Activity of MK-3118, a new oral glucan synthase inhibitor, tested against Candida spp. by two international methods (CLSI and EUCAST)

Novel antifungals and treatment approaches to tackle resistance and improve outcomes of invasive fungal disease

SUMMARYFungal infections are on the rise, driven by a growing population at risk and climate change. Currently available antifungals include only five classes, and their utility and efficacy in antifungal treatment are limited by one or more of innate or acquired resistance in some fungi, poor penetration into "sequestered" sites, and agent-specific side effect which require frequent patient reassessment and monitoring. Agents with novel mechanisms, favorable pharmacokinetic (PK) profiles including good oral bioavailability, and fungicidal mechanism(s) are urgently needed. Here, we provide a comprehensive review of novel antifungal agents, with both improved known mechanisms of actions and new antifungal classes, currently in clinical development for treating invasive yeast, mold (filamentous fungi), Pneumocystis jirovecii infections, and dimorphic fungi (endemic mycoses). We further focus on inhaled antifungals and the role of immunotherapy in tackling fungal infections, and the specific PK/pharmacodynamic profiles, tissue distributions as well as drug-drug interactions of novel antifungals. Finally, we review antifungal resistance mechanisms, the role of use of antifungal pesticides in agriculture as drivers of drug resistance, and detail detection methods for antifungal resistance.

A Mini-Review of In Vitro Data for Candida Species, Including C. auris, Isolated during Clinical Trials of Three New Antifungals: Fosmanogepix, Ibrexafungerp, and Rezafungin

This mini-review summarizes the clinical outcomes and antifungal susceptibility results, where available, for three new antifungals, including fosmanogepix, ibrexafungerp, and rezafungin, against Candida isolates cultured from patients in clinical trials. When reported, most of the data were generated by the Clinical and Laboratory Standards Institute (CLSI) broth microdilution method or by both the CLSI and European Committee on Antimicrobial Susceptibility Testing (EUCAST) methodologies. For fosmanogepix, we summarize the in vitro data for C. auris isolates from 9 patients and for Candida spp. cultured from 20 patients in two clinical trials. Ibrexafungerp has also been evaluated in several clinical trials. From conference proceedings, a total of 176 Candida isolates were evaluated in the FURI and CARES studies, including 18 C. auris isolates (CARES study). However, MIC data are not available for all clinical isolates. Results from the ReSTORE rezafungin phase 3 clinical study also included in vitro results against Candida spp., but no patients with C. auris infections were included. In conclusion, this mini-review summarizes insights regarding clinical outcomes and the in vitro activity of three new antifungals against Candida spp. cultured from patients in clinical trials.

In vitro activity of ibrexafungerp against clinically relevant echinocandin-resistant Candida strains

Invasive candidiasis is a major hospital-acquired infection. Usually, echinocandins are considered first-line treatment. However, resistant phenotypes have emerged. Ibrexafungerp (IBX) is a new antifungal substance with potent anti-Candida activity. We challenged IBX with a library of 192 pheno-/genotypically echinocandin-resistant Candida isolates, focusing on the substance susceptibility, its activity on certain FKS hotspot (HS) mutated strains, and applying WTULs (wild-type upper limits). Therefore, a 9-year-old strain and patient data collection provided by the German National Reference Center for Invasive Fungal Infections were analyzed. Species identification was confirmed through ITS-sequencing. Molecular susceptibility testing was performed by sequencing HS of the FKS gene. Anidulafungin (AND) and IBX EUCAST-broth-microdilution was conducted. The four most common echinocandin-resistance mediating mutations were found in Candida glabrata [112/192 isolates; F659-(43×) and S663-(48×)] and Candida albicans [63/192 isolates; F641-(15×) and S645-(39×)]. Mutations at the HS-start sequence were associated with higher IBX MIC-values (F659 and F641 (MIC 50/90 mg/L: >4/>4 and 2/4 mg/L) in comparison to AND (F659 and F641 (MIC 50/90: 1/4 and 0.25/1 mg/L). MIC-values in HS-center mutations were almost equal [MIC50/90 in S663: 2/4 (AND and IBX); in S645: 0.5/1 (AND) and 0.25/1 (IBX) mg/L]. In total, 61 vs 78 of 192 echinocandin-resistant isolates may be classified as IBX wild type by applying WTULs, whereas the most prominent effect was seen in C. albicans [48% (30/63) vs 70% (44/63)]. IBX shows in vitro activity against echinocandin-resistant Candida and thus is an addition to the antifungal armory. However, our data suggest that this effect is more pronounced in C. albicans and strains harboring mutations, affecting the HS-center.

The Role of Novel Antifungals in the Management of Candidiasis: A Clinical Perspective

Mucosal and invasive candidiasis can be challenging to treat in the setting of drug intolerance, antifungal resistance, drug-drug interactions, or host immune status. Antifungals with novel mechanisms of action and distinct pharmacokinetic/pharmacodynamic properties have been developed in recent years. Rezafungin is an echinocandin with high-tissue penetration and an extended half-life that allows for once-weekly administration, making it a convenient treatment option for invasive candidiasis while obviating the need for central catheter placement. Ibrexafungerp is an oral glucan synthase inhibitor that is active against most echinocandin-resistant Candida species. At present, it is approved for the treatment of acute vulvovaginal candidiasis and is under investigation as an oral step-down therapy following initial treatment with an echinocandin for cases of invasive candidiasis. Oteseconazole is a long-acting tetrazole that exhibits a higher affinity for the fungal enzyme CYP51, resulting in a potentially lower risk of drug-drug interactions and side effects compared to other azoles. It is currently approved for the treatment of recurrent vulvovaginal candidiasis. Fosmanogepix has a novel mechanism of action and potent activity against several Candida strains resistant to other antifungals. Due to its considerable bioavailability and tissue penetration, it holds promise as a potential treatment option in patients with invasive candidiasis, including those with chorioretinitis or meningitis. Results from clinical trials and observational studies will further delineate the role of these agents in the management of candidiasis. As the usage of these novel antifungals becomes widespread, we expect to acquire a greater understanding of their efficacy and potential benefits.

Natural Enfumafungin Analogues from Hormonema carpetanum and Their Antifungal Activities

Ibrexafungerp, an inhibitor of fungal β-(1,3)-d-glucan synthase, represents the first new class of antifungals to be approved in the last 20 years. Ibrexafungerp is a semisynthetic derivative of the naturally occurring triterpene glycoside enfumafungin. In order to search for new analogues of enfumafungin and to probe its biosynthesis, we undertook a reinvestigation of Hormonema carpetanum, which led to the isolation of two new analogues, enfumafungins B and C, together with enfumafungin. Due to the presence of a hemiacetal moiety in the structure, the enfumafungins appear as a mixture of two interconverting epimers during both the purification process and NMR data acquisition. The structure elucidation, including the differentiation of 25S* and 25R* epimers, was completed by combined analyses of NMR and MS spectroscopic data. The discovery of enfumafungins B and C may have implications for enfumafungin biosynthesis. The antifungal activity of enfumafungins B and C was significantly lower than that of enfumafungin, suggesting that the C-2 substituents and the C-19 carboxy acid are important for activity. Molecular docking simulations revealed significant hydrogen bond interactions between enfumafungins and β-(1,3)-d-glucan synthase, which may be useful for developing new antifungal agents.

Combatting antifungal resistance: Paradigm shift in the diagnosis and management of onychomycosis and dermatomycosis

Antifungal resistance has become prevalent worldwide. Understanding the factors involved in spread of resistance allows the formulation of strategies to slow resistance development and likewise identify solutions for the treatment of highly recalcitrant fungal infections. To investigate the recent explosion of resistant strains, a literature review was performed focusing on four main areas: mechanisms of resistance to antifungal agents, diagnosis of superficial fungal infections, management, and stewardship. The use of traditional diagnostic tools such as culture, KOH analysis and minimum inhibitory concentration values on treatment were investigated and compared to the newer techniques such as molecular methods including whole genome sequencing, and polymerase chain reaction. The management of terbinafine-resistant strains is discussed. We have emphasized the need for antifungal stewardship including increasing surveillance for resistant infection.

Emerging Diagnostics and Therapeutics for Invasive Fungal Infections

Recently, there have been significant advances in the diagnosis and management of invasive fungal infections. Compared with traditional fungal diagnostics, molecular assays promise improved sensitivity and specificity, the ability to test a range of samples (including noninvasive samples, ie, blood), the detection of genetic mutations associated with antifungal resistance, and the potential for a faster turnaround time. Antifungals in late-stage clinical development include agents with novel mechanisms of action (olorofim and fosmanogepix) and new members of existing classes with distinct advantages over existing antifungals in toxicity, drug-drug interactions, and dosing convenience (oteseconazole, opelconazole, rezafungin, ibrexafungerp, encochleated amphotericin B).

A Comprehensive Overview of the Antibiotics Approved in the Last Two Decades: Retrospects and Prospects

Due to the overuse of antibiotics, bacterial resistance has markedly increased to become a global problem and a major threat to human health. Fortunately, in recent years, various new antibiotics have been developed through both improvements to traditional antibiotics and the discovery of antibiotics with novel mechanisms with the aim of addressing the decrease in the efficacy of traditional antibiotics. This manuscript reviews the antibiotics that have been approved for marketing in the last 20 years with an emphasis on the antibacterial properties, mechanisms, structure-activity relationships (SARs), and clinical safety of these antibiotics. Furthermore, the current deficiencies, opportunities for improvement, and prospects of antibiotics are thoroughly discussed to provide new insights for the design and development of safer and more potent antibiotics.

EUCAST Ibrexafungerp MICs and Wild-Type Upper Limits for Contemporary Danish Yeast Isolates

Ibrexafungerp is a novel triterpenoid antifungal that inhibits glucan synthase and thus fungal cell wall synthesis. We examined the in vitro activity against contemporary clinical yeast, investigated inter-laboratory and intra-laboratory variability, suggested wild-type upper-limit values (WT-UL), and compared in vitro activity of ibrexafungerp to five licensed antifungals. Susceptibility to ibrexafungerp and comparators was investigated prospectively for 1965 isolates (11,790 MICs) and repetitively for three QC strains (1764 MICs) following the EUCAST E.Def 7.3.2 method. Elevated ibrexafungerp/echinocandin MICs prompted FKS sequencing. Published ibrexafungerp EUCAST MIC-distributions were retrieved and aggregated for WT-UL determinations following EUCAST principles. Ibrexafungerp MICs were ≤2 mg/L except against C. pararugosa, Cryptococcus and some rare yeasts. Modal MICs (mg/L) were 0.06/0.125/0.25/0.5/0.5/0.5/0.5/1/2 for C. albicans/C. dubliniensis/C. glabrata/C. krusei/C. parapsilosis/C. tropicalis/S. cerevisiae/C. guilliermondii/C. lusitaniae and aligned within ±1 dilution with published values. The MIC ranges for QC strains were: 0.06–0.25/0.5–1/0.125–0.5 for CNM-CL-F8555/ATCC6258/ATCC22019. The WT-UL (mg/L) were: 0.25/0.5/1/1/2 for C. albicans/C. glabrata/C. krusei/C. parapsilosis/C. tropicalis. Adopting these, non-wild-type rates were 0.3%/0.6%/0%/8%/3% for C. albicans/C. glabrata/C. krusei/C. parapsilosis/C. tropicalis and overall lower than for comparators except amphotericin B. Five/six non-wild-type C. albicans/C. glabrata were echinocandin and Fks non-wild-type (F641S, F659del or F659L). Eight C. parapsilosis and three C. tropicalis non-wild-type isolates were echinocandin and Fks wild-type. Partial inhibition near 50% in the supra-MIC range may explain variable MICs. Ibrexafungerp EUCAST MIC testing is robust, although the significance of paradoxical growth for some species requires further investigation. The spectrum is broad and will provide an oral option for the growing population with azole refractory infection.

Ibrexafungerp: A new triterpenoid antifungal

DISCLAIMER

In an effort to expedite the publication of articles, AJHP is posting manuscripts online as soon as possible after acceptance. Accepted manuscripts have been peer-reviewed and copyedited, but are posted online before technical formatting and author proofing. These manuscripts are not the final version of record and will be replaced with the final article (formatted per AJHP style and proofed by the authors) at a later time.

PURPOSE

The pharmacology, microbiology, pharmacokinetics, pharmacodynamics, efficacy, safety, and role of ibrexafungerp in the treatment of fungal infections are reviewed.

SUMMARY

Ibrexafungerp is the first triterpenoid antifungal. Similarly to echinocandins, it inhibits the synthesis of 1,3-β-d-glucan. However, it binds to a different site on the enzyme than echinocandins, resulting in limited cross-resistance. Ibrexafungerp exerts concentration-dependent fungicidal activity against Candida species and retains in vitro activity against most fluconazole-resistant strains. It is also active against Aspergillus species. Ibrexafungerp has been shown to be safe and effective in the treatment of vulvovaginal candidiasis caused by Candida albicans in phase 2 and phase 3 clinical trials. It is approved for vulvovaginal candidiasis in adult and postmenarchal pediatric females and is given as two 150-mg tablets orally, administered 12 hours apart. Ibrexafungerp is contraindicated in pregnancy. The most commonly reported adverse reactions were diarrhea, nausea, abdominal pain, dizziness, and vomiting. Ibrexafungerp should be avoided with strong or moderate CYP3A inducers, and the dose should be reduced with strong CYP3A inhibitors. Ibrexafungerp may be useful for patients who are not able to receive fluconazole or prefer oral therapy for the treatment of vulvovaginal candidiasis. However, it is more expensive than the 150-mg tablet of generic fluconazole, which is the current standard of care for vulvovaginal candidiasis. Clinical trials are ongoing for recurrent and complicated vulvovaginal candidiasis as well as invasive candidiasis and pulmonary aspergillosis.

CONCLUSION

Ibrexafungerp is an alternative to fluconazole for the treatment of vulvovaginal candidiasis in nonpregnant females. It has the potential to be useful for recurrent and complicated vulvovaginal candidiasis as well as certain invasive fungal infections.

Pharmacodynamics, Mechanisms of Action and Resistance, and Spectrum of Activity of New Antifungal Agents

Several new antifungals are currently in late-stage development, including those with novel pharmacodynamics/mechanisms of action that represent new antifungal classes (manogepix, olorofim, ATI-2307, GR-2397). Others include new agents within established classes or with mechanisms of action similar to clinically available antifungals (ibrexafungerp, rezafungin, oteseconazole, opelconazole, MAT2203) that have been modified in order to improve certain characteristics, including enhanced pharmacokinetics and greater specificity for fungal targets. Many of the antifungals under development also have activity against Candida and Aspergillus strains that have reduced susceptibility or acquired resistance to azoles and echinocandins, whereas others demonstrate activity against species that are intrinsically resistant to most clinically available antifungals. The tolerability and drug–drug interaction profiles of these new agents also appear to be promising, although the number of human subjects that have been exposed to many of these agents remains relatively small. Overall, these agents have the potential for expanding our antifungal armamentarium and improving clinical outcomes in patients with invasive mycoses.

Novel agents in the treatment of invasive fungal infections in solid organ transplant recipients

PURPOSE OF REVIEW

Recipients of solid organ transplants (SOTs) suffer a significant burden of invasive fungal infections (IFIs). The emergence of drug-resistant fungi and toxicities of currently used antifungal agents as well as drug-drug interactions with immunosuppressants make their treatment challenging. This review discusses selected novel antifungal agents in the development pipeline that can currently be used through clinical trials or may be commercially available in the near future.

RECENT FINDINGS

These agents in development have novel pharmacokinetics and pharmacodynamics, expanded spectra of activity and excellent safety profiles.

SUMMARY

The properties of novel antifungal agents have the potential to expand the therapeutic options for IFIs in recipients of SOTs.

In Vitro Antifungal Activity of Ibrexafungerp (SCY-078) Against Contemporary Blood Isolates From Medically Relevant Species of Candida: A European Study

Background

Ibrexafungerp (SCY-078) is the newest oral and intravenous antifungal drug with broad activity, currently undergoing clinical trials for invasive candidiasis.

Objective

The aim of this study was to assess the in vitro activity of ibrexafungerp and comparators against a collection of 434 European blood isolates of Candida.

Methods

Ibrexafungerp, caspofungin, fluconazole, and micafungin minimum inhibitory concentrations (MICs) were collected from 12 European laboratories for 434 blood isolates, including 163 Candida albicans, 108 Candida parapsilosis, 60 Candida glabrata, 40 Candida tropicalis, 29 Candida krusei, 20 Candida orthopsilosis, 6 Candida guilliermondii, 2 Candida famata, 2 Candida lusitaniae, and 1 isolate each of Candida bracarensis, Candida catenulata, Candida dubliniensis, and Candida kefyr. MICs were determined by the EUCAST broth microdilution method, and isolates were classified according to recommended clinical breakpoints and epidemiological cutoffs. Additionally, 22 Candida auris from different clinical specimens were evaluated.

Results

Ibrexafungerp MICs ranged from 0.016 to ≥8 mg/L. The lowest ibrexafungerp MICs were observed for C. albicans (geometric MIC 0.062 mg/L, MIC range 0.016–0.5 mg/L) and the highest ibrexafungerp MICs were observed for C. tropicalis (geometric MIC 0.517 mg/L, MIC range 0.06–≥8 mg/L). Modal MICs/MIC₅₀s (mg/L) against Candida spp. were 0.125/0.06 for C. albicans, 0.5/0.5 for C. parapsilosis, 0.25/0.25 for C. glabrata, 0.5/0.5 for C. tropicalis, 1/1 for C. krusei, 4/2 for C. orthopsilosis, and 0.5/0.5 for C. auris. Ibrexafungerp showed activity against fluconazole- and echinocandin-resistant isolates. If adopting wild-type upper limits, a non-wild-type phenotype for ibrexafungerp was only observed for 16/434 (3.7%) isolates: 11 (4.6%) C. parapsilosis, 4 (5%) C. glabrata, and 1 (2.5%) C. tropicalis.

Conclusion

Ibrexafungerp showed a potent in vitro activity against Candida.

Drug Resistance and Novel Therapeutic Approaches in Invasive Candidiasis

Candida species are the leading cause of invasive fungal infections worldwide and are associated with acute mortality rates of ~50%. Mortality rates are further augmented in the context of host immunosuppression and infection with drug-resistant Candida species. In this review, we outline antifungal drugs already in clinical use for invasive candidiasis and candidaemia, their targets and mechanisms of resistance in clinically relevant Candida species, encompassing not only classical resistance, but also heteroresistance and tolerance. We describe novel antifungal agents and targets in pre-clinical and clinical development, including their spectrum of activity, antifungal target, clinical trial data and potential in treatment of drug-resistant Candida. Lastly, we discuss the use of combination therapy between conventional and repurposed agents as a potential strategy to combat the threat of emerging resistance in Candida.

OUP accepted manuscript

Candida auris is an emerging, multi drug resistant fungal pathogen that has caused infectious outbreaks in over 45 countries since its first isolation over a decade ago, leading to in-hospital crude mortality rates as high as 72%. The fungus is also acclimated to disinfection procedures and persists for weeks in nosocomial ecosystems. Alarmingly, the outbreaks of C. auris infections in Coronavirus Disease-2019 (COVID-19) patients have also been reported. The pathogenicity, drug resistance and global spread of C. auris have led to an urgent exploration of novel, candidate antifungal agents for C. auris therapeutics. This narrative review codifies the emerging data on the following new/emerging antifungal compounds and strategies: antimicrobial peptides, combinational therapy, immunotherapy, metals and nano particles, natural compounds, and repurposed drugs. Encouragingly, a vast majority of these exhibit excellent anti- C. auris properties, with promising drugs now in the pipeline in various stages of development. Nevertheless, further research on the modes of action, toxicity, and the dosage of the new formulations are warranted. Studies are needed with representation from all five C. auris clades, so as to produce data of grater relevance, and broader significance and validity.

Pharmacokinetics and Pharmacodynamics of Ibrexafungerp

On 2 June, 2021, the US Food and Drug Administration approved ibrexafungerp (formerly MK-3118 and SCY-078) for the treatment of vulvovaginal candidiasis, also known as vaginal yeast infection. Ibrexafungerp is the first drug approved in a novel antifungal class in more than two decades, and the Food and Drug Administration’s decision was based on positive results from two pivotal phase III studies in which oral ibrexafungerp proved both safe and effective in patients with vulvovaginal candidiasis. The decision was also based on substantial preclinical and clinical work in both the pharmacokinetics and pharmacodynamics of ibrexafungerp. This paper reviews that research and looks ahead to explore how this novel antifungal agent may be used in the future to address the expanding problem of drug-resistant mycotic infections.

Our pursuit for effective antifungal agents targeting fungal cell wall components, where are we?

Invasive mycotic infections account for an unacceptably high mortality rates in humans. These infections are initiated by the fungal cell wall which mediates host-fungi interactions. The cell wall is fused to the physiology of fungi, and it is involved in essential functions in the entire cell functionality. Components of the cell wall are synthesised and modified in the cell wall space by the activities of cell wall proteins through a range of signalling pathways that have only been described in many fungi, therefore making them suitable drug targets. The echinocandins class of cell wall-active drugs block cell wall β-1,3-glucan biosynthesis through inhibiting the catalytic subunit of the synthetic protein complex. Resistance to echinocandins can be through the acquisition of single nucleotide polymorphisms and/or through activation of cell wall signalling pathways resulting in altered cell wall proteome and elevated chitin content in the cell wall. Countering the cell wall remodelling process will enhance the effectiveness of β-1,3-glucan-active antifungal agents. Cell surface proteins are also important antifungal targets which can be used to develop rapid and robust diagnostics and more effective therapeutics. The cell wall remains a crucial target in fungi that needs to be harnessed to combat mycotic infections.

In vitro activity of ibrexafungerp against Candida species isolated from blood cultures. Determination of wild-type populations using the EUCAST method

OBJECTIVES

Ibrexafungerp is a new oral glucan synthase inhibitor with in vivo and in vitro activity against Candida spp., including echinocandin- and azole-resistant isolates. We studied the in vitro activity of ibrexafungerp against Candida species isolated from blood cultures and assessed wild-type upper limits against the five Candida species most frequently associated to candidaemia.

METHODS

Isolates (n = 958) causing incident episodes of candidaemia in patients admitted to Gregorio Marañón hospital (Madrid, Spain) between January 2007 and April 2021 were studied. Antifungal susceptibility to ibrexafungerp, fluconazole, micafungin and anidulafungin was tested (EUCAST E.Def 7.3.2) and wild-type upper limits determined against C. albicans (n = 462), C. glabrata (n = 120), C. parapsilosis (n = 249), C. tropicalis (n = 73) and C. krusei (n = 24). fksgene sequencing was carried out in non-wild-type isolates.

RESULTS

Ibrexafungerp showed antifungal in vitro activity against the studied isolates. Wild-type upper limits for ibrexafungerp were >0.25 mg/L against C. albicans, >1 mg/L against C. parapsilosis, C. glabrata, and C. tropicalis, and >2 mg/L against C. krusei. Percentages of ibrexafungerp non-wild-type isolates were low (C. parapsilosis and C. krusei, 0%; C. albicans, 0.22% (1/462); C. glabrata, 0.83% (1/120); and C. tropicalis, 1.37% (1/73)). Ibrexafungerp proved in vitro activity against fluconazole- or echinocandin-resistant isolates.

DISCUSSION

We show in vitro activity of ibrexafungerp against the tested Candida species. Furthermore, we provide ibrexafungerp wild-type upper limits, which allows defining the wild-type populations of the five most relevant Candida species.

Ibrexafungerp: First Approval

Ibrexafungerp (BREXAFEMME^®) is an orally active triterpenoid antifungal drug being developed by SCYNEXIS, Inc. for the treatment of fungal infections. The inhibition of β-1,3-D glucan synthetase by ibrexafungerp compromises the integrity of fungal cell walls. Ibrexafungerp has been recently approved for the treatment of vulvovaginal candidiasis (VVC), and it is the first novel antifungal drug class to be approved in more than 20 years. Development for the treatment of recurrent VVC and invasive fungal infections is ongoing. This article summarizes the milestones in the development of ibrexafungerp leading to this first approval for the treatment of VVC in adult and postmenarchal paediatric females.

Natural products targeting the synthesis of β(1,3)-D-glucan and chitin of the fungal cell wall. Existing drugs and recent findings

BACKGROUND

During the last three decades systemic fungal infections associated to immunosuppressive therapies have become a serious healthcare problem. Clinical development of new antifungals is an urgent requirement. Since fungal but not mammalian cells are encased in a carbohydrate-containing cell wall, which is required for the growth and viability of fungi, the inhibition of cell wall synthesizing machinery, such as β(1,3)-D-glucan synthases (GS) and chitin synthases (CS) that catalyze the synthesis of β(1-3)-D-glucan and chitin, respectively, represent an ideal mode of action of antifungal agents. Although the echinocandins anidulafungin, caspofungin and micafungin are clinically well-established GS inhibitors for the treatment of invasive fungal infections, much effort must still be made to identify inhibitors of other enzymes and processes involved in the synthesis of the fungal cell wall.

PURPOSE

Since natural products (NPs) have been the source of several antifungals in clinical use and also have provided important scaffolds for the development of semisynthetic analogues, this review was devoted to investigate the advances made to date in the discovery of NPs from plants that showed capacity of inhibiting cell wall synthesis targets. The chemical characterization, specific target, discovery process, along with the stage of development are provided here.

METHODS

An extensive systematic search for NPs against the cell wall was performed considering all the articles published until the end of 2020 through the following scientific databases: NCBI PubMed, Scopus and Google Scholar and using the combination of the terms "natural antifungals" and "plant extracts" with "fungal cell wall".

RESULTS

The first part of this review introduces the state of the art of the structure and biosynthesis of the fungal cell wall and considers exclusively those naturally produced GS antifungals that have given rise to both existing semisynthetic approved drugs and those derivatives currently in clinical trials. According to their chemical structure, natural GS inhibitors can be classified as 1) cyclic lipopeptides, 2) glycolipids and 3) acidic terpenoids. We also included nikkomycins and polyoxins, NPs that inhibit the CS, which have traditionally been considered good candidates for antifungal drug development but have finally been discarded after enduring unsuccessful clinical trials. Finally, the review focuses in the most recent findings about the growing field of plant-derived molecules and extracts that exhibit activity against the fungal cell wall. Thus, this search yielded sixteen articles, nine of which deal with pure compounds and seven with plant extracts or fractions with proven activity against the fungal cell wall. Regarding the mechanism of action, seven (44%) produced GS inhibition while five (31%) inhibited CS. Some of them (56%) interfered with other components of the cell wall. Most of the analyzed articles refer to tests carried out in vitro and therefore are in early stages of development.

CONCLUSION

This report delivers an overview about both existing natural antifungals targeting GS and CS activities and their mechanisms of action. It also presents recent discoveries on natural products that may be used as starting points for the development of potential selective and non-toxic antifungal drugs.

How urgent is the need for new antifungals?

Introduction: Invasive fungal infection carries a high morbidity, mortality and economic cost. In recent times, a rising incidence of fungal infection and antifungal resistance is occurring which has prompted the development of novel antifungal agents.Areas covered:In this perspective, the authors describe the current status of registered antifungals and their limitations in the treatment of invasive fungal infection. They also go on to describe the new antifungal agents that are in the clinical stage of development and how they might be best utilized in patient care in the future.Expert opinion: The antifungal drug development pipeline has responded to a growing need for new agents to effectively treat fungal disease without concomitant toxicity or issues with drug tolerance. Olorofim (F901318), ibrexafungerp (SCY-078), fosmanogepix (APX001), rezafungin (CD101), oteseconazole (VT-1161), encochleated amphotericin B (MAT2203), nikkomycin Z (NikZ) and ATI-2307 are all in the clinical stage of development and offer great promise in offering clinicians better agents to treat these difficult infections.

Enhancing the production of amphotericin B by Strepyomyces nodosus in a 50-ton bioreactor based on comparative genomic analysis

Amphotericin, as an important macrolide antibiotic, is synthesized by Streptomyces nodosus. A high-yield S. nodosus ZJB2016050 was obtained by mutagenesis in our lab with the advantages of high yield, short fermentation cycle and few by-products, which was more suitable for industrial production. The fermentation differences in 50-tons bioreactor between S. nodosus ATCC14899 and S. nodosus ZJB2016050 were compared. The amphotericin B (AmB) yield of S. nodosus ZJB2016050 was 9.73 mg/g at 96 h, which was 30% higher than that of S. nodosus ATCC14899. The by-product amphotericin A (AmA) production of S. nodosus ZJB2016050 was 78% lower than that of S. nodosus ATCC14899. By performing whole-genome sequencing of S. nodosus ZJB2016050 and comparative genome analysis with the wild-type S. nodosus ATCC14899, it was found that the two strains have high synteny, but each has a special gene fragment. The genes functions of fragment were identified in the amino acid transport and metabolism, carbohydrate metabolism and lipid transport and metabolism. The gene functions of SNP (single nucleotide polymorphism) genes were identified in amino acid transport and metabolism, carbohydrate metabolism, coenzyme metabolism and secondary metabolites biosynthesis. The difference in signal-regulation and transcription may be the main reason for the differences between these two strains. Three GntR family egulatory factors of S. nodosus ATCC14899 may reduce the synthesis of amphotericin. Based on the analysis of comparative genomes, the effects of corn oil in S. nodosus ATCC14899 and S. nodosus ZJB2016050 were also compared. The results showed that corn oil can promote the fermentation of S. nodosus ZJB2016050. The S. nodosus ZJB2016050 may degrade fatty acids faster, and the degraded acyl-coenzyme can be used to synthesize amphotericin.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02844-2.

Emerging Issues in Antifungal Resistance

Invasive fungal diseases continue to cause substantial mortality in the enlarging immunocompromised population. It is fortunate that the field has moved past amphotericin B deoxycholate as the only available antifungal drug but despite new classes of antifungal agents both primary and secondary drug resistance in molds and yeasts abound. From the rise of multiple-drug-resistant Candida auris to the agrochemical selection of environmental azole-resistant Aspergillus fumigatus, it is and will be critical to understand antifungal drug resistance and both prevent and treat it with new strategies and agents.

Ibrexafungerp Demonstrates In Vitro Activity against Fluconazole-Resistant Candida auris and In Vivo Efficacy with Delayed Initiation of Therapy in an Experimental Model of Invasive Candidiasis

Candida auris is an emerging pathogen that has rapidly spread to many countries on multiple continents. Invasive infections caused by this species are associated with significant mortality, and treatment options are limited due to antifungal resistance. Ibrexafungerp is the first-in-class member of the triterpenoids, which inhibit the production of (1,3)-β-d-glucan and can be administered orally. We evaluated the in vitro activity and in vivo efficacy of ibrexafungerp against C. auris Antifungal susceptibility was tested by broth microdilution against 54 C. auris isolates. Neutropenic mice were intravenously infected with a clinical isolate, and a 7-day treatment course was begun 24 h postinoculation with vehicle control, ibrexafungerp (20, 30, and 40 mg/kg orally twice daily), fluconazole (20 mg/kg orally once daily), or caspofungin (10 mg/kg intraperitoneally once daily). Fungal burden was assessed by colony counts in the kidneys on day 8 and on day 21 or as mice became moribund in the survival arm. Ibrexafungerp demonstrated consistent activity, with MICs ranging between 0.25 and 2 μg/ml against all isolates. Marked improvements in survival were observed in mice treated with the higher doses of ibrexafungerp and caspofungin. Similarly, reductions in kidney fungal burden were also observed in these groups. No improvements in survival or reductions in fungal burden were observed with fluconazole, consistent with the in vitro resistance of the isolate used to establish infection to this azole. These results demonstrate that ibrexafungerp is effective in vivo against C. auris even when the start of therapy is delayed.

An overview of current and emerging antifungal pharmacotherapy for invasive fungal infections

Introduction: Invasive fungal infections (IFIs) remain a significant cause of morbidity and mortality despite significant advancements in currently available therapy. With a flush pipeline of investigational antifungals, the clinician must identify appropriate roles of currently available therapies, potential advantages of emerging antifungals, and shortcomings in the evolving clinical evidence.Areas covered: Standard and developing treatment approaches for IFIs with currently available antifungals are summarized with a focus on invasive candidiasis and invasive aspergillosis. Emerging investigational antifungals are discussed in depth, including mechanisms of action, fungal activity, clinical evidence, and ongoing research. An opinion on the impact and potential role of therapy for emerging antifungals of interest is also provided.Expert opinion: Despite advances and clinical studies optimizing antifungal use, current therapies fall short in preventing IFI morbidity and mortality. Further optimization of currently available antifungals may improve outcomes; however, novel agents are required for historically difficult-to-treat infections, transitions to oral treatment, minimizing adverse drug effects, decreasing drug interactions, and ultimately improving patient quality of life. Emerging antifungals may positively revolutionize the treatment of IFIs.

An evaluation of ibrexafungerp for the treatment of invasive candidiasis: the evidence to date

INTRODUCTION

Invasive fungal infections, especially candidemia and invasive candidiasis, cause significant morbidity and mortality. The epidemiology of candida infections have changed dramatically due to an increase in risk factors associated with the development of infection and the emergence of resistant isolates such as C. glabrata and C. auris. This has prompted the search for novel and effective antifungals.

AREAS COVERED

The results of in vitro studies evaluating the activity of ibrexafungerp against Candida species are reviewed and the pharmacokinetic/pharmacodynamic properties are highlighted. Available results and safety data from limited clinical studies are discussed.

EXPERT OPINION

Ibrexafungerp demonstrates potent in vitro activity against susceptible and resistant Candida species, including echinocandin-resistant C. glabrata and multidrug-resistant C. auris. It also offers the flexibility of a parenteral and an oral preparation, minimal adverse effects, and low drug-drug interactions. In Phase 2/3 clinical trials, ibrexafungerp appears to have excellent clinical activity in patients with candidemia, invasive candidiasis, and mucosal candidiasis. Although there are several ongoing clinical trials, ibrexafungerp appears to be a promising agent and an important addition to the antifungal armamentarium necessary to treat emerging and resistant pathogens, including several of the Candida species.

Ibrexafungerp: A novel oral glucan synthase inhibitor

Ibrexafungerp is a novel glucan synthase inhibitor currently undergoing phase II and phase III clinical trials. This compound has demonstrated in vitro activity against clinically important fungal pathogens including Candida spp. and Aspergillus spp. It is able to retain activity against many echinocandin-resistant strains of Candida due to differential avidity for the target site compared to echinocandins. In vivo animal models have demonstrated efficacy in murine models of invasive candidiasis, aspergillosis, and pneumocystis. Due to high bioavailability, it can be administered both orally and intravenously. A favorable drug interaction and tolerability profile is observed with this compound. This review summarizes existing data that have either been published or presented at international symposia.

Ibrexafungerp: An orally active β-1,3-glucan synthesis inhibitor

We previously reported medicinal chemistry efforts that identified MK-5204, an orally efficacious β-1,3-glucan synthesis inhibitor derived from the natural product enfumafungin. Further extensive optimization of the C2 triazole substituent identified 4-pyridyl as the preferred replacement for the carboxamide of MK-5204, leading to improvements in antifungal activity in the presence of serum, and increased oral exposure. Reoptimizing the aminoether at C3 in the presence of this newly discovered C2 substituent, confirmed that the (R) t-butyl, methyl aminoether of MK-5204 provided the best balance of these two key parameters, culminating in the discovery of ibrexafungerp, which is currently in phase III clinical trials. Ibrexafungerp displayed significantly improved oral efficacy in murine infection models, making it a superior candidate for clinical development as an oral treatment for Candida and Aspergillus infections.

Antimicrobial Peptides: a New Frontier in Antifungal Therapy

Invasive fungal infections in humans are generally associated with high mortality, making the choice of antifungal drug crucial for the outcome of the patient. The limited spectrum of antifungals available and the development of drug resistance represent the main concerns for the current antifungal treatments, requiring alternative strategies. Antimicrobial peptides (AMPs), expressed in several organisms and used as first-line defenses against microbial infections, have emerged as potential candidates for developing new antifungal therapies, characterized by negligible host toxicity and low resistance rates. Most of the current literature focuses on peptides with antibacterial activity, but there are fewer studies of their antifungal properties. This review focuses on AMPs with antifungal effects, including their in vitro and in vivo activities, with the biological repercussions on the fungal cells, when known. The classification of the peptides is based on their mode of action: although the majority of AMPs exert their activity through the interaction with membranes, other mechanisms have been identified, including cell wall inhibition and nucleic acid binding. In addition, antifungal compounds with unknown modes of action are also described. The elucidation of such mechanisms can be useful to identify novel drug targets and, possibly, to serve as the templates for the synthesis of new antimicrobial compounds with increased activity and reduced host toxicity.

MSG-10: a Phase 2 study of oral ibrexafungerp (SCY-078) following initial echinocandin therapy in non-neutropenic patients with invasive candidiasis

OBJECTIVES

To evaluate the safety and efficacy of two dosing regimens of oral ibrexafungerp (formerly SCY-078), a novel orally bioavailable β-glucan synthase inhibitor, in subjects with invasive candidiasis versus the standard of care (SOC) and to identify the dose to achieve target exposure (15.4 μM·h) in >80% of the intended population.

METHODS

In a multinational, open-label study, patients with documented invasive candidiasis were randomized to receive step-down therapy to one of three treatment arms: two dosing regimens of novel oral ibrexafungerp or the SOC treatment following initial echinocandin therapy. Plasma samples were collected to evaluate exposure by population pharmacokinetic (PK) modelling. Safety was assessed throughout the study and global response at the end of treatment.

RESULTS

Out of 27 subjects enrolled, 7 received ibrexafungerp 500 mg, 7 received ibrexafungerp 750 mg and 8 received the SOC. Five did not meet criteria for randomization. Population PK analysis indicated that an ibrexafungerp 750 mg regimen is predicted to achieve the target exposure in ∼85% of the population. The rate of adverse events was similar among patients receiving ibrexafungerp or fluconazole. Similar favourable response rates were reported among all groups: 86% (n = 6) in the ibrexafungerp 750 mg versus 71% (n = 5) in both the fluconazole and ibrexafungerp 500 mg treatment arms. The one subject treated with continued micafungin had a favourable global response.

CONCLUSIONS

The oral ibrexafungerp dose estimated to achieve the target exposure in subjects with invasive candidiasis is 750 mg daily. This dose was well tolerated and achieved a favourable global response rate, similar to the SOC.

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.

Combination Therapy with Ibrexafungerp (Formerly SCY-078), a First-in-Class Triterpenoid Inhibitor of (1→3)-β-d-Glucan Synthesis, and Isavuconazole for Treatment of Experimental Invasive Pulmonary Aspergillosis

Ibrexafungerp (formerly SCY-078) is a semisynthetic triterpenoid and potent (1→3)-β-d-glucan synthase inhibitor. We investigated the in vitro activity, pharmacokinetics, and in vivo efficacy of ibrexafungerp (SCY) alone and in combination with antimold triazole isavuconazole (ISA) against invasive pulmonary aspergillosis (IPA). The combination of ibrexafungerp and isavuconazole in in vitro studies resulted in additive and synergistic interactions against Aspergillus spp. Plasma concentration-time curves of ibrexafungerp were compatible with linear dose proportional profile. In vivo efficacy was studied in a well-established persistently neutropenic New Zealand White (NZW) rabbit model of experimental IPA. Treatment groups included untreated control (UC) rabbits and rabbits receiving ibrexafungerp at 2.5 (SCY2.5) and 7.5 (SCY7.5) mg/kg of body weight/day, isavuconazole at 40 (ISA40) mg/kg/day, or combinations of SCY2.5+ISA40 and SCY7.5+ISA40. The combination of SCY+ISA produced an in vitro synergistic interaction. There were significant in vivo reductions of residual fungal burden, lung weights, and pulmonary infarct scores in SCY2.5+ISA40, SCY7.5+ISA40, and ISA40 treatment groups versus those of the SCY2.5-treated, SCY7.5-treated, and UC (P < 0.01) groups. Rabbits treated with SCY2.5+ISA40 and SCY7.5+ISA40 had prolonged survival in comparison to that of the SCY2.5-, SCY7.5-, ISA40-treated, or UC (P < 0.05) groups. Serum galactomannan index (GMI) and (1→3)-β-d-glucan levels significantly declined in animals treated with the combination of SCY7.5+ISA40 in comparison to those of animals treated with SCY7.5 or ISA40 (P < 0.05). Ibrexafungerp and isavuconazole combination demonstrated prolonged survival, decreased pulmonary injury, reduced residual fungal burden, and lower GMI and (1→3)-β-d-glucan levels in comparison to those of single therapy for treatment of IPA. These findings provide an experimental foundation for clinical evaluation of the combination of ibrexafungerp and an antimold triazole for treatment of IPA.

New and Promising Chemotherapeutics for Emerging Infections Involving Drug-resistant Non-albicans Candida Species

Fungal infections are a veritable public health problem worldwide. The increasing number of patient populations at risk (e.g. transplanted individuals, cancer patients, and HIV-infected people), as well as the use of antifungal agents for prophylaxis in medicine, have favored the emergence of previously rare or newly identified fungal species. Indeed, novel antifungal resistance patterns have been observed, including environmental sources and the emergence of simultaneous resistance to different antifungal classes, especially in Candida spp., which are known for the multidrug-resistance (MDR) profile. In order to circumvent this alarming scenario, the international researchers' community is engaged in discovering new, potent, and promising compounds to be used in a near future to treat resistant fungal infections in hospital settings on a global scale. In this context, many compounds with antifungal action from both natural and synthetic sources are currently under clinical development, including those that target either ergosterol or β(1,3)-D-glucan, presenting clear evidence of pharmacologic/pharmacokinetic advantages over currently available drugs against these two well-known fungal target structures. Among these are the tetrazoles VT-1129, VT-1161, and VT-1598, the echinocandin CD101, and the glucan synthase inhibitor SCY-078. In this review, we compiled the most recent antifungal compounds that are currently in clinical trials of development and described the potential outcomes against emerging and rare Candida species, with a focus on C. auris, C. dubliniensis, C. glabrata, C. guilliermondii, C. haemulonii, and C. rugosa. In addition to possibly overcoming the limitations of currently available antifungals, new investigational chemical agents that can enhance the classic antifungal activity, thereby reversing previously resistant phenotypes, were also highlighted. While novel and increasingly MDR non-albicans Candida species continue to emerge worldwide, novel strategies for rapid identification and treatment are needed to combat these life-threatening opportunistic fungal infections.

Hope on the Horizon: Novel Fungal Treatments in Development

The treatment of invasive fungal infections remains challenging due to limitations in currently available antifungal therapies including toxicity, interactions, restricted routes of administration, and drug resistance. This review focuses on novel therapies in clinical development, including drugs and a device. These drugs have novel mechanisms of action to overcome resistance, and some offer new formulations providing distinct advantages over current therapies to improve safety profiles and reduce interactions. Among agents that target the cell wall, 2 glucan synthesis inhibitors are discussed (rezafungin and ibrexafungerp), as well as fosmanogepix and nikkomycin Z. Agents that target the cell membrane include 3 fourth-generation azoles, oral encochleated amphotericin B, and aureobasidin A. Among agents with intracellular targets, we will review olorofim, VL-2397, T-2307, AR-12, and MGCD290. In addition, we will describe neurapheresis, a device used as adjunctive therapy for cryptococcosis. With a field full of novel treatments for fungal infections, the future looks promising.

Multiresistant Fusarium Pathogens on Plants and Humans: Solutions in (from) the Antifungal Pipeline?

The fungal genus Fusarium contains numerous plant pathogens causing considerable economic losses. In addition, Fusarium species are emerging as opportunistic human pathogens causing both superficial and systemic infections. Appropriate treatment of Fusarium infections in a clinical setting of neutropenia is currently not available. ESCMID and ECMM joint guidelines, following the majority of published studies, suggest early therapy with amphotericin B and voriconazole, in conjunction with surgical debridement and reversal of immunosuppression. In this review, we elaborate on the trans-kingdom pathogenicity of Fusarium. Intrinsic resistance to several antifungal drugs and the evolution of antifungal resistance over the years are highlighted. Recent studies present novel compounds that are effective against some pathogenic fungi including Fusarium. We discuss the robust and dynamic antifungal pipeline, including results from clinical trials as well as preclinical data that might appear beneficial for patients with invasive fusariosis.

Antifungal drugs: What brings the future?

The high burden and growing prevalence of invasive fungal infections (IFIs), the toxicity and interactions associated with current antifungal drugs, as well as the increasing resistance, ask for the development of new antifungal drugs, preferably with a novel mode of action. Also, the availability of oral or once-weekly alternatives would enable ambulatory treatment resulting in an improved patient's comfort and therapy adherence. However, only one new azole and two new posaconazole-formulations were marketed over the last decade. This review focuses on the antifungal drugs in the pipeline undergoing clinical evaluation. First, the newest azole, isavuconazole, with its improved safety profile and reduction in DDIs, will be discussed. Moreover, there are two glucan synthase inhibitors (GSIs) in the antifungal pipeline: rezafungin (CD101), a long-acting echinocandin with an improved stability that enables once weekly administration, and SCY-078, an orally available GSI with efficacy against azole- and echinocandin resistant isolates. A new oral formulation of amphotericin B will also be presented. Moreover, the first representative of a new antifungal class, the orotomides, with a broad spectrum and no cross-resistance with current antifungal classes, will be discussed. Finally, an overview of other antifungals that are still in earlier clinical development phases, is provided.

Activity of a novel 1,3-beta-D-glucan Synthase Inhibitor, Ibrexafungerp (formerly SCY-078), Against Candida glabrata

Ibrexafungerp (formerly SCY-078), a novel glucan synthase inhibitor with oral availability, was evaluated for activity against Candida glabrata Susceptibility of clinical strains to Ibrexafungerp was determined by microdilution and time kill assays. The MIC range against wild type strains was 1-2 μg/mL. IBX was also active against the majority of echinocandin-resistant strains. Time kill studies showed a 4 to 6-log reduction in growth at concentrations of 0.25 to 4 μg/ml at 24 and 48 hr.

Targeting the fungal cell wall: current therapies and implications for development of alternative antifungal agents

Fungal infections are a worldwide problem associated with high morbidity and mortality. There are relatively few antifungal agents, and resistance has emerged within these pathogens for the newest antifungal drugs. As the fungal cell wall is critical for growth and development, it is one of the most important targets for drug development. In this review, the currently available cell wall inhibitors and suitable drug candidates for the treatment of fungal infections are explored. Future studies of the fungal cell wall and compounds that have detrimental effects on this important outer structural layer could aid in antifungal drug discovery and lead to the development of alternative cell wall inhibitors to fill gaps in clinical therapies for difficult-to-treat fungal infections.

Drugs in Clinical Development for Fungal Infections

Despite increasing rates of invasive fungal infections being reported globally, only a single antifungal drug has been approved during the last decade. Resistance, toxicity, drug interactions and restricted routes of administration remain unresolved issues. This review focuses on new antifungal compounds which are currently in various clinical phases of development. We discuss two azoles with a tetrazole moiety that allows selective activity against the fungal CYP: VT-1161 for Candida infections and VT-1129 for cryptococcal meningoencephalitis. We also discuss two glucan synthesis inhibitors: CD101, an echinocandin with an increased half-life, and SCY-078 with oral bioavailability and increased activity against echinocandin-resistant isolates. Among the polyenes, we discuss MAT023, an encochleated amphotericin B formulation that allows oral administration. Two novel classes of antifungal drugs are also described: glycosylphosphatidylinositol inhibitors, and the leading drug APX001, which disrupt the integrity of the fungal wall; and the orotomides, inhibitors of pyrimidine synthesis with the leading drug F901318. Finally, a chitin synthesis inhibitor and progress on human monoclonal antifungal antibodies are discussed.

The fungal cell wall as a target for the development of new antifungal therapies

In the past three decades invasive mycoses have globally emerged as a persistent source of healthcare-associated infections. The cell wall surrounding the fungal cell opposes the turgor pressure that otherwise could produce cell lysis. Thus, the cell wall is essential for maintaining fungal cell shape and integrity. Given that this structure is absent in host mammalian cells, it stands as an important target when developing selective compounds for the treatment of fungal infections. Consequently, treatment with echinocandins, a family of antifungal agents that specifically inhibits the biosynthesis of cell wall (1-3)β-D-glucan, has been established as an alternative and effective antifungal therapy. However, the existence of many pathogenic fungi resistant to single or multiple antifungal families, together with the limited arsenal of available antifungal compounds, critically affects the effectiveness of treatments against these life-threatening infections. Thus, new antifungal therapies are required. Here we review the fungal cell wall and its relevance in biotechnology as a target for the development of new antifungal compounds, disclosing the most promising cell wall inhibitors that are currently in experimental or clinical development for the treatment of some invasive mycoses.

Oral glucan synthase inhibitor SCY-078 is effective in an experimental murine model of invasive candidiasis caused by WT and echinocandin-resistant Candida glabrata

Background

Echinocandins are recommended as first-line therapy against Candida glabrata infections, although increased resistance to this class has been reported worldwide and they are currently only available for parenteral administration. SCY-078 is an investigational glucan synthase inhibitor that is orally available.

Objectives

To evaluate the in vivo efficacy of SCY-078 in an experimental model of invasive candidiasis due to WT and echinocandin-resistant C. glabrata isolates.

Methods

Neutropenic ICR mice were inoculated intravenously with a WT isolate (SCY-078 and caspofungin MICs 0.25 and 0.125 mg/L, respectively) or an echinocandin-resistant isolate (SCY-078 and caspofungin MICs 1 and 0.5 mg/L, respectively). Treatment with placebo, SCY-078 (8, 30 or 40 mg/kg orally every 12 h) or caspofungin (1 mg/kg by intraperitoneal injection once daily) began 24 h later. Kidney fungal burden was measured on day 8 post-inoculation.

Results

Significant reductions in kidney fungal burden were observed with 30 mg/kg SCY-078 against both isolates and with the 40 mg/kg dose against the echinocandin-resistant isolate. These results were supported by SCY-078 plasma concentration data at the higher doses, where levels above the MICs for both isolates were observed 12 h after the last oral dose. Reductions in fungal burden were also observed with caspofungin against the WT isolate, but not against the resistant isolate.

Conclusions

SCY-078 demonstrated in vivo efficacy against infections caused by both WT and echinocandin-resistant C. glabrata isolates in this experimental model. This orally available glucan synthase inhibitor has potential as a therapy against echinocandin-resistant C. glabrata infections.

SCY-078, a Novel Fungicidal Agent, Demonstrates Distribution to Tissues Associated with Fungal Infections during Mass Balance Studies with Intravenous and Oral [14C]SCY-078 in Albino and Pigmented Rats

SCY-078, a fungicidal β-1,3-glucan synthesis inhibitor administered as intravenous or oral [¹⁴C]SCY-078 to rats, was distributed primarily into tissues associated with invasive fungal disease (kidney, lung, liver, spleen, bone marrow, muscle, vaginal tissue, and skin) to levels exceeding those in plasma. Oral fraction absorbed was ∼40%. Elimination was primarily via bile and feces (∼90%) and urine (∼1.5%). Mean half-time was ∼8 h. Quantitative whole-body autoradiography showed a rapid distribution at 8 h and elimination by 168 h postdose.

Antifungal drugs: New insights in research & development

The need for better antifungal therapy is commonly accepted in view of the high mortality rates associated with systemic infections, the low number of available antifungal classes, their associated toxicity and the increasing number of infections caused by strains with natural or acquired resistance. The urgency to expand the range of therapeutic options for the treatment of fungal infections has led researchers in recent decades to seek alternative antifungal targets when compared to the conventional ones currently used. Although new potential targets are reported, translating the discoveries from bench to bedside is a long process and most of these drugs fail to reach the patients. In this review, we discuss the development of antifungal drugs focusing on the approach of drug repurposing and the search for novel drugs for classical targets, the most recently described gene targets for drug development, the possibilities of immunotherapy using antibodies, cytokines, therapeutic vaccines and antimicrobial peptides.

Lack of Impact by SCY-078, a First-in-Class Oral Fungicidal Glucan Synthase Inhibitor, on the Pharmacokinetics of Rosiglitazone, a Substrate for CYP450 2C8, Supports the Low Risk for Clinically Relevant Metabolic Drug-Drug Interactions

SCY‐078, the first in a new class of β 1,3‐glucan synthesis inhibitors, is being developed as an oral and intravenous antifungal treatment for Candida and Aspergillus species fungal infections. In vitro, studies indicated SCY‐078 is an inhibitor of cytochrome P450 (CYP) 2C8 with markedly lower effect over other CYP isozymes. To examine clinically relevant effects of the potential interaction with SCY‐078, this phase 1, open‐label, 2‐period crossover study evaluated the pharmacokinetic parameters of rosiglitazone, a sensitive substrate of CYP2C8 metabolism, in the absence and presence of SCY‐078 dosed to therapeutically relevant SCY‐078 concentration exposure after repeat dosing. Healthy adult subjects were randomized to 2 treatment sequences: a single oral 4‐mg rosiglitazone dose alone on day 1 or a 1250‐mg SCY‐078 loading dose on day 1 followed by a once‐daily 750‐mg SCY‐078 dose for an additional 7 days (reflecting the clinical regimen evaluated during phase 2 studies for infections by Candida species) and concurrent administration of a single oral 4‐mg rosiglitazone dose on day 3, before alternating following a ≥10‐day washout. The exposure to SCY‐078 observed in this study was in line with the intended exposure for treatment of invasive fungal infections. The 90% confidence intervals for rosiglitazone exposure geometric mean ratios were within the prespecified no effect interval of 0.70‐1.43. Additionally, maximum concentration values for rosiglitazone and its metabolite, N‑desmethylrosiglitazone, were not significantly affected by co‐administration with SCY‐078. Overall, rosiglitazone exposure was not impacted to a clinically meaningful extent with co‐administration of therapeutically relevant SCY‐078 concentration exposure after repeat dosing. The results are indicative of low risk for interaction of SCY‐078 with drugs metabolized via the CYP family of enzymes.

The novel oral glucan synthase inhibitor SCY-078 shows in vitro activity against sessile and planktonic Candida spp

Objectives

We studied the antifungal activity of SCY-078 (an orally bioavailable 1,3-β -d- glucan synthesis inhibitor), micafungin and fluconazole against the planktonic and sessile forms of 178 Candida and non- Candida isolates causing fungaemia in patients recently admitted to a large European hospital.

Methods

The in vitro activity of SCY-078, micafungin and fluconazole against the planktonic form of the isolates was assessed using EUCAST EDef 7.3 and CLSI M27-A3. Antibiofilm activity was assessed using the XTT reduction assay.

Results

SCY-078 and micafungin showed potent in vitro activity against Candida and non- Candida isolates. The in vitro activity of both drugs was similar, but SYC-078 displayed significantly lower MIC values than micafungin against Candida parapsilosis and non- Candida isolates, whereas micafungin displayed significantly lower MIC values for the remaining species ( P  <0.001). In contrast, SCY-078 and micafungin showed essentially the same activity against the biofilms with the exception of Candida glabrata , in which the micafungin sessile MIC values were significantly lower ( P  <0.001). These observations were confirmed by assessing biofilm structure by scanning electron microscopy after antifungal treatment.

Conclusions

Our study showed that the high in vitro activity of SCY-078 against invasive Candida isolates in both sessile and planktonic forms is comparable to that of micafungin.