Repurposing and optimization of drugs for discovery of novel antifungals

Tackling multi-drug resistant fungi by efflux pump inhibitors

The emergence of multidrug-resistant fungi is of grave concern, and its infections are responsible for significant deaths among immunocompromised patients. The treatment of fungal infections primarily relies on a clinical class of antibiotics, including azoles, polyenes, echinocandins, polyketides, and a nucleotide analogue. However, the incidence of fungal infections is increasing as the treatment for human and plant fungal infections overlaps with antifungal drugs. The need for new antifungal agents acting on different targets than known targets is undeniable. Also, the pace at which loss of fungal susceptibility to antibiotics cannot be undermined. There are several modes by which fungi can develop resistance to antibiotics, including reduced drug uptake, drug target alteration, and a reduction in the cellular concentration of the drug due to active extrusions and biofilm formation. The efflux pump's overexpression in the fungi primarily reduced the antibiotic's concentration to a sub-lethal concentration, thus responsible for developing resistant fungus strains. Several strategies are used to check antibiotic resistance in multi-drug resistant fungi, including synthesizing antibiotic analogs and giving antibiotics in combination therapies. Among them, the efflux pump protein inhibitors are considered potential adjuvants to antibiotics and can block the efflux of antibiotics by inhibiting efflux pump protein transporters. Moreover, it can sensitize the antifungal drugs to multi-drug resistant fungi with overexpressed efflux pump proteins. This review discusses the natural lead molecules, repurposable drugs, and formulation strategies to overcome the efflux pump activity in the fungi.

Strategies of Pharmacological Repositioning for the Treatment of Medically Relevant Mycoses

Fungal infections represent a worldwide concern for public health, due to their prevalence and significant increase in cases each year. Among the most frequent mycoses are those caused by members of the genera Candida, Cryptococcus, Aspergillus, Histoplasma, Pneumocystis, Mucor, and Sporothrix, which have been treated for years with conventional antifungal drugs, such as flucytosine, azoles, polyenes, and echinocandins. However, these microorganisms have acquired the ability to evade the mechanisms of action of these drugs, thus hindering their treatment. Among the most common evasion mechanisms are alterations in sterol biosynthesis, modifications of drug transport through the cell wall and membrane, alterations of drug targets, phenotypic plasticity, horizontal gene transfer, and chromosomal aneuploidies. Taking into account these problems, some research groups have sought new therapeutic alternatives based on drug repositioning. Through repositioning, it is possible to use existing pharmacological compounds for which their mechanism of action is already established for other diseases, and thus exploit their potential antifungal activity. The advantage offered by these drugs is that they may be less prone to resistance. In this article, a comprehensive review was carried out to highlight the most relevant repositioning drugs to treat fungal infections. These include antibiotics, antivirals, anthelmintics, statins, and anti-inflammatory drugs.

Uniqueness of Candida auris cell wall in morphogenesis, virulence, resistance, and immune evasion

Candida auris has drawn global attention due to its alarming multidrug resistance and the emergence of pan resistant strains. C. auris poses a significant risk in nosocomial candidemia especially among immunocompromised patients. C. auris showed unique virulence characteristics associated with cell wall including cell polymorphism, adaptation, endurance on inanimate surfaces, tolerance to external conditions, and immune evasion. Notably, it possesses a distinctive cell wall composition, with an outer mannan layer shielding the inner 1,3-β glucan from immune recognition, thereby enabling immune evasion and drug resistance. This review aimed to comprehend the association between unique characteristics of C. auris's cell wall and virulence, resistance mechanisms, and immune evasion. This is particularly relevant since the fungal cell wall has no human homology, providing a potential therapeutic target. Understanding the complex interactions between the cell wall and the host immune system is essential for devising effective treatment strategies, such as the use of repurposed medications, novel therapeutic agents, and immunotherapy like monoclonal antibodies. This therapeutic targeting strategy of C. auris holds promise for effective eradication of this resilient pathogen.

Hydroxychloroquine an Antimalarial Drug, Exhibits Potent Antifungal Efficacy Against Candida albicans Through Multitargeting

Candida albicans is the primary etiological agent associated with candidiasis in humans. Unrestricted growth of C. albicans can progress to systemic infections in the worst situation. This study investigates the antifungal activity of Hydroxychloroquine (HCQ) and mode of action against C. albicans. HCQ inhibited the planktonic growth and yeast to hyphal form morphogenesis of C. albicans significantly at 0.5 mg/ml concentration. The minimum inhibitory concentrations (MIC50) of HCQ for C. albicans adhesion and biofilm formation on the polystyrene surface was at 2 mg/ml and 4 mg/ml respectively. Various methods, such as scanning electron microscopy, exploration of the ergosterol biosynthesis pathway, cell cycle analysis, and assessment of S oxygen species (ROS) generation, were employed to investigate HCQ exerting its antifungal effects. HCQ was observed to reduce ergosterol levels in the cell membranes of C. albicans in a dose-dependent manner. Furthermore, HCQ treatment caused a substantial arrest of the C. albicans cell cycle at the G0/G1 phase, which impeded normal cell growth. Gene expression analysis revealed upregulation of SOD2, SOD1, and CAT1 genes after HCQ treatment, while genes like HWP1, RAS1, TEC1, and CDC 35 were downregulated. The study also assessed the in vivo efficacy of HCQ in a mice model, revealing a reduction in the pathogenicity of C. albicans after HCQ treatment. These results indicate that HCQ holds for the development of novel antifungal therapies.

Asymmetric Synthesis of Triazole Antifungal Agents Enabled by an Upgraded Strategy for the Key Epoxide Intermediate

A streamlined and efficient approach to the key epoxide intermediate for the asymmetric synthesis of triazole antifungal agents is presented. This synthesis highlights a P(NMe2)3-mediated nonylidic olefination of α-keto ester, ensuring the exclusive formation of the requisite (Z)-alkene, followed by a highly enantioselective Jacobsen epoxidation to establish the two vicinal stereocenters in a single step. The versatility of this strategy is exemplified through the efficient synthesis of efinaconazole and ravuconazole.

Design, Synthesis, and Antifungal/Anti-Oomycete Activities of Novel 1,2,4-Triazole Derivatives Containing Carboxamide Fragments

Plant diseases caused by pathogenic fungi or oomycetes seriously affect crop growth and the quality and yield of products. A series of novel 1,2,4-triazole derivatives containing carboxamide fragments based on amide fragments widely used in fungicides and the commercialized mefentrifluconazole were designed and synthesized. Their antifungal activities were evaluated against seven kinds of phytopathogenic fungi/oomycete. Results showed that most compounds had similar or better antifungal activities compared to mefentrifluconazole's inhibitory activity against Physalospora piricola, especially compound 6h (92%), which possessed outstanding activity. Compound 6h (EC50 = 13.095 μg/mL) showed a better effect than that of mefentrifluconazole (EC50 = 39.516 μg/mL). Compound 5j (90%) displayed outstanding anti-oomycete activity against Phytophthora capsici, with an EC50 value of 17.362 μg/mL, far superior to that of mefentrifluconazole (EC50 = 75.433 μg/mL). The result of molecular docking showed that compounds 5j and 6h possessed a stronger affinity for 14α-demethylase (CYP51). This study provides a new approach to expanding the fungicidal spectrum of 1,2,4-triazole derivatives.

Structure-activity relationships of bensulfuron methyl and its derivatives as novel agents against drug-resistant Candida auris

With the emergence of the human pathogen Candida auris as a threat to human health, there is a strong demand to identify effective medicines to prevent the harm caused by such drug-tolerant human fungi. Herein, a series of 33 new derivatives of bensulfuron methyl (BSM) were synthesized and characterized by 1 H NMR, 13 C NMR, and HRMS. Among the target compounds, 8a possessed the best Ki value of 1.015 μM against C. auris acetohydroxyacid synthase (CauAHAS) and an MIC value of 6.25 μM against CBS10913, a clinically isolated strain of C. auris. Taken together the structures of BSM and the synthesized compounds, it was found that methoxy groups at both meta-position of pyrimidine ring are likely to provide desirable antifungal activities. Quantum calculations and molecular dockings were performed to understand the structure-activity relationships. The present study has hence provided some interesting clues for the discovery of novel antibiotics with this distinct mode of action.

Antifungal therapy of Candida biofilms: Past, present and future

Virtually all Candida species linked to clinical candidiasis are capable of forming highly resistant biofilms on different types of surfaces, which poses an additional significant threat and further complicates therapy of these infections. There is a scarcity of antifungal agents, and their effectiveness, particularly against biofilms, is limited. Here we provide a historical perspective on antifungal agents and therapy of Candida biofilms. As we reflect upon the past, consider the present, and look towards the future of antifungal therapy of Candida biofilms, we believe that there are reasons to remain optimistic, and that the major challenges of Candida biofilm therapy can be conquered within a reasonable timeframe.

The role of methionine synthases in fungal metabolism and virulence

Methionine synthases (MetH) catalyse the methylation of homocysteine (Hcy) with 5-methyl-tetrahydrofolate (5, methyl-THF) acting as methyl donor, to form methionine (Met) and tetrahydrofolate (THF). This function is performed by two unrelated classes of enzymes that differ significantly in both their structures and mechanisms of action. The genomes of plants and many fungi exclusively encode cobalamin-independent enzymes (EC.2.1.1.14), while some fungi also possess proteins from the cobalamin-dependent (EC.2.1.1.13) family utilised by humans. Methionine synthase's function connects the methionine and folate cycles, making it a crucial node in primary metabolism, with impacts on important cellular processes such as anabolism, growth and synthesis of proteins, polyamines, nucleotides and lipids. As a result, MetHs are vital for the viability or virulence of numerous prominent human and plant pathogenic fungi and have been proposed as promising broad-spectrum antifungal drug targets. This review provides a summary of the relevance of methionine synthases to fungal metabolism, their potential as antifungal drug targets and insights into the structures of both classes of MetH.

Searching for new antifungals for the treatment of cryptococcosis

There is a consensus that the antifungal repertoire for the treatment of cryptococcal infections is limited. Standard treatment involves the administration of an antifungal drug derived from natural sources (i.e., amphotericin B) and two other drugs developed synthetically (i.e., flucytosine and fluconazole). Despite treatment, the mortality rates associated with fungal cryptococcosis are high. Amphotericin B and flucytosine are toxic, require intravenous administration, and are usually unavailable in low-income countries because of their high cost. However, fluconazole is cost-effective, widely available, and harmless with regard to its side effects. However, fluconazole is a fungistatic agent that has contributed considerably to the increase in fungal resistance and frequent relapses in patients with cryptococcal meningitis. Therefore, there is an unquestionable need to identify new alternatives or adjuvants to conventional drugs for the treatment of cryptococcosis. A potential antifungal agent should be able to kill cryptococci and "bypass" the virulence mechanism of the yeast. Furthermore, it should have fungicidal action, low toxicity, high selectivity, easily penetrate the central nervous system, and widely available. In this review, we describe cryptococcosis, its conventional therapy, and failures arising from the use of drugs traditionally considered to be the reference standard. Additionally, we present the approaches used for the discovery of new drugs to counteract cryptococcosis, ranging from the conventional screening of natural products to the inclusion of structural modifications to optimize anticryptococcal activity, as well as drug repositioning and combined therapies.

Design, Synthesis, and Evaluation of Antifungal Bioactivity of Novel Pyrazole Carboxamide Thiazole Derivatives as SDH Inhibitors

Agricultural production is seriously threatened by plant pathogens. The development of new fungicides with high efficacy and low toxicity is urgently needed. In this study, a series of pyrazole carboxamide thiazole derivatives were designed, synthesized, and evaluated for their antifungal activities against nine plant pathogens in vitro. Bioassay results showed that most compounds (3i, 5i, 6i, 7i, 9i, 12i, 16i, 19i, and 23i) exhibited good antifungal activities against Valsa mali. In particular, compounds 6i and 19i exhibited better antifungal activities against Valsa mali with EC₅₀ values of 1.77 and 1.97 mg/L, respectively, than the control drug boscalid (EC₅₀ = 9.19 mg/L). Additionally, compound 23i exhibited excellent inhibitory activity against Rhizoctonia solani, with an EC₅₀ value of 3.79 mg/L. Compound 6i at 40 mg/L showed a satisfactory in vivo protective effect against Valsa mali. Scanning electron microscopy analyses revealed that compound 6i could significantly damage the surface morphology to interfere with the growth of Valsa mali. In molecular docking, the results showed that compound 6i interacts with TRP O: 173, SER P: 39, TYR Q: 58, and ARG P: 43 of succinate dehydrogenase (SDH) through hydrogen bonding and σ-π interaction, and its binding mode is similar to that of boscalid and SDH. The enzyme activity experiment also further verified its action mode. Our studies suggested that pyrazole carboxamide thiazole derivative 6i provided a valuable reference for the further development of succinate dehydrogenase inhibitors.

New antifungal strategies: drug combination and co-delivery

The growing occurrence of invasive fungal infections and the mounting rates of drug resistance constitute a significant menace to human health. Antifungal drug combinations have garnered substantial interest for their potential to improve therapeutic efficacy, reduce drug doses, reverse, or ameliorate drug resistance. A thorough understanding of the molecular mechanisms underlying antifungal drug resistance and drug combination is key to developing new drug combinations. Here we discuss the mechanisms of antifungal drug resistance and elucidate how to discover potent drug combinations to surmount resistance. We also examine the challenges encountered in developing such combinations and discuss prospects, including advanced drug delivery strategies.

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.

In silico approach towards polyphenols as targeting glucosamine-6-phosphate synthase for Candida albicans

Candida albicans is one of the most common species of fungus with life-threatening systemic infections and a high mortality rate. The outer cell wall layer of C. albicans is packed with mannoproteins and glycosylated polysaccharide moieties that play an essential role in the interaction with host cells and tissues. The glucosamine-6-phosphate synthase enzyme produces N-acetylglucosamine, which is a crucial chemical component of the cell wall of Candida albicans. Collectively, these components are essential to maintain the cell shape and for infection. So, its disruption can have serious effects on cell growth and morphology, resulting in cell death. Hence, it is considered a good antifungal target. In this study, we have performed an in silico approach to analyze the inhibitory potential of some polyphenols obtained from plants. Those can be considered important in targeting against the enzyme glucosamine-6-phosphate synthase (PDB-2VF5). The results of the study revealed that the binding affinity of complexes theaflavin and 3-o-malonylglucoside have significant docking scores and binding free energy followed by significant ADMET parameters that predict the drug-likeness property and toxicity of polyphenols as potential ligands. A molecular dynamic simulation was used to test the validity of the docking scores, and it showed that the complex remained stable during the period of the simulation, which ranged from 0 to 100 ns. Theaflavins and 3-o-malonylglucoside may be effective against Candida albicans using a computer-aided drug design methodology that will further enable researchers for future in vitro and in vivo studies, according to our in silico study.Communicated by Ramaswamy H. Sarma.