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1. WO1992014448 - BINDING OF GROWTH HORMONE TO LIPOSOMES

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[ EN ]

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 natural ly- 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.

lήlhil e 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.