Phase behaviour of amphotericin B multilamellar vesicles

Interactions of the antimalarial amodiaquine with lipid model membranes

A detailed molecular description of the mechanism of action of the antimalarial drug amodiaquine (AQ) is still an open issue. To gain further insights on that, we studied the interactions of AQ with lipid model membranes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) by spin labeling electron spin resonance (ESR) and differential scanning calorimetry (DSC). Both techniques indicate a coexistence of an ordered DPPS-rich domain with a disordered DPPC-rich domain in the binary DPPC/DPPS system. We found that AQ slightly lowered the melting transition temperatures associated to both domains and significantly increased the enthalpy change of the whole DPPC/DPPS phase transition. DSC and ESR data also suggest that AQ increases the number of DPPC molecules in the DPPC-rich domains. AQ also causes opposing ordering effects on different regions of the bilayer: while the drug increases the ordering of the lipid acyl chains from carbon 7 to 16, it decreases the order parameter of the lipid head group and of carbon 5. The gel phase was mostly affected by the presence of AQ, suggesting that AQ is able to influence more organized lipid domains. Moreover, the effects of AQ and cholesterol on lipid acyl chain ordering and mobility were compared at physiological temperature and, in a general way, they are similar. Our results suggest that the quinoline ring of AQ is located completely inside the lipid bilayers with its phenol ring and the tertiary amine directed towards the head group region. The nonspecific interaction between AQ and DPPC/DPPS bilayers is a combination of electrostatic and hydrophobic interactions.

High sensitivity differential scanning calorimetry investigation of the interaction between liposomes, lactate dehydrogenase and tyrosinase

High sensitivity differential scanning calorimetry (HSDSC) has been used to study the interaction of the model proteins lactate dehydrogenase (LDH) and tyrosinase with dimyristoylphosphatidylcholine (DMPC) liposomes, and relate this to the thermal and physical stability of the proteins. On heating, both LDH and tyrosinase denatured irreversibly in a time-dependent manner and modified the phase transition behaviour of DMPC liposomes at all concentrations investigated. The most marked effects occurred for the pretransition rather than the main phospholipid phase transition. The effects on the bilayer are likely to result from electrostatic interactions of the hydrophilic proteins with the head-groups of DMPC molecules, whilst due to their hydrophilic nature they do not penetrate into the bilayer. Tyrosinase is more highly ionised than LDH at the pH of the investigation, which may explain why tyrosinase has a greater effect than LDH on the HSDSC scans at mg/ml protein concentrations.

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.

Amphotericin B–phospholipid covalent conjugates: dependence of membrane-permeabilizing activity on acyl-chain length

The interaction between amphotericin B and phospholipid upon forming ion channels across a biomembrane was investigated using their covalent conjugates. The membrane permeabilizing activity was greatly affected by the chain length of the fatty acyl groups, suggesting that their interaction is involved in ion channel assemblages.

Polyene antibiotic amphotericin B in monomolecular layers: spectrophotometric and scanning force microscopic analysis

Monolayers of amphotericin B (AmB) and monolayers composed of AmB and dipalmitoylphosphatidylcholine (DPPC) were formed at the argon-water interface and deposited on a solid support by means of the Langmuir-Blodgett technique. The hypsochromic shift observed in the absorption spectra of monolayers is indicative of aggregated structures of AmB. The exciton splitting theory allowed us to calculate the distance between neighboring molecules in the aggregates as 7.8 A. Scanning force microscopy of the AmB monolayers revealed the formation of a homogeneous monolayer composed of molecules separated by a distance of 6-8 A. Microscopy also reveals the formation of cylindrical structures of AmB with a diameter close to 17 A (internal diameter close to 6 A) in the monolayers containing additionally 10 mol% DPPC.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Spectrophotometric analysis of organisation of dipalmitoylphosphatidylcholine bilayers containing the polyene antibiotic amphotericin B

Amphotericin B (AmB) is a polyene antibiotic widely used in the treatment of deep-seated fungal infections. The mode of action of AmB is directly related to the effect of the drug on the lipid phase of biomembranes. In the present work the effect of AmB on the properties of lipid bilayers formed with dipalmitoylphosphatidylcholine (DPPC) and the effect of the lipid phase on the molecular organisation of AmB were studied with application of spectrophotometry in the UV-Vis region. The absorption spectra of AmB in lipid membranes display a complex structure with hypsochromically and bathochromically shifted bands indicative of formation of molecular aggregates of the drug. Formation of molecular aggregates was analysed at different concentrations of the drug in the lipid phase in the range 0.05--5 mol% and at different temperatures in the range 5--55 degrees C. The aggregation level of AmB in the ordered phase of DPPC displayed a minimum corresponding to a concentration of 1 mol% with respect to the lipid. An increase in the aggregation level was observed in the temperature region corresponding to the main phase transition. The structure of molecular aggregates of AmB is analysed on the basis of spectroscopic effects in terms of the exciton splitting model. Analysis of the position of the absorption maximum of AmB in the lipid phase of DPPC in terms of the theory of solvatochromc effects makes it possible to ascribe the refractive indices n=1.40 and n=1.49 to the hydrophobic core of the membrane in the L(alpha) and the P(beta)' phase respectively. Analysis of the aggregation of AmB in the lipid phase in relation to the physical state of the membrane reveals that the temperature range of the main phase transition of a lipid cluster in the immediate vicinity of AmB depends on its concentration. The termination of the phase transition temperature, as read from the AmB aggregation, varies between 42 degrees C at 1 mol% AmB in DPPC and 49 degrees C at 5 mol% AmB in DPPC. The exciton splitting theory applied to the analysis of the spectroscopic data makes it possible to calculate the diameter of the AmB pore as 2.8 A in the gel phase and 3.6 A in the fluid phase of the DPPC membrane, on the assumption that the pore is formed by nine AmB molecules.

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).

Effect of amphotericin B on dipalmitoylphosphatidylcholine membranes: calorimetry, ultrasound absorption and monolayer technique studies

Amphotericin B (AmB) is a popular drug frequently applied in the treatment of mycosis. Differential scanning calorimetry (DSC), ultrasound absorption and monomolecular layer technique were applied to study the effect of AmB on organisation of dipalmitoylphosphatidylcholine (DPPC) membranes. DSC-determined enthalpy of the main phase transition of DPPC liposomes was found to be a sensitive parameter to monitor AmB-DPPC interaction. The enthalpy of the phase transition decreases with the increase in molar fraction of AmB incorporated to membranes. The exceptionally sharp decrease in the enthalpy of the transition was observed in the membranes containing 5-7 mol% AmB. Ultrasound absorption-monitored main phase transition of DPPC is very broad under the presence of 5 mol% AmB showing destabilisation and disorganisation of a membrane structure. These findings are discussed in comparison to monomolecular layer study of two-component DPPC-AmB system. Analysis of the surface pressure-molecular area isotherms of compressing DPPC-AmB films at the air-water interface shows pronounced increase in mean molecular area at AmB concentrations corresponding to those found to destabilise DPPC membranes of liposomes. Disorganisation of lipid bilayers due to the presence of AmB in concentrations below 10 mol% with respect to lipid is discussed in terms of toxicity and side effects of this drug.

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.

Protein location in liposomes, a drug carrier: a prediction by differential scanning calorimetry

Location of protein drugs in lipid carriers often determines the stability, loading efficiency, and release rate of these drugs from the carriers following administration. On the basis of conventional differential scanning calorimetry (DSC) measurements, Papahadjopoulos et al. (Biochim. Biphys. Acta 1975, 401, 317-335) proposed that proteins can be classified into three categories depending on their effects on the thermotropic behavior of the lipids, e.g., transition temperature and enthalpy. Interactions are usually electrostatic, hydrophobic, or their combination. The nature of these interactions are reflected by changes in various thermotropic parameters. Our study aims to test the validity of Papahadjopoulos' classification. Hydrophilic ribonuclease A, cytochrome c, and superoxide dismutase (SOD), as well as hydrophobic cyclosporin A, are used as model proteins. Neutral lipids, e.g., dipalmitoylphosphatidylcholine, and/or negatively charged lipids, e.g., dipalmitoylphosphatidylglycerol (DPPG), are used to prepare liposomes. Results from conventional and high-sensitivity DSC are compared. High-sensitivity DSC gives significant, more reproducible results. We find that the classification of Papahadjopoulos et al. needs to be modified. No hydrophilic proteins bind to liposomes exclusively on the surface by electrostatic interactions, and some degree of penetration is observed in most cases. An unexpected binding between SOD and DPPG liposomes is observed. The binding of SOD to negatively charged lipids may account, at least in part, for its ability to protect lipid membranes against oxygen-mediated injury.

Immunological effects of amphotericin B and liposomal amphotericin B on splenocytes from immune-normal and immune-compromised mice

The immunological effects of amphotericin B and liposomal amphotericin B were studied in vitro by measuring B- and T-lymphocyte proliferation on splenocytes from immune-normal, cyclosporine-compromised, and cyclophosphamide-compromised mice. Cellular viability of cells from immune-normal mice was also evaluated. The concentrations used (0, 0.5, 1, 2, 4, 8, and 16 micrograms/ml) encompassed clinically relevant doses. Amphotericin B consistently reduced the abilities of B cells and T cells to proliferate, especially when administered at higher than clinically relevant doses. Direct cytotoxicity probably played only a minor role, since viability studies showed that, compared with its liposomal analog, amphotericin B reduced the number of viable cells by no more than 10%. Clinically relevant doses of liposomal amphotericin B (A. S. Janoff, L. T. Boni, M. C. Popescu, S. R. Minchey, P. R. Cullis, T. D. Madden, T. Tarashi, S. M. Gruner, E. Shyamsunder, M. W. Tate, R. Mendelsohn, and D. Bonner, Proc. Natl. Acad. Sci. USA 85:6122-6126, 1988; R. Mehta, G. Lopez-Berestein, R. Hopfer, K. Mills, and R. L. Juliano, Biochim. Biophys. Acta 770:230-234, 1984) did not inhibit any of the immune parameters examined. Liposomes may, therefore, be a useful means of delivering more drug to a host infected with a fungal organism without further compromising the patient's already suppressed immune system.

Clinical experience with multilamellar liposomal amphotericin B in patients with proven and suspected fungal infections

Over a 3-year period, an unsonicated multilamellar vesicle preparation containing a low ratio of amphotericin B (5 mole %) was used as a routine alternative to amphotericin B-deoxycholate in treating 17 patients with a variety of systemic fungal infections representative of those commonly encountered on a tertiary care centre infectious disease service. Patient acceptability and convenience of administration were noteworthy. In 6/7 patients who had been given the liposomal drug after experiencing severe side effects (primarily hypokalemia and marked elevation of serum creatinine) on the non-liposomal form, the problems that had led to institution of the liposomal drug were reversed during treatment. However, multilamellar liposomal amphotericin B at conventional dosage was not without detectable toxicity in this patient population. Three transplant patients receiving cyclosporin at the same time as liposomal amphotericin B experienced a rise in serum creatinine, and 4 patients became hypokalemic during treatment: none of these effects was severe or required discontinuation of therapy. One or more liver enzymes rose measurably in 7 patients during treatment with liposomal amphotericin B, but remained unchanged or actually decreased in the remaining patients.