MFAME, N-methyl-N-D-fructosyl amphotericin B methyl ester, a new amphotericin B derivative of low toxicity: relationship between self-association and effects on red blood cells

Emerging Role of Sphingolipids in Amphotericin B Drug Resistance

Invasive fungal infections in humans are common in people with compromised immune systems and are difficult to treat, resulting in high mortality. Amphotericin B (AmB) is one of the main antifungal drugs available to treat these infections. AmB binds with plasma membrane ergosterol, causing leakage of cellular ions and promoting cell death. The increasing use of available antifungal drugs to combat pathogenic fungal infections has led to the development of drug resistance. AmB resistance is not very common and is usually caused by changes in the amount or type of ergosterol or changes in the cell wall. Intrinsic AmB resistance occurs in the absence of AmB exposure, whereas acquired AmB resistance can develop during treatment. However, clinical resistance arises due to treatment failure with AmB and depends on multiple factors such as the pharmacokinetics of AmB, infectious fungal species, and host immune status. Candida albicans is a common opportunistic pathogen that can cause superficial infections of the skin and mucosal surfaces, thrush, to life-threatening systemic or invasive infections. In addition, immunocompromised individuals are more susceptible to systemic infections caused by Candida, Aspergillus, and Cryptococcus. Several antifungal drugs with different modes of action are used to treat systemic to invasive fungal infections and are approved for clinical use in the treatment of fungal diseases. However, C. albicans can develop a variety of defenses against antifungal medications. In fungi, plasma membrane sphingolipid molecules could interact with ergosterol, which can lead to the alteration of drug susceptibilities such as AmB. In this review, we mainly summarize the role of sphingolipid molecules and their regulators in AmB resistance.

Design, Synthesis, and Structure-Activity Relationship Studies of Bisamide Derivatives of Amphotericin B with Potent Efficacy and Low Toxicity

Amphotericin B (AMB, 1) is the most powerful antibiotic in treating potentially life-threatening invasive fungal infections (IFIs), though severe toxicity derived from self-aggregation greatly limits its clinical application. Herein, we applied a bisamidation strategy at the C16-COOH and C3'-NH₂ to improve the therapeutic properties by suppressing self-aggregation. It was found that basic amino groups at the residue of C16 amide were beneficial to activity, while lipophilic fragments contributed to toxicity reduction. Additionally, N-methyl-amino acetyl and amino acetyl moieties at C3' amide could help keep the fungistatic effectiveness. The modification work culminated in the discovery of 36 (ED₅₀ = 0.21 mg/kg), which exerted a 1.5-fold stronger antifungal efficacy than amphamide, the optimal derivative theretofore, in mice, low self-aggregation propensity, and thus low acute toxicity. With the improvement in therapeutic index and good PK profile, 36 is promising for further development as a second-generation polyene antifungal agent.

Enhancing the production of amphotericin B by Strepyomyces nodosus in a 50-ton bioreactor based on comparative genomic analysis

Amphotericin, as an important macrolide antibiotic, is synthesized by Streptomyces nodosus. A high-yield S. nodosus ZJB2016050 was obtained by mutagenesis in our lab with the advantages of high yield, short fermentation cycle and few by-products, which was more suitable for industrial production. The fermentation differences in 50-tons bioreactor between S. nodosus ATCC14899 and S. nodosus ZJB2016050 were compared. The amphotericin B (AmB) yield of S. nodosus ZJB2016050 was 9.73 mg/g at 96 h, which was 30% higher than that of S. nodosus ATCC14899. The by-product amphotericin A (AmA) production of S. nodosus ZJB2016050 was 78% lower than that of S. nodosus ATCC14899. By performing whole-genome sequencing of S. nodosus ZJB2016050 and comparative genome analysis with the wild-type S. nodosus ATCC14899, it was found that the two strains have high synteny, but each has a special gene fragment. The genes functions of fragment were identified in the amino acid transport and metabolism, carbohydrate metabolism and lipid transport and metabolism. The gene functions of SNP (single nucleotide polymorphism) genes were identified in amino acid transport and metabolism, carbohydrate metabolism, coenzyme metabolism and secondary metabolites biosynthesis. The difference in signal-regulation and transcription may be the main reason for the differences between these two strains. Three GntR family egulatory factors of S. nodosus ATCC14899 may reduce the synthesis of amphotericin. Based on the analysis of comparative genomes, the effects of corn oil in S. nodosus ATCC14899 and S. nodosus ZJB2016050 were also compared. The results showed that corn oil can promote the fermentation of S. nodosus ZJB2016050. The S. nodosus ZJB2016050 may degrade fatty acids faster, and the degraded acyl-coenzyme can be used to synthesize amphotericin.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02844-2.

Insight into Structural Characteristics of Protein-Substrate Interaction in Pimaricin Thioesterase

As a polyene antibiotic of great pharmaceutical significance, pimaricin has been extensively studied to enhance its productivity and effectiveness. In our previous studies, pre-reaction state (PRS) has been validated as one of the significant conformational categories before macrocyclization, and is critical to mutual recognition and catalytic preparation in thioesterase (TE)-catalyzed systems. In our study, molecular dynamics (MD) simulations were conducted on pimaricin TE-polyketide complex and PRS, as well as pre-organization state (POS), a molecular conformation possessing a pivotal intra-molecular hydrogen bond, were detected. Conformational transition between POS and PRS was observed in one of the simulations, and POS was calculated to be energetically more stable than PRS by 4.58 kcal/mol. The structural characteristics of PRS and POS-based hydrogen-bonding, and hydrophobic interactions were uncovered, and additional simulations were carried out to rationalize the functions of several key residues (Q29, M210, and R186). Binding energies, obtained from MM/PBSA calculations, were further decomposed to residues, in order to reveal their roles in product release. Our study advanced a comprehensive understanding of pimaricin TE-catalyzed macrocyclization from the perspectives of conformational change, protein-polyketide recognition, and product release, and provided potential residues for rational modification of pimaricin TE.

Redesign of antifungal polyene glycosylation: engineered biosynthesis of disaccharide-modified NPP

Polyene macrolides such as nystatin A1 and amphotericin B have been known to be potent antifungal antibiotics for several decades. Because the therapeutic application of polyenes is restricted by severe side effects such as nephrotoxicity, various chemical and biological studies to modify the polyene structure have been conducted to develop less-toxic polyene antifungals. A newly discovered nystatin-like polyene compound NPP was shown to contain an aglycone that was identical to nystatin but harbored a unique di-sugar moiety, mycosaminyl-N-acetyl-glucosamine, which led to higher solubility and reduced hemolytic toxicity. Additionally, a NPP-specific second sugar extending gene, nppY, was recently identified to be responsible for the transfer of a second sugar, N-acetyl-glucosamine, in NPP biosynthesis. In this study, we investigated biosynthesis of the glycoengineered NPP analog through genetic manipulation of the NPP A1 producer, Pseudonocardia autotrophica KCTC9441. NypY is another second sugar glycosyltransferase produced by Pseudonocardia sp. P1 that is responsible for the transfer of a mannose to the mycosaminyl sugar residue of nystatin. We blocked the transfer of a second sugar through nppY disruption, then expressed nypY in P. autotrophica △nppY mutant strain. When compared with nystain A1 and NPP A1, the newly engineered mannosylated NPP analog showed reduced in vitro antifungal activity, while exhibiting higher nephrotoxical activities against human hepatocytes. These results suggest for the first time that not only the number of sugar residues but also the type of extended second sugar moiety could affect biological activities of polyene macrolides.

Antifungal Therapy: New Advances in the Understanding and Treatment of Mycosis

The high rates of morbidity and mortality caused by fungal infections are associated with the current limited antifungal arsenal and the high toxicity of the compounds. Additionally, identifying novel drug targets is challenging because there are many similarities between fungal and human cells. The most common antifungal targets include fungal RNA synthesis and cell wall and membrane components, though new antifungal targets are being investigated. Nonetheless, fungi have developed resistance mechanisms, such as overexpression of efflux pump proteins and biofilm formation, emphasizing the importance of understanding these mechanisms. To address these problems, different approaches to preventing and treating fungal diseases are described in this review, with a focus on the resistance mechanisms of fungi, with the goal of developing efficient strategies to overcoming and preventing resistance as well as new advances in antifungal therapy. Due to the limited antifungal arsenal, researchers have sought to improve treatment via different approaches, and the synergistic effect obtained by the combination of antifungals contributes to reducing toxicity and could be an alternative for treatment. Another important issue is the development of new formulations for antifungal agents, and interest in nanoparticles as new types of carriers of antifungal drugs has increased. In addition, modifications to the chemical structures of traditional antifungals have improved their activity and pharmacokinetic parameters. Moreover, a different approach to preventing and treating fungal diseases is immunotherapy, which involves different mechanisms, such as vaccines, activation of the immune response and inducing the production of host antimicrobial molecules. Finally, the use of a mini-host has been encouraging for in vivo testing because these animal models demonstrate a good correlation with the mammalian model; they also increase the speediness of as well as facilitate the preliminary testing of new antifungal agents. In general, many years are required from discovery of a new antifungal to clinical use. However, the development of new antifungal strategies will reduce the therapeutic time and/or increase the quality of life of patients.

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.

Polyene macrolide biosynthesis in streptomycetes and related bacteria: recent advances from genome sequencing and experimental studies

The polyene macrolide group includes important antifungal drugs, to which resistance does not arise readily. Chemical and biological methods have been used in attempts to make polyene antibiotics with fewer toxic side effects. Genome sequencing of producer organisms is contributing to this endeavour, by providing access to new compounds and by enabling yield improvement for polyene analogues obtained by engineered biosynthesis. This recent work is also enhancing bioinformatic methods for deducing the structures of cryptic natural products from their biosynthetic enzymes. The stereostructure of candicidin D has recently been determined by NMR spectroscopy. Genes for the corresponding polyketide synthase have been uncovered in several different genomes. Analysis of this new information strengthens the view that protein sequence motifs can be used to predict double bond geometry in many polyketides.Chemical studies have shown that improved polyenes can be obtained by modifying the mycosamine sugar that is common to most of these compounds. Glycoengineered analogues might be produced by biosynthetic methods, but polyene glycosyltransferases show little tolerance for donors other than GDP-α-D-mycosamine. Genome sequencing has revealed extending glycosyltransferases that add a second sugar to the mycosamine of some polyenes. NppY of Pseudonocardia autotrophica uses UDP-N-acetyl-α-D-glucosamine as donor whereas PegA from Actinoplanes caeruleus uses GDP-α-D-mannose. These two enzymes show 51 % sequence identity and are also closely related to mycosaminyltransferases. These findings will assist attempts to construct glycosyltransferases that transfer alternative UDP- or (d)TDP-linked sugars to polyene macrolactones.

Thermodynamics and kinetics of amphotericin B self-association in aqueous solution characterized in molecular detail

Amphotericin B (AmB) is a potent but toxic drug commonly used to treat systemic mycoses. Its efficiency as a therapeutic agent depends on its ability to discriminate between mammalian and fungal cell membranes. The association of AmB monomers in an aqueous environment plays an important role in drug selectivity, as oligomers formed prior to membrane insertion - presumably dimers - are believed to act differently on fungal (ergosterol-rich) and mammalian (cholesterol-rich) membranes. In this work, we investigate the initial steps of AmB self-association by studying the structural, thermodynamic and spectral properties of AmB dimers in aqueous medium using molecular dynamics simulations. Our results show that in water, the hydrophobic aggregation of AmB monomers yields almost equiprobable populations of parallel and antiparallel dimers that rapidly interconvert into each other, and the dipole-dipole interaction between zwitterionic head groups plays a minor role in determining the drug's tendency for self-aggregation. A simulation of circular dichroism (CD) spectra indicates that in experimental measurements, the signature CD spectrum of AmB aggregates should be attributed to higher-order oligomers rather than dimers. Finally, we suggest that oligomerization can impair the selectivity of AmB molecules for fungal membranes by increasing their hydrophobic drive for non-specific membrane insertion.

New Rimocidin/CE-108 Derivatives Obtained by a Crotonyl-CoA Carboxylase/Reductase Gene Disruption in Streptomyces diastaticus var. 108: Substrates for the Polyene Carboxamide Synthase PcsA

The rimJ gene, which codes for a crotonyl-CoA carboxylase/reductase, lies within the biosynthetic gene cluster for two polyketides belonging to the polyene macrolide group (CE-108 and rimocidin) produced by Streptomyces diastaticus var. 108. Disruption of rimJ by insertional inactivation gave rise to a recombinant strain overproducing new polyene derivatives besides the parental CE-108 (2a) and rimocidin (4a). The structure elucidation of one of them, CE-108D (3a), confirmed the incorporation of an alternative extender unit for elongation step 13. Other compounds were also overproduced in the fermentation broth of rimJ disruptant. The new compounds are in vivo substrates for the previously described polyene carboxamide synthase PcsA. The rimJ disruptant strain, constitutively expressing the pcsA gene, allowed the overproduction of CE-108E (3b), the corresponding carboxamide derivative of CE-108D (3a), with improved pharmacological properties.

Novel Nystatin A₁ derivatives exhibiting low host cell toxicity and antifungal activity in an in vitro model of oral candidosis

Opportunistic oral infections caused by Candida albicans are frequent problems in immunocompromised patients. Management of such infections is limited due to the low number of antifungal drugs available, their relatively high toxicity and the emergence of antifungal resistance. Given these issues, our investigations have focused on novel derivatives of the antifungal antibiotic Nystatin A1, generated by modifications at the amino group of this molecule. The aims of this study were to evaluate the antifungal effectiveness and host cell toxicity of these new compounds using an in vitro model of oral candidosis based on a reconstituted human oral epithelium (RHOE). Initial studies employing broth microdilution, revealed that against planktonic C. albicans, Nystatin A1 had lower minimal inhibitory concentration than novel derivatives. However, Nystatin A1 was also markedly more toxic against human keratinocyte cells. Interestingly, using live/dead staining to assess C. albicans and tissue cell viability after RHOE infection, Nystatin A1 derivatives were more active against Candida with lower toxicity to epithelial cells than the parent drug. Lactate dehydrogenase activity released by the RHOE indicated a fourfold reduction in tissue damage when certain Nystatin derivatives were used compared with Nystatin A1. Furthermore, compared with Nystatin A1, colonisation of the oral epithelium by C. albicans was notably reduced by the new polyenes. In the absence of antifungal agents, confocal laser scanning microscopy showed that C. albicans extensively invaded the RHOE. However, the presence of the novel derivatives greatly reduced or totally prevented this fungal invasion.

Engineered biosynthesis of disaccharide-modified polyene macrolides

Recent work has uncovered genes for two glycosyltransferases that are thought to catalyze mannosylation of mycosaminyl sugars of polyene macrolides. These two genes are nypY from Pseudonocardia sp. strain P1 and pegA from Actinoplanes caeruleus. Here we analyze these genes by heterologous expression in various strains of Streptomyces nodosus, producer of amphotericins, and in Streptomyces albidoflavus, which produces candicidins. The NypY glycosyltransferase converted amphotericins A and B and 7-oxo-amphotericin B to disaccharide-modified forms in vivo. The enzyme did not act on amphotericin analogs lacking exocyclic carboxyl or mycosamine amino groups. Both NypY and PegA acted on candicidins. This work confirms the functions of these glycosyltransferases and provides insights into their acceptor substrate tolerance. Disaccharide-modified polyenes may have potential as less toxic antibiotics.

The effect of sterols on amphotericin B self-aggregation in a lipid bilayer as revealed by free energy simulations

Amphotericin B (AmB) is an effective but toxic antifungal drug, known to increase the permeability of the cell membrane, presumably by assembling into transmembrane pores in a sterol-dependent manner. The aggregation of AmB molecules in a phospholipid bilayer is, thus, crucial for the drug's activity. To provide an insight into the molecular nature of this process, here, we report an atomistic molecular dynamics simulation study of AmB head-to-head dimerization in a phospholipid bilayer, a possible early stage of aggregation. To compare the effect of sterols on the thermodynamics of aggregation and the architecture of the resulting AmB-AmB complexes, free energy profiles for the dimerization in ergosterol- or cholesterol-containing and sterol-free membranes are derived from the simulations. These profiles demonstrate that although AmB dimers are formed in all the systems studied, they are significantly less favorable in the bilayer with ergosterol than in the cholesterol-containing or sterol-free ones. We investigate the structural and energetic determinants of this difference and discuss its consequences for the AmB mechanism of action.

Biosynthesis and pathway engineering of antifungal polyene macrolides in actinomycetes

Polyene macrolides are a large family of natural products typically produced by soil actinomycetes. Polyene macrolides are usually biosynthesized by modular and large type I polyketide synthases (PKSs), followed by several steps of sequential post-PKS modifications such as region-specific oxidations and glycosylations. Although known as powerful antibiotics containing potent antifungal activities (along with additional activities against parasites, enveloped viruses and prion diseases), their high toxicity toward mammalian cells and poor distribution in tissues have led to the continuous identification and structural modification of polyene macrolides to expand their general uses. Advances in in-depth investigations of the biosynthetic mechanism of polyene macrolides and the genetic manipulations of the polyene biosynthetic pathways provide great opportunities to generate new analogues. Recently, a novel class of polyene antibiotics was discovered (a disaccharide-containing NPP) that displays better pharmacological properties such as improved water-solubility and reduced hemolysis. In this review, we summarize the recent advances in the biosynthesis, pathway engineering, and regulation of polyene antibiotics in actinomycetes.

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.

The pcsA gene from Streptomyces diastaticus var. 108 encodes a polyene carboxamide synthase with broad substrate specificity for polyene amides biosynthesis

Two structurally related polyene macrolides are produced by Streptomyces diastaticus var. 108: rimocidin (3a) and CE-108 (2a). Both bioactive metabolites are biosynthesized from the same pathway through type I polyketide synthases by choosing a starter unit either acetate or butyrate, resulting in 2a or 3a formation, respectively. Two additional polyene amides, CE-108B (2b) and rimocidin B (3b), are also produced "in vivo" when this strain was genetically modified by transformation with engineered SCP2*-derived vectors carrying the ermE gene. The two polyene amides, 2b and 3b, showed improved pharmacological properties, and are generated by a tailoring activity involved in the conversion of the exocyclic carboxylic group of 2a and 3a into their amide derivatives. The improvement on some biological properties of the resulting polyenes, compared with that of the parental compounds, encourages our interest for isolating the tailoring gene responsible for the polyene carboxamide biosynthesis, aimed to use it as tool for generating new bioactive compounds. In this work, we describe the isolation from S. diastaticus var. 108 the corresponding gene, pcsA, encoding a polyene carboxamide synthase, belonging to the Class II glutamine amidotransferases and responsible for "in vivo" and "in vitro" formation of CE-108B (2b) and rimocidin B (3b). The fermentation broth from S. diastaticus var. 108 engineered with the appropriate pcsA gene construction, showed the polyene amides to be the major bioactive compounds.

Influence of a lipid bilayer on the conformational behavior of amphotericin B derivatives - A molecular dynamics study

Amphotericin B (AmB) is an effective but very toxic antifungal antibiotic. In our laboratory a series of AmB derivatives of improved selectivity of action was synthesized and tested. To understand molecular basis of this improvement, comparative conformational studies of amphotericin B and its two more selective derivatives were carried out in an aqueous solution and in a lipid membrane. These molecular simulation studies revealed that within a membrane environment the conformational behavior of the derivatives differs significantly from the one observed for the parent molecule. Possible reasons for such a difference are analyzed. Furthermore, we hypothesize that the observed conformational transition within the polar head of AmB derivatives may lead to destabilization of antibiotic-induced transmembrane channels. Consequently, the selective toxicity of the derivatives should increase as ergosterol-rich liquid-ordered domains are more rigid and conformationally ordered than their cholesterol-containing counterparts, and as such may better support less stable channel structure.

The in vitro characterization of polyene glycosyltransferases AmphDI and NysDI

The overproduction, purification, and in vitro characterization of the polyene glycosyltransferases (GTs) AmphDI and NysDI are reported. A novel nucleotidyltransferase mutant (RmlA Q83D) for the chemoenzymatic synthesis of unnatural GDP-sugar donors in conjunction with polyene GT-catalyzed sugar exchange/reverse reactions allowed the donor and acceptor specificities of these novel enzymes to be probed. The evaluation of polyene GT aglycon and GDP-sugar donor specificity revealed some tolerance to aglycon structural diversity, but stringent sugar specificity, and culminated in new polyene analogues in which L-gulose or D-mannose replace the native sugar D-mycosamine.

Engineered synthesis of 7-oxo- and 15-deoxy-15-oxo-amphotericins: insights into structure-activity relationships in polyene antibiotics

Site-directed mutagenesis and gene replacement were used to inactivate two ketoreductase (KR) domains within the amphotericin polyketide synthase in Streptomyces nodosus. The KR12 domain was inactivated in the DeltaamphNM strain, which produces 16-descarboxyl-16-methyl-amphotericins. The resulting mutant produced low levels of the expected 15-deoxy-15-oxo analogs that retained antifungal activity. These compounds can be useful for further chemical modification. Inactivation of the KR16 domain in the wild-type strain led to production of 7-oxo-amphotericin A and 7-oxo-amphotericin B in good yield. 7-oxo-amphotericin B was isolated, purified, and characterized as the N-acetyl methyl ester derivative. 7-oxo-amphotericin B had good antifungal activity and was less hemolytic than amphotericin B. These results indicate that modification at the C-7 position can improve the therapeutic index of amphotericin B.

Interactions of amphotericin B derivatives with lipid membranes—A molecular dynamics study

Amphotericin B (AmB) is a well-known polyene macrolide antibiotic used to treat systemic fungal infections. AmB targets more efficiently fungal than animal membranes. However, there are only minor differences in the mode of action of AmB against both types of membranes, which is a source of AmB toxicity. In this work, we analyzed interactions of two low toxic derivatives of AmB (SAmE and PAmE), synthesized in our laboratory, with lipid membranes. Molecular dynamics simulations of the lipid bilayers containing ergosterol (fungal cells) or cholesterol (animal cells) and the studied antibiotic molecules were performed to compare the structural and dynamic properties of AmB derivatives and the parent drug inside the membrane. A number of differences was found for AmB and its derivatives' behavior in cholesterol- and ergosterol-containing membranes. We found that PAmE and SAmE can penetrate deeper into the hydrophobic region of the membrane compared to AmB. Modification of the amino and carboxyl group of AmB also resulted in the conformational transition within the antibiotic's polar head. Wobbling dynamics differentiation, depending on the sterol present, was discovered for the AmB derivatives. These differences may be interpreted as molecular factors responsible for the improved selectivity observed macroscopically for the studied AmB derivatives.

Modulation of amphotericin B membrane interaction by cholesterol and ergosterol--a molecular dynamics study

Amphotericin B (AmB) is a well-known polyene macrolide antibiotic used to treat systemic fungal infections. According to a well-documented hypothesis, molecules of AmB form ionic membrane channels that are responsible for chemotherapeutic action. These channels disturb the barrier function of the cell membrane which, in consequence, leads to cell death. The presence of sterols in the cell membrane is necessary for full manifestation of the antibiotic's ionophoric activity, at least in vivo. Ergosterol-containing fungal membranes are targeted more efficiently by AmB than mammalian membranes containing cholesterol. However, a similar level of disturbance of fungal and mammalian membranes is responsible for serious toxicity of the antibiotic. Due to the importance of AmB and lack of better antifungal alternatives, the search for new less toxic derivatives of this antibiotic still continues. Therefore, studies of the AmB-membrane interaction are very important. The present work constitutes a continuation of a broad program of study on AmB mode of action in our group. In particular, molecular dynamics simulations of AmB monomers inside the bilayers of three different compositions (pure dimiristoylphosphatidylcholine (DMPC) and DMPC bilayer containing approximately 25 mol % of cholesterol or ergosterol) were carried out. In general, analysis of generated trajectories resulted in identifying many significant differences in the behavior of AmB monomers depending on the membrane environment. In particular, it was established that the antibiotic increases the internal order of DMPC bilayer containing 25 mol % of cholesterol, while it has no effect on the order of the bilayer with the same amount of ergosterol. Performed calculations also revealed that relatively rigid and elongated AmB molecules exhibit higher affinity toward the sterol-containing lo phases and, therefore, may be cumulated in ordered membrane domains (e.g., lipid rafts). Since the partition coefficient between the ld and lo phase appears to be greater in the case of the ergosterol- compared to cholesterol-containing membrane, this effect can be also discussed as the possible origin of AmB-selective toxicity and indirect sterol involvement in expression of AmB activity.

Interaction of amphotericin B and its selected derivatives with membranes: molecular modeling studies

Amphotericin B (AmB) is a well-known antifungal antibiotic that has been used in the clinic for about five decades. Despite its chemotherapeutic importance, AmB is quite toxic and many efforts have been made to improve its pharmacological properties, e.g., by chemical modifications. The lipid membrane is a molecular target for AmB, however, due to heterogeneity of its components, the molecular mechanism of AmB action is still unclear. The lack of this knowledge hinders rational designing of new and less toxic AmB derivatives. Our review is a critical presentation of the current understanding of AmB molecular mechanism of action at the membrane level. Except the experimental approach, the extensive overview of molecular modeling studies, performed mostly in our lab, is presented. The results of interactions between AmB or some of its derivatives and lipid model membranes are discussed. In our studies, different biomembrane models and different associate states of the antibiotic were included. Presented molecular modeling approach is especially valuable with regard to a new paradigm of the structure of lipid membrane containing liquid-ordered domains. Hopefully, all these complementary experimental/computational approaches are going to reach the point at which a new hypothesis about molecular mechanism of AmB activity and selectivity will be put forward.

Selective activity of polyene macrolides produced by genetically modified Streptomyces on Trypanosoma cruzi

The growth inhibitory effects on Trypanosoma cruzi of several natural tetraene macrolides and their derivatives were studied and compared with that of amphotericin B. All tetraenes strongly inhibited in vitro multiplication. Proliferation of epimastigotes was arrested by all these drugs at < or =3.6 microM, which were also active on amastigotes proliferating in fibroblasts. Compared with amphotericin B, the compounds were less effective but also less toxic, showing no effect on the proliferation of J774 and NCTC 929 mammalian cells at concentrations active against the parasites. CE-108B (a polyene amide) appeared to be an especially potent trypanocidal compound, with strong in vivo trypanocidal activity and very low or no toxic side effects, and thus should be considered for further studies.

A tailoring activity is responsible for generating polyene amide derivatives in Streptomyces diastaticus var. 108

We recently characterized rimocidin B (3b) and CE-108B (4b) as two polyene amides with improved pharmacological properties, produced by genetically modified Streptomyces diastaticus var. 108. In this work, genetic and biochemical analysis of the producer strain show that the two amides are derived from the parental polyenes rimocidin (3a) and CE-108 (4a) by a post-PKS modification of the free side chain carboxylic acid. This modification is mediated by an amidotransferase activity operating after the biosynthesis of rimocidin (3a) and CE-108 (4a) are completed. Two polyenes, intermediates of the biosynthetic pathway of rimocidin (3a) and CE-108 (4a), were also isolated and shown to have some improved pharmacological properties compared with the final products.

Two polyene amides produced by genetically modified Streptomyces diastaticus var. 108

Streptomyces diastaticus var. 108, a newly isolated strain, was recently characterized as a producer of two polyene macrolide antibiotics (rimocidin and CE-108), and the biosynthetic gene cluster was partially characterized. When the producer strain was genetically modified by transformation with some engineered SCP2*-derived vectors carrying the ermE gene, two previously uncharacterized macrolides were detected in the fermentation broth of the recombinant strain and chemically characterized as the amides of the parental polyene carboxylic acids. The biological activity and some in vitro toxicity assays showed that this chemical modification resulted in pharmaceuticals with improved biological properties compared with the parental products.

How does the N-acylation and esterification of amphotericin B molecule affect its interactions with cellular membrane components—the Langmuir monolayer study

This work presents the results of Langmuir monolayers study of two amphotericin B derivatives obtained by N-acylation (N-acetylamphotericin B, Ac-AmB) and esterification (amphotericin B methyl ester, AME) of the parent AmB molecule. The main objective of present investigations was to examine the strength and nature of interactions of Ac-AmB and AME with natural membrane components as compared to AmB, and verify the monolayer results with biological studies in vitro. Our experiments were based on surface pressure-area measurements of mixed monolayers formed by the investigated antibiotics and sterols/DPPC. The interactions were analyzed with the following dependencies: compression modulus-surface pressure, mean molecular area-composition, excess molecular area-composition and excess free energy-composition plots. It has been found that both Ac-AmB and AME form monolayers of a liquid expanded state and their stability is highest as compared to AmB films. The investigated compounds mix in monolayers with natural membrane components within the whole range of the antibiotic mole fraction. The quantitative analysis of the interactions of the investigated antibiotics with sterols and DPPC as well as sterols/DPPC interactions allow us to verify the monolayer results with biological results. A good correlation between both kinds of studies has been found.

Biosynthesis of amphotericin derivatives lacking exocyclic carboxyl groups

Amphotericin B is a medically important antifungal antibiotic that is also active against human immunodeficiency virus, Leishmania parasites, and prion diseases. The therapeutic use of amphotericin B is restricted by severe side effects that can be moderated by liposomal formulation or structural alteration. Chemical modification has shown that suppression of charge on the exocyclic carboxyl group of amphotericin B substantially reduces toxicity. We report targeted deletions of the amphN cytochrome P450 gene from the chromosome of the amphotericin-producing bacterium Streptomyces nodosus. The mutant strains produced amphotericin analogues in which methyl groups replace the exocyclic carboxyl groups. These compounds retained antifungal activity and had reduced hemolytic activity.

N-(1-Piperidinepropionyl)amphotericin B methyl ester (PAME)— a new derivative of the antifungal antibiotic amphotericin B: Searching for the mechanism of its reduced toxicity

N-(1-piperidinepropionyl)amphotericin B methyl ester (in short, PAME), a low-toxicity amphotericin B derivative, has been investigated in Langmuir monolayers at the air/water interface alone and in mixtures with cellular membrane sterols (a mammalian sterol, cholesterol, and a fungal sterol, ergosterol) and a model phospholipid (DPPC). The analysis of the strength of interaction between PAME and both sterols as well as DPPC was based, on surface pressure measurements and analysis of the isothermal compressibility (C(s)(-1)), the mean area per molecule (A(12)), the excess free energy of mixing (DeltaG(Exc)) and the total free energy of mixing (DeltaG(M)). It has been found that the interactions between PAME and sterols are attractive; however, their strength is significantly weaker for mixtures of PAME with cholesterol than with ergosterol. This casts light on the improved selectivity of PAME toward fungal cells. The strongest interactions, found for PAME/DPPC mixtures, proved an important role of DPPC in the mechanism of reduced toxicity of PAME as compared to amphotericin B. Due to stable complex formation between PAME and DPPC the antibiotic is immobilized with DPPC molecules, which reduces the concentration of free antibiotic, which is capable of interacting with membrane sterols.

Interactions of amphotericin B derivative of low toxicity with biological membrane components—the Langmuir monolayer approach

Amphotericin B (AmB)--a polyene macrolide antibiotic--exhibits strong antifungal activity, however, is known to be very toxic to mammalian cells. In order to decrease AmB toxicity, a number of its derivatives have been synthesized. Basing on in vitro and in vivo research, it was evidenced that one of AmB derivatives, namely N-methyl-N-D-fructopyranosylamphotericin B methyl ester (in short MF-AME) retained most of the antifungal activity of the parent antibiotic, however, exhibited dramatically lower animal toxicity. Therefore, MF-AME seems to be a very promising modification product of AmB. However, further development of this derivative as potential new antifungal drug requires the elucidation of its molecular mechanism of reduced toxicity, which was the aim of the present investigations. Our studies were based on examining the binding energies by determining the strength of interaction between MF-AME and membrane sterols (ergosterol-fungi sterol, and cholesterol-mammalian sterol) and DPPC (model membrane phospholipid) using the Langmuir monolayer technique, which serves as a model of cellular membrane. Our results revealed that at low concentration the affinity of MF-AME to ergosterol is considerably stronger as compared to cholesterol, which correlates with the improved selective toxicity of this drug. It is of importance that the presence of phospholipids is essential since--due to very strong interactions between MF-AME and DPPC--the antibiotic used in higher concentration is "immobilized" by DPPC molecules, which reduces the concentration of free antibiotic, thus enabling it to selectively interact with both sterols.

Biosynthesis of the polyene macrolide antibiotic nystatin in Streptomyces noursei

The polyene macrolide antibiotic nystatin, produced commercially by the bacterium Streptomyces noursei, is an important antifungal agent used in human therapy for treatment of certain types of mycoses. Early studies on nystatin biosynthesis in S. noursei provided important information regarding the precursors utilised in nystatin biosynthesis and factors affecting antibiotic yield. New insights into the enzymology of nystatin synthesis became available after the gene cluster governing nystatin biosynthesis in S. noursei was cloned and analysed. Six large polyketide synthase proteins were implicated in the formation of the nystatin macrolactone ring, while other enzymes, such as P450 monooxygenases and glycosyltransferase, were assumed responsible for ring "decoration". The latter data, supported by analysis of the polyene mixture synthesised by the nystatin producer, helped elucidate the complete nystatin biosynthetic pathway. This information has proved useful for engineered biosynthesis of novel nystatin analogues, suggesting a plausible route for the generation of potentially safer and more efficient antifungal drugs.

Interaction of amphotericin B and its low toxic derivative, N-methyl-N-D-fructosyl amphotericin B methyl ester, with fungal, mammalian and bacterial cells measured by the energy transfer method

Amphotericin B (AMB) derivative, N-methyl-N-D-fructosyl amphotericin B methyl ester (MFAME) retains the broad antifungal spectrum and potency of the parent antibiotic, whereas its toxicity towards mammalian cells is reduced by about two orders of magnitude. The purpose of this work was to find out whether the differences observed in the toxicity of MFAME and native AMB are due to the differential drugs affinity to fungal and mammalian cell membranes. Comparative studies on AMB and MFAME biological activity and their affinity to fungal, mammalian and bacterial cells were performed. The interaction of AMB and MFAME with cells have been studied by fluorescence method based on the energy transfer between membrane fluorescent probe (donor) and the polyenic chromophore of the antibiotic (acceptor) simultaneously present in the cell membrane. The amount of the antibiotic bound to cells was indicated by the extent of fluorescence quenching of 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) or 1,6-diphenyl-1,3,5-hexatriene (DPH) by polyenic chromophore of the antibiotic. The results obtained indicate that binding extent and characteristics for both antibiotics are comparable in the three types of cells studied. Dramatically lower toxicity of MFAME as compared to AMB towards mammalian cells is not related to the antibiotic-cell affinity, but rather to different consequences of these interactions for cells, reflected in membrane permeabilization. MFAME is definitely less effective than parent AMB in the permeabilizing species formation in mammalian cell membrane.

Molecular aspects of the interaction between amphotericin B and a phospholipid bilayer: molecular dynamics studies

Amphotericin B (AmB) is a polyene macrolide antibiotic used to treat systemic fungal infections. The molecular mechanism of AmB action is still only partly characterized. AmB interacts with cell-membrane components and forms membrane channels that eventually lead to cell death. The interaction between AmB and the membrane surface can be regarded as the first (presumably crucial) step on the way to channel formation. In this study molecular dynamics simulations were performed for an AmB-lipid bilayer model in order to characterize the molecular aspects of AmB-membrane interactions. The system studied contained a box of 200 dimyristoylphosphatidylcholine (DMPC) molecules, a single AmB molecule placed on the surface of the lipid bilayer and 8,065 water molecules. Two molecular dynamics simulations (NVT ensemble), each lasting 1 ns, were performed for the model studied. Two different programs, CHARMM and NAMD2, were used in order to test simulation conditions. The analysis of MD trajectories brought interesting information concerning interactions between polar groups of AmB and both DMPC and water molecules. Our studies show that AmB preferentially took a vertical position, perpendicular to the membrane surface, with no propensity to enter the membrane. Our finding may suggest that a single AmB molecule entering the membrane is very unlikely.

Antifungal therapy--state of the art at the beginning of the 21st century

The most relevant information on the present state of the art of antifungal chemotherapy is reviewed in this chapter. For dermatomycoses a variety of topical antifungals are available, and safe and efficacious systemic treatment, especially with the fungicidal drug terbinafine, is possible. The duration of treatment can be drastically reduced. Substantial progress in the armamentarium of drugs for invasive fungal infections has been made, and a new class of antifungals, echinocandins, is now in clinical use. The following drugs in oral and/or intravenous formulations are available: the broad spectrum polyene amphotericin B with its new "clothes"; the sterol biosynthesis inhibitors fluconazole, itraconazole, and voriconazole; the glucan synthase inhibitor caspofungin; and the combination partner flucytosine. New therapy schedules have been studied; combination therapy has found a significant place in the treatment of severely compromised patients, and the field of prevention and empiric therapy is fast moving. Guidelines exist nowadays for the treatment of various fungal diseases and maintenance therapy. New approaches interfering with host defenses or pathogenicity of fungal cells are being investigated, and molecular biologists are looking for new targets studying the genomics of pathogenic fungi.

Relative aggregation state and hemolytic activity of amphotericin B encapsulated by poly(ethylene oxide)-block-poly(N-hexyl-L-aspartamide)-acyl conjugate micelles: effects of acyl chain length

We systematically altered the chemical structure of the core-forming poly(L-amino acid) block of an amphiphilic diblock copolymer series based on poly(ethylene oxide)-block-poly(N-hexyl-L-aspartamide), PEO-b-p(N-HA), acyl esters by varying the length of the attached acyl side chain. Drug-loaded micelles were prepared in good yield by a modified solvent evaporation procedure. In addition, the relative aggregation state and hemolytic activity of encapsulated amphotericin B (AmB) were analyzed by absorption spectroscopy. The length of the attached acyl side chain in PEO-b-p(N-HA) acyl ester micelles modulates the relative aggregation state of encapsulated AmB. Furthermore, acyl chain length appears to have a profound influence on the time-dependent hemolytic profile of encapsulated AmB toward bovine erythrocytes. For all acyl conjugate micelle-AmB formulations, the onset of hemolysis is delayed relative to free AmB. Particularly in the case of stearate ester micelles, the incomplete and gradual build-up of hemolysis might reflect the sustained release of drug over a period of 24 h. Based on the corresponding absorption spectrum, we speculate that encapsulated AmB may interact strongly with stearate side chains, resulting in sustained release. Via chemical manipulation of the core-forming region, it may be possible to fine-tune the release of encapsulated AmB from PEO-b-p(N-HA)-acyl ester micelles.

Comparative molecular dynamics simulations of amphotericin B-cholesterol/ergosterol membrane channels

Amphotericin B (AmB) is a very effective anti-fungal polyene macrolide antibiotic whose usage is limited by its toxicity. Lack of a complete understanding of AmB's molecular mechanism has impeded attempts to design less toxic AmB derivatives. The antibiotic is known to interact with sterols present in the cell membrane to form ion channels that disrupt membrane function. The slightly higher affinity of AmB toward ergosterol (dominant sterol in fungal cells) than cholesterol (mammalian sterol) is regarded as the most essential factor on which antifungal chemotherapy is based. To study these differences at the molecular level, two realistic model membrane channels containing molecules of AmB, sterol (cholesterol or ergosterol), phospholipid, and water were studied by molecular dynamics (MD) simulations. Comparative analysis of the simulation data revealed that the sterol type has noticeable effect on the properties of AmB membrane channels. In addition to having a larger size, the AmB channel in the ergosterol-containing membrane has a more pronounced pattern of intermolecular hydrogen bonds. The interaction between the antibiotic and ergosterol is more specific than between the antibiotic and cholesterol. These observed differences suggest that the channel in the ergosterol-containing membrane is more stable and, due to its larger size, would have a higher ion conductance. These observations are in agreement with experiments.