Drug Repurposing of Haloperidol: Discovery of New Benzocyclane Derivatives as Potent Antifungal Agents against Cryptococcosis and Candidiasis

Mechanisms of Azole Potentiation: Insights from Drug Repurposing Approaches

The emergence of azole resistance and tolerance in pathogenic fungi has emerged as a significant public health concern, emphasizing the urgency for innovative strategies to bolster the efficacy of azole-based treatments. Drug repurposing stands as a promising and practical avenue for advancing antifungal therapy, with the potential for swift clinical translation. This review offers a comprehensive overview of azole synergistic agents uncovered through drug repurposing strategies, alongside an in-depth exploration of the mechanisms by which these agents augment azole potency. Drawing from these mechanisms, we delineate strategies aimed at enhancing azole effectiveness, such as inhibiting efflux pumps to elevate azole concentrations within fungal cells, intensifying ergosterol synthesis inhibition, mitigating fungal cell resistance to azoles, and disrupting biological processes extending beyond ergosterol synthesis. This review is beneficial for the development of these potentiators, as it meticulously examines instances and provides nuanced discussions on the mechanisms underlying the progression of azole potentiators through drug repurposing strategies.

The evolution of antifungal therapy: Traditional agents, current challenges and future perspectives

Fungal infections kill more than 3 million people every year. This high number reflects the significant challenges that treating these diseases worldwide presents. The current arsenal of antifungal drugs is limited and often accompanied by high toxicity to patients, elevated treatment costs, increased frequency of resistance rates, and the emergence of naturally resistant species. These treatment challenges highlight the urgency of developing new antifungal therapies, which could positively impact millions of lives each year globally. Our review offers an overview of the antifungal drugs currently available for treatment, presents the status of new antifungal drugs under clinical study, and explores ahead to future candidates that aim to help address this important global health issue.

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.

Design, synthesis and inhibitory activity against Candida albicans of a series of derivatives with 5-nitrofuran scaffold

The increasing resistance of Candida albicans against the currently available antifungal drugs has exerted enormous damage to human health. To develop novel and efficient antifungal agents with unique structure, a series of derivatives containing 5-nitrofuran scaffold (33 examples) were designed, synthesized, and screened the in vitro antifungal activities. Bioassay results disclosed that 5-nitrofuran derivatives could dramatically inhibit the growth of six strains of Candida albicans, particularly the drug-resistant clinical ones. There were ten kinds of compounds exhibited stronger inhibitory activities against tested fungi than those of fluconazole. For all tested fungi, B5 showed the highest activity with the MIC80 values of 0.25-8 µg/mL. The results of cytotoxicity assay displayed that B5 hardly influenced the growth of HL-7702 cell lines, consequently, it was safe for people and animals. The preliminary exploration of antifungal mechanism documented that B5 could destroy the morphology of tested fungi, facilitate the formation of reactive oxygen species, ultimately inhibited the proliferation of the tested fungi. In conclusion, a new and safe lead compound was successfully developed for the treatment of Candida albicans infection.

Halogenated Antimicrobial Agents to Combat Drug-Resistant Pathogens

Antimicrobial resistance presents us with a potential global crisis as it undermines the abilities of conventional antibiotics to combat pathogenic microbes. The history of antimicrobial agents is replete with examples of scaffolds containing halogens. In this review, we discuss the impacts of halogen atoms in various antibiotic types and antimicrobial scaffolds and their modes of action, structure-activity relationships, and the contributions of halogen atoms in antimicrobial activity and drug resistance. Other halogenated molecules, including carbohydrates, peptides, lipids, and polymeric complexes, are also reviewed, and the effects of halogenated scaffolds on pharmacokinetics, pharmacodynamics, and factors affecting antimicrobial and antivirulence activities are presented. Furthermore, the potential of halogenation to circumvent antimicrobial resistance and rejuvenate impotent antibiotics is addressed. This review provides an overview of the significance of halogenation, the abilities of halogens to interact in biomolecular settings and enhance pharmacological properties, and their potential therapeutic usages in preventing a postantibiotic era. SIGNIFICANCE STATEMENT: Antimicrobial resistance and the increasing impotence of antibiotics are critical threats to global health. The roles and importance of halogen atoms in antimicrobial drug scaffolds have been established, but comparatively little is known of their pharmacological impacts on drug resistance and antivirulence activities. This review is the first to extensively evaluate the roles of halogen atoms in various antibiotic classes and pharmacological scaffolds and to provide an overview of their ability to overcome antimicrobial resistance.

An update on the global treatment of invasive fungal infections

Fungal infections are a serious problem affecting many people worldwide, creating critical economic and medical consequences. Fungi are ubiquitous and can cause invasive diseases in individuals mostly living in developing countries or with weakened immune systems, and antifungal drugs currently available have important limitations in tolerability and efficacy. In an effort to counteract the high morbidity and mortality rates associated with invasive fungal infections, various approaches are being utilized to discover and develop new antifungal agents. This review discusses the challenges posed by fungal infections, outlines different methods for developing antifungal drugs and reports on the status of drugs currently in clinical trials, which offer hope for combating this serious global problem.

Azole potentiation in Candida species

Fungal infections are rising, with over 1.5 billion cases and more than 1 million deaths recorded each year. Among these, Candida infections are frequent in at-risk populations and the rapid development of drug resistance and tolerance contributes to their clinical persistence. Few antifungal drugs are available, and their efficacy is declining due to the environmental overuse and the expansion of multidrug-resistant species. One way to prolong their utility is by applying them in combination therapy. Here, we highlight recently described azole potentiators belonging to different categories: natural, repurposed, or novel compounds. We showcase examples of molecules and discuss their identified or proposed mode of action. We also emphasise the challenges in azole potentiator development, compounded by the lack of animal testing, the overreliance on Candida albicans and Candida auris, as well as the limited understanding of compound efficacy.

Discovery of Novel Myristic Acid Derivatives as N-Myristoyltransferase Inhibitors: Design, Synthesis, Analysis, Computational Studies and Antifungal Activity

In recent years, N-Myristoyltransferase (NMT) has been identified as a new target for the treatment of fungal infections. It is observed that at present, there are increased rates of morbidity and mortality due to fungal infections. Hence, a series of novel myristic acid derivatives were designed via molecular docking studies and ADMET studies by targeting NMT (N-Myristoyltransferase). The designed myristic acid derivatives were synthesized by converting myristic acid into myristoyl chloride and coupling it with aryl amines to yield corresponding myristic acid derivatives. The compounds were purified and characterized via FTIR, NMR and HRMS spectral analyses. In this study, we carried out a target NMT inhibition assay. In the NMT screening assay results, the compounds 3u, 3m and 3t showed better inhibition compared to the other myristic acid derivatives. In an in vitro antifungal evaluation, the myristic acid derivatives were assessed against Candida albicans and Aspergillus niger strains by determining their minimal inhibitory concentrations (MIC₅₀). The compounds 3u, 3k, 3r and 3t displayed superior antifungal capabilities against Candida albicans, and the compounds 3u, 3m and 3r displayed superior antifungal capabilities against Aspergillus niger compared to the standard drug FLZ (fluconazole). Altogether, we identified a new series of antifungal agents.

Antifungal Development and the Urgency of Minimizing the Impact of Fungal Diseases on Public Health

Fungal infections are a major public health problem resulting from the lack of public policies addressing these diseases, toxic and/or expensive therapeutic tools, scarce diagnostic tests, and unavailable vaccines. In this Perspective, we discuss the need for novel antifungal alternatives, highlighting new initiatives based on drug repurposing and the development of novel antifungals.

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.

Novel avenues for identification of new antifungal drugs and current challenges

INTRODUCTION

: Some of otherwise useful fungi are pathogenic to humans, and unfortunately, the number of these pathogens is increasing. In addition to common skin infections, these opportunistic pathogens are able to cause severe, often incurable, systemic mycoses.

AREAS COVERED

: The number of antifungal drugs is limited, especially drugs that can be used for systemic administration, and resistance to these drugs is very common. This review summarizes various approaches to the discovery and development of new antifungal drugs, provides an overview of the most important molecules in terms of basic (laboratory) research and compounds currently in clinical trials, and focuses on drug repurposing strategy, while providing an overview of drugs of other indications that have been tested in vitro for their antifungal activity for possible expansion of antifungal drugs and/or support of existing antimycotics.

EXPERT OPINION

: Despite the limitations of the research of new antifungal drugs by pharmaceutical manufacturers, in addition to innovated molecules based on clinically used drugs, several completely new small entities with unique mechanisms of actions have been identified. The identification of new molecular targets that offer alternatives for the development of new unique selective antifungal highly effective agents has been an important outcome of repurposing of non-antifungal drugs to antifungal drug. Also, given the advances in monoclonal antibodies and their application to immunosuppressed patients, it may seem possible to predict a more optimistic future for antifungal therapy than has been the case in recent decades.

Repurposing and optimization of drugs for discovery of novel antifungals

Although fungal diseases are a major and growing public health concern, there are only four major classes of drug to treat primary fungal pathogens. The pipeline of new antifungals in clinical development is relatively thin compared with other disease classes. One approach to rapidly identify and provide novel treatment options is to repurpose existing drugs as antifungals. However, such proposed drug-repurposing candidates often suffer suboptimal efficacy and pharmacokinetics (PK) for fungal diseases. Herein, we briefly review the current antifungal drug pipeline and recent approaches to optimize existing drugs into novel molecules with unique modes of action relative to existing antifungal drug classes.

Jumonji Histone Demethylase Inhibitor JIB-04 as a Broad-Spectrum Antifungal Agent

Invasive fungal infections are emerging as a global public health problem. The lack of effective antifungal drugs is the bottleneck of clinical antifungal treatment. To identify novel antifungal agents with new mechanisms of action, JIB-04, a Jumonji histone demethylase inhibitor, was identified to possess broad-spectrum antifungal activity by a cell-based screen. Particularly, JIB-04 effectively inhibited Jumonji demethylase activity and ergosterol biosynthesis of Cryptococcus neoformans cells, leading to in vitro and in vivo anti-Cryptococcus activity. It also significantly inhibited the virulence factors of C. neoformans including biofilm, melanin, capsule, and surface hydrophobicity. Thus, JIB-04 was validated as a potent antifungal agent for the treatment of cryptococcal meningitis and Jumonji histone demethylase was preliminarily identified as a potential target for the development of novel antifungal therapeutics.

Small molecule based anti-virulence approaches against Candida albicans infections

Fungi are considered "silent killers" due to the difficulty of, and delays in diagnosis of infections and lack of effective antifungals. This challenge is compounded by the fact that being eukaryotes, fungi share several similarities with human cellular targets, creating obstacles to drug discovery. Candida albicans, a ubiquitous microbe in the human body is well-known for its role as an opportunistic pathogen in immunosuppressed people. Significantly, C. albicans is resistant to all the three classes of antifungals that are currently clinically available. Over the past few years, a paradigm shift has been recommended in the management of C. albicans infections, wherein anti-virulence strategies are considered an alternative to the discovery of new antimycotics. Small molecules, with a molecular weight <900 Daltons, can easily permeate the cell membrane and modulate the signal transduction pathways to elicit desired virulence inhibitory actions against pathogens. This review dissects in-depth, the discoveries that have been made with small-molecule anti-virulence approaches to tackle C. albicans infections.

Repurposing pantoprazole and haloperidol as efflux pump inhibitors in azole resistant clinical Candida albicans and non-albicans isolates

Candida species have a major role in nosocomial infections leading to high morbidity and mortality. Increased resistance to various antifungals, especially azoles is a significant problem. One of the main mechanisms for azole resistance is the up-regulation of efflux pump genes including CDR1 and MDR1. In the current study, clinical Candida isolates were identified to the species level and the antifungal susceptibility (AFS) of different Candida species was determined by disk diffusion method. Furthermore, the main mechanisms of azole resistance were investigated. Finally, haloperidol and pantoprazole were tested for their potential synergistic effect against fluconazole-resistant isolates. One hundred and twenty-two Candida clinical isolates were used in this study. 70 isolates were Candida albicans (57.4%), the non-albicans Candida species include: C. krusei (20.5%), C. tropicalis (6.6%), C. parapsilosis (5.7%), C. dubliniensis (4.9%) and C. glabrata (4.9%). The AFS testing showed that resistance to fluconazole and voriconazole were 13.1% (n = 16) and 9.8% (n = 12), respectively. Among the 16 resistant isolates, eight isolates (50%) were strong biofilm producers, seven (43.8 %) formed intermediate biofilm and one had no biofilm. All resistant strains overexpressed efflux pumps. Using RT-PCR, the efflux genes CDR1, MDR1 and ABC2 were over-expressed in azole resistant isolates. Haloperidol-fluconazole and pantoprazole-fluconazole combinations reduced the MIC of fluconazole in resistant isolates. The current study showed an increase in azole resistance of Candida species. The majority of resistant isolates form biofilm, and overexpress efflux pumps. Pantoprazole and Haloperidol showed a noteworthy effect as efflux pump inhibitors which oppose the fluconazole resistance in different Candida species.

Construction and activity evaluation of novel benzodioxane derivatives as dual-target antifungal inhibitors

Ergosterol exert the important function in maintaining the fluidity and osmotic pressure of fungal cells, and its key biosynthesis enzymes (Squalene epoxidase, SE; 14 α-demethylase, CYP51) displayed the obvious synergistic effects. Therefore, we expected to discover the novel antifungal compounds with dual-target (SE/CYP51) inhibitory activity. In the progress, we screened the different kinds of potent fragments based on the dual-target (CYP51, SE) features, and the method of fragment-based drug discovery (FBDD) was used to guide the construction of three different series of benzodioxane compounds. Subsequently, their chemical structures were synthesized and evaluated. These compounds displayed the obvious biological activity against the pathogenic fungal strains. Notably, target compounds 10a-2 and 22a-2 possessed the excellent broad-spectrum anti-fungal activity (MIC₅₀, 0.125-2.0 μg/mL) and the activity against drug-resistant strains (MIC₅₀, 0.5-2.0 μg/mL). Preliminary mechanism studies have confirmed that these compounds effectively inhibited the dual-target (SE/CYP51) activity, they could cause fungal rupture and death by blocking the bio-synthetic pathway of ergosterol. Further experiments discovered that compounds 10a-2 and 22a-2 also maintained a certain of anti-fungal effect in vivo. In summary, this study not only provided the new dual-target drug design strategy and method, but also discover the potential antifungal compounds.

Repurposing Antipsychotics for Cancer Treatment

Cancer is a leading cause of death worldwide, with approximately 19 million new cases each year. Lately, several novel chemotherapeutic drugs have been introduced, efficiently inhibiting tumor growth and proliferation. However, developing a new drug is a time- and money-consuming process, requiring around 1 billion dollars and nearly ten years, with only a minority of the initially effective anti-cancer drugs experimentally finally being efficient in human clinical trials. Drug repurposing for cancer treatment is an optimal alternative as the safety of these drugs has been previously tested, and thus, in case of successful preclinical studies, can be introduced faster and with a lower cost into phase 3 clinical trials. Antipsychotic drugs are associated with anti-cancer properties and, lately, there has been an increasing interest in their role in cancer treatment. In the present review, we discussed in detail the in-vitro and in-vivo properties of the most common typical and atypical antipsychotics, along with their mechanism of action.

Construction and activity evaluation of novel dual-target (SE/CYP51) anti-fungal agents containing amide naphthyl structure

With the increase of fungal infection and drug resistance, it is becoming an urgent task to discover the highly effective antifungal drugs. In the study, we selected the key ergosterol bio-synthetic enzymes (Squalene epoxidase, SE; 14 α-demethylase, CYP51) as dual-target receptors to guide the construction of novel antifungal compounds, which could achieve the purpose of improving drug efficacy and reducing drug-resistance. Three different series of amide naphthyl compounds were generated through the method of skeleton growth, and their corresponding target products were synthesized. Most of compounds displayed the obvious biological activity against different Candida spp. and Aspergillus fumigatus. Among of them, target compounds 14a-2 and 20b-2 not only possessed the excellent broad-spectrum anti-fungal activity (MIC₅₀, 0.125-2 μg/mL), but also maintained the anti-drug-resistant fungal activity (MIC₅₀, 1-4 μg/mL). Preliminary mechanism study revealed the compounds (14a-2, 20b-2) could block the bio-synthetic pathway of ergosterol by inhibiting the dual-target (SE/CYP51) activity, and this finally caused the cleavage and death of fungal cells. In addition, we also discovered that compounds 14a-2 and 20b-2 with low toxic and side effects could exert the excellent therapeutic effect in mice model of fungal infection, which was worthy for further in-depth study.

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.

Docusate-Based Ionic Liquids of Anthelmintic Benzimidazoles Show Improved Pharmaceutical Processability, Lipid Solubility, and in Vitro Activity against Cryptococcus neoformans

As the existing therapeutic modalities for the treatment of cryptococcal meningitis (CM) have suboptimal efficacy, repurposing existing drugs for the treatment of CM is of great interest. The FDA-approved anthelmintic benzimidazoles, albendazole, mebendazole, and flubendazole, have demonstrated potent but variable in vitro activity against Cryptococcus neoformans, the predominant fungal species responsible for CM. We performed molecular docking studies to ascertain the interaction of albendazole, mebendazole, and flubendazole with a C. neoformans β-tubulin structure, which revealed differential binding interactions and explained the different in vitro efficacies reported previously and observed in this investigation. Despite their promising in vitro efficacy, the repurposing of anthelmintic benzimidazoles for oral CM therapy is significantly hampered due to their high crystallinity, poor pharmaceutical processability, low and pH-dependent solubility, and drug precipitation upon entering the intestine, all of which result in low and variable oral bioavailability. Here, we demonstrate that the anthelmintic benzimidazoles can be transformed into partially amorphous low-melting ionic liquids (ILs) with a simple metathesis reaction using amphiphilic sodium docusate as a counterion. In vitro efficacy studies on a laboratory reference and a clinical isolate of C. neoformans showed 2- to 4-fold lower IC90 values for docusate-based ILs compared to the pure anthelmintic benzimidazoles. Furthermore, using a C. neoformans strain with green fluorescent protein (GFP)-tagged β-tubulin and albendazole and its docusate IL as model candidates, we showed that the benzimidazoles and their ILs reduce the viability of C. neoformans by interfering with its microtubule assembly. Unlike pure anthelmintic benzimidazoles, the docusate-based ILs showed excellent solubility in organic solvents and >30-fold higher solubility in bioavailability-enhancing lipid vehicles. Finally, the docusate ILs were successfully incorporated into SoluPlus, a self-assembling biodegradable polymer, which upon dilution with water formed polymeric micelles with a size of <100 nm. Thus, the development of docusate-based ILs represents an effective approach to improve the physicochemical properties and potency of anthelmintic benzimidazoles to facilitate their repurposing and preclinical development for CM therapy.

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.

Drug Repurposing in Medical Mycology: Identification of Compounds as Potential Antifungals to Overcome the Emergence of Multidrug-Resistant Fungi

Immunodepression, whether due to HIV infection or organ transplantation, has increased human vulnerability to fungal infections. These conditions have created an optimal environment for the emergence of opportunistic infections, which is concomitant to the increase in antifungal resistance. The use of conventional antifungal drugs as azoles and polyenes can lead to clinical failure, particularly in immunocompromised individuals. Difficulties related to treating fungal infections combined with the time required to develop new drugs, require urgent consideration of other therapeutic alternatives. Drug repurposing is one of the most promising and rapid solutions that the scientific and medical community can turn to, with low costs and safety advantages. To treat life-threatening resistant fungal infections, drug repurposing has led to the consideration of well-known and potential molecules as a last-line therapy. The aim of this review is to provide a summary of current antifungal compounds and their main resistance mechanisms, following by an overview of the antifungal activity of non-traditional antimicrobial drugs. We provide their eventual mechanisms of action and the synergistic combinations that improve the activity of current antifungal treatments. Finally, we discuss drug repurposing for the main emerging multidrug resistant (MDR) fungus, including the Candida auris, Aspergillus or Cryptococcus species.

Mechanistic Understanding of Candida albicans Biofilm Formation and Approaches for Its Inhibition

In recent years, the demand for novel antifungal therapies has increased several- folds due to its potential to treat severe biofilm-associated infections. Biofilms are made by the sessile microorganisms attached to the abiotic or biotic surfaces, enclosed in a matrix of exopolymeric substances. This results in new phenotypic characteristics and intrinsic resistance from both host immune response and antimicrobial drugs. Candida albicans biofilm is a complex association of hyphal cells that are associated with both abiotic and animal tissues. It is an invasive fungal infection and acts as an important virulent factor. The challenges linked with biofilm-associated diseases have urged scientists to uncover the factors responsible for the formation and maturation of biofilm. Several strategies have been developed that could be adopted to eradicate biofilm-associated infections. This article presents an overview of the role of C. albicans biofilm in its pathogenicity, challenges it poses and threats associated with its formation. Further, it discusses strategies that are currently available or under development targeting prostaglandins, quorum-sensing, changing surface properties of biomedical devices, natural scaffolds, and small molecule-based chemical approaches to combat the threat of C. albicans biofilm. This review also highlights the recent developments in finding ways to increase the penetration of drugs into the extracellular matrix of biofilm using different nanomaterials against C. albicans.

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.

Lead optimization generates selenium-containing miconazole CYP51 inhibitors with improved pharmacological profile for the treatment of fungal infections

A series of selenium-containing miconazole derivatives were identified as potent antifungal drugs in our previous study. Representative compound A03 (MIC = 0.01 μg/mL against C.alb. 5314) proved efficacious in inhibiting the growth of fungal pathogens. However, further study showed lead compound A03 exhibited potential hemolysis, significant cytotoxic effect and unfavorable metabolic stability and was therefore modified to overcome these drawbacks. In this article, the further optimization of selenium-containing miconazole derivatives resulted in the discovery of similarly potent compound B17 (MIC = 0.02 μg/mL against C.alb. 5314), exhibiting a superior pharmacological profile with decreased rate of metabolism, cytotoxic effect and hemolysis. Furthermore, compound B17 showed fungicidal activity against Candida albicans and significant effects on the treatment of resistant Candida albicans infections. Meanwhile, compound B17 not only could reduce the ergosterol biosynthesis pathway by inhibiting CYP51, but also inhibited biofilm formation. More importantly, compound B17 also shows promising in vivo efficacy after intraperitoneal injection and the PK study of compound B17 was evaluated. In addition, molecular docking studies provide a model for the interaction between the compound B17 and the CYP51 protein. Overall, we believe that these selenium-containing miconazole compounds can be further developed for the potential treatment of fungal infections.

Non-antifungal drugs inhibit growth, morphogenesis and biofilm formation in Candida albicans

The increased resistance/tolerance of Candida infections to antimicrobial treatment can be attributed to biofilm-associated cells. A way to overcome this situation is to re-purpose non-anti-fungal drugs that could be active against fungi. We have explored the potential of a small library of eighteen non-antifungal drugs used in different human diseases. Candida albicans was cultured in the presence and absence of different concentrations of these drugs. Subsequently, inhibition of growth, germ tube formation, adhesion, and biofilm development were studied. Out of eighteen drug molecules, six showed a reduction in planktonic and biofilm growth in a dose-dependent manner and three drugs inhibited germ tube formation. This study shows the potential of non-antifungal drugs for the development of new anti-Candida agents.

Broad-Spectrum Antifungal Agents: Fluorinated Aryl- and Heteroaryl-Substituted Hydrazones

Fluorinated aryl- and heteroaryl-substituted monohydrazones displayed excellent broad-spectrum activity against various fungal strains, including a panel of clinically relevant Candida auris strains relative to a control antifungal agent, voriconazole (VRC). These monohydrazones displayed less hemolysis of murine red blood cells than that of VRC at the same concentrations, possessed fungicidal activity in a time-kill study, and exhibited no mammalian cell cytotoxicity. In addition, these monohydrazones prevented the formation of biofilms that otherwise block antibiotic effectiveness and did not trigger the development of resistance when exposed to C. auris AR Bank # 0390 over 15 passages.

[Screening of potential non-azole inhibitors of lanosterol14-alpha demethylase (CYP51) of Candida fungi]

Currently, opportunistic fungi of the genus Candida are the main causative agents of mycoses, which are especially severe upon condition of acquired immunodeficiency. The main target for the development of new antimycotics is the cytochrome P450 51 (CYP51) of the pathogenic fungus. Due to the widespread distribution of Candida strains resistancy to inhibitors of the azole class, the screening for CYP51 inhibitors both among non-azole compounds and among clinically used drugs repurposing as antimycotics is becoming urgent. To identify potential inhibitors from the non-azole group, an integrated approach was applied, including bioinformatics analysis, computer molecular modeling, and a surface plasmon resonance (SPR) technology. Using in silico modeling, the binding sites for acetylsalicylic acid, ibuprofen, chlorpromazine and haloperidol (this compounds, according to the literature, showed antimycotic activity) were predicted in the active site of CYP51 of Candida albicans and Candida glabrata. The Kd values of molecular complexes of acetylsalicylic acid, ibuprofen and haloperidol with CYP51, determined by SPR analysis, ranged from 18 μM to 126 μM. It was also shown that structural derivatives of haloperidol, containing various substituents, could be positioned in the active site of CYP51 of Candida albicans with the possible formation of coordination bonds between the hydroxyl groups of the derivatives and the iron atom in the heme of CYP51. Thus, the potential basic structures of non-azole compounds have been proposed, which can be used for the design of new CYP51 inhibitors of Candida fungi.

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.

Antifungal Drug Repurposing

Control of fungal pathogens is increasingly problematic due to the limited number of effective drugs available for antifungal therapy. Conventional antifungal drugs could also trigger human cytotoxicity associated with the kidneys and liver, including the generation of reactive oxygen species. Moreover, increased incidences of fungal resistance to the classes of azoles, such as fluconazole, itraconazole, voriconazole, or posaconazole, or echinocandins, including caspofungin, anidulafungin, or micafungin, have been documented. Of note, certain azole fungicides such as propiconazole or tebuconazole that are applied to agricultural fields have the same mechanism of antifungal action as clinical azole drugs. Such long-term application of azole fungicides to crop fields provides environmental selection pressure for the emergence of pan-azole-resistant fungal strains such as Aspergillus fumigatus having TR34/L98H mutations, specifically, a 34 bp insertion into the cytochrome P450 51A (CYP51A) gene promoter region and a leucine-to-histidine substitution at codon 98 of CYP51A. Altogether, the emerging resistance of pathogens to currently available antifungal drugs and insufficiency in the discovery of new therapeutics engender the urgent need for the development of new antifungals and/or alternative therapies for effective control of fungal pathogens. We discuss the current needs for the discovery of new clinical antifungal drugs and the recent drug repurposing endeavors as alternative methods for fungal pathogen control.