BINDING OF GROWTH HORMONE TO LIPOSOMES

Abstract: Crosslinking reagents glutaraldehyde and ethylene glycol diglycidyl ether have been used to bind epidermal growth factor ("EGF") to liposomal surfaces without utilizing the disulfide bridge linkage of EGF and, therefore, not adversely affecting the functionality of EGF. The crosslinking occurs through the amine residues offered by EGF and liposomes having phosphatidylethanolamine. The resulting EGF-modified liposomes offer potential as a microscopic drug delivery system.

Description: BINDING OF GROWTH HORMONE TO LIPOSOMES < BACKGROUND OF THE INVENTION The present invention relates to the preparation of microscopic drug delivery systems (HDDS) utilizing 5 drug-encapsulating Dioadhesive liposomes. Microscopic drug delivery systems (MODS) have Deen developed for improved drug administration relative to administration of drugs in their free form. Drug-loaded HDDS can perform as sustained or controlled release drug depots. By 10 providing a mutual protection of the drug and the biological environment, MDDS reduces drug degradation or inactivation. As a system for controlled release of a drug, MDDS improves drug efficacy and allows reduction in the frequency of dosing. Since the phar aco inetics of free drug release from depots of 15 MDDS are different than from directly-administered drug, MDDS provides an additional measure to reduce toxicity and undesirable side effects. MDDS is divided into two basic classes: particulate systems, such as cells, microspheres, viral envelopes and 20 liposomes; or nonparticulate systems which are macromolecules such as proteins or synthetic polymers. Liposomes have been studied as drug carriers and offer a range of advantages relative to other MDDS systems. Composed of naturally- occurriny materials which are bioco patible and biodegradable, 25 liposomes are used to encapsulate biologically active materials for a variety of purposes. Having a variety of layers, sizes, surface charges and compositions, numerous procedures for liposomal preparation and for drug encapsulation within them have been developed, some of which have been scaled up to 30 industrial levels. Liposomes can be designed to act as sustained release drug depots and, in certain applications, aid drug access across cell membranes. Their ability to protect encapsulated drugs and various other characteristics make liposomes a popular choice in developing MDDS, with respect to the previous practices of free drug admin stration. Despite the advantages offered, utilization of drug-encapsulating liposomes does pose some difficulties. For example, liposomes as MDDS have limited targeting abilities, limited retention and stability in circulation, potential toxicity upon chronic administration and inability to extravasate. In recent years, successful attempts have been made to bind different substances to liposomes. For example, binding of chymotrypsin to liposomes has been studied as a model for binding substances to liposomal surfaces. Recognizing substances, including antibodies, glycoproteins and lectins have been bound to liposomal surfaces in an attempt to confer target specificity to the liposomes. Concentrating on systemic applications and in vivo studies, these previous efforts discuss methods of binding recognizing substances with liposomes and the effectiveness of such modified l posomes. Although the Donding of these recognizing substances to liposomes occurred, the resulting modified liposomes did not perform as hoped, particularly during in vivo studies. Other difficulties are presented when utilizing these recognizing suostances. For example, antibodies can be patient specific and, therefore, add cost to the drug therapy. It has also oeen previously learned that liposomes can be modified by binding epidermal growth factor (EGF) to the liposomal surface as a recognizing substance. EGF stimulates cell growth and proliferation through the interaction with the EGF receptor. The EGF receptors are distributed on the cell surface of various organs and are present in burns, wounds, and other designated targets of MDDS such as ocular, dermal and tumors. Accordingly, EGF-modified liposomes potentially offer efficiency as drug carriers to target sites expressing the EGF receptors. EGF is a polypeptide. It has been reported that the biological activity of EGF is dependent upon the conservation of the native conformation of EGF, to which the disulfide bonds are critical. Previous work in binding EGF to liposomes has capitalized on the existence of the disulfide bonds. Specifically, EGF has been bound to the liposomal surface by the disulfide bridge linkage using a heterobifunctional crosslinking reagent, N-hydroxysuccinimidyl-3-(2-pyridyldithio) propionate. Although the resulting EGF-modified liposomes have been suggested as potential drug carriers for systemic applications, the complex chemistry of this process results in byproducts whose effect on drug delivery and toxicity are unknown. EGF is bound covalently to discrete sites on the liposome surfaces. The number and surface density of these sites will be dictated by the liposome formulation and the liposome type. The liposomal surfaces may also have sites for noncovalent association. Covalent binding is essential as noncovalent binding might result in dissociation of EGF from the liposomes at the site of administration since the liposomes and the bioadhesive counterparts of the target site (that is, the bioadnesive matter) compete for EGF. Such dissociation would reverse the administered modified liposomes into regular, non-modified liposomes, thereby defeating the purpose of administration of the modified liposomes. To form covalent conjugates of recognizing substances and liposomes, crosslinking reagents have been studied for effectiveness and biocompatibility. One such reagent is glutaraldehyde (GAD). Through the complex chemistry of crosslinking by GAD, linkage of the amine residues of the recognizing substance and liposomes is established. Previous efforts have studied the binding of chymotrypsin and liposomes witn GAD as the crosslinking reagent. SUMMARY OF INVENTION According to the present invention, methodologies have been developed and crosslinking reagents have been identified to bind growth hormones, preferably epidermal growth factor (EGF), to liposomal surfaces. More specifically, crosslinking reagents have been identified which crosslink amine residues on the liposomal surfaces to the residues offered by EGF. The crosslinking reagents include glutaraldehyde (GAD) and bifunctional oxirane (OXR), preferably ethylene glycol diglycidyl ether (EGDE). By modifying regular liposomes through covalent bonding certain growth hormones to the liposomal surface, the growth hormones are recognizing substances which can be utilized as an adhesive or glue to attach the modified liposome onto a target area. These "bioadhesive" liposomes offer potential advantages as a MDDS for the administration of drugs which is further disclosed in my concurrently filed applications. DETAILED DESCRIPTION According to the present invention, growth hormones, preferably EGF, have oeen covalently bound to liposomal surfaces as a recognizing substance through the crosslinking of amine residues. Liposomes, in particular, multilamellar vesicles (MLV), icroemulsified liposomes (MEL) or large unila ellar vesicles (LUVET), each containing phosphatidylethanolamine (PE), have been prepared by establisned procedures. Although urogasterone and EGF are recognized as biological equivalents, both purified urogasterone or EGF mouse were used as recognizing substances, with the results reported in Examples 1 and 2. In the specification and claims the term "EGF" means either urogasterone or epidermal growth factor regardless of the source. Growth hormone was linked to the PE-liposomes. In the Examples, the EGF and lipids are assayed by traces of radioactive or fluorescent labels included in each formulation. Alternatively, the lipids are assayed by colorimetric methods. The determination of the non-labeled and the fluorescent-labeled EGF can also be performed by the Lowry procedure previously reported. The "level of covalent binding" as reported in the Examples and Tables 1 and 2, is defined as the quantity of ligand or EGF bound to a given quantity of lipid in the final product since the most accurate quantitative measure of liposomes is in terms of lipid quantities. For a given lipid quantity, different liposome types will yield different quantities of liposome. Therefore, similar initial ratios of EGF to lipid for different liposome types should not be expected to yield the same level of binding. Another factor which would yield different results for different liposomes even under tne same initial EGF to lipid ratios, is the differences in particle size, therefore in curvature, number and accessibility of PE sites on the surface of the liposome. Therefore, comparisons among l posome types should be avoided. Tne effects of the increase in the EGF/lipid ratios in the presence of a crosslinking reagent are shown in Tables 1 and 2. Generally, an increase in the level of binding occurs with the increase in initial EGF/lipid ratios regardless of which crosslinking reagent is used. At the lower end of the EGF/lipid ratios, the level of covalent binding increases significantly. Beyond initial concentration ratios of 25 ng EGF/uMoles lipid, tne increase of binding is less significant. Noncovalent!y bound product is removed as excess unreacted material and does not appear in the reported results. Example One EGF is added to a PE-liposome sample and the mixture is buffered by a phosphate buffer saline solution (PBS) to pH of 7.2. Concentration ratios of EGF to lipid are shown in Table 1. Aliquots from a 25% solution of the crosslinking reagent GAD are added at a ratio of lOul per 1 ml EGF/PE-l poso e mixture. Incubation for a desired period is completed at either room temperature without stirring or at 37°C with stirring. Depending upon the liposome used, excess unreacted material was removed through centπ ' fugation and washings, column chromatography or dialysis against PBS. TABLE 1 EGF-LIPOSOME CROSSLINKING BY GAD ng EGF/uMOLES LIPID (a) LIPOSOME TYPE INITIAL FINAL MLV MLV MLV MEL MEL MEL LUVET 0.016 0.003 (a) EGF assayed by a radioactive tracer. Example Two EGF is crosslinked with PE-liposome samples following the same procedure as in Example 1. Concentration ratios of labeled EGF to lipid are shown in Table 2. TABLE 2 EGF-LIPOSOME CROSSLINKING BY GAD ng EGF/uMOLES LIPID (a) (a) EGF assayed by a fluorescent tracer. Example Three Reaction mixtures of EGF and PE-liposomes were prepared as above and buffered by PBS to pH 7 or by 0.5N carbonate buffer to pH 9. Concentrations ratios of EGF to lipid are shown in Table 3. The crosslinking reagent EGDE was added in 0.2 - 1.0 ml volumes to buffered reaction mixtures of 2.5 - 3.0 ml volumes. Incubation periods were completed for 10-24 hours at 37°C with stirring. Depending upon the liposome used, excess unreacted material was removed through ultracentnfugations and washings or dialysis against PBS. TABLE 3 EGF-LIPOSOME CROSSLINKING BY EGDE (a) EGF assayed by a radioactive tracer. (b) Initial ratios were increased by decreasing lipid concentration. From these results, the preferred pH of 9 and quantity of crosslinking reagent of 500 mg has been determined. lήlhile the preferred embodiments have been described, various modifications and substitutions may be made without departing from the scope of the invention. For example, the mouse EGF and human urogasterone used in the disclosed examples could be substituted with growth factors from other natural or synthetic sources. Accordingly, it is to be understood that the invention has been described by way of illustration and not limitation.

Claims: CLAIMS What I claim is: 1. A process for producing modified liposomes comprising the steps of: (a) providing a reaction vessel containing a liposome component having liposome and phosphatidylethanolamine; 5 (b) adding a recognizing substance component of a growth factor to the reaction vessel ; (c) buffering the reaction mixture of the liposome and recognizing substance components; (d) adding a crosslinking reagent to the buffered ° reaction mixture; and (e) allowing the buffered reaction mixture with reagent to incubate for a period of time sufficient for the modified liposome to form. 2. The process of claim 1 wherein the liposome is selected from the group consisting of multilamellar vesicles, microemulsified liposomes and large unilamellar vesicles. 3. The process of claim 1 wherein the growth factor is epidermal growth factor. 4. The process of claim 1 wherein the growth factor is urogasterone. 5. The process of claim 1 wherein the crosslinking reagent is glutaraldehyde. 6. The process of claim 1 wherein the crosslinking reagent is a bifunctional oxirane. 7. The process of claim 1 where in the crosslinking reagent is ethylene glycol diglycidyl ether. 8. The process of claim 1 wherein the recognizing substance is labeled with a fluorescent marker. 9. The process of claim 1 wherein the recognizing substance is labeled with a iodinated marker. 10. A modified liposome produced according to the method of claim 1. 11. A process for covalently bonding a recognizing substance to a liposome comprising the steps of: (a) providing a reaction vessel containing a liposome component having liposome and phosphatidylethanolamine; 5 (b) adding a recognizing substance component of growth factor to the reaction vessel; (c) buffering the reaction mixture of the liposome and recognizing substance components; (d) adding a crosslinking reagent to the buffered 10 reaction mixture; (e) allowing the buffered reaction mixture with reagent to incubate for a period of time sufficient for the covalent bondi g of amine residues of the recognizing substance and iposome components; and I 5 (f) separation and removal of covalently bound recognizing substance liposome components from excess unreacted and noncovalently bound materials. 12. Tne process of claim 11 wherein the liposome is selected from the group consisting of multilamellar vesicles, microemulsified liposomes and large unilamellar vesicles. 13. The process of claim 11 wherein the growth factor is epidermal growth factor. 14. The process of claim 11 wherein the growth factor is urogasterone. 15. The process of claim 11 wherein the crosslinking reagent is glutaraldehyde. 16. The process of claim 11 wherein the crosslinking reagent is a bifunctional oxirane. 17. The process of claim 11 where in the crosslinking reagent is ethylene glycol diglycidyl ether. 18. The process of claim 11 wherein the recognizing substance component is labeled with a fluorescent marker. 19. The process of claim 11 wherein the recognizing substance component is labeled with a iodinated marker. 20. A modified liposome produced according to the method of claim 11. 21. A modified liposome comprising a liposome component covalently linked to a recognizing substance component by a crosslinking reagent. 22. The modified liposome of claim 21 wherein the liposome component includes phosphatidylethanolamine. 23. The modified liposome of claim 21 wherein the liposome component is selected from the group consisting of multilamellar vesicles, microemulsified liposomes and large unilamellar vesicles. 24. The modified liposome of claim 21 wherein the recognizing substance component is a growth factor. 25. The modified liposome of claim 21 wherein the recognizing substance component is an epidermal growth factor. 26. The modified liposome of claim 21 wherein the recognizing substance component is urogasterone. 27. The modified liposome of claim 21 wherein the crosslinking reagent is glutaraldehyde. 28. The modified liposome of claim 21 wherein the crosslinking reagent is a bifunctional oxirane. 29. The modified liposome of claim 21 wherein the crosslinking reagent is ethylene glycol diglycidyl ether. 30. The modified liposome of claim 21 wherein the recognizing substance component is labeled with a fluorescent marker. 31. The modified liposome of claim 21 wherein the recognizing substance component is labeled with a iodinated marker. 32. The modified liposome of claim 21 wherein the crosslinking reagent is selected from a group of reagents linking amine residues of the liposome component and the recognizing substance component.

Inventors: MARGALIT RIMONA, MARGALIT, RIMONA
Applicants: BAXTER INT, BAXTER INTERNATIONAL INC.
Application year: 1991
Main classification: A61K9
IPC codes: A61K9, A61K38