Preliminary data on the encapsulation of biologically active materials in liposomes

Multilamellar and unilamellar phospholipid liposomes were prepared and investigated as regards their properties and the capacity of encapsulating biologically active materials, such as CrO4-(2-)ions, basic dyes, proteins, DNA, as well as Sendai virus particles. The efficiency of encapsulation ranged from 10% for CrO4-(2-)ions to about 80% for toluidine blue; it was found to depend not only on the type of encapsulated material, but also on the method used for liposome preparation and on liposome composition.

[Ultrastructural organization of liposomes and isolated glycoproteins of enveloped viruses obtained by using the nonionic detergent MESK]

Materials on comparative ultrastructural analysis of isolated glycoproteins (GP) of influenza, parainfluenza, and Venezuelan equine encephalomyelitis viruses as well as liposomes reconstructed on their basis prepared with the use of MECK, a nonionic detergent, are presented. Original virions, subvirion particles, and isolated GP were characterized by typical morphology and remained intact in highly purified and concentrated suspensions owing to the use of the sparing and effective method of treatment with MECK. After removal of the detergent typical micellar aggregates were shown to form from GP monomers for all three viruses under study. The forming rosettes morphologically and biologically are similar to those isolated from ortho-, paramyxo-, and togaviruses using the Triton X-100 detergent. Stages of liposome formation and interaction with cell membrane were studied electron microscopically. A common feature for liposomes from GP of all the three viruses consisted in the formation of two morphological variants of structural complexes: filaments of various lengths and composition in them of GP and vesicles, the arrays of GP in which were similar or identical to those of viral membrane. Liposomes from Sendai virus GP possessed hemolytic activity, however, their interaction with the cell studied by means of the immunoferritine label occurred not by fusion but apparently as a result of destruction of the cell membrane at the sites of protein F contact with plasmalemma and formation in the latter of defects leading to erythrocyte lysis.

[Liposome preparation by a reverse-phase evaporation method and freezing-thawing]

A method for preparing liposomes containing Na3Ca-14C-DTPA by means of phase-reverse evaporation and freezing-thawing is described. Freezing-thawing is required for transformation of the system formed after phase-reverse evaporation at a lipid concentration of over 20-30 mg/ml to liposomal suspension. Liposomes thus obtained contain 10-14 microliters aqueous phase per mumol phosphatidylcholine, their diameter varies between 0.2 and 1.5 micron (the mean value 0.45-0.54 micron). Increasing the concentration of lipids, it is possible to include into liposomes up to 70-80% of 14C-DTPA in solution. The method is believed to be fit for preparing the liposomal forms of other drug hydrophilic compounds.

[Production of liposomal preparations of streptomycin and dihydrostreptomycin]

The effect of the conditions of the formation of liposomal preparations of streptomycin and dihydrostreptomycin on incorporation of the antibiotics into the liposomes was studied. The liposomes were obtained with the detergent (cholate) method modified by the authors. The modification implies preliminary application of a 20 per cent antibiotic buffer solution on columns for gel filtration of a mixed mycellar antibiotic solution (20 per cent). The volume ratio of the preliminary applied buffer solution and the mixed mycellar solution is higher than 4:1. The new procedure is simple, readily reproduced, providing formation of the liposomal preparations characterized by high levels of the antibiotic incorporation (about 300 micrograms per 1 mg of lecithin). The preparations are stable with regard to the antibiotic release. The liposomes do not aggregate on storage for a long period (more than 6 months). The liposomal preparations thus formed may be useful as intravenous dosage forms of the antibiotics.

Effects of ethanol on the Escherichia coli plasma membrane

The effects of ethanol on the fluidity of Escherichia coli plasma membranes were examined by using a variety of fluorescent probes: 1,6-diphenyl-1,3,5-hexatriene, perylene, and a set of n-(9-anthroyloxy) fatty acids. The anthroyloxy fatty acid probes were used to examine the fluidity gradient across the width of the plasma membrane and artificial membranes prepared from lipid extracts of plasma membranes. Ethanol caused a small decrease in the polarization of probes primarily located near the membrane surface. In comparison, hexanol decreased the polarization of probes located more deeply in the membrane. Temperature had a large effect on probes located at all depths. The effects of ethanol on E. coli membranes from cells grown with or without ethanol were also examined. Plasma membranes isolated from cells grown in the presence of ethanol were more rigid than those from control cells. In contrast to plasma membranes, artificial membranes prepared from lipid extracts of ethanol-grown cells were more fluid than those from control cells. These differences are explained by analyses of membrane composition. Membranes from cells grown in the presence of ethanol are more rigid than those from control cells due to a decrease in the lipid-to-protein ratio. This change more than compensates for the fluidizing effect of ethanol and the ethanol-induced increase in membrane C18:1 fatty acid which occurs during growth. Our results suggest that the regulation of the lipid-to-protein ratio of the plasma membrane may be an important adaptive response of E. coli to growth in the presence of ethanol.

Plasma membrane-mediated leakage of liposomes induced by interaction with murine thymocytic leukemia cells

The interaction of liposomes with BW 5147 murine thymocytic leukemia cells was studied using fluorescent probes (entrapped carboxyfluorescein and fluorescent phosphatidylethanolamine) in conjunction with a Ficoll-Paque discontinous gradient system for rapid separation of liposomes from cells. Reversible liposomal binding to discrete sites on the BW cell surface was found to represent the major form of interaction; uptake of intact liposomal contents by a process such as liposome-BW cell membrane fusion was found to apparently represent a minor pathway of interaction (2%). Liposomal lysis was found to be associated with the process of liposomal binding (perhaps as a result of the binding itself). Lysis was followed by release of the entrapped carboxyfluorescein into the media and its subsequent uptake by the cells. This lysis was shown to be dependent upon discrete membrane-associated sites that have some of the properties of proteins. The results of these studies suggest that liposomal binding to the cells, subsequent lysis of the liposomes and cellular uptake of their contents should be seriously considered in all studies of liposome-cell interactions as an alternate mode of interaction to the four modes (fusion, endocytosis, adsorption and lipid exchange) previously emphasized in the literature.

[Isolation and low-temperature preservation of liposomes containing rifampicin]

The optimal conditions for preparations of rifampicin-containing liposomes were determined with the methods of mechanical shaking, gas dispersion and and reversible phases. It was found that the percentage of rifampicin incorporation into liposomes depended on the molar ratio of the antibiotic to the lipid (the optimal ratio was 1 : 10), the size and structure of liposomes, the amount of cholesterol added and the lipid membrane charge. Incorporation of rifampicin amounted to 16.1 +/- 2.4, 39.2 +/- 3.2 and 60.5 +/- 2.9 per cent with respect to neutral lecithin multilamellar liposes, liposomes prepared with the gas dispersion method and liposomes prepared with the method of reversible phases, respectively. Cholesterol in a molar ratio to lecithin equal to 2 : 5 or higher and dicetyl phosphate imparting the negative charge to the membrane had an inhibitory effect on the drug uptake by liposomes, while stearyl amine having the positive charge had a stimulating effect. The effect of the cryoprotectors glucose, polyvinylpyrrolidone, poly-ethylene glycole-400 and glycerol on low-temperature preservation and storage of rifampicin-containing liposomes was studied. It was shown that 10--15 per cent solutions of sucrose and glucose had the highest cryoprotective effect, when the two-stage method of freezing was used. It provided almost 84 per cent preservation of liposomal rifampicin. Electron microscopy showed that after defrosting liposomes no significant changes in the size and structure of lipid membranes were observed.

Induction of syngeneic tumour-specific immunity by liposomes reconstituted with L2C tumour-cell antigens

Liposomes composed of phosphatidylcholine and phosphatidylserine were reconstituted by detergent dialysis with 3 M K Cl extracts of L2C tumour cells. Liposomes containing L2C antigens were as antigenic as intact tumour cells in the elicitation of delayed-type hypersensitivity responses in strain-2 guinea-pigs previously immunized against L2C tumours. Soluble L2C antigens were devoid of immunoprotective activity, whereas the reconstituted liposomes were capable of protecting animals against up to approximately 100 times the minimal lethal dose of tumour cells. Moreover, the reconstituted liposomes were as antigenic and immunoprotective as viable (irradiated) cells.

Antitumor activity of a water-insoluble compound entrapped in liposomes on L1210 leukemia in mice

The use of sonicated phospholipid vesicles (liposomes) as carriers of 2-[3'-(methoxycarbonylamino)-phenyl]-3-phenyl-6-methoxycarbonylamino-4-(3H)- quinazolone (NSC-251635), a water-insoluble antimitotic compound, was investigated in mice. NSC-251635 was incorporated in egg yolk lecithin, cholesterol, and stearylamine (4:3:1) liposomes. In vitro, NSC-251635 in suspension or entrapped in liposomes was not toxic for L1210 cells. In vivo, after ip or iv injections to CDF1 mice bearing intraperitoneal or intravenous L1210 leukemia, NSC-251635 was active only when it was incorporated in the liposomes and not when it was given as a suspension in Klucel or in saline. The NSC-251635 liposome preparation induced significantly prolonged survival of the treated animals.

[Isolation of skeletal muscle plasmatic membranes as closed vesicles]

Ultrasonic treatment of skeletal muscle sarcolemma preparations in 0.25 M sucrose, 0.1 M NaCl, 0.001 M tris-HCl-buffer (pH 7.4) induces formation of closed osmotically active vesicles which may be isolated by centrifugation of a sonicated suspension within the sucrose density gradient. The vesicles retain the enzymic activity peculiar to the initial sarcolemma preparations.

The polymyxin sensitivity and the phosphatidyl ethanolamine content of the Vibrio cholerae membranes

The phospholipid composition of three different strains of Vibrio cholerae was determined by quantitative two-dimensional thin-layer chromatography. The polymyxin sensitivity of the whole organisms or of the liposomes derived from the total phospholipids of these organisms depended solely on the phosphatidyl ethanolamine content of the system concerned and could be quantitatively related by the equations Y1 = 1.074 X1 - 9.828 and Y2 = 1.22 X2 - 34.47 where Y represents the maximum lysis (%)/hr and X, the % phosphatidyl ethanolamine content of the system concerned, the suffix 1 and 2 corresponding to the liposomal and the whole organisms respectively. The analysis revealed the requirement of a threshold amount of phosphatidyl ethanolamine for the expression of polymyxin action on the system concerned.

Lipid A from endotoxin: antigenic activities of purified fractions in liposomes

Isolation of lipid A by acid hydrolysis of Shigella flexneri lipopolysaccharide resulted in a product that consisted of a heterogeneous mixture of bands when visualized by thin layer chromatography. Differential extraction with ethyl acetate and chloroform, or extraction with EDTA, followed by chloroform-methanol-water (Bligh-Dyer extraction), or a combination of both extraction schemes, resulted in partial purification of immunologically active lipid A. Eight fractions were purified further by preparative thin layer chromatography, and each of the fractions had phosphate, carbohydrate, and esterified fatty acids. Upon incorporation into liposomes, five of the eight purified fractions reacted with anti-lipid A serum, but the three fractions with the most number of esterified fatty acids failed to react with anti-lipid A serum. At least one fraction that originally was unreactive with anti-lipid A serum became reactive as a hapten inhibitor upon removal of esterified fatty acids by alkaline hydrolysis. Alkali-treated fractions from "unreactive" and "reactive" lipid A had similar activities as hapten inhibitors. Our data suggest that lipid A can exist in multiple forms that differ by the number and placement, and possibly by the type, of fatty acids linked to the carbohydrate of lipid A. Highly acylated forms of lipid A do not react with antiserum against the unpurified lipid A mixture, but removal of fatty acids does expose immunoreactive groups.

Gel filtration of egg phosphatidylcholine vesicles

The abilities of Sepharose 2B (Pharmacia), Controlled Pore Glass (Electro-Nucleonics) and Bio-Gel A150m (Bio-Rad) to purify small unilamellar vesicles prepared by sonication and the ethanol-injection methods were compared. The Bio-Gel causes complete aggregation of the sonicated vesicles and partial aggregation of the ethanol-injection vesicles. Both Sepharose and Controlled Pore Glass are acceptable for purifying vesicles from multilamellar liposomes; however, neither will separate the vesicles from sonication by-products which might be formed.

Reconstitution of lipid vesicles associated with HVJ (Sendai virus) sikes. Purification and some properties of vesicles containing nontoxic fragment A of diphtheria toxin

A mixture of HVJ (Sendai virus) spike proteins, the nontoxic fragment A of diphtheria toxin, lecithin, and cholesterol was solubilized in sucrose solution containing a nonionic neutral detergent. The liposomal vesicles which formed on removal of the detergent by dialysis were purified by gel filtration and centrifugation on a sucrose gradient. The resulting purified vesicles had hemagglutinating activity, hemolytic activity and, after solubilization, the enzymic activity of fragment A. The vesicles had no cell fusion activity. Electron microscopy showed that both the outside and inside of membranes of the vesicles were associated with the spikes. When the vesicles were freeze-fractured, no large aggregates of particles were seen on either face. Such fragment A-containing lipid vesicles (liposomes) with HVJ spikes bound to mamalian cell membrane and released their fragment A into the cytoplasm causing cell death. Neither fragment A-containing liposomes without spikes nor empty liposomes with spikes were toxic.

Lack of counteracting effect of liposomes on benserazide-induced hyperprolactinemia

Benserazide induces an increase of serum prolactin in man, possibly as the result of an impairment of the dopamine effect on the pituitary and/or on the outer median eminence caused by the inhibition on L-dopa decarboxylase. On the other hand, liposomes obtained from bovine brain cortex phospholipids reduced serum prolactin possibly through an effect of phosphatidylserine on dopamine biosynthesis at the level of tyrosine hydroxylase. Benserazide, given orally (125 mg) to 5 normal subjects, induced an increase of serum prolactin that did not change when 300 mg of phospholipid liposomes were given intravenously 60 min later. An increase of L-dopa synthesis does not seen to be capable to overcome the effects of the decarboxylase inhibition.

[Effect of the structure of liposomal membrane surface layer on the binding of phenothiazines]

By using a fluorescent 3-metoxybenzatrone probe the binding of the phenothiazine series drugs (chlorpromazine, triftazine, aethaperazine) with phospholipid vesicles--liposomes, was investigated. Cholesterol is shown not to affect the binding of these drugs with liposomes. The surface charge of liposomes influences only the combination with the chlorpromazine membranes. With a higher positive charge of the membranes the binding of chlorpromazine diminishes in force.