CA1330532C

CA1330532C - Manufacturing unilamellar lipid vesicles

Info

Publication number: CA1330532C
Application number: CA000572961A
Authority: CA
Inventors: Donald F. H. Wallach; Jean Philippot
Original assignee: Micro Pak Inc
Current assignee: Novavax Inc
Priority date: 1987-07-28
Filing date: 1988-07-25
Publication date: 1994-07-05
Anticipated expiration: 2011-07-05
Also published as: EP0374181A4; US4853228A; NZ225510A; WO1989000812A1; EP0374181A1; ZA885450B

Abstract

ABSTRACT OF THE INVENTION

Disclosed is a new type of unilamellar lipid vesicle made from polyoxyethylene acyl ethers or sorbitan alkyl esters. These unilamellar vesicles are made with easily obtained, relatively inexpensive materials and the vesicles are more stable then conventional unilamellar lipid vesicles. A delivery system for drugs or other molecules is also disclosed.

Description

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1 ~a~RQUN~ OF THE-LgyEE~IQ~

The present invention relates large lipid vesicular membrane structures having single bilayers -surrounding aqueous interiors. More specifically, the present invention relates to unilamellar lipid vesicles having large encapsulation efficiency and captured volume. The large unilamellar vesicles of the invention are useful, for e~ample, as carriers for hydrophilic, biologically active molecules.
Liposomes or lipid vesicles composed of lipid bilayers enclosing an interior aqueous volume are known to be useful as delivery systems for, or carriers of, various substances. There are three --general types of liposomes: multilamellar vesicles ~MLVs) composed of more than one concentric lipid bilayer separated by a multiplicity of enclosed volumes; small unilamellar vesicles (S Ws) composed of a single lipid bilayer enclosing a single interior -volume and having a diameter of less than 0.2 ~;
and large unilamellar vesicles ~L W s) composed of a single bilayer enclosing a single interior volume and ha~ing a diameter of qreater than 0.450 ~, and preferably greater than 1.0 ~.

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Lipid vesicles can be characterized by a number of functional properties. Properties of importance include captured volume, a measure of the amount of solvent trapped within the vesicles; and encapsulation efficiency (Ee)~ a measure of the amount of the material to be encapsulated enclosed entirely within the vesicle's internal volume. The captured volume is defined as the concentration of the aqueous fraction inside the vesicle divided by the concentration of lipid in the vesicle, normally expressed as l/mole lipid. The encapsulation efficiency is defined by the equation Ee=C'/C ~ -l/CL, where C' is the final molar concentration of the molecule to be encapsulated within the lipid vesicle, C is the initial molar concentration of the molecule in its solvent, and CL is the concentration of lipid in the vesicle.

For some uses, e.g., carrying drugs to a specific tissue without dose-related tosicity problems, the vesicles which encapsulate or trap the largest amount of a desired material, and which, upon injection, are the most successful in reaching the targeted tissue are the most valuable. Each type of lipid vesicle is well suited for a different purpose. For e~ample, MLVs are particularly useful for capturing lipophilic materials because of their small agueous lipid ratio, whereas S W s, although having low encapsulation efficiencies have the widest tissue access by virtue of their size. L W s, although very poor in encapsulating hydrophobic or lipophilic materials because of their large aqueous to lipid ratio, have large captured volumes ~ ~ ' ~ j ~ ,~ . . . ' :. '~ ".,: . :
' ;' .. ~,: , ' : ' 133~32 (approximately 35 l/mole lipid) and high encapsulation efficiencies for hydrophilic materials (40-50%). Therefore, L W s are often the vehicles of choice for transporting hydrophilic, biologically active molecules.

In an effort to create stable, nonantigenic structures havinq properties similar to natural phospholipid membranes, liposomes have traditionally been synthesized primarily from phospholipids.
However, construction of lipid vesicles solely from phospholipids does not accurately recreate the native environment of a membrane since natural membranes also contain proteins and glycoproteins. In ~-addition, phospholipid structures are notoriously unstable in vivo. Factors responsible for this instability include degradation by phospholipases and plasma lipoproteins such as HDL, and by autocatalyzed perosidation. L W s, in particular tend to be particularly susceptible to such lytic digestion and peroxidation since they have only a single lipid bilayer surrounding a large a~ueous interior.
' ' As well as having the aforementioned stability problems, phospholipids are expensive, making their cost in large scale preparation of liposome operations prohibitive.

In an attempt to solve some of the problems associated with the use of phospholipids in the construction of liposomes, it has been demonstrated that lipid membrane structures, and primarily NLYs, :
can be prepared from other amphiphilic molecules ' ~f . ,~: ,: ' , ., ' ' 1 33~r,~2 including fatty acids, dialkyl dimethyl ammonium compounds, and dialkyl amphiphiles with nonionic and zwitterionic head groups. L~Oreal has disclosed the construction of multilamellar vesicles out of ~ -synthetic amphiphiles such as alkyl ethers of certain polyglycerols and polyoxyethylene, while others have attempted the synthesis of similar vesicles from aliphatic lipids and digalactosyl diglyceride. y-~;

Although these efforts have met with varying 10 degrees of success, none have solved all of the foregoing problems.

Accordingly, it is an object of the present invention to provide structurally stable L W s with high captured volume and high encapsulation lS efficiency for hydrophilic molecules.

It is an additional object of the invention to provide L W s which are constructed from lipids which are inexpensive, readily available, biocompatible, and biodegradable.

It is another object of the invention to provide L W s composed of synthetic, non-phospholipid amphiphilic molecules which are stable, and which are capable of encapsulating hydrophilic molecules. ~;-. : ~.. :
It is yet another object of the invention to provide L W s which are capable of encapsulating and transporting hydrophilic molecules.

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It is a further object of the invention to provide a delivery system for hydrophilic molecules consisting of an L W comprising a surfactant, a ~
sterol, a charge-producing amphiphile, and a -targeting molecule.
. ~ . , These and other objects and features of the ~-~
invention will be apparent from the detailed description and claims which follow.

S~MMARY OF THE INVENTION

The present invention features a large lipid vesicular membrane structure having a single lipid bilayer surrounding an aqueous interior. Lipids useful in the construction of these vesicles include surfactants such as polyo~yethylene acyl ethers, preferably having the structure Rl-o- ( cH2-cH2-o- ) m-H

where Rl is CH3-(CH2)n, n ranges from 11 to 15, and m ranges from 2 to 4. Although other polyoxyethylene ethers can be used, the most preferred materials are polyoxyethylene (2) cetyl ether and polyoxyethylene (4) lauryl ether. -An alternative group of lipids which are ~ -useful in the invention are the sorbitan alkyl esters having the structure ~ ;~

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¦ O H H H O

H H O H O H
H H

where R2 is CH3-(CH2)s, and x ranges from 11 to 15. Although other sorbitan esters can be used, the most preferred materials are sorbitan laurate and sorbitan monopalmitate.

It has been determined that the inclusion of sterols in the construction of the L W s of the present invention helps to buffer the thermotropic phase transition of the membrane layer, i.e., it enables the lipid membrane structure to be less susceptible to temperature changes in the region of the transition temperature. The sterols also insure optimal vesicle size and increase bilayer stability.
Sterols useful in the construction of this invention include cholesterol, its chemical analogs, and its derivatives. ~ --Vesicles of this invention may also include a negative or positive charge-producing amphiphile.
Exemplary negative charge-producing materials include ~ -~
dicetyl phosphate, cetyl sulfate, phosphatydic acid, phosphatidyl serine, and mixtures thereof, while ` -~-exemplary positive charge-producing materials include long chain amines, quaternary ammonium compounds, and mixtures thereof.
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^` 13~32 The invention may additionally feature a targeting molecule, preferably a hydrophilic molecule. Commonly utilized hydrophilic targeting molecules include immunoglobulins, lectins, and peptide hormones.

The present invention further provides a delivery system for a biologically active hydrophilic material. This system consists of a large unilamellar vesicle having a lipid bilayer surrounding a substantially aqueous interior and enclosing the hydrophilic material. The lipid bilayer is formed of a surfactant selected from the group consisting of polyoxyethylene acyl ethers and sorbitan alkyl esters, a sterol selected from the 15 group consisting of cholesterol, its chemical ;
analogs, and its derivatives, and a charge-producing amphiphile, and a targeting molecule. Preferably, the lipid bilayer includes approximately 46-48 parts of the surfactant, 46-48 parts of the sterol, 4-8 parts of the charge-producing amphiphile and less than one part of the targeting molecule, most preferably in the ratios of 47.5:47.5:5:<1, respectively. The hydrophilic material enclosed within the lipid bilayer is preferably selected from a group consisting of nucleic acids, immunological adjuvants, enzymes, hormones, lymphokines, blood proteins, pesticides, contrast dyes, and radioactive marker molecules.

~ he following description and esamples will 30 more fully illustrate the invention.

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133~32 DESCRIPTION

The L W s of the present invention are preferably manufactured from a subclass of polyoxyethylene acyl ethers and from certain æorbitan alkyl esters.

Polyosyethylene acyl ethers for use in the invention have the structure Rl-O-(CH2--CH2-~)m~H

wherein Rl is CH3-(CH2)n, n ranges from 11 to ~ . ;
17, preferably 11-15, and m refers to the number of .:.
polyoxyethylene units in the hydrophilic moiety, commonly 2 to 4. The terminal hydroxyl group can be substituted with a number of functional residues, modifying the properties as desired or for use as a targeting molecule. Selected polyosyethylene acyl ethers are available in commerce from a number of :~ :
manufacturers, for example as the BRIJ series of -:.
surfactants manufactured by ICI Americas, Inc.

Of the readily available polyosyethylene ~: :
ethers tested to date, polyoxyethylene 2 cetyl ether gives L W s with the highest (40-50%) encapsulation efficiency, as monitored by the encapsulation of ~ :
[1,2-14C]polyethylene glycol 4,000 (molecular weight = 4,000).
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' ' 1 3 ~ 2 g The sorbitan acyl esters have the structure O :

¦ O H ¦ H H O

H H O H O H
H H

where R2 is CH3-(CH2)x, and x ranges from 11 to 15. Although other sorbitan esters can be used, 10 the most preferred materials are sorbitan laurate and sorbitan monopalmitate.

Selected sorbitan esters are availa~le in ;~
commerce from many manufacturers, for esample, as the ~PAN series of surfactants manufac~ursd by ICI
15 Americas, Inc. Sorbitan laurate and sorbitan monopalmitate are preferred to construct L W s.

Various molecules may be associated with the surfactants for the purpose of modifying the physical properties and permeabilities of the lipid membrane 20 structures. Of particular importance are the sterols such as cholesterol, its chemical analogs, or derivatives which buffer the thermotropic phase transition of the membrane layer, insure optimal size, and increase membrane stability. These effects 25 are enhanced by use of charge-bearing agents. A
nega~ive surface charge can be provided by including a small prop~rtion of dicetyl phosphate or cetyl sulphate in the lipid vesicle during formation, and a positive charge by including a small proportion of .... , - . : ~ ~ . .
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~3~532 long chain amines or quaternary ammonium derivatives, such cetyltrimethylammonium.

A number of different targeting molecules can be accommodated in the vesicle to provide L Ws 5 with an affinity for cells having surface architecture recognized by the molecules or for cells bearing receptors for such molecules. Hydrophilic targeting molecules such as immunoglobulins, lectins, and peptide hormones can be coupled to the vesicles in a variety of ways. A preferred method is by covalent linking of the targeting molecule to a palmitate chain and inserting the acyl chain into the lipid bilayer. Other methods include covalent coupling of the targeting molecule to the amino group of phosphatidylethanolamine in place of palmitate by means of a bifunctional reagent or via spacer molecules to the -CH2OH groups of some of the polyoxyethylene head groups.

Non-limiting examples of the kinds of 20 materials which might be administered to humans, -lower animals, and plants by lipid membrane structures described by the present invention are~
.
(1) Small viruses (e.g. for vaccination), plasmids, and other forms of nucleic acids;
(2) Immunological adjuvants such as muramyl dipeptide;
(3) Enzymes for enzyme replacement therapy and diagnostic testing;

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133~532 (4) Peptide hormones and growth factors, including insulin, glucagon, pituitary hormones, hypothalamic hormones, angiogenic, epithelial and epidermal qrowth factors;

(5) Lymphokines which are normally degraded rapidly, such as interleukin-2 and interferon;
t6) Blood proteins, such as hemoglobin, as a basis of artificial blood;
(7) Water soluble plant hormones;
(8) Water soluble pesticides;
(9) Radionucleotides for diagnosis;
(10) Contrast dyes for radiological diagnosis.

To form the L W s of the present invention, the surfactant capable of forming the large unilamellar lipid membrane structures, together with any other lipophilic substances, including biologica~ly active molecules which are lipophilic, are initially dissolved in an appropriate, generally -non-polar solvent or solvent mixture. A 2:1 misture ;
of chloroform and methanol has been found to be particularly suitable for this purpose. The organic solvent is removed by evaporation under reduced pressure, or from a round-bottom flask under a stream of inert gas, such as nitrogen, generally at between 20 and 60 C.

The residue is then hydrated with an aqueous liquid, typically a buffered solution, which should contain an hydrophilic biologically active molecules ; ,~ ~ : . : ', 133~532 which are to be incorporated. Hydration is carried out at a temperature above the transition temperature of the polyosyethylene acyl ethers (or sorbitan esters), normally about 39C. The misture is solubilized by addition and agitation of a detergent of low molecular weight and high critical micelle ~ -concentration, such as octyl glucoside, in 5 to 10-~old molar escess with respect to the lipid. ~ ~
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The solubilizing detergent is then removed, thereby resulting in the formation of L W s.
Detergent removal can be accomplished in a number of ways: e.g., by (1) overnight dialysis against an escess of the aqueous phase buffer, (2) overnight dialysis against a much smaller volume of detergent buffer containing hydrophobic affinity resin, or (3) direct passage over hydrophobic affinity beads.
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The LW s can be separated from unincluded material by esclusion chromatography in which the L W s and associated material appear in the void 20 volume. The L W s can then be suspended in any -suitable electrolytic buffer for subsequent use.

The following esamples will demonstrate the capacity of large unilamellar lipid membrane structures composed primarily of inexpensive readily, available surfactants to act as carriers for molecules of biological importance.

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133~532 In this example, large unilamellar vesicle lipid membrane structures are prepared by the following procedure, originally set forth by 5 Philippot et al. utilizing phospholipids. The materials for preparing the large unilamellar vesicular lipid membrane structures and their proportions are given in Table I. A small amount of tl,2-14C] polyethylene glycol was included to allow 10 determination of encapsulation efficiency.
~ ~ ' TABLE I

_____________ __ :
~omDonent Concentration ~-Polyoxyethylene (2) cetyl ether 0.0140 mmoles Cholesterol 0.0120 mmoles 15 Dicetyl phosphate 0.0015 mmoles Polyethylene glycol 4,000 106 cpm Octyl glucoside 0.270 mmoles Physiological buffer 0.5 ml ______________________________________________________ In accordance with the method used to 20 construct the vesicles of this invention, the polyoxyethylene (2) cetyl ether (Brij 52, ICI
Americas), cholesterol, and dicetyl phosphate were co-dissolved in 0.2Sml of a chloroform:methanol (2:1) solution, and the solvent was removed under a stream 25 of nitrogen from a round-bottom flask. One-half ml of physiological buffer (10mM Hepes, lmM EGTA, 150mM

14 13~0~3~ - ~

NaCl, pH 7.4) containing octyl glucoside and :
[1,2-14C] polyethylene glycol 4,000 tracer, was then added, and the mixture solubilized by agitation :
at 60C. The clear, solubilized misture was 5 transferred to a dialysis bag (1 cm wide) permeable ~ .
to octyl glucoside, and dialyzed over night against 100 ml of the physiological buffer containing 9 mg :
Bio-Beads SM2 (Biorad, Richmond, CA) /~ole of octyl glucoside.

The resulting residue constituted a stable, opalescent suspension. Using radioactive polyethylene glycol as a marker for exclusion chromatography on ACA44 (IBF, France) or Sephacryl .
Sl,000 (Pharmacia), the encapsulation efficiently in three experiments was found to lie between 47 and 50%. Electron microscopy revealed L W s with a diameter of greater than 0.450 ~ and a few very ~:
large vesicles with diameters greater than 1.00 ~. -There was no detectable loss of encapsulated material after 24 hours at ambient temperature in physiological buffer. After 1 week at ambient temperature in physiological buffer, there was a loss of about 33%.

Removal of detergent by direct passage of 25 the clear, solubilized mixture over Bio-Beads SM2 gave a lower encapsulating efficiency, approsimately 10-20%. These results are comparable to the results of Philippot et al. using the same procedure for phospholipid L W s. Philippot demonstrated that the 30 rapid e~traction procedure for removal of ,... .. . . .

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solubilizing detergents yields smaller phospholipid L W s having lower encapæulation efficiencies.

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In this example, hemoglobin-containing L W s were prepared. The materials employed and their proportions are given in ~able II.
TABLE II
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Component Concentration Polyoxyethylene (2) cetyl ether 0.0140 mmoles 10 Cholesterol 0.0120 mmoles Dicetyl phosphate 0.0015 mmoles Polyethylene glycol 4,000 106 cpm Octyl glucoside 0.270 mmoles Human hemoglobin 0.02 g 15 Physiological buffer ` 0.5 ml ______________________________________________________ ~
-, To obtain the hemoglobin for this esample, the erythrocytes of freshly drawn human blood were packed by high-speed centrifugation, the buffy coat removed, and the red cells washed three times in physiological saline. The erythrocytes were then lysed by repeated freezing and thawing, and the erythrocyte membranes removed by high-speed ultracentrifugation.

The polyoxyethylene (2) cetyl ether, cholesterol, and dicetyl phosphate were co-dissolved ,' i "
`~;` " '~' ~ ' ' ' S; ' ' ~

in 0.25ml of a chloroform:methanol (2:1) solution, and the solvent removed by stream of nitrogen gas.
Physiological buffer (0.5 ml) containing the free hemoglobin, the octyl glucoside, and the polyethylene glycol 4,000 tracer, was then added, and the mi~ture solubilized by agitation at 37C. The clear, bright red, solubilized mixture was then transferred to a dialysis bag (1 cm wide) permeable to octyl glucoside, and dialyzed against 10 ml buffered medium ;
containing 9 mg Bio-Beads/~mole of octyl glucoside.
.
After overnight dialysis, the residue constituted a stable, bright-red, opalescent suspension. Upon exclusion chromatography on Biol-Gel A-15m, the hemoglobin-colored L W s could be seen to elute in the void volume. Measurements of encapsulation efficiency gave values of between 17 and 23%. ;

EXAMPLE 3 ~
.
In this e~ample, L W s were prepared from æorbitan lauryl ester by the method used to construct the vesicles of this invention. The materials used in preparing the LUVs and their proportions are given in Table IV. A small amount of tl,2-14C]
polyethylene glycol was included to allow 25 determination of encapsulation efficiency.

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TABLE III
______________________________________________________ . , Componen~ Concentration Sorbitan lauryl ester 0.0140 mmoles Cholesterol 0.0120 mmoles Dicetyl phosphate 0.0015 mmoles Polyethylene glycol 4,000 106 cpm Octyl glucoside 0.270 mmoles Physiological buffer 0.5 ml ______________________________________________________ The sorbitan lauryl ester (SPAN 20, ICI
10 Americas~, cholesterol, and dicetyl phosphate were co-dissolved in 1 ml of a chloroform: methanol (2:1) solution in a round bottomed tube, and the solvent was removed by a stream of nitrogen. Physiological buffer (0.5 ml) containing octyl glucoside and 15 polyethylene glycol 4,000 tracer, was then added, and the misture solubilized by agitation at 37C. The clear, solubilized misture was then transferred to a dialysis bag (1 cm wide) permeable to octyl glucoside and dialyzed against 100 ml buffered medium containing 9 mg Bio-Beads/y mole of octyl glucoside.

After overnight dialysis, the residue in the dialysis bag constituted a stable, opalescent suspension. Using radioactive polyethylene glycol as a marker for exclusion chromatography on Biogel 25 A-15m, the encapsulation efficiency in three experiments was found to lie between 40 and 50%.

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!A,, ~ . . .: , -18- 1 3~ 0 ~ 3 2 The L W s prepared from polyo~yethylene (2) cetyl ether or sorbitan esters by dialysis or by exclusion chromatography contain about 10 and 35 -liters entrapped volume per mole of lipid. These results are comparable to the results obtained by Philippot et al. (1984) using phospholipids. The -~
values obtained by the dialysis method are considerably larger than those achieved by applying the reverse phase evaporation method described for phospholipids in U.S. Patent No. 4,235,871.

L W s which still fall within the scope of ' ' the invention can be made from a variety of surfactants. However, a change in the æurfactant component may result in a change in the æpecific encapsulation efficiency of the vesicle.

The invention may be embodied in other æpecific formæ without departing from the æpirit and æcope thereof. Accordingly, other embodimentæ are within the following claimæ.

What is claimed is:

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Claims

1. A large unilamellar vesicle comprising a surfactant selected from a group consisting of polyoxyethylene alkyl ethers having the structure R1-O-(CH2-CH2-O-)m-H
wherein R1 is CH3-(CH2)n, n ranges from 11 to 15 and m ranges from 2 to 4 and sorbitan alkyl esters having the structure wherein R2 is CH3-(CH2)x and x ranges from 11 to 15 and a sterol, whereby said surfactant comprises substantially all the lipid in the structue of said large unilamellar lipid vesicle.

2. The large unilamellar vesicle of claim 1 wherein said polyoxyethylene alkyl ether comprises polyoxyethylene (2) cetyl ether.

3. The large unilamellar vesicle of claim 1 wherein said polyoxyethylene alkyl ether comprises polyoxyethylene (4) lauryl ether.

4. The large unilamellar vesicle of claim 1 wherein said sterol is selected from a group consisting of cholesterol, its chemical analogs, and its derivatives.

5. The large unilamellar vesicle of claim 4 further comprising a charge-producing amphiphile.

6. The large unilamellar vesicle of claim 5 wherein said amphiphile is a negative charge-producing material selected from a group consisting of dicetyl phosphate, cetyl sulphate, phosphatydic acid, phosphatidyl serine, and mixtures thereof.

7. The large unilamellar vesicle of claim 5 wherein said amphiphile is a positive charge-producing material selected from a group consisting of long chain amines, quaternary ammonium compounds, and mixtures thereof.

8. The large unilamellar vesicle of claim 1 further comprising a hydrophilic targeting molecule.

9. The large unilamellar vesicle of claim 8 wherein said hydrophilic targeting molecule is selected from a group consisting of immunoglobulins, lectins, and peptide hormones.

10. The large unilamellar vesicle of claim 1 wherein said sorbitan alkyl ester comprises sorbitan laurate.

11. The large unilamellar vesicle of claim 1 wherein said sorbitan alkyl ester comprises sorbitan monopalmitate.

12. A delivery system for a hydrophilic material, said system consisting of a large unilamellar vesicle having a lipid bilayer surrounding a subtantially aqueous interior, said lipid bilayer comprising a nonphospholipid surfactant, a sterol, a targeting molecule, and a charge-producing amphiphile, said surfactant being selected from a group consisting of polyoxyethylene alkyl ethers having the structure R1-O-(CH2-CH2-O-)m-H
wherein R1 is CH3-(CH2)n, n ranges from 11 to 15, and m ranges from 2 to 4, and sorbitan alkyl esters having the structure wherein R2 is CH3-(CH2)X and x ranges from 11 to 15 said surfactant comprising substantially all the lipid in the structure of said vesicle.

13. The delivery system of claim 12 wherein said polyoxyethylene alkyl ether comprises polyoxyethylene 2-cetyl ether.

14. The delivery system of claim 12 wherein said polyoxyethylene alkyl ether comprises polyoxyethylene (4) lauryl ether.

15. The delivery system of claim 12 wherein said sorbitan alkyl ester comprises sorbitan laurate.

16. The delivery system of claim 12 wherein said sorbitan alkyl ester comprises sorbitan monopalmitate.

17. The delivery system of claim 12 wherein said sterol comprises cholesterol or its derivatives.

18. The delivery system of claim 12 wherein said amphiphile is a negative charge-producing material selected from a group consisting of dicetyl phosphate, cetyl sulphate, phosphatydic acid, phosphatidyl serine, and mixtures thereof.

19. The delivery system of claim 12 wherein said amphiphile is a positive charge-producing material selected from a group consisting of long chain amines, quaternary ammonium compounds, and mixtures thereof.

20. The delivery system of claim 12 wherein said target molecule comprises a hydrophilic targeting molecule.

21. The delivery system of claim 20 wherein said hydrophilic targeting molecule is selected from a group consisting of immunoglobulins, lectins, and peptide hormones.

22 22. The delivery system of claim 12 wherein said hydrophilic material is enclosed within said lipid bilayer, said lipid bilayer comprising:
approximately 46-48 parts of said surfactant;
approximately 46-48 parts of a sterol selected from the group consisting of cholesterol, its chemical analogs, and its derivatives;
approximately 4-8 parts of a charge-producing amphiphile; and less than one part of a targeting molecule.

23. The delivery system of claim 22 wherein said hydrophilic material is selected from the group consisting of nucleic acids, immunological adjuvants, enzymes, hormones, lymphokines, blood proteins, pesticides, contrast dyes, and radioactive marker molecules.