Structural analyses of Candida albicans sterol 14α-demethylase complexed with azole drugs address the molecular basis of azole-mediated inhibition of fungal sterol biosynthesis

Construction of antifungal dual-target (SE, CYP51) pharmacophore models and the discovery of novel antifungal inhibitors

Fungal infections and drug-resistance are rapidly increasing with the deterioration of the external environment. Squalene cyclooxygenase (SE) and 14α-demethylase (CYP51) are considered to be important antifungal targets, and the corresponding pharmacophore models can be used to design and guide the discovery of novel inhibitors. Therefore, the common feature pharmacophore model (SE inhibitor) and structure-based pharmacophore model (CYP51 receptor) were constructed using different methods in this study. Then, appropriate organic fragments were selected and superimposed onto the pharmacophore features, and compounds 5, 6 and 8 were designed and produced by linking these organic fragments. It is noteworthy that compound 8 can simultaneously match the features of both the SE and CYP51 pharmacophores. Further analysis found that these compounds exhibit a potent antifungal activity. Preliminary mechanistic studies revealed that compound 8 could undergo dual-target inhibition (SE and CYP51) of Candida albicans. This study proved the rationale of pharmacophore models (SE and CYP51), which can guide the design and discovery of new antifungal inhibitors.

In Vitro Activities of the Novel Investigational Tetrazoles VT-1161 and VT-1598 Compared to the Triazole Antifungals against Azole-Resistant Strains and Clinical Isolates of Candida albicans

The fungal Cyp51-specific inhibitors VT-1161 and VT-1598 have emerged as promising new therapies to combat fungal infections, including Candida spp. To evaluate their in vitro activities compared to other azoles, MICs were determined by Clinical and Laboratory Standards Institute (CLSI) method for VT-1161, VT-1598, fluconazole, voriconazole, itraconazole, and posaconazole against 68 C. albicans clinical isolates well characterized for azole resistance mechanisms and mutant strains representing individual azole resistance mechanisms. VT-1161 and VT-1598 demonstrated potent activity (geometric mean MICs ≤0.15 μg/ml) against predominantly fluconazole-resistant (≥8 μg/ml) isolates. However, five of 68 isolates exhibited MICs greater than six dilutions (>2 μg/ml) to both tetrazoles compared to fluconazole-susceptible isolates. Four of these isolates likewise exhibited high MICs beyond the upper limit of the assay for all triazoles tested. A premature stop codon in ERG3 likely explained the high-level resistance in one isolate. VT-1598 was effective against strains with hyperactive Tac1, Mrr1, and Upc2 transcription factors and against most ERG11 mutant strains. VT-1161 MICs were elevated compared to the control strain SC5314 for hyperactive Tac1 strains and two strains with Erg11 substitutions (Y132F and Y132F&K143R) but showed activity against hyperactive Mrr1 and Upc2 strains. While mutations affecting Erg3 activity appear to greatly reduce susceptibility to VT-1161 and VT-1598, the elevated MICs of both tetrazoles for four isolates could not be explained by known azole resistance mechanisms, suggesting the presence of undescribed resistance mechanisms to triazole- and tetrazole-based sterol demethylase inhibitors.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Precise genome editing using a CRISPR-Cas9 method highlights the role of CoERG11 amino acid substitutions in azole resistance in Candida orthopsilosis

Background

Azoles are one of the main antifungal classes for the treatment of candidiasis. In the current context of emerging drug resistance, most studies have focused on Candida albicans, Candida glabrata or Candida auris but, so far, less is known about the underlying mechanisms of resistance in other species, including Candida orthopsilosis.

Objectives

We investigated azole resistance in a C. orthopsilosis clinical isolate recovered from a patient with haematological malignancy receiving fluconazole prophylaxis.

Methods

Antifungal susceptibility to fluconazole was determined in vitro (CLSI M27-A3) and in vivo (in a Galleria mellonella model of invasive candidiasis). The CoERG11 gene was then sequenced and amino acid substitutions identified were mapped on the predicted 3D structure of CoErg11p. A clustered regularly interspaced short palindromic repeat-Cas9 (CRISPR-Cas9) genome-editing strategy was used to introduce relevant mutations into a fluconazole-susceptible C. orthopsilosis isolate.

Results

Compared with unrelated C. orthopsilosis isolates, the clinical isolate exhibited both in vitro and in vivo fluconazole resistance. Sequencing of the CoERG11 gene identified several amino acid substitutions, including two possibly involved in fluconazole resistance (L376I and G458S). Both mutations mapped close to the active site of CoErg11p. Engineering these mutations in a different genetic background using CRISPR-Cas9 demonstrated that G458S, but not L376I, confers resistance to fluconazole and voriconazole.

Conclusions

Our data show that the G458S amino acid substitution in CoERG11p, but not L376I, contributes to azole resistance in C. orthopsilosis. In addition to highlighting the potential of CRISPR-Cas9 technology for precise genome editing in the field of antifungal resistance, we discuss some points that are critical to improving its efficiency.

Design, synthesis, and antimicrobial evaluation of 1,4-dihydroindeno[1,2-c]pyrazole tethered carbohydrazide hybrids: exploring their in silico ADMET, ergosterol inhibition and ROS inducing potential

A series of new 1,4-dihydroindeno[1,2-c]pyrazole tethered carbohydrazide hybrids (5a-u) were designed, synthesized and evaluated for their antimicrobial activity. Compounds 5d, 5g, 5j, 5k and 5q demonstrated significant activity against the entire panel of test pathogens. Further, compounds 5d and 5g exhibited significant anti-Candida activity. These potential hybrids (5d and 5g) also exhibited promising ergosterol biosynthesis inhibition against Candida albicans, which was further validated through molecular docking studies. Furthermore, compounds 5d and 5g caused intracellular ROS accumulation in C. albicans MTCC 3017 and were non-toxic to normal human lung cell line MRC5. In silico studies revealed that they demonstrated drug likeness and an appreciable pharmacokinetic profile. Overall, the findings demonstrate that 5d and 5g may be considered as promising leads for further development of new antifungal drugs.

The Evolution of Azole Resistance in Candida albicans Sterol 14α-Demethylase (CYP51) through Incremental Amino Acid Substitutions

Recombinant Candida albicans CYP51 (CaCYP51) proteins containing 23 single and 5 double amino acid substitutions found in clinical strains and the wild-type enzyme were expressed in Escherichia coli and purified by Ni²⁺-nitrilotriacetic acid agarose chromatography. Catalytic tolerance to azole antifungals was assessed by determination of the concentration causing 50% enzyme inhibition (IC₅₀) using CYP51 reconstitution assays. The greatest increase in the IC₅₀ compared to that of the wild-type enzyme was observed with the five double substitutions Y132F+K143R (15.3-fold), Y132H+K143R (22.1-fold), Y132F+F145L (10.1-fold), G307S+G450E (13-fold), and D278N+G464S (3.3-fold). The single substitutions K143R, D278N, S279F, S405F, G448E, and G450E conferred at least 2-fold increases in the fluconazole IC₅₀, and the Y132F, F145L, Y257H, Y447H, V456I, G464S, R467K, and I471T substitutions conferred increased residual CYP51 activity at high fluconazole concentrations. In vitro testing of select CaCYP51 mutations in C. albicans showed that the Y132F, Y132H, K143R, F145L, S405F, G448E, G450E, G464S, Y132F+K143R, Y132F+F145L, and D278N+G464S substitutions conferred at least a 2-fold increase in the fluconazole MIC. The catalytic tolerance of the purified proteins to voriconazole, itraconazole, and posaconazole was far lower and limited to increased residual activities at high triazole concentrations for certain mutations rather than large increases in IC₅₀ values. Itraconazole was the most effective at inhibiting CaCYP51. However, when tested against CaCYP51 mutant strains, posaconazole seemed to be the most resistant to changes in MIC as a result of CYP51 mutation compared to itraconazole, voriconazole, or fluconazole.

Lepidine B & E as New Target Inhibitors from Lepidium Sativum Seeds Against Four Enzymes of the Pathogen Candida albicans: In Vitro and In Silico Studies

BACKGROUND AND OBJECTIVE

The present paper aims to study the inhibition of Candida albicans growth as candidiasis treatment, using seeds of Lepidium sativum as source.

METHODS

In vitro assays were carried out on the antifungal activity of three kinds of extracts from L. sativum seeds against four strains of C. albicans, then testing the same phytochemicals on the inhibition of Lipase (LCR). A new in silico study was achieved using molecular docking, with Autodock vina program, to find binding affinity of two important and major lepidine alkaloids (lepidine E and B) towards the four enzymes secreted by C. albicans as target drugs, responsible of vitality and virulence of this yeast cells: Lipase, Serine/threonine phosphatase, Phosphomannose isomerase and Sterol 14-alpha demethylase (CYP51).

RESULTS

The results of the microdillution assay show that the hexanic and alkaloidal extracts have an antifungal activity with MICs: 2.25 mg/ml and 4.5mg/ml, respectively. However, Candida rugosa lipase assay gives a remarkable IC50 values for the hexanic extract (1.42± 0.04 mg/ml) followed by 1.7± 0.1 and 2.29 ± 0.09 mg/ml of ethyl acetate and alkaloidal extracts respectively. The molecular docking confirms a significant correlation between C. albicans growth and inhibition of crucial enzymes involved in the invasion mechanism and cellular metabolisms, for the first time there were an interesting and new positive results on binding modes of lepidine E and B on the four studied enzymes.

CONCLUSION

Through this work, we propose Lepidine B & E as potent antifungal drugs.

Synthesis and Anticandidal Activities of Some Aryl (5-Chloro-Benzofuran- 2-yl) Ketoximes

Background:

In this study, some aryl (5-chloro-benzofuran-2-yl) ketoximes and their ethers were synthesised to evaluate their antifungal activity against C. albicans, C. glabrata, C. krusei, and C. parapsilosis. </p><p> Methods: The structure elucidation of the compounds was performed by IR, 1H-NMR, 13C-NMR and HR-MS spectroscopic data. ADME parameters of synthesised compounds 2a-2d, 3a-3d, 4a-4d were predicted by an in-silico study and it was determined that all synthesised compounds may have a good pharmacokinetic profile. </p><p> Results: In the anticandidal activity studies, compounds 2c and 3c were found to be the most active compounds. The effect of compound 2c, on ergosterol biosynthesis of C. albicans, was determined by using the LC-MS-MS method. </p><p> Conclusion: It was also docked in the active site of the lanosterol 14&#945;-demethylase enzyme, and shown that there is a strong interaction between compound 2c and enzyme.

Fungal Lanosterol 14α-demethylase: A target for next-generation antifungal design

The cytochrome P450 enzyme lanosterol 14α-demethylase (LDM) is the target of the azole antifungals used widely in medicine and agriculture as prophylaxis or treatments of infections or diseases caused by fungal pathogens. These drugs and agrochemicals contain an imidazole, triazole or tetrazole substituent, with one of the nitrogens in the azole ring coordinating as the sixth axial ligand to the LDM heme iron. Structural studies show that this membrane bound enzyme contains a relatively rigid ligand binding pocket comprised of a deeply buried heme-containing active site together with a substrate entry channel and putative product exit channel that reach to the membrane. Within the ligand binding pocket the azole antifungals have additional affinity determining interactions with hydrophobic side-chains, the polypeptide backbone and via water-mediated hydrogen bond networks. This review will describe the tools that can be used to identify and characterise the next generation of antifungals targeting LDM, with the goal of obtaining highly potent broad-spectrum fungicides that will be able to avoid target and drug efflux mediated antifungal resistance.

Cytochrome P450 research and The Journal of Biological Chemistry

In honor of the 100th birthday of Dr. Herbert Tabor, JBC's Editor-in-Chief for 40 years, I will review here JBC's extensive coverage of the field of cytochrome P450 (P450) research. Research on the reactions catalyzed by these enzymes was published in JBC before it was even realized that they were P450s, i.e. they have a "pigment" with an absorption maximum at 450 nm. After the P450 pigment discovery, reported in JBC in 1962, the journal proceeded to publish the methods for measuring P450 activities and many seminal findings. Since then, the P450 field has grown extensively, with significant progress in characterizing these enzymes, including structural features, catalytic mechanisms, regulation, and many other aspects of P450 biochemistry. JBC has been the most influential journal in the P450 field. As with many other research areas, Dr. Tabor deserves a great deal of the credit for significantly advancing this burgeoning and important topic of research.

A Structural View on Medicinal Chemistry Strategies against Drug Resistance

The natural phenomenon of drug resistance is a widespread issue that hampers the performance of drugs in many major clinical indications. Antibacterial and antifungal drugs are affected, as well as compounds for the treatment of cancer, viral infections, or parasitic diseases. Despite the very diverse set of biological targets and organisms involved in the development of drug resistance, the underlying molecular mechanisms have been identified to understand the emergence of resistance and to overcome this detrimental process. Detailed structural information on the root causes for drug resistance is nowadays frequently available, so next-generation drugs can be designed that are anticipated to suffer less from resistance. This knowledge-based approach is essential for fighting the inevitable occurrence of drug resistance.

Synthesis and Antifungal Potential of Some Novel Benzimidazole-1,3,4-Oxadiazole Compounds

Discovery of novel anticandidal agents with clarified mechanisms of action, could be a rationalist approach against diverse pathogenic fungal strains due to the rise of resistance to existing drugs. In support to this hypothesis, in this paper, a series of benzimidazole-oxadiazole compounds were synthesized and subjected to antifungal activity evaluation. In vitro activity assays indicated that some of the compounds exhibited moderate to potent antifungal activities against tested Candida species when compared positive control amphotericin B and ketoconazole. The most active compounds 4h and 4p were evaluated in terms of inhibitory activity upon ergosterol biosynthesis by an LC-MS-MS method and it was determined that they inhibited ergosterol synthesis concentration dependently. Docking studies examining interactions between most active compounds and lanosterol 14-α-demethylase also supported the in vitro results.

The Fungal CYP51s: Their Functions, Structures, Related Drug Resistance, and Inhibitors

CYP51 (Erg11) belongs to the cytochrome P450 monooxygenase (CYP) superfamily and mediates a crucial step of the synthesis of ergosterol, which is a fungal-specific sterol. It is also the target of azole drugs in clinical practice. In recent years, researches on fungal CYP51 have stepped into a new stage attributing to the discovery of crystal structures of the homologs in Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. This review summarizes the functions, structures of fungal CYP51 proteins, and the inhibitors targeting these homologs. In particular, several drug-resistant mechanisms associated with the fungal CYP51s are introduced. The sequences and crystal structures of CYP51 proteins in different fungal species are also compared. These will provide new insights for the advancement of research on antifungal agents.

Azole Resistance Reduces Susceptibility to the Tetrazole Antifungal VT-1161

Tetrazole antifungals designed to target fungal lanosterol 14α-demethylase (LDM) appear to be effective against a range of fungal pathogens. In addition, a crystal structure of the catalytic domain of Candida albicans LDM in complex with the tetrazole VT-1161 has been obtained. We have addressed concern about artifacts that might arise from crystallizing VT-1161 with truncated recombinant CYP51s and measured the impact on VT-1161 susceptibility of genotypes known to confer azole resistance. A yeast system was used to overexpress recombinant full-length Saccharomyces cerevisiae LDM with a C-terminal hexahistidine tag (ScLDM6×His) for phenotypic analysis and crystallographic studies with VT-1161 or with the widely used triazole drug posaconazole (PCZ). We determined the effect of characterized mutations in LDM on VT-1161 activity and identified drug efflux pumps from fungi, including key fungal pathogens, that efflux VT-1161. The relevance of these yeast-based observations on drug efflux was verified using clinical isolates of C. albicans and Candida glabrata VT-1161 binding elicits a significant conformational difference between the full-length and truncated enzymes not found when posaconazole is bound. Susceptibility to VT-1161 is reduced by ATP-binding cassette (ABC) and major facilitator superfamily (MFS) drug efflux pumps, the overexpression of LDM, and mutations within the drug binding pocket of LDM that affect interaction with the tertiary alcohol of the drug.

CYP51 as drug targets for fungi and protozoan parasites: past, present and future

The efficiency of treatment of human infections with the unicellular eukaryotic pathogens such as fungi and protozoa remains deeply unsatisfactory. For example, the mortality rates from nosocomial fungemia in critically ill, immunosuppressed or post-cancer patients often exceed 50%. A set of six systemic clinical azoles [sterol 14α-demethylase (CYP51) inhibitors] represents the first-line antifungal treatment. All these drugs were discovered empirically, by monitoring their effects on fungal cell growth, though it had been proven that they kill fungal cells by blocking the biosynthesis of ergosterol in fungi at the stage of 14α-demethylation of the sterol nucleus. This review briefs the history of antifungal azoles, outlines the situation with the current clinical azole-based drugs, describes the attempts of their repurposing for treatment of human infections with the protozoan parasites that, similar to fungi, also produce endogenous sterols, and discusses the most recently acquired knowledge on the CYP51 structure/function and inhibition. It is our belief that this information should be helpful in shifting from the traditional phenotypic screening to the actual target-driven drug discovery paradigm, which will rationalize and substantially accelerate the development of new, more efficient and pathogen-oriented CYP51 inhibitors.

Synthesis and Evaluation of New 1,3,4-Thiadiazole Derivatives as Potent Antifungal Agents

With the goal of obtaining a novel bioactive compound with significant antifungal activity, a series of 1,3,4-thiadiazole derivatives (3a–3l) were synthesized and characterized. Due to thione-thiol tautomerism in the intermediate compound 2, type of substitution reaction in the final step was determined by two-dimensional (2D) NMR. In vitro antifungal activity of the synthesized compounds was evaluated against eight Candida species. The active compounds 3k and 3l displayed very notable antifungal effects. The probable mechanisms of action of active compounds were investigated using an ergosterol quantification assay. Docking studies on 14-α-sterol demethylase enzyme were also performed to investigate the inhibition potency of compounds on ergosterol biosynthesis. Theoretical absorption, distribution, metabolism, and excretion (ADME) predictions were calculated to seek their drug likeness of final compounds. The results of the antifungal activity test, ergosterol biosynthesis assay, docking study, and ADME predictions indicated that the synthesized compounds are potential antifungal agents, which inhibit ergosterol biosynthesis probably interacting with the fungal 14-α-sterol demethylase.

Crystal Structures of Full-Length Lanosterol 14α-Demethylases of Prominent Fungal Pathogens Candida albicans and Candida glabrata Provide Tools for Antifungal Discovery

Targeting lanosterol 14α-demethylase (LDM) with azole drugs provides prophylaxis and treatments for superficial and disseminated fungal infections, but cure rates are not optimal for immunocompromised patients and individuals with comorbidities. The efficacy of azole drugs has also been reduced due to the emergence of drug-resistant fungal pathogens. We have addressed the need to improve the potency, spectrum, and specificity for azoles by expressing in Saccharomyces cerevisiae functional, recombinant, hexahistidine-tagged, full-length Candida albicans LDM (CaLDM6×His) and Candida glabrata LDM (CgLDM6×His) and determining their X-ray crystal structures. The crystal structures of CaLDM6×His, CgLDM6×His, and ScLDM6×His have the same fold and bind itraconazole in nearly identical conformations. The catalytic domains of the full-length LDMs have the same fold as the CaLDM6×His catalytic domain in complex with posaconazole, with minor structural differences within the ligand binding pocket. Our structures give insight into the LDM reaction mechanism and phenotypes of single-site CaLDM mutations. This study provides a practical basis for the structure-directed discovery of novel antifungals that target LDMs of fungal pathogens.

Heterologous Expression of Full-Length Lanosterol 14α-Demethylases of Prominent Fungal Pathogens Candida albicans and Candida glabrata Provides Tools for Antifungal Discovery

Targeting lanosterol 14α-demethylase (LDM) with azole drugs provides prophylaxis and treatments for superficial and disseminated fungal infections, but cure rates are modest for immunocompromised patients and individuals with comorbidities. The efficacy of azole drugs has also been reduced due to the emergence of drug-resistant fungal pathogens. We have addressed these problems by expressing in Saccharomyces cerevisiae functional, hexahistidine-tagged, full-length Candida albicans LDM (CaLDM6×His) and Candida glabrata LDM (CgLDM6×His) for drug discovery purposes and determining their X-ray crystal structures. Compared with S. cerevisiae overexpressing LDM6×His (ScLDM6×His), the reduced susceptibility of CgLDM6×His to all azole drugs tested correlated with its level of overexpression. In contrast, the reduced susceptibility to short-tailed (fluconazole and voriconazole) but not medium-tailed (VT-1161) or long-tailed azoles (itraconazole and posaconazole) indicates CaLDM6×His works best when coexpressed with its cognate NADPH-cytochrome P450 reductase (CaNcp1A) rather than the host reductase (ScNcp1). Overexpression of LDM or Ncp1 modified the ergosterol content of yeast and affected growth inhibition by the polyene antibiotic amphotericin B. Affinity-purified recombinant Candida LDMs bind carbon monoxide and show tight type II binding of a range of azole drugs, including itraconazole, posaconazole, fluconazole, and voriconazole. This study provides a practical basis for the phenotype-, biochemistry-, and structure-directed discovery of novel antifungals that target LDMs of fungal pathogens.

Binding of a physiological substrate causes large-scale conformational reorganization in cytochrome P450 51

Sterol 14α-demethylases (CYP51s) are phylogenetically the most conserved cytochromes P450, and their three-step reaction is crucial for biosynthesis of sterols and serves as a leading target for clinical and agricultural antifungal agents. The structures of several (bacterial, protozoan, fungal, and human) CYP51 orthologs, in both the ligand-free and inhibitor-bound forms, have been determined and have revealed striking similarity at the secondary and tertiary structural levels, despite having low sequence identity. Moreover, in contrast to many of the substrate-promiscuous, drug-metabolizing P450s, CYP51 structures do not display substantial rearrangements in their backbones upon binding of various inhibitory ligands, essentially representing a snapshot of the ligand-free sterol 14α-demethylase. Here, using the obtusifoliol-bound I105F variant of Trypanosoma cruzi CYP51, we report that formation of the catalytically competent complex with the physiological substrate triggers a large-scale conformational switch, dramatically reshaping the enzyme active site (3.5-6.0 Å movements in the FG arm, HI arm, and helix C) in the direction of catalysis. Notably, our X-ray structural analyses revealed that the substrate channel closes, the proton delivery route opens, and the topology and electrostatic potential of the proximal surface reorganize to favor interaction with the electron-donating flavoprotein partner, NADPH-cytochrome P450 reductase. Site-directed mutagenesis of the amino acid residues involved in these events revealed a key role of active-site salt bridges in contributing to the structural dynamics that accompanies CYP51 function. Comparative analysis of apo-CYP51 and its sterol-bound complex provided key conceptual insights into the molecular mechanisms of CYP51 catalysis, functional conservation, lineage-specific substrate complementarity, and druggability differences.

Sterol 14α-Demethylase Structure-Based Design of VNI (( R)- N-(1-(2,4-Dichlorophenyl)-2-(1 H-imidazol-1-yl)ethyl)-4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzamide)) Derivatives To Target Fungal Infections: Synthesis, Biological Evaluation, and Crystallographic Analysis

Because of the increase in the number of immunocompromised patients, the incidence of invasive fungal infections is growing, but the treatment efficiency remains unacceptably low. The most potent clinical systemic antifungals (azoles) are the derivatives of two scaffolds: ketoconazole and fluconazole. Being the safest antifungal drugs, they still have shortcomings, mainly because of pharmacokinetics and resistance. Here, we report the successful use of the target fungal enzyme, sterol 14α-demethylase (CYP51), for structure-based design of novel antifungal drug candidates by minor modifications of VNI [( R)- N-(1-(2,4-dichlorophenyl)-2-(1 H-imidazol-1-yl)ethyl)-4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzamide)], an inhibitor of protozoan CYP51 that cures Chagas disease. The synthesis of fungi-oriented VNI derivatives, analysis of their potencies to inhibit CYP51s from two major fungal pathogens ( Aspergillus fumigatus and Candida albicans), microsomal stability, effects in fungal cells, and structural characterization of A. fumigatus CYP51 in complexes with the most potent compound are described, offering a new antifungal drug scaffold and outlining directions for its further optimization.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Current treatment options for vulvovaginal candidiasis caused by azole-resistant Candida species

INTRODUCTION

Clinicians are increasingly challenged by patients with refractory vulvovaginal candidiasis (VVC) caused by azole-resistant Candida species. Fluconazole resistant C.albicans is a growing and perplexing problem following years of indiscriminate drug prescription and unnecessary drug exposure and for which there are few therapeutic alternatives. Regrettably, although the azole class of drugs has expanded, new classes of antifungal drugs have not been forthcoming, limiting effective treatment options in patients with azole resistant Candida vaginitis.

AREAS COVERED

This review covers published data on epidemiology, pathophysiology and treatment options for women with azole-resistant refractory VVC.

EXPERT OPINION

Fluconazole resistant C.albicans adds to the challenge of azole resistant non-albicans Candida spp. Both issues follow years of indiscriminate drug prescription and unnecessary fluconazole exposure. Although an understanding of azole resistance in yeast has been established, this knowledge has not translated into useful therapeutic advantage. Treatment options for such women with refractory symptoms are extremely limited. New therapeutic options and strategies are urgently needed to meet this challenge of azole drug resistance.

Structure-based virtual screening and ADME/T-based prediction analysis for the discovery of novel antifungal CYP51 inhibitors

With the increasing incidence of pathogenic fungi and drug-resistant fungi in clinic, it has become very important to develop the novel rate-limiting enzyme 14α-demethylase (CYP51) as an antifungal inhibitor. In this study, a method involving structure-based virtual screening was employed. First, a publicly available database was obtained from the Dow Chemical Company, and the database was screened by the designed pharmacophore model of CYP51 inhibitors. Then, the pharmacophore search hits were docked into the CYP51 crystal structure. Finally, sixteen compounds were selected for in vitro antifungal inhibition assay, and most of the compounds showed a certain degree of antifungal activity. In particular, compounds 3, 4, and 9 exhibited significant antifungal and anti-drug resistance activities by blocking the synthesis of ergosterol. The molecular docking and ADME/T properties of the compounds 3, 4, and 9 were further predicted, and the results indicated that they can form hydrophobic and coordination interactions with the active sites of CYP51. At the same time, compounds 4 and 9 showed promising drug-like properties. This study reveals that the compounds can be further optimized and developed as lead compounds.

Effects of Acetone Fraction From Buchenavia tomentosa Aqueous Extract and Gallic Acid on Candida albicans Biofilms and Virulence Factors

A promising anti-Candida activity of Buchenavia tomentosa extracts was recently described. In the present work, experiments were carried out to determine the fraction with higher antifungal activity from a B. tomentosa extract. Acetone fraction (AF) was obtained from the aqueous extract from dried leaves (5 min/100°C) and it was the most effective one. Gallic acid (GA) was identified by electrospray ionization mass spectrometry (ESI–MS) and also chosen to perform antifungal tests due to its promising activity on Candida albicans. Minimal inhibitory and fungicidal concentrations (MIC and MFC) were determined by broth microdilution technique. The effect on virulence factors of C. albicans was evaluated, and the cytotoxicity was determined. MIC₅₀ and MIC₉₀ values were both equal to 0.625 mg ml^-1 for AF and 2.5 and 5 mg ml^-1, respectively, for GA. AF and GA showed ability to inhibit C. albicans adherence and to disrupt 48 h-biofilm. AF and GA were effective in reducing the formation of hyphae of C. albicans SC5314. AF and GA decreased adherence of C. albicans to oral epithelial cells. AF and GA showed slight to moderate toxicity to Vero cells. This result suggests further studies for topic use of these compounds. AF, which contains a combination of several molecules, presented greater potential of antimicrobial activity than GA, with lower values of MIC and lower cytoxicity.

The Synthesis of Molecular Docking Studies, In Vitro Antimicrobial and Antifungal Activities of Novel Dipeptide Derivatives Based on N-(2-(2-Hydrazinyl-2-oxoethylamino)-2-oxoethyl)-nicotinamide

A series of linear dipeptide derivatives (4–10) were prepared and evaluated as antimicrobial agents via the synthesis of N-(2-(2-hydrazinyl-2-oxoethylamino)-2-oxoethyl) nicotinamide (4). Compound 4 was reacted with 4-chlorobenzaldehyde or 4-hydroxybenzaldehyde, to give the hydrazones 5 and 6, respectively. On the other hand, Compound 4 was coupled with phenylisocyanate or methylisothiocyanate to give Compounds 7 and 8, respectively. The latter compounds (7 and 8) were coupled with chloroacetic acid to give oxazolidine (9) and thiazolidine (10), respectively. The newly synthesized dipeptide compounds were confirmed by means of their spectral data. The antimicrobial activity of the newly synthesized compounds 4–10 was evaluated by agar well diffusion, and they showed good activity. Compounds 4, 5, and 9 gave the most promising activity in this study. Most of the tested compounds possessed MIC values ranging from 50 to 500 µg/mL. Furthermore, docking studies were carried out on enoyl reductase from E. coli and cytochrome P450 14 α-sterol demethylase (Cyp51) from Candida albicans active sites. The MolDock scores of the seven tested compounds ranged between −117 and −171 and between −107 and −179, respectively.

Impact of Homologous Resistance Mutations from Pathogenic Yeast on Saccharomyces cerevisiae Lanosterol 14α-Demethylase

Fungal infections frequently affect immunodeficient individuals and are estimated to kill 1.35 million people per annum. Azole antifungals target the membrane-bound cytochrome P450 monooxygenase lanosterol 14α-demethylase (CYP51; Erg11p). Mutations in CYP51 can render the widely used triazole drugs less effective. The Candida albicans CYP51 mutation G464S and the double mutation Y132F G464S (Y140F and G464S by Saccharomyces cerevisiae numbering) as well as the CYP51A G54E/R/W mutations of Aspergillus fumigatus (G73E/R/W by S. cerevisiae numbering) have been reproduced in a recombinant C-terminal hexahistidine-tagged version of S. cerevisiae CYP51 (ScErg11p6×His). Phenotypes and X-ray crystal structures were determined for the mutant enzymes. Liquid microdilution assays showed that the G464S mutation in ScErg11p6×His conferred no difference in the susceptibility of yeast to triazole drugs but in combination with the Y140F mutation gave a 4-fold reduction in susceptibility to the short-tailed triazole fluconazole. The ScErg11p6×His Y140F G464S mutant was unstable during purification and was not crystallized. The ScErg11p6×His G73E/R/W mutations conferred increased susceptibly to all triazoles tested in liquid microdilution assays. High-resolution X-ray crystal structures reveal two different conformations of the ligand itraconazole, including a previously unseen conformation, as well as interactions between the tail of this triazole and the E/W73 residue. This study shows that S. cerevisiae CYP51 adequately represents some but not all mutations in CYP51s of pathogenic fungi. Insight into the molecular mechanisms of resistance mutations in CYP51 will assist the development of inhibitors that will overcome antifungal resistance.

The binding mechanism between azoles and FgCYP51B, sterol 14α-demethylase of Fusarium graminearum

BACKGROUND

Fusarium graminearum is the main pathogen of Fusarium head blight (FHB), a worldwide plant disease and a major disease of wheat in China. Control of FHB is mainly dependent on the application of demethylase inhibitor (DMI) fungicides. Fungal sterol 14α-demethylase enzymes (CYP51) are the main target for DMI fungicides. A molecular modeling study and biological evaluation were performed to investigate the binding mechanism between azoles and CYP51B in F. graminearum.

RESULTS

A homology model based on the crystal structure of Aspergillus fumigatus was built. Molecular docking and molecular dynamics (MD) simulations were then used to identify the optimum binding mode of propiconazole (PRP), diniconazole (DIN), triadimenol (TRL), tebuconazole (TEC) and triadimefon (TRN) with FgCYP51B. Furthermore, the binding free energy of the five protein-inhibitor complexes was calculated using molecular mechanics generalized Born surface area and Poisson-Boltzmann surface area (MM-GB/PBSA) methods. Key residues in the selective binding of azoles to FgCYP51B were recognized by per-residue free energy decomposition analysis. The five ligands have a similar binding mode in the active pocket. The binding free energy to the enzyme for inhibitors PRP and TEC is more favorable than that of TRN, TRL and DIN. Furthermore, the amino acid residues Phe511, Val136, Ile374, Ala308, Ser312 and Try137 of FgCYP51B are key residues interacting with azoles fungicides. From the experimental evaluation, the 50% effective concentration (EC₅₀ ) values for PRP, TEC, DIN, TRL and TRN are 0.024, 0.047, 0.148, 0.154 and 0.474 mg L^-1 , respectively. These five molecules exhibit potential inhibitory activity against CYP51B protein from F. graminearum.

CONCLUSION

Azole fungicides for FgCYP51B should possess more hydrophobic groups interacting with residues Phe511, Val136, Ile374, Ala308, Ser312 and Tyr137. PRP and TEC are preferable for the control of FHB than DIN, TRL and TRN. © 2017 Society of Chemical Industry.

New 1,5 and 2,5-disubstituted tetrazoles-dependent activity towards surface barrier of Candida albicans

A series of novel tetrazole derivatives was synthetized using N-alkylation or Michael-type addition reactions, and screened for their fungistatic potential against Candida albicans (the lack of endpoint = 100%). Among them, the selected compounds 2d, 4b, and 6a differing in substituents at the tetrazole ring were non-toxic to Galleria mellonella larvae in vivo and exerted slight toxicity against Caco-2 in vitro (CC₅₀ at 256 μg/mL). An antagonistic effect of tetrazole derivatives 2d, 4b, and 6a respectively in combination with Fluconazole was shown using the checker board and colorimetric methods (fractional inhibitory concentration indexes FICIs >1). The most active 2d and 6a displayed an inverse relation between MICs in the presence of exogenous ergosterol, the effect was opposite to Itraconazole and Amphotericin B. The differences between 6a's and 2d's action mode were noted. Combining both flow cytometry and fluorescence image analyses respectively showed the complexity of planktonic and biofilm cell demise mode under the tetrazole derivatives tested. The following evidences for 6a's interaction with fungal membrane were noted: necrosis-like programmed cell death (97.03 ± 0.88), DNA denaturation (no laddering), mitochondrial damage (XTT assay), reduced adhesion to human epithelium (>50% at 0.0313 μg/mL, p ≤ .05), irregular deposit of chitin, and attenuated morphogenesis in mature biofilm. The treatment with 6a reduced pathogenicity of C. albicans during infection in G. mellonella. Contrariwise, 2d enhancing fungal adhesion displayed mechanism targeted to the cell wall (due to the presence of 3-chloropropyl clubbed with aryltetrazole) in the presence of osmotic protector. Under 2d, the accidental cell death (88.60% ± 4.81) was observed. In conclusion, all tetrazole derivatives were obtained in satisfactory yields (60-95%) using efficient, simple and not expensive methods. Fungistatic and slightly anticancer tetrazole derivatives with the novel action mode can circumvent an appearance of antifungal-resistant strains. These results indicate that they are worthy of further studies.

CYP51 is an essential drug target for the treatment of primary amoebic meningoencephalitis (PAM)

Primary Amoebic Meningoencephalitis (PAM) is caused by Naegleria fowleri, a free-living amoeba that occasionally infects humans. While considered "rare" (but likely underreported) the high mortality rate and lack of established success in treatment makes PAM a particularly devastating infection. In the absence of economic inducements to invest in development of anti-PAM drugs by the pharmaceutical industry, anti-PAM drug discovery largely relies on drug 'repurposing'-a cost effective strategy to apply known drugs for treatment of rare or neglected diseases. Similar to fungi, N. fowleri has an essential requirement for ergosterol, a building block of plasma and cell membranes. Disruption of sterol biosynthesis by small-molecule inhibitors is a validated interventional strategy against fungal pathogens of medical and agricultural importance. The N. fowleri genome encodes the sterol 14-demethylase (CYP51) target sharing ~35% sequence identity to fungal orthologues. The similarity of targets raises the possibility of repurposing anti-mycotic drugs and optimization of their usage for the treatment of PAM. In this work, we (i) systematically assessed the impact of anti-fungal azole drugs, known as conazoles, on sterol biosynthesis and viability of cultured N. fowleri trophozotes, (ii) identified the endogenous CYP51 substrate by mass spectrometry analysis of N. fowleri lipids, and (iii) analyzed the interactions between the recombinant CYP51 target and conazoles by UV-vis spectroscopy and x-ray crystallography. Collectively, the target-based and parasite-based data obtained in these studies validated CYP51 as a potentially 'druggable' target in N. fowleri, and conazole drugs as the candidates for assessment in the animal model of PAM.

Structural insights into impact of Y134F mutation and discovery of novel fungicidal compounds against CYP51 in Puccinia triticina

Sterol 14α-Demethylase Cytochrome P450 (CYP51) protein involved in ergosterol biosynthesis pathways turn out to be a crucial target for the fungicidal compound. However, the recognition mechanism and dynamic behavior of CYP51 in wheat leaf rust pathogen, Puccinia triticina, is still obscure. Previously, a mutation at position 134 (Y134F) was reported in five European isolates of P. triticina, conversely, structural basis of this mutation remains unclear. To address this problem, three-dimensional structure of CYP51 protein from P. triticina was successfully built using homology modeling approach. To assess the protein structure stability, wild and mutant-type CYP51 proteins bound with azole fungicide was subjected to 50 ns molecular dynamics (MD) simulations run. Observably, the comparative protein-ligand interaction analysis and binding free energy results revealed that impact of the mutation on the thermodynamics and conformational stability of the CYP51 protein was negligible. In addition, we carried out structure-based virtual screening and identified potent novel fungicidal compounds from four different databases and libraries. Consequently, through MD simulation and thermodynamic integration, four novel compounds such as CoCoCo54211 (CoCoCo database), ZINC04089470 (ZINC database), Allyl pyrocatechol 3,4 diacetate (Natural compound library), and 9-octadecenoic acid (Traditional Chinese Medicine database) has been predicted as potent fungicidal compound against CYP51 with XPGlide docking score of -11.41, -13.64, -7.40, and -6.55 kcal/mol, respectively. These compounds were found to form hydrogen bonds with heme group of CYP51, subsequently disturbing the stability and survival of fungus and can be used to control leaf rust in wheat.

Syntheses of Novel 4-Substituted N-(5-amino-1H-1,2,4-triazol-3-yl)pyridine-3-sulfonamide Derivatives with Potential Antifungal Activity

Candidiasis represent a serious threat for patients with altered immune responses. Therefore, we have undertaken the synthesis of compounds comprising a pyridine-3-sulfonamide scaffold and known antifungally active 1,2,4-triazole substituents. Thus a series of novel 4-substituted N-(5-amino-1H-1,2,4-triazol-3-yl)pyridine-3-sulfonamides have been synthesized by multistep reactions starting from 4-chloropyridine-3-sulfonamide via N′-cyano-N-[(4-substitutedpyridin-3-yl)sulfonyl]carbamimidothioates which were further converted with hydrazine hydrate to the corresponding 1,2,4-triazole derivatives 26–36. The final compounds were evaluated for antifungal activity against strains of the genera Candida, Geotrichum, Rhodotorula, and Saccharomycess isolated from patients with mycosis. Many of them show greater efficacy than fluconazole, mostly towards Candida albicans and Rhodotorula mucilaginosa species, with MIC values ≤ 25 µg/mL. A docking study of the most active compounds 26, 34 and 35 was performed showing the potential mode of binding to Candida albicans lanosterol 14α-demethylase. Also in vitro cytotoxicity of selected compounds have been evaluated on the NCI-60 cell line panel.

A CTG Clade Candida Yeast Genetically Engineered for the Genotype-Phenotype Characterization of Azole Antifungal Resistance in Human-Pathogenic Yeasts

A strain of the opportunistic pathogenic yeast Candida lusitaniae was genetically modified for use as a cellular model for assessing by allele replacement the impact of lanosterol C14α-demethylase ERG11 mutations on azole resistance. Candida lusitaniae was chosen because it is susceptible to azole antifungals, it belongs to the CTG clade of yeast, which includes most of the Candida species pathogenic for humans, and it is haploid and easily amenable to genetic transformation and molecular modeling. In this work, allelic replacement is targeted at the ERG11 locus by the reconstitution of a functional auxotrophic marker in the 3' intergenic region of ERG11 Homologous and heterologous ERG11 alleles are expressed from the resident ERG11 promoter of C. lusitaniae, allowing accurate comparison of the phenotypic change in azole susceptibility. As a proof of concept, we successfully expressed in C. lusitaniae different ERG11 alleles, either bearing or not bearing mutations retrieved from a clinical context, from two phylogenetically distant yeasts, C. albicans and Kluyveromyces marxianusCandida lusitaniae constitutes a high-fidelity expression system, giving specific Erg11p-dependent fluconazole MICs very close to those observed with the ERG11 donor strain. This work led us to characterize the phenotypic effect of two kinds of mutation: mutation conferring decreased fluconazole susceptibility in a species-specific manner and mutation conferring fluconazole resistance in several yeast species. In particular, a missense mutation affecting amino acid K143 of Erg11p in Candida species, and the equivalent position K151 in K. marxianus, plays a critical role in fluconazole resistance.

Crystal Structure of the New Investigational Drug Candidate VT-1598 in Complex with Aspergillus fumigatus Sterol 14α-Demethylase Provides Insights into Its Broad-Spectrum Antifungal Activity

Within the past few decades, the incidence and complexity of human fungal infections have increased, and therefore, the need for safer and more efficient, broad-spectrum antifungal agents is high. In the study described here, we characterized the new tetrazole-based drug candidate VT-1598 as an inhibitor of sterol 14α-demethylase (CYP51B) from the filamentous fungus Aspergillus fumigatus VT-1598 displayed a high affinity of binding to the enzyme in solution (dissociation constant, 13 ± 1 nM) and in the reconstituted enzymatic reaction was revealed to have an inhibitory potency stronger than the potencies of all other simultaneously tested antifungal drugs, including fluconazole, voriconazole, ketoconazole, and posaconazole. The X-ray structure of the VT-1598/A. fumigatus CYP51 complex was determined and depicts the distinctive binding mode of the inhibitor in the enzyme active site, suggesting the molecular basis of the improved drug potency and broad-spectrum antifungal activity. These data show the formation of an optimized hydrogen bond between the phenoxymethyl oxygen of VT-1598 and the imidazole ring nitrogen of His374, the CYP51 residue that is highly conserved across fungal pathogens and fungus specific. Comparative structural analysis of A. fumigatus CYP51/voriconazole and Candida albicans CYP51/VT-1161 complexes supports the role of H bonding in fungal CYP51/inhibitor complexes and emphasizes the importance of an optimal distance between this interaction and the inhibitor-heme iron interaction. Cellular experiments using two A. fumigatus strains (strains 32820 and 1022) displayed a direct correlation between the effects of the drugs on CYP51B activity and fungal growth inhibition, indicating the noteworthy anti-A. fumigatus potency of VT-1598 and confirming its promise as a broad-spectrum antifungal agent.