 1 shows the structure of the monomers in alginate. Alginate monomers: M J5-D-mannopyranuronic acid and G = α-L-gulopyranuronic acid. Top: Illustration of the Haworth formulas. Bottom: The most probable ring conformation of the alginate residues; <4>Ci for the M residues and "C* for the G residues. Figure 2 shows part of an alginate chain with the monomers: M - p-D-mannopyranuronic acid and G = a-L-gulopyranuronic acid. Figure 3 (A) is a light microscopy image of Mn <2+->alginate beads with added Ba2+ in the gelling solution (alginate with 65% G, scale bar is 100 µm). (B) - (E) show Mn<2+->release from the alginate beads according to the invention. Medicine bottles filled with Mn<2 +->alginate beads (B) 1 minute, (C) 20 hours, (D) 40 hours and (E) 60 hours after removing the beads from the incubation solution.Scale bar is 1 cm.

Figure 4 is a graphical representation showing the normalized MR signal intensity

in an ROI in the central part of medicine bottles containing a selection of different alginate beads with manganese as one of the cross-linking ions. The signal was normalized to the original signal intensity for each individual experiment. Description of the different alginate types: Mpyr = 40% G-alginate from Macrocystis pyriferia (high M-alginate) - see example 2 below.

PT180 = 65% G-alginate from Laminaria hyperborea (high G-alginate) - see example 1 below.

Dry sample 4 = freeze-dried Mpyr alginate (high M alginate) - see example 2 below.

Large balls = PT180 with ball diameter -500 micrometers - see example 1 below.

Small balls = PT 180 with ball diameter ~200 micrometers - see example 1 below.

Figure 5. MR images from the same laboratory rat at different time points after injection of 10-15 Mn<2+->alginate spheres into the corpus vitreum. Note the enhancement of the optic nerve from the retina to the contralateral superior colliculus.

The structure of alginate.

Alginates are linear, binary copolymers of α-L-gulopyranuronic acid (G) and its C-5 epimer, /?-D-mannopyranuronic acid (M) (Figure 1).

The relative amount of the monomers as well as their sequential arrangement along the polymer chain varies widely from one alginate to another. Alginate is a true block copolymer consisting of homopolymeric regions of M and G (designated M-blocks and G-blocks respectively), interrupted by regions with alternating structure designated MG-blocks (Figure 2).

Alginates are isolated from marine brown algae and are used in the pharmaceutical industry, the cosmetics industry, the textile industry and the food industry. The most common areas of application are based on the alginates' polyelectrolytic properties, which form the basis for their gel-forming properties.

The commercially available sodium alginates are water soluble. When such alginates are added to a solution containing polyvalent ions, for example bivalent alkaline earth metal ions such as Ca<2+>, Sr<2+>, Mn<2+> and Ba2+, alginate gels with a defined shape are formed. This is a consequence of ionic cross-linking of several alginate chains.

Alginate can bind polyvalent cations selectively in the order:

Alginate's affinity for polyvalent cations, and consequently its gel-forming properties, depends on the monomer composition and the distribution of G units along the chain. Diaxially connected G-units (G-blocks) form binding sites for cations that cross-link G-blocks from different alginate chains. Thus, long G-blocks and a high content of G-blocks give an alginate with high affinity and selectivity for polyvalent cations. The alginate's G content will depend on the source from which the alginate is isolated. The usual, commercially available alginates have a G content of 40-60%. Furthermore, the content of G in, for example, sodium alginates can be increased by epimerization using mannuronan C-5 epimerases (for example AlgE4) which convert M to G, cf. example 4 below. Procedures for transferring M to G are described in Svanem, BIG. Skjåk-Bræk, G. Ertesvåg, H. Valla, S. Cloning and expression of three new Azotobacter vinelandii genes closely related to a previously described gene family encoding mannuronan C-5-epimerases. J Bacteriol 1999; 181: 68-77 and Ertesvåg, H. Høidal, HK. Hals, IK. Rian, A. Doseth, B. Valla, S. A family of modular type mannuronan C-5-epimerase genes Controls alginate

structure in Azotobacter vinelandii. MolMicrobiol 1995; 16: 719-731.

A gel can be defined as a cross-linked polymer that has been allowed to swell in water. In an ionic gel, the degree of swelling will essentially be determined by the osmotic potential of the dissociable ions, which at equilibrium is in balance with the cross-binding forces in the gel network. By replacing some of the bivalent crosslinking ions in an alginate gel with dissociable monovalent ions (such as Na<*>), the number of crosslinks in the gel will be reduced, while the osmotic potential will increase and the gel will swell. The release of the contrast-enhancing agent and the cross-linking ions can be manipulated by varying the type and concentration of the gel-forming ions used, as well as by varying the G content of the alginate.

By varying the concentration of the cross-linking ions, a delivery system is obtained with a predetermined release rate for the image enhancing agent, for example manganese. For example, barium and strontium form stronger alginate gels than calcium. Alginate gels or spheres produced using Br<2*> and/or Sr<2*> are thus more stable. This leads to alginate gels/balls that swell to a lesser extent in physiological solutions, and consequently slow exchange of ions and slow release of contrast agent et. Depending on the specific need, one can fabricate a delivery system that is adjusted for the specifically desired release rate of the contrast enhancing agent, eg manganese. Furthermore, the use of Br<2*> is specifically preferable if a slow release of the manganese ions is required. If, on the other hand, a faster release of manganese is necessary, Ca<2*> is preferably used as a cross-linking agent.

The alginates to be used for the production of gels according to the present invention are highly purified and have a G content in the range 40-80%. The alginates and their G content will be selected depending on the intended application. The manganese dose and release rate will thus also be adjusted by adjusting the G content in the alginate backbone, the size of the spheres <p>g number, and by adding other divalent ions to the gel forming solutions. Alginates with a high G content form gels or spheres with stronger binding of manganese. This leads to slow release of manganese. In contrast, alginates with a low G content form gels or spheres with looser binding of manganese, and gels/spheres with a faster release of manganese are obtained compared to alginate gels/spheres with a higher G content.

In general, gels with the highest mechanical strength are produced from alginates with a high content of G and the use of gel-forming ions with high selectivity, for example Ba<2+> and Sr<2+>.

Microenvironments in the tissues with different ability to transport manganese away from the alginate capsule (into cells and through the extracellular space) will require different doses and release rates for manganese to obtain optimal MR image quality per scan time unit (high temporal and/or spatial resolution). Furthermore, specific areas for implantation of the delivery device (for example, in the brain) may limit the bullet size or number of bullets tolerated. The present invention is flexible in this regard, in that it allows the selection of:

i) the composition of the alginate core (G content),

ii) alginate cross-linking ion(s),

iii) alginate sphere size,

iv) injection preparation (wet/dry), and

v) number of alginate beads injected.

Examples of alginate spheres that differ in terms of backbone composition (G content), injection preparation (wet/dry) and size are shown in Figure 4.

Production of the supply system according to the invention.

The present delivery system, which comprises alginate with a G content of approximately 40-80%, can be produced by cross-linking the alginate chains and consequently by forming alginate gels or alginate balls which are cross-linked with one or more suitable cross-linking agents. According to the invention, the crosslinking agent is also a contrast enhancing agent, for example a paramagnetic ion such as manganese or a manganese isotope for MR or PET imaging. The contrast enhancing agent is preferably Mn<2+> or <52m>Mn. When administered by injection or implantation, the contrast-enhancing agent will be released from the alginate gel or alginate beads.

According to one embodiment of the invention, alginate gels are crosslinked with Mn<2*> alone or together with other bivalent ions according to methods well known to those skilled in the art. The following steps can be followed to produce the alginate gel: 1) Addition of an aqueous alginate solution to an aqueous salt solution of MnClj or Mn<2*>, if desired together with other polyvalent, gel-forming cations, whereby a gel is instantly formed. 2) Wash the gel using a weak salt solution or physiological solution.

The concentration range for the alginate solution used in the production of the alginate gels is preferably 0.8-5% (weight/volume). Furthermore, this solution is preferably added to a solution of polyvalent, gel-forming cations with a total concentration of 50 mM-1.2 M. Non-limiting examples of such polyvalent gel-forming cations are Ba<2*>, Ca<2*> or Sr <2*>.

A preferred method for producing an alginate gel according to it

. present invention, is by the following steps:

1) Dropwise addition of an aqueous solution of Na-alginate of 1.5-2% (w/v) to a solution of MnCh (0.5-1 M) and CaCk or BaCb (1-50 mM) using a electrostatic sphere generator, whereby alginate gel spheres with a diameter of 0.15-1 mm and with a narrow size distribution are formed. 2) Wash the gel beads 3-6 times with physiological salt solution to remove excess gel-forming ions.

In vitro and in vivo experiments.

The performance of the present invention has been demonstrated by both in vitro and in vivo experiments.

In vitro, the experiment consisted of investigating the effect of manganese leaching on the NMR signal from NaCl solution surrounding the alginate gel spheres (Figures 3 and 4). The NMR signal dropped when the MRI pulse sequence described above was used.

This drop in the NMR signal was mainly due to the effects of manganese on the T2<*->avsl opening (dipolar effects), as the original T2<*> in a pure NaCl solution is of the order of 1 second in this context (increased Lorentzian NMR- line width). Manganese will act as a T2<*->contrast agent in this situation. The results thus show that manganese will be released from the slow release delivery system according to the present invention.

In vivo, the initial T2<*> is much shorter (in, for example, brain tissue) than in pure salt solution, in the region of 0.01 seconds. Consequently, in animal experiments, the release of manganese will mainly affect Tl relaxation and produce shorter Tl relaxation times for water in nearby tissues, leading to enhanced signal in Tl-weighted MR images (Figure 5). The in vivo experiments performed on animals were very similar to those performed by Watanabe T. et al. (in Magnetic Resonance in Medicine 46: 424-429 (2001)) just a few years ago. Watanabe et al. used a pure MnCh solution that was injected into the corpus vitreum of laboratory rats to achieve MRI contrast enhancement of the optic nerve. This contrast enhancement is specific and has until now only been achieved by the introduction of manganese.

Contrast agent-releasing designs

The controlled delivery system of the present invention may also comprise conventional pharmaceutically acceptable adjuvants, carriers, excipients or diluents which are well known to those skilled in the art.

The present formulation can be prepared as wet formulations suitable for subcutaneous, intramuscular or interstitial injection/implantation, for example for intracortical administration. In one embodiment of the invention, a preferred contrast-releasing formulation comprises the delivery system according to the invention and saline solution.

In another embodiment of the invention, the contrast medium releasing formulation is a dry formulation. Dry formulations according to the present invention can be prepared by freeze-drying alginate gels/balls. The dry formulation according to the invention is preferably administered by implanting the formulation in a desired site.

The medical practitioner or clinician, with the support of the present disclosure, can adapt an appropriate route of administration and contrast-enhancing condition, such as the required release rate of manganese, for the specific patient and for the purpose of MR or PET imaging.

EXAMPLES

Preparation of Slow Release Contrast Agent Delivery System Alginate spheres were successfully prepared from a series of different alginate types (see examples below) ranging in size from 200 to 1000 mm (Figure 3A). After washing the beads to remove the concentrated MnCh incubation solution, a series of Tl-weighted MRI scans were performed to detect the efflux of manganese from the beads into a 0.9% NaCl solution (Figures 3B-E). The MR signal intensity fell as a function of time in an exponential manner, leveling off after approximately 30-40 hours after the washing procedure was initiated (at the present stage, no rigorous analysis has been performed on the kinetics of this process). Curves showing the normalized signal intensity for a set of different Mn<2+->alginate beads are shown in Figure 4.

Nuclear magnetic resonance

In vitro

Saturated manganese-containing alginate gels from medicine bottles with high molar manganese concentration were washed in 3-6 consecutive steps in 0.9% NaCl medical injection solution to remove ions that did not participate in the gel structure. The volume of alginate gel was approximately 1% of the volume of incubation solution and washing solution. The final bottles containing approximately 1% gel volume in 0.9% NaCl solution were placed upright in the isocenter of the 2.35 T MR spectrometer for MR imaging (Figure 3). A transversal image section was selected to follow the MR image signal intensity as a function of time after initiation of the washing procedure. The MR imaging pulse sequence was a Tl-weighted spin echo (MSME, based on the original ASPECT3000 2D MSME from Bruker AG, Germany) with the following main parameters: FOV = 5 cm x 5 cm, matrix = 128 x 128, slice thickness « 3 mm, TR = 200 ms, TE = 10 ms, NEX = 4, total acquisition time ~2 minutes. Imaging was repeated every 30 minutes for up to 60 hours after the start of washing.

In vivo

Two laboratory rats (female Sprague-Dawley rats, -200 g) were injected with 10-15 alginate gel capsules in the corpus vitrum of the left eye. Released manganese will be taken up by retinal ganglion cells and transported along the axons of the optic nerve (see Watanabe T. et al.: Magnetic Resonance in Medicine 46:424-429 (2001)). 24 hours after injection of alginate, the rats underwent MR imaging using the same MR scanner and a Tl-weighted 3D pulse sequence (FLASH, see Haase A. et al.: Journal of Magnetic Resonance 67: 258-266 (1986) ) with the following main parameters: FOV « 5 cm x 5 cm x 2 cm, matrix = 256x256 x 64 (voxel = 195 mm x 195 mm x 312 mm), TR = 15 ms, TE = 4.2 ms, FA = 25 degrees , total acquisition time = 32 minutes. Approval from the appropriate authorities was granted for the animal experiments. The animals were anesthetized with 0.05 ml/g body weight subcutaneous injection of a mixture of Hypnorm and Dormicum in sterile water (ratio 1:1:2) throughout the experiment.

Example 1: Alginate gel microspheres with Mn<2*> and Ba<2*>

A 1.8% w/v solution of highly purified sodium alginate with a G content of 65%

(PT 180) was dropped into one solution of 1 M MnCh and 1 mM BaCh using an electrostatic bead generator. Spheres formed instantly. The diameter of the spheres was approximately 500 µm. After gelation, the gelling solution was removed using a filter and the beads were washed three times in large volumes of 0.9% NaCl. The pre-treated alginate gel microspheres were further used for in vitro and in vivo experiments, and the results are shown in Figures 4 and 5 respectively.

Example 2: Freeze-dried aiginate gel beads with Mn<2*> and Ba<3*>

A 1.8% w/v solution of sodium alginate with a G content of 40% (Mpyr) was dropped into a solution of 1 M MnCh and 1 mM BaCh using an electrostatic bead generator. Spheres formed instantly. The diameter of the spheres was approximately 200 µm. After gelation, the gel-forming solution was removed using a filter, and the spheres were washed twice in large volumes of 0.9% NaCl. The gel spheres were then freeze-dried and transferred to saline just prior to MR imaging .For in vitro results, see Figure 4.

Example 3: Alginate gel microspheres in Mn<2+> and Ca<2+>

A 1.8% w/v solution of sodium alginate with a G content of 70% was dropped into a solution of 50 mM CaCh using an electrostatic bead generator. Spheres with a diameter of approximately 200 µm were formed instantly. The beads were then transferred to a 1 M MnCh solution and incubated for a few hours for further gelation. The gelling solution was removed using a filter and the beads were washed three times in large volumes of 0.9% NaCl.

Example 4: Alginate gel balls of epimerized high-G alginate

A high-G sodium alginate with 75% G was prepared by epimerizing a 65% G alginate using a mannuronan C-5 epimerase (AlgE4) that converts M blocks to MG blocks. A 1.8% solution of the epimerized material was dropped into a solution of 1 M MnCh and 1 mM BaCh using an electrostatic bead generator. Spheres with a diameter of approximately 200 µm were formed instantly. The gelling solution was removed using a filter and the beads were washed three times in large volumes of 0.9% NaCl.

The results described above thus show that a delivery system for slow release according to the present invention can be used for localized and stationary (for example limited) slow release of manganese in anatomical sites in situ to follow the development of, for example, ruptures and lesions in the central nervous system (CNS ).

In conclusion, a new contrast-enhancing delivery system for use in medical imaging has been presented. The properties of the present invention imply the possibility of designing tailor-made preparations for a number of areas of application.

Claims (15)

1. Controlled delivery system for contrast agent comprising alginate, characterized in that the content of the monomer α-L-gulopyranuronic acid in the alginate is approximately 40-80% and that the alginate is cross-linked with an agent which is both a contrast-enhancing agent and a cross-linking agent, and in which said agent is released after administration. 2. Controlled supply system according to claim 1, characterized in that the cross-linking and contrast-enhancing agent is an isotope or a paramagnetic ion. 3. Controlled supply system according to claim 2, characterized in that the paramagnetic ion is Mn<2*>.; 4. Checked according to claim 2, characterized in that the isotope is <52m>Mn.; 5. Controlled supply system according to any one of claims 1-4, characterized in that it comprises further cross-linking agents.; 6. Controlled supply system according to claim 5, characterized in that said further cross-linking agents are bivalent ions, preferably Ba<2+>, Ca<2+> or Sr<2*>. 7. Controlled supply system according to any one of claims 1-6, characterized in that the content of α-L-gulopyranuronic acid is over 60% (weight/volume), for example over 65% (weight/volume), 70% (weight/volume) or 75% (weight/volume). 8. Controlled supply system according to any one of claims 1-6, characterized in that the content of α-L-gulopyranuronic acid is approximately 65%-80% (weight/volume), for example approximately 70% (weight/volume) - 80% (weight/volume) or 75% (weight/volume) - 80% (weight/volume). 9. Controlled delivery system according to any one of claims 1-8, characterized in that the contrast-enhancing agent is released after administration by injection or implantation. 10. Controlled supply system according to any one of claims 1-9, characterized in that the alginate is in the form of alginate balls, preferably with a size in the range from approximately 0.15 to 1.00 mm. 11. Contrast agent-releasing preparation, characterized in that it comprises the delivery system according to any of claims 1-10 and, if desired, pharmaceutically acceptable excipients, carriers, adjuvants and/or diluents. 12. Preparation according to claim 11, characterized in that it includes salt solution. 13. Preparation according to claim 11, characterized in that the alginate is freeze-dried. 14. Preparation according to any of claims 11-12, characterized in that it is administered by injection/implantation, for example by subcutaneous, intramuscular or interstitial injection/implantation. 15. Preparation according to claim 11 and claim 13, characterized in that it is administered by implantation.