Link:  Pharm/Biotech Resources

Title:  Stimulus sensitive gel with radioisotope and methods of making

United States Patent:  6,869,588

Issued:  March 22, 2005

Inventors:  Weller; Richard E. (Selah, WA); Lind; Michael A. (Kent, WA); Fisher; Darrell R. (Richland, WA); Gutowska; Anna (Richland, WA); Campbell; Allison A. (Kennewick, WA)

Assignee:  Battelle Memorial Institute (Richland, WA)

Appl. No.:  853507

Filed:  May 9, 2001

Abstract

The present invention is a thermally reversible stimulus-sensitive gel or gelling copolymer radioisotope carrier that is a linear random copolymer of an [meth-]acrylamide derivative and a hydrophilic comonomer, wherein the linear random copolymer is in the form of a plurality of linear chains having a plurality of molecular weights greater than or equal to a minimum gelling molecular weight cutoff. Addition of a biodegradable backbone and/or a therapeutic agent imparts further utility. The method of the present invention for making a thermally reversible stimulus-sensitive gelling copolymer radionuclcide carrier has the steps of: (a) mixing a stimulus-sensitive reversible gelling copolymer with an aqueous solvent as a stimulus-sensitive reversible gelling solution; and (b) mixing a radioisotope with said stimulus-sensitive reversible gelling solution as said radioisotope carrier. The gel is enhanced by either combining it with a biodegradable backbone and/or a therapeutic agent in a gelling solution made by mixing the copolymer with an aqueous solvent.

Description of the Invention

FIELD OF THE INVENTION

The present invention relates generally to a stimulus sensitive gel containing a radioisotope or radionuclide and method of making. As used herein, the term "stimulus sensitive gel" is a polymer solution that gels upon a change in stimulus. A stimulus includes but is not limited to temperature, pH, ionic strength, solvent composition, sheer stress or a combination of these factors. The preferred gel is generally a reversible gel, more specifically; the gel is a random copolymer of an [meth-]acrylamide derivative with a hydrophilic comonomer. As used herein, the term [meth]-acrylamide denotes methacrylamide, acrylamide, or combinations thereof. As used herein, the terms "radioisotope" and "radionuclide" are synonymous.

BACKGROUND OF THE INVENTION

Radiolabelling as a method of diagnosis or treatment has been in use for many years. The continuing challenge has been to maximize concentration of the radioisotope in the area or region of interest, diseased tissue or tumor, while minimizing the concentration of the radioisotope in other areas and thereby minimizing damage to healthy tissues.

The paper by S. Ning, K. Trisler, D. M. Brown, N. Y. Yu, S. Kanekal, M. J. Lundsten, S. J. Knox: "Intratumoral radioimmunotherapy of a human colon cancer xenograft using a sustained-release gel", Radiotherapy and Oncology, 39, 179-189, 1996 discusses an intratumoral injectable gel drug delivery system for local administration of radio-immunotherapy. The injectable gel was a collagen-based drug delivery system designed for intratumoral administration. The study demonstrated that intratumoral delivery of radiolabeled antibodies using the collagen gel system markedly increased the retention of radioisotope in the tumors, enhanced the antitumor efficacy, and reduced the systemic toxicity compared to systemic administration of the radiolabeled antibody. Ning et al. teach the use of injectible collagen gels that are not stimuli-sensitive. Moreover, these collagen gels neither fully perfuse tumor tissue nor do they hold the radioisotope within the collagen gel matrix. Thus, the radioisotope is attached to an antibody for perfusing and binding to the tumor tissue. Lack of perfusion of the collagen gel and limited range of radioisotope decay products require that the radioisotope leave the collagen gel matrix to achieve close proximity with tumor tissue to achieve the therapeutic effect.

In the paper PRELIMINARY EXPERIENCE OF INFUSIONAL BRACHYTHERAPY USING COLLOIDAL ³² P, S E Order, J A Siegel, R Principato, L S Zieger, E Johnson, P Lang, R Lustig C Kroprowski, P E Wallner, Annals Academy of Medicine, May 1996, Vol. 25, No. 3, an infusion by a needle into a tumor was done without the need for an arterial catheter and eliminating the need for hospitalization. This paper reports using dexamethasone (Decadron) to overcome intratumoral resistance followed by macroaggregated albumin then colloidal chromic phosphate ³² P followed by more macroaggregated albumin injected into the tumor. Sufficient radiation emitted by the radioisotope leads to tumor cell killing and remission of solid cancers. However, disadvantages of this method include the serial injections and leakage of ³² P from the tumor.

There is need in the art for a method of introducing a radioisotope into a localized area with a single or multiple injection(s) as well as a need for a local delivery system with little or reduced leakage of the radioisotope.

Stimulus-sensitive reversible hydrogels are herein defined as copolymer-solvent systems that undergo a transition between a solution and a gel state in response to the external stimuli such as temperature, pH, ionic strength, solvent composition, sheer stress or a combination of these factors. A reversible stimuli-sensitive gel is one in which the transition is reversed upon reversal of the stimulus. A well known example of a reversible hydrogel is an aqueous solution of gelatin that is in a solution state at high temperatures (e.g. 80^oC.) and forms a gel at lower temperatures (e.g., 20^oC.). Other examples of reversible gels involve aqueous solutions of agarose and kappa-carrageenan that gel in response to the temperature change, and aqueous solutions of alginate that gel in response to the increased concentration of calcium ions. Reversible hydrogel systems are used in food and pharmaceutical industries as thickeners and suspending agents.

Some specific reversible gelling copolymers were also investigated as drug delivery systems and tissue engineering polymer matrices. High viscosity aqueous solutions containing 20 (or more) wt % of block copolymers of polyethylene oxide and polypropylene oxide, e.g. Poloxamer 407 and Pluronic F68 (Poloxamer 188) exhibit reverse thermal gelation. Solutions of Poloxamer 407 have been investigated for intraocular administration. Solutions containing 25 and 30 wt % of Poloxamer 407 have been prepared and the force needed to inject them through a 25 GA needle was investigated. It was concluded that a liquid-gel transition occurred inside the needle, due to the heat transfer between the needle walls and the surroundings. [J. Juhasz, A. Cabana, A. Ait-Kadi, EVALUATION OF THE INJECTION FORCE OF POLOXAMER 407 GELS FOR INTRAOCULAR ADMINISTRATION, Pharm.Res., 13, No.9, 1996, Symposium Supplement, S-276].

In another example, 25 wt % aqueous solution of Pluronic F68 was mixed with articular chondrocyte cells suspension at 4^oC. and injected subcutaneously in nude and immunocompetent rabbit. In both cases, the cells entrapped in the copolymer formed tissue with histological appearance of hyaline cartilage. It was concluded that thermally reversible Pluronic F68 gel can serve as an effective injectable matrix for tissue engineering. [C. A. Vacanti, et al., Proceedings of Tissue Engineering Society, Orlando, Fla., 1996].

An example of a pH-reversible hydrogel, investigated as an in situ gelling system for ophthalmic use is the aqueous solution of, a poly(acrylic acid)polymer, which undergoes a pH-mediated phase transition at concentrations above 0.1 wt %. The solution also contains hydroxypropyl methylcellulose, a viscosity enhancing agent. [Pharm.Res., 13, No.9, 1996, Symposium Supplement].

A new vehicle for topical and mucosal delivery, based on reversible gelation, was developed as an interpenetrating polymer network (IPN) of poly(acrylic acid) and a block copolymer of poly(ethylene oxide)/poly(propylene oxide). When heated from ambient to body temperature the network exhibited a significant viscosity increase from a viscous liquid to a gel-like consistency. It was concluded that at higher temperature, reduced release rates of active ingredients from the network were observed due to the increased viscosity of the IPN. [E. S. Ron, et al., A NEW VEHICLE FOR TOPICAL AND MUCOSAL DRUG DELIVERY, Pharm.Res., 13, No.9, 1996, Symposium Supplement, S-299].

All gels containing the copolymers of poly(ethylene oxide)/poly(propylene oxide), i.e., Poloxamer 407, Pluronic F68 (Poloxamer 188), an IPN of poly(acrylic acid) and a block copolymer of poly(ethylene oxide)/poly(propylene oxide), and combinations thereof exhibit a limited, concentration dependent, stability of the gel state. The gels formed from these copolymers become liquids upon dilution (as for example due to the dilution with body fluids after peritoneal injection). Additionally, all the above examples of reversible hydrogels exhibit high initial viscosity in a liquid state, i.e., before the gelling transition.

Accordingly there is a need for a reversible gel that only reverses when a specific stimulus is reversed and does not reverse upon introduction of a different stimulus (e.g. dilution). Moreover, there is a need for a reversible gel that has a lower initial viscosity.

The U.S. Pat. No. 5,262,055 to Bae et al. discusses an artificial pancreas utilizing reversible gels based on NiPAAM and its copolymers. These polymers and copolymers do not reverse upon dilution and they have a lower initial viscosity. However, the NiPAAM homopolymer of Bae et al. forms a dense gel with minimal water content (i.e. exhibits substantial syneresis).

Accordingly, there remains a need for a thermally reversible gel without substantial syneresis.

Polymers exhibiting phase transitions in water have many potential uses for drug delivery as stated in GRAFT COPOLYMERS THAT EXHIBIT TEMPERATURE-INDUCED PHASE TRANSITIONS OVER A WIDE RANGE OF pH, G. Chen, A S Hoffman, Nature, Vol 373, 5 Jan. 1995 (pp49-52). In this paper, the authors further describe a temperature sensitive polymer that phase separates with a change in temperature or pH. Chen and Hoffman use graft copolymers having side chains of a temperature sensitive homopolymer, the oligo-N-isopropylacrylamide, grafted onto a pH sensitive backbone homopolymer of acrylic acid. The authors describe the phase separation of the graft copolymer investigated by a cloud point determination in dilute solutions. However, a dilute solution cannot produce a reversible gelation of these graft copolymers. Chen and Hoffman also mention random copolymers of N-isopropylacrylamide and acrylic acid as exhibiting a phase separation, however, there is no description of the intention to study the possibility of reversible gelation in more concentrated solutions of these random copolymers.

Thus, there is a need for a stimulus sensitive gel with radioisotope that is useful in infusional brachytherapy.

SUMMARY OF THE INVENTION

The present invention is a radioisotope carrier made by combining a stimulus sensitive gel with either an aqueous insoluble or confined radioisotope. A preferred stimulus sensitive gel is a thermally reversible gel or thermally reversible gelling copolymer that is preferably a random copolymer of an [meth-]acrylamide derivative and a hydrophilic comonomer, wherein the random copolymer is in the form of a plurality of linear chains having a plurality of molecular weights greater than or equal to a minimum gelling molecular weight cutoff. The thermally reversible gelling copolymer is enhanced by either combining it with a therapeutic agent in an aqueous solution containing the thermally reversible gelling copolymer, and/or by grafting the thermally reversible gelling copolymer to a biodegradable backbone. The stimulus sensitive gel may also be selected from biodegradable polymers, for example polysaccharides, polypeptides and combinations thereof; cellulose derivatives including but not limited to hydroxypropylmethyl cellulose; other polymers such as agar, gelatin, chitosan, alginate in combination with a slow gelling agent for example calcium sulfate and combinations thereof.

The method of the present invention for making a radioisotope carrier has the steps of:

(a) mixing a stimulus-sensitive gelling polymer with an aqueous solvent as a stimulus-sensitive gelling solution; and

(b) mixing an aqueous non-soluble or confined radioisotope with the stimulus-sensitive reversible gelling solution as the radioisotope carrier.

Aqueous non-soluble radioisotope is a radioisotope in a colloidal or precipitate form, for example radioisotope insoluble salt, e.g. yttrium phosphate, radium sulfate, and combinations thereof. Confined radioisotope is radioisotope in a chelator, glass particle, polymer particle or other binding compound.

A preferred stimulus-sensitive gelling polymer is a thermally reversible gelling copolymer, preferably made according to the steps of:

(a) mixing an [meth-]acrylamide derivative with a hydrophilic comonomer in a solvent with an initiator forming a reaction mixture;

(b) polymerizing the reaction mixture and forming a first random copolymer having a plurality of linear chains having a plurality of molecular weights; and

(c) purifying the polymerized first random copolymer and obtaining a second random copolymer having a plurality of molecular weights greater than or equal to a minimum gelling molecular weight cutoff. The method has the further steps of combining the thermally reversible gelling copolymer with a radioisotope in an aqueous solution containing the thermally reversible gelling copolymer.

It will be apparent to one of skill in the art of radiotherapy that an image enhancer or contrast agent may be added to the stimulus sensitive gelling polymer, for example as used in nuclear medicine imaging, ultrasonic imaging, and magnetic resonance imaging (MRI).

Advantages of the present invention include (1) the stimuli-sensitive gel of the present invention exhibits a thermodynamic stability, and when geled, will not reverse to the liquid state upon dilution but may reverse to the liquid state only in response to a stimulus change. Moreover, the stimuli-sensitive gel of the present invention in a solution state has lower initial viscosity more suitable for tissue perfusion.

It is an object of the present invention to provide a radioisotope carrier.

It is a further object of the present invention to provide a method of making a radioisotope carrier.

It is a further object of the present invention to provide a biodegradable stimuli-sensitive polymer useful for a radioisotope carrier.

The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention is a radioisotope carrier having a stimulus-sensitive gelling polymer mixed with aqueous solvent and with a radioisotope that is either aqueous non-soluble or confined as the radioisotope carrier.

The radioisotope is preferably an alpha and/or beta emitter with a short half life. More specifically, the radioisotope is selected from the group of yttrium-90, indium-111, radium-223, actinium-225, bismuth-212, bismuth-213, scandium-47, astatine-211, rhenium-186, rhenium-188, iodine-131, iodine-124, lutetium-177, holinium-166, samarium-153, copper-64, copper-67, phosphorus-32 and combinations thereof.

In a preferred embodiment, the radioisotope carrier further has a radioisotope confine. The purpose of the radioactive confine is to minimize or prevent migration of the radioisotope to healthy tissue areas. The radioisotope confine may be for example chelators or complexing agents, capsules and combinations thereof. Preferred isotope/chelator combinations are yttrium-90 or indium-111 with 1,4,7,10-tetraazacyclododecane-N,N',N",N'"-tetraacetic acid (DOTA), derivatives of DOTA; radium-223 with tetra-t-butyl-calix[4]arene-crown-6-dicarboxylic acid (TBBCDA), derivatives of TBBCDA; actinium-225 with 5,11,17,23-tetra-t-butyl-25,26,27,28-tetrakis(carboxymethoxy)-calix[4]aren e (TBTC), derivatives of TBTC; and bismuth-212, bismuth-213 with 5,11,17,23,29,35-hexa-t-butyl-37,38,39,40,41,42-hexakis(carboxymethoxy)-ca lix[6]arene (HBHC), derivatives of HBHC, diethylenetriamine-pentaacetic acid (DTPA), or ethylenediaminetetraacetic acid (EDTA), derivatives of DTPA, and combinations thereof.

The radioisotope confine may be glass beads and/or polymer beads.

The stimulus-sensitive gelling polymer may be selected from biodegradable polymers, for example polysaccharides, polypeptides and combinations thereof; cellulose derivatives including but not limited to hydroxypropylmethyl cellulose; other polymers such as agar, gelatin, collagen, chitosan, alginate with a slow gelling agent for example calcium phosphate, gelling copolymer and combinations thereof. A preferred polymer is a polymer that is useful as a gel that forms without substantial syneresis when the stimulus-sensitive polymer is in an aqueous solution. Syneresis is defined as water expelled from a polymer matrix upon gelation. Substantial syneresis is more than about 10 wt % water expelled from the polymer matrix. According to the present invention, it is preferred that the syneresis be less than about 10 wt %, more preferably less than about 5 wt % and most preferably less than about 2 wt %. Substantially no syneresis is syneresis of less than about 2 wt %, preferably 0 wt %.

The thermally reversible copolymer is a linear random copolymer of an [meth-]acrylamide derivative and a hydrophilic comonomer wherein the linear random copolymer is in the form of a plurality of linear chains having a plurality of molecular weights greater than or equal to a minimum gelling molecular weight cutoff. According to the present invention, the minimum gelling molecular weight cutoff is at least several thousand and is preferably about 12,000. The presence of a substantial amount of copolymer or polymer chains having molecular weights less than the minimum gelling molecular weight cutoff results in a milky solution that does not gel. Further, the amount of hydrophilic comonomer in the linear random copolymer is preferably less than about 10 mole %, more preferably less than about 6 mole % and most preferably about 2-5 mole %. The structure of linear chains is not cross linked. Moreover, the linear random copolymer structure is one in which a linear chain 100 is shared by randomly alternating portions of the [meth-]acrylamide derivative 102 and the hydrophilic comonomer 104 as depicted in FIG. 1.

The [meth-]acrylamide derivative is an N,N'-alkyl substituted [meth-]acrylamide including but not limited to N-isopropyl[meth-]acrylamide, N,N'-diethyl[meth-]acrylamide, N-[meth-]acryloylpyrrolidine, N-ethyl[meth-]acrylamide, and combinations thereof.

The hydrophilic comonomer is any hydrophilic comonomer that co-polymerizes with the [meth-]acrylamide derivative.

Preferred hydrophilic comonomers are hydrophilic [meth-]acryl-compounds including but not limited to carboxylic acids, [meth-]acrylamide, hydrophilic [meth-]acrylamide derivatives, hydrophilic [meth-]acrylic acid esters. The carboxylic acid may be, for example, acrylic acid, methacrylic acid and combinations thereof. The hydrophilic acrylamide derivatives include but are not limited to N,N-diethyl[meth-]acrylamide, 2-[N,N-dimethylamino]ethyl[meth-]acrylamide, 2-[N,N-diethylamino]ethyl[meth-]acrylamide, or combinations thereof. The hydrophilic [meth-]acrylic esters include but are not limited to 2-[N,N-diethylamino]ethyl[meth-]acrylate, 2-[N,N-dimethylamino]ethyl[meth-]acrylate, and combinations thereof.

According to the present invention, the stimulus-sensitive polymer may be mixed with an aqueous solvent to form a stimulus-sensitive gelling solution, or reversible gelling solution. The aqueous solvent includes but is not limited to water and aqueous salt solutions. The salt solution is preferably a phosphate buffered saline solution for medical use.

The method of making the thermally reversible polymer according to the present invention has the steps of:

(a) mixing an [meth-]acrylamide derivative with a hydrophilic comonomer in a reaction solvent with an initiator forming a reaction mixture;

(b) polymerizing the reaction mixture and forming a first linear random copolymer having a plurality of linear chains having a plurality of molecular weights; and

(c) isolating and purifying the polymerized first linear random copolymer and obtaining a second linear random copolymer having a plurality of molecular weights greater than or equal to a minimum gelling molecular weight cutoff.

The alternatives for the [meth-]acrylamide derivative and the hydrophilic comonomer have been set forth above and are not repeated here.

The reaction solvent may be aqueous or non-aqueous. The preferred aqueous solvent is simply water. Alternatively, the aqueous solvent is a salt solution. The non-aqueous solvent may be a hydrocarbon including but not limited to oxygenated hydrocarbon solvent, for example dioxane, chlorinated hydrocarbon solvent, for example chloroform, an aromatic hydrocarbon, for example benzene. Precipitation of the polymer occurs during polymerization in benzene. Dioxane is the preferred solvent because there is no precipitation during copolymerization thereby imparting greater uniformity of composition of the random copolymer (NiPAAM/AAc).

The amount of aqueous solvent with respect to [meth-]acrylamide derivative is preferably about 80 wt %, but may range from about 30 wt % to about 98 wt %. The amount of non-aqueous solvent with respect to the [meth-]acrylamide derivative is preferably about 80 wt % but may range from about 30 wt % to about 98 wt %.

The initiator may be any free radical initiator compatible with the [meth-]acrylamide derivative. The preferred initiator is 2,2'-azobis-isobutyrolnitrile (AIBN). The amount of the initiator with respect to the reaction mixture of solvent and polymer is preferably about 0.1 wt % but may range from about 0.01 wt % to about 2 wt %.

A reversible gelling solution is made by mixing the thermally reversible polymer with an aqueous solution. The amount of aqueous solution with respect to polymer is from about 70 wt % to about 99 wt %, preferably about 98 wt % for NiPAAm/AAc to achieve a nonresorbable reversible gel with substantially no syneresis. The aqueous solution is preferably a salt solution.

In addition to the nonresorbable reversible gel composed of a linear random copolymer of N-isopropyl[meth-]acrylamide and [meth-]acrylic acid described in this invention, a biodegradable (resorbable) copolymer exhibiting similar gelation properties is obtained by grafting of the oligo [meth-]acrylamide derivative side chains on a biodegradable homopolymer backbone of, e.g., poly(amino acid). Preferred oligo [meth-]acrylamide derivative side chains include N,N-alkyl substituted [meth-]acrylalmide derivatives, linear random copolymer of [meth-]acrylamide derivative and hydrophylic comonomer, and combinations thereof. Techniques of grafting of oligo-N-isopropyl[meth]acrylamide side chains on a nonbiodegradable pH-sensitive homopolymer backbone are described (Chen and Hoffman). The technique(s) of Chen and Hoffman were used herein to graft the oligo-N-isopropyl[meth-]acrylamide side chains on an alternative biodegradable homopolymer backbone such as poly(amino acid). The first step of the synthesis is either the free-radical homopolymerization or the random copolymerization of the oligo-N-isopropyl[meth-]acrylamide side chains by free radical homopolymerization using an amino-terminated chain transfer agent, for example 2-aminoethanethiol hydrochloride. The next step is the coupling of the amino-terminated macromer to the carboxyl moieties of the biodegradable backbone using the activation reagent, e.g., dicyclohexyl carbodiimide. Other biodegradable backbones such as poly(phosphazenes) and poly(caprolactone) may also be grafted with the oligo-N-isopropyl[meth-]acrylamide side chains using similar synthetic techniques. The reaction solvent is non-aqueous, preferably a hydrocarbon, for example chloroform, dichloromethane, N,N'-dimethylformamide or combinations thereof.

The resorbable and/or non-resorbable stimulus-sensitive gel(s) of the present invention is/are useful as a radioisotope carrier for infusional brachytherapy.

One or more contrast or imaging agents may be added to the stimulus sensitive gelling polymer. For nuclear medicine imaging, any gamma-emitting radioisotope may be added to the gel polymer as an imaging agent or contrast agent. The common types of gamma source imaging systems are gamma-cameras (Anger cameras), single-photon emission computed tomography (SPECT), and positron-emission tomography (PET). Preferred imaging agents for gamma cameras would be technetium-99m (and any of the standard chemical forms of Tc-99m, such as pertechnetate). The preferred chemical form of Tc-99m may be an insoluble material, such as Tc-99m-sulfur colloid (a common liver scanning agent). Other gamma emitters include but are not limited to indium-111, rhenium-186, rhenium-188, thallium-201, gallium-67, yttrium-91, and iodine-131, and combinations thereof. Positron-emission tomography systems use positron-emitting radioisotopes and detect the twin 0.511 keV photons that accompany radioactive decay. Examples of radioisotopes that could be added for positron-emission tomography include fluorine-18, copper-64, arsenic-74, and zirconium-89, ioding-124, and yttrium-86. A typical amount of photon-emitter added is one that will provide approximately 500,000 counts per two-minute imaging time (0.3 to 10 millicuries).

For ultrasonic imaging, any ultrasound contrast-enhancement agent could be added to render the gel polymer more imageable using ultrasonic detection. Examples include commercially available echocontrast products such as Albunex.RTM. (registered trade mark, Molecular Biosystems) and Optison.TM. (trade mark of Molecular Biosystems), which are manufactured for and distributed by Mallinckrodt Medical, St. Louis, Mo. Albunex.RTM. is an ultrasound contrast agent prepared by sonicating 5% human serum albumin to produce stable, air-filled, albumin-coated microspheres. It is an effective ultrasound contrast agent for use during echocardiography and other ultrasound radiological procedures. Optison.TM. is an ultrasound contrast agent containing human serum albumin with octofluoropropane. Each milliliter of echocontrast agent contains about 700 million microspheres. The amount of contrast agent added could be approximately 1 to 5 percent by weight of the gel polymer.

For magnetic resonance imaging (MRI), any paramagnetic material used for contrast-enhancement may be used. An example includes a relaxation agent gadolinium-chelate (gadolinium-DTPA or gadolinium-EDTA). The stable (nonradioactive) form of gadolinium is preferred. The amount of gadolinium contrast agent added would be a few parts per thousand by weight (millimolar concentrations).

Claim 1 of 25 Claims

We claim:

1. A radioisotope carrier, comprising:

(a) a stimulus-sensitive methacrylamide gelling polymer in an aqueous solution which gelling polymer changes from a first fluent state to a second, less fluent state upon exposure to an increase in temperature, a change in ionic strength, and/or a change in pH, wherein the gelling polymer remains in the second, less fluent state upon dilution; and

(b) an aqueous insoluble or confined radioisotope mixed with said gelling polymer as said radioisotope carrier.

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