Effect of the modifications of ionizable groups of amphotericin B on its ability to form complexes with sterols in hydroalcoholic media

The interaction of amphotericin B and some of its semisynthetic derivatives with cholesterol and ergosterol has been tested in 1:4 (v/v) ethanol/water mixture by circular dichroism and absorption spectroscopy. The effect of the chemical modification of the 'ionizable head' of the antibiotic, the pH of the medium, and the sterol/antibiotic ratio has been studied. The results obtained show that in the presence of the sterols, amphotericin B forms several spectroscopically different species. A high extent of polyene-sterol interaction is observed for: (i) amphotericin B in neutral or acidic media, (ii) esters and amides at neutral or alkaline media, (iii) N-acyl derivatives only in acidic medium. The extent of interaction at neutral pH is highly correlated with the biological activity of compounds tested. The implication of these findings on the nature of the forces responsible for the antibiotic-sterol interaction is discussed.

Isoprenoid-mediated changes in the glycerophospholipid molecular species of the sterol auxotrophic fungus Lagenidium giganteum

The mosquito pathogenic fungus Lagenidium giganteum (Oomycetes: Lagenidiales) is a sterol auxotroph that can grow vegetatively in the absence of these compounds, but requires an exogenous source of sterols to enter its sexual and asexual reproductive cycles. Electrospray mass spectrometry (MS) and electrospray MS/MS were used to examine three major glycerophospholipid molecular species--glycerophosphocholine (GPC), glycerophosphoethanolamine (GPE) and glycerophosphoinositol (GPI)--from fungal mycelium and nuclei grown in defined medium with and without isoprenoids which induce (cholesterol and ergosterol) or do not induce (squalene, cholestane) reproduction. Testosterone supplementation of defined media inhibited growth of L. giganteum, so the effect of this steroid on phospholipid metabolism could not be assessed. Mycelium grown in defined media supplemented with these isoprenoids produced significantly different quantities of total phospholipid relative to unsupplemented media and to each other, ranging from a mean of 292 micrograms phosphate per g wet weight for cholesterol-supplemented media to 56 micrograms phosphate per g wet weight for mycelium grown in the presence of squalene. A very large percentage of the GPC (69-80 mol%) and GPI (74-79 mol%) molecular species from mycelia and nuclei contained ether linkages. GPE molecular species had 13-20 mol% ether-containing moieties. The elevated levels of ether lipids may be related to the sterol auxotrophic nature of the fungus. Isoprenoid supplementation of defined growth media resulted in many significant changes in molecular species for all three lipid classes. Significant differences (P < 0.05) in the percentage of total cell ether lipids in GPC and GPE were generated by isoprenoid supplements to culture media. Mycelium grown in the presence of the two sterols which induce asexual and sexual reproduction in L. giganteum, cholesterol and ergosterol, had a significantly greater percentage of ether-containing GPE moieties. The glycerolipid species from nuclei isolated from cultures grown with cholesterol and ergosterol were similar to the composition of nuclei isolated from fungus cultured in defined medium without any supplement or supplemented with squalene. The nuclear membrane from mycelia grown in cholestane-supplemented media, however, had a very different glycerophospholipid composition relative to either whole cells or nuclei from cells grown on other media. It appears that one of the reasons that cyclic isoprenoids such as cholestane do not induce fungal reproduction is that they drastically alter the nuclear membrane glycerophospholipid composition.

Efficacy of unilamellar liposomal amphotericin B in treatment of pulmonary aspergillosis in persistently granulocytopenic rabbits: the potential role of bronchoalveolar D-mannitol and serum galactomannan as markers of infection

A model of primary pulmonary aspergillosis in rabbits was developed to reproduce the persistent levels of profound granulocytopenia and the histopathologic features of bronchopneumonia, vascular invasion, and hemorrhagic infarction encountered in humans. D-mannitol was detectable in bronchoalveolar lavage fluid by gas-liquid chromatography/mass spectroscopy, and galactomannan was measurable in serum by latex agglutination immunoassay. A pharmacokinetically distinctive unilamellar vesicle formulation of liposomal amphotericin B, 5 mg/kg/day intravenously, compared with high-dose conventional desoxycholate amphotericin B, 1 mg/kg/day intravenously, was more effective in preventing nephrotoxicity, increasing survival, reducing the number of viable organisms, and decreasing tissue injury due to Aspergillus organisms. Thus, D-mannitol in lavage fluid and galactomannan in serum may be useful markers of pulmonary aspergillosis, and liposomal amphotericin B was significantly more effective and safer than desoxycholate amphotericin B for treatment of pulmonary aspergillosis in profoundly granulocytopenic rabbits.

Frequent fusion of liposomes to a positively charged planar bilayer without calcium ions

A novel positively charged planar bilayer membrane was formed from a mixture containing 20% cationic lipid, 1,2-dioleoyloxy-3-(trimethylammonio)propane, and neutral phospholipid mixture of 56% phosphatidylethanolamine and 24% egg phosphatidylcholine. The basic properties of the bilayer were essentially the same as those previously reported for neutral and negatively charged lipid bilayers. Using the positively charged bilayer in addition to neutral and negatively charged bilayers, the effects of charge of the planar bilayers upon vesicle-planar membrane fusion were investigated by measuring the fusion, to the bilayers, of liposomes containing nystatin-ergosterol channels and carrying a net negative charge. The tendency for fusion was evaluated in terms of the time elapsed before the first fusion event (denoted fusion time). In the absence of calcium ions, a fusion time of about 1 min was measured with the positively charged planar bilayers, and about 5 and over 15 min with the neutral and negatively charged planar bilayers, respectively. These results indicated that the vesicle-planar membrane fusion without calcium ions is greatly enhanced by the presence of cationic lipids in the planar bilayers, and suggested the usefulness of cationic lipid bilayers.

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.

Reconstituting channels into planar membranes: a conceptual framework and methods for fusing vesicles to planar bilayer phospholipid membranes

Protocols to reconstitute channels into planar bilayers via fusion methods have now been developed. The greater the intravesicular pressures generated, the greater is the fusion. These pressures can be calculated exactly for any experimental configuration. For some of the configurations, adding nystatin channels to the vesicle membrane will greatly aid fusion. The configurations of the 1990 Method (Figs. 4 and 5) are optimal for fusing vesicles that are reconstituted with ion-selective channels to planar membranes. Greater binding, and ultimately greater fusion, is achieved by ejecting vesicles directly at the membrane rather than by simply adding material to the cis compartment.

Sterol specific inactivation of gramicidin A induced membrane cation permeability

Channel inactivation, a time-dependent decrease of the high-cationic permeability induced by gramicidin A, has been found both in cholesterol containing red blood cell membranes and lipid bilayers (Schagina et al., (1989) Biochim. Biophys. Acta 978, 145-150). The rate of channel inactivation strongly depends on the phospholipid to cholesterol molar ratio of the membrane. The channel inactivation is suggested to be the result of an interaction between gramicidin and cholesterol in a stoichiometry of 1:5. Cholesterol dependent inactivation is shown also for gramicidin A analogs: tryptophan-N-formylated gramicidin A, o-pyromellitilgramicidin and malonylbisdesformylgramicidin. When cholesterol in the membrane is substituted by sitosterol, the inactivation of gramicidin-induced cation permeability is preserved, while in the presence of either ergosterol or 7-dehydrocholesterol no indication of the channel inactivation is observed. Thus, the structure of the 'B', ring, not the apolar tail of the sterol molecule, appears to be important in the inactivation process.

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.

A screening method for antifungal substances using Saccharomyces cerevisiae strains resistant to polyene macrolides

Strains of Saccharomyces cerevisiae FL200 capable of growing on a solid medium containing a mixture of polyene macrolide antibiotics (nystatin, 40 micrograms/ml, amphotericin B, 40 micrograms/ml, pimaricin, 150 micrograms/ml and RP9971 antibiotic, 10 micrograms/ml) have been isolated after successive selection steps. The mutant strains, PR13 and PRC24, are 10 to 100 times more resistant than the polyene macrolide antibiotics. When 4% Tween 80 is added to the medium, resistance to these antifungal drugs is further increased. In addition, strain PRC24, derived from strain PR13, is resistant to a non-polyene macrolide antifungal antibiotic, cycloheximide. In contrast, PR13 and PRC24 are both highly susceptible to a large range of compounds, including non-polyenic antifungal, antitumor and antibacterial agents. These particular characteristics make these strains useful for the rapid detection of antifungal compounds of the polyene macrolide and cycloheximide types, as well as for the recognition of antimitotic substances.

Comparative conformational analysis of cholesterol and ergosterol by molecular mechanics

A comparative conformational analysis of cholesterol and ergosterol has been carried out using molecular mechanics methods. These studies are aimed at giving a better understanding of the molecular nature of the interaction of these sterols with polyene macrolide antibiotics. Structures of cholesterol and ergosterol determined by X-ray methods have been used as initial geometries of these molecules for force field calculations. The calculation of steric energy has also been made for conformations which do not appear in the crystal. The latter conformers have different conformations of the side chain as well as different conformations of rings A and D. The rotational barriers around bonds C17-C20 and C20-C22 have also been calculated. The results obtained on differences and similarities in the conformations of cholesterol and ergosterol allow us to postulate a mechanism for differential interaction with the antibiotics. The relatively rigid side chain of ergosterol (stretched molecule) in comparison with the flexible side chain of cholesterol (bent molecule), allows better intermolecular contact of the first sterol molecule with a polyene macrolide and in consequence facilitates complex formation involving Van der Waal's forces.

Polyene--sterol interaction and selective toxicity

From permeability experiments carried out with series of amphotericin B derivatives in both biological and model membranes, it was concluded that derivatives, whose carboxyl group at the C18 position is blocked by substitution, are much more efficient at inducing permeability in ergosterol-containing than in cholesterol-containing membranes, whereas derivatives whose carboxyl group is free and ionizable are equally efficient in both membranes types. Binding measurements on erythrocyte membranes showed that all amphotericin B derivatives simply partition between membrane lipids and aqueous medium, according to their lipid solubility. There is no relationship between binding and efficiency in inducing permeability. Permeability studies carried out on lipidic vesicles containing various sterols showed that: 1) derivatives having their carboxyl free induced permeability of the 'channel' type, regardless of the sterol present, and no detectable permeability in sterol-free membranes; 2) derivatives whose carboxyl group is blocked induce channels only in membranes containing ergosterol or sterols having an alkyl side chain identical to that of ergosterol. In the presence of other sterols or in sterol-free membranes, their ionophoric activity is poor and always of the 'mobile-carrier' type. A model of polyene-sterol interaction is proposed, accounting for the data obtained with both biological and model membranes.

Effects of the detergent sucrose monolaurate on binding of amphotericin B to sterols and its toxicity for cells

Amphotericin B (AmB) is a potent antifungal agent used to treat patients with systemic mycoses. The cytotoxicity of AmB is related to its binding to membrane sterols and its clinical usefulness is based on its greater affinity to ergosterol, the fungal sterol, compared to the mammalian cell sterol, cholesterol (1-3). Here we report that sucrose monolaurate (L.S.) decreased the binding of AmB to cholesterol without interfering with its binding to ergosterol. Furthermore, the toxicity of AmB for mouse erythrocytes (RBC) and cultured mouse fibroblasts, L-929, cells was significantly decreased by low concentrations of L.S., whereas under the same conditions, its toxicity for Candida albicans was unaffected. We observed a very good correlation between the spectroscopic and cell studies. The results reported here on the effects of L.S. on the selectivity of AmB toxicity for fungal cells compared to animal cells and the relative nontoxic nature of sugar esters suggest a potential for compounds of this type to enhance the therapeutic index of AmB.

Studies on application of a lead acetate solution as filter to the photochemical reaction of ergosterol to improve the yield of vitamin D

Since a lead acetate solution can remove most of the ultraviolet (UV) light in the range below 275 nm which usually gives undesirable by-products in the photochemical conversion of ergosterol to vitamin D, it is useful as a filter solution for the reaction to obtain higher yield of vitamin D. When a 5% lead acetate solution was used as the filter, the yield of vitamin D was 20-25% higher than that without using filter solution.

On the existence of an amphotericin B--sterol complex in lipid vesicles and in propanol-water systems

The amphotericin B (AmB) - ergosterol complex, formed by interaction of the antibiotic with ergosterol-containing phospholipid vesicles, is associated with the lipid bilayer. It has been shown by circular dichroism studies that the AmB-ergosterol complex formed in water-propanol binary mixtures has a similar structure to that observed in phospholipid vesicles. A positive cooperativity is found for the interaction of AmB with ergosterol. The similar AmB-cholesterol complex is much less stable and rearranges rapidly to a different conformation.

Association of polyene antibiotics with sterols

Molecular interaction between amphotericin B and sterols in non-aqueous solutions was examined quantitatively by spectroscopic methods in order to support the view point that selectivity of amphotericin B is more pronounced in the presence of ergosterol than of cholesterol. The most likely association complexes between ergosterol and amphotericin B are 4:1, 6:1 stoichiometric complex when the concentrations of amphotericin B are 3.93 x 10(-4) M, 1.94 x 10(-4) M respectively. The presence of 3 beta-OH group is necessary but not enough for the association with amphotericin B. It appears that the extent of spectral change of amphotericin B induced by complexing sterol is greater for ergosterol than cholesterol.

In-vivo studies of amphotericin B liposomes derived from proliposomes: effect of formulation on toxicity and tissue disposition of the drug in mice

The repeat dose toxicity of various liposomal formulations containing amphotericin B has been determined in mice. In general, small liposomes (e.g. 100-150 nm) were found to be more toxic than their large counterparts (e.g. about 2000 nm). However, the repeat dose toxicity of small liposomes could be diminished substantially by the inclusion of sterol (i.e. ergosterol) into the liposomal membranes. Tissue accumulation studies of amphotericin B after repeat dosing may be a useful adjunct to formulation development.

Fluorescence studies of the interaction of trichorzianine A IIIc with model membranes

The biological activity and the chemical structure of the lipophilic peptides, trichorzianines, suggested that these substances could act on membrane permeability. The interaction of a major component of trichorzianines, trichorzianine A IIIc (TA IIIc), a 19-residue peptaibol containing tryptophanol as C-terminal amino-alcohol, with some synthetic phospholipid vesicles (egg phosphatidylcholine (egg PC), dipalmitoylphosphatidylcholine (DPPC) and sterol-containing egg PC) was studied by fluorescence spectroscopy. TA IIIc was found to bind to lipid vesicles either in liquid-crystalline or gel state. The accessibility to the aqueous phase of the embedded peptide was examined for various phospholipid compositions by fluorescence quenching experiments. We found that incorporation of TA IIIc in egg PC vesicles leads to reduced accessibility of the C-terminal tryptophanol to external quenchers, whereas when sterols are present in the bilayer, this accessibility is higher, consistent with a higher exposure of the chromophore to the aqueous phase. TA IIIc was shown to induce leakage of vesicular entrapped material. Incorporation of sterols in the bilayer seems to influence the position of the bound peptide within the bilayer but not its action on the membrane permeability.

Concentration- and time-dependence of amphotericin-B induced permeability changes across ergosterol-containing liposomes

The effect of amphotericin B on the permeability properties of liposomes prepared by reverse-phase evaporation was examined by using an osmotic method. This study has revealed that the magnitude and type of the alterations in permeability induced by amphotericin B in liposomes made of egg phosphatidylcholine and ergosterol depend not only on the amphotericin B concentration in the external aqueous solution but also on the time elapsed after mixing. Thus, low amphotericin B concentrations (from 0.2 to 1.2 microM) led to, (1) an small increment of the total extent of shrinkage of liposomes suspended in non-electrolytes such as urea or salts like KNO3, (2) an enhancement of urea and salt permeabilities at the same time scale at which volume changes were measured (ms to s), (3) a maximal blocking by tetraethylammonium of amphotericin B-induced urea permeability and (4) an enhancement of glucose permeability but only after liposomes were incubated with amphotericin B for some minutes before mixing. The high amphotericin B concentration regime (beyond 1.2 microM) led to, (1) a decrease of the total extent of shrinkage of liposomes immediately after rapid mixing of liposomes with urea solutions containing amphotericin B and (2) a 50% reduction of the tetraethylammonium blocking of amphotericin B-induced urea permeability. These results are explained by assuming that amphotericin B may form in ergosterol-containing liposomes two types of active channel differing in internal diameter.

Minor and trace sterols in marine invertebrates 55. The isolation, structure elucidation and synthesis of ergosta-5, 24(28), 25-trien-3 beta-ol

The isolation, structure determination and synthesis of ergosta-5, 24(28), 25-trien-3 beta-ol, as well as the synthesis of its 28-14C analog--a possible biosynthetic precursor of several marine sterols--is described.

Enhancement of amphotericin B selectivity by antibiotic incorporation into gel state vesicles. A circular dichroism and permeability study

The permeability induced in cholesterol-or ergosterol-containing phospholipid vesicles by Amphotericin B incorporated into Dipalmitoyl Phosphatidylcholine vesicles has been studied in parallel with Circular Dichroism spectroscopy measurements. In our conditions, Amphotericin B is 5 to 10 times more selective to ergosterol- than to cholesterol-containing vesicles. Such a large difference is not observed when Amphotericin B is directly added to the vesicles suspension as its solution in organic solvent.

Dissociation kinetics and equilibrium binding properties of polyene antibiotic complexes with phosphatidylcholine/sterol vesicles

The interactions of sonicated vesicles with the polyene antibiotics amphotericin B, candicidin, mediocidin , and a water-soluble, guanidine derivative of amphotericin B were examined by UV-visible spectroscopy at concentrations below which the polyenes become self-associated. The association constants, Kapp, and the numbers of binding sites per sterol or phospholipid molecule (n) were determined at 30 degrees C and pH 7.4. A single class of binding sites was found, with no evidence of cooperativity. For the binding of mediocidin , amphotericin B, and the guanidine derivative with phosphatidylcholine (PC), PC/cholesterol, and PC/ergosterol vesicles, Kapp was in the range of (1.0-3.0) X 10(6) M-1; Kapp was higher for candicidin-vesicle interaction, reaching 9.0 X 10(6) M-1 with PC/ergosterol vesicles. Binding of the guanidine derivative of amphotericin B to PC vesicles lacking sterol was extensive (n = 0.46); since the other polyenes, which have low aqueous solubilities, had n less than 0.05, positive charges in the mycosamine moiety appear to enhance the extent of polyene antibiotic interaction with the glycerophospholipid head group. Higher values of n (and, therefore, of nKapp ) were found with sterol-containing than with sterol-free vesicles, suggestive of penetration of the polyenes toward the interior of the bilayer when sterol is present. For binding to PC/sterol vesicles, nKapp followed the order of candicidin greater than guanidine derivative of amphotericin B greater than amphotericin B much greater than mediocidin . The values of n and nKapp were appreciably higher for amphotericin B-ergosterol than for amphotericin B-cholesterol interaction in vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

A spectroscopic technique for evaluating sterol-aponin interactions and implications for management of Ptychodiscus brevis red tides

A double-beam quadruple-cell spectrophotometric technique is described that can be used to evaluate the interaction between sterols and HPLC fractions from Nannochloris sp. which are cytolytic (aponin) toward the red tide organism. The interaction with ergosterol but not beta-sitosterol is consistent with complex formation and also with the reduction of cytolytic activity of aponin in the presence of ergosterol, as previously noted. Implications for the mechanism of aponin cytolytic activity are considered.