Long-circulating immunoliposomal amphotericin B against invasive pulmonary aspergillosis in mice

DectiSomes: C-type lectin receptor-targeted liposomes as pan-antifungal drugs

Combatting the ever-increasing threat from invasive fungal pathogens faces numerous fundamental challenges, including constant human exposure to large reservoirs of species in the environment, the increasing population of immunocompromised or immunosuppressed individuals, the unsatisfactory efficacy of current antifungal drugs and their associated toxicity, and the scientific and economic barriers limiting a new antifungal pipeline. DectiSomes represent a new drug delivery platform that enhances antifungal efficacy for diverse fungal pathogens and reduces host toxicity for current and future antifungals. DectiSomes employ pathogen receptor proteins - C-type lectins - to target drug-loaded liposomes to conserved fungal cognate ligands and away from host cells. DectiSomes represent one leap forward for urgently needed effective pan-antifungal therapy. Herein, we discuss the problems of battling fungal diseases and the state of DectiSome development.

Multicompartment systems: A putative carrier for combined drug delivery and targeting

In this review, we discuss recent developments in multicompartment systems commonly referred to as vesosomes, as well as their method of preparation, surface modifications, and clinical potential. Vesosomal systems are able to entrap more than one drug moiety and can be customized for site-specific delivery. We focus in particular on the possible reticuloendothelial system (RES) - mediated accumulation of vesosomes, and their application in tumor targeting, as areas for further investigation.

Antifungal Liposomes Directed by Dectin-2 Offer a Promising Therapeutic Option for Pulmonary Aspergillosis

Invasive fungal diseases cause millions of deaths each year. There are currently approximately 300,000 acute cases of aspergillosis, most of which result from a pulmonary infection of immunocompromised patients by the common soil organism and opportunistic pathogen Aspergillus fumigatus. Patients are treated with antifungal drugs, such as amphotericin B (AmB). However, AmB has serious limitations due to human organ toxicity. AmB is slightly less toxic if loaded in liposomes, such as AmBisome or AmB-loaded liposomes (AmB-LLs). Even with antifungal therapy, recurrent infections are common, and 1-year fatality rates may exceed 50%. We have previously shown that coating AmB-LLs with the extracellular oligomannan-binding domain of the C-type lectin receptor Dectin-2 (DEC2-AmB-LLs) effectively targets DEC2-AmB-LLs to cell walls, exopolysaccharide matrices, and biofilms of fungal pathogens in vitro. In vitro, DEC2-AmB-LLs reduce the effective dose of AmB for 95% inhibition and killing of A. fumigatus 10-fold compared to that of untargeted AmB-LLs. Herein we tested the antifungal activity of DEC2-AmB-LLs relative to that of untargeted AmB-LLs in immunosuppressed mice with pulmonary aspergillosis. Remarkably, DEC2-AmB-LLs bound 30-fold more efficiently to A. fumigatus at sites of infection in the lungs. Furthermore, Dectin-2-targeted liposomes delivering AmB at a dose of 0.2 mg/kg of body weight significantly reduced the fungal burden in lungs compared to results with untargeted AmB-LLs at 0.2 mg/kg and micellar voriconazole at 20 mg/kg and prolonged mouse survival. By dramatically increasing the efficacy of antifungal drugs at low doses, targeted liposomes have the potential to create a new clinical paradigm to treat diverse fungal diseases.

Surface engineering of nanomaterials with phospholipid-polyethylene glycol-derived functional conjugates for molecular imaging and targeted therapy

In recent years, phospholipid-polyethylene glycol-derived functional conjugates have been widely employed to decorate different nanomaterials, due to their excellent biocompatibility, long blood circulation characteristics, and specific targeting capability. Numerous in vivo studies have demonstrated that nanomedicines peripherally engineered with phospholipid-polyethylene glycol-derived functional conjugates show significantly increased selective and efficient internalization by target cells/tissues. Targeting moieties including small-molecule ligands, peptides, proteins, and antibodies are generally conjugated onto PEGylated phospholipids to decorate liposomes, micelles, hybrid nanoparticles, nanocomplexes, and nanoemulsions for targeted delivery of diagnostic and therapeutic agents to diseased sites. In this review, the synthesis methods of phospholipid-polyethylene glycol-derived functional conjugates, biophysicochemical properties of nanomedicines decorated with these conjugates, factors dominating their targeting efficiency, as well as their applications for in vivo molecular imaging and targeted therapy were summarized and discussed.

Dectin-1-Targeted Antifungal Liposomes Exhibit Enhanced Efficacy

Aspergillus species cause pulmonary invasive aspergillosis resulting in nearly 100,000 deaths each year. Patients at the greatest risk of developing life-threatening aspergillosis have weakened immune systems and/or various lung disorders. Patients are treated with antifungals such as amphotericin B (AmB), caspofungin acetate, or triazoles (itraconazole, voriconazole, etc.), but these antifungal agents have serious limitations due to lack of sufficient fungicidal effect and human toxicity. Liposomes with AmB intercalated into the lipid membrane (AmB-LLs; available commercially as AmBisome) have severalfold-reduced toxicity compared to that of detergent-solubilized drug. However, even with the current antifungal therapies, 1-year survival among patients is only 25 to 60%. Hence, there is a critical need for improved antifungal therapeutics. Dectin-1 is a mammalian innate immune receptor in the membrane of some leukocytes that binds as a dimer to beta-glucans found in fungal cell walls, signaling fungal infection. Using a novel protocol, we coated AmB-LLs with Dectin-1's beta-glucan binding domain to make DEC-AmB-LLs. DEC-AmB-LLs bound rapidly, efficiently, and with great strength to Aspergillus fumigatus and to Candida albicans and Cryptococcus neoformans, highly divergent fungal pathogens of global importance. In contrast, untargeted AmB-LLs and bovine serum albumin (BSA)-coated BSA-AmB-LLs showed 200-fold-lower affinity for fungal cells. DEC-AmB-LLs reduced the growth and viability of A. fumigatus an order of magnitude more efficiently than untargeted control liposomes delivering the same concentrations of AmB, in essence decreasing the effective dose of AmB. Future efforts will focus on examining pan-antifungal targeted liposomal drugs in animal models of disease.IMPORTANCE The fungus Aspergillus fumigatus causes pulmonary invasive aspergillosis resulting in nearly 100,000 deaths each year. Patients are often treated with antifungal drugs such as amphotericin B (AmB) loaded into liposomes (AmB-LLs), but all antifungal drugs, including AmB-LLs, have serious limitations due to human toxicity and insufficient fungal cell killing. Even with the best current therapies, 1-year survival among patients with invasive aspergillosis is only 25 to 60%. Hence, there is a critical need for improved antifungal therapeutics. Dectin-1 is a mammalian protein that binds to beta-glucan polysaccharides found in nearly all fungal cell walls. We coated AmB-LLs with Dectin-1 to make DEC-AmB-LLs. DEC-AmB-LLs bound strongly to fungal cells, while AmB-LLs had little affinity. DEC-AmB-LLs killed or inhibited A. fumigatus 10 times more efficiently than untargeted liposomes, decreasing the effective dose of AmB. Dectin-1-coated drug-loaded liposomes targeting fungal pathogens have the potential to greatly enhance antifungal therapeutics.

Rodent Models of Invasive Aspergillosis due to Aspergillus fumigatus: Still a Long Path toward Standardization

Invasive aspergillosis has been studied in laboratory by the means of plethora of distinct animal models. They were developed to address pathophysiology, therapy, diagnosis, or miscellaneous other concerns associated. However, there are great discrepancies regarding all the experimental variables of animal models, and a thorough focus on them is needed. This systematic review completed a comprehensive bibliographic analysis specifically-based on the technical features of rodent models infected with Aspergillus fumigatus. Out the 800 articles reviewed, it was shown that mice remained the preferred model (85.8% of the referenced reports), above rats (10.8%), and guinea pigs (3.8%). Three quarters of the models involved immunocompromised status, mainly by steroids (44.4%) and/or alkylating drugs (42.9%), but only 27.7% were reported to receive antibiotic prophylaxis to prevent from bacterial infection. Injection of spores (30.0%) and inhalation/deposition into respiratory airways (66.9%) were the most used routes for experimental inoculation. Overall, more than 230 distinct A. fumigatus strains were used in models. Of all the published studies, 18.4% did not mention usage of any diagnostic tool, like histopathology or mycological culture, to control correct implementation of the disease and to measure outcome. In light of these findings, a consensus discussion should be engaged to establish a minimum standardization, although this may not be consistently suitable for addressing all the specific aspects of invasive aspergillosis.

Uptake and intracellular processing of PEG-liposomes and PEG-immunoliposomes by kupffer cells in vitro 1 *

Specific targeting of drugs to for instance tumors or sites of inflammation may be achieved by means of immunoliposomes carrying site-specific antibodies on their surface. The presence of these antibodies may adversely affect the circulation kinetics of such liposomes as a result of interactions with cells of the mononuclear phagocyte system (MPS), mainly represented by macrophages in liver and spleen. The additional insertion of poly(ethylene glycol) chains on the surface of the immunoliposomes may, however, attenuate this effect. We investigated the influence of surface-coupled rat or rabbit antibodies and of PEG on the uptake of liposomes by rat Kupffer cells in culture with (3)H-cholesteryloleyl ether as a metabolically stable marker. Additionally, we assessed the effects of surface-bound IgG and PEG on the intracellular processing of the liposomes by the Kupffer cells, based on a double-label assay using the (3)H-cholesteryl ether as an absolute measure for liposome uptake and the hydrolysis of the degradable marker cholesteryl-(14)C-oleate as relative measure of degradation. Attachment of both rat and rabbit antibodies to PEG-free liposomes caused a several-fold increase in apparent size. The uptake by Kupffer cells, however, was 3-4 fold higher for the rat than for the rabbit IgG liposomes. The presence of PEG drastically reduced the difference between these liposome types. Uptake of liposomes without antibodies amounted to only about 10% (non-PEGylated) or less (PEGylated) of that of the immunoliposomes. In contrast to the marked effects of IgG and PEG on Kupffer cell uptake, the rate of intracellular processing of the liposomes remained virtually unaffected by the presence of these substances on the liposomal surface. These observations are discussed with respect to the design of optimally formulated liposomal drug preparations, combining maximal therapeutic efficacy with minimal toxicity.

Application of poly(ethylene glycol)-distearoylphosphatidylethanolamine (PEG-DSPE) block copolymers and their derivatives as nanomaterials in drug delivery

Poly(ethylene glycol)–distearoylphosphatidylethanolamine (PEG-DSPE) block copolymers are biocompatible and amphiphilic polymers that can be widely utilized in the preparation of liposomes, polymeric nanoparticles, polymer hybrid nanoparticles, solid lipid nanoparticles, lipid–polymer hybrid nanoparticles, and microemulsions. Particularly, the terminal groups of PEG can be activated and linked to various targeting ligands, which can prolong the circulation time, improve the drug bioavailability, reduce undesirable side effects, and especially target specific cells, tissues, and even the intracellular localization in organelles. This review herein aims to describe recent developments in drug carriers exploiting PEG-DSPE block copolymers and their derivatives, and the incorporation of different ligands to the end groups of PEG-DSPE to target delivery, focusing on their modification approaches, advantages, applications, and the probable associated drawbacks.

Efficacy of combination antifungal therapy with intraperitoneally administered micafungin and aerosolized liposomal amphotericin B against murine invasive pulmonary aspergillosis

Targeted intrapulmonary delivery of drugs may reduce systemic toxicity and improve treatment efficacy. In the current study, we evaluated the effects of a combination treatment consisting of inhalation of aerosolized liposomal amphotericin B (L-AMB) with intraperitoneal administration of micafungin (MCFG) against murine invasive pulmonary aspergillosis. The combination of aerosolized L-AMB with intraperitoneal MCFG significantly improved the survival rate, and the fungal burdens and histopathology findings after this treatment were superior to those of the control and both monotherapy groups.

Delivery strategies to target therapies to inflammatory tissue

BACKGROUND

Inflammation plays a key role in many chronic disease processes as well as an acute role in injury and wound healing. Various cell types are recruited from the bloodstream to the inflamed site through adhesion molecules, cytokines, chemokines and others.

OBJECTIVES

This review examines many drug-targeting strategies that make use of these molecules or signaling pathways, and seeks to describe certain commonalities irrespective of the disease process or agent to be delivered.

METHODS

A survey of the literature, primarily within the last year, was performed. Search words included 'drug targeting' and 'inflammation' and of those, the scope was refined to include those studies that specifically sought to modify or ameliorate an aspect of the inflammatory process in the treatment of a disease.

RESULTS/CONCLUSION

Inflammation plays a key role in many diseases, and many similar targets (such as adhesion molecules) are the focus of the treatment of those diseases.

Optimizing efficacy of Amphotericin B through nanomodification

The polyene antibiotic Amphotericin B (AMB) is one of the first therapeutic agents to be marketed commercially as nanosized formulations in which the drug is associated with lipids as liposomes or complexes. In this way, its renal toxicity is reduced and its therapeutic index improved. This review summarizes the particular properties of AMB which justify this type of formulation and the early work leading up to their development. The clinical results obtained in the treatment of fungal infections are reviewed and their activity against leishmaniasis is also evoked. Some newer formulations of AMB, based on both lipids and polymers are described. In particular, their potential by the oral and pulmonary routes are discussed. Finally, the development of targeted systems to deliver the drug to specific cells and tissues is considered.

Optimizing efficacy of amphotericin B through nanomodification

Fungal infections and leishmaniasis are an important cause of morbidity and mortality in immunocompromised patients. The macrolide polyene antibiotic amphotericin B (AmB) has long been recognized as a powerful fungicidal and leishmanicidal drug. A conventional intravenous dosage form of AmB, AmB- deoxycholate (Fungizone or D-AmB), is the most effective clinically available for treating fungal and parasitic (leishmaniasis) infections. However, the clinical efficacy of AmB is limited by its adverse effects mainly nephrotoxicity. Efforts to lower the toxicity are based on synthesis of AmB analogues such as AmB esters or preparation of AmB-lipid associations in the forms of liposomal AmB (L-AmB or AmBisome), AmB lipid complex (Abelcet or ABLC), AmB colloidal dispersion (Amphocil or ABCD), and intralipid AmB. These newer formulations are substantially more expensive, but allow patients to receive higher doses for longer periods of time with decreased renal toxicity than conventional AmB. Modifications of liposomal surface in order to avoid RES uptake, thus increased targetability has been attempted. Emulsomes and other nanoparticles are special carrier systems for intracellular localization in macrophage rich organs like liver and spleen. Injectable nano-carriers have important potential applications as in site-specific drug delivery.

Hydroxylated analogues of the orally active broad spectrum antifungal, Sch 51048 (1), and the discovery of posaconazole [Sch 56592; 2 or (S,S)-5]

As part of a detailed study, the syntheses, biological activities, and pharmacokinetic properties of hydroxylated analogues of the previously described broad spectrum antifungal agents, Sch 51048 (1), Sch 50001 (3), and Sch 50002 (4), are described. Based on an overall superior profile, one of the alcohols, Sch 56592 (2), was selected for clinical studies.

Monoclonal immunoglobulin G1 directed against Aspergillus fumigatus cell wall glycoprotein protects against experimental murine aspergillosis

Most of the biological functions related to pathogenicity and virulence reside in the fungal cell wall, which, being the outermost part of the cell, mediates the host-fungus interplay. For these reasons much effort has focused on the discovery of useful inhibitors of cell wall glucan, chitin, and mannoprotein biosynthesis. In the absence of a wide-spectrum, safe, and potent antifungal agent, a new strategy for antifungal therapy is directed towards the development of monoclonal antibodies (MAbs). In the present study the MAb A9 (immunoglobulin G1 [IgG1]) was identified from hybridomas raised in BALB/c mice immunized with cell wall antigen of Aspergillus fumigatus. The immunoreactive epitopes for this IgG1 MAb appeared to be associated with a peptide moiety, and indirect immunofluorescence microscopy revealed its binding to the cell wall surface of hyphae as well as with swollen conidia. MAb A9 inhibited hyphal development as observed by MTT [3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay (25.76%), reduced the duration of spore germination, and exerted an in vitro cidal effect against Aspergillus fumigatus. The in vivo protective efficacy of MAb A9 was also evaluated in a murine model of invasive aspergillosis, where a reduction in CFU (>4 log(10) units) was observed in kidney tissue of BALB/c mice challenged with A. fumigatus (2 x 10(5) CFU/ml) and where enhanced mean survival times (19.5 days) compared to the control (7.1 days) and an irrelevant MAb (6.1 days) were also observed.

Pharmacological agents in development for invasive aspergillosis

The urgent medical need for new potent antifungal agents in the management of invasive aspergillosis (IA) has resulted in the development of several compounds which may be of value in the future for the treatment or prophylaxis of IA. In the past years, several novel types of drugs have been discovered and developed, some of which are already in late-stage clinical trials and ready to enter the market. This paper discusses the antifungal agents, classified by their mode of action, that are currently available and the agents which are still in development for treatment or prevention of IA.

Long-circulating sterically stabilized liposomes as carriers of agents for treatment of infection or for imaging infectious foci

Liposomes are considered as potential carriers for biologically active compounds. One evident drawback of 'classical' liposomes is their fast elimination by cells of the mononuclear phagocyte system (MPS), primarily by liver and spleen. An important breakthrough in this respect is the development of long-circulating liposomes among which liposomes coated with polyethyleneglycol (PEG), the so-called 'sterically stabilized' liposomes (SSL). An important characteristic of SSL is that their prolonged blood residence time and infectious target localization is relatively independent of the lipid dose, particle size or lipid composition of the bilayer. SSL are applied as carriers of antimicrobial agents to achieve infectious target localization, to reduce side effects, or to serve as a micro-reservoir in the circulation. In addition, radiolabelled SSL are used to image infectious and inflammatory foci.

Synthesis and characterisation of a new amphotericin B-methoxypoly(ethylene glycol) conjugate

The reaction of methoxypoly(ethylene glycol)-4-nitrophenyl carbonate with amphotericin B has been used to prepare a new conjugate of amphotericin B (mPEG-AmB). A preliminary screening of in vitro antifungal activity has suggested that mPEG-AmB possesses a similar effect and a similar spectrum of activity as the conventional amphotericin B formulated with sodium desoxycholate.

The role of murine models in the development of antifungal therapy for systemic mycoses

Animal testing is crucial to the development of new antifungal compounds. This review describes the role that murine and other animal models have played in the development of three classes of antifungal agents: the polyenes, the triazoles and the echinocandins and the ways in which these models have been either the positive link in the path from in vitro studies to the patient, or have foreclosed later clinical evaluation. Efficacy studies in particular mycoses are discussed, as well as studies designed to determine whether combinations of antifungal drugs may have value over single agents. Copyright 2000 Harcourt Publishers Ltd.

Amphotericin B preparations: a maximum tolerated dose in severe invasive fungal infections?

Availability of lipid formulations of amphotericin B has opened up the possibility of treating invasive fungal infections in immunocompromised patients with high doses of this antifungal agent. Evidence is emerging to suggest that lipid formulations may have heightened efficacy compared to conventional amphotericin B. The issue of optimal dosage has been a neglected area. This article reviews published data accrued from clinical, open-label, salvage, and other studies, and finds little support that the use of high doses of lipid formulations are more efficacious than lower doses. The response rates for invasive fungal infection from most studies are predictably around 56%, irrespective of the lipid formulation and dose used. Animal models provide evidence that low doses of a lipid formulation are as successful in reducing fungal dissemination and in prolonging survival as higher doses, although concomitant tissue fungal eradication is not as effectively achieved by the lower doses (survival-mycologic eradication dissociation). Kinetic studies performed in the clinically relevant setting of critically ill patients give further support to the use of low doses, since levels of liposomal amphotericin B at all dosages between 1 and 4 mg/kg/day are similar and above maximum inhibitory concentrations for commonly encountered fungi. There has only been one prospective randomised study designed with the primary end-point of comparing two dosages of an amphotericin B lipid formulation on clinical response and survival. That European Organization for Research and Treatment of Cancer (EORTC) study concluded that liposomal amphotericin B given at 1 mg was as efficacious as 4 mg/kg/day in treating neutropenic patients with invasive pulmonary aspergillosis. There are a multitude of unanswered questions concerning dosing, and their answers are confounded by difficulties in performing clinical trials and the multiplicity of factors other than antifungal chemotherapy that influence outcome. Maximum tolerated dose studies using existing lipid formulations, or perhaps with the newer formulations such as pegylated immunoliposomal amphotericin B, could be performed to shed light on this difficult area.

Spectroscopic investigation of the molecular state of nystatin encapsulated in liposomes

The stability and spectral properties of nystatin-encapsulating liposomes, composed of various combinations of dipalmitoyl phosphatidylcholine (DPPC), cholesterol (CH) and distearoyl-N-(monomethoxy poly(ethylene glycol)succinyl) phosphatidylethanolamine (DSPE-PEG), were studied in order to elucidate the molecular state and localization of nystatin encapsulated in liposomes. Localization of nystatin at the surface region of the liposomal membrane was investigated by PEG/dextran two-phase partition and measurement of the fluorescence quenching of nystatin by p-xylene-bis-pyridinium bromide (DPX). In DPPC/DSPE-PEG liposomes and DPPC/CH/DSPE-PEG liposomes, containing 151 and 160 mcg nystatin per mg lipid, respectively, nystatin appeared to be present at the surface region of the liposomal membranes. Self-quenching of nystatin fluorescence was observed in DPPC/CH and DPPC/CH/DSPE-PEG liposomes even at low encapsulated amounts, suggesting the localization of nystatin in CH-incorporating membranes. In CH-free liposomes, nystatin molecules were at first delocalized in the membranes and then self-associated at a higher level of encapsulation. Absorption and circular dichroism (CD) spectra were also measured to examine the monomeric and aggregated states of nystatin in liposomes. High encapsulation efficacy was observed in DPPC and DPPC/DSPE-PEG liposomes, but the highest stability and retention of nystatin in liposomes were observed in DPPC/CH/DSPE-PEG liposomes, evaluated in terms of the nystatin and calcein release from nystatin-encapsulating liposomes in vitro. From the results, possible encapsulation mechanisms of nystatin in liposomes narrowed down to the following three points; interaction with lipid membrane, adsorption on the liposomal surface and complex formation with DSPE-PEG.

Therapeutic applications of colloidal drug carriers

Colloidal drug carriers such as liposomes and nanoparticles can be used to improve the therapeutic index of both established and new drugs by modifying their distribution, and thus increasing their efficacy and/or reducing their toxicity. This is because the drug distribution then follows that of the carrier, rather than depending on the physicochemical properties of the drug itself. If these delivery systems are carefully designed with respect to the target and the route of administration, they may provide one solution to some of the delivery problems posed by new classes of active molecules, such as peptides and proteins, genes and oligonucleotides. They may also offer alternative modes for more conventional drugs, such as highly hydrophobic small molecules. This review discusses the use of colloidal, particulate carrier systems (25 nm to 1 µm in diameter) in such applications.

Efficacy of liposomal amphotericin B with prolonged circulation in blood in treatment of severe pulmonary aspergillosis in leukopenic rats

The therapeutic efficacy of long-circulating polyethylene glycol-coated liposomal amphotericin B (AMB) (PEG-AMB-LIP) was compared with that of AMB desoxycholate (Fungizone) in a model of severe invasive pulmonary aspergillosis in persistently leukopenic rats as well as in temporarily leukopenic rats. PEG-AMB-LIP treatment (intravenous administration) consisted of a single, or double (every 72 h), or triple (every 72 h) dose of 10 mg of AMB/kg of body weight, a double dose (every 72 h) of 14 mg of AMB/kg, or a 5-day treatment (every 24 h) with 6 mg/kg/dose. AMB desoxycholate was administered for 10 consecutive days at 1 mg of AMB/kg/dose. Treatment was started 30 h after fungal inoculation, at which time mycelial growth was firmly established. Both persistently and temporarily leukopenic rats died between 4 and 9 days after Aspergillus fumigatus inoculation when they were left untreated or after treatment with a placebo. In persistently leukopenic rats, a single dose of PEG-AMB-LIP (10 mg/kg) was as effective as the 10-day treatment with AMB desoxycholate (at 1 mg/kg/dose) in significantly prolonging the survival of rats infected with A. fumigatus and in reducing the dissemination of A. fumigatus to the liver. Prolongation of PEG-AMB-LIP treatment (double or triple dose or 5-day treatment) did not further improve efficacy. For temporarily leukopenic rats no major advances in efficacy were achieved compared to those for persistently leukopenic rats, probably because the leukocyte numbers in blood were restored too late in the course of infection.

Molecular localization and state of amphotericin B in PEG liposomes

We investigated the molecular localization and state of amphotericin B (AmB) encapsulated in polyethylene glycol (PEG)-coated liposomes. AmB-encapsulating PEG-liposomes composed of dipalmitoylphosphatidylcholine (DPPC), cholesterol (CH) and distearoyl-N-(monomethoxy poly(ethylene glycol)succinyl) phosphatidylethanolamine (DSPE-PEG, average MW of the PEG chain 2000) were prepared by hydration with 9% sucrose solution and extrusion. The amount of AmB encapsulated in the liposomes increased with incorporation of DSPE-PEG and decreased with that of CH. The molecular localization and state of AmB were investigated by PEG/dextran two-phase partition, potassium permeability measurement, fluorescence quenching measurement and circular dichroism (CD) spectroscopy. The results suggest that there are two types of AmB localization in PEG-liposomes, one of which corresponds to the complex of AmB with DSPE-PEG on the membrane surface, while the other corresponds to the pore form of AmB in the hydrophobic core of the liposomal membrane. AmB in PEG liposomes was present in both aggregated and monomeric states.

Possibility of active targeting to tumor tissues with liposomes

In terms of active targeting by immunoliposomes, two anatomical compartments are considerable for targeting sites. One is located a readily accessible site in intravascular, and another is a much less accessible target site located in the extravascular. However, it was made clear that the active targeting with immunoliposomes is determined by two kinetically competing processes, such as binding to the target site and uptake by the RES. To overcome these contradictions, we have designed a new type of long-circulating immunoliposome, which was PEG-immunoliposome attached antibodies at the distal end of PEG chain, so called the pendant type immunoliposome. The pendant type immunoliposome showed much higher targetability than the ordinary immunoliposomes to both targeting sites of lung endothelial cells and solid tumor tissue. This is due to the free PEG chains (not linked to the antibody) effectively avoiding the RES uptake of liposomes, resulting in elevated the blood concentration and enhanced the target binding of immunoliposomes. The presence of free PEG does not interfere with the binding of the terminally linked antibody to the antigen. For targeting to the vascular endothelial surface in the lung, 34A antibody, which is highly specific to mouse pulmonary endothelial cells, was conjugated to make the pendant type immunoliposomes (34A-PEG-ILP). 34A-PFG-ILP showed significantly higher targeting degree than the ordinary type of immunoliposomes. For targeting to the solid tumor tissue, Fab' fragment of 21B2 antibody which is anti-human CFA and transferrin (TF) were used. Both pendant type immunoliposomes (Fab'-PFG-ILP and TF-PEG-ILP) showed the low RES uptake and the long circulation time, and resulted in enhanced accumulation of the liposomes in the solid tumor. TF-PEG-ILP was internalized into tumor cells with receptor mediated endocytosis, after extravasation into tumor tissue. The pendant type immunoliposome can escape from the gaps between adjacent endothelial cells and openings at the vessel termini during tumor angiogenesis by passive convective transport much rather than ligand directed targeting. Active targeting to tumor tissue with the pendant type immunoliposome is particularly important for many highly toxic anticancer drugs for cancer chemotherapy. An ultimate goal of pendant type immunoliposome is the incorporation of a fusogenic molecule that would induce fusion of liposome following their binding to the target cells or their internalization by endocytosis. Such liposomal formulations should be useful for endocytotic internalization of plasmid DNA and other bioactive materials.

Encapsulation characteristics of nystatin in liposomes: effects of cholesterol and polyethylene glycol derivatives

In this study, we characterized the encapsulation of amphipathic nystatin into liposomes with or without cholesterol (CH) and a polyethylene glycol derivative, distearoyl-N-(monomethoxy poly(ethylene glycol)succinyl)phosphatidylethanolamine (DSPE-PEG). The highest encapsulation efficacy of nystatin into liposomes (151 microg nystatin/mg lipid) was obtained with a cholesterol-free lipid composition containing 6 mol% of DSPE-PEG. The encapsulation efficacy was decreased by the incorporation of CH and improved by the incorporation of DSPE-PEG. In liposomes composed of dipalmitoylphosphatidylcholine (DPPC)/CH (2:1, mol/mol), the highest encapsulation efficacy of nystatin liposomes (84 microg/mg lipid) was achieved by the addition of DSPE-PEG and hydration with 9% sucrose solution, as compared with 13 microg/mg lipid without DSPE-PEG. The encapsulated amount increased with increasing amount of DSPE-PEG used and plateaued at 6 mol% of DSPE-PEG. The optimum molecular weight of PEG in DSPE-PEG was 2000 and a larger molecular weight resulted in lower encapsulation. The incorporation of CH affected the self-association of nystatin with lipid membranes, which was detected by fluorescence measurement. The molecular interaction between an amino group in nystatin and a phosphate group in DSPE-PEG plays an important role in efficient encapsulation of nystatin. Finally, the encapsulation characteristics of nystatin were compared with those of amphotericin B (AmB). Nystatin more readily associated with CH-free lipid membranes, but, AmB more readily interacted with DSPE-PEG. The results indicated that the differences in the molecular association of AmB or nystatin with lipids or DSPE-PEG are reflected in the encapsulation characteristics in liposomes.

Aspergillus fumigatus and aspergillosis

Aspergillus fumigatus is one of the most ubiquitous of the airborne saprophytic fungi. Humans and animals constantly inhale numerous conidia of this fungus. The conidia are normally eliminated in the immunocompetent host by innate immune mechanisms, and aspergilloma and allergic bronchopulmonary aspergillosis, uncommon clinical syndromes, are the only infections observed in such hosts. Thus, A. fumigatus was considered for years to be a weak pathogen. With increases in the number of immunosuppressed patients, however, there has been a dramatic increase in severe and usually fatal invasive aspergillosis, now the most common mold infection worldwide. In this review, the focus is on the biology of A. fumigatus and the diseases it causes. Included are discussions of (i) genomic and molecular characterization of the organism, (ii) clinical and laboratory methods available for the diagnosis of aspergillosis in immunocompetent and immunocompromised hosts, (iii) identification of host and fungal factors that play a role in the establishment of the fungus in vivo, and (iv) problems associated with antifungal therapy.

Enhanced encapsulation of amphotericin B into liposomes by complex formation with polyethylene glycol derivatives

PURPOSE

A highly efficient method was developed for the encapsulation of amphotericin B (AmB) in liposomes, and the mechanism involved was characterized.

METHODS

AmB was encapsulated in dipalmitoylphosphatidylcholine/ cholesterol (DPPC/CH, 2:1) liposomes after complex formation with distearoyl-N-(monomethoxy poly(ethylene glycol)succinyl) phosphatidylethanolamine (DSPE-PEG). Hydration of lipids was done with 9% sucrose solution.

RESULTS

The encapsulated amount of AmB was 111 microg/mg lipid, which was much higher than that obtained by the same method without DSPE-PEG (14 microg/mg lipid). The amount encapsulated increased with amount of DSPE-PEG used and with PEG molecular weight. Encapsulation efficacy was also influenced by the type of PEG derivatives used and by the modification of AmB, suggesting the involvement of complex formation between AmB and DSPE-PEG. Absorption and 31P-NMR spectral analyses indicated that interactions between the amino and phosphate groups and between the polyene and PEG moieties in AmB and DSPE-PEG, respectively, play an important role in the complex formation.

CONCLUSIONS

Complex formation of AmB with DSPE-PEG allows the highly efficient encapsulation of the drug in liposomes. This simple technique should be applicable to other hydrophobic drugs.