Fate of cholesterol-rich liposomes after subcutaneous injection into rats

Nanoparticle Vaccines Adopting Virus-like Features for Enhanced Immune Potentiation

Synthetic nanoparticles play an increasingly significant role in vaccine design and development as many nanoparticle vaccines show improved safety and efficacy over conventional formulations. These nanoformulations are structurally similar to viruses, which are nanoscale pathogenic organisms that have served as a key selective pressure driving the evolution of our immune system. As a result, mechanisms behind the benefits of nanoparticle vaccines can often find analogue to the interaction dynamics between the immune system and viruses. This review covers the advances in vaccine nanotechnology with a perspective on the advantages of virus mimicry towards immune potentiation. It provides an overview to the different types of nanomaterials utilized for nanoparticle vaccine development, including functionalization strategies that bestow nanoparticles with virus-like features. As understanding of human immunity and vaccine mechanisms continue to evolve, recognizing the fundamental semblance between synthetic nanoparticles and viruses may offer an explanation for the superiority of nanoparticle vaccines over conventional vaccines and may spur new design rationales for future vaccine research. These nanoformulations are poised to provide solutions towards pressing and emerging human diseases.

Lymphatic targeting of zidovudine using surface-engineered liposomes

The present investigation was aimed at lymphatic targeting of zidovudine (ZDV)-loaded surface-engineered liposomes (SE liposomes). Surface of liposomes was engineered by incorporation of charges (positive or negative) and site-specific ligand (mannose) in order to enhance localization to lymphatics, specifically to lymph node and spleen. Positively and negatively charged nanosized SE liposomes (120 +/- 10 nm) were prepared using stearylamine (SA) and dicetyl phosphate (DCP), respectively, while ligand-coated SE liposomes were prepared using mannose-terminated SA (mannose conjugate). The SE liposomes were characterized for shape and surface morphology, size, entrapment efficiency, and in vitro drug release. All the SE liposomes formulations showed biphasic ZDV release, whereas mannose-coated liposomes (MAN-Lip) significantly reduced (p < 0.05) drug release compared with conventional liposome (Lip). The organ distribution pattern of the SE liposomes exhibited significant reduction in free ZDV concentration in serum, whereas significantly increased quantity was detected in the spleen and lymph nodes (p < 0.05). Fluorescent microscopy suggested enhanced uptake and localization of the SE liposomes in the lymph nodes and spleen, which were in the order: mannose coated > negatively charged > positively charged > Lip. Thus, the SE liposomes appeared to be promising novel vesicular system for enhanced targeting of ZDV to lymphatics, in AIDS chemotherapy.

Liposome-mediated DNA Immunisation via the Subcutaneous Route

Compared to naked DNA immunisation, entrapment of plasmid-based DNA vaccines into liposomes by the dehydration-rehydration method has shown to enhance both humoural and cell-mediated immune responses to encoded antigens administered by a variety of routes. In this paper, we have investigated the application of liposome-entrapped DNA and their cationic lipid composition on such potency after subcutaneous immunisation. Plasmid pI.18Sfi/NP containing the nucleoprotein (NP) gene of A/Sichuan/2/87 (H3N2) influenza virus in the pI.18 expression vector was incorporated by the dehydration-rehydration method into liposomes composed of 16 micromol egg phosphatidylcholine (PC), 8 micromoles dioleoyl phosphatidylethanolamine (DOPE) or cholesterol (Chol) and either the cationic lipid 1,2-diodeoyl-3-(trimethylammonium) propane (DOTAP) or cholesteryl 3-N-(dimethyl amino ethyl) carbamate (DC-Chol). This method, entailing mixing of small unilamellar vesicles (SUV) with DNA, followed by dehydration and rehydration, yielded incorporation values of 90-94% of the DNA used. Mixing or rehydration of preformed cationic liposomes with 100 microg plasmid DNA also led to similarly high complexation values (92-94%). In an attempt to establish differences in the nature of DNA association with these various liposome preparations their physico-chemical characteristics were investigated. Studies on vesicle size, zeta potential and gel electrophoresis in the presence of the anion sodium dodecyl sulphate (SDS) indicate that, under the conditions employed, formulation of liposomal DNA by the dehydration-rehydration generated submicron size liposomes incorporating most of the DNA in a manner that prevents DNA displacement through anion competition. The bilayer composition of these dehydration-rehydration vesicles (DRV(DNA)) can also further influence these physico-chemical characteristics with the presence of DOPE within the liposome bilayer resulting in a reduced vesicle zeta potential. Subcutaneous liposome-mediated DNA immunisation employing two DRV(DNA) formulations as well as naked DNA revealed that humoural responses (immunoglobulin total IgG, and subclasses IgG1 and 1gG2a) engendered by the plasmid encoded NP were substantially higher after dosing twice, 28 days apart with 10 microg liposome-entrapped DNA compared to naked DNA. At all time points measured, mice immunised with naked DNA showed no greater immune response compared to the control, non-immunised group. In contrast, as early as day 49, responses were significantly higher in mice injected with DNA entrapped in DRV liposomes containing DOTAP compared to the control group and mice immunised with naked DNA. By day 56, all total IgG responses from mice immunised with both DRV formulations were significantly higher. Comparison between the DRV formulations revealed no significant difference in immune responses elicited except at day 114, where the humoural responses of the group injected with liposomal formulation containing DC-Chol dropped to significantly lower levels that those measured in mice which received the DOTAP formulation. Similar results were found when the IgG1 and IgG2a subclass responses were determined. These results suggest that, not only can DNA be effectively entrapped within liposomes using the DRV method but that such DRV liposomes containing DNA may be a useful system for subcutaneous delivery of DNA vaccines.

Poly(propyleneimine) dendrimer and dendrosome mediated genetic immunization against hepatitis B

The purpose of the present research work is to explore the potential of dendrosomes in genetic immunization against hepatitis B. Plasmid DNA encoding pRc/CMV-HBs[S] (5.6 kb), encoding the small region of the hepatitis B surface antigen, was complexed with 5th generation poly(propyleneimine) dendrimer (PPI) in different ratios. Transfection of CHO cells revealed that a ratio of 1:50 for pDNA:PPI was optimum for transfection. Results of cytotoxicity studies showed that the toxicity of PPI-DNA complex was significantly (p<0.05) higher for PPI 75 and PPI 100 as compared to the other PPI-DNA complexes. PPI 50 was employed for preparation of dendrosomes by reverse phase evaporation method. The dendrosomal formulation DF3 was found to possess optimum vesicle size, zeta potential and entrapment efficiency. In vitro production of HBsAg in CHO cells showed that DF3 possess maximum transfection efficiency. In vivo immunization studies were carried out by giving a single intramuscular injection of 10 microg of plasmid DNA (pDNA) or its dendrimeric or dendrosomal formulation to female Balb/c mice, followed by estimation of total IgG, IgG(1), IgG(2a), IgG(2b), biweekly. DF3 was found to elicit maximum immune response in terms of total IgG and its subclasses under study as compared to PPI 50 and pDNA at all time points. Animals immunized with DF3 developed very high cytokine level. Higher level of IFN-gamma suggests that the immune response was strictly Th1 mediated. Our observations clearly prove the superiority of dendrosomes over PPI-DNA complex and pDNA for genetic immunization against hepatitis B.

Role of nanotechnology in pharmaceutical product development

A number of new molecular entities (NMEs) selected for full-scale development based on their safety and pharmacological data suffer from undesirable physicochemical and biopharmaceutical properties, which lead to poor pharmacokinetics and distribution after in vivo administration. An optimization of the preformulation studies to develop a dosage form with proper drug delivery system to achieve desirable pharmacokinetic and toxicological properties can aid in the accelerated development of these NMEs into therapies. Nanoparticulate drug delivery systems show a promising approach to obtain desirable druglike properties by altering the biopharmaceutics and pharmacokinetics properties of the molecule. Apart from the advantages of enhancing potential for systemic administration, nanoparticulate drug delivery systems can also be used for site-specific delivery, thus alleviating unwanted toxicity due to nonspecific distribution, improve patient compliance, and provide favorable clinical outcomes. This review summarizes some of the parameters and approaches that can be used to evaluate nanoparticulate drug delivery systems in early stages of formulation development.

Induction of a cytotoxic T lymphocyte (CTL) response to plasmid DNA delivered via Lipodine liposomes

We have previously shown that liposome-mediated plasmid DNA immunisation may be a preferred alternative to the use of naked DNA. Lipodine DNA formulations consist of liposomes containing entrapped DNA plasmid by the dehydration-rehydration (DRV) method. Such liposome formulations are distinct from liposomes with externally complexed DNA in that the majority of the DNA is "internal" to the liposome structure and hence protected from DNAase degradation. Previous studies on the immune response induced by DNA vaccines entrapped in Lipodine have focused on the humoural response. In the present study, we have expanded the analysis profile in order to include the cytotoxic T lymphocyte (CTL) component of the immune response. We have analysed the immune response induced by DNA entrapped in Lipodine compared to that induced by DNA alone when delivered subcutaneously, a route of administration not normally inducing significant plasmid DNA mediated immune activation. Our results indicate that delivery of a small dose of plasmid DNA in Lipodine results in an improved antibody response to the plasmid encoded antigen and a strong antigen specific CTL response compared to that induced by DNA delivered alone.

Final report on the safety assessment of Lecithin and Hydrogenated Lecithin

Lecithin is a naturally occurring mixture of the diglycerides of stearic, palmitic, and oleic acids, linked to the choline ester of phosphoric acid, commonly called phosphatidylcholine. Hydrogenated Lecithin is the product of controlled hydrogenation of Lecithin. Bilayers of these phospholipids in water may form liposomes, a spherical structure in which the acyl chains are inside and not exposed to the aqueous phase. Lecithin and Hydrogenated Lecithin are used in a large number of cosmetic formulations as skin conditioning agents-miscellaneous and as surfactant-emulsifying agents. Hydrogenated Lecithin is also used as a suspending agent-nonsurfactant. Historical data on concentration of use of Lecithin reveals that 0.1% to 1.0% is the concentration range most frequently seen, with concentrations up to 50% reported for two moisturizing products. A solution of 65% Lecithin is currently reported to be used at concentrations up to 3% in cosmetics. Nonocclusive application of Lecithin-containing liposomes to murine skin resulted in 30% penetration to the subdermis. In piglet skin, the same application resulted in 99% accumulating in the stratum corneum. In general, liposomes are considered effective in capturing other compounds inside their spherical structure and delivering any such captured compound through the skin barrier. As a result, caution should be exhibited in formulating cosmetic products that contain these ingredients in combination with other ingredients whose safety is based on their lack of absorption or where dermal absorption is a concern. Lecithin is virtually nontoxic in acute oral studies, short-term oral studies, and subchronic dermal studies in animals. Lecithin is not a reproductive toxicant, nor is it mutagenic in several assays. In an oral carcinogenicity study, brain neoplasms were found in mice exposed to Lecithin. In a subcutaneous carcinogenicity study, no neoplasms were found in mice and rats exposed to Lecithin. Adverse reactions to Lecithin in a metered-dose inhaler have been reported. Lecithin and Hydrogenated Lecithin were generally nonirritating and nonsensitizing in animal and human skin. Based on the available data, Lecithin and Hydrogenated Lecithin are safe as used in rinse-off cosmetic products; they may be safely used in leave-on products at concentrations up to 15%, the highest concentration tested in clinical irritation and sensitization studies; but the safety of use could not be substantiated in cosmetic products likely to be inhaled. Because of the possibility of formation of nitrosamines, these ingredients should not be used in cosmetic products in which N-nitroso compounds may be formed.

Development of systems for targeting the regional lymph nodes for diagnostic imaging: in vivo behaviour of colloidal PEG-coated magnetite nanospheres in the rat following interstitial administration

PURPOSE

Nanoparticles can be utilised for targeting drugs to the regional lymph nodes or as diagnostic agents. The surface modification of magnetite nanospheres with poly(ethylene glycol) (PEG) has been assessed by in vitro characterisation and in vivo studies following subcutaneous administration to the rat.

METHODS

Magnetite nanospheres were prepared with a grafted PEG layer using various PEG lengths from 350 to 1,000 Da. Thermogravimetric analysis was utilised to measure the adsorbed amount of PEG. Colloid stability was confirmed by measurement of the particle size and electrophoretic mobility. The kinetics of injection site drainage and lymph node retention were determined 2 hours after subcutaneous administration, for nanospheres coated with PEG lengths of 350, 550. 750, and 1,000 Da. For the 750 PEG coated nanospheres, the kinetics of distribution was determined over a 48-hour time course.

RESULTS

The distribution of the nanospheres was modified and the lymph node localisation enhanced by altering the surface coverage of PEG on the magnetic surface.

CONCLUSIONS

PEG-coated magnetite nanospheres with different surface characteristics can be utilised to target a diagnostic agent to regional lymph nodes.

Liposome-mediated DNA vaccination: the effect of vesicle composition

Liposome-entrapped DNA has been shown to enhance the potency of DNA vaccines, possibly by facilitating uptake of the plasmid by antigen-presenting cells (APC). In this paper, we have investigated the influence of the liposomal composition and surface charge on such potency. Plasmid DNA pRc/CMV HBS encoding the S (small) region of hepatitis B surface antigen was entrapped within cationic liposomes of various compositions and surface charges with high efficiency (88-97% of the amount used) by the dehydration-rehydration method that generates dehydration-rehydration vesicles (DRV). Cryo-electron microscopy revealed that DNA-containing DRV (DRV(DNA)) were multilamellar. In immunisation studies, female Balb/c mice were given two to four intramuscular injections of 10 microg naked or liposome-entrapped pRc/CMV HBS and bled at time intervals. Results indicate that the lipid composition of the DRV(DNA) influences the strength of the humoural response (immunoglobulin (Ig)G subclasses) with inclusion of dioleoyl phosphatidylethanolamine (DOPE) or phosphatidylethanolamine (PE) in the liposomal structure contributing to greater responses. DRV(DNA) in which the DOPE or PE were omitted or substituted with cholesterol led to significant reduction of humoural responses against the encoded antigen. Replacing phosphatidylcholine (PC) in the DRV(DNA) with the high-melting distearoyl phosphatidylcholine also contributed to lower responses. In other experiments, IgG responses were monitored in mice immunised with pRc/CMV HBS entrapped in DRV composed of PC and DOPE as before but incorporating increasing amounts of DOTAP (1-16 micromol). Maximal IgG responses were observed at 10 weeks after the first of four injections and suggested a trend of higher responses when 4 or 8 micromol DOTAP was present in the DRV(DNA) formulation. Cell-mediated immunity (measured in terms of endogenous antigen-specific splenic interferon-gamma) in mice immunised with pRc/CMV HBS entrapped in liposomes composed of PC, DOPE and DOTAP (16:8:4 molar ratio) was much greater than in animals treated with naked plasmid. These results indicate that liposome-mediated DNA immunisation is more effective than the use of naked DNA, and also suggest that the presence of fusogenic phosphatidylethanolamine in DRV in conjunction with a low-melting phosphatidylcholine and an appropriate content of cationic lipid might contribute to more effective liposomal DNA vaccines. The notion that liposomes improve immune responses to the plasmid-encoded vaccine by facilitating the latter's uptake by APC was supported by the observation that in Balb/c mice injected intramuscularly with liposome-entrapped pCMV. Enhanced green fluorescent protein, expression of the gene in terms of fluorescence intensity in the draining lymph nodes, was much greater than in animals treated with the naked plasmid.

Bioavailability of a small unilamellar low-clearance liposomal amikacin formulation after extravascular administration

Amikacin in small, low-clearance liposomes (MiKasome) has prolonged plasma and tissue residence and in vivo activity against extracellular infections, including Klebsiella pneumonia and Pseudomonas endocarditis. Small liposomes may cross endothelial barriers, and enter the systemic circulation after extravascular administration. We compared the systemic bioavailability (F) of low-clearance liposomal amikacin in rats following intravenous (i.v.), intraperitoneal (i.p.), intramuscular (i.m.) and subcutaneous (s.c.) injection (20 mg/kg) and intratracheal (i.t.) instillation (10 mg/kg). Drug-containing liposomes were extensively absorbed after i.p. (F = 87-146%) and i.t. (F = 64%) administration, with maximum amikacin plasma concentrations of 171 micrograms/ml at 9 h and 80 micrograms/ml at 18 h, respectively. Absorption was slower and less extensive following s.c. (plasma Tmax: 20.3 micrograms/ml at 48 h) and i.m. (plasma Tmax: 49.6 micrograms/ml at 19 h) injection, but a significant fraction (12-27%) of the liposomes was absorbed. The plasma AUCs of liposomal amikacin exceeded the AUC of conventional i.v. amikacin by at least 25-fold for all routes. Amikacin AUCs in regional lymph nodes exceeded plasma AUCs by 4-fold after s.c. and i.m. injection of liposomal amikacin. AUCs in tissues surrounding the injection sites were 20- and 191-fold higher than plasma AUCs after i.m. and s.c. injection, respectively. Thus, small low-clearance liposomes produced sustained levels of liposome-encapsulated amikacin in plasma, local tissues and lymph nodes after extravascular administration, suggesting applications in perioperative prophylaxis, pneumonias and intralesional therapy as well as sustained systemic delivery of encapsulated drugs.

Liposome-entrapped plasmid DNA: characterisation studies

Plasmid DNA pRc/CMV HBS (5.6 kb) (100 microg) encoding the S (small) region of hepatitis B surface antigen was incorporated by the dehydration-rehydration method into liposomes composed of 16 micromol egg phosphatidylcholine (PC), 8 micromol dioleoylphosphatidylcholine (DOPE) and 1, 2-diodeoyl-3-(trimethylammonium)propane (DOTAP) (cationic liposomes) or phosphatidylglycerol (anionic liposomes) in a variety of molar ratios. The method, entailing mixing of small unilamellar vesicles (SUV) with the DNA, followed by dehydration and rehydration, yielded incorporation values of 95-97 and 48-54% of the DNA used, respectively. Mixing of preformed cationic liposomes with 100 microg plasmid DNA also led to high complexation values of 73-97%. As expected, the association of DNA with preformed anionic liposomes was low (9%). Further work with cationic PC/DOPE/DOTAP liposomes attempted to establish differences in the nature of DNA association with the vesicles after complexation and the constructs generated by the process of dehydration/rehydration. Several lines of evidence obtained from studies on vesicle size and zeta-potential, fluorescent microscopy and gel electrophoresis in the presence of the anion sodium dodecyl sulphate (SDS) indicate that, under the conditions employed, interaction of DNA with preformed cationic SUV as above, or with cationic SUV made of DOPE and DOTAP (1:1 molar ratio; ESCORT Transfection Reagent), leads to the formation of large complexes with externally bound DNA. For instance, such DNA is accessible to and can be dissociated by competing anionic SDS molecules. However, dehydration of the DNA-SUV complexes and subsequent rehydration, generates submicron size liposomes incorporating most of the DNA in a fashion that prevents DNA displacement through anion competition. It is suggested that, in this case, DNA is entrapped within the aqueous compartments, in between bilayers, presumably bound to the cationic charges.

The influence of the route of administration and liposome composition on the potential of liposomes to protect tissue against local toxicity of two antitumor drugs

The present paper reports on the influence of the route of administration and liposome stability on the protective effect of liposome encapsulation of two model antitumor agents, mitoxantrone and doxorubicin. The results demonstrate that liposome encapsulation can protect surrounding tissue from the cytotoxic effects of the drugs after subcutaneous (s.c.) and intramuscular (i.m.) administration. The route of administration is an important factor influencing tissue damage. Liposomal mitoxantrone caused much less tissue irritation after im injection than after s.c. injection. Liposome stability is also an important factor. Liposomes composed of 'fluid-state' phospholipids only delayed the damaging effects of doxorubicin when injected sc. Liposomes with a more rigid nature were much more effective in preventing local tissue damage over a longer period of time when administered sc. Results suggest that slow release of liposome-associated drugs may eventually cause severe local tissue damage. The incorporation of the hydrophilic lipid derivative distearoylphosphatidylethanolamine-poly(ethyleneglycol) (PEG-PE) had no apparent effect on the protective effect of liposomes after sc administration.

Lymphatic uptake and biodistribution of liposomes after subcutaneous injection: III. Influence of surface modification with poly(ethyleneglycol)

PURPOSE

The aim of the present paper was to assess the effect of inclusion of distearoylphosphatidylethanolamine-poly(ethyleneglycol) (DSPE-PEG) into liposomal bilayers on the lymphatic uptake and lymph node localization of liposomes after subcutaneous administration.

METHODS

[3H]-Cholesteryloleylether labeled liposomes of various composition and sizes were injected s.c. into the dorsal side of the foot of rats. At several time-points after injection, blood levels of liposomes were determined. Lymphatic uptake from the s.c. site of injection and lymph node localization in regional lymph nodes were determined at the end of the 52 h observation period.

RESULTS

The results demonstrate that inclusion of DSPE-PEG into several types of liposomes has only a modest effect on lymphatic uptake. Also lymph node localization is only slightly affected by PEG-mediated steric stabilization.

CONCLUSIONS

Factors other than the presence of a steric barrier are more important in determining lymphatic uptake from the s.c. injection site. The observation that lymph node localization was only slightly affected by PEG-coating strongly suggests that macrophage uptake is not the only important mechanism of lymph node localization of s.c. administered liposomes.

Lymphatic uptake and biodistribution of liposomes after subcutaneous injection. II. Influence of liposomal size, lipid compostion and lipid dose

The present paper reports on the results of a systematic study on liposome variables potentially affecting lymphatic disposition and biodistribution of liposomes after sc injection. Liposomal size was found to be the most important factor influencing lymphatic uptake and lymph node localization of sc administered liposomes. Lymphatic uptake from the s.c. injection site of small liposomes (about 0.04 microm) was relatively high (76% of the injected dose (%ID)) as compared to large, non-sized liposomes, which remained almost completely at the site of injection. Small liposomes were less efficiently retained by regional lymph nodes than larger liposomes. Liposomal lipid composition did not influence lymphatic uptake with one exception: Lymphatic uptake was decreased in case of neutral liposomes composed of (DPPC). Lymph node localization was substantially enhanced by inclusion of phosphatidylserine (PS) into the liposomal bilayers. Saturation of lymphatic uptake and lymph node localization did not occur over a large liposomal lipid dose range, illustrating the efficient performance of lymph nodes in capturing s.c. administered particles.

Preparation of biodegradable, surface engineered PLGA nanospheres with enhanced lymphatic drainage and lymph node uptake

PURPOSE

Nanospheres can be utilised for the targeting of drugs and diagnostic agents to the regional lymph nodes. The surface modification of model polystyrene, (PS), and poly(lactide-co-glycolide),(PLGA), nanospheres by poly(lactide)-poly(ethylene glycol), (PLA:PEG), copolymers has been assessed by in vitro characterisation and in vivo biodistribution studies following subcutaneous administration of the nanospheres to the rat.

METHODS

Three PLA:PEG copolymers were investigated, with PEG chain lengths of 750, 2000 and 5000 Da. The PLA:PEG copolymers were either coated onto the surface of PS and PLGA nanospheres or used as a co-precipitate in the formation of PLGA-PLA:PEG nanospheres. Coating of the nanospheres was confirmed by an increase in their particle size and a corresponding decrease in the surface potential. The kinetics of injection site drainage and lymph node retention was determined over a 24 hour time course for naked, coated and co-precipitated nanosphere systems.

RESULTS

Dependent on the surface characteristics, the distribution of the nanospheres can be significantly modified and the lymph node localisation dramatically enhanced by coating their surfaces with PLA:PEG copolymers or by producing co-precipitate nanospheres of PLGA and PLA:PEG.

CONCLUSIONS

A fully biodegradable nanosphere system has been developed with excellent lymph node targeting characteristics.

Morphological observations on the fate of liposomes in the regional lymph nodes after footpad injection into rats

Multilamellar liposomes composed of equimolar egg phosphatidylcholine and cholesterol and containing carboxyfluorescein or colloidal gold were injected subcutaneously into the footpad of the hind-leg of rats. The draining popliteal lymph nodes of animals killed at time intervals after injection were then dissected and sections examined by fluorescence microscopy (carboxyfluorescein), light microscopy using an immunogold silver kit to enhance gold particles or by transmission electron microscopy. Morphological observations confirmed that subcutaneously injected liposomes accumulate in large numbers in the draining lymph node. The majority of liposomes arrived at the subcapsular sinuses, probably via afferent lymphatic vessels, as such, i.e., in a non-cell bound form. Subsequently, liposomes were dispersed throughout the lymph node either by permeation as free vesicles along the sinuses or by cells involved in vesicle uptake. The majority of such cells were free macrophages, littoral cells and reticular cells (fixed macrophages). Once within cells, liposomes were seen digested by the lysosomal apparatus with varying loss of their lamellar structure, leaving free gold particles within the lysosomes.

The immunological adjuvant and vaccine carrier properties of liposomes

Animal immunization studies by numerous laboratories have shown that liposomes promote humoural and cell-mediated immunity to a wide spectrum of bacterial, protozoan and viral antigens as well as tumour cell antigens, venoms and allergens. Adjuvanticity depends on liposomal structural characteristics which determine vesicle fate in vivo and, thus, the mode of antigen interaction with antigen-presenting cells. Adjuvanticity is further promoted by receptor mediated targeting of liposomes to macrophages, or the presence of other adjuvants including cytokines. The immunoadjuvant function of liposomes is supplemented by their ability to act as a carrier for co-entrapped B and T-cell epitopes, thus eliminating the need for a carrier protein. Recently, a technique has been developed for the entrapment of live or attenuated microbial vaccines into giant liposomes under conditions which retain their viability. Liposomes containing microbial vaccines (together with other soluble antigens or cytokines) could be used as carriers of vaccines in cases where there is a need to prevent interaction of vaccines with maternal antibodies or preformed antibodies to vaccine impurities.

Liposomes in drug delivery. Clinical, diagnostic and ophthalmic potential

Liposomes (phospholipid-based vesicles) have been investigated since 1970 as a system for the delivery or targeting of drugs to specific sites in the body. Because of their structural versatility in terms of size, composition, surface charge, bilayer fluidity and ability to incorporate almost any drug regardless of solubility, or to carry on their surface cell-specific ligands, liposomes have the potential to be tailored in a variety of ways to ensure the production of formulations that are optimal for clinical use. This includes controlled retention of entrapped drugs in the presence of biological fluids, controlled vesicle residence in the blood circulation or other compartments in the body, and enhanced vesicle uptake by target cells. Accumulated in vivo evidence, particularly in areas such as cancer chemotherapy, antimicrobial therapy, vaccines, diagnostic imaging and the treatment of ophthalmic disorders has indicated clearly that some liposome-entrapped drugs and vaccines exhibit superior pharmacological properties to those observed with conventional formulations. Such work has encouraged the application of liposomes in the treatment of diseases in humans. A large number of trials in patients with cancer or infections suggest that certain liposomal drug formulations are likely to prove clinically useful.

Liposomes for the transformation of eukaryotic cells

Gene therapy of human disease is a method of treatment under active development. DNA-loaded liposomes exhibit great promise for use in this field. Liposome-based transfection vectors have many inherent advantages that will likely lead to their wide in vivo use. Vectors with low toxicity and a high degree of targetability can now be easily prepared. These vectors are also free of the length constraints governing retroviral vectors. In this review we discuss recent developments in the use of liposomes for transfection of eukaryotic cells.

Characterization of Leishmania major antigen-liposomes that protect BALB/c mice against cutaneous leishmaniasis

Leishmania major antigen-liposomes prepared as dehydration-rehydration vesicles (DRV) and composed of equimolar amounts of L-alpha-distearoyl phosphatidylcholine and cholesterol confer high-level host-protective immunity against virulent homologous challenge to susceptible BALB/c mice. Physical and antigenic characterization of these protective liposomes is described. Both empty and L. major antigen-DRV were multilamellate and heterogeneous in size, ranging from 0.10 to 2.00 microns. Although the liposomes were made by using a crude mixture of promastigote antigens, lipophosphoglycan covered the liposome surface; this was demonstrated by immunogold electron microscopy. Application of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (immunoblot) analysis revealed preferential entrapment of the 63-kilodalton promastigote protease (gp63) into the DRV. We suggest that our L. major antigen-DRV merit further study because of their preferential entrapment of these two host protective antigens together with their long in vivo half-life. In addition, this report illustrates that intravenous or subcutaneous immunization of BALB/c mice with the same limited subset of protective antigens, predominantly lipophosphoglycan and gp63, within DRV liposomes leads to either protection and low splenic interleukin-3 production or to nonprotection and high splenic interleukin-3 production, respectively. This was consistent with our hypothesis that differential antigen presentation after administration of the same immunogen by the intravenous or the subcutaneous route results in differential T-cell activation.

Immunological adjuvants: a role for liposomes

Recent technological advances have resulted in the production of safe subunit and synthetic small peptide vaccines. These vaccines are weakly or non-immunogenic and cannot, therefore, be used effectively in the absence of immunological adjuvants (agents that can induce strong immunity to antigens). Owing to the toxicity of adjuvants, only one (aluminium salts) has hitherto been licensed for use in humans, and it is far from ideal. In this article, Gregory Gregoriadis discusses the use of liposomes as an alternative safe, versatile, universal adjuvant that can induce humoral- and cell-mediated immunity to antigens when administered parenterally or enterally.

Liposomes as carriers of antitumor agents: toward a clinical reality

For several years, liposomes have been explored as carriers of antitumor agents. Liposomes can be administered intravenously to carry lipid-soluble drugs, to improve drug stability, to target organs with fenestrated capillaries, to achieve an intravascular slow drug-release system, and to reduce drug levels in certain organs sensitive to toxicity. They also have significant potential for local therapy when administered subcutaneously or intraperitoneally. The preclinical development of liposome-entrapped antitumor agents has been, in general, slowed by cumbersome formulation problems. However, during the last few years, new drug loading techniques and a better selection of the drugs for liposome entrapment have resulted in pharmaceutically acceptable liposomal formulations or antitumor agents. Three preparations have been approved by the Federal Drug Administration, two containing doxorubicin, and the other a new lipophilic cisplatin analogue [cis-bis-neodecanoato-trans-R,R-1,2-diamino-cyclohexane platinum (II)]. In preclinical studies, these preparations were shown to be less toxic than the parent compounds and more active in models of liver micrometastases. Clinical Phase I and II studies with these formulations are now in progress. Although liposomal antitumor agents have no established role in the anticancer armamentarium at this stage, the information available suggests that they may improve the therapeutic index or broaden the applications of available antitumor agents and possibly act as carriers for newly designed liposome-dependent antitumor agents.

Cancer chemotherapy administered by activated carbon particles and liposomes

In cancer chemotherapy, a specific drug delivery system is expected since many anticancer drugs show toxicity against not only cancer cells but also against normal tissues. The dosage form comprising anticancer drugs adsorbed on activated carbon particles or encapsulated in liposomes is developed as a drug-delivery system which enhances the therapeutic efficacy and reduces the adverse effects. The dosage forms are versatile in size and electric charge, so that large amounts of the drugs are distributed to the "targeted" organs or tissues and lesser amounts are distributed to the whole body. The dosage forms are designed to release the drugs slowly for a long time at local sites. Through this process, practical use of the dosage forms as an anticancer drug carrier results in an enhancement of anticancer efficacy on the local lesion and a decrease of systemic toxicity.

Kinetics and disposition of fluorescein-labelled liposomes in healthy human subjects

The in vivo kinetics and organ uptake of multilamellar liposomes have been studied in healthy volunteers. Sodium fluorescein-containing liposomes composed of equimolar amounts of egg phosphatidylocholine and cholesterol were injected into a peripheral vein in 4 healthy subjects. Blood samples collected from the femoral artery, hepatic vein and pulmonary artery, were analysed for liposomal dye content. The results, showing involvement of the reticuloendothelial system (RES) in the removal of liposomes, confirmed those previously obtained with radiolabelled preparations. Use of an innocuous liposomal marker (sodium fluorescein) and conventional vascular catheterization techniques, as employed here, may provide a reliable and clinically acceptable approach to establishing disease-induced changes in the kinetics of uptake of drug-containing liposomes by the RES, and thus help in the design of protocols for effective treatment.

Increased stimulation of alkaline phosphatase by small doses of colchicine entrapped in liposomes. A biochemical test to detect effective liposome-hepatocyte interaction

The subcutaneous administration of colchicine encapsulated in small unilamellar vesicles reduced the initial toxicity peak and maintained for several days an adequate level of the drug in the liver. Colchicine is an excellent marker for effective liposome-hepatocyte interaction since it fulfills the following criteria: (a) When taken up by the hepatocytes within liposomes, it is active and induces the synthesis of alkaline-phosphatase two to three times over control values. The injection of at least ten times more free colchicine is necessary to attain a similar induction. (b) If released from extracellular liposomes, colchicine is cleared rapidly from the circulation. The results show that liposomes, in spite of their reduced aqueous compartment (approximately 1.0 microliter/mumole of lipid), can achieve clinical utility when administered subcutaneously because of their efficient interaction with parenchymal cells and their continual arrival from the injection site.

The absorption of phospholipid vesicles by perfused rat liver depends on vesicle surface charge

Two types of sonicated vesicle have been prepared from dipalmitoylphosphatidylcholine (DPPC) by incorporation of phosphatidylinositol (PI) to give negatively charged vesicles and stearylamine to give positively charged vesicles. The absorption of the vesicles by rat liver has been investigated by perfusion techniques. A steady state of vesicle absorption is rapidly established in approx. 2 min and the initial rates of absorption decrease with PI content of the vesicles and increase with stearylamine content. In the steady state, the uptake of vesicles by the liver is similarly dependent on vesicle charge, being inhibited by PI and enhanced by incorporation of stearylamine in the vesicles. Fractionation of the liver into subcellular fractions following perfusion showed that most of the vesicular lipid could be found associated with a nuclear (plus plasma membrane) fraction. The suppression of vesicle absorption by PI may be of value as a means of bypassing the liver in relation to the use of vesicles as a delivery system.

Enhanced delivery to target cells by heat-sensitive immunoliposomes

Heat-sensitive immunoliposomes are capable of releasing the entrapped content at the target cell surface upon a brief heating to the phase transition temperature of the liposome membrane. In this study we have examined the delivery efficiency of drugs entrapped in heat-sensitive immunoliposomes. Immunoliposomes composed of dipalmitoyl phosphatidylcholine with entrapped [3H]uridine were incubated with target cells at 4 degrees C. The cell-liposome mixture was then heated to 41 degrees C and the uptake of [3H]uridine into the intracellular pool of phosphorylated uridine-containing molecules was measured. The immunoliposomes showed maximal release of the uridine at 41 degrees C, the phase transition temperature of dipalmitoyl phosphatidylcholine liposomes. The largest accumulation of [3H]uridine in the target cells also took place at 41 degrees C. The initial level of uptake of [3H]uridine released from immunoliposomes by heating was greatly enhanced over that observed for free [3H]uridine and [3H]uridine released from liposomes without attached antibody. The nucleoside uptake inhibitors nitrothiobenzylinosine, dipyridamole, and unlabeled uridine were able to inhibit uptake of [3H]uridine released from immunoliposomes. This supports the hypothesis that the enhanced uptake is due to a heat-induced release of [3H]uridine at the cell surface followed by transport and phosphorylation of [3H]uridine by the target cells. These results indicate the feasibility of using the heat-sensitive immunoliposomes as a target-specific drug delivery system.

Tissue distribution of liposomes exhibiting long half-lives in the circulation after intravenous injection

The previously established direct relationship between long half-life of uncharged small unilamellar liposomes in the circulation of injected animals and reduced permeability to entrapped solutes in the presence of blood plasma was investigated further. It was found that vesicle size and surface charge override state of membrane permeability in determining rates of vesicle clearance. Thus, half-lives of liposomes that were practically impermeable in plasma were much shorter for larger vesicles or vesicles with a negative charge on their surface. 111In-labelled bleomycin-containing small unilamellar liposomes with long half-lives accumulated in the liver after injection to a much lesser extent (e.g., 26%) than similar liposomes (55% of the dose) exhibiting a shorter half-life. Much of the injected long-lived liposomes (about 35%) were recovered in the carcass of the animals. Scanning of the carcasses revealed quantitative accumulation of radioactivity in the bones, presumably the phagocytic cells of the bone marrow. Long-lived liposomes appear suitable for drug delivery to, or imaging of the bone marrow.