Interaction of the polyene antibiotic amphotericin B with model membranes: differences between small and large unilamellar vesicles

Amphotericin B assembles into seven-molecule ion channels: An NMR and molecular dynamics study

Amphotericin B, an antifungal drug with a long history of use, forms fungicidal ion-permeable channels across cell membranes. Using solid-state nuclear magnetic resonance spectroscopy and molecular dynamics simulations, we experimentally elucidated the three-dimensional structure of the molecular assemblies formed by this drug in membranes in the presence of the fungal sterol ergosterol. A stable assembly consisting of seven drug molecules was observed to form an ion conductive channel. The structure is somewhat similar to the upper half of the barrel-stave model proposed in the 1970s but substantially different in the number of molecules and in their arrangement. The present structure explains many previous findings, including structure-activity relationships of the drug, which will be useful for improving drug efficacy and reducing adverse effects.

An Amphotericin B Derivative Equally Potent to Amphotericin B and with Increased Safety

Amphotericin B is the most potent antimycotic known to date. However due to its large collateral toxicity, its use, although long standing, had been limited. Many attempts have been made to produce derivatives with reduced collateral damage. The molecular mechanism of polyene has also been closely studied for this purpose and understanding it would contribute to the development of safe derivatives. Our study examined polyene action, including chemical synthesis, electrophysiology, pharmacology, toxicology and molecular dynamics. The results were used to support a novel Amphotericin B derivative with increased selectivity: L-histidine methyl ester of Amphotericin B. We found that this derivative has the same form of action as Amphotericin B, i.e. pore formation in the cell membrane. Its reduced dimerization in solution, when compared to Amphotericin B, is at least partially responsible for its increased selectivity. Here we also present the results of preclinical tests, which show that the derivative is just as potent as Amphotericin B and has increased safety.

High-throughput and sensitive assay for amphotericin B interaction with lipid membrane on the model membrane systems by surface plasmon resonance

In the present study, we developed a high-throughput and sensitive assay for interactions of amphotericin B (AmB) with two model lipid membranes, which mimicked mammal cell membrane and fungal membrane using surface plasmon resonance (SPR). The binding kinetics of AmB to the membrane could be analyzed by multiple sensorgrams obtained at different AmB concentrations, indicating that the binding properties could be clarified for an approximately 7-fold concentration range. AmB showed an approximately 18-fold higher affinity for ergosterol-containing membrane than for cholesterol-containing membrane. We also optimized the procedure for the reproducible immobilization of liposome containing the sterols and the estimation of binding kinetics of AmB to the lipid membranes, and the sensitivity of AmB to membrane interaction was 20-fold higher, compared with the reported method. The throughput of the established method for the binding kinetics characterization was calculated to be 10 compounds a day. The results demonstrate that the established SPR method could be a valuable tool for predicting selective binding to sterol-containing membranes.

Synthesis-enabled functional group deletions reveal key underpinnings of amphotericin B ion channel and antifungal activities

Amphotericin B is the archetype for small molecules that form transmembrane ion channels. However, despite extensive study for more than five decades, even the most basic features of this channel structure and its contributions to the antifungal activities of this natural product have remained unclear. We herein report that a powerful series of functional group-deficient probes have revealed many key underpinnings of the ion channel and antifungal activities of amphotericin B. Specifically, in stark contrast to two leading models, polar interactions between mycosamine and carboxylic acid appendages on neighboring amphotericin B molecules are not required for ion channel formation, nor are these functional groups required for binding to phospholipid bilayers. Alternatively, consistent with a previously unconfirmed third hypothesis, the mycosamine sugar is strictly required for promoting a direct binding interaction between amphotericin B and ergosterol. The same is true for cholesterol. Synthetically deleting this appendage also completely abolishes ion channel and antifungal activities. All of these results are consistent with the conclusion that a mycosamine-mediated direct binding interaction between amphotericin B and ergosterol is required for both forming ion channels and killing yeast cells. The enhanced understanding of amphotericin B function derived from these synthesis-enabled studies has helped set the stage for the more effective harnessing of the remarkable ion channel-forming capacity of this prototypical small molecule natural product.

Direct interaction between amphotericin B and ergosterol in lipid bilayers as revealed by 2H NMR spectroscopy

Although amphotericin B (AmB) is thought to exert its antifungal activity by forming transmembrane ion-permeable self-assemblies together with ergosterol, no previous study has directly proven AmB-ergosterol interaction. To establish the interaction, we measured (2)H NMR using deuterium-labeled sterols and AmB. The (2)H NMR spectra of deuterated ergosterol in palmitoyloleoylphosphatidylcholine (POPC) bilayers showed that fast axial diffusion of erogosterol was almost completely inhibited by the coexistence of AmB. Conversely, cholesterol mobility in POPC membrane was essentially unchanged with or without AmB. These results unequivocally demonstrate that ergosterol has significant interaction with AmB in POPC bilayers. In addition, we examined the mobility of AmB using deuterium-labeled AmB, and found that, although AmB is almost immobilized in sterol-free and cholesterol-containing POPC membranes, a certain ratio of AmB molecules acquires mobility in the presence of ergosterol. The similar mobility of AmB and ergosterol in POPC bilayers confirmed the idea of the direct intermolecular interaction between ergosterol and AmB.

Amphotericin B Lipid Complex (Abelcet) in the treatment of invasive mycoses: the North American experience

Abelcet, or Amphotericin B lipid Complex, is unique formulation, comprising an equimolar mixture of amphotericin B complexed with two lipids. In preclinical studies, Abelcet was clearly demonstrated to be less toxic than amphotericin B desoxycholate and to be effective in models where amphotericin B was ineffective at its maximum tolerated dose. Pharmacokinetic studies in animals also showed that the concentration of Abelcet in blood is similar or reduced compared to levels seen with conventional amphotericin B, with accumulation in the liver, lungs and spleen. Phase I clinical trials determined the optimum tolerated dose of Abelcet to be 5 mg/kg d-1. Data are now available for 228 cases (including 51 paediatric cases) of invasive fungal infection treated with Abelcet in an open-label emergency-release protocol. All patients had to have failed on previous amphotericin B or other conventional antifungals, or to have unacceptable toxicity on amphotericin B, or underlying renal disease, or nephrotoxicity due to other drugs. Abelcet was administered at a dose of 5 mg/kg d-1 for 4 wk. Approximately one-third of patients had candidiasis, one-third aspergillosis and one-third other infections, including fusariosis. Of 183 cases evaluable for response, 126 (69%) had a clinical response (cure or improvement) which was mycologically confirmed in 55% (61/110 tested). Results in paediatric cases were similar to or better than those seen in the group as a whole. When comparisons were made between cases with different types of infection, underlying disease/immunosuppressive disorder, and degree of neutropenia, the response rates were very consistent from group to group. Treatment with Abelcet was well tolerated and mean serum creatinine levels actually declined during therapy, particularly in patients with pre-existing renal dysfunction.

Complex formation of amphotericin B in sterol-containing membranes as evidenced by surface plasmon resonance

Amphotericin B (AmB) is a membrane-active antibiotic that increases the permeability of fungal membranes. Thus, the dynamic process of its interaction with membranes poses intriguing questions, which prompted us to elaborate a quick and reliable method for real-time observation of the drug's binding to phospholipid liposomes. We focused on surface plasmon resonance (SPR) and devised a new modification method of sensor chips, which led to a significant reduction in the level of nonspecific binding of the drug in a control lane. With this method in hand, we examined the affinity of AmB for various membrane preparations. As expected, AmB exhibited much higher affinity for sterol-containing palmitoyloleoylphosphatidylcholine membranes than those without sterol. The sensorgrams recorded under various conditions partly fitted theoretical curves, which were based on three interaction models. Among those, a two-state reaction model reproduced well the sensorgram of AmB binding to an ergosterol-containing membrane; in this model, two states of membrane-bound complexes, AB and AB*, are assumed, which correspond to a simple binding to the surface of the membrane (AB) and formation of another assembly in the membrane (AB*) such as an ion channel complex. Kinetic analysis demonstrated that the association constant in ergosterol-containing POPC liposomes is larger by 1 order of magnitude than that in the cholesterol-containing counterpart. These findings support the previous notion that ergosterol stabilizes the membrane-bound assembly of AmB.

Conformation and Position of Membrane-Bound Amphotericin B Deduced from NMR in SDS Micelles

Amphotericin B (AmB) is known to self-assemble to form an ion channel across lipid bilayer membranes. To gain insight into the conformation of AmB in lipidic environments, AmB in SDS micelles was subjected to high-resolution NMR and CD measurements, and the NMR-derived conformation thus obtained was refined by molecular mechanics calculations. These results indicate that AmB in SDS micelles is conformationally fixed particularly for the macrolide moiety. Paramagnetic relaxation experiments with the use of Mn2+ reveal that AmB is shallowly embedded in the micelle with the polyhydroxyl chain being close to the water interface and the side of polyene portion facing to the micelle interior. CD measurements demonstrate that AmB is in a monomeric form in SDS micelles. The structure of AmB in the micelles obtained in the present study may reproduce the initial stage of membrane interaction of AmB prior to the assembly formation in biomembranes.

Membrane interaction of amphotericin B as single-length assembly examined by solid state NMR for uniformly 13C-enriched agent

The membrane interaction of amphotericin B (AmB), one of the most important anti-fungal drugs, was investigated by solid state NMR measurements of uniformly 13C-enriched AmB, which was prepared by the culture of the drug-producing microorganism in the presence of [u-13C6]glucose. All the 13C NMR signals of AmB upon binding to DLPC membrane were successfully assigned on the basis of the 13C-13C correlation spectrum. 13C-31P RDX (Rotational-Echo Double Resonance for X-clusters) experiments clearly revealed the REDOR dephasing effects for carbon atoms residing in the both terminal parts, whereas no dephasing was observed for the middle parts including polyolefinic C20-C33 and hydroxyl-bearing C8/C9 parts. These observations suggest that AmB binds to DLPC membrane with a high affinity to the phospholipid and spans the membrane with a single molecular length.

Large Molecular Assembly of Amphotericin B Formed in Ergosterol-Containing Membrane Evidenced by Solid-State NMR of Intramolecular Bridged Derivative

Amphotericin B (AmB 1) is known to assemble and form an ion channel across biomembranes. We have recently reported that conformation-restricted derivatives of AmB 2-4 show different ergosterol preferences in ion-channel assays, which suggested that the orientation of the mycosamine strongly affects the sterol selectivity of AmB. The data allowed us to assume that compound 3 showing the highest selectivity would reflect the active conformation of AmB in the channel assembly. In this study, to gain further insight into the active conformation of AmB, we prepared a new intramolecular-bridged derivative 5, where the linker encompassed a hydrophilic glycine moiety. The derivative has almost equivalent ion-channel activity to those of AmB and 3. The antifungal activity of 5 compared with 3 improves significantly, possibly because the increasing hydrophilicity in the linker enhances the penetrability through the fungal cell wall. Conformation of 5 was well converged and very similar to that of 3, thus further supporting the notion that the conformations of these derivatives reproduce the active structure of AmB in the channel complex. Then we used the derivative to probe the mobility of AmB in the membrane by solid-state NMR. To measure dipolar couplings and chemical shift anisotropies, we incorporated [1-(13)C,(15)N]glycine into the linker. The results indicate that 5 is mostly immobilized in ergosterol-containing DMPC bilayers, implying formation of large aggregates of 5. Meanwhile some fraction of 5 remains mobile in sterol-free DMPC bilayers, suggesting promotion of AmB aggregation by ergosterol.

Mycosamine orientation of amphotericin B controlling interaction with ergosterol: sterol-dependent activity of conformation-restricted derivatives with an amino-carbonyl bridge

Amphotericin B (AmB 1) is known to assemble together and form an ion channel across biomembranes. The antibiotic consists of mycosamine and macrolactone moieties, whose relative geometry is speculated to be determinant for the drug's channel activity and sterol selectivity. To better understand the relationship between the amino-sugar orientation and drug's activity, we prepared conformation-restricted derivatives 2-4, in which the amino and carboxyl groups were bridged together with various lengths of alkyl chains. K+ influx assays across the lipid-bilayer membrane revealed that ergosterol selectivity was markedly different among derivatives; short-bridged derivative 2 almost lost the selectivity, while 3 showed higher ergosterol preference than AmB itself. Monte Carlo conformational analysis of 2-4 based on NOE-derived distances indicated that the amino-sugar moiety of 2 comes close to C41 because of the short bridge, whereas those of 3 and 4 are pointing outward. The mutual orientation of the amino-sugar moiety and macrolide ring is so rigid in derivatives 2 and 3 that these conformations should be unchanged upon complex formation in lipid membranes. These results strongly suggest that the large difference in sterol preference between derivatives 2 and 3 is ascribed to the different orientation of amino-sugar moieties. These findings allowed us to propose a simple model accounting for AmB-sterol interactions, in which hydrogen bonding between 2'-OH of AmB and 3beta-OH of ergosterol plays an important role.

Cholesterol and ergosterol influence nystatin surface aggregation: relation to pore formation

Nystatin interaction with liposomes mimicking fungal and mammalian membranes (ergosterol- and cholesterol-containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) large unilamellar vesicles, respectively) was studied by fluorescence spectroscopy. The activity of this antibiotic was also measured using a pyranine fluorescence detected K+/H+ exchange assay. Nystatin mean fluorescence lifetime varied with the antibiotic concentration and ergosterol content (0-30 mol%) of the lipid vesicles. It sharply increased from 5 to 37 ns upon reaching 100 molecules per liposome, reporting nystatin oligomerization in the membrane. Concomitantly, spectral alterations typical of excitonic coupling were detected and there was a pronounced increase in the initial rate of pore formation by nystatin. These findings suggest that nystatin exerts its antibiotic activity via a two-stage mechanism: at low antibiotic concentrations, surface-adsorbed monomeric antibiotic molecules perturb the lipid packing, changing the permeability properties of the ergosterol-rich liposomes. Upon reaching a critical threshold, nystatin mode of action switches to the classical model of transmembrane aqueous channel formation. In the presence of cholesterol-containing POPC liposomes, neither nystatin spectroscopic properties, nor the kinetics of K+ efflux varied with the antibiotic concentration suggesting that in this case the first stage of antibiotic mode of action always prevails or the assemblies formed by nystatin and cholesterol are very loose.

Salvage treatment with amphotericin B in progressive human alveolar echinococcosis

Most patients with alveolar echinococcosis are diagnosed at a late stage when the disease has advanced to unresectable hepatic lesions. These patients require lifelong therapy with benzimidazoles, the only medical treatment currently available. To date, no treatment option remains for patients with benzimidazole intolerance or treatment failure. Amphotericin B was recently shown to exert antiparasitic activity in vitro. Here, we report the efficacy of amphotericin B in human alveolar echinococcosis. In three patients with extensive disease and without further treatment options, disease progression had been documented over several months. They were treated with amphotericin B intravenously at a dose of 0.5 mg/kg of body weight three times per week. Follow-up parameters were physical examination, laboratory parameters, and imaging techniques. Amphotericin B treatment effectively halted parasite growth in all three patients. The antiparasitic effect was most evident by spontaneous closure of cutaneous fistulae in two patients and by constant size of parasitic lesions during treatment, as assessed radiologically. Metabolic activity in parasitic areas was visualized by positron emission tomography and significantly decreased during treatment. However, progressive affection of the heart in one patient could not be stopped. All patients currently continue on amphotericin B and have been treated for 25, 17, and 14 months, respectively. We introduce amphotericin B as salvage treatment for alveolar echinococcosis patients with intolerance or resistance to benzimidazoles, as it effectively suppresses parasite growth. Amphotericin B is not parasitocidal; therefore long-term treatment has to be anticipated.

Cooperative partition model of nystatin interaction with phospholipid vesicles

Nystatin is a membrane-active polyene antibiotic that is thought to kill fungal cells by forming ion-permeable channels. In this report we have investigated nystatin interaction with phosphatidylcholine liposomes of different sizes (large and small unilamellar vesicles) by time-resolved fluorescence measurements. Our data show that the fluorescence emission decay kinetics of the antibiotic interacting with gel-phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine vesicles is controlled by the mean number of membrane-bound antibiotic molecules per liposome, <A>. The transition from a monomeric to an oligomeric state of the antibiotic, which is associated with a sharp increase in nystatin mean fluorescence lifetime from approximately 7-10 to 35 ns, begins to occur at a critical concentration of 10 nystatin molecules per lipid vesicle. To gain further information about the transverse location (degree of penetration) of the membrane-bound antibiotic molecules, the spin-labeled fatty acids (5- and 16-doxyl stearic acids) were used in depth-dependent fluorescence quenching experiments. The results obtained show that monomeric nystatin is anchored at the phospholipid/water interface and suggest that nystatin oligomerization is accompanied by its insertion into the membrane. Globally, the experimental data was quantitatively described by a cooperative partition model which assumes that monomeric nystatin molecules partition into the lipid bilayer surface and reversibly assemble into aggregates of 6 +/- 2 antibiotic molecules.

Effect of amphotericin B on larval growth of Echinococcus multilocularis

Alveolar echinococcosis is caused by the parasitic cestode Echinococcus multilocularis. Benzimidazoles, namely, mebendazole and albendazole, are the only drugs available for the treatment of inoperable alveolar echinococcosis. At present, no therapeutic alternative is available for patients with progressive disease under treatment or for patients who are unable to tolerate the side effects of the benzimidazoles. In addition, benzimidazoles are only parasitostatic for E. multilocularis. Thus, new therapeutic options are of paramount importance. In the present study we examined the in vitro effect of amphotericin B on E. multilocularis larvae. E. multilocularis metacestodes grown in the peritoneal cavities of Mongolian gerbils were transferred into a culture system. Vesicles budded from the tissue blocks and increased in number and size during the first 5 weeks. After 6 weeks drugs were added and deleterious effects on the vesicles were observed macroscopically and microscopically. By use of this in vitro tissue culture model we demonstrated that amphotericin B effectively inhibits the growth of E. multilocularis metacestodes. This destructive effect was significantly more rapid with amphotericin B than with the benzimidazoles. Cyclic treatment was effective in suppressing parasite growth. However, amphotericin B appears to be parasitostatic for E. multilocularis larvae, and regrowth occurs even after extended periods. In summary, amphotericin B constitutes the first promising alternative for the treatment of alveolar echinococcosis in cases of intolerance or resistance to benzimidazoles. It holds promise as an effective treatment option for otherwise fatal courses of disease.

Amphotericin B/emulsion admixture interactions: an approach concerning the reduction of amphotericin B toxicity

Mixing Fungizone with a fat emulsion used for nutritional purpose (Intralipid or Lipofundin ) was reported to decrease Amphotericin B (AmB) toxicity in clinical use. In an effort to understand the reason for this phenomenon, spectral and morphological analyses were done for the Fungizone and Fungizone /Lipofundin admixture (FLmix). The absorption spectra analyses showed that not only Fungizone but also FLmix presented spectra that were concentration dependent. Moreover, the spectra of FLmix remained stable until the concentration of 5 x 10(-7) M, and only at 5 x 10(-8) M did they become similar in shape to the Fungizone spectra. Morphological studies revealed that even though emulsion droplets with or without Fungizone presented the same particle size, the former was less electron dense compared with Lipofundin alone. These results suggest a kind of association between Fungizoneand Lipofundin that remains over the whole range of concentrations. This hypothesis was confirmed by in vitro studies in which FLmix presented an important selectivity against human and fungal cells compared with Fungizone. These findings suggest that parenteral emulsions should be able to reduce the AmB toxicity probably by changing the AmB self-association state by binding it with emulsion droplets.

The effects of amphotericin B on pure and ergosterol- or cholesterol-containing dipalmitoylphosphatidylcholine bilayers as viewed by 2H NMR

Amphotericin B (AmB) is a widely used polyene antibiotic to treat systemic fungal infections. This drug is known to be lethal to fungal cells but it has also side effect toxicity on mammalian cells. The mechanism of action of AmB is thought to be related to the difference of the main sterol present in the mammalian and the fungal cells, namely cholesterol and ergosterol, respectively. The effect of AmB has been investigated on pure dipalmitoylphosphatidylcholine (DPPC) and on cholesterol- and ergosterol-containing DPPC bilayers by 2H NMR spectroscopy. The 2H NMR results first confirm that AmB forms a complex with sterol-free DPPC bilayers, the interaction causing the structurization of the lipids and the increase of the gel-to-lamellar fluid DPPC phase transition temperature with increasing concentration of the antibiotic. The results also show that the effects of AmB on cholesterol- and ergosterol-containing DPPC bilayers are remarkably different. On one hand, the drug causes an increase of the orientational order of the lipid acyl chains in cholesterol-containing membranes, mostly in high cholesterol content membranes. On the other hand, the addition of AmB disorders the DPPC acyl chains when ergosterol is present. This is thought to be due to the direct complexation of the ergosterol by AmB, causing the sterol ordering effect to be weaker on the lipids.

Cholesterol markedly reduces ion permeability induced by membrane-bound amphotericin B

It is widely accepted that amphotericin B (AmB) together with sterol makes a mixed molecular assemblage in phospholipid membrane. By adding AmB to lipids prior to preparation of large unilamellar vesicles (LUV), we directly measured the effect of cholesterol on assemblage formation by AmB without a step of drug's binding to phospholipid bilayers. Potassium ion flux assays based on 31P-nuclear magnetic resonance (NMR) clearly demonstrated that cholesterol markedly inhibits ion permeability induced by membrane-bound AmB. This could be accounted for by a membrane-thickening effect of cholesterol since AmB actions are known to be markedly affected by the thickness of membrane. Upon addition of AmB to an LUV suspension, the ion flux gradually increased with increasing molar ratios of cholesterol up to 20 mol%. These biphasic effects of cholesterol could be accounted for, at least in part, by the ordering effect of cholesterol.

Amphotericin B interactions with a DOPC monolayer. Electrochemical investigations

A model lipid membrane consisting of a monolayer of dioleoyl phosphatidylcholine (DOPC) adsorbed onto a Hg electrode has been used to study the interaction between the lipid and different formulations of Amphotericin B (AmB) [Fungizone (FZ), Heated Fungizone (HFZ), and Abelcet]. The lipid organizational order was measured by electrochemical methods [capacitance and metal ion (Tl(+)) reduction], characterizing the change in lipid order due to interaction with the drug. The mean size and number density of pores formed in the monolayer were estimated by fitting the reduction current transients to a random array of microelectrode model. This method was shown sensitive for investigation of the interaction of drugs with the DOPC monolayer. Abelcet was found to have a smaller disruptive effect on lipid order than FZ and HFZ. The formulations used to solubilize the AmB were also studied. Sodium deoxycholate used as a solubilizer in FZ displayed significant influence on lipid order similar to that observed for Abelcet. The lipid complex, used in Abelcet, did not significantly perturb the DOPC monolayer order. The lipid complex used in Abelcet may have an annealing or healing effect that buffers the disruption possible due to AmB.

Oligonucleotide delivery by a cationic derivative of the polyene antibiotic amphotericin B. II: study of the interactions of the oligonucleotide/cationic vector complexes with lipid monolayers and lipid unilamellar vesicles

We report a study of the behavior of oligodeoxyribonucleotide (ODN)/amphotericin B3-(N'-dimethylamino)propylamide (AMA) complexes, in the presence of lipid monolayers and large unilamellar vesicles. This study follows the recent discovery of the capacity of AMA, as a new cationic vector, to enhance ODN cellular uptake and efficacy. It aims at investigating the internalization mode of a nucleic acid by AMA. A first study at the air-water interface of AMA and AMA/ODN by surface pressure measurement shows that only free AMA would adsorb at the air-water interface. Second, in the presence of zwitterionic phospholipid- and sterol-containing mixture, ODN-AMA interactions in solution would be higher than lipid-AMA interactions at the interface. In monolayer or with large unilamellar vesicles, AMA monomers adsorb mainly at the phospholipid interface. These results favor a crossing mechanism through AMA channel formation, despite the size of ODN.

Binding of nystatin and amphotericin B with sterol-free L-dilauroylphosphatidylcholine bilayers resulting in the formation of dichroic lipid superstructures

Interactions of multilamellar vesicles (MLV) of dilauroylphosphatidylcholine (DLPC) with the polyene antibiotics, amphotericin B (AmB) and nystatin (Ny), were followed by circular dichroism (CD). These interactions proceed with both antibiotics through a slow association with high [DLPC]/[antibiotic] stoichiometric molar ratios (> or = 130), at room temperature for which DLPC membranes are in a fluid state. Microscopic investigations of the spatial distributions of the antibiotic and the MLV in the mixtures revealed that MLV form clusters inside which the antibiotic is strongly concentrated and lipid superstructures appear. Concomitantly with the appearance of these superstructures a DLPC dichroic signal emerges. This observation indicates that the chiral properties of antibiotic oligomers can induce a chirality of the DLPC molecules which are bound to them. These results support the hypothesis of a recent molecular modeling of AmB oligomers which postulates that their chiral properties result from a chiral assemblage of antibiotic molecules (Millié et al., J. Phys. Chem. B, in press).

The structuring effects of amphotericin B on pure and ergosterol- or cholesterol-containing dipalmitoylphosphatidylcholine bilayers: a differential scanning calorimetry study

Amphotericin B (AmB) is the most widely used polyene antibiotic to treat systemic fungal infections which affect an increasing number of immunocompromised patients. It is generally thought that AmB forms pores within the fungi membranes by interacting with ergosterol, the main sterol of fungi. However, it also interacts with the cholesterol contained in mammalian cells, hence its toxicity. In order to have a better understanding of the interactions prevailing between AmB and sterols, differential scanning calorimetry was used to study various mixtures incorporating from 6.5 to 25 mol% of AmB in pure dipalmitoylphosphatidylcholine (DPPC) vesicles and in ergosterol- or cholesterol-containing DPPC vesicles. The sterol concentration was kept constant at 12.5 mol% with respect to the phospholipid. Our results show that three phases co-exist when AmB is dispersed in the pure phospholipid. One corresponds to the phospholipid phase alone. The two others are characterised by a broad transition at temperatures higher than the main transition temperature of the pure phospholipid, corresponding to the drug in interaction with the aliphatic chains of the lipid. The fact that the transition temperatures of these additional components are higher than that of the pure phospholipid suggests that AmB interacts strongly with the aliphatic chains of the lipid, consistent with the idea prevailing in the literature that AmB by itself may form pores in a lipid matrix. When AmB interacts with cholesterol-containing bilayers the thermograms also present three components. Upon increasing the concentration of AmB, though, an important broadening of these components is observed which is explained in terms of destabilisation of the organisation of the aliphatic chains. The situation is strikingly different if ergosterol is present in the lipid matrix. The thermograms remain unmodified as the concentration of AmB is increased and a broad transition, now involving only two components when the thermograms are decomposed, is observed. An analysis of the results shows that various interacting units, e.g. AmB+DPPC and (AmB+ergosterol)+DPPC, are present within the membrane. These units involve the phospholipid and hence contribute to its structurisation. The important differences between the thermograms obtained with the ergosterol- as compared to the cholesterol-containing bilayers, in spite of the structural similarity of these two sterols, provides strong evidence for the selectivity of interaction of AmB with ergosterol as compared to cholesterol. It is thus clear that the action of AmB on cholesterol- as compared to ergosterol-containing membranes results from different mechanisms. Finally, UV-visible spectra of AmB in pure as well as sterol-containing DPPC vesicles show the presence of absorption bands that give support to the interpretation derived from the calorimetric data.

Association of polyene antibiotics with sterol-free lipid membranes. II. Hydrophobic binding of nystatin to dilauroylphosphatidylcholine bilayers

Interaction of nystatin A1 with multilamellar vesicles (MLV) of dilauroylphosphatidylcholine (DLPC), observed either by adding nystatin to preformed MLV (mixtures I) or by incorporating it during the formation of vesicles (mixtures II, inner lamellas of MLV in contact with nystatin) was investigated for 0.002 < or = nystatin/DLPC = R(A) < or = 0.20, by four complementary methods. The main results were: (i) Ultraviolet absorption and circular dichroism (CD) spectra of mixtures I revealed the occurrence of a saturable association with a stoichiometry (R(A) = 0.007 +/- 0.002) constant between 3 and 33 degrees C. (ii) By differential scanning calorimetry, thermograms of the two types of mixtures were similar only when water was in great excess. In the opposite (e.g., (H2O)/(DLPC) = R(W) < or = 300), mixture II thermograms displayed two features, upshifted by about 6.5 degrees C with respect to the sharp peak observed with mixture I, resembling those obtained for pure DLPC when the low-temperature phase was the subgel phase. For this R(W), the nystatin absolute concentrations were those for which nystatin form superaggregates as revealed by the nystatin CD spectra. It is proposed that these superaggregates are excluded from the interlamellar spacings of MLV and exert a pumping action on the interlamellar water. The subsequent dehydration of the inner lamellas is thought to convert them into the subgel state. (iii) 2H-NMR spectra of sn-2-perdeuterated DLPC MLV + nystatin mixtures II, confirmed such a temperature shift of the main transition. They showed, in addition, an ordering of the aliphatic chains immediately above the transition temperature, equivalent to a bilayer thickening of 2 A.

Carrier effects on biological activity of amphotericin B

Amphotericin B (AmB), the drug of choice for the treatment of most systemic fungal infections, is marketed under the trademark Fungizone, as an AmB-deoxycholate complex suitable for intravenous administration. The association between AmB and deoxycholate is relatively weak; therefore, dissociation occurs in the blood. The drug itself interacts with both mammalian and fungal cell membranes to damage cells, but the greater susceptibility of fungal cells to its effects forms the basis for its clinical usefulness. The ability of the drug to form stable complexes with lipids has allowed the development of new formulations of AmB based on this property. Several lipid-based formulations of the drug which are more selective in damaging fungal or parasitic cells than mammalian cells and some of which also have a better therapeutic index than Fungizone have been developed. In vitro investigations have led to the conclusion that the increase in selectivity observed is due to the selective transfer of AmB from lipid complexes to fungal cells or to the higher thermodynamic stability of lipid formulations. Association with lipids modulates AmB binding to lipoproteins in vivo, thus influencing tissue distribution and toxicity. For example, lipid complexes of AmB can be internalized by macrophages, and the macrophages then serve as a reservoir for the drug. Furthermore, stable AmB-lipid complexes are much less toxic to the host than Fungizone and can therefore be administered in higher doses. Experimentally, the efficacy of AmB-lipid formulations compared with Fungizone depends on the animal model used. Improved therapeutic indices for AmB-lipid formations have been demonstrated in clinical trials, but the definitive trials leading to the selection of an optimal formulation and therapeutic regimen have not been done.

Osmotic stress sensitizes sterol-free phospholipid bilayers to the action of Amphotericin B

We have tested the ability of Amphotericin B to form ion channels/defects in osmotically stressed large unilamellar vesicles (LUV) using pyranine fluorescence detected ion/H+ exchange. We found that sterol-free LUV exhibit greatly increased sensitivity to AmB channel formation in the soluble oligomer state (> 0.5 microM) under modestly hypoosmotic conditions (< 100 delta mosM). These vesicles are completely insensitive under isoosmotic conditions. The related antibiotics, Amphotericin B methyl ester and Nystatin showed almost no activity under hypoosmotic conditions in the absence of sterol. This difference may be attributable to differences in solution oligomeric states. Experiments with KCl and CaCl2 internal buffers demonstrate that these sterol-free AmB membrane disruptions are highly selective for monovalent cations (K+) over anions (Cl-), ruling out massive lysis or unselective membrane defects caused by osmotic pressure. Thus, AmB seems to be acting as a 'molecular harpoon', an expression coined to describe substances which can selectively target osmotically stressed, strained or highly curved membranes. These results may provide a rationale for AmB's reported anti-HIV activity and reported activity against sterol-free small unilamellar vesicles (highly curved membranes) as well as the reduced activity of liposomal drug delivery systems toward cholesterol-containing and sterol-free membranes (fewer soluble oligomers).

Convex-concave curvatures in bilayers of dipalmitoylphosphatidylcholine and cholesterol induced by amphotericin B/deoxycholate after prolonged storage

Freeze-fracture investigations on the influence of amphotericin B/deoxycholate on multilamellar vesicles (MLV) of DPPC containing cholesterol have revealed a new phase structure. Alternating convex and concave curvatures are observed after storage of the vesicles at temperatures below 25 degrees C for at least 4 weeks. Three types of these patterns occur, a small-dimensional (repeat distance approximately 100 nm), an intermediate-dimensional (repeat distance approximately 400 nm) and a large-dimensional (repeat distance approximately 700 nm). The types can be formed on the same bilayer side by side. Additionally, the types differ in the morphology of the tops. In the case of the small-dimensional type the shape of the top can be described as a circular flat plane or opening and in the other cases as a hemispherical cap. The large dimensional type differs from the others by involvement of bilayer stacks. The formation of this new phase after prolonged storage could be confirmed by DSC measurements. The new structure can be explained in the framework of bicontinuous cubic phases and periodically curved bilayer structures. From the electron micrographs a lo (liquid ordered) phase is suggested.

Lipid-amphotericin B complex structure in solution: a possible first step in the aggregation process in cell membranes

The interactions between the polyene antibiotic amphotericin B with dipalmitoylphosphatidylcholine were investigated in vesicles (using circular dichroism) and in chloroform solution (using circular dichroism and 1H, 13C, and 31P nuclear magnetic resonance). The results show that amphotericin B readily aggregates in vesicles and that the extent of aggregation depends on the lipid:drug concentration ratio. Introduction of sterol molecules into the membrane hastens the process of aggregation of amphotericin B. In chloroform solutions amphotericin B strongly interacts with phospholipid molecules to form a stoichiometric complex. The results suggest that there are interactions between the conjugated heptene stretch of amphotericin B and the methylene groups of lipid acyl chains, while the sugar moiety interacts with the phosphate head group by the formation of a hydrogen bond. A model is proposed for the lipid-amphotericin B complex, in which amphotericin B interacts equally well with the two lipid acyl chains, forming a 1:1 complex.

Effects of aggregation and solvent on the toxicity of amphotericin B to human erythrocytes

In aqueous suspensions of amphotericin B (AmB), a polyene antibiotic and antifungal agent, three forms of AmB coexist: monomers, water-soluble oligomers, and non-water-soluble aggregates. The toxicity of the water-soluble self-associated form of AmB compared with that of the non-water-soluble self-associated form was tested by measuring induction of K+ leakage from human erythrocytes, using different suspensions containing the antibiotic and phosphate-buffered saline. These suspensions were obtained from various stock solutions of the antibiotic in dimethyl formamide or dimethyl sulfoxide. Their circular dichroism spectra around 340 nm, indicative of the degree of AmB self-association, were strongly dependent on the concentration of organic solvent in the suspensions. The nonsoluble self-associated form was separated from the water-soluble form by centrifugation. The nonsoluble form was favored by a high concentration of AmB of the stock solution. The kinetics of AmB-induced K+ leakage from human erythrocytes also appeared to be strongly dependent on the AmB concentration of the stock solution being much weaker with concentrated stock solutions. It was concluded that the only form of AmB toxic to human erythrocytes is the water-soluble self-associated form (in contrast with fungal cells on which the monomeric form is also active). This result may be important in the design of new less toxic AmB derivatives and in the understanding of the mechanism of action of liposomal AmB.

Emerging role of lipids of Candida albicans, a pathogenic dimorphic yeast

It is clear that C. albicans lipids have gained tremendous importance in recent years. In addition to being a barrier for entrance of various metabolites, it also provides the site of action for the synthesis of enzyme(s) involved in cell wall morphogenesis and antifungal action. While alterations in lipid composition during a yeast to mycelia transition have been observed, in most of the studies, lipid fluctuations reported could have been due to various environmental factors involved in the induction of morphogenesis [4,5]. A clear understanding of lipid biosynthesis and metabolic blocks due to antifungal action is likely to shed further light on selective interactions of antifungals. Despite the multifacet role of lipids in various functions of this pathogenic yeast, their exact involvement is poorly understood. The situation is little better with regard to ergosterol and its metabolism. Ergosterol is, indeed, important for anti-candidal activity and appears to be involved in the morphogenesis of C. albicans. The fluctuation in phospholipid composition have led to altered properties of plasma membrane namely, membrane fluidity, transport activities and drug sensitivity, which suggest that-a critical level of individual phospholipid is important for proper functioning of the plasma membrane. What the exact role is of individual phospholipid is far from clear. Many unanswered questions relating to the role of PI and sphingomyelin in signal transduction, involvement of phospholipases in the maintenance of phospholipid composition, and role of lipid transfer proteins in assembly and asymmetry of lipids are some aspects which merit further work.

A sequential mechanism for the formation of aqueous channels by amphotericin B in liposomes. The effect of sterols and phospholipid composition

The kinetics of formation of amphotericin B (AmB) aqueous pores in ergosterol-containing DMPC or egg-PC liposomes was investigated using a stopped-flow method. The formation of aqueous pores by AmB occurred very rapidly (in milliseconds to seconds depending of the AmB concentration), and it was always preceded by the formation of transient, non-aqueous pre-pore structures. As anticipated, these non-aqueous pre-pore structures made the liposomes more permeable to urea without at the same time leading to a decrease of the reflection coefficient of urea or to an enhancement of glucose permeability. However, when liposomes were composed of egg-PC and cholesterol, the formation of non-aqueous and aqueous channels by AmB occurred after a lag time of several minutes. Such a time lag for AmB action was not observed in cholesterol-containing DMPC liposomes, an indication that the phospholipid composition is an important parameter in the formation of non-aqueous channels by AmB. Both non-aqueous and aqueous channels were always formed at lower concentrations of AmB in liposomes containing ergosterol while higher concentrations were needed in cholesterol-containing liposomes. Measurements of the permeabilizing effect of AmB on liposomes prepared without sterols indicate that non-aqueous channels were formed in DMPC (but not in egg-PC) at polyene concentrations identical to that found for cholesterol-containing liposomes. No evidence of the formation of aqueous channels by AmB was found in pure DMPC liposomes. These data are consistent with the concept that AmB forms non-aqueous channels without the direct participation of sterol molecules. The initially formed non-aqueous channels subsequently interact with the sterols in the membrane to form aqueous channels, having an enlarged diameter. This sequential mechanism for the formation of AmB aqueous pores in liposomes provides a rationale for the understanding of the effect of both the phospholipid composition and type of sterol in the interaction of AmB with natural membranes and artificial bilayers.

Permeabilizing action of filipin III on model membranes through a filipin-phospholipid binding

The binding of the pentaene antibiotic filipin to egg-yolk phosphatidylcholine (EPC) and dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles, has been studied by ultraviolet (UV) absorption and circular dichroism (CD). A stoichiometry of one molecule of filipin for five molecules of phospholipid was demonstrated by CD when phospholipids were in fluid phase. The similarity of the CD spectra with EPC and DMPC established a similar filipin-phospholipid assemblage in both membranes. We therefore postulated that filipin incorporation leads to the formation of gel-like domains in fluid EPC membranes as previously demonstrated for fluid DMPC membranes (Milhaud, J., Mazerski J., Bolard, J. and Dufoure, E.J. (1989) Eur. Biophys. J. 17, 151-158). The release of fluorescent probes (carboxyfluorescein (CF) or calcein (CC)), entrapped in EPC small unilamellar vesicles (SUV), due to the action of filipin, was followed by fluorescence and CD measurements concomitantly. The following observations were made. (1) The percentage of released probe, as a function of the filipin/phospholipid molar ratios, was the same whether or not membranes contained cholesterol. (2) The permeabilization of vesicles proceeded concomitantly with filipin-phospholipid binding while filipin-cholesterol binding leveled off. (3) The release of the content of vesicles occurred by an all-or-none mechanism leaving the depleted vesicles intact. From these observations and from the previous structural findings, a new interpretation of the action of filipin is proposed. Precluding any disruptive effect, inducement of permeability would result from the high intrinsic permeability of the interfacial region at the boundaries of the gel-like domains corresponding to the filipin-phospholipid aggregates. Additionally, we obtained the permeability coefficients for the anionic forms of CC and CF across EPC SUV, 0.6.10(-10) cm s-1 and 2.10(-10) cm s-1, respectively, as compared to 2.5.10(-14) cm s-1 for the counterion Na+ (Hauser, H, Oldani, D. and Phillips, M.C. (1973) Biochemistry 12, 4507-4517).

A kinetic method for measuring functional delivery of amphotericin B by drug delivery systems

The human toxicity of amphotericin B can be considerably reduced by associating the drug with liposomes of varying lipid compositions. Some lipid compositions are much more effective than others. We show that a simple kinetic fluorescence assay using pyranine as an indirect probe of amphotericin-induced K+ currents may be used to study different liposomal drug delivery systems in vitro. We find that lipid mixtures composed of DMPC/DMPG/amphotericin at a 7:3:1 mole ratio show very slow functional delivery with a preference for ergosterol over cholesterol-containing membrane vesicles. On the other hand, amphotericin delivered from egg phosphatidylcholine liposomes lead to 100-fold increases in K+ leakage at one-fifth the amphotericin concentration of the 7:3:1 system. The egg phosphatidylcholine system as well as micellar amphotericin also show a slight selectivity towards cholesterol-containing vesicles over ergosterol. These results are consistent with previous clinical and in vitro cellular studies and this technique may prove valuable in screening of other delivery systems.

Phase behaviour of amphotericin B multilamellar vesicles

Because side effect profiles and key physical properties of liposomal amphotericin B reflect the molecular nature of the hydrated preparations, effort has been directed toward understanding this nature. We describe here an examination by differential scanning calorimetry in the region of the main transition of the phase behaviour of amphotericin B multilamellar liposomes used investigationally for patient treatment. Liposomes were composed of 7:3 dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (7:3 DMPC/DMPG) containing up to 33 mol% drug. Preparations in which pure DMPC or pure 1-oleoyl-2-stearoylphosphatidylcholine (OSPC) was substituted for 7:3 DMPC/DMPG were subjected to the same measurements for comparison. The DSC-derived partial phase diagrams were similar to those previously recorded using EPR spectroscopy for unsonicated liposomes of 7:3 DMPC/DMPG containing amphotericin B, and for mixtures with different pure saturated and unsaturated phosphatidylcholines (Grant, C.W.M., et al. (1989) Biochim. Biophys. Acta 984, 11-20). Fluidization onset temperatures for liposome host matrices were relatively unaffected by drug compared to the temperatures of completion. This effect was particularly marked for the unsaturated phospholipid matrix. Partial phase diagrams were interpreted as demonstrating that amphotericin B has a tendency to separate into a rigid phase within the membrane. This is consistent with molecular modelling considerations which suggest that amphotericin B may exist as oligomers in a phospholipid matrix. Drug-induced alterations of DSC melting profiles for the phospholipid bilayers studied were less extensive than those reported for partially sonicated preparations of 7:3 DMPC/DMPG (Janoff, A.S., et al. (1989) Proc. Natl. Acad. Sci. USA 85, 6122-6126). Melting profiles obtained did not change upon further sample incubation, suggesting that the hydrated preparation represented a thermodynamically stable form.

Enhanced action of amphotericin B on Leishmania mexicana resulting from heat transformation

A comparative study of the effect of the polyene antibiotic amphotericin B (AmB) on the viability of Leishmania mexicana promastigotes before and after their transformation by heat into amastigotelike forms was carried out. The kinetics of cell death were followed by spectrofluorometry with the nucleic acid-binding compound ethidium bromide. It was found that the rapid killing effect that is exerted by AmB on Leishmania promastigotes was even faster after their transformation into amastigotelike forms. Binding studies of AmB to Leishmania membranes by circular dichroism indicated that heat transformation modified it from noncooperative to cooperative binding, decreasing the amount of antibiotic that bound to the membranes. Thus, the increased rate of ethidium bromide incorporation into transformed cells was not related either to the amount of AmB bound or to an increased amount of ergosterol in the membrane (the ergosterol/phospholipid ratio was four times smaller after heat shock). An increase in the Mg2+ content of the external aqueous solution was able to prevent the AmB-induced incorporation of ethidium bromide into Leishmania promastigotes to a greater extent (Ki = 13.8 mM) than it was into heat-transformed cells (Ki = 64 mM), suggesting that there were significant changes at the Leishmania cell surface on heat transformation. The significance of these results for understanding the mechanism of action of AmB on sensitive organisms is discussed.

Amphotericin B and Nystatin show different activities on sterol-free vesicles

It has generally been assumed that the polyene antibiotics Nystatin and Amphotericin B cause membrane damage by the same mechanism. However, using kinetic fluorescence methods we have found that AmB and Nystatin have very different activities on sterol-free dioleoyl phosphatidylcholine and egg phosphatidylcholine small unilamellar vesicles. At very low AmB concentrations (less than 1/1000 lipids in egg phosphatidylcholine) significant K+ permeability enhancement is observed. However, even at very high Nystatin to lipid ratios (1/100) very little K+ current is induced, particularly in dioleoyl phosphatidylcholine vesicles. The novel technique described here uses a K+/H+ exchange mechanism to detect minute transmembrane K+ currents by monitoring internal membrane vesicle pH changes with pyranine.

Interaction of amphotericin B and its N-fructosyl derivative with murine thymocytes: a comparative study using fluorescent membrane probes

The polyene antibiotics amphotericin B (AmB) and N-(1-deoxy-D-fructos-1-yl)amphotericin (N-Fru-AmB) have different activity towards murine thymocytes (N-Fru-AmB is less toxic but is a potent immunomodulator). The interactions of the drugs with these cells have been studied by fluorescence methods. Fluorescence energy transfer from 1-[4-(trimethylammonio) phenyl]-6-phenylhexa-1,3,5-triene, p-toluenesulfonate (TMA-DPH) to polyenes was used to follow the binding of the two drugs to the plasma membrane. The results, confirmed by circular dichroism measurements, indicate that at saturation the ratio AmB bound/plasma membrane lipid is low (less than 1 molecule of polyene for 170 lipids). The slightly higher binding of AmB as compared to N-Fru-AmB demonstrates that affinity of the antibiotic for plasma membrane does not account for the activity of the polyenes towards lymphocytes. The effect of the two polyenes on membrane fluidity was studied by steady-state fluorescence anisotropy. The results suggest that AmB strongly perturbs the structure of the membrane whereas only a slight decrease of the anisotropy is observed with N-Fru-AmB in the range of concentration where the biological activity has been demonstrated. Polyene location was further investigated by comparing the energy transfer efficiency obtained with TMA-DPH and with the parental compound 1,6-diphenylhexa-1,3,5-triene, p-toluene sulfonate (DPH). While AmB binds to plasma membrane, as well as to intracellular structures, N-Fru-AmB seems to accumulate into the cell and bind to intracellular membrane structures.

The polyene antibiotic amphotericin B acts as a Ca2+ ionophore in sterol-containing liposomes

Amphotericin B (AmB) was shown to induce a Ca2+ influx across ergosterol- and cholesterol-containing large unilamellar liposomes, by following spectrophotometrically the formation of the Arsenazo III-Ca2+ complex. At equivalent antibiotic concentrations the Ca2+ influx was much more extensive through ergosterol-containing membranes (almost 100% with 1 microM AmB, 160 microM lipid) than through cholesterol-containing membranes (below 0.5 microM the influx of Ca2+ was negligible). In the presence of ergosterol-containing membranes the initial rate of Ca2+ influx had the same linear dependence on the ratio antibiotic/lipid whatever the lipid concentration, which was not the case in cholesterol-containing membranes. These results suggest that the channels responsible for the AmB-induced Ca2+ permeability across cholesterol- and ergosterol-containing liposomes have different structures.