Application of differential spectra in the ultraviolet-visible region to study the formation of amphotericin B-sterol complexes

Antifungal activity of eugenol analogues. Influence of different substituents and studies on mechanism of action

Twenty one phenylpropanoids (including eugenol and safrole) and synthetic analogues, thirteen of them new compounds, were evaluated for antifungal properties, first with non-targeted assays against a panel of human opportunistic pathogenic fungi. Some structure-activity relationships could be observed, mainly related to the influence of an allyl substituent at C-4, an OH group at C-1 and an OCH₃ at C-2 or the presence of one or two NO₂ groups in different positions of the benzene ring. All active compounds were tested in a second panel of clinical isolates of C. albicans and non-albicans Candida spp., Cryptococcus neoformans and dermatophytes. The eugenol derivative 4-allyl-2-methoxy-5-nitrophenol (2) was the most active structure against all strains tested, and therefore it was submitted to targeted assays. These studies showed that the antifungal activity of 2 was not reversed in the presence of an osmotic support such as sorbitol, suggesting that it does not act by inhibiting the fungal cell wall synthesis or assembly. On the other hand, the Ergosterol Assay showed that 2 did not bind to the main sterol of the fungal membrane up to 250 µg mL⁻¹. In contrast, a 22% of fungal membrane damage was observed at concentrations = 1 × MIC and 71% at 4× MIC, when 2 was tested in the Cellular Leakage assay. The comparison of log P and MICs for all compounds revealed that the antifungal activity of the eugenol analogues would not to be related to lipophilicity.

On the possibility of the amphotericin B-sterol complex formation in cholesterol- and ergosterol-containing lipid bilayers: a molecular dynamics study

Amphotericin B (AmB) is a well-known membrane-active antibiotic that has been used to treat systemic fungal infections for more than 45 years. Therapeutic application of AmB is based on the fact that it is more active against ergosterol-containing membranes of fungal cells than against mammalian membranes with cholesterol. In this paper, we examine the hypothesis according to which the selectivity of the AmB's membrane action originates from its different ability to form the binary complexes with the relevant sterols. To this end, molecular dynamics simulations were performed for systems containing the preformed models of AmB/sterol complexes embedded in lipid bilayers containing either cholesterol or ergosterol. The initial structures of the studied binary associates were selected on the basis of a systematic scan of all possible mutual positions and orientations of the two molecules. The results obtained demonstrate that in general the complexes with ergosterol are more stable on the 100 ns time scale. Furthermore, on the basis of motional correlation analysis, taking into account the effects of lipid environment, we propose that, within the sterol-enriched liquid-ordered membrane phases, AmB molecules exhibit a greater tendency to bind ergosterol than cholesterol. The analysis of the interactions suggests that this affinity difference is of enthalpic origin and may arise from the considerable difference in the energy of the van der Waals interactions between AmB and the two types of sterols. Thus, our current results: (i) support the hypothesis that binary AmB/sterol complexes form within a lipid membrane and (ii) suggest that the higher toxicity may at least partly be attributed to the higher affinity of AmB for ergosterol than for cholesterol within a lipid membrane environment.

Evidence for the mechanism of action of the antifungal phytolaccoside B isolated from Phytolacca tetramera Hauman

Phytolaccoside B (1), an antifungal monodesmoside triterpenoid glycoside isolated from berries of Phytolacca tetramera Hauman (Phytolaccaceae), alters the morphology of yeasts and molds. The malformations were similar to those produced by enfumafungin, a known inhibitor of (1-->3)-beta-D-glucan synthase, an enzyme that catalyzes the synthesis of (1-->3)-beta-D-glucan, one of the major polymers of the fungal cell wall. However, enzymatic assays revealed that 1 did not inhibit (1-->3)-beta-D-glucan synthase, but it did produce a notable enhancement of the chitin synthase 1 activity and, concomitantly, a rise in chitin, another important polymer of the fungal cell walls. This finding was corroborated by fluorescence microscopy and also by quantification of the chitin. In addition, a 2-fold increase in the thickness of the fungal cell wall was observed with transmission electronic microscopy. On the other hand, 1 neither bound to ergosterol nor caused hemolysis of red blood cells, although some fungal membrane damage was observed at the MIC of 1.

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.

Effect of fasting on temporal variation in the nephrotoxicity of amphotericin B in rats

Evidence for temporal variation in the nephrotoxicity of amphotericin B was recently reported in experimental animals. The role of food in these variations was determined by studying the effect of a short fasting period on the temporal variation in the renal toxicity of amphotericin B. Twenty-eight normally fed and 28 fasted female Sprague-Dawley rats were used. Food was available ad libitum to the fed rats, while the fasted animals were fasted 12 h before and 24 h after amphotericin B injection to minimize stress for the animals. Water was available ad libitum to both groups of rats, which were maintained on a 14-h light, 10-h dark regimen (light on at 0600 h). Renal toxicity was determined by comparing the levels of excretion of renal enzyme and the serum creatinine and blood urea nitrogen (BUN) levels at the time of the maximal (0700 h) or the minimal (1900 h) nephrotoxicity after the intraperitoneal administration of a single dose of dextrose (5%; control group) or amphotericin B (50 mg/kg of body weight; treated group) to the rats. The nephrotoxicities obtained after amphotericin B administration at both times of day were compared to the nephrotoxicities observed for time-matched controls. In fed animals, the 24-h urinary excretion of N-acetyl-beta-D-glucosaminidase and beta-galactosidase was significantly higher when amphotericin B was injected at 0700 and 1900 h. The excretion of these two enzymes was reduced significantly (P < 0.05) in fasting rats, and this effect was larger at 0700 h (P < 0.05) than at 1900 h. The serum creatinine level was also significantly higher (P < 0.05) in fed animals treated at 0700 h than in fed animals treated at 1900 h. Fasting reduced significantly (P < 0.05) the increase in the serum creatinine level, and this effect was larger in the animals treated at 0700 h. Similar data were obtained for BUN levels. Amphotericin B accumulation was significantly higher (P < 0.05) in the renal cortexes of fed rats than in those of fasted animals, but there was no difference according to the time of injection. These results demonstrated that fasting reduces the nephrotoxicity of amphotericin B and that food availability is of crucial importance in the temporal variation in the renal toxicity of amphotericin B in rats.

Amphotericin B lipid complex

OBJECTIVE

To evaluate the published data on the effectiveness and safety of amphotericin B lipid complex (ABLC) for the treatment of invasive mycosis and to evaluate data describing the pharmacologic properties and pharmacokinetic behavior of ABLC in both animals and humans.

DATA SOURCE

A MEDLINE search was conducted to identify literature published from 1965 to January 1997 for amphotericin B deoxycholate (DCAB) and ABLC. In addition, preliminary data published as abstracts and presented at national conferences on infectious disease and hematology within the last 6 years were also included in this review.

STUDY SELECTION

Both human and animal studies were reviewed. Animal and in vitro studies were selected to evaluate the pharmacologic and toxicologic properties of ABLC. For the evaluation of the efficacy, safety, and pharmacokinetic behavior of ABLC, large, well-controlled studies were reviewed. In addition, data from open-label and emergency use protocols were also included in the review.

DATA EXTRACTION

The study and analytical methods, results, and conclusions of the selected studies were evaluated. Pharmacokinetic data for both ABLC and DCAB that were derived from human subjects were also evaluated.

DATA SYNTHESIS

DCAB has been the cornerstone for the treatment of invasive mycosis, even though it has a narrow therapeutic index. Infusion-related toxicities (e.g., fever, chills, rigors) are likely due to DCAB stimulation of cytokine and prostaglandin synthesis. Conversely, nephrotoxicity, the primary non-infusion-related toxicity, likely results from the nonselective cytotoxic interaction between DCAB and cholesterol-containing mammalian cells. ABLC represents a new approach to improving the therapeutic index of DCAB. Mammalian cytotoxicity is attenuated by complexing amphotericin B to a mixture of phospholipids. This alters the affinity of amphotericin B and decreases its selective transfer from the complex to cholesterol-containing mammalian cells. Fungi also possess lipase, which improves the selective transfer from the complex to ergosterol-containing cell membranes. In humans, the lipid formulation increases the volume of distribution of amphotericin B. Thus, compared with DCAB, larger doses of ABLC can be administered for a longer duration with less nephrotoxicity. However, the prevalence of infusion-related toxicities associated with ABLC is similar to that of DCAB. Whether the alteration in distribution improves efficacy by improving tissue concentrations of amphotericin B has not been determined. The cost of this agent will limit its use.

CONCLUSIONS

ABLC has been shown to be at least as effective as DCAB, and it has been well tolerated in the clinical studies to date. Despite large dosages and extended courses of administration, there is little nephrotoxicity associated with its use. However, the cost of this agent will limit its use to the treatment of refractory mycosis or to cases where DCAB is contraindicated due to significant renal insufficiency.

The effect of aggregation state of amphotericin-B on its interactions with cholesterol- or ergosterol-containing phosphatidylcholine monolayers

Amphotericin B (AmB) is the most effective antibiotic used in the treatment of systemic fungal infections. It is generally thought that the activity of this drug results from its interaction with ergosterol, the main sterol of fungi membranes. However, AmB also interacts with cholesterol, the major sterol of mammal membranes, thus limiting the usefulness of this drug due to its relatively high toxicity. The aim of the present work is to study the molecular basis of the interactions of AmB with these sterols contained in a DOPC film by using the monolayer technique. Two different concentrations of the sterols in the film (13 and 30%) at an initial surface pressure of 30 mN/m were studied, which correspond to conditions found in various biological membranes. Four concentrations of AmB in the subphase, ranging from a molecularly dispersed to a highly aggregated state of the drug were studied. Our results show that the monomeric form of AmB interacts with the ergosterol containing film solely. On the other hand, when AmB is dispersed as a pre-micellar or as a highly aggregated state in the subphase, a very significant selectivity of its interactions between the two sterols is observed which is shown in our experimental results by a difference of 8 mN/m in the surface pressure when AmB interacts with ergosterol as compared to cholesterol. We show that the activity of AmB is most likely related to the micellar form of the antibiotic. In addition, we observe that upon increasing the amount of ergosterol in the film, the insertion of AmB is largely promoted, results that are discussed in terms of the molecular organization of the sterols within the monolayer film. We show that these results provide a better understanding of the action of AmB (activity/toxicity) at the membrane level.

Ion coordination in the amphotericin B channel

The antifungal polyene antibiotic amphotericin B forms channels in lipid membranes that are permeable to ions, water, and nonelectrolytes. Anion, cation, and ion pair coordination in the water-filled pore of the "barrel" unit of the channels was studied by molecular dynamics simulations. Unlike the case of the gramicidin A channel, the water molecules do not create a single-file configuration in the pore, and some cross sections of the channel contain three or four water molecules. Both the anion and cation are strongly bound to ligand groups and water molecules located in the channel. The coordination number of the ions is about six. The chloride has two binding sites in the pore. The binding with water is dominant; more than four water molecules are localized in the anion coordination sphere. Three motifs of the ion coordination were monitored. The dominant motif occurs when the anion is bound to one ligand group. The ion is bound to two or three ligand groups in the less favorable configurations. The strong affinity of cations to the channel is determined by the negatively charged ligand oxygens, whose electrostatic field dominates over the field of the hydrogens. The ligand contribution to the coordination number of the sodium ion is noticeably higher than in the case of the anion. As in the case of the anion, there are three motifs of the cation coordination. The favorable one occurs when the cation is bound to two ligand oxygens. In the less favorable cases, the cation is bound to three or four oxygens. In the contact ion pair, the cation and anion are bound to two ligand oxygens and one ligand hydrogen, respectively. There exist intermediate solvent-shared states of the ion pair. The average distances between ions in these states are twice as large as that of the contact ion pair. The stability of the solvent-shared state is defined by the water molecule oriented along the electrostatic field of both ions.

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.

Theoretical study of the complexation of amphotericin B with sterols

The aim of this present work was the study of the intermolecular complexes between amphotericin B (AmB) and either cholesterol or ergosterol. In such complexes the intermolecular interaction energy mainly proceeds from both Van der Waals and H-bonding (via water molecules) forces. Our calculations have shown that the Van der Waals forces slightly favor the AmB-ergosterol complex. Several relative positions of the sterol with regard to AmB lead to energy minima: sterol may be either in contact with the AmB polar head or repelled towards the end of the macrolide ring. It appeared that the role played by some water molecules was to maintain the sterol close to the AmB polar head.

Comparative theoretical study of the conformations of amphotericin methyl ester and amphotericin B polar heads in the presence of water

Amphotericin methyl ester (AmE) is an interesting derivative of amphotericin B (AmB) because of its enhancement of selectivity against the fungicells. Both AmB and AmE molecules differ by the structure of their polar heads. This work deals with a theoretical study of conformations of the polar head of AmE in the presence of hydration water molecules. The results will be compared with our previous work concerning AmB.

A reverse-phase HPLC assay for measuring the interaction of polyene macrolide antifungal agents with sterols

A quick and simple affinity chromatography method for gauging the interaction of polyene antifungal agents with sterols has been developed. The required affinity columns are prepared from a standard C-18 reverse-phase HPLC column by injecting a measured quantity of sterol under conditions where it is completely retained. After the assay, the sterol is eluted with a less polar solvent and the column reused. By comparing the elution volume of a polyene injected onto the sterol-free column (Ve) with that of the polyene injected onto the sterol-doped column (V), an association constant (Ka) for the polyene-sterol complex was determined. Association constants of different amphotericin B-sterol and pimaricin-sterol complexes were determined and correlated with the polyene's ability to induce membrane permeability and its antifungal properties. This procedure provides a new tool for screening polyene macrolides for antifungal therapy.

Treatment of murine cryptococcal meningitis with an SCH 39304-amphotericin B combination

Cryptococcal meningitis was induced in BALB/c mice by intracerebral infection with Cryptococcus neoformans. Drug therapy was initiated 1 day later, with mice receiving amphotericin B (AMB), SCH 39304, combination therapy, or no drug therapy (controls). Most, but not all, combinations showed additive benefits, significantly prolonging survival and reducing organism counts in tissues compared with those in controls and groups which received the drugs independently. Optimum protection was obtained when a single dose of 10 mg of AMB per kg of body weight was combined with a fairly narrow SCH 39304 dose range. AMB antagonism did not occur with any regimen tested. AMB-azole combinations may be reasonable alternatives for patients who fail standard cryptococcosis therapeutic regimens.

The influence of Myrj 59 on the solubility, toxicity and activity of amphotericin B

The effect of Myrj 59 (a polyoxyethyleneglycol derivative of stearic acid) on amphotericin B (Am B) solubility, toxicity and activity has been investigated. We showed that Myrj 59 could solubilize the antibiotic. Moreover, it also decreased and abolished the haemolytic activity of the drug by increasing the resistance of the red blood cells and impairing the interaction of Am B with the cellular membrane cholesterol, but it did not modify the in-vitro antifungal activity of the drug. On the other hand, Myrj 59 did not decrease the acute in-vivo toxicity of the drug (LD50 and nephrotoxicity). In a previous study we have shown that a polyoxyethleneglycol derivative of cholesterol could solubilize Am B and was able to decrease the in-vitro and in-vivo toxicity of the antibiotic without altering its in-vitro antifungal activity. The results of the present study suggest that the cholesterol moiety of the surfactant is not necessary to decrease the in-vitro lytic activity of the drug but could play a role in the reduction of the in-vivo toxicity.

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

A mild detergent, the monolauryl ester of sucrose (LS), at concentrations which ranged from 0.008 to 0.03%, enhanced amphotericin B (AmB) toxicity against Saccharomyces cerevisiae and Cryptococcus neoformans cells. At higher concentrations, 0.06 to 2.5%, LS inhibited AmB effects on these two fungi. We analyzed changes in the absorption spectrum of AmB induced by LS at these two concentration ranges by comparing ratios (R values) of AmB absorbance at 409 nm, the wavelength characteristic of non-aggregated (monomeric) AmB, to absorbance at 328 nm, the wavelength characteristic of aggregated AmB. Low concentrations of LS caused a decrease in R, whereas the higher LS concentrations increased R. Therefore, LS had concentration-dependent dual effects on the antifungal activity of AmB which correlated with shifts in the physical states of AmB. The concentration range of LS required to inhibit the antifungal effects of AmB was about 1000-fold greater than the previously reported concentrations required to inhibit AmB toxicity to mammalian cells (Gruda, I., Gauthier, E., Elberg, S., Brajtburg, J. and Medoff, G. (1988) Biochem. Biophys. Res. Commun. 154, 954-958). This suggests that LS may be a useful agent to decrease AmB toxicity to host cells without affecting the antifungal effects. Moreover, increase in AmB toxicity induced by low concentrations of LS suggests the possibility that synergistic interaction between fatty acid esters and polyene antibiotics may have therapeutic value.

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.

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.

How do the polyene macrolide antibiotics affect the cellular membrane properties?

In the 1970's great strides were made in understanding the mechanism of action of amphotericin B and nystatin: the formation of transmembrane pores was clearly demonstrated in planar lipid monolayers, in multilamellar phospholipid vesicles and in Acholeplasma laidlawii cells and the importance of the presence and of the nature of the membrane sterol was analyzed. For polyene antibiotics with shorter chains, a mechanism of membrane disruption was proposed. However, recently obtained data on unilamellar vesicles have complicated the situation. It has been shown that: membranes in the gel state (which is not common in cells), even if they do not contain sterols may be made permeable by polyene antibiotics, several mechanisms may operate, simultaneously or sequentially, depending on the antibiotic/lipid ratio, the time elapsed after mixing and the mode of addition of the antibiotic, there is a rapid exchange of the antibiotic molecules between the vesicles. Although pore formation is apparently involved in the toxicity of amphotericin B and nystatin, it is not the sole factor which contributes to cell death, since K+ leakage induced by these antibiotics is separate from their lethal action. The peroxidation of membrane lipids, which has been demonstrated for erythrocytes and Candida albicans cells in the presence of amphotericin B, may play a determining role in toxicity concurrently with colloid osmotic effect. On the other hand, it has been shown that the action of polyene antibiotics on cells is not always detrimental: at sub-lethal concentrations these drugs stimulate either the activity of some membrane enzymes or cellular metabolism. In particular, some cells of the immune system are stimulated. Furthermore, polyene antibiotics may act synergistically with other drugs, such as antitumor or antifungal compounds. This may occur either by an increased incorporation of the drug, under the influence of a polyene antibiotic-induced change of membrane potential, for example, or by a direct interaction of both drugs. That fungal membranes contain ergosterol while mammalian cell membranes contain cholesterol, has generally been considered the basis for the selective toxicity of amphotericin B and nystatin for fungi. Actually, in vitro studies have not always borne out this assumption, thereby casting doubt on the use of polyene antibiotics as antifungal agents in mammalian cell culture media.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of the polyene antibiotic etruscomycin with large unilamellar lipid vesicles: binding and proton permeability inducement

The effect of the polyene antibiotic etruscomycin on the permeability of large unilamellar lipid vesicles was investigated. Proton leakage was induced in egg-yolk phosphatidylcholine (EPC) vesicles only when sterol was present in the membrane; the extent of leakage was limited. High etruscomycin/lipid ratios (R) were necessary (R greater than 0.1). Higher percentages of sterol increased the permeability, slightly more strongly for ergosterol than for cholesterol. Dipalmitoylphosphatidylcholine (DPPC) vesicles were more sensitive to permeability inducement, even in the absence of sterol in the bilayer (inducement for R greater than 0.06). The interactions of etruscomycin with the vesicles were examined by circular dichroism, fluorescence and 31P-NMR. In the range of antibiotic concentration where permeability was induced, R greater than 0.1 for EPC vesicles, R greater than 0.06 for DPPC vesicles, etruscomycin exhibited characteristic circular dichroism spectra independent of the presence of sterol. Under the same conditions, 31P-NMR and fluorescence studies indicated a destruction or a fusion of the vesicle bilayer. At lower etruscomycin concentrations (R less than 0.03), the etruscomycin circular dichroism spectra were different, indicating that the interaction with membranes containing ergosterol differed from that with membranes containing cholesterol. From correlating the increase in fluorescence intensity with this interaction, as well as from exchange experiments, it was inferred that etruscomycin at a low antibiotic/lipid ratio is more strongly bound to ergosterol-containing vesicles than to cholesterol-containing vesicles. These results and their comparison with the results obtained with other polyene antibiotics indicate that at low R etruscomycin resembles amphotericin rather than filipin in its preferential binding to ergosterol-containing vesicles. At higher R, that is in conditions where permeability is induced, the selectivity is different. The corresponding mechanism seems not to involve the formation of an etruscomycin-sterol channel, since the hydrophobic chain of the complex would be too short to form a channel.

Relative avidity of etruscomycin to cholesterol and ergosterol

Two methods, biological and spectroscopic, were used to determine the avidity of the polyene antibiotic Etruscomycin for cholesterol and ergosterol. The biological method consisted of measuring the inhibitory potency of both sterols on the Etruscomycin-induced damage to erythrocytes and fungi. The spectroscopic method consisted of recording of series of differential spectra in a number of solvents of different composition. The results obtained showed that cholesterol protected erythrocytes and candida albicans against the damaging action of Etruscomycin more efficiently than ergosterol did and that Etruscomycin-cholesterol complexes were more resistant to interruption by organic solvents than Etruscomycin-ergosterol complexes. These results and their comparison with the results obtained with other polyene antibiotics indicate that Etruscomycin resembles filipin in that it binds more avidly to cholesterol than to ergosterol. This implies that the length of the hydrophobic chain rather than the presence of the amino sugar determines sterol preference. The spectral method that we used can have general application for the quantitative measurement of complex formation between polyenes and sterols.

Equilibrium binding of amphotericin B and its methyl ester and borate complex to sterols

Scatchard analysis of the binding of amphotericin B to sterols in egg phosphatidylcholine/sterol vesicles revealed that amphotericin B was bound to ergosterol by approximately one order of magnitude more tightly than to cholesterol. Amphotericin B methyl ester formed a tighter complex with each of the sterols investigated than did amphotericin B, but the difference in the apparent binding constants towards ergosterol and cholesterol was not as large as with amphotericin B. Amphotericin B borate formed weaker complexes with vesicle-bound sterols than amphotericin B and amphotericin B methyl ester.

Increase in colony-forming units of Candida albicans after treatment with polyene antibiotics

Polyene antibiotics, at concentrations which do not cause detectable toxic effect, induce an increase in the number of colon-forming units of yeast cells of Candida albicans. This effect, which we attribute to an increase in plating efficiency, is probably caused by binding of the polyenes to fatty acids in the cell wall of fungi.