Discovery of Novel Fungal Lanosterol 14α-Demethylase (CYP51)/Histone Deacetylase Dual Inhibitors to Treat Azole-Resistant Candidiasis

Discovery of new fungal jumonji H3K27 demethylase inhibitors for the treatment of Cryptococcus neoformans and Candida auris infections

Invasive fungal infections have become a serious public health problem. To tackle the challenges of limited efficacy in antifungal therapy and severe drug resistance, antifungal drugs with new mechanisms of action are urgently needed. Our previous study identified JIB-04 to be an inhibitor of fungal histone demethylase (HDM). To promote target validation and inhibitor design, herein a series of new JIB-04 derivatives were designed and synthesized. After the establishment of structure-activity relationship, compound A4 was identified to possess potent antifungal activity against Cryptococcus neoformans and Candida auris. Compared to lead compound JIB-04, compound A4 was a more potent HDM inhibitor and exhibited better water solubility, virulence factors inhibitory activity and in vivo antifungal potency. Collectively, this study further confirmed that fungal HDMs were potential antifungal targets and compound A4 was a promising antifungal lead compound.

Lanosterol 14α-Demethylase (CYP51)/Heat Shock Protein 90 (Hsp90) Dual Inhibitors for the Treatment of Invasive Candidiasis

Invasive candidiasis has attracted global attention with a high incidence and mortality. Current antifungal drugs are limited by unfavorable therapeutic efficacy, significant hepatorenal toxicity, and the development of drug resistance. Herein, we designed the first generation of lanosterol 14α-demethylase (CYP51)/heat shock protein 90 (Hsp90) dual inhibitors on the basis of antifungal synergism. Among them, dual inhibitor MM4 exhibited potent in vitro and in vivo antifungal activity against Candida albicans and effectively inhibited important fungal virulence factors (e.g., hyphae, biofilm). Therefore, CYP51/Hsp90 dual inhibitors show great promise in the development of novel antifungal drugs to combat invasive candidiasis.

Discovery of highly potent triazole derivatives with broad-spectrum antifungal activity based on Iodiconazole

The widespread use of broad-spectrum antibiotics, the growing number of immunocompromised individuals, and the emergence of drug-resistant strains have resulted in the increasing incidence and mortality of invasive fungal infections. Azole drugs are the primary treatment for invasive fungal infections, and Iodiconazole is a potent azole drug with strong antifungal activity, but its stability is poor. In order to improve stability, a series of triazole compounds containing ethynyl group were designed and synthesized. Most of the compounds showed strong inhibitory activity against pathogenic fungi, among which compound 20l showed excellent inhibitory activity against pathogenic fungi and drug-resistant fungi. Importantly, and the stability of 20l (T1/2 = 30.2 min) was obviously improved compared with Iodiconazole (T1/2 = 4.39 min). In addition, the preferred compound 20l can prevent fungal phase transition and the formation of fungal biofilm, and show satisfactory fungicidal activity. In addition, the compound 20l was almost non-toxic to mammalian HUVEC cell and 293T cell. In vivo pharmacokinetic studies showed that 20l had acceptable pharmacokinetic properties. These results strongly demonstrate that compound 20l was worth further investigation as a potential antifungal inhibitor.

Epigenetic Regulation of Fungal Secondary Metabolism

Secondary metabolism is one of the important mechanisms by which fungi adapt to their living environment and promote survival and reproduction. Recent studies have shown that epigenetic regulation, such as DNA methylation, histone modifications, and non-coding RNAs, plays key roles in fungal secondary metabolism and affect fungal growth, survival, and pathogenicity. This review describes recent advances in the study of epigenetic regulation of fungal secondary metabolism. We discuss the way in which epigenetic markers respond to environmental changes and stimulate the production of biologically active compounds by fungi, and the feasibility of these new findings applied to develop new antifungal strategies and optimize secondary metabolism. In addition, we have deliberated on possible future directions of research in this field. A deeper understanding of epigenetic regulatory networks is a key focus for future research.

Antifungal Activity and Multi-Target Mechanism of Action of Methylaervine on Candida albicans

The discovery of a lead compound against Candida albicans is urgently needed because of the lack of clinically available antifungal drugs and the increase in drug resistance. Herein, a β-carboline alkaloid methylaervine (MET) exhibited potential activity against C. albicans (MIC = 16-128 μg/mL), no hemolytic toxicity, and a low tendency to induce drug resistance. An antifungal mechanism study indicated that MET effectively inhibited the biofilm formation and disrupted the mature biofilm. Moreover, filamentation formation and spore germination were also weakened. The electron microscopy analysis revealed that MET could damage the cell structure, including the cell wall, membrane, and cytoplasm. In particular, the permeability and integrity of the cell membrane were destroyed. When it entered the fungi cell, it interfered with the redox homeostasis and DNA function. Overall, MET can inhibit the growth of C. albicans from multiple channels, such as biofilm, filamentation, cell structure, and intracellular targets, which are difficult to mutate at the same time to generate drug resistance. This work provides a promising lead compound for the creation of new antifungal agents against C. albicans.

Updates on Mechanisms of Cytochrome P450 Catalysis of Complex Steroid Oxidations

Cytochrome P450 (P450) enzymes dominate steroid metabolism. In general, the simple C-hydroxylation reactions are mechanistically straightforward and are generally agreed to involve a perferryl oxygen species (formally FeO3+). Several of the steroid transformations are more complex and involve C-C bond scission. We initiated mechanistic studies with several of these (i.e., 11A1, 17A1, 19A1, and 51A1) and have now established that the dominant modes of catalysis for P450s 19A1 and 51A1 involve a ferric peroxide anion (i.e., Fe3+O2¯) instead of a perferryl ion complex (FeO3+), as demonstrated with 18O incorporation studies. P450 17A1 is less clear. The indicated P450 reactions all involve sequential oxidations, and we have explored the processivity of these multi-step reactions. P450 19A1 is distributive, i.e., intermediate products dissociate and reassociate, but P450s 11A1 and 51A1 are highly processive. P450 17A1 shows intermediate processivity, as expected from the release of 17-hydroxysteroids for the biosynthesis of key molecules, and P450 19A1 is very distributive. P450 11B2 catalyzes a processive multi-step oxidation process with the complexity of a chemical closure of an intermediate to a locked lactol form.

Discovery of Novel α,β-Unsaturated Amide Derivatives as Candidate Antifungals to Overcome Fungal Resistance

In our previous study, coumarin-containing CYP51 inhibitor A32 demonstrated potent antiresistance activity. However, compound A32 demonstrated unsatisfied metabolic stability, necessitating modifications to overcome these limitations. In this study, α,β-unsaturated amides were used to replace the unstable coumarin ring, which increased metabolic stability by four times while maintaining antifungal activity, including activity against resistant strains. Subsequently, the sterol composition analysis and morphological observation experiments indicated that the target of these novel compounds is lanosterol 14α-demethylase (CYP51). Meanwhile, biofilm growth was inhibited and resistance genes (ERG11, CDR1, CDR2, and MDR1) expression was downregulated to find out how the antiresistance works. Importantly, compound C07 demonstrated the capacity to stimulate reactive oxygen species, thus displaying potent fungicidal activity. Moreover, C07 exhibited encouraging effectiveness in vivo following intraperitoneal administration. Additionally, the most potent compound C07 showed satisfactory pharmacokinetic properties and low toxicity. These α,β-unsaturated amide derivatives, particularly C07, are potential candidates for treating azole-resistant candidiasis.

Establishment of a genetic transformation system for cordycipitoid fungus Cordyceps chanhua

Cordyceps chanhua is a well-known edible and medicinal mushroom with a long history of use in China, and it contains a variety of secondary metabolites with interesting bioactive ingredients. However, recent researches have mainly focused on cultivation conditions, secondary metabolite compositions and pharmacological activities of C. chanhua, the lack of an efficient and stable genetic transformation system has largely limited further research on the relationship between secondary metabolites and biosynthetic gene clusters in C. chanhua. In this study, single-factor experiments were used to compare the effects of different osmotic stabilizers, enzyme concentrations and enzyme digestion times on protoplast yield, and we found that the highest yield of 5.53 × 108 protoplasts/mL was obtained with 0.7 M mannitol, 6 mg/mL snail enzyme and 4 h of enzyme digestion time, and the regeneration rate of protoplasts was up to approximately 30% using 0.7 M mannitol as an osmotic stabilizer. On this basis, a PEG-mediated genetic transformation system of C. chanhua was successfully established for the first time, which lays the foundation for further genetic transformation of C. chanhua.

Fluconazole Analogs and Derivatives: An Overview of Synthesis, Chemical Transformations, and Biological Activity

Fluconazole (2-(2,4-difluorophenyl)-1,3-bis(1H-1,2,4-triazol-1-yl)propan-2-ol), which was patented in 1981 and introduced for commercial use in 1988, is a widely utilized antifungal drug whose mechanism of action involves inhibition of the activity of 14-α lanosterol demethylase. Its safety and effectiveness have established it as one of the most frequently employed antifungal agents. Resistance to azole antifungal drugs is becoming more common. It may be related to a mutation of the gene encoding the enzyme. To address this issue, molecules with modifications in three main regions of fluconazole, namely the hydroxyl group, the aromatic ring, and the 1,2,4-triazole rings, have been synthesized in an attempt to create more potent antifungal drugs. These modifications aim at enhancing the effectiveness against microorganisms and improving pharmacokinetic parameters and safety profiles of the synthesized compounds. The present review explores the synthesis of fluconazole derivatives, accompanied by insights into the results of biological studies evaluating the therapeutic effects of these compounds.

New thymol-derived triazole exhibits promising activity against Trichophyton rubrum

Fungal infections have emerged worldwide, and azole antifungals are widely used to control these infections. However, the emergence of antifungal resistance has been compromising the effectiveness of these drugs. Therefore, the objective of this study was to evaluate the antifungal and cytotoxic activities of the nine new 1,2,3 triazole compounds derived from thymol that were synthesized through Click chemistry. The binding mode prediction was carried out by docking studies using the crystallographic structure of Lanosterol 14α-demethylase G73E mutant from Saccharomyces cerevisiae. The new compounds showed potent antifungal activity against Trichophyton rubrum but did not show relevant action against Aspergillus fumigatus and Candida albicans. For T. rubrum, molecules nº 5 and 8 showed promising results, emphasizing nº 8, whose fungicidal and fungistatic effects were similar to fluconazole. In addition, molecule nº 8 showed low toxicity for keratinocytes and fibroblasts, concluding that this compound demonstrates promising characteristics for developing a new drug for dermatophytosis caused by T. rubrum, or serves as a structural basis for further research.

Strategies of targeting CYP51 for IFIs therapy: Emerging prospects, opportunities and challenges

CYP51, a monooxygenase associated with the sterol synthesis pathway, is responsible for the catalysis of the 14-methyl hydroxylation reaction of lanosterol precursors. This enzyme is widely present in microorganisms, plants, and mammals. In mammals, CYP51 plays a role in cholesterol production, oligodendrocyte formation, oocyte maturation, and spermatogenesis. In fungal cells, CYP51 is an enzyme that synthesizes membrane sterols. By inhibiting fungal CYP51, ergosterol synthesis can be inhibited and ergosterol membrane fluidity is altered, resulting in fungal cell apoptosis. Thus, targeting CYP51 is a reliable antifungal strategy with important implications for the treatment of invasive fungal infections (IFIs). Many CYP51 inhibitors have been approved by the FDA for clinical treatment. However, several limitations of CYP51 inhibitors remain to be resolved, including fungal resistance, hepatotoxicity, and drug-drug interactions. New broad-spectrum, anti-resistant, highly selective CYP51 inhibitors are expected to be developed to enhance clinical efficacy and minimize adverse effects. Herein, we summarize the structural features and biological functions of CYP51 and emphatically analyze the structure-activity relationship (SAR) and therapeutic potential of different chemical types of small-molecule CYP51 inhibitors. We also discuss the latest progress of novel strategies, providing insights into new drugs targeting CYP51 for clinical practice.

Novel Nitric Oxide Donor-Azole Conjugation Strategy for Efficient Treatment of Cryptococcus neoformans Infections

Invasive fungal infections (IFIs) such as cryptococcal meningitis (CM) remain a serious health issue worldwide due to drug resistance closely related to biofilm formation. Unfortunately, available antifungal drugs with ideal safety and promising potency are still lacking; thus, the research of new candidate and therapeutic approach is urgently needed. As an important gas messenger molecule, nitric oxide (NO) shows vital inhibition on various microorganism biofilms. Hence, three series of novel NO-donating azole derivatives were designed and synthesized, and the in vitro antifungal activity as well as the mechanism of action was investigated. Among them, 3a and 3e displayed excellent antifungal activity against Cryptococcus neoformans and biofilm depending on the release of NO. Moreover, a more stable analogue 3h of 3a demonstrated markedly anti-CM effects via intranasal dropping, avoiding the first-pass effects and possessing a better brain permeability bypass blood-brain barrier. These results present a promising antifungal candidate and intranasal dropping approach for the treatment of CM, warranting further studies.

Developing Novel Coumarin-Containing Azoles Antifungal Agents by the Scaffold Merging Strategy for Treating Azole-Resistant Candidiasis

The extensive use of antifungal drugs has resulted in severe drug resistance, making clinical treatment of fungal infections more difficult. Biofilm inhibitors can overcome drug resistance by inhibiting fungal biofilm formation. In this study, some coumarins with antibiofilm activity were merged into CYP51 inhibitors to produce novel molecules possessing potent antiresistance activity. As expected, most compounds exhibited excellent in vitro antifungal activity against pathogenic fungi, especially fluconazole-resistant candidiasis. Then, their mechanism was confirmed by sterol composition analysis and morphological observation. Biofilm inhibition and down-regulation of resistance-related genes were employed to confirm the compounds' antiresistance mechanisms. Significantly, compound A32 demonstrated fungicidal activity against fluconazole-resistant strain 904. Most importantly, compound A32 showed potent in vivo antifungal activity against pathogenic fungi and fluconazole-resistant strains. Preliminary pharmacokinetic and toxicity tests demonstrated that the compounds possessed favorable druggability. Taken together, compound A32 represents a promising lead to develop novel antifungal agents for treating azole-resistant candidiasis.

Discovery of Pyrazolone Carbothioamide Derivatives as Inhibitors of the Pdr1-KIX Interaction for Combinational Treatment of Azole-Resistant Candidiasis

Candida glabrata has emerged as an important opportunistic pathogen of invasive candidiasis due to increasing drug resistance. Targeting Pdr1-KIX interactions with small molecules represents a potential strategy for treating drug-resistant candidiasis. However, effective Pdr1-KIX inhibitors are rather limited, hindering the validation of target druggability. Here, new Pdr1-KIX inhibitors were designed and assayed. Particularly, compound B8 possessed a new chemical scaffold and exhibited potent KIX binding affinity, leading to enhanced synergistic efficacy with fluconazole to treat resistant C. glabrata infection (FICI = 0.28). Compound B8 acted by inhibiting the efflux pump and down-regulating resistance-associated genes through blocking the Pdr1-KIX interaction. Compound B8 exhibited excellent in vitro and in vivo antifungal potency in combination with fluconazole against azole-resistant C. glabrata. It also had direct antifungal effect to treat C. glabrata infection, suggesting new mechanisms of action independent of Pdr1-KIX inhibition. Therefore, compound B8 represents a promising lead compound for antifungal drug development.

Recent advances in antifungal drug development targeting lanosterol 14α-demethylase (CYP51): A comprehensive review with structural and molecular insights

Fungal infections are posing serious threat to healthcare system due to emerging resistance among available antifungal agents. Among available antifungal agents in clinical practice, azoles (diazole, 1,2,4-triazole and tetrazole) remained most effective and widely prescribed antifungal agents. Now their associated side effects and emerging resistance pattern raised a need of new and potent antifungal agents. Lanosterol 14α-demethylase (CYP51) is responsible for the oxidative removal of 14α-methyl group of sterol precursors lanosterol and 24(28)-methylene-24,25-dihydrolanosterol in ergosterol biosynthesis hence an essential component of fungal life cycle and prominent target for antifungal drug development. This review will shed light on various azole- as well as non-azoles-based derivatives as potential antifungal agents that target fungal CYP51. Review will provide deep insight about structure activity relationship, pharmacological outcomes, and interactions of derivatives with CYP51 at molecular level. It will help medicinal chemists working on antifungal development in designing more rational, potent, and safer antifungal agents by targeting fungal CYP51 for tackling emerging antifungal drug resistance.

Discovery of Novel 1,2,3,4-Tetrahydrobenzofuro[2,3-c]pyridine Histone Deacetylase Inhibitors for Efficient Treatment of Hepatocellular Carcinoma

The development of histone deacetylase (HDAC) inhibitors for treating hematologic malignancies has been widely investigated, while their role in hepatocellular carcinoma (HCC) remains unexplored. In this study, we employed a scaffold-hopping design and a multicomponent synthesis approach to develop a novel series of 1,2,3,4-tetrahydrobenzofuro[2,3-c]pyridines as HDAC inhibitors. There were a total of 29 compounds achieved with flexible linkers and zinc-binding groups, wherein compound 12k was identified as a promising candidate with good HDAC inhibitory activity, pharmacokinetic profiles, and potency. It exhibited significant therapeutic efficacy in HCC cell lines (IC50 = 30 nM for Bel-7402) and xenograft models (76% inhibition for Bel-7402 xenografts, P.O. at 20 mg/kg, QOD, for 14 days) and was found to upregulate the acetylation of histone H3 and α-tubulin, leading to apoptosis and autophagy in HCC models. Molecular docking studies indicated a unique T-shaped conformation of 12k with the catalytic domain of HDAC1. Therefore, this work provides a new structure design for HDAC inhibitors and also offers a promising treatment for HCC.

Discovery of BRD4-HDAC Dual Inhibitors with Improved Fungal Selectivity and Potent Synergistic Antifungal Activity against Fluconazole-Resistant Candida albicans

Over the past several decades, invasive fungal infections, especially candidiasis, have caused dramatic morbidity and mortality due to ineffective antifungal drugs and severe drug resistance. Herein, new BRD4-histone deacetylase (HDAC) inhibitors were designed to restore the susceptibility of Candida albicans (C. albicans) to fluconazole (FLC). Interestingly, several compounds showed excellent selectivity against fungal HDACs. In particular, compound B2 showed excellent synergistic effect with FLC against resistant C. albicans (FICI = 0.063) with high selectivity against fungal HDACs (SI = 1653) and low cytotoxicity. Compound B2 effectively synergized with FLC and prevented biofilm formation and morphological transition in resistant C. albicans, potentiating the antifungal activity of FLC in vivo and significantly reducing kidney fungal loads. Thus, this drug combination is promising in the treatment of resistant C. albicans infections.

Target- and prodrug-based design for fungal diseases and cancer-associated fungal infections

Invasive fungal infections (IFIs) are emerging as a serious threat to public health and are associated with high incidence and mortality. IFIs also represent a frequent complication in patients with cancer who are undergoing chemotherapy. However, effective and safe antifungal agents remain limited, and the development of severe drug resistance further undermines the efficacy of antifungal therapy. Therefore, there is an urgent need for novel antifungal agents to treat life-threatening fungal diseases, especially those with new mode of action, favorable pharmacokinetic profiles, and anti-resistance activity. In this review, we summarize new antifungal targets and target-based inhibitor design, with a focus on their antifungal activity, selectivity, and mechanism. We also illustrate the prodrug design strategy used to improve the physicochemical and pharmacokinetic profiles of antifungal agents. Dual-targeting antifungal agents offer a new strategy for the treatment of resistant infections and cancer-associated fungal infections.

Design, synthesis and biological activity of hybrid antifungals derived from fluconazole and mebendazole

A novel series of triazole alcohol antifungals bearing a 5-benzoylbenzimidazol-2-ylthio side chain have been designed and synthesized as hybrids of fluconazole (a typical triazole antifungal) and mebendazole (an anthelmintic agent with antifungal activity). The title compounds were synthesized via the reaction of an appropriate oxirane and desired 2-mercaptobenzimidazole. Although there was possibility for formation of different N-substituted or S-substituted products, the structures of final compounds were assigned as thioether congeners by using ¹³C NMR spectroscopy. The SAR analysis of the primary lead compounds (series A) was conducted by simplifying the 5-benzoylbenzimidazol-2-ylthio residue to the benzimidazol-2-ylthio (series B) or benzothiazol-2-ylthio side chain (series C), and modification of halogen substituent on the phenethyl-triazole scaffold. In general, series A (compounds 4a-e) containing 5-benzoylbenzimidazole scaffold showed better antifungal activity against Candida spp. and Cryptococcus neoformans than related benzimidazole and benzothiazole derivatives. The better results were obtained with the 4-chloro derivative 4b displaying MICs <0.063-1 μg/mL. Although, removing benzoyl group from compound 4b had negative effect on the activity, optimization of phenethyl-triazole scaffold by desired halogen substituent resulted in compound 5c being as potent as 4b. In vitro and in silico ADMET evaluations of the most promising compounds 4b and 5c indicated that the selected compounds have desirable ADMET properties in comparison to standard drug fluconazole. Docking simulation study demonstrated that the benzimidazol-2-ylthio moiety is responsible for the potent antifungal activity of these compounds.

Controlling antifungal activity with light: Optical regulation of fungal ergosterol biosynthetic pathway with photo-responsive CYP51 inhibitors

Invasive fungal infections (IFIs) have been associated with high mortality, highlighting the urgent need for developing novel antifungal strategies. Herein the first light-responsive antifungal agents were designed by optical control of fungal ergosterol biosynthesis pathway with photocaged triazole lanosterol 14α-demethylase (CYP51) inhibitors. The photocaged triazoles completely shielded the CYP51 inhibition. The content of ergosterol in fungi before photoactivation and after photoactivation was 4.4% and 83.7%, respectively. Importantly, the shielded antifungal activity (MIC₈₀ ≥ 64 μg/mL) could be efficiently recovered (MIC₈₀ = 0.5-8 μg/mL) by light irradiation. The new chemical tools enable optical control of fungal growth arrest, morphological conversion and biofilm formation. The ability for high-precision antifungal treatment was validated by in vivo models. The light-activated compound A1 was comparable to fluconazole in prolonging survival in Galleria mellonella larvae with a median survival of 14 days and reducing fungal burden in the mouse skin infection model. Overall, this study paves the way for precise regulation of antifungal therapy with improved efficacy and safety.

BET-HDAC Dual Inhibitors for Combinational Treatment of Breast Cancer and Concurrent Candidiasis

Breast cancer is susceptible to Candida infections, and candidiasis has an enhancing effect on the progression and metastasis of tumor. Breast cancer and concurrent candidiasis represent a significant challenge in clinical therapy. Herein, a series of novel small molecule inhibitors simultaneously targeting bromodomain and extra-terminal (BET) and histone deacetylase (HDAC) were designed for combinational treatment of breast cancer and resistant Candida albicans infections. Among them, compounds 13c and 17b exhibited excellent and balanced inhibitory activity against both BET family proteins BRD4 and HDAC1. As compared with BRD4 or HDAC1 inhibitors, dual inhibitors 13c and 17b displayed improved in vivo antitumor efficacy in MDA-MB-231 breast cancer xenograft models. Notably, they synergized with fluconazole (FLC) to effectively reduce the kidney fungal burden in a murine model of disseminated candidiasis. Thus, the BET-HDAC dual inhibitors represented a novel therapeutic strategy for combinational treatment of breast cancer and concurrent candidiasis.

Efficient selective targeting of Candida CYP51 by oxadiazole derivatives designed from plant cuminaldehyde

Candida infection represents a global threat with associated high resistance and mortality rate. Azoles such as the triazole drug fluconazole are the frontline therapy against invasive fungal infections; however, the emerging multidrug-resistant strains limit their use. Therefore, a series of novel azole UOSO1-15 derivatives were developed based on a modified natural scaffold to combat the evolved resistance mechanism and to provide improved safety and target selectivity. The antifungal screening against C. albicans and C. auris showed that UOSO10 and 12-14 compounds were the most potent derivatives. Among them, UOSO13 exhibited superior potent activity with MIC50 values of 0.5 and 0.8 μg mL-1 against C. albicans and C. auris compared to 25 and 600 μg mL-1 for fluconazole, respectively. UOSO13 displayed significant CaCYP51 enzyme inhibition activity in a concentration-dependent manner with an IC50 10-fold that of fluconazole, while exhibiting no activity against human CYP50 enzyme or toxicity to human cells. Furthermore, UOSO13 caused a significant reduction of Candida ergosterol content by 70.3% compared to a 35.6% reduction by fluconazole. Homology modeling, molecular docking, and molecular dynamics simulations of C. auris CYP51 enzyme indicated the stability and superiority of UOSO13. ADME prediction indicated that UOSO13 fulfils the drug-likeness criteria with good physicochemical properties.

Small molecules for combating multidrug-resistant superbug Candida auris infections

Candida auris is emerging as a major global threat to human health. C. auris infections are associated with high mortality due to intrinsic multi-drug resistance. Currently, therapeutic options for the treatment of C. auris infections are rather limited. We aim to provide a comprehensive review of current strategies, drug candidates, and lead compounds in the discovery and development of novel therapeutic agents against C. auris. The drug resistance profiles and mechanisms are briefly summarized. The structures and activities of clinical candidates, drug combinations, antifungal chemosensitizers, repositioned drugs, new targets, and new types of compounds will be illustrated in detail, and perspectives for guiding future research will be provided. We hope that this review will be helpful to prompting the drug development process to combat this fungal pathogen.

Drug design strategies for the treatment azole-resistant candidiasis

INTRODUCTION

Despite the availability of novel antifungals and therapeutic strategies, the rate of global mortality linked to invasive fungal diseases from fungal infection remains high. Candida albicans account for the most invasive mycosis produced by yeast. Thus, the current arsenal of medicinal chemists is focused on finding new effective agents with lower toxicity and broad-spectrum activity. In this review article, recent efforts to find effective agents against azole-resistant candidiasis, a common fungal infection, are covered.

AREAS COVERED

Herein, the authors outlined all azole-based compounds, dual target, and new scaffolds (non-azole-based compounds) which were effective against azole-resistant candidiasis. In addition, the mechanism of action and SAR studies were also discussed, if the data were available.

EXPERT OPINION

The current status of fungal infections and the drawbacks of existing drugs have encouraged scientists to find novel scaffolds based on different methods like virtual screening and fragment-based drug discovery. Machine learning and in-silico methods have found their role in this field and experts are hopeful to find novel scaffolds/compounds by using these methods.

Multitargeted anti-infective drugs: resilience to resistance in the antimicrobial resistance era

The standard drug discovery paradigm of single molecule – single biological target – single biological effect is perhaps particularly unsuitable for anti-infective drug discovery. This is due to the rapid evolution of resistance likely to be observed with single target drugs. Multitargeted anti-infective drugs are likely to be superior due to their lower susceptibility to target-related resistance mechanisms. Strathclyde minor groove binders are a class of compounds which have been developed by adopting the multitargeted anti-infective drugs paradigm, and their effectiveness against a wide range of pathogenic organisms is discussed. The renaming of this class to Strathclyde nucleic acid binders is also presented due to their likely targets including both DNA and RNA.

Role of Azole Drugs in Promoting Fungal Cell Autophagy Revealed by an NIR Fluorescence-Based Theranostic Probe

Autophagy, a widespread degradation system in eukaryotes, plays an important role in maintaining the homeostasis of the cellular environment and the recycling of substances. Optical probes for the tracking of autophagy can be used as an effective tool not only to visualize the autophagy process but also to study autophagy-targeted drugs. Various molecule probes for autophagy of cancer cells emerge but are very limited for that of fungal cells, resulting in the lack of research on antifungal drugs targeting autophagy. To address this issue, we report an azole NIR fluorescence-based theranostic probe AF-1 with antifungal activity that is sensitive to autophagy-associated pH. The unique design of this probe lies in the introduction of both the pH-sensitive fluorophore with a detection range matching the pH range of the autophagy process and the conserved core structural fragment of azole drugs, providing a strategy to investigate the relationship between antifungal drug action and autophagy. As such, AF-1 exhibited excellent spectral properties and was found to target and induce the autophagy of the fungal cell membrane while maintaining moderate antifungal activity. Of note, using this theranostic probe as both a dye and drug, the autophagy process of fungi was visualized in a ratiometric manner, revealing the role of azole antifungal drugs in promoting autophagy to induce fungal cell apoptosis.

New Peptide Based Fluconazole Conjugates with Expanded Molecular Targets

Infections of Candida spp. etiology are frequently treated with azole drugs. Among azoles, the most widely used in the clinical scenario remains fluconazole (FLC). Promising results in treatment of dangerous, systemic Candida infections demonstrate the advantages of combined therapies carried out with combinations of at least two different antifungal agents. Here, we report five conjugates composed of covalently linked FLC and cell penetrating or antimicrobial peptide: TP10-7-NH₂, TP10-NH₂, LFcinB(2-11)-NH₂, LFcinB[Nle^1,11]-NH₂, and HLopt2-NH₂, with aspects of design, chemical synthesis and their biological activities. Two of these compounds, namely FLCpOH-TP10-NH₂ and FLCpOH-TP10-7-NH₂, exhibit high activity against reference strains and fluconazole-resistant clinical isolates of C. albicans, including strains overproducing drug transporters. Moreover, both of them demonstrate higher fungicidal effects compared to fluconazole. Analysis performed with fluorescence and scanning electron microscopy as well as flow cytometry indicated the cell membrane as a molecular target of synthesized conjugates. An important advantage of FLCpOH-TP10-NH₂ and FLCpOH-TP10-7-NH₂ is their low cytotoxicity. The IC₉₀ value for the human cells after 72 h treatment was comparable to the MIC₅₀ value after 24 h treatment for most strains of C. albicans. In reported conjugates, FLC was linked to the peptide by its hydroxyl group. It is worth noting that conjugation of FLC by the nitrogen atom of the triazole ring led to practically inactive compounds. Two compounds produced by us and reported herein appear to be potential candidates for novel antifungal agents.

Structure-Guided Discovery of the Novel Covalent Allosteric Site and Covalent Inhibitors of Fructose-1,6-Bisphosphate Aldolase to Overcome the Azole Resistance of Candidiasis

Fructose-1,6-bisphosphate aldolase (FBA) represents an attractive new antifungal target. Here, we employed a structure-based optimization strategy to discover a novel covalent binding site (C292 site) and the first-in-class covalent allosteric inhibitors of FBA from Candida albicans (CaFBA). Site-directed mutagenesis, liquid chromatography-mass spectrometry, and the crystallographic structures of APO-CaFBA, CaFBA-G3P, and C157S-2a4 revealed that S268 is an essential pharmacophore for the catalytic activity of CaFBA, and L288 is an allosteric regulation switch for CaFBA. Furthermore, most of the CaFBA covalent inhibitors exhibited good inhibitory activity against azole-resistant C. albicans, and compound 2a11 can inhibit the growth of azole-resistant strains 103 with the MIC₈₀ of 1 μg/mL. Collectively, this work identifies a new covalent allosteric site of CaFBA and discovers the first generation of covalent inhibitors for fungal FBA with potent inhibitory activity against resistant fungi, establishing a structural foundation and providing a promising strategy for the design of potent antifungal drugs.

Heat shock protein 90 (Hsp90)/Histone deacetylase (HDAC) dual inhibitors for the treatment of azoles-resistant Candida albicans

Clinical treatment of candidiasis has suffered from increasingly severe drug resistance and limited efficacy. Thus, novel strategies to deal with drug resistance are highly desired to develop effective therapeutic agents. Herein, dual inhibition of heat shock protein 90 (Hsp90) and histone deacetylase (HDAC) was validated as a new strategy to potentiate efficacy of fluconazole against resistant Candida albicans infections. The first generation of Hsp90/HDAC dual inhibitors were designed as synergistic enhancers to treat azoles-resistant candidiasis. In particular, compound J5 exhibited fungal-selective inhibitory effects on Hsp90 and HDACs, leading to low toxicity and excellent in vitro (FICI = 0.266) and in vivo synergistic antifungal potency to treat fluconazole resistant candidiasis. Antifungal-mechanistic investigation revealed that compound J5 suppressed important virulence factors and down-regulated expression of resistance-associated genes. Therefore, Hsp90/HDAC dual inhibitors represent a new strategy for the development of novel antifungal therapeutics to combat azole-resistant candidiasis.

Mode of action and efficacy of quinolinic acid for the control of Ceratocystis fimbriata on sweet potato

BACKGROUND

Ceratocystis fimbriata is a hazardous fungal pathogen able to cause black rot disease on sweet potato. The management of C. fimbriata strongly relies on the use of toxic fungicides, and there is a lack of efficient alternative strategies.

RESULTS

The antifungal properties of quinolinic acid (QA) were studied for the first time, indicating that QA shows selective antifungal activity against C. fimbriata. QA inhibited completely the mycelial growth of C. fimbriata at less than 0.8 mg mL^-1 concentration (pH 4), and was able to produce alterations in the fungal cell wall, and to impede spore agglutination and mycelium formation. QA significantly reduced the concentration of ergosterol, and was able to associate to iron (II), suggesting that QA may be a lanosterol 14-α demethylase inhibitor. In preventive applications, QA reduced the disease incidence of C. fimbriata on sweet potato by 75%, achieving higher control efficacy in comparison with commercial fungicides prochloraz and carbendazim.

CONCLUSIONS

The first selective antifungal agent against C. fimbriata was discovered in this work, and showed suitable antifungal properties for the management of black rot disease. © 2021 Society of Chemical Industry.

Design, Synthesis, and In Vitro and In Vivo Evaluation of Novel Fluconazole-Based Compounds with Promising Antifungal Activities

Demand has arisen for developing new azole antifungal agents with the growth of the resistant rate of infective fungal species to current azole antifungals in recent years. Accordingly, the present study reports the synthesis of novel fluconazole (FLC) analogues bearing urea functionality that led to discovering new azole agents with promising antifungal activities. In particular, compounds 8b and 8c displayed broad-spectrum activity and superior in vitro antifungal capabilities compared to the standard drug FLC against sensitive and resistant Candida albicans (C. albicans). The highly active compounds 8b and 8c had potent antibiofilm properties against FLC-resistant C. albicans species. Additionally, these compounds exhibited very low toxicity for three mammalian cell lines and human red blood cells. Time-kill studies revealed that our synthesized compounds displayed a fungicidal mechanism toward fungal growth. Furthermore, a density functional theory (DFT) calculation, additional docking, and independent gradient model (IGM) studies were performed to analyze their structure-activity relationship (SAR) and to assess the molecular interactions in the related target protein. Finally, in vivo results represented a significant reduction in the tissue fungal burden and improvements in the survival rate in a mice model of systemic candidiasis along with in vitro and in silico studies, demonstrating the therapeutic efficiency of compounds 8b and 8c as novel leads for candidiasis drug discovery.

Fungal Lysine Deacetylases in Virulence, Resistance, and Production of Small Bioactive Compounds

The growing number of immunocompromised patients begs for efficient therapy strategies against invasive fungal infections. As conventional antifungal treatment is increasingly hampered by resistance to commonly used antifungals, development of novel therapy regimens is required. On the other hand, numerous fungal species are industrially exploited as cell factories of enzymes and chemicals or as producers of medically relevant pharmaceuticals. Consequently, there is immense interest in tapping the almost inexhaustible fungal portfolio of natural products for potential medical and industrial applications. Both the pathogenicity and production of those small metabolites are significantly dependent on the acetylation status of distinct regulatory proteins. Thus, classical lysine deacetylases (KDACs) are crucial virulence determinants and important regulators of natural products of fungi. In this review, we present an overview of the members of classical KDACs and their complexes in filamentous fungi. Further, we discuss the impact of the genetic manipulation of KDACs on the pathogenicity and production of bioactive molecules. Special consideration is given to inhibitors of these enzymes and their role as potential new antifungals and emerging tools for the discovery of novel pharmaceutical drugs and antibiotics in fungal producer strains.

Design, synthesis and evaluation of novel 5-phenylthiophene derivatives as potent fungicidal of Candida albicans and antifungal reagents of fluconazole-resistant fungi

A series of 5-phenylthiophene derivatives with novel structures were designed and synthesized to combat the increasing incidence of susceptible and drug-resistant fungal infections. The antifungal activity of the synthesized compounds was assessed against seven susceptible strains and six fluconazole-resistant strains. It is especially encouraging that compounds 17b and 17f displayed significant antifungal activities against all tested strains. Furthermore, the potent compounds 17b and 17f could prevent the formation of fungi biofilms and 17f displayed satisfactory fungicidal activity. Preliminary mechanistic studies showed that the potent antifungal activity of compound 17f stemmed from inhibition of C. albicans CYP51. In addition, Compounds 17b and 17f were almost nontoxic to mammalian A549, MCF-7, and THLE-2 cells. These results strongly suggested that compounds 17b and 17f are promising as novel antifungal drugs.

Effects of Hsp90 Inhibitor Ganetespib on Inhibition of Azole-Resistant Candida albicans

Candida albicans is the most common fungal pathogen. Recently, drug resistance of C. albicans is increasingly severe. Hsp90 is a promising antifungal target to overcome this problem. To evaluate the effects of Hsp90 inhibitor ganetespib on the inhibition of azole-resistant C. albicans, the microdilution checkerboard method was used to measure the in vitro synergistic efficacy of ganetespib. The XTT/menadione reduction assay, microscopic observation, and Rh6G efflux assay were established to investigate the effects of ganetespib on azole-resistant C. albicans biofilm formation, filamentation, and efflux pump. Real-time RT-PCR analysis was employed to clarify the mechanism of antagonizing drug resistance. The in vivo antifungal efficacy of ganetespib was determined by the infectious model of azole-resistant C. albicans. Ganetespib showed an excellent synergistic antifungal activity in vitro and significantly inhibited the fungal biofilm formation, whereas it had no inhibitory effect on fungal hypha formation. Expression of azole-targeting enzyme gene ERG11 and efflux pump genes CDR1, CDR2, and MDR1 was significantly down-regulated when ganetespib was used in combination with FLC. In a mouse model infected with FLC-resistant C. albicans, the combination of ganetespib and FLC effectively reversed the FLC resistance and significantly decreased the kidney fungal load of mouse.

Molecular targets for antifungals in amino acid and protein biosynthetic pathways

Fungi cause death of over 1.5 million people every year, while cutaneous mycoses are among the most common infections in the world. Mycoses vary greatly in severity, there are long-term skin (ringworm), nail or hair infections (tinea capitis), recurrent like vaginal candidiasis or severe, life-threatening systemic, multiorgan infections. In the last few years, increasing importance is attached to the health and economic problems caused by fungal pathogens. There is a growing need for improvement of the availability of antifungal drugs, decreasing their prices and reducing side effects. Searching for novel approaches in this respect, amino acid and protein biosynthesis pathways appear to be competitive. The route that leads from amino acid biosynthesis to protein folding and its activation is rich in enzymes that are descriptive of fungi. Blocking the action of those enzymes often leads to avirulence or growth inhibition. In this review, we want to trace the principal processes of fungi vitality. We present the data of genes encoding enzymes involved in amino acid and protein biosynthesis, potential molecular targets in antifungal chemotherapy, and describe the impact of inhibitors on fungal organisms.

Discovery of novel purinylthiazolylethanone derivatives as anti-Candida albicans agents through possible multifaceted mechanisms

An unprecedented amount of fungal and fungal-like infections has recently brought about some of the most severe die-offs and extinctions due to fungal drug resistance. Aimed to alleviate the situation, new effort was made to develop novel purinylthiazolylethanone derivatives, which were expected to combat the fungal drug resistance. Some prepared purinylthiazolylethanone derivatives possessed satisfactory inhibitory action towards the tested fungi, among which compound 8c gave a MIC value of 1 μg/mL against C. albicans. The active molecule 8c was able to kill C. albicans with undetectable resistance as well as low hematotoxicity and cytotoxicity. Furthermore, it could hinder the growth of C. albicans biofilm, thus avoiding the occurrence of drug resistance. Mechanism research manifested that purinylthiazolylethanone derivative 8c led to damage of cell wall and membrane disruption, so protein leakage and the cytoplasmic membrane depolarization were observed. On this account, the activity of fungal lactate dehydrogenase was reduced and metabolism was impeded. Meanwhile, the increased levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) disordered redox equilibrium, giving rise to oxidative damage to fungal cells and fungicidal effect.

Lanosterol 14α-demethylase (CYP51)/histone deacetylase (HDAC) dual inhibitors for treatment of Candida tropicalis and Cryptococcus neoformans infections

Invasive fungal infections remain a challenge due to lack of effective antifungal agents and serious drug resistance. Discovery of antifungal agents with novel antifungal mechanism is important and urgent. Previously, we designed the first CYP51/HDAC dual inhibitors with potent activity against resistant Candida albicans infections. To better understand the antifungal spectrum and synergistic mechanism, herein new CYP51/HDAC dual inhibitors were designed which showed potent in vitro and in vivo antifungal activity against C. neoformans and C. tropicalis infections. Antifungal mechanism studies revealed that the CYP51/HDAC dual inhibitors acted by inhibiting various virulence factors of C. tropicalis and C. neoformans and down-regulating resistance-associated genes. This study highlights the potential of CYP51/HDAC dual inhibitors as a promising strategy for the discovery of novel broad-spectrum antifungal agents.

Pyrimidinetrione-imidazoles as a Unique Structural Type of Potential Agents towards Candida Albicans: Design, Synthesis and Biological Evaluation

Substantial morbidity and mortality of fungal infections have aroused concerns all over the world, and common Candida spp. currently bring about severe systemic infections. A series of pyrimidinetrione-imidazole conjugates as potentially antifungal agents were developed. Bioassays manifested that 4-fluobenzyl pyrimidinetrione imidazole 5 f exerted favorable inhibition towards C. albicans (MIC=0.002 mM), being 6.5 folds more active than clinical antifungal drug fluconazole (MIC=0.013 mM). Preliminary mechanism research indicated that compound 5 f could not only depolarize membrane potential but also permeabilize the membrane of C. albicans. Molecular docking was operated to simulate the interaction mode between molecule 5 f and CYP51. In addition, hybrid 5 f might form 5 f-DNA supramolecular complex via intercalating into DNA. The interference of membrane and DNA might contribute to its fungicidal capacity with no obvious tendency to induce the resistance against C. albicans. Conjugate 5 f endowed good blood compatibility as well as low cytotoxicity towards HeLa and HEK-293T cells.

Discovery of Piperidol Derivatives for Combinational Treatment of Azole-Resistant Candidiasis

Effective strategies are needed to deal with invasive fungal infections caused by drug-resistant fungi. Previously, we designed a series of antifungal benzocyclane derivatives based on the drug repurposing of haloperidol. Herein, further structural optimization and antifungal mechanism studies were performed, leading to the discovery of new piperidol derivative B2 with improved synergistic antifungal potency, selectivity, and water solubility. In particular, the combination of compound B2 and fluconazole showed potent in vitro and in vivo antifungal activity against azole-resistant Candida albicans. Compound B2 inhibited important virulence factors by regulating virulence-associated genes and improved the efficacy of fluconazole by down-regulating the CYP51-coding gene and efflux pump gene. Taken together, the piperidol derivative B2 represents a promising lead compound for the combinational treatment of azole-resistant candidiasis.

CAP rigidification of MS-275 and chidamide leads to enhanced antiproliferative effects mediated through HDAC1, 2 and tubulin polymerization inhibition

The study focuses on the prudent design and synthesis of anilide type class I HDAC inhibitors employing a functionalized pyrrolo[2,3-d]pyrimidine skeleton as the surface recognition part. Utilization of the bicyclic aromatic ring to fabricate the target compounds was envisioned to confer rigidity to the chemical architecture of MS-275 and chidamide. In-vitro enzymatic and cellular assays led to the identification of compound 7 as a potent inhibitor of HDAC1 and 2 isoform that exerted substantial cell growth inhibitory effects against human breast MDA-MB-231, cervical HeLa, breast MDA-MB-468, colorectal DLD1, and colorectal HCT116 cell lines with an IC₅₀ values of 0.05-0.47 μM, better than MS-275 and chidamide. In addition, the anilide 7 was also endowed with a superior antiproliferative profile than MS275 and chidamide towards the human cutaneous T cell lymphoma (HH and HuT78), leukemia (HL60 and KG-1), and HDACi sensitive/resistant gastric cell lines (YCC11 and YCC3/7). Exhaustive exploration of the construct 7 confirmed it to be a microtubule-targeting agent that could trigger the cell-cycle arrest in mitosis. In pursuit of extracting the benefits of evidenced microtubule-destabilizing activity of the anilide 7, it was further evaluated against non-small-cell lung cancer cell lines as well as the multiple-drug resistant uterine cancer cell line (MES-SA/Dx5) and overwhelmingly positive results in context of inhibitory effects were attained. Furthermore, molecular modelling studies were performed and some key interactions of the anilide 7 with the amino acid residues of the active site of HDAC1 isoform and tubulin were figured out.

Novel Carboline Fungal Histone Deacetylase (HDAC) Inhibitors for Combinational Treatment of Azole-Resistant Candidiasis

Due to the evolution and development of antifungal drug resistance, limited efficacy of existing drugs has led to high mortality in patients with serious fungal infections. To develop novel antifungal therapeutic strategies, herein a series of carboline fungal histone deacetylase (HDAC) inhibitors were designed and synthesized, which had potent synergistic effects with fluconazole against resistant Candida albicans infection. In particular, compound D12 showed excellent in vitro and in vivo synergistic antifungal efficacy with fluconazole to treat azole-resistant candidiasis. It cooperated with fluconazole in reducing the virulence of C. albicans by blocking morphological mutual transformation and inhibiting biofilm formation. Mechanism studies revealed that the reversion of drug resistance was due to downregulation of the expression of the azole target gene ERG11 and efflux gene CDR1. Taken together, fungal HDAC inhibitor D12 offered a promising lead compound for combinational treatment of azole-resistant candidiasis.

Novel naphthylamide derivatives as dual-target antifungal inhibitors: Design, synthesis and biological evaluation

Fungal infections have become a serious medical problem due to the high infection rate and the frequent emergence of drug resistance. Squalene epoxidase (SE) and 14α-demethylase (CYP51) are considered as the important antifungal targets, they can show the synergistic effect on antifungal therapy. In the study, a series of active fragments were screened through the method of De Novo Link, and these active fragments with the higher Ludi_Scores were selected, which can show the obvious binding ability with the dual targets (SE, CYP51). Subsequently, three series of target compounds with naphthyl amide scaffolds were constructed by connecting these core fragments, and their structures were synthesized. Most of compounds showed the antifungal activity in the treatment of pathogenic fungi. It was worth noting that compounds 10b-5 and 17a-2 with the excellent broad-spectrum antifungal properties also exhibited the obvious antifungal effects against drug-resistant fungi. Preliminary mechanism study has proved these target compounds can block the biosynthesis of ergosterol by inhibiting the activity of dual targets (SE, CYP51). Furthermore, target compounds 10-5 and 17a-2 with low toxicity side effects also demonstrated the excellent pharmacological effects in vivo. The molecular docking and ADMET prediction were performed, which can guide the optimization of subsequent lead compounds.

Dual inhibitors of histone deacetylases and other cancer-related targets: A pharmacological perspective

Epigenetic enzymes histone deacetylases (HDACs) are clinically validated anticancer drug targets which have been studied intensively in the past few decades. Although several drugs have been approved in this field, they are still limited to a subset of hematological malignancies (in particular T-cell lymphomas), with therapeutic potential not fully realized and the drug-resistance occurred after a certain period of use. To maximize the therapeutic potential of these classes of anticancer drugs, and to extend their application to solid tumors, numerous combination therapies containing an HDACi and an anticancer agent from other mechanisms are currently ongoing in clinical trials. Recently, dual targeting strategy comprising the HDACs component has emerged as an alternative approach for combination therapies. In this perspective, we intend to gather all HDACs-containing dual inhibitors related to cancer therapy published in literature since 2015, classify them into five categories based on targets' biological functions, and discuss the rationale why dual acting agents should work better than combinatorial therapies using two separate drugs. The article discusses the pharmacological aspects of these dual inhibitors, including in vitro biological activities, pharmacokinetic studies, in vivo efficacy studies, as well as available clinical trials. The review of the current status and advances should provide better analysis for future opportunities and challenges of this field.

Recent Progress in the Discovery of Antifungal Agents Targeting the Cell Wall

Due to the limit of available treatments and the emergence of drug resistance in the clinic, invasive fungal infections are an intractable problem with high morbidity and mortality. The cell wall, as a fungi-specific structure, is an appealing target for the discovery and development of novel and low-toxic antifungal agents. In an attempt to accelerate the discovery of novel cell wall targeted drugs, this Perspective will provide a comprehensive review of the progress made to date on the development of fungal cell wall inhibitors. Specifically, this review will focus on the targets, discovery process, chemical structures, antifungal activities, and structure-activity relationships. Although two types of cell wall antifungal agents are clinically available or in clinical trials, it is still a long way for the other cell wall targeted inhibitors to be translated into clinical applications. Future efforts should be focused on the identification of inhibitors against novel conserved cell wall targets.