GB2134869A

GB2134869A - Method of preparing liposomes and products produced thereby

Info

Publication number: GB2134869A
Application number: GB08304165A
Authority: GB
Inventors: Cheryl Vanessa Groom; Peter Timmins
Original assignee: ER Squibb and Sons LLC
Current assignee: ER Squibb and Sons LLC
Priority date: 1983-02-15
Filing date: 1983-02-15
Publication date: 1984-08-22
Also published as: JPS59173133A; JPH0551338B2; GB8304165D0; ZA84908B

Abstract

A method for preparing stable liposome precursors in the form of water-soluble carrier materials coated with thin films of liposome components includes the steps of dissolving at least one liposome-forming amphipathic lipid, optionally at least one biologically active compound, and, optionally, at least one adjuvant in a suitable solvent and employing the resulting solution to coat a water-soluble carrier material to form thin films of liposome components thereon. Upon exposing the coated carrier material to water, the thin films of liposome components hydrate and the carrier material dissolves to give liposome preparations.

Description

SPECIFICATION Method of preparing liposomes and products produced thereby Field of the Invention The present invention relates to a method for preparing particulate water-soluble carrier materials coated with thin films of liposome components, which coated carrier materials are employed to form liposome preparations, and to intermediates and products produced in such method.

Background of the Invention Liposomes are widely described in the literature and their structure is well known. They are formed by amphipathic molecules such as the class II polar lipids, that is, phosphatidyl cholines, ethanolamines and serines, sphingomyelins, cardiolipins, plasmalogens, phosphatidic acids and cerebrosides. Liposomes are formed when phospholipids or other suitable amphipathic molecules are allowed to swell in water or aqueous solutions to form liquid crystals usually of multilayer structure comprised of many bilayers separated from each other by aqueous material.

Another type of liposome is known consisting of a single bilayer encapsulating aqueous material which may also be referred to as a unilamellar vesicle. "If water-soluble materials are included in the aqueous phase during the swelling of the lips they become entrapped between the lipid bilayers.

Alternatively, lipid soluble materials may be dissolved in the lipid and, hence, may be incorporated into the lipid bilayers themselves," Ryman, B. E., "The Use of Liposomes as Carriers of Drugs and Other Cell-Modifying Molecules," Proc.

6th Int'l. Congr. Pharmacol. 5,91(1976), published in "Drug Applications," Clinical Pharmacology, Vol. 5, pp. 91-103, Pergamon Press (1975).

In recent years there has been much interest in the use of liposomes as carriers of compounds which are of interest because of one or other biological property, for example, medicaments, proteins, enzymes, hormones and diagnostic agents, hereinafter referred to as "biologically active compounds." Liposomes have been suggested as carriers for drugs, see Ryman, supra at page 91 and Gregoriadis, G., "Enzyme or Drug Entrapment in Liposomes; Possible Biomedical Application," lnsolubllized Enzymes, Ed.

M. Salmona et al, Raven Press, N.T. 1974, pp.165-177.

Water-soluble materials are encapsulated in the aqueous spaces between the bimolecular layers. Lipid soluble materials are incorporated into the lipid layers although polar head groups may protrude from the layer into the aqueous space. The encapsulation of these compounds can be achieved by a number of methods. The method most commonly used involves casting a thin film of phospholipid onto the walls of a flask by evaporation of an organic solvent. When this film is dispersed in a suitable aqueous medium multilamellar liposomes are formed (also referred to as coarse liposomes). Upon suitable sonication, the coarse liposomes form smaller similarly closed vesicles.

Water-soluble biologically active compounds are usually incorporated by dispersing the cast film with an aqueous solution of the compound.

The unencapsulated compound is then removed by centrifugation, chromatography, dialysation or some other suitable procedure. Lipid-soluble biologically active compounds are usually incorporated by dissolving them in the organic solvent with the phospholipid prior to casting the film. Providing the solubility of these compounds in the lipid phase is not exceeded or the amount present is not in excess of that which can be bound to the lipid, liposomes prepared by the above method usually contain most of the compound bound in the lipid bilayers; separation of the liposomes from unencapsulated material is not required.Other methods of preparing liposomes have been described although these are mainly specialized methods producing unilamellar liposomes and include reverse-phase evaporation of an organic solvent from a water-in-oil emulsion of phospholipid, infusion of organic solutions of phospholipid into large volumes of aqueous phase and detergent removal from mixed micelles of detergent and lipid.

Aqueous liposome dispersons only have limited physical stability. The liposomes can aggregate and precipitate as sediment. Although this sediment may be redispersed, the size distribution may be different from that of the original dispersion. This may be overcome to some extent by incorporation of charged lipids into the liposomes. In addition, on storage the biologically active compounds may be lost into the external aqueous phase which restricts the potential of these preparations as practical dosage forms. This is particularly notable for low molecular weight water-soluble compounds but lipid soluble compounds too can partition into the external aqueous medium. If the volume of the aqueous medium is large, this loss can be significant.In addition, depending upon the type of lipid and biologically active compound present in the liposome, there is the potential for chemical degradation of the lipid components and/or the biologically active components in the aqueous dispersion.

These factors restrict the use of liposomes as practical carriers of biologically active compounds.

One solution suggested for overcoming the limited physical stability of liposomes is to prepare and store the lipid/biologically active compound film and then disperse the film to form liposomes just prior to administration. However, unit dose film preparation presents serious practical difficulties in that the containers would require a high surface area to facilitate solvent evaporation and deposition of a thin film suitable for rapid rehydration to form liposomes readily. This type of container by virtue of its bulk would present severe storage problems. Other methods suggested for preparing liposome components in a solid form for storage have included freeze-drying the prepared aqueous liposome suspension as described in U.S.Patents No. 4,229,3760 and 4,247,411 and by freeze-drying the liposome components from a suitable organic solvent as described in U.S. Patent No. 4,311,712. These freeze-dried preparations result in a porous matrix of iiposome components which is easily hydrated.

Brief Description of the Invention In accordance with the present invention, a method is provided for preparing thin films of liposome components, which are not subject to the physical stability problems set out above, and which may be employed to form liposome preparations immediately prior to administration.

The method of the present invention includes the steps of forming a solution of at least one liposome-forming amphipathic lipid, optionally, at least one biologically active compound, and, optionally, at least one adjuvant, and employing the so-formed solution to coat a suitable watersoluble particulate carrier material.

In addition, in accordance with the present invention, a method is provided for forming a liposome preparation which method includes the step of exposing the water-soluble particulate material coated with the thin film of liposome components to water thereby causing the thin film of liposome components to hydrate and the carrier material to dissolve to give a liposome preparation similar to that prepared by hydration of cast films with a solution of the carrier material.

Further, in accordance with the present invention, there is provided the intermediate formed above which is comprised of the relatively stable particulate water-soluble carrier material coated with a thin film of liposome components and which is useful for forming the liposome preparation.

The problems associated with the physical stability of liposome dispersions on storage may be overcome by forming the aqueous dispersion of the coated powdered carrier material prior to administration. Additionally, the chemical integrity of the biologically active compounds and lipid components may be protected in the coated powdered preparations by the incorporation of antioxidants therein or packing the coated powdered material under inert atmospheres, for example.

Detailed Description of the Preferred Embodiments In carrying out the method of the invention for preparing the particulate water-soluble carrier materials coated with a thin film of liposomal components, at least one liposome forming amphipathic lipid, optionally, at least one biologically active compound, and, optionally, at least one adjuvant are dissolved in a solvent and this solution is used to coat a suitable watersoluble carrier material. For low melting point liposome-forming amphipathic lipids (that is having a melting point below 500C), the optional biologically active compound and optional adjuvant may be directly dissolved in the lipid and this solution used to coat a suitable carrier material.

The lipid will be present in the solution (containing a separate solvent), to be used to coat the carrier material, in an amount of within the range of from about 1 to about 25% by weight, depending upon the solubility of the lipid in the solvent or solvent mixture used, and preferably from about 2.5 to about 12.5% by weight of such solution. The optional biologically active compound and optional adjuvant material will be present in the coating solution in varying amounts depending upon the nature of the particular compound and/or material employed.

The ratio of lipid to optional biologically active compound in the coating solution will depend upon the lipid solubility or binding of the biologically active compound used. Thus, the coating to be applied to the carrier material will normally contain a weight ratio of lipid: optional biologically active compound of within the range of from about 5:1 to about 1000:1 and preferably from about 10:1 to about 200:1 depending upong the particular biologically active compound to be employed. For example, where the biologically active compound is an anti-infective, such as an antibotic or an anti-fungal agent, the lipid will be present in a weight ratio to the biologically active compound of within the range of from about 5:1 to about 1000:1 and preferably from about 10:1 to about 300:1.Where the biologically active compound is a contrast agent, the lipid will be present in a weight ratio to the contrast agent in an amount of within the range of from about 5:1 to about 1000:1 and preferably from about 10:1 to about 200:1.

The amounts of optional adjuvant material and biologically active material employed in the coating will comprise amounts conventionally employed in forming liposomes.

The amount of coating applied to the carrier material will depend upon physical characteristics of the carrier material such as surface area and isotonicity requirements. Thus, the coating will normally be present in a weight ratio to carrier material in an amount of within the range of from about 0.03 :1 to about 0.3 :1 and preferably from about 0.05:1 to about 0.2:1.

Any amphipathic lipid which is known to be suitable for preparing liposomes by known methods can be used in the method of this invention. Thus, a wide variety of lipids may be used but non-immunogenic and biodegradable lipids would be preferred. Examples of suitable lipids are the phospholipids, for example, natural lecithins, such as egg lecithin or soya bean lecithin, or synthetic lecithins such as saturated synthetic lecithins, for example, dimyristoyl phosphatidyl choline, dipalmitoyl phosphatidyl choline or distearoyl phosphoatidyl choline or unsaturated synthetic lecithins, such as dioleyl phosphatidyl choline or dilinoleyl phosphatidyl choline, with egg lecithin or soya bean lecithin being preferred.

The biologically active compound employed in the present invention may be any compound of biological interest, for example, the compound may be a medicament, such as an anti-infective, for example, amphotericin B and benzyl penicillin, anti-tumor agents, such as 5-fluorouracil, methotrexate, actinomycin D, enzyme, hormone, contrast agent or marker compound.

Examples of contrast agents suitable for use in the present invention include, but are not limited to the following: N,N'-bis[2-hydroxyl-1- (hydroxymethyl)ethyl]-5-[(2-hydroxy- 1 oxopropyl)-amino]-2,4,6-triiodo- 1 ,3- benzenedicarboxamide (Bracco 15,000), metrizamide, diatrizoic acid, sodium diatrizoate, meglumine diatrizoate, acetrizoic acid and its soluble cationic salts, diprotrizoic acid and its soluble inorganic and organic cationic salts, iodamide, sodium iodipamide, meglumine iodipamide, iodohippuric acid and its soluble salts, iodomethamic acid and its soluble salts, iodopyracetiodo-2-pyridone-N-acetic acid and its soluble salts, 3,5-diiodo-4-pyridone-N-acetic acid (iodopyracet), 3,5-diiodo-4-pyridone-N-acetic acid diethanolamine salt, iodo-2-pyridone-N-acetic acid and its amine salts, iothalamic acid and its soluble salts, methanesulfonic acid, metrizoic acid and its soluble salts, sodium ipodate, ethiodized oil, iopanoic acid, iocetamic acid, tyropanoate sodium, iopydol, iophenoxic acid, iophendyiate, and other chemically related iodinated contrast agents. Unless indicated otherwise, where applicable, the contrast agents which may be employed herein include inorganic, organic and cationic salts of the above contrast agent, such as the potassium salt, calcium salt, lithium salt, arginin salt, cystein salt, glycin salt, glycyl glycin salt, N-methyl glucosamine salt and other non-toxic aliphatic and alicyclic amines employed in preparing water soluble salts. Other X-ray contrast agents which may be employed herein are disclosed in German Offenlegungsschrift DT 2935-195.

The final liposome preparation containing a contrast agent prepared by the method of the invention may be employed as described in U.S.

Patent No. 4,192,859 which is incorporated herein by reference.

Other proteins and drugs available for use herein as optional biologically active compounds include steroids such as hydrocortisone, colchicine, insulin, cyclic AMP and a-thiodeoxyguanosine, chelating agents and cell modifying substances, such as antigens and interferon inducers.

The present invention is particularly useful in the case of lipid-soluble or lipid-bound biologically active compounds (which include some water soluble compounds, such as proteins).

The method of this invention, like other methods of preparing liposomes, will result in partial incorporation of water-soluble biologically active compounds. Usually the formation of liposomes containing this type of compound is foliowed by removal of the unencapsulated material; however, in some instances coadministration of unencapsulated and liposomally entrapped biologically-active compounds may be advantageous.

The optional adjuvants suitable for use in the present invention may be: a) substances which are known to provide a negative charge on the liposomes, for example, egg phosphatidic acid or dicetyl phosphate.

b) substances known to provide a positive charge, for example, stearyl amine, or stearyl amine acetate.

c) substances shown to affect the physical properties of the liposomes in a more desirable way; for example, sterols such as cholesterol, ergosterol, phytosterol, sitosterol, sitosterol pyrogiutamate, 7-dehydrocholesterol, lanosterol, orcaprolactam, will affect membrane rigidity.

d) substances known to have antioxidant properties to improve the chemical stability of the particulate carrier coated with liposome components, such as tocopherol, propyl gallate, ascorbyl palmitate, or butylated hydroxy toluene.

Suitable solvents for use in dissolving or aiding in dissolution of the above-mentioned mixture of ingredients include, but are not limited to, ethanol, methanol, chloroform, dichloromethane, diethyl ether, carbon tetrachloride, ethyl acetate, dioxane, cyclohexane and the like with methanol, ethanol or chloroform being preferred.

The carrier material to be coated may be watersoluble material which is suitable for intravanous use, for example, sodium, chloride, lactose, dextrose, and sucrose. It may be advantageous to use micronized forms of the carrier materials (that is, having an average particle size of less than about 10 microns) as the high surface area would facilitate the hydration of the liposomal components. The amount of carrier material used may be adjusted so that the final reconstituted suspension is iso-osmotic with the blood, although for small volume injections this may not be necessary. As a suitable aqueous medium for dispersion distilled water, isotonic saline or buffer solution may be used.

The liposomal components (excluding the biologically active compound) preferably are binary mixtures of lecithin and a sterol selected from the group listed hereinabove, or ternary mixtures of lecithin, dicetyl phosphate, and a sterol selected from the group listed hereinabove, in the preferred molar ratios of 7 :2:1, respectively. The molar percentage of lecithin may range from about 55 to about 95% and the sterol from about about 55 to about 95% on a binary mixture. The molar percentage of lecithin may range from about 50% to about 80%, the dicetyl phosphate from about 0 to about 30%, and the sterol from about 5% to about 30%, based on a ternary lipid mixture. Lecithin is employed to take advantage of its property of swelling in salt solutions.Dicetyl phosphate has the property of imparting a negative charge to the lipid membranes so that the mutual repulsive action of opposing channel surfaces widens the channels.

The components which constitute the liposomal mixture are commercially available or may readily be prepared.

Coating of the carrier material may be achieved by applying solutions of liposomal components dissolved either in the lipid phase or in a suitable organic solvent. Alternatively, the carrier material may be suspended in the solution of liposomal components and spray dried. By suitable containment and sterilization of component materials, sterile carrier material coated with the liposomal components is produced. The coated carrier material of the invention may be packed in sterile unit dose vials under aseptic conditions and reconstituted immediately prior to use by the physician.

The following Examples represent preferred embodiments of the present invention.

EXAMPLE 1 Five hundred and ten milligrams (510 mg) of dextrose (anhydrous) were placed in a 100 ml round bottom flask and 40 mg of egg lecithin dissolved in 3 ml of chloroform added in 3 x 1 ml portions. After each portion addition, the solvent was removed under vacuum from the rotating flask. Microscopic examination of the powder showed that the dextrose particles were coated with lipid material.

The so-formed coated carrier may be packaged and stored in sterile unit dose vials under aseptic conditions and reconstituted immediately prior to use.

Distilled water (5 ml) was added to a portion of the above dextrose preparation coated with phospholipid (225 mg) in a vial and the mixture heated to 600C for one minute, then agitated by hand to achieve the final dispersion. The size distribution of the liposome preparation was log normal as determined using a Coulter Counter with a mass median volume equivalent diameter of 5.3 ym and a geometric standard deviation of 1.58.

The above liposome preparation is similar to those prepared by hydration of cast films with a solution of the carrier material.

EXAMPLE 2 Egg lecithin (2.0 g), ergosterol (0.5 g) and amphotericin B (50.0 mg) were dissolved in methanol (10 ml). Lactose (13.0 g) was placed in a 250 ml round bottom flask and the above solution added in 2 ml portions. After each portion addition, the solvent was removed under vacuum from the rotating flask. Microscopic examination of the powder showed that the lactose particles were coated with lipid material.

The so-formed coated carrer may be packaged and stored in sterile unit dose vials under aseptic conditions and reconstituted immediately prior to use by the physician as follows.

Water for injection (10 ml) was added to a portion of the above lactose preparation coated with liposomal components (0.775 g) in a vial and the mixture heated to about 700C in a water bath to aid dissolution of the carrier material. Shaking of the vial caused the preparation to disperse resulting in a milky dispersion. The size distribution of the liposomes formed was lognormal as determined using a Coulter counter with a mass median volume equivalent diameter of 2.5 ssm and a geometric standard deviation of 1.56.

The liposome preparation containing the amphotericin B is similar to those prepared by hydration of cast films with a solution of the carrier material.

Claims

1. A method for preparing a stable liposome precursor in the form of a thin film of liposomal components coated on a water-soluble particulate carrier material, which comprises forming a solution of at least one liposome-forming lipid, optionally, at least one biologically active compound, and, optionally, at least one adjuvant which imparts advantageous properties to the final liposome preparation, and coating a particulate water-soluble carrier material with the so-formed solution to form a thin film of liposomal components on said carrier material.

2. A method according to Claim 1 wherein the solution is formed by dissolving said lipid, optionally, said biologically active compound and, optionally, said adjuvant in one or more organic solvents.

3. A method according to Claim 2 wherein the organic solvent is ethanol, methanol, chloroform, dichloromethane, diethyl ether, carbon tetrachloride, ethyl acetate, dioxane or cyclohexane.

4. A method according to Claim 3 wherein the solvent is methanol, ethanol or chloroform.

5. A method according to Claim 1 wherein the solution is formed by dissolving the optional biologically active compound and optional adjuvant in said lipid, said lipid being of the low melting point liposome-forming type.

6. A method according to any preceding claim, wherein the particulate carrier material is coated with said solution by suspending the carrier material in the solution of liposomal components and spray drying the coated carrier material.

7. A method according to any preceding claim wherein the lipid is a phospholipid.

8. A method according to Claim 7 wherein the phospholipid is a natural or synthetic lecithin.

9. A method according to any preceding claim wherein said solution includes a biologically active compound which is a medicament, contrast agent, enzyme, hormone or marker compound.

10. A method according to any one of Claims 1 to 8 wherein the biologically active compound is amphotericin B.

11. A method according to any preceding claim wherein the adjuvant is egg phosphatidic acid, dicetyl phosphate, or stearyl amine.

12. A method according to any one of Claims 1 to 10 wherein the adjuvant is a sterol.

13. A method according to Claim 11 wherein the adjuvant also includes a sterol selected from cholesterol, phytosterol, ergosterol, sitosterol, sitosterol pyrogluamate, 7-dehydrocholesterol, lanosterol and caprolactam.

14. A method according to any preceding claim wherein said carrier material is a water-soluble carrier suitable for intravenous use.

1 5. A method according to Claim 14 wherein said carrier is sodium chloride, lactose, dextrose or sucrose.

1 6. A method according to Claim 1 wherein the lipid is egg lecithin, the adjuvant is ergosterol or cholesterol, and the carrier material is lactose.

1 7. A stable liposome precursor which when mixed with water forms a liposomal preparation; comprising a water-soluble particulate carrier material coated with a thin film comprised of at least one liposome-forming lipid, optionally at least one biologically active compound and, optionally, at least one adjuvant whichEmparts advantageous properties to the final liposome preparation.

1 8. A stable liposome precursor according to Claim 1 7 including at least one biologically active compound.

19. A stable liposome precursor which when mixed with water forms a liposomal preparation, comprising water-soluble particulate carrier material coated with a thin film comprised of at least one liposome-forming lipid, optionally, at least one biologically active compound and, optionally, at least one adjuvant which imparts advantageous properties to the final liposome preparation, said precursor having been prepared by a method according to any one of Claims 1-16.

20. A method for preparing a liposome preparation which comprises exposing a liposome precursor according to Claim 17, 18 or 19 to water.

21. A liposome preparation prepared by a method according to Claim 20.

22. A method according to Claim 1, substantially as described in either of the Examples.

23. A stable liposome precursor according to Claim 1 7 or 1 9 substantially as described in either of the Examples.

24. A liposome preparation according to Claim 21 substantially as described in either of the Examples.