C2'-OH of amphotericin B plays an important role in binding the primary sterol of human cells but not yeast cells

Formulation Strategies to Overcome Amphotericin B Induced Toxicity

Fungal infection poses a major global threat to public health because of its wide prevalence, severe mortality rate, challenges involved in diagnosis and treatment, and the emergence of drug-resistant fungal strains. Millions of people are getting affected by fungal infection, and around 3.8 million people face death per year due to fungal infection, as per the latest report. The polyene antibiotic AmB has an extensive record of use as a therapeutic moiety against systemic fungal infection and leishmaniasis since 1960. AmB has broad-spectrum fungistatic and fungicidal activity. AmB exerts its therapeutic activity at the cellular level by binding to fungal sterol and forming hydrophilic pores, releasing essential cellular components and ions into the extracellular fluid, leading to cell death. Despite using AmB as an antifungal and antileishmanial at a broad scale, its clinical use is limited due to drug-induced nephrotoxicity resulting from binding the aggregated form of the drug to mammalian sterol. To mitigate AmB-induced toxicity and to get better anti-fungal therapeutic outcomes, researchers have developed nanoformulations, self-assembled formulations, prodrugs, cholesterol- and albumin-based AmB formulations, AmB-mAb combination therapy, and AmB cochleates. These formulations have helped to reduce toxicity to a certain extent by controlling the aggregation state of AmB, providing sustained drug release, and altering the physicochemical and pharmacokinetic parameters of AmB. Although the preclinical outcome of AmB formulations is quite satisfactory, its parallel result at the clinical level is insignificant. However, the safety and efficacy of AmB therapy can be improved at the clinical stage by continuous investigation and collaboration among researchers, clinicians, and pharmaceutical companies.

A Formal Synthesis of (+)-Hannokinol Using a Chiral Horner-Wittig Reagent

Herein, we report a concise and efficient formal synthesis of (+)-hannokinol. Key to this new strategy is the use of a chiral Horner-Wittig reagent, readily available from 2-deoxy-D-ribose, to introduce the chiral 1,3-diol motif.

A Study on the Effect of Quaternization of Polyene Antibiotics' Structures on Their Activity, Toxicity, and Impact on Membrane Models

Polyene antibiotics have been used in antifungal therapy since the mid-twentieth century. They are highly valued for their broad spectrum of activity and the rarity of pathogen resistance to their action. However, their use in the treatment of systemic mycoses often results in serious side-effects. Recently, there has been a renewed interest in the development of new antifungal drugs based on polyenes, particularly due to the emergence of highly dangerous pathogenic strains of fungi, such as Candida auris, and the increased incidence of mucormycosis. Considerable understanding has been established regarding the structure-biological activity relationships of polyene antifungals. Yet, no previous studies have examined the effect of introducing quaternized fragments into their molecular structure. In this study, we present a series of amides of amphotericin B, nystatin, and natamycin bearing a quaternized group in the side chain, and discuss their biological properties: antifungal activity, cytotoxicity, and effects on lipid bilayers that mimic fungal and mammalian cell membranes. Our research findings suggest that the nature of the introduced quaternized residue plays a more significant role than merely the introduction of a constant positive charge. Among the tested polyenes, derivatives 4b, 5b, and 6b, which contain a fragment of N-methyl-4-(aminomethyl)pyridinium in their structure, are particularly noteworthy due to their biological activity.

Origins of Selectivity in Glycosylation Reactions with Saccharosamine Donors

A combination of DFT calculations and experiments is used to describe how the selection of a promoter can control the stereochemical outcome of glycosylation reactions with the deoxy sugar saccharosamine. Depending on the promoter, either α- or β-linked reactive intermediates are formed. These studies show that differential modes of activation lead to the formation of distinct intermediates that undergo highly selective reactions through an SN2-like mechanism.

Nanogel Particles Based on Modified Nucleosides and Oligosaccharides as Advanced Delivery System

This work addresses the challenge of delivering bioactive molecules by designing biocompatible nanogel particles (NGPs) utilizing rationally modified nature-sourced building blocks: capryl-oligochitosan and oxidized inosine. Capryl substituents endowed the resultant NGPs with membrane-penetration capabilities, while purine-containing inosine allowed H-bond/π-π/π-cation interactions. The prepared NGPs were complexed with carboxyfluorescein-labeled single-stranded oligonucleotide (FAM-oligo) and DsRed-encoding plasmid DNA. The successful delivery of FAM-oligo to the cell cytoplasm of the Nicotiana benthamiana plant was observed. Alexa 555-labeled bovine serum albumin (Alexa 555-BSA) was also efficiently encapsulated and delivered to the plant. In addition to delivering FAM-oligo and Alexa 555-BSA separately, NGPs also successfully co-delivered both biomolecules to the plant. Finally, NGPs successfully encapsulated the drug amphotericin B and reduced its toxicity while maintaining its efficacy. The presented findings suggest that NGPs may become a promising platform for the advanced delivery of bioactive molecules in various applications.

Tuning sterol extraction kinetics yields a renal-sparing polyene antifungal

Decades of previous efforts to develop renal-sparing polyene antifungals were misguided by the classic membrane permeabilization model1. Recently, the clinically vital but also highly renal-toxic small-molecule natural product amphotericin B was instead found to kill fungi primarily by forming extramembraneous sponge-like aggregates that extract ergosterol from lipid bilayers2-6. Here we show that rapid and selective extraction of fungal ergosterol can yield potent and renal-sparing polyene antifungals. Cholesterol extraction was found to drive the toxicity of amphotericin B to human renal cells. Our examination of high-resolution structures of amphotericin B sponges in sterol-free and sterol-bound states guided us to a promising structural derivative that does not bind cholesterol and is thus renal sparing. This derivative was also less potent because it extracts ergosterol more slowly. Selective acceleration of ergosterol extraction with a second structural modification yielded a new polyene, AM-2-19, that is renal sparing in mice and primary human renal cells, potent against hundreds of pathogenic fungal strains, resistance evasive following serial passage in vitro and highly efficacious in animal models of invasive fungal infections. Thus, rational tuning of the dynamics of interactions between small molecules may lead to better treatments for fungal infections that still kill millions of people annually7,8 and potentially other resistance-evasive antimicrobials, including those that have recently been shown to operate through supramolecular structures that target specific lipids9.

Discovery of amphotericin B, an antifungal drug as tyrosinase inhibitor with potent anti-melanogenic activity

Tyrosinase, a rate-limiting enzyme for melanin production, has been the most efficient target for the development of depigmenting agents. Although hydroquinone, kojic acid, and arbutin are the most well-known tyrosinase inhibitors, their adverse effects are inevitable. In the present study, an in silico drug repositioning combined with experimental validation was performed to search for novel potent tyrosinase inhibitors. Docking-based virtual screening results revealed that, among the 3210 FDA-approved drugs available in the ZINC database, amphotericin B, an antifungal drug exhibited the highest binding efficiency against human tyrosinase. Results from tyrosinase inhibition assay demonstrated that amphotericin B could inhibit the activity of mushroom and cellular tyrosinases, especially from MNT-1 human melanoma cells. Molecular modeling results revealed that amphotericin B/human tyrosinase complex exhibited high stability in an aqueous environment. Melanin assay results demonstrated that amphotericin B significantly suppressed melanin production in α-MSH-induced B16F10 murine melanoma and MNT-1 human melanoma cell lines better than the known inhibitor, kojic acid. Mechanistically, amphotericin B treatment significantly activated ERK and Akt signaling pathways, resulting in the decreased expression of MITF and tyrosinase. The obtained results may pursue pre-clinical and clinical studies to examine the possibility of using amphotericin B as an alternative treatment for hyperpigmentation disorders.

Segmental Dynamics of Membranous Cholesterol are Coupled

Cholesterol promotes the structural integrity of the fluid cell membrane and interacts dynamically with many membrane proteins to regulate function. Understanding site-resolved cholesterol structural dynamics is thus important. This long-standing challenge has thus far been addressed, in part, by selective isotopic labeling approaches. Here we present a new 3D solid-state NMR (SSNMR) experiment utilizing scalar 13C-13C polarization transfer and recoupling of the 1H-13C interactions in order to determine average dipolar couplings for all 1H-13C vectors in uniformly 13C-enriched cholesterol. The experimentally determined order parameters (OP) agree exceptionally well with molecular dynamics (MD) trajectories and reveal coupling among several conformational degrees of freedom in cholesterol molecules. Quantum chemistry shielding calculations further support this conclusion and specifically demonstrate that ring tilt and rotation are coupled to changes in tail conformation and that these coupled segmental dynamics dictate the orientation of cholesterol. These findings advance our understanding of physiologically relevant dynamics of cholesterol, and the methods that revealed them have broader potential to characterize how structural dynamics of other small molecules impact their biological functions.

A Theoretical Analysis of Interaction Energies and Intermolecular Interactions between Amphotericin B and Potential Bioconjugates Used in the Modification of Nanocarriers for Drug Delivery

Amphotericin B (AmB) is an antibiotic with a wide spectrum of action and low multidrug resistance, although it exhibits self-aggregation, low specificity, and solubility in aqueous media. An alternative for its oral administration is its encapsulation in polymers modified with bioconjugates. The aim of the present computational research is to determine the affinity between AmB and six bioconjugates to define which one could be more suitable. The CAM-B3LYP-D3/6-31+G(d,p) method was used for all computational calculations. The dimerization enthalpy of the most stable and abundant systems at pH = 7 allows obtaining this affinity order: AmB_1,2-distearoyl-sn-glycerol-3-phosphorylethanolamine (DSPE) > AmB_γ-cyclodextrin > AmB_DSPEc > AmB_retinol > AmB_cholesterol > AmB_dodecanol, where DSPEc is a DSPE analog. Quantum theory of atoms in molecules, the non-covalent interactions index, and natural bond orbital analysis revealed the highest abundance of noncovalent interactions for AmB-DSPE (51), about twice the number of interactions of the other dimers. Depending on the interactions’ strength and abundance of the AmB-DSPE dimer, these are classified as strong: O-H---O (2), N-H---O (3) and weak: C-H---O (25), H---H (18), C-H---C (3). Although the C-H---O hydrogen bond is weak, the number of interactions involved in all dimers cannot be underestimated. Thus, non-covalent interactions drive the stabilization of copolymers, and from our analysis, the most promising candidates for encapsulating are DSPE and γ-cyclodextrin.

Semisynthetic Amides of Amphotericin B and Nystatin A1: A Comparative Study of In Vitro Activity/Toxicity Ratio in Relation to Selectivity to Ergosterol Membranes

Polyene antifungal amphotericin B (AmB) has been used for over 60 years, and remains a valuable clinical treatment for systemic mycoses, due to its broad antifungal activity and low rate of emerging resistance. There is no consensus on how exactly it kills fungal cells but it is certain that AmB and the closely-related nystatin (Nys) can form pores in membranes and have a higher affinity towards ergosterol than cholesterol. Notably, the high nephro- and hemolytic toxicity of polyenes and their low solubility in water have led to efforts to improve their properties. We present the synthesis of new amphotericin and nystatin amides and a comparative study of the effects of identical modifications of AmB and Nys on the relationship between their structure and properties. Generally, increases in the activity/toxicity ratio were in good agreement with increasing ratios of selective permeabilization of ergosterol- vs. cholesterol-containing membranes. We also show that the introduced modifications had an effect on the sensitivity of mutant yeast strains with alterations in ergosterol biosynthesis to the studied polyenes, suggesting a varying affinity towards intermediate ergosterol precursors. Three new water-soluble nystatin derivatives showed a prominent improvement in safety and were selected as promising candidates for drug development.

Effect of Post-Polyketide Synthase Modification Groups on Property and Activity of Polyene Macrolides

The biosynthesis of polyene macrolides, which are natural products produced by soil actinomycetes, have been extensively explored, and recent studies have focused on the effects of post-polyketide synthase (PKS) modifications to polyene macrolides on toxicity, water solubility, and antifungal activity. For example, there are interactions between glycosyl, carboxyl, and hydroxyl or epoxy groups generated in the post-PKS modification steps; salt bridges will be formed between carboxylate and ammonium on the mycosamine; and water bridges will be formed between hydroxy and hydroxyl on mycosamine. These interactions will affect their water solubility and substrate-recognition specificity. This review summarizes research related to these post-PKS modification groups and discusses some genetic engineering operation problems and solutions that may be encountered when modifying these post-PKS modification groups. In addition, this review provides a basis for the structural research of polyene macrolide antibiotics and contributes to comprehensive and systematic knowledge, and it may thus encourage researchers to develop novel antifungal drugs with higher therapeutic indexes and medical values.

Total Synthesis of Cryptoconcatone D via Construction of 1,3-Diol Units Using Chiral Horner-Wittig Reagents

The modular synthesis of 1,3-polyols using a chiral phosphine oxide building block is reported. This versatile building block works in a repetitive way for the stereocontrolled synthesis of a tetraol key intermediate, which serves for the first total synthesis of the potentially anti-inflammatory natural product cryptoconcatone D. A new route toward the chiral building block is also presented: Starting from 2-deoxy-d-ribose, the optimized sequence now makes the use of the building block more attractive to practicing chemists again.

Natamycin sequesters ergosterol and interferes with substrate transport by the lysine transporter Lyp1 from yeast

Natamycin is a polyene macrolide, widely employed to treat fungal keratitis and other yeast infections as well as to protect food products against fungal molds. In contrast to other polyene macrolides, such as nystatin or amphotericin B, natamycin does not form pores in yeast membranes, and its mode of action is not well understood. Here, we have employed a variety of spectroscopic methods, computational modeling, and membrane reconstitution to study the molecular interactions of natamycin underlying its antifungal activity. We find that natamycin forms aggregates in an aqueous solution with strongly altered optical properties compared to monomeric natamycin. Interaction of natamycin with model membranes results in a concentration-dependent fluorescence increase which is more pronounced for ergosterol- compared to cholesterol-containing membranes up to 20 mol% sterol. Evidence for formation of specific ergosterol-natamycin complexes in the bilayer is provided. Using nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, we find that natamycin sequesters sterols, thereby interfering with their well-known ability to order acyl chains in lipid bilayers. This effect is more pronounced for membranes containing the sterol of fungi, ergosterol, compared to those containing mammalian cholesterol. Natamycin interferes with ergosterol-dependent transport of lysine by the yeast transporter Lyp1, which we propose to be due to the sequestering of ergosterol, a mechanism that also affects other plasma membrane proteins. Our results provide a mechanistic explanation for the selective antifungal activity of natamycin, which can set the stage for rational design of novel polyenes in the future.

Indications that the Antimycotic Drug Amphotericin B Enhances the Impact of Platelets on Aspergillus

Platelets are currently thought to harbor antimicrobial functions and might therefore play a crucial role in infections, e.g., those caused by Aspergillus or mucormycetes. The incidence of invasive fungal infections is increasing, particularly during the coronavirus disease 2019 (COVID-19) pandemic, and such infections continue to be life-threatening in immunocompromised patients. For this reason, the interaction of antimycotics with platelets is a key issue to evaluate modern therapeutic regimens. Amphotericin B (AmB) is widely used for the therapy of invasive fungal infections either as deoxycholate (AmB-D) or as a liposomal formulation (L-AmB). We showed that AmB strongly activates platelets within a few minutes. AmB concentrations commonly measured in the blood of patients were sufficient to stimulate platelets, indicating that this effect is highly relevant in vivo. The stimulating effect was corroborated by a broad spectrum of platelet activation parameters, including degranulation, aggregation, budding of microparticles, morphological changes, and enhanced adherence to fungal hyphae. Comparison between the deoxycholate and the liposomal formulation excluded the possibility that the liposomal part of L-Amb is responsible for these effects, as no difference was visible. The induction of platelet activation and alteration by L-AmB resulted in the activation of other parts of innate immunity, such as stimulation of the complement cascade and interaction with granulocytes. These mechanisms might substantially fuel the antifungal immune reaction in invasive mycoses. On the other hand, thrombosis and excessive inflammatory processes might occur via these mechanisms. Furthermore, the viability of L-AmB-activated platelets was consequently decreased, a process that might contribute to thrombocytopenia in patients.

- Amphotericin B: A drug of choice for Visceral Leishmaniasis

Visceral leishmaniasis or Kala-azar is a vector-borne disease caused by an intracellular parasite of the genus leishmania. In India, Amphotericin B (AmB) is a first-line medication for treating leishmaniasis. After a large-scale resistance to pentavalent antimony therapy developed in Bihar state, it was rediscovered as an effective treatment for Leishmania donovani infection. AmB which binds to the ergosterol of protozoan cells causes a change in membrane integrity resulting in ions leakage, and ultimately leading to cell death. The treatment effect of liposomal AmB can be seen more quickly than deoxycholate AmB because, it has some toxic effects, but liposomal AmB is significantly less toxic. Evidence from studies suggested that ABLC (Abelcet) and ABCD (Amphotec) are as effective as L-AmB but Liposomal form (Ambisome) is a more widely accepted treatment option than conventional ones. Nevertheless, the world needs some way more efficient antileishmanial drugs that are less toxic and less expensive for people living with parasitic infections caused by Leishmania. So, academics, researchers, and sponsors need to focus on finding such drugs. This review provides a summary of the chemical, pharmacokinetic, drug-target interactions, stability, dose efficacy, and many other characteristics of the AmB and their various formulations. We have also highlighted the clinically significant aspects of PKDL and VL co-infection with HIV/TB.

Amphotericin B Tamed by Salicylic Acid

Although Amphotericin B (AmB) is considered as the "gold standard" treatment for deep fungal infections, owing to its excellent antifungal effect, it often causes severe hemolytic toxicity and nephrotoxicity, which limits its clinical use. We designed and synthesized AmB derivatives by attaching salicylic acid (SA) to the carboxyl group and confirmed their structures using ¹H NMR, ¹³C NMR, HR-MS, and IR. We evaluated its biological activity in vitro and measured its ultraviolet-visible (UV-vis) absorption spectrum. The AmB-SA conjugates exhibited good antifungal effects against Candida albicans, Candida glabrata, and Cryptococcus neoformans compared with AmB, and the renal cytotoxicity toward HEK 293T cells in vitro was significantly reduced, with almost no nephrotoxicity in the therapeutic window of the drug. At the same time, the hemolytic toxicity was significantly reduced. Therefore, modification of AmB by introducing SA is an effective strategy to maintain the broad antifungal activity of AmB and reduce its cytotoxicity. These AmB derivatives could be applied in clinical therapy in the future.

Benzylic Dehydroxylation of Echinocandin Antifungal Drugs Restores Efficacy against Resistance Conferred by Mutated Glucan Synthase

Each year, infections caused by fungal pathogens claim the lives of about 1.6 million people and affect the health of over a billion people worldwide. Among the most recently developed antifungal drugs are the echinocandins, which noncompetitively inhibit β-glucan synthase, a membrane-bound protein complex that catalyzes the formation of the main polysaccharide component of the fungal cell wall. Resistance to echinocandins is conferred by mutations in FKS genes, which encode the catalytic subunit of the β-glucan synthase complex. Here, we report that selective removal of the benzylic alcohol of the nonproteinogenic amino acid 3S,4S-dihydroxy-l-homotyrosine of the echinocandins anidulafungin and rezafungin, restored their efficacy against a large panel of echinocandin-resistant Candida strains. The dehydroxylated compounds did not significantly affect the viability of human-derived cell culture lines. An analysis of the efficacy of the dehydroxylated echinocandins against resistant Candida strains, which contain mutations in the FKS1 and/or FKS2 genes of the parental strains, identified amino acids of the Fks proteins that are likely to reside in proximity to the l-homotyrosine residue of the bound drug. This study describes the first example of a chemical modification strategy to restore the efficacy of echinocandin drugs, which have a critical place in the arsenal of antifungal drugs, against resistant fungal pathogens.

Catalytic Stereoselective Synthesis of 2-Deoxy α-glycosides Using Glycosyl Ortho-[1-(p-MeOPhenyl)Vinyl]Benzoates (PMPVB) Donors

2-Deoxy Glycosyl Ortho-[1-(p-MeOPhenyl)Vinyl]Benzoates (PMPVB) donors have been presented as stable, reactive glycosyl donors for the synthesis of 2-deoxy α-glycosides. The donors react under Brønsted acid conditions to provide the 2-deoxy-α-glycosides...

Fungicidal amphotericin B sponges are assemblies of staggered asymmetric homodimers encasing large void volumes

Amphotericin B (AmB) is a powerful but toxic fungicide that operates via enigmatic small molecule-small molecule interactions. This mechanism has challenged the frontiers of structural biology for half a century. We recently showed AmB primarily forms extramembranous aggregates that kill yeast by extracting ergosterol from membranes. Here, we report key structural features of these antifungal 'sponges' illuminated by high-resolution magic-angle spinning solid-state NMR, in concert with simulated annealing and molecular dynamics computations. The minimal unit of assembly is an asymmetric head-to-tail homodimer: one molecule adopts an all-trans C1-C13 motif, the other a C6-C7-gauche conformation. These homodimers are staggered in a clathrate-like lattice with large void volumes similar to the size of sterols. These results illuminate the atomistic interactions that underlie fungicidal assemblies of AmB and suggest this natural product may form biologically active clathrates that host sterol guests.

Quest for the Molecular Basis of Improved Selective Toxicity of All-Trans Isomers of Aromatic Heptaene Macrolide Antifungal Antibiotics

Three aromatic heptaene macrolide antifungal antibiotics, Candicidin D, Partricin A (Gedamycin) and Partricin B (Vacidin) were subjected to controlled cis-trans→ all trans photochemical isomerization. The obtained all-trans isomers demonstrated substantially improved in vitro selective toxicity in the Candida albicans cells: human erythrocytes model. This effect was mainly due to the diminished hemotoxicity. The molecular modeling studies on interactions between original antibiotics and their photoisomers with ergosterol and cholesterol revealed some difference in free energy profiles of formation of binary antibiotic/sterol complexes in respective membrane environments. Moreover, different geometries of heptaene: sterol complexes and variations in polyene macrolide molecule alignment in cholesterol-and ergosterol-containing membranes were found. None of these effects are of the crucial importance for the observed improvement of selective toxicity of aromatic heptaene antifungals but each seems to provide a partial contribution.

Well-Tolerated Amphotericin B Derivatives That Effectively Treat Visceral Leishmaniasis

Chemotherapy against the neglected tropical disease visceral leishmaniasis (VL) is suboptimal with only four licensed drugs. Amphotericin B (AmB), despite its toxicity, remained a second line drug for a long time. However, the demonstration that liposomal AmB is highly effective against VL propelled it, despite its cost, to a first line drug in many countries. While several ongoing efforts are aiming at finding cheaper and stable AmB-formulations, an alternative strategy is the development of less-toxic AmB derivatives. We show here that two less-toxic AmB derivatives with the carboxylate at position 16 of AmB derivatized to a methyl urea (AmB-MU) or amino urea (AmB-AU) are active in vitro against Leishmania donovani, both as free-living parasites as well as their intracellular form. Both less-toxic derivatives, similarly to AmB, target the ergosterol pathway of L. donovani. While the AmB-AU derivative showed female-specific liver toxicity in vivo, the AmB-MU derivative was well-tolerated and more effective than AmB against experimental VL. These studies are an important step for improving AmB-based therapy against a prevalent parasitic disease.

Improving the Cellular Selectivity of a Membrane-Disrupting Antimicrobial Agent by Monomer Control and by Taming

Antimicrobial resistance represents a significant world-wide health threat that is looming. To meet this challenge, new classes of antimicrobial agents and the redesign of existing ones will be required. This review summarizes some of the studies that have been carried out in my own laboratories involving membrane-disrupting agents. A major discovery that we made, using a Triton X-100 as a prototypical membrane-disrupting molecule and cholesterol-rich liposomes as model systems, was that membrane disruption can occur by two distinct processes, depending on the state of aggregation of the attacking agent. Specifically, we found that monomers induced leakage, while attack by aggregates resulted in a catastrophic rupture of the membrane. This discovery led us to design of a series of derivatives of the clinically important antifungal agent, Amphotericin B, where we demonstrated the feasibility of separating antifungal from hemolytic activity by decreasing the molecule’s tendency to aggregate, i.e., by controlling its monomer concentration. Using an entirely different approach (i.e., a “taming” strategy), we found that by covalently attaching one or more facial amphiphiles (“floats”) to Amphotericin B, its aggregate forms were much less active in lysing red blood cells while maintaining high antifungal activity. The possibility of applying such “monomer control” and “taming” strategies to other membrane-disrupting antimicrobial agents is briefly discussed.

Branched amphotericin functional poly(N-isopropyl acrylamide): an antifungal polymer

Branched poly(N-isopropylacrylamide) was functionalized with Amphotericin B (AmB) at the chain ends to produce an antifungal material. The polymer showed antifungal properties against AmB-sensitive strains of Candida albicans, Fusarium keratoplasticum and Aspergillus flavus (minimal inhibitory concentration ranged from 5 to 500 µg ml^-1) but was not effective against an AmB resistant strain of C. albicans nor against Candida tropicalis. The polymer end groups bound to the AmB target, ergosterol, and the fluorescence spectrum of a dye used as a solvatochromic probe, Nile red, was blue shifted indicating that segments of the polymer became desolvated on binding. The polymer was less toxic to corneal and renal epithelial cells and explanted corneal tissue than the free drug. Also, the polymer did not induce reactive oxygen species release from peripheral blood mononuclear cells, nor did it cause a substantial release of the proinflammatory cytokines, tumour necrosis factor-α and interleukin-1β (at 0.5 mg ml^-1).

Structural and Bioactivity Characterization of Filipin Derivatives from Engineered Streptomyces filipinensis Strains Reveals Clues for Reduced Haemolytic Action

The rise in the number of immunocompromised patients has led to an increased incidence of fungal infections, with high rates of morbidity and mortality. Furthermore, misuse of antifungals has boosted the number of resistant strains to these agents; thus, there is urgent need for new drugs against these infections. Here, the in vitro antifungal activity of filipin III metabolic intermediates has been characterized against a battery of opportunistic pathogenic fungi—Candida albicans, Candida glabrata, Candida krusei, Cryptococcus neoformans, Trichosporon cutaneum, Trichosporon asahii, Aspergillus nidulans, Aspergillus niger, and Aspergillus fumigatus—using the Clinical and Laboratory Standards Institute broth microdilution method. Structural characterization of these compounds was undertaken by mass spectrometry (MS) and nuclear magnetic resonance (NMR) following HPLC purification. Complete NMR assignments were obtained for the first time for filipins I and II. In vitro haemolytic assays revealed that the haemolytic action of these compounds relies largely on the presence of a hydroxyl function at C26, since derivatives lacking such moiety show remarkably reduced activity. Two of these derivatives, 1′-hydroxyfilipin I and filipin I, show decreased toxicity towards cholesterol-containing membranes while retaining potent antifungal activity, and could constitute excellent leads for the development of efficient pharmaceuticals, particularly against Cryptococcosis.

New Approaches for Cryptococcosis Treatment

Cryptococcosis is an important opportunistic infection and a leading cause of meningitis in patients with HIV infection. The antifungal pharmacological treatment is limited to amphotericin B, fluconazole and 5- flucytosine. In addition to the limited pharmacological options, the high toxicity, increased resistance rate and difficulty of the currently available antifungal molecules to cross the blood-brain barrier hamper the treatment. Thus, the search for new alternatives for the treatment of cryptococcal meningitis is extremely necessary. In this review, we describe the therapeutic strategies currently available, discuss new molecules with antifungal potential in different phases of clinical trials and in advanced pre-clinical phase, and examine drug nanocarriers to improve delivery to the central nervous system.

Antifungal resistance in Aspergillus terreus: A current scenario

Invasive aspergillosis caused by intrinsically resistant non-fumigatus Aspergillus species displays a poor outcome in immunocompromised patients. The polyene antifungal amphotericin B (AmB) remains to be "gold standard" in the treatment of invasive fungal infections. Aspergillus terreus is innately resistant to AmB, in vivo and in vitro. Till now, the exact mode of action in polyene resistance is not well understood. This review highlights the underlying molecular basis of AmB resistance in A. terreus, displaying data obtained from AmB susceptible A. terreus and AmB resistant A. terreus strains. The effect of AmB on main cellular and molecular functions such as fungal respiration and stress response pathways will be discussed in detail and resistance mechanisms will be highlighted. The fungal stress response machinery seems to be a major player in the onset of AmB resistance in A. terreus.

Fixing the Unfixable: The Art of Optimizing Natural Products for Human Medicine

Molecules isolated from natural sources including bacteria, fungi, and plants are a long-standing source of therapeutics that continue to add to our medicinal arsenal today. Despite their potency and prominence in the clinic, complex natural products often exhibit a number of liabilities that hinder their development as therapeutics, which may be partially responsible for the current trend away from natural product discovery, research, and development. However, advances in synthetic biology and organic synthesis have inspired a new generation of natural product chemists to tackle powerful undeveloped scaffolds. In this Perspective, we will present case studies demonstrating the historical and current focus on making targeted, but significant, changes to natural product scaffolds via biosynthetic gene cluster manipulation, total synthesis, semisynthesis, or a combination of these methods, with a focus on increasing activity, decreasing toxicity, or improving chemical and pharmacological properties.

Nanostructured cochleates: a multi-layered platform for cellular transportation of therapeutics

Among lipid-based nanocarriers, multi-layered cochleates emerge as a novel delivery system because of prevention of oxidation of hydrophobic and hydrophilic drugs, enhancement in permeability, and reduction in dose of drugs. It also improves oral bioavailability and increases the safety of a drug by targeting at a specific site with less side effects. Nanostructured cochleates are used as a carrier for the delivery of water-insoluble or hydrophobic drugs of anticancer, antiviral and anti-inflammatory action. This review article focuses on different methods for preparation of cochleates, mechanism of formation of cochleates, mechanism of action like cochleate undergoes macrophagic endocytosis and release the drug into the systemic circulation by acting on membrane proteins, phospholipids, and receptors. Advanced methods such as calcium-substituted and β-cyclodextrin-based cochleates, novel techniques include microfluidic and modified trapping method. Cochleates showed enhancement in oral bioavailability of amphotericin B, delivery of factor VII, oral mucosal vaccine adjuvant-delivery system, and delivery of volatile oil. In near future, cochleate will be one of the interesting delivery systems to overcome the stability and encapsulation efficiency issues associated with liposomes. The current limiting factors for commercial preparation of cochleates involve high cost of manufacturing, lack of standardization, and specialized equipments.

ERG6 and ERG2 Are Major Targets Conferring Reduced Susceptibility to Amphotericin B in Clinical Candida glabrata Isolates in Kuwait

Candida glabrata is intrinsically less susceptible to azoles, and resistance to echinocandins and reduced susceptibility (RS) to amphotericin B (AMB) have also been detected. The molecular mechanisms of RS to AMB were investigated in C. glabrata strains in Kuwait by sequence analyses of genes involved in ergosterol biosynthesis. A total of 1,646 C. glabrata isolates were tested by Etest, and results for 12 selected isolates were confirmed by reference broth microdilution. PCR sequencing of three genes (ERG2, ERG6, and ERG11) was performed for all isolates with RS to AMB (RS-AMB isolates) and 5 selected wild-type C. glabrata isolates by using gene-specific primers. The total cell sterol content was analyzed by gas chromatography-mass spectrometry. The phylogenetic relationship among the isolates was investigated by multilocus sequence typing. Wild-type isolates contained only synonymous mutations in ERG2, ERG6, or ERG11, and the total sterol content was similar to that of the reference strains. A nonsynonymous ERG6 mutation (AGA48AAA, R48K) was found in both RS-AMB and wild-type isolates. Four RS-AMB isolates contained novel nonsense mutations at Trp286, Tyr192, and Leu341, and 2 isolates contained a nonsynonymous mutation in ERG6 (V126F or C198F); and the sterol content of these isolates was consistent with ERG6 deficiency. Two other RS-AMB isolates contained a novel nonsynonymous ERG2 mutation (G119S or G122S), and their sterol content was consistent with ERG2 deficiency. Of 8 RS-AMB isolates, 1 fluconazole-resistant isolate also contained nonsynonymous Y141H plus L381M mutations, while 7 isolates contained only synonymous mutations in ERG11 All isolates with ERG6, ERG2, and ERG11 mutations were genotypically distinct strains. Our data show that ERG6 and ERG2 are major targets conferring RS-AMB in clinical C. glabrata isolates.

Aspergillus terreus: Novel lessons learned on amphotericin B resistance

The polyene antifungal amphotericin B (AmB) exerts a powerful and broad activity against a vast array of fungi and in general displays a remarkably low rate of antimicrobial resistance. Aspergillus terreus holds an exceptional position among the Aspergilli due to its intrinsic AmB resistance, in vivo and in vitro. Until now, the underlying mechanisms of polyene resistance were not well understood. This review will highlight the molecular basis of A. terreus and AmB resistance recently gained and will display novel data on the mode of action of AmB. A main focus is set on fundamental stress response pathways covering the heat shock proteins (Hsp) 90/Hsp70 axis, as well as reactive oxygen species detoxifying enzymes in response to AmB. The effect on main cellular functions such as fungal respiration will be addressed in detail and resistance mechanisms will be highlighted. Based on these novel findings we will discuss new molecular targets for alternative options in the treatment of invasive infections due to A. terreus.

Methods for 2-Deoxyglycoside Synthesis

Deoxy-sugars often play a critical role in modulating the potency of many bioactive natural products. Accordingly, there has been sustained interest in methods for their synthesis over the past several decades. The focus of much of this work has been on developing new glycosylation reactions that permit the mild and selective construction of deoxyglycosides. This Review covers classical approaches to deoxyglycoside synthesis, as well as more recently developed chemistry that aims to control the selectivity of the reaction through rational design of the promoter. Where relevant, the application of this chemistry to natural product synthesis will also be described.

Getting the Jump on the Development of Bullfrog Oil Microemulsions: a Nanocarrier for Amphotericin B Intended for Antifungal Treatment

Amphotericin B (AmB), a potent antifungal drug, presents physicochemical characteristics that impair the development of suitable dosage forms. In order to overcome the AmB insolubility, several lipid carriers such as microemulsions have been developed. In this context, the bullfrog oil stands out as an eligible oily phase component, since its cholesterol composition may favor the AmB incorporation. Thus, the aim of this study was to develop a microemulsion based on bullfrog oil containing AmB. Moreover, its thermal stability, antifungal activity, and cytotoxicity in vitro were evaluated. The microemulsion formulation was produced using the pseudo-ternary phase diagram (PTPD) approach and the AmB was incorporated based on the pH variation technique. The antifungal activity was evaluated by determination of minimal inhibitory concentration (MIC) against different species of Candida spp. and Trichosporon asahii. The bullfrog oil microemulsion, stabilized with 16.8% of a surfactant blend, presented an average droplet size of 26.50 ± 0.14 nm and a polydispersity index of 0.167 ± 0.006. This system was able to entrap AmB up to 2 mg mL^-1. The use of bullfrog oil as oily phase allowed an improvement of the thermal stability of the system. The MIC assay results revealed a growth inhibition for different strains of Candida spp. and were able to enhance the activity of AmB against T. asahii. The microemulsion was also able to reduce the AmB toxicity. Finally, the developed microemulsion showed to be a suitable system to incorporate AmB, improving the system's thermal stability, increasing the antifungal activity, and reducing the toxicity of this drug.

Solid-State NMR of highly 13C-enriched cholesterol in lipid bilayers

Cholesterol (Chol) is vital for cell function as it is essential to a myriad of biochemical and biophysical processes. The atomistic details of Chol's interactions with phospholipids and proteins is therefore of fundamental interest, and NMR offers unique opportunities to interrogate these properties at high resolution. Towards this end, here we describe approaches for examining the structure and dynamics of Chol in lipid bilayers using high levels of ¹³C enrichment in combination with magic-angle spinning (MAS) methods. We quantify the incorporation levels and demonstrate high sensitivity and resolution in 2D ¹³C-¹³C and ¹H-¹³C spectra, enabling de novo assignments and site-resolved order parameter measurements obtained in a fraction of the time required for experiments with natural abundance sterols. We envision many potential future applications of these methods to study sterol interactions with drugs, lipids and proteins.

Unique aggregation of conjugated amphotericin B and its interaction with lipid membranes

The purpose of this paper was to investigate the aggregation of amphotericin B (AMB) and AMB-arabinogalactan conjugate (AMB-AGC), and the interactions of these drugs with free and membrane-embedded sterols. Aggregation of AMB and AMB-AGC was studied by circular dichroic (CD) and UV absorbance spectroscopic techniques. The effect of liposomes on the spectra was utilized to investigate the interactions of aggregates with membrane-embedded sterols. Interaction with free sterols was studied by measuring sterols' effect on AMB/AMB-AGC susceptibility test. The results demonstrated that AMB-AGC forms unique aggregates in aqueous solution which differ from those formed by free AMB. Ergosterol and cholesterol embedded in liposomes, affected the CD spectra obtained for both AMB and AMB-AGC, indicating interactions of these sterols with both drugs. Interaction with both cholesterol and ergosterol resulted in an increase of AMB-AGC's minimal inhibitory concentration (MIC) in Candida albicans. In conclusion, AMB-AGC forms unique aggregates in aqueous solution; these aggregates interact with membrane-embedded cholesterol and ergosterol and with free sterols. These results indicate that the selectivity of AMB-AGC to fungal cells may not occur due to inability to bind cholesterol but probably as a result of this unique aggregation. Understanding this mechanism may help to develop a safer AMB formulation for therapy.

Toward Generalization of Iterative Small Molecule Synthesis

Small molecules have extensive untapped potential to benefit society, but access to this potential is too often restricted by limitations inherent to the customized approach currently used to synthesize this class of chemical matter. In contrast, the "building block approach", i.e., generalized iterative assembly of interchangeable parts, has now proven to be a highly efficient and flexible way to construct things ranging all the way from skyscrapers to macromolecules to artificial intelligence algorithms. The structural redundancy found in many small molecules suggests that they possess a similar capacity for generalized building block-based construction. It is also encouraging that many customized iterative synthesis methods have been developed that improve access to specific classes of small molecules. There has also been substantial recent progress toward the iterative assembly of many different types of small molecules, including complex natural products, pharmaceuticals, biological probes, and materials, using common building blocks and coupling chemistry. Collectively, these advances suggest that a generalized building block approach for small molecule synthesis may be within reach.

Design of amphotericin B oral formulation for antifungal therapy

Amphotericin B (AmB) remains the "gold standard" for systemic antifungal therapy, even though new drugs are emerging as the attractive antifungal agents. Since AmB has negligible oral absorption as a consequence of its unfavorable physicochemical characterizations, its use is restricted to parenteral administration which is accompanied by severe side effects. As greater understanding of the gastrointestinal tract has developed, the advanced drug delivery systems are emerging with the potential to overcome the barriers of AmB oral delivery. Much research has demonstrated that oral AmB formulations such as lipid formulations may have beneficial therapeutic efficacy with reduced adverse effects and suitable for clinical application. Here we reviewed the different formulation strategies to enhance oral drug efficacy, and discussed the current trends and future perspectives for AmB oral administration in the treatment of antifungal infections.

Applications of Nonenzymatic Catalysts to the Alteration of Natural Products

The application of small molecules as catalysts for the diversification of natural product scaffolds is reviewed. Specifically, principles that relate to the selectivity challenges intrinsic to complex molecular scaffolds are summarized. The synthesis of analogues of natural products by this approach is then described as a quintessential "late-stage functionalization" exercise wherein natural products serve as the lead scaffolds. Given the historical application of enzymatic catalysts to the site-selective alteration of complex molecules, the focus of this Review is on the recent studies of nonenzymatic catalysts. Reactions involving hydroxyl group derivatization with a variety of electrophilic reagents are discussed. C-H bond functionalizations that lead to oxidations, aminations, and halogenations are also presented. Several examples of site-selective olefin functionalizations and C-C bond formations are also included. Numerous classes of natural products have been subjected to these studies of site-selective alteration including polyketides, glycopeptides, terpenoids, macrolides, alkaloids, carbohydrates, and others. What emerges is a platform for chemical remodeling of naturally occurring scaffolds that targets virtually all known chemical functionalities and microenvironments. However, challenges for the design of very broad classes of catalysts, with even broader selectivity demands (e.g., stereoselectivity, functional group selectivity, and site-selectivity) persist. Yet, a significant spectrum of powerful, catalytic alterations of complex natural products now exists such that expansion of scope seems inevitable. Several instances of biological activity assays of remodeled natural product derivatives are also presented. These reports may foreshadow further interdisciplinary impacts for catalytic remodeling of natural products, including contributions to SAR development, mode of action studies, and eventually medicinal chemistry.

Improved recovery and biological activities of an engineered polyene NPP analogue in Pseudonocardia autotrophica

NPP A1 produced by Pseudonocardia autotrophica is a unique disaccharide-containing polyene macrolide. NPP A1 was reported to have higher water solubility and lower hemolytic toxicity than nystatin A1 while retaining its antifungal activity. An engineered NPP A1 analogue, NPP A2, was generated by inactivation of the nppL gene, encoding a P450 monooxygenase in P. autotrophica. The resulting compound exhibited the corresponding chemical structure of NPP A1 but lacked a C10 hydroxyl group. In this study, newly developed crystallization recovery methods for NPP A2 purification, followed by an evaluation of in vitro antifungal activity and hemolytic activity, were performed. The crystallization methods were designed to eliminate the undesired viscous impurities encountered during the NPP A2 purification process, resulting in improved purity from 5.3 to 83.5% w/w. NPP A2 isolated from the improved purification process also exhibited two times higher antifungal activity and 1.8 times higher hemolytic toxicity than those of NPP A1. These results suggest that the minor structural modification of disaccharide-containing polyene macrolides, such as removing a C10 hydroxyl group, might require an alternative recovery process, such as crystallization, to confirm its improved biological activity.

Restored iron transport by a small molecule promotes absorption and hemoglobinization in animals

Multiple human diseases ensue from a hereditary or acquired deficiency of iron-transporting protein function that diminishes transmembrane iron flux in distinct sites and directions. Because other iron-transport proteins remain active, labile iron gradients build up across the corresponding protein-deficient membranes. Here we report that a small-molecule natural product, hinokitiol, can harness such gradients to restore iron transport into, within, and/or out of cells. The same compound promotes gut iron absorption in DMT1-deficient rats and ferroportin-deficient mice, as well as hemoglobinization in DMT1- and mitoferrin-deficient zebrafish. These findings illuminate a general mechanistic framework for small molecule-mediated site- and direction-selective restoration of iron transport. They also suggest that small molecules that partially mimic the function of missing protein transporters of iron, and possibly other ions, may have potential in treating human diseases.

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.

Fungicide Xylaria sp. BCC 1067 extract induces reactive oxygen species and activates multidrug resistance system in Saccharomyces cerevisiae

AIM

To investigate antifungal potential of Xylaria sp. BIOTEC culture collection (BCC) 1067 extract against the model yeast Saccharomyces cerevisiae.

MATERIALS & METHODS

Antifungal property of extract, reactive oxygen species levels and cell survival were determined, using selected deletion strains.

RESULTS

Extract showed promising antifungal effect with minimal inhibitory concentration₁₀₀ and minimal fungicidal concentration of 500 and 1000 mg/l, respectively. Strong synergy was observed with fractional inhibitory concentration index value of 0.185 for the combination of 60.0 and 0.5 mg/l of extract and ketoconazole, respectively. Extract-induced intracellular reactive oxygen species levels in some oxidant-prone strains and mediated plasma membrane rupture. Antioxidant regulator Yap1, efflux transporter Pdr5 and ascorbate were pivotal to protect S. cerevisiae from extract cytotoxicity.

CONCLUSION

Xylaria sp. BCC 1067 extract is a potentially valuable source of novel antifungals.

New facets of antifungal therapy

Invasive fungal infections remain a major cause of morbidity and mortality in immunocompromised patients, and such infections are a substantial burden to healthcare systems around the world. However, the clinically available armamentarium for invasive fungal diseases is limited to 3 main classes (i.e., polyenes, triazoles, and echinocandins), and each has defined limitations related to spectrum of activity, development of resistance, and toxicity. Further, current antifungal therapies are hampered by limited clinical efficacy, high rates of toxicity, and significant variability in pharmacokinetic properties. New antifungal agents, new formulations, and novel combination regimens may improve the care of patients in the future by providing improved strategies to combat challenges associated with currently available antifungal agents. Likewise, therapeutic drug monitoring may be helpful, but its present use remains controversial due to the lack of available data. This article discusses new facets of antifungal therapy with a focus on new antifungal formulations and the synergistic effects between drugs used in combination therapy.