Concentration dependent dual effect of the monolauryl ester of sucrose on the antifungal activity and absorption spectra of amphotericin B (Fungizone)

Polyene Antibiotics Physical Chemistry and Their Effect on Lipid Membranes; Impacting Biological Processes and Medical Applications

This review examined a collection of studies regarding the molecular properties of some polyene antibiotic molecules as well as their properties in solution and in particular environmental conditions. We also looked into the proposed mechanism of action of polyenes, where membrane properties play a crucial role. Given the interest in polyene antibiotics as therapeutic agents, we looked into alternative ways of reducing their collateral toxicity, including semi-synthesis of derivatives and new formulations. We follow with studies on the role of membrane structure and, finally, recent developments regarding the most important clinical applications of these compounds.

Amphotericin B Formulations and Drug Targeting

Amphotericin B is a low-soluble polyene antibiotic which is able to self-aggregate. The aggregation state can modify its activity and pharmacokinetical characteristics. In spite of its high toxicity it is still widely employed for the treatment of systemic fungal infections and parasitic disease and different formulations are marketed. Some of these formulations, such as liposomal formulations, can be considered as classical examples of drug targeting. The pharmacokinetics, toxicity and activity are clearly dependent on the type of amphotericin B formulation. New drug delivery systems such as liposomes, nanospheres and microspheres can result in higher concentrations of AMB in the liver and spleen, but lower concentrations in kidney and lungs, so decreasing its toxicity. Moreover, the administration of these drug delivery systems can enhance the drug accessibility to organs and tissues (e.g., bone marrow) otherwise inaccessible to the free drug. During the last few years, new AMB formulations (AmBisome, Abelcet, and Amphotec) with an improved efficacy/toxicity ratio have been marketed. This review compares the different formulations of amphotericin B in terms of pharmacokinetics, toxicity and activity and discusses the possible drug targeting effect of some of these new formulations.

The effect of serum albumin on the aggregation state and toxicity of amphotericin B

Studies have shown that the dose-limiting toxicity of amphotericin B (AmB), a key drug for systemic mycoses, depends on its self-aggregation state. In a step toward understanding the various factors in blood mediating the toxicity of AmB, we have investigated the effect of serum albumin, the most abundant plasma protein, on the aggregation state of AmB using absorption spectroscopy. The critical aggregation concentration (CAC) of AmB, which coincides with its concentration at the onset of toxicity (hemolysis), was 1.1 microM, but rose in proportion to the level of serum albumin (1.0 to 4.0% w/v). The CAC of AmB was 8.0 microM at 4.0% w/v serum albumin, which is considerably higher than peak therapeutic levels of AmB in plasma (i.e., 2.0 microM). Serum albumin (4.0% w/v) lowered the degree of aggregation of AmB (size of aggregates) above the CAC and increased its solubility. The results suggest that serum albumin attenuates the toxicity of AmB at a membrane level by affecting its aggregation state. In this way, serum albumin in blood may balance deleterious effects of AmB mediated by serum low-density lipoproteins.

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.

Modulation of amphotericin B activity by association with mannose ester

The biological and molecular properties of a new formulation of Amphotericin B complexed with the surfactant palmitoyl mannose were studied in in vitro as well as in in vivo situations. The properties analyzed include toxicity towards two types of mammalian cells and four fungi strains, effect on macrophage activity, inflammatory properties, acute toxicity in mice and spectral behavior in presence of foetal calf serum or 6% propanol. The results demonstrate that, in presence of palmitoyl mannose, the cytotoxicity of AmB is decreased towards both, fungal and mammalian cells while its fungistatic potential is increased, its inflammatory properties are conserved and its acute toxicity is significantly diminished. These effects can be potentially explained by the formation of a complex between AmB and the sugar ester that impedes the interaction of the drug with either serum components or cell membrane constituents. The overall properties of AmB in the complex would be expected to favor an increase in the immunoadjuvant properties of the drug, a more localized inflammation during fungal infection and consequently a better therapeutic efficiency.

Fast atom bombardment mass spectrometry of sucrose monocaprate and sucrose monolaurate

Fast Atom Bombardment (FAB) ionization of sucrose monocaprate and sucrose monolaurate in the presence of Na+ ions shows the formation of both [M+Na]+ and [M - H]- ions. The [M+Na]+ ions undergo charge-remote fragmentations when collisionally activated at high translational energy. These charge-remote fragmentations are useful for the structural determination of the acyclic part of the glycolipid. In the negative ion mode both sucrose esters yield anions which can be attributed to the saccharide and lipid components of these glycolipids. Structural characterization of the fatty acid can be achieved as the released carboxylate anions undergo charge-remote fragmentations that are consistent with the ion chemistry of [M - H]- anions from FAB-ionized free fatty acids.

Structure-activity study of inhibition of amphotericin B (Fungizone) binding to sterols, toxicity to cells, and lethality to mice by esters of sucrose

The effects of four monoesters of sucrose with different acyl chain lengths (palmitate, C16; myristate, C14; laurate, C12; and caprate, C10) on the aggregation state of amphotericin B (AmB), its binding to cholesterol and ergosterol, its toxicity to cells, and its lethality to mice were determined. In solution, all four of these esters inhibited AmB binding to cholesterol more than to ergosterol; this effect correlated with the ester-induced shift from the mainly aggregated form of AmB to the mainly monomeric form. In experiments with cells, the esters inhibited the toxicity of AmB to mouse erythrocytes and cultured mouse fibroblast L-929 cells more than its toxicity to Candida albicans cells. When injected intravenously with AmB, these esters decreased AmB lethality to mice. In all of these assays, the ester with the shortest chain length (caprate) was much less potent than the other three esters. Our results indicate a correlation between in vitro and in vivo assays and suggest that the in vitro and in vivo selectivity of AmB may be enhanced by surface-active agents which modulate the aggregation state of AmB.

The effect of surfactants on the aggregation state of amphotericin B

We have studied the effect of two surfactants, one non-ionic, lauryl sucrose (LS) and the other ionic, sodium deoxycholate (DOC), on the aggregation state of amphotericin B (AmB) and its selectivity towards ergosterol and cholesterol. It is shown that the addition of these surfactants has very similar effects on the AmB micelles. Below the critical micellar concentration of the surfactants, mixed micelles with AmB are first formed as a result of the penetration of the surfactant molecules into the AmB micelles. At higher concentrations of the surfactant molecules, the micellar structure is completely destroyed and AmB is found as monomers in solution. When the concentration of the surfactant is further increased, micelles of the surfactant molecules are built up, AmB remaining in monomeric form. However, the critical micellar concentration of LS is modified by the presence of AmB in solution, while that of DOC is not affected, thereby indicating that the interactions of AmB with LS are stronger than those of DOC with AmB. We also show that both surfactants enhance the selectivity of the AmB binding to sterols at exactly the concentrations of the surfactants which induce the monomerization of the antibiotic. It is observed that the maximal selectivity is found at a concentration of the surfactants corresponding to their particular CMC in presence of the antibiotic.