Preparation and use of liposomes in the treatment of microbial infections

Liposomes as Antibiotic Delivery Systems: A Promising Nanotechnological Strategy against Antimicrobial Resistance

Antimicrobial drugs are key tools to prevent and treat bacterial infections. Despite the early success of antibiotics, the current treatment of bacterial infections faces serious challenges due to the emergence and spread of resistant bacteria. Moreover, the decline of research and private investment in new antibiotics further aggravates this antibiotic crisis era. Overcoming the complexity of antimicrobial resistance must go beyond the search of new classes of antibiotics and include the development of alternative solutions. The evolution of nanomedicine has allowed the design of new drug delivery systems with improved therapeutic index for the incorporated compounds. One of the most promising strategies is their association to lipid-based delivery (nano)systems. A drug's encapsulation in liposomes has been demonstrated to increase its accumulation at the infection site, minimizing drug toxicity and protecting the antibiotic from peripheral degradation. In addition, liposomes may be designed to fuse with bacterial cells, holding the potential to overcome antimicrobial resistance and biofilm formation and constituting a promising solution for the treatment of potential fatal multidrug-resistant bacterial infections, such as methicillin resistant Staphylococcus aureus. In this review, we aim to address the applicability of antibiotic encapsulated liposomes as an effective therapeutic strategy for bacterial infections.

Nanomaterials in the Prevention, Diagnosis, and Treatment of Mycobacterium Tuberculosis Infections

Despite the tremendous advancements that have been made in biomedical research, Mycobacterium tuberculosis (TB) still remains one of the top 10 causes of death worldwide, outpacing the Human Immunodeficiency Virus as a leading cause of death from an infectious disease. In the light of such significant disease burden, tremendous efforts have been made worldwide to stem this burgeoning spread of disease. The use of nanomaterials in TB management has increased in the past decade, particularly in the areas of early TB detection, prevention, and treatment. Nanomaterials have been proven to be efficacious in the rapid and accurate detection of TB pathogens. Novel nanocarriers have also shown tremendous promise in improving drug delivery, potentially enhancing drug concentrations in target organs while at the same time, reducing treatment frequency. In addition, the engineering of antigen nanocarriers represents an exciting front in TB research, potentially paving the way for the successful development of a new class of effective TB vaccines. This article discusses epidemiology and pathogenesis of TB infections, current TB therapeutics, advanced nanomaterials for anti-TB drug delivery, and TB vaccines. In addition, challenges and future perspectives in developing safe and effective nanomaterials in TB diagnosis and therapy are also presented.

Application of liposomes in medicine and drug delivery

Liposomes provide an established basis for the sustainable development of different commercial products for treatment of medical diseases by the smart delivery of drugs. The industrial applications include the use of liposomes as drug delivery vehicles in medicine, adjuvants in vaccination, signal enhancers/carriers in medical diagnostics and analytical biochemistry, solubilizers for various ingredients as well as support matrices for various ingredients and penetration enhancers in cosmetics. In this review, we summarize the main applications and liposome-based commercial products that are currently used in the medical field.

Investigation of the antibacterial activity and efflux pump inhibitory effect of co-loaded piperine and gentamicin nanoliposomes in methicillin-resistant Staphylococcus aureus

OBJECTIVE

Antibiotic resistance has stimulated the research for developing novel strategies that can prevent bacterial growth. Methicillin-resistant Staphylococcus aureus (MRSA), regarded as one of the most serious antibiotic-resistant bacteria which has been conventionally recognized as a nosocomial pathogen.

MATERIALS AND METHODS

Nanoliposomal formulations of piperine and gentamicin were prepared by dehydration-rehydration (DRV) method and characterized for size, zeta potential and encapsulation efficiency. Antibactericidal activities of liposomal and free forms were evaluated against MRSA ATCC 43300 by the determination of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and fractional inhibitory concentration index (FICI). The time-kill studies were carried out to evaluate the potency of antibacterial agents. The effect of piperine on bacterial efflux pumps was also investigated.

RESULTS

MIC values of gentamicin and piperine were 32 and 100 µg/mL, respectively. Synergetic effects were observed by the combination of gentamicin and piperine and FICI was determined to be 0.5. Following incorporation of gentamicin into liposomal gentamicin and liposomal combination, the MIC values were reduced 16- and 32-fold, respectively. MBC values of gentamicin reduced 4 and 8 times following incorporation into gentamicin and combination liposomes, respectively. In comparison with vancomycin, liposomal combination was more effective in bacterial inhibition and killing. Liposomal combination was the most effective preparations in time-kill study. Our findings indicated that liposomal piperine was able to inhibit the efflux pump sufficiently.

CONCLUSION

The results of this study revealed that liposomal combination is a powerful nano-antibacterial agent to eradicate MRSA infection. This dual-loaded formulation was an effective approach for eradication of MRSA.

Nanoliposome-based antibacterial drug delivery

Although most bacterial infectious diseases can be treated successfully with the remarkable array of antibiotics, the microbial pathogens continue to be one of the most critical health challenges worldwide. One of the common efforts in addressing this issue lies in improving the existing antibacterial delivery systems since inefficient delivery can lead to poor therapeutic outcome of the administered drug. Recently, nanoliposomal systems have been widely used as promising strategies to overcome these challenges due to their unique set of properties. This article tries to briefly summarize the current studies that have taken advantage of liposomal nanoparticles as carriers to deliver antibacterial agents. The reviewed investigations demonstrate the immense potential of liposomal nanoparticles as carriers for antibiotic delivery and highlight the latent promise in this class of vehicles for treatment of bacterial infections. The future of these promising approaches lies in the development of more efficient techniques for preparing liposomal nanoparticles with great potential in effective and selective targeting of antibiotics to bacterial cells for eradication as well as the highest safety for human host.

Nanotechnology and pulmonary delivery to overcome resistance in infectious diseases

Used since ancient times especially for the local treatment of pulmonary diseases, lungs and airways are a versatile target route for the administration of both local and systemic drugs. Despite the existence of different platforms and devices for the pulmonary administration of drugs, only a few formulations are marketed, partly due to physiological and technological limitations.

Respiratory infections represent a significant burden to health systems worldwide mainly due to intrahospital infections that more easily affect immune-compromised patients. Moreover, tuberculosis (TB) is an endemic infectious disease in many developing nations and it has resurged in the developed world associated with the human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic.

Currently, medicine faces the specter of antibiotic resistance. Besides the development of new anti-infectious drugs, the development of innovative and more efficient delivery systems for drugs that went off patent appears as a promising strategy pursued by the pharmaceutical industry to improve the therapeutic outcomes and to prolong the utilities of their intellectual property portfolio. In this context, nanotechnology-based drug delivery systems (nano-DDS) emerged as a promising approach to circumvent the limitations of conventional formulations and to treat drug resistance, opening the hypothesis for new developments in this area.

Liposome: classification, preparation, and applications

Liposomes, sphere-shaped vesicles consisting of one or more phospholipid bilayers, were first described in the mid-60s. Today, they are a very useful reproduction, reagent, and tool in various scientific disciplines, including mathematics and theoretical physics, biophysics, chemistry, colloid science, biochemistry, and biology. Since then, liposomes have made their way to the market. Among several talented new drug delivery systems, liposomes characterize an advanced technology to deliver active molecules to the site of action, and at present, several formulations are in clinical use. Research on liposome technology has progressed from conventional vesicles to 'second-generation liposomes', in which long-circulating liposomes are obtained by modulating the lipid composition, size, and charge of the vesicle. Liposomes with modified surfaces have also been developed using several molecules, such as glycolipids or sialic acid. This paper summarizes exclusively scalable techniques and focuses on strengths, respectively, limitations in respect to industrial applicability and regulatory requirements concerning liposomal drug formulations based on FDA and EMEA documents.

Lipophilic-formulated gold porphyrin nanoparticles for chemotherapy

Lipophilic formulation is an invaluable technique for the delivery of cancer drugs. Incorporation of poorly soluble and toxic compounds into a lipophilic carrier vehicle improves both the stability and compatibility in blood and body fluids. Currently, although a large proportion of novel cancer drugs are poorly water soluble, most existing drug carriers are only able to encapsulate hydrophilic drugs. As the ultimate goal of drug delivery (in particular cancer drug delivery) is to achieve high therapeutic effect with minimal toxicity, it would thus be beneficial to invest substantial efforts in the development of lipophilic carrier systems. Here we describe our technique to synthesize a lipophilic carrier for hydrophobic and toxic potent cancer drugs, such as gold(III) porphyrin.

A comparison of the in vitro antimicrobial activity of liposomes containing meropenem and gentamicin

The antimicrobial activity of eight cationic, two neutral and three anionic liposome compositions containing meropenem and gentamicin was tested in vitro in broth and serum medium. The cationic formulations showed better antibacterial efficacy against both Gram-positive and Gram-negative bacteria than the anionic and neutral ones, regardless of the encapsulated drug. The most effective formulations were the cationic PC/DOPE/DOTAP 3:4:3 and PC/Chol/DOTAP 3:4:3, as the MICs with meropenem were 2 to 4 times lower than those of the free drug.

Protection against H1, H5, H6 and H9 influenza A infection with liposomal matrix 2 epitope vaccines

The recent emergence of multiple avian influenza A subtypes that cause human disease (i.e., H5N1, H9N2 and H7N7), coupled with the fear that one of these strains might precipitate a new pandemic, underscores the need to develop new technological approaches to immunization which elicit protective immune responses against multiple subtypes of influenza A. In response to this demand, several matrix 2 protein ectodomain segments (M2eA) corresponding to the H1N1, H5N1 and H9N2 influenza strains were formulated using a novel liposome-based vaccine technology and evaluated as potential immunogens for developing a "universal" influenza vaccine. Mice immunized with liposomal M2eA survived homologous challenges with H1N1 (100% survival) or H9N2 (80% survival) influenza strains. There were significant reductions in their lung viral load as well as in immunized mice challenged with the H5N1 subtype. The mice vaccinated with an M2eA segment corresponding to the H1N1 and H6N2 (a reassortant influenza A virus carrying the M2eA from PR8/34) strains elicited elevated IgG ELISA antibody titers to this M2eA epitope segment and antiserum from these immunized mice provided passive protection (100% survival) to naïve mice receiving a lethal dose of H6N2 influenza virus. These results provide the first evidence that recombinant M2eA epitopes to multiple subtypes elicited immune protection against a homologous challenge and provides further evidence in favor of the development of a "universal" influenza vaccine based on M2eA.

Long term improvement in the treatment of canine leishmaniosis using an antimony liposomal formulation

Pharmacokinetic and clinical effectiveness of liposome-encapsulated N-methylglucamine antimoniate (LMA) was performed in dogs suffering from experimental leishmaniosis. LMA was compared with N-methylglucamine antimoniate (MGA), the same drug in its free form. Sb plasma concentrations for LMA were always higher than those for MGA. Mean residence time (MRT), half-life time (t(1/2)) and clearance (Cl) showed that Sb was eliminated slower after liposome administration. The high volume of distribution (Vd) obtained with LMA suggests that Sb could achieve therapeutic concentrations in parasite-infected tissues. Average plasma concentration at steady state (Css(ave)) shows that Sb body concentrations after LMA treatment (9.8 mg/kg Sb, each 24h) would be effective in Leishmania infantum canine infection. Comparing LMA with MGA in a 1-year follow-up we observed no relapses for LMA and total protein and gammaglobulin concentrations were within normal range, while for MGA both began to rise 3 months after treatment. Use of antimonial liposomal formulations may restore effectiveness to an existing drug and reduce toxicity.

Targeted delivery of antibiotics using liposomes and nanoparticles: research and applications

This review examines current technologies for increasing the bioavailability of antibiotics by means of liposomes or nanoparticles. The main focus is on liposomes. These carriers were preferentially developed because their composition is compatible with biological constituents. Biodegradable polymers in the form of colloidal particles have also been used and show promise for future applications in antimicrobial chemotherapy. The in vivo behaviour of both types of carriers and consequently their therapeutic potential, are determined by their route of administration. Conventional carrier strategies permit the mononuclear phagocyte system to be targeted by intravenous injection of antibiotics. Stealthy strategies avoid major uptake by these cells and extend the systemic presence of these carriers. The purpose of this review is to provide background information in antibiotic targeting gathered from papers published over the last twenty years. It seems clear that such drug carriers (liposomes, nanoparticles) allow increased drug concentration at infected sites but reduce drug toxicity.

Effective treatment of acute and chronic murine tuberculosis with liposome-encapsulated clofazimine

The therapeutic efficacy of liposomal clofazimine (L-CLF) was studied in mice infected with Mycobacterium tuberculosis Erdman. Groups of mice were treated with either free clofazimine (F-CLF), L-CLF, or empty liposomes twice a week for five treatments beginning on day 1 (acute), day 21 (established), or day 90 (chronic) postinfection. One day after the last treatment, the numbers of CFU of M. tuberculosis in the spleen, liver, and lungs were determined. F-CLF at the maximum tolerated dose of 5 mg/kg of body weight was ineffective; however, 10-fold-higher doses of L-CLF demonstrated a dose response with significant CFU reduction in all tissues without any toxic effects. In acutely infected mice, 50 mg of L-CLF/kg reduced CFU 2 to 3 log units in all three organs. In established or chronic infection, treated mice showed no detectable CFU in the spleen or liver and 1- to 2-log-unit reduction in the lungs. A second series of L-CLF treatments cleared M. tuberculosis in all three tissues. L-CLF appears to be bactericidal in the liver and spleen, which remained negative for M. tuberculosis growth for 2 months. Thus, L-CLF could be useful in the treatment of tuberculosis.

The capacity of a combined liposomal hepatitis B and C vaccine to stimulate humoral and cellular responses in mice

Combined hepatitis B surface antigen and hepatitis C antigen were encapsulated into 1, 2, and 5 microm discrete liposomes and then lyophilized. Groups of adolescent CD-1 mice were given a single 0.3 mL oral dose of these liposomes containing 50 microg/mL hepatitis B surface antigen and hepatitis C antigen, 50 microg/mL of the same antigens or liposomes alone. Animals in each group were sacrificed every 2 weeks for 10 weeks and the humoral response investigated by enzyme-linked immunosorbent assay (ELISA) and the cellular response by splenic lymphocyte proliferation to 10 microg of either antigen. Seroconversion to both antigens in the mice receiving liposomal antigens occurred in 87.5% of animals sacrificed at 4 weeks and later. One animal (12.5%) receiving antigen alone seroconverted to hepatitis B virus at 6 weeks, but all animals receiving liposomes alone remained negative. Proliferation indexes (PI) greater than 3 were observed in all animals receiving liposomal antigens, with the greatest response seen at 10 weeks. PI was less than 2 for all animals in the other two groups. Thus, a single oral dose of liposomes of three sizes containing both hepatitis B and C antigens given to mice resulted in rapid seroconversion and a progressive robust cellular immune response, whereas the antigens alone or liposomes without antigen did not.

Role of liposomes in selective proliferation of splenic lymphocytes

The effects of free and encapsulated allergens of Artemisia scoparia pollen on lymphocyte proliferation and immunoglobulin production in BALB/c mice were investigated. Splenic lymphocytes from mice immunized with liposome entrapped allergen (LEA) elicited a marked proliferative response upon in vitro stimulation with both free and encapsulated allergen in comparison to mice immunized with free allergen (FA). The serum immunoglobulin profile of mice administered LEA revealed a predominance of IgG1 antibodies concomitant with an enhancement of IgG2a, IgG2b, IgG3 and IgM responses and suppression of IgE responses. However immunization with FA resulted in significant production of IgE responses and low levels of IgG antibodies. The differential ability of free and encapsulated allergens to selectively induce immunoglobulin isotypes suggests that different presentation and T cell differentiation pathways may be followed by FA and LEA in the immune system. Proliferation studies involving macrophage depletion demonstrated that macrophages play an obligatory role in the processing of LEA. Analysis of cytokine production in sera of immunized mice (FA/LEA) revealed that LEA induced significant IFN-gamma responses and lower IL-4 responses than mice immunized with FA. The results of the present study indicate that liposomes synergise the proliferation by the antigen incorporated in it and polarizes the response towards Th1 type of cytokine production. The immunoadjuvant and immunomodulation property of liposomes make it an efficient vehicle for effective immunotherapy.

Therapeutic efficacy of liposomal clofazimine against Mycobacterium avium complex in mice depends on size of initial inoculum and duration of infection

The therapeutic efficacy of liposomal clofazimine (L-CLF) against Mycobacterium avium complex (MAC) was evaluated in the acute and chronic infection models of the beige mouse (C57BL/6J bgj bgj). The maximum tolerated dose of L-CLF was inversely proportional to the infection level. L-CLF showed higher antibacterial activity than free clofazimine. Treatment with 25 mg of L-CLF per kg of body weight (intravenously) was started at days 1, 8, 15, and 22 postinfection and was studied at three levels of MAC infection (10(4), 10(5), and 10(6) bacilli/mouse). L-CLF treatment caused a significant (P < 0.05 to 0.001) reduction in the numbers of viable bacteria in lung, liver, and spleen at all infection levels, irrespective of time of treatment. However, the best results were obtained when an already established infection was treated (day 22). The organ-related differences in response to the treatment were also affected by the level of infection. A marked reduction in the numbers of CFU was observed in the lungs of mice with lower infection levels, whereas liver and spleen were treated more efficiently at higher infection levels. These studies might help in evaluations of host responses to therapy.

Liposome encapsulation of clofazimine reduces toxicity in vitro and in vivo and improves therapeutic efficacy in the beige mouse model of disseminated Mycobacterium avium-M. intracellulare complex infection

Disseminated infections caused by the Mycobacterium avium-M. intracellulare complex (MAC) are the most frequent opportunistic bacterial infections in patients with AIDS. MAC isolates are resistant to many of the standard antituberculous drugs. Failure to obtain significant activities of certain drugs is due to difficulty in achieving high concentrations at the sites where the infections reside. New and improved agents for the treatment of mycobacterial infections are therefore required. Earlier, the anti-MAC activities of various agents in free or liposomal form were studied; liposomes were used as drug carriers to ultimately target the drugs to macrophages where mycobacterial infections reside. Clofazimine was chosen for further studies because it could be effectively encapsulated and its activity was well maintained in liposomal form. The present studies with both erythrocytes and macrophages as the model systems show that liposomal drug is far less toxic in vitro than the free drug. The in vivo toxicity of clofazimine was also significantly reduced after liposome encapsulation. The therapeutic efficacies of free and liposomal drugs were compared in a beige mouse model of disseminated MAC infection. An equivalent dose of liposomal drug (10 mg/kg of body weight) was more effective in eliminating the bacterial from the various organs studied, particularly from the liver. Moreover, because of the reduced toxicity of liposomal drug, higher doses could be administered, resulting in a significant reduction in the numbers of CFU in the liver, spleen, and kidneys. The data demonstrate that liposomal clofazimine is highly effective in the treatment of MAC infections, even if the treatment is initiated after a disseminated infection has been established. The present studies thus suggest the potential usefulness of liposomal clofazimine for the treatment of disseminated MAC infections.

Encapsulation of vancomycin and gentamicin within cationic liposomes for inhibition of growth of Staphylococcus epidermidis

Liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC), cholesterol (Chol) and dimethyldioctadecylammonium bromide (DDAB). The cationic vesicles adsorb to biofilms of the skin-associated bacteria Staphylococcus epidermidis, which have a negative charge. Encapsulation of the antibacterial drug vancomycin into such liposomes enhanced its activity relative to the free agent. The effectiveness of the preparation was dependent on the fluidity of the liposomal membrane and on the level of drug entrapment within the aqueous core of the vesicles. The aminoglycoside antibiotic gentamicin was also encapsulated within similar liposomes but was less effective, possibly due to its slow passage through the membrane. The liposomal vancomycin preparation has potential medical use in treating bacterial infections of foreign body biomedical devices (e.g. catheters), with either topical or intravenous administration.

Antibacterial activity of liposomal amikacin against Pseudomonas aeruginosa in vitro

The influence of liposomal amikacin on Pseudomonas aeruginosa was studied. P. aeruginosa clinical isolates caused release of encapsulated amikacin from liposomes. The liposomal amikacin proved to be active as bactericidal agent after 3 h of incubation with P. aeruginosa. Incubation of P. aeruginosa with liposomal amikacin resulted in inhibition of the growth when equivalent of 2 MIC was added but not when equivalent of 1 MIC was added. Susceptibility of bacterial isolates to the liposomal amikacin varied with bacterial strain used, but generally encapsulation of amikacin did not enhance their antibacterial activity.

Liposomes as delivery systems in the prevention and treatment of infectious diseases

Research on the potential application of liposomes in the prevention and treatment of infectious diseases has focussed on improvement of the therapeutic index of antimicrobial drugs and immunomodulators and on stimulation of the immune response to otherwise weak antigens in vaccines composed of purified micro-organism subunits. In this review current approaches in this field are outlined. The improved therapeutic index of antimicrobial drugs after encapsulation in liposomes is a result of enhanced drug delivery to infected tissue or infected cells and/or a reduction of drug toxicity of potentially toxic antibiotics. Liposomal encapsulation of immunomodulators that activate macrophages aims at reducing the toxicity of these agents and targeting them to the cells of the mononuclear phagocyte system in order to increase the nonspecific resistance of the host against infections. Studies on the immunogenicity of liposomal antigens have demonstrated that liposomes can potentiate the humoral and cell mediated immunity to a variety of antigens.

Liposomal muramyl tripeptide phosphatidylethanolamine (MTP-PE) promotes haemopoietic recovery in irradiated mouse

Pretreatment of C57B1/6 mouse with the macrophage activator muramyl tripeptide phosphatidylethanolamine encapsulated in liposomes (MTP-PE/MLV) induced haemopoietic recovery in subsequently irradiated mouse. An optimal endoCFU-S survival was observed when 200 micrograms MTP-PE/MLV was administered i.p. 24 h before irradiation. MTP-PE/MLV did not affect the day 8 exogenous CFU-S survival in the bone marrow immediately after irradiation. However, 3, 6, 9 and 14 days after irradiation the number of day 8 CFU-S was almost 2 to 4-fold higher in the bone marrow of the MTP-PE/MLV injected mouse. Also, recovery of the GM-CFC pools in femoral bone marrow after irradiation proceeded at a faster rate in the MTP-PE/MLV-treated animal than in control groups. After a single i.p. injection of MTP-PE/MLV to the non-irradiated mouse, the number of CFU-S in bone marrow was not significantly different from controls, whereas the number of GM-CSC was significantly increased. In addition, the percentage of day 8 CFU-S and GM-CFC in S-phase of the cell cycle was significantly increased, as was colony-stimulating activity present in the serum of treated animals. Pretreatment with MTP-PE/MLV protected the C57Bl/6 mouse in a dose-dependent manner from the lethal effects of ionizing radiation. A single dose (100 or 200 micrograms) injected i.p. 24 h, or 100 micrograms MTP-PE/MLV injected i.v. 24 h before 9.5 Gy gamma-rays protected 47, 85 and 59% of C57B1/6 mouse, respectively. The dose reduction factor in the case when the MTP-PE/MLV (200 micrograms per mouse) was administered i.p. at that time was 1.17 (95% CL 1.13, 1.21). Combined administration of MTP-PE/MLV (24 h) and indomethacin (24 and 3 h) to mouse prior to irradiation exerted an additional radioprotective effect.

Treatment of murine candidiasis and cryptococcosis with amphotericin B incorporated into egg lecithin-bile salt mixed micelles

Amphotericin B (AmB) with deoxycholate (Fungizone) and AmB incorporated into mixed micelles (AmB-mixMs) composed of egg lecithin with glycocholate, deoxycholate, or taurocholate were compared as treatments for murine infections. For mice infected with Candida albicans, treatment consisted of a single intravenous injection; for mice infected with Cryptococcus neoformans, treatment consisted of two intravenous injections. The maximal tolerated doses of AmB as Fungizone were 1.25 mg/kg of body weight in mice with candidiasis and 2.5 mg/kg of body weight in mice with cryptococcosis. The AmB-mixMs were nontoxic to mice at doses of 80 and 100 mg/kg of body weight and were therapeutically more active than the maximal tolerated dose of Fungizone in both models of infection. However, when Fungizone or AmB-mixMs were administered at equivalent doses of AmB, AmB-mixMs were more active in treating murine candidiasis, whereas Fungizone was more active in treating murine cryptococcosis.

In vitro activities of free and liposomal drugs against Mycobacterium avium-M. intracellulare complex and M. tuberculosis

We compared MICs and MBCs of various free- and liposome-incorporated antimicrobial agents against several patient isolates of Mycobacterium avium-M. intracellulare complex and certain American Type Culture Collection strains of M. avium, M. intracellulare, and Mycobacterium tuberculosis. Seven of 19 agents were selected for incorporation into liposomes. The MICs of these agents for 50 and 90% of isolates tested (MIC50s and MIC90s, respectively) ranged from 0.5 to 62 micrograms/ml. Members of the M. avium-M. intracellulare complex were resistant to killing by most of the other agents tested in the free form. However, clofazimine, resorcinomycin A, and PD 117558 showed complete killing of bacteria at concentrations ranging from 8 to 31 micrograms/ml, represented as MBC90s. Among the liposome-incorporated agents, clofazimine and resorcinomycin A had the highest killing effects (MBC90s, 8 and 16 micrograms/ml, respectively). Furthermore, both free and liposome-incorporated clofazimine had equivalent growth-inhibitory and killing effects on all American Type Culture Collection strains of M. avium, M. intracellulare, and M. tuberculosis tested. These results show that the antibacterial activities of certain drugs, particularly those of clofazimine and resorcinomycin, were maintained after the drugs were incorporated into liposomes.

Tissue distribution and cellular distribution of liposomes encapsulating muramyltripeptide phosphatidyl ethanolamide. Tissue and cellular distribution of LE-MTPPE

In previous studies it was shown that administration of liposome-encapsulated MTPPE (LE-MTPPE) led to resistance against Klebsiella pneumoniae infection. To get more insight in the cell types that are involved in this by LE-MTPPE induced antibacterial resistance, the tissue distribution of liposomes encapsulating MTPPE and the distribution over the cells in the main target organs were investigated. After intravenous injection of the liposomes in mice a substantial amount was recovered from liver and spleen and a smaller amount from the lung. In the liver 83% of the liposomes was taken up by the macrophages. In the spleen also most liposomes were taken up by macrophages of the red and white pulp as well as by dendrocytes. The liver and spleen were also the organs in which, after intravenous inoculation, K. pneumoniae was trapped. It was observed that cells containing LE-MTPPE often had not taken up bacteria. Most bacteria, about 73%, were found in cells not containing liposomes. The capacity of the liposome-containing cells to take up bacteria did not change with time. This suggests that the by LE-MTPPE immunostimulating effect is due to the production of cytokines by the cells that take up LE-MTPPE. These cytokines might stimulate other cells to the killing of bacteria.

Liposomes as carriers of antimicrobial agents or immunomodulatory agents in the treatment of infections

Targeting of antimicrobial agents by means of liposomes is under investigation and may be of importance in the treatment of infections that prove refractory to conventional forms of antimicrobial treatment. The ability to achieve a significantly longer residence time of liposomes in plasma and limited uptake of liposomes by the mononuclear phagocyte system opens up new areas of investigation and potential therapeutic application. By manipulating the liposomal composition, rates of uptake and intracellular degradation can be influenced and thereby the rates at which liposome-encapsulated agents are released and become available to exert their therapeutic action. With respect to the targeting of macrophage modulators at the mononuclear phagocyte system by means of liposomes for maximal stimulation of the nonspecific antimicrobial resistance, experimental evidence is now available of the potential usefulness of liposomes as carriers of these agents. This approach may also be of importance for the potentiation of treatment of severe infections.

Enhanced localization of liposomes with prolonged blood circulation time in infected lung tissue

In an experimental model of unilateral pneumonia caused by Klebsiella pneumoniae in rats we investigated whether intravenous administration of liposomes with prolonged blood circulation time resulted in significant localization of liposomes in infected lung tissue. Liposomes (100 nm) composed of hydrogenated phosphatidylinositol:hydrogenated phosphatidylcholine:cholesterol (molar ratio, 1:10:5) radiolabeled with gallium-67-deferoxamine showed relatively long blood circulation time. The degree of localization of these long circulating liposomes in the infected left lung was significantly higher compared to that of localization of 110 nm egg phosphatidylglycerol:egg phosphatidylcholine:cholesterol (molar ratio, 1:10:5) liposomes which exhibited relatively short blood circulation time. At 16 h after administration of the long circulating liposomes (when 10% of the injected dose was still present in the bloodstream) localization of liposomes in the infected left lung was increased up to 10-fold compared to the left lung of uninfected rats, and appeared to be highly correlated with the intensity of the infection. In the uninfected right lung the localization of long circulating liposomes was not increased. The degree of localization of liposomes in the infected tissue is dependent on the residence time of liposomes in the blood compartment. The extent of localization of long circulating liposomes in infected tissue appeared to be dependent on the liposomal dose administered.

Liposomes and nanoparticles in the treatment of intracellular bacterial infections

The treatment of infections caused by obligate or facultative intracellular microorganisms is difficult because most of the available antibiotics have either poor intracellular diffusion and retention or reduced activity at the acidic pH of the lysosomes. The need for antibiotics with greater intracellular efficacy led to the development of endocytosable drug carriers, such as liposomes and nanoparticles, which mimic the entry path of the bacteria by penetrating the cells into phagosomes or lysosomes. This Review assesses the potential of liposomes and nanoparticles in the targeted antibiotic therapy of intracellular bacterial infections and diseases and the pharmaceutical advantages and limitations of these submicron delivery systems.

Liposomes and pulmonary alveolar macrophages: functional and morphologic interactions

In vitro toxicity of liposomes and their functional and morphologic interactions with rat pulmonary alveolar macrophage (AMs) were investigated using viability (trypan blue exclusion), phagocytic and killing activity (uptake and digestion of live S. cerevisiae), surface adherence, respiratory burst (nitro-blue tetrazolium reduction), and morphometry (computerized image analysis) as indicators. Liposome stability in physiologic solutions and uptake of liposome-encapsulated carboxyfluorescein (CF) by AMs was assessed by fluorescence spectroscopy and microscopy. Liposomes made from saturated phospholipids and cholesterol were stable, whereas liposomes consisting of unsaturated phospholipids without cholesterol lost 30% to 40% of their content over 24 h. However, CF uptake was highest with unsaturated phospholipid preparations, whereas uptake of the three other formulations was comparable. Although liposome exposure did not affect macrophage viability, a reduction in the number of phagocytizing macrophages to 73% of control was noted after 24-h incubation with the highest lipid concentration tested (10 mumol/ml). Phagocytic killing was similar under all circumstances observed. The fraction of intracellularly killed yeast ranged from 32% to 42% for both control and experimental samples. An increase in cell surface area from 166.1 +/- 39.9 microns 2 on day O (n = 709) to 196.3 +/- 57.6 microns 2 on day 1 (n = 516) and 211.2 +/- 48.0 microns 2 on day 4 (n = 834) was observed after liposome treatment. The corresponding average cell areas of control samples did not change during the observation period. There was no net cell loss of adherence from monolayers as determined by protein assay. The respiratory burst, indicating generation of intracellular superoxide, was also similar--84% to 92% of experimental and control cells under all conditions showed a strong nitro-blue tetrazolium reduction. In summary, in vitro exposure of AMs to large concentrations of liposomes, although producing an increase in macrophage size, was not associated with aberrant macrophage morphologic features, function, or toxicity for the parameters examined.

Liposomes as vehicle for allergen presentation in the immunotherapy of allergic diseases

Liposomes are non-toxic, biodegradable and weakly immunogenic lipid vesicles which can be used as immunomodulating agents. In the present study, multilamellar vesicles (MLV) and small unilamellar vesicles (SUV) were used to incorporate an allergenic protein from Artemisia scoparia pollen. MLV incorporated more allergenic protein than SUV. To assess the immunomodulating effect of allergen entrapped in liposomes, Swiss strain mice (made IgE responders) were injected with either free allergen or liposome-entrapped allergen (LEA) and their immune response was measured in terms of specific IgG and specific IgE levels. Results indicated that specific IgE response was significantly lower in mice injected LEA (P less than 0.02) than in mice injected free allergenic protein. Although specific IgG response was higher in mice injected LEA, there was no statistically significant difference between the two groups. Potential use of liposomes as non-immunogenic biocompatible vehicle for antigen presentation in immunotherapy will be discussed.

Liposomes for the sustained drug release in vivo

New lipidic carriers suitable for the sustained drug release in vivo are presented. They consist of middle sized, compact phospholipid vesicles with one or up to few lipid bilayers which are sterically stabilized with a small amount of large-head phospholipids. As an example, phosphatidylcholine (PC) liposomes casted with up to 10 mol% of phosphatidylethanolamine with a covalently attached polyethyleneglycol 5000 headgroup (PE-PEG) are discussed. Such vesicles exhibit a very long circulation time after an i.v. administration in mice; the improvement over pure phosphatidylcholine liposomes within the first 24 h exceeds 8000%, at this point nearly 25% of the applied PE-PEG liposomes being still in the circulation. This advantage is a consequence of reduced phagocytosis of the lipidic carriers, as shown by an in vitro assay with blood monocyte cells in the flow cytometric experiments. For example, after 6 h incubation with THP-1 monocyte cells in human plasma the difference between the uptake of standard distearoylphosphatidylcholine (DSPC) and novel liposomes containing 10% distearoylphosphatidylethanolamine-PEG is by 1000%. Vesicles with 2.5 mol% DSPE-PEG are also taken-up via phagocytosis relatively slowly. But the latter vesicles, moreover, retain most of the enclosed model-drug carboxyfluorescein after an incubation in plasma. The rate of permeation of the encapsulated substance from such DSPE-PEG liposomes is below 2.4% per h. This is by approximately a factor of two less than for pure DSPC liposomes; vesicles with a higher PE-PEG content are inferior in this respect. Long circulation time and high retention of the newly developed liposomes open up ways for the future systemic use as such stabilized drug carriers for the therapeutic applications in vivo.

Pharmacokinetics and in vivo activity of liposome-encapsulated gentamicin

Gentamicin sulfate was encapsulated in liposomes composed solely of egg phosphatidylcholine and administered via intravenous injection to rats and mice. The total gentamicin activity (regardless of whether it was free or liposome associated) in serum and selected tissues was determined for 24 h (serum) or up to 15 weeks (tissues) by using a microbiological assay. The mean half-lives in serum of a single 20-mg/kg dose of free (nonencapsulated) gentamicin in mice and rats were estimated to be 1.0 and 0.6 h, respectively, whereas a similar dose of encapsulated drug had apparent mean half-lives of 3.8 h in mice and 4.0 h in rats. In both species, the apparent half-life in serum of the liposomal formulation increased as the dose increased. Liposome encapsulation resulted in higher and more prolonged activity in organs rich in reticuloendothelial cells (especially spleen and liver). In acute septicemia infections in mice, the liposomal formulation showed enhanced prophylactic activity (as determined by calculation of the 50% protective dose). In a model of murine salmonellosis, liposomal gentamicin greatly enhanced survival when given as a single dose (10 mg/kg) at 1 or 2 days after infection as well as up to 7 days before infection.

Affinity of amphotericin B for phosphatidylcholine vesicles as a determinant of the in vitro cellular toxicity of liposomal preparations

Candida albicans and human erythrocytes were treated with liposomal amphotericin B (AmB) obtained by incubation of free AmB with small unilamellar vesicles (SUV) composed of unsaturated fatty acyl chains phosphatidylcholine (egg-yolk PC). Cellular effects were determined by changes in the K+ internal content of cells and in the number of colonies formed by fungal cells or as hemolysis, measured as a decrease in haemoglobin retention by erythrocytes. Dose-response curves were obtained by two procedures: either the ratio of AmB to phospholipids was kept constant over the AmB concentration range used (R = 10(-2] or the phospholipid concentration was kept constant (C = 0.2 mM) and the concentration of AmB varied. The liposomal preparations of AmB were as active against fungi as AmB in dimethylsulfoxide but less active (internal K+ changes) or inactive (hemolysis) against erythrocytes. On the other hand the binding of AmB to the SUV, as a function of the AmB concentration, was monitored by circular dichroism, fluorescence and UV absorption, in the two conditions used for the cellular studies. The amount of AmB bound when the total concentration of antibiotic was 2.10(-7) M was very low but increased with concentration and reached 90% at 10(-5) M. In all the assays we used, the anticellular effects could be attributed to the levels of AmB remaining free (unbound to the lipids). The variations of these levels with total AmB concentration could therefore explain the increased selectivity of liposomal AmB in toxicity against fungi and erythrocytes as compared to that of AmB added as a solution in dimethylsulfoxide. Indeed fungal cells are sensitive to low concentrations of AmB in dimethylsulfoxide; at these concentrations, in liposomal preparations, AmB is not bound to phospholipids and therefore as active as if added in dimethylsulfoxide. By contrast erythrocytes are only sensitive to much higher concentrations of AmB in dimethylsulfoxide; at these concentrations AmB is almost totally bound to phospholipids and therefore much less active.

Liposome-encapsulated-amikacin therapy of Mycobacterium avium complex infection in beige mice

Efficacy of liposome-encapsulated amikacin and free amikacin against Mycobacterium avium complex was evaluated in the beige mouse (C57BL/6J-bgJ/bgJ) acute infection model. Approximately 10(7) viable M. avium complex serotype 1 cells for which the MIC of amikacin was 8 micrograms/ml were given intravenously. Treatment was started with encapsulated or free amikacin at approximately 110 or 40 mg/kg of body weight 7 or 14 days later. In the former experiment, treatment was given two or three times per week. In the latter experiment, treatment was given daily for 5 days. The animals were sacrificed 5 days after the last dose. Liver, spleen, and lung were homogenized, and viable cell counts were determined on 7H10 agar. An analysis of variance and subsequent Tukey HSD (honestly significant difference) tests indicated that both encapsulated and free amikacin significantly reduced viable cell counts in each of the organs compared with counts in the control group. Compared with free amikacin, encapsulated amikacin significantly reduced viable cell counts in the liver and spleen. Liposome encapsulation of an active agent appears to be a promising therapeutic approach to M. avium complex infection.