SAR Studies on Aromatic Acylhydrazone-Based Inhibitors of Fungal Sphingolipid Synthesis as Next-Generation Antifungal Agents

Structural Simplification of Luotonin F: Discovery of Quinoline Derivatives as Novel Antifungal Agents for Plant Protection

Inspired by the natural quinoline alkaloid Luotonin F, a series of quinoline-3-hydrazide derivatives were designed and synthesized by structure simplification and evaluated for their fungicidal activities against plant phytopathogenic fungi. The bioassay results showed that these compounds exhibited broad-spectrum and highly efficient antifungal activities in vitro. Among them, compound W9 exhibited broad-spectrum inhibitory activities against R. solani, S. sclerotiorum, B. cinerea, F. graminearum, F. oxysporum, and P. capsici with EC50 values of 0.471, 0.752, 0.570, 0.329, 0.960, and 0.504 μg/mL, respectively. In particular, compound W9 was more effective against B. cinerea than the reference drugs Pyrimethanil (EC50 = 3.54 μg/mL) and Boscalid (EC50 = 1.37 μg/mL). Therefore, the preliminary mechanism of action of compound W9 on B. cinerea was explored. The results showed that it could affect the mycelial morphology, disrupt the integrity of the cell membrane, and increase the reactive oxygen species content. Furthermore, compound W9 exhibited excellent in vivo protection efficacy against B. cinerea compared to the reference drug Pyrimethanil. Thus, compound W9 is expected to be a novel broad-spectrum fungicidal candidate, exhibiting excellent fungicidal activities, which is worthy of further investigation.

Evaluation of antifungal spectrum of Cupferron against Candida albicans

Candida albicans is an opportunistic yeast accounting for about 50-90 % of all cases of candidiasis in humans, ranging from superficial to systemic potentially life-threatening infections. The presence of several virulence factors, including biofilm, hyphal transition, and proteolytic enzymes production, worsens the fungal infections burden on healthcare system resources. Hence, developing new bioactive compounds with antifungal activity is a pressing urgence for the scientific community. In this perspective, we evaluated the anti-Candida potential of the N-Nitroso-N-phenylhydroxylamine ammonium salt (cupferron) against standard and clinical C. albicans strains. Firstly, the in vitro cytotoxicity of cupferron was checked in the range 400-12.5 μg/mL against human microglial cells (HMC-3). Secondly, its antifungal spectrum was explored via disk diffusion test, broth-microdilution method, and time-killing curve analysis, validating the obtained results through scanning electron microscopy (SEM) observations. Additionally, we evaluated the cupferron impact on the main virulence determinants of Candida albicans. At non-toxic concentrations (100-12.5 μg/mL), the compound exerted interesting anti-Candida activity, registering a minimum inhibitory concentration (MIC) between 50 and 100 μg/mL against the tested strains, with a fungistatic effect until 100 μg/mL. Furthermore, cupferron was able to counteract fungal virulence at MIC and sub-MIC values (50-12.5 μg/mL). These findings may propose cupferron as a new potential antifungal option for the treatment of Candida albicans infections.

An allosteric inhibitor of the PhoQ histidine kinase with therapeutic potential against Salmonella infection

BACKGROUND

The upsurge of antimicrobial resistance demands innovative strategies to fight bacterial infections. With traditional antibiotics becoming less effective, anti-virulence agents or pathoblockers, arise as an alternative approach that seeks to disarm pathogens without affecting their viability, thereby reducing selective pressure for the emergence of resistance mechanisms.

OBJECTIVES

To elucidate the mechanism of action of compound N'-(thiophen-2-ylmethylene)benzohydrazide (A16B1), a potent synthetic hydrazone inhibitor against the Salmonella PhoP/PhoQ system, essential for virulence.

MATERIALS AND METHODS

The measurement of the activity of PhoP/PhoQ-dependent and -independent reporter genes was used to evaluate the specificity of A16B1 to the PhoP regulon. Autokinase activity assays with either the native or truncated versions of PhoQ were used to dissect the A16B1 mechanism of action. The effect of A16B1 on Salmonella intramacrophage replication was assessed using the gentamicin protection assay. The checkerboard assay approach was used to analyse potentiation effects of colistin with the hydrazone. The Galleria mellonella infection model was chosen to evaluate A16B1 as an in vivo therapy against Salmonella.

RESULTS

A16B1 repressed the Salmonella PhoP/PhoQ system activity, specifically targeting PhoQ within the second transmembrane region. A16B1 demonstrates synergy with the antimicrobial peptide colistin, reduces the intramacrophage proliferation of Salmonella without being cytotoxic and enhances the survival of G. mellonella larvae systemically infected with Salmonella.

CONCLUSIONS

A16B1 selectively inhibits the activity of the Salmonella PhoP/PhoQ system through a novel inhibitory mechanism, representing a promising synthetic hydrazone compound with the potential to function as a Salmonella pathoblocker. This offers innovative prospects for combating Salmonella infections while mitigating the risk of antimicrobial resistance emergence.

Combination therapy of itraconazole and an acylhydrazone derivative (D13) for the treatment of sporotrichosis in cats

Acylhydrazone (AH) derivatives represent a novel category of anti-fungal medications that exhibit potent activity against Sporothrix sp., both in vitro and in a murine model of sporotrichosis. In this study, we demonstrated the anti-fungal efficacy of the AH derivative D13 [4-bromo-N'-(3,5-dibromo-2-hydroxybenzylidene)-benzohydrazide] against both planktonic cells and biofilms formed by Sporothrix brasiliensis. In a clinical study, the effect of D13 was then tested in combination with itraconazole (ITC), with or without potassium iodide, in 10 cats with sporotrichosis refractory to the treatment of standard of care with ITC. Improvement or total clinical cure was achieved in five cases after 12 weeks of treatment. Minimal abnormal laboratory findings, e.g., elevation of alanine aminotransferase, were observed in four cats during the combination treatment and returned to normal level within a week after the treatment was ended. Although highly encouraging, a larger and randomized controlled study is required to evaluate the effectiveness and the safety of this new and exciting drug combination using ITC and D13 for the treatment of feline sporotrichosis.

IMPORTANCE

This paper reports the first veterinary clinical study of an acylhydrazone anti-fungal (D13) combined with itraconazole against a dimorphic fungal infection, sporotrichosis, which is highly endemic in South America in animals and humans. Overall, the results show that the combination treatment was efficacious in ~50% of the infected animals. In addition, D13 was well tolerated during the course of the study. Thus, these results warrant the continuation of the research and development of this new class of anti-fungals.

From Synergy to Monotherapy: Discovery of Novel 2,4,6-Trisubstituted Triazine Hydrazone Derivatives with Potent Antifungal Potency In Vitro and In Vivo

Invasive fungal infections pose a serious threat to public health and are associated with high mortality and incidence rates. The development of novel antifungal agents is urgently needed. Based on hit-to-lead optimization, a series of 2,4,6-trisubstituted triazine hydrazone compounds were designed, synthesized, and biological evaluation was performed, leading to the identification of compound 28 with excellent in vitro synergy (FICI range: 0.094-0.38) and improved monotherapy potency against fluconazole-resistant Candida albicans and Candida auris (MIC range: 1.0-16.0 μg/mL). Moreover, 28 exhibited broad-spectrum antifungal activity against multiple pathogenic strains. Furthermore, 28 could inhibit hyphal and biofilm formation, which may be related to its ability to disrupt the fungal cell wall. Additionally, 28 significantly reduced the CFU in a mouse model of disseminated infection with candidiasis at a dose of 10 mg/kg. Overall, the triazine-based hydrazone compound 28 with low cytotoxicity, hemolysis, and favorable ADME/T characteristics represents a promising lead to further investigation.

Advancements and challenges in antifungal therapeutic development

Over recent decades, the global burden of fungal disease has expanded dramatically. It is estimated that fungal disease kills approximately 1.5 million individuals annually; however, the true worldwide burden of fungal infection is thought to be higher due to existing gaps in diagnostics and clinical understanding of mycotic disease. The development of resistance to antifungals across diverse pathogenic fungal genera is an increasingly common and devastating phenomenon due to the dearth of available antifungal classes. These factors necessitate a coordinated response by researchers, clinicians, public health agencies, and the pharmaceutical industry to develop new antifungal strategies, as the burden of fungal disease continues to grow. This review provides a comprehensive overview of the new antifungal therapeutics currently in clinical trials, highlighting their spectra of activity and progress toward clinical implementation. We also profile up-and-coming intracellular proteins and pathways primed for the development of novel antifungals targeting their activity. Ultimately, we aim to emphasize the importance of increased investment into antifungal therapeutics in the current continually evolving landscape of infectious disease.

SAR study of N'-(Salicylidene)heteroarenecarbohydrazides as promising antifungal agents

Clinically available antifungal drugs have therapeutic limitations due to toxicity, narrow spectrum of activity, and intrinsic or acquired drug resistance. Thus, there is an urgent need for new broad-spectrum antifungal agents with low toxicity and a novel mechanism of action. In this context, we have successfully identified several highly promising lead compounds, i.e., aromatic N'-(salicylidene)carbohydrazides, exhibiting excellent antifungal activities against Cryptococcus neoformans, Candida albicans, Aspergillus fumigatus and several other fungi both in vitro and in vivo. Building upon these highly promising results, 71 novel N'-(salicylidene)heteroarenecarbohydrazides 5 were designed, synthesized and their antifungal activities examined against fungi. Based on the SAR study, four highly promising lead compounds, i.e., 5.6a, 5.6b, 5.7b and 5.13a were identified, which exhibited excellent potency against C. neoformans, C. albicans and A. fumigatus, and displayed impressive time-kill profiles against C. neoformans with exceptionally high selectivity indices (SI ≥ 500). These four lead compounds also showed synergy with clinical antifungal drugs, fluconazole, caspofungin (CS) and amphotericin B against C. neoformans. For the SAR study, we also employed quantitative structure-activity relationship (QSAR) analysis by taking advantage of the accumulated data on a large number of aromatic and heteroaromatic N'-(salicylidene)carbohydrazides, which successfully led to rational design and selection of promising compounds for chemical synthesis and biological evaluation.

Update on fungal lipid biosynthesis inhibitors as antifungal agents

Fungal diseases today represent a world-wide problem. Poor hygiene and decreased immunity are the main reasons behind the manifestation of this disease. After COVID-19, an increase in the rate of fungal infection has been observed in different countries. Different classes of antifungal agents, such as polyenes, azoles, echinocandins, and anti-metabolites, as well as their combinations, are currently employed to treat fungal diseases; these drugs are effective but can cause some side effects and toxicities. Therefore, the identification and development of newer antifungal agents is a current need. The fungal cell comprises many lipids, such as ergosterol, phospholipids, and sphingolipids. Ergosterol is a sterol lipid that is only found in fungal cells. Various pathways synthesize all these lipids, and the activities of multiple enzymes govern these pathways. Inhibiting these enzymes will ultimately impede the lipid synthesis pathway, and this phenomenon could be a potential antifungal therapy. This review will discuss various lipid synthesis pathways and multiple antifungal agents identified as having fungal lipid synthesis inhibition activity. This review will identify novel compounds that can inhibit fungal lipid synthesis, permitting researchers to direct further deep pharmacological investigation and help develop drug delivery systems for such compounds.

Discovery of novel salicylaldehyde derivatives incorporating an α-methylene-γ-butyrolactone moiety as fungicidal agents

BACKGROUND

Plant diseases caused by phytopathogenic fungi and oomycetes pose a serious threat to ensuring crop yield and quality. Finding novel fungicidal candidates based on natural products is one of the critical methods for developing effective and environmentally friendly pesticides. In this study, a series of salicylaldehyde derivatives containing an α-methylene-γ-butyrolactone moiety were designed, synthesized, and their fungicidal activities were evaluated.

RESULTS

The bioassay studies indicated that compound C3 displayed an excellent in vitro activity against Rhizoctonia solani with a half-maximal effective concentration (EC50 ) value of 0.65 μg/mL, higher than that of pyraclostrobin (EC50  = 1.44 μg/mL) and comparable to that of carbendazim (EC50  = 0.33 μg/mL). For Valsa mali and Phytophthora capsici, compound C3 also showed good fungicidal activities with EC50 values of 0.91 and 1.33 μg/mL, respectively. In addition, compound C3 exhibited promising protective in vivo activity against R. solani (84.1%) at 100 μg/mL, which was better than that of pyraclostrobin (78.4%). The pot experiment displayed that compound C3 had 74.8% protective efficacy against R. solani at 200 μg/mL, which was comparable to that of validamycin (78.2%). The antifungal mode of action research indicated that compound C3 could change the mycelial morphology and ultrastructure, increase cell membrane permeability, affect respiratory metabolism by binding to complex III, and inhibit the germination and formation of sclerotia, thereby effectively controlling the disease.

CONCLUSION

The present study provides support for the application of these salicylaldehyde derivatives as promising potential pesticides with remarkable and broad-spectrum fungicidal activities against phytopathogenic fungi and oomycetes in crop protection. © 2023 Society of Chemical Industry.

Design, synthesis and antifungal activity of novel 1,4-benzoxazin-3-one derivatives containing an acylhydrazone moiety

A series of 1,4-benzoxazin-3-one derivatives containing an acylhydrazone moiety were designed, synthesized and evaluated for their in vitro antifungal activities against Gibberella zeae, Pellicularia sasakii, Phytophthora infestans, Capsicum wilt, and Phytophthora capsica. The structures of target compounds were characterized by 1H NMR, 13H NMR, 19F NMR and HRMS. The preliminary antifungal evaluation of all target compounds showed that some target compounds possessed moderate to good activities against G. zeae, P. sasakii, P. infestans and C. wilt. Among them, compounds 5L and 5o exhibited noticeable inhibition effects against G. zeae with the EC50 values (effective concentration for 50% activity) of 20.06 and 23.17 μg/ml, respectively, which were even nearly double effective than that of hymexazol (40.51 μg/ml). Meanwhile, compound 5q displayed a notable inhibitory effect toward P. sasakii, with the EC50 value of 26.66 μg/ml, which was better than that of hymexazol (32.77 μg/ml). In addition, compound 5r yielded the EC50 value of 15.37 μg/ml against P. infestans, which was less than those of hymexazol (18.35 μg/ml) and carbendazim (34.41 μg/ml). Eventually, compound 5p showed higher inhibitory effect against C. wilt, with EC50 value of 26.76 μg/ml, which was better than that of hymexazol (>50 μg/ml).

Design, Synthesis, and Bioactivity Study of Novel Tryptophan Derivatives Containing Azepine and Acylhydrazone Moieties

Based on the scaffolds widely used in drug design, a series of novel tryptophan derivatives containing azepine and acylhydrazone moieties have been designed, synthesized, characterized, and evaluated for their biological activities. The bioassay results showed that the target compounds possessed moderate to good antiviral activities against the tobacco mosaic virus (TMV), among which compounds 5c, 6a, 6h, 6t, 6v, and 6y exhibited higher inactivation, curative, and protection activities in vivo than that of ribavirin (40 ± 1, 37 ± 1, 39 ± 2% at 500 mg/L). Especially, 6y showed comparable activities to that of ningnanmycin (57 ± 2, 55 ± 3, 58 ± 1% at 500 mg/L). Meanwhile, we were pleased to find that almost all these derivatives showed good larvicidal activities against Plutella xylostella. Meanwhile, these derivatives also showed a broad spectrum of fungicidal activities.

Development and research progress of anti-drug resistant fungal drugs

With the widespread use of immunosuppressive agents and the increase in patients with severe infections, the incidence of fungal infections worldwide has increased year by year. The fungal pathogens Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus cause a total of more than 1 million deaths each year. Long-term use of antifungal drugs can easily lead to fungal resistance, and the prevalence of drug-resistant fungi is a major global health challenge. In order to effectively control global fungal infections, there is an urgent need for new drugs that can exert effective antifungal activity and overcome drug resistance. We must promote the discovery of new antifungal targets and drugs, and find effective ways to control drug-resistant fungi through different ways, so as to reduce the threat of drug-resistant fungi to human life, health and safety. In the past few years, certain progress has been made in the research and development of antifungal drugs. In addition to summarizing some of the antifungal drugs currently approved by the FDA, this review also focuses on potential antifungal drugs, the repositioned drugs, and drugs that can treat drug-resistant bacteria and fungal infections, and provide new ideas for the development of antifungal drugs in the future.

Optimization and Evaluation of Novel Antifungal Agents for the Treatment of Fungal Infection

Due to the increased morbidity and mortality by fungal infections and the emergence of severe antifungal resistance, there is an urgent need for new antifungal agents. Here, we screened for antifungal activity in our in-house library through the minimum inhibitory concentration test and derived two hit compounds with moderate antifungal activities. The hit compounds' antifungal activities and drug-like properties were optimized by substituting various aryl ring, alkyl chain, and methyl groups. Among the optimized compounds, 22h was the most promising candidate with good drug-like properties and exhibited potent fast-acting fungicidal antifungal effects against various fungal pathogens and synergistic antifungal activities with some known antifungal drugs. Additionally, 22h was further confirmed to disturb fungal cell wall integrity by activating multiple cell wall integrity pathways. Furthermore, 22h exerted significant antifungal efficacy in both the subcutaneous infection mouse model and ex vivo human nail infection model.

Antifungal activity of Acylhydrazone derivatives against Sporothrix spp

Sporotrichosis is an emerging mycosis caused by members of the genus Sporothrix The disease affects humans and animals, particularly cats, which plays an important role in the zoonotic transmission. Feline sporotrichosis treatment options include itraconazole (ITC), potassium iodide and amphotericin B, drugs usually associated with deleterious adverse reactions and refractoriness in cats, especially when using ITC. Thus, affordable, non-toxic and clinically effective anti-Sporothrix agents are needed. Recently, acylhydrazones (AH), molecules targeting vesicular transport and cell cycle progression, exhibited a potent antifungal activity against several fungal species and displayed low toxicity when compared to the current drugs. In this work, the AH derivatives D13 and SB-AF-1002 were tested against Sporothrix schenckii and Sporothrix brasiliensis Minimal inhibitory concentrations of 0.12 - 1 μg/mL were observed for both species in vitro D13 and SB-AF-1002 showed an additive effect with itraconazole. Treatment with D13 promoted yeast disruption with release of intracellular components, as confirmed by transmission electron microscopy of S. brasiliensis exposed to the AH derivatives. AH-treated cells displayed thickening of the cell wall, discontinuity of the cell membrane and an intense cytoplasmic degeneration. In a murine model of sporotrichosis, treatment with AH derivatives was more efficient than ITC, the drug of choice for sporotrichosis. The results of the preliminary clinical study in cats indicate that D13 is safe and has potential to become a therapeutic option for sporotrichosis when associated to ITC. Our results expand the antifungal broadness of AH derivatives and suggest that these drugs could be exploited to combat sporotrichosis.

The Future of Antifungal Drug Therapy: Novel Compounds and Targets

Fungal infections are a universal problem and are routinely associated with high morbidity and mortality rates in immunocompromised patients. Existing therapies comprise five different classes of antifungal agents, four of which target the synthesis of ergosterol and cell wall glucans. However, the currently available antifungals have many limitations, including poor oral bioavailability, narrow therapeutic indices, and emerging drug resistance resulting from their use, thus making it essential to investigate the development of novel drugs which can overcome these limitations and add to the antifungal armamentarium. Advances have been made in antifungal drug discovery research and development over the past few years as evidenced by the presence of several new compounds currently in various stages of development. In the following minireview, we provide a comprehensive summary of compounds aimed at one or more novel molecular targets. We also briefly describe potential pathways relevant for fungal pathogenesis that can be considered for drug development in the near future.

Fungal sphingolipids: role in the regulation of virulence and potential as targets for future antifungal therapies

INTRODUCTION

The antifungal therapy currently available includes three major classes of drugs: polyenes, azoles and echinocandins. However, the clinical use of these compounds faces several challenges: while polyenes are toxic to the host, antifungal resistance to azoles and echinocandins has been reported.

AREAS COVERED

Fungal sphingolipids (SL) play a pivotal role in growth, morphogenesis and virulence. In addition, fungi possess unique enzymes involved in SL synthesis, leading to the production of lipids which are absent or differ structurally from the mammalian counterparts. In this review, we address the enzymatic reactions involved in the SL synthesis and their relevance to the fungal pathogenesis, highlighting their potential as targets for novel drugs and the inhibitors described so far.

EXPERT OPINION

The pharmacological inhibition of fungal serine palmitoyltransferase depends on the development of specific drugs, as myriocin also targets the mammalian enzyme. Inhibitors of ceramide synthase might constitute potent antifungals, by depleting the pool of complex SL and leading to the accumulation of the toxic intermediates. Acylhydrazones and aureobasidin A, which inhibit GlcCer and IPC synthesis, are not toxic to the host and effectively treat invasive mycoses, emerging as promising new classes of antifungal drugs.

Antifungal Drug Development: Targeting the Fungal Sphingolipid Pathway

Fungal infections are becoming more prevalent and problematic due to the continual rise of immune deficient patients as well as the progressive development of drug resistance towards currently available antifungal drugs. There has been a significant increase in the development of antifungal compounds with a similar mechanism of action of current drugs. In contrast, there has been very little progress in developing compounds inhibiting totally new fungal targets or/and fungal pathways. This review focuses on novel compounds recently discovered to target the fungal sphingolipids and their metabolizing enzymes.

Preclinical Evaluation of Acylhydrazone SB-AF-1002 as a Novel Broad-Spectrum Antifungal Agent

The incidence of invasive fungal infections is rising due to the increase in susceptible populations. Current clinically available drugs have therapeutic limitations due to toxicity, a narrow spectrum of activity, and, more importantly, the consistent rise of fungal species that are intrinsically resistant or that develop resistance due to prolonged therapy. Thus, there is an urgent need for new broad-spectrum antifungal agents with low toxicity and a novel mechanism of action. We previously reported a new class of potent antifungal compounds, acylhydrazones, that target the fungal sphingolipid pathway. Based upon our initial lead molecules, (E)-N'-(5-bromo-2-hydroxybenzylidene)-2-methylbenzohydrazide and D13, we performed a structure-activity relationship study, synthesizing ca. 300 new compounds. Of these, 5 compounds were identified to be the most promising for further studies, based on their broad-spectrum activity and low toxicity in mammalian cells lines. Among these top 5 lead compounds, we report here the impressive in vivo activity of 2,4-dibromo-N'-(5-bromo-2-hydroxybenzylidene)benzohydrazide (SB-AF-1002) in several models of systemic fungal infection. Our data show that SB-AF-1002 is efficacious and outperforms current standard-of-care drugs in models of invasive fungal infections, such as cryptococcosis, candidiasis, and aspergillosis. Specifically, animals treated with SB-AF-1002 not only survived the infection but also showed a clearing of fungal cells from key organs. Moreover, SB-AF-1002 was very effective in an aspergillosis model as a prophylactic therapy. SB-AF-1002 also displayed acceptable pharmacokinetic properties in mice, similar to those of the parent compound, D13. These results clearly indicate that our novel acylhydrazones constitute a new class of highly potent and efficacious antifungal agents which warrant further development for the treatment of invasive fungal infections.

Antifungal Drugs

We reviewed the licensed antifungal drugs and summarized their mechanisms of action, pharmacological profiles, and susceptibility to specific fungi. Approved antimycotics inhibit 1,3-β-d-glucan synthase, lanosterol 14-α-demethylase, protein, and deoxyribonucleic acid biosynthesis, or sequestrate ergosterol. Their most severe side effects are hepatotoxicity, nephrotoxicity, and myelotoxicity. Whereas triazoles exhibit the most significant drug–drug interactions, echinocandins exhibit almost none. The antifungal resistance may be developed across most pathogens and includes drug target overexpression, efflux pump activation, and amino acid substitution. The experimental antifungal drugs in clinical trials are also reviewed. Siderophores in the Trojan horse approach or the application of siderophore biosynthesis enzyme inhibitors represent the most promising emerging antifungal therapies.