Antimicrobial activity of manogepix, a first-in-class antifungal, and comparator agents tested against contemporary invasive fungal isolates from an international surveillance programme (2018-2019)

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.

Scedosporiosis and lomentosporiosis: modern perspectives on these difficult-to-treat rare mold infections

SUMMARYAlthough Scedosporium species and Lomentospora prolificans are uncommon causes of invasive fungal diseases (IFDs), these infections are associated with high mortality and are costly to treat with a limited armamentarium of antifungal drugs. In light of recent advances, including in the area of new antifungals, the present review provides a timely and updated overview of these IFDs, with a focus on the taxonomy, clinical epidemiology, pathogenesis and host immune response, disease manifestations, diagnosis, antifungal susceptibility, and treatment. An expansion of hosts at risk for these difficult-to-treat infections has emerged over the last two decades given the increased use of, and broader population treated with, immunomodulatory and targeted molecular agents as well as wider adoption of antifungal prophylaxis. Clinical presentations differ not only between genera but also across the different Scedosporium species. L. prolificans is intrinsically resistant to most currently available antifungal agents, and the prognosis of immunocompromised patients with lomentosporiosis is poor. Development of, and improved access to, diagnostic modalities for early detection of these rare mold infections is paramount for timely targeted antifungal therapy and surgery if indicated. New antifungal agents (e.g., olorofim, fosmanogepix) with novel mechanisms of action and less cross-resistance to existing classes, availability of formulations for oral administration, and fewer drug-drug interactions are now in late-stage clinical trials, and soon, could extend options to treat scedosporiosis/lomentosporiosis. Much work remains to increase our understanding of these infections, especially in the pediatric setting. Knowledge gaps for future research are highlighted in the review.

Management of invasive candidiasis: A focus on rezafungin, ibrexafungerp, and fosmanogepix

Management of invasive fungal infections is challenging with growing antifungal resistance. Broad antifungal use has resulted in greater intrinsic and acquired resistance among Candida spp. It is important for clinicians to recognize the relationship between host susceptibility, site of infection, Candida resistance profiles, specific drug pharmacokinetics and pharmacodynamics, and the role of novel antifungal agents. This narrative review covers the role of rezafungin, ibrexafungerp, and fosmanogepix in the management of invasive candidiasis (IC). The PubMed Database, Embase, and ClinicalTrials.gov were searched between January 2006 and January 2024 using the following terms: rezafungin, CD101, ibrexafungerp, SCY-078, fosmanogepix, APX001, candidemia, and invasive candidiasis. Review articles, prospective clinical trials, and observational studies published in the English language were reviewed. Studies evaluating pharmacology, pharmacokinetics, efficacy, and safety in animals and humans were also reviewed. Promising data continues to emerge in support of novel drug therapies for IC and candidemia. Rezafungin possesses a unique pharmacodynamic profile that might be advantageous compared to other echinocandins, with a practical, once-weekly dosing interval. Ibrexafungerp, currently approved for vulvovaginal candidiasis, has been studied off-label for use in IC and candidemia, and initial data is encouraging. Lastly, fosmanogepix, a mechanistically novel, investigational antifungal agent, may be a potential future option in the management of IC and candidemia. Future research is needed to evaluate the potential use of these agents among diverse patient populations.

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.

Fosmanogepix: The Novel Anti-Fungal Agent's Comprehensive Review of in Vitro, in Vivo, and Current Insights From Advancing Clinical Trials

Fosmanogepix, a prodrug of Manogepix (MGX), is a groundbreaking antifungal agent with broad-spectrum activity against yeasts, including Cryptococcus and Candida, as well as molds. It exhibits effectiveness against drug-resistant strains, such as Candida strains resistant to echinocandins and Aspergillus strains resistant to azoles. Furthermore, fosmanogepix shows activity against pathogens that typically resist other classes of drugs, such as Scedosporium, Lomentospora prolificans, and Fusarium, although its efficacy against Mucorales varies. In animal models, fosmanogepix has demonstrated notable effectiveness against disseminated infections caused by various Candida species, Coccidioides immitis, and Fusarium solani. It has also shown efficacy in pulmonary infection models involving Aspergillus fumigatus, Aspergillus flavus, Scedosporium prolificans, Scedosporium apiospermum, and Rhizopus arrhizus. Clinical trials have revealed excellent oral bioavailability (>90%), enabling a seamless transition between intravenous and oral formulations without compromising blood concentrations. Fosmanogepix exhibits favorable profiles in terms of drug interactions, tolerability, and extensive distribution in various tissues, making it an appealing choice for treating invasive fungal infections. This comprehensive review aims to examine the outcomes of published data on fosmanogepix, encompassing in vitro, in vivo, and clinical investigations.

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.

In vitro activity of manogepix and comparators against infrequently encountered yeast and mold isolates from the SENTRY Surveillance Program (2017-2022)

Manogepix is a potent new antifungal agent targeting the fungal Gwt1 enzyme. Manogepix has previously demonstrated potent in vitro activity against clinical isolates of both Candida (except Candida krusei) and Aspergillus species. This study determined the in vitro activity of manogepix and comparators against a large collection of infrequently encountered yeast and molds. Manogepix demonstrated potent in vitro activity against infrequently encountered yeasts exhibiting elevated MIC values to other drug classes, including Candida spp. (MIC50/90, 0.008/0.12 mg/L), Saprochaete clavata (Magnusiomyces clavatus) (MIC50/90, 0.03/0.06 mg/L), Magnusiomyces capitatus (MICrange, 0.016-0.06 mg/L), Rhodotorula minuta (MIC, 0.016 mg/L), and Rhodotorula mucilaginosa (MIC50/90, 0.03/0.12 mg/L). Similarly, manogepix was active against infrequently encountered mold isolates and strains exhibiting elevated MIC/MEC values to echinocandins, azoles, and amphotericin B, including Coprinopsis cinerea (MEC, 0.004 mg/L), Fusarium spp. (MEC50/90, 0.016/0.06 mg/L), Fusarium (Gibberella) fujikuroi species complex (MEC50/90, 0.016/0.03 mg/L), Lomentospora prolificans (MEC50/90, 0.03/0.06 mg/L), Microascus cirrosus (MEC, 0.008 mg/L), Paecilomyces spp. (MEC50/90, ≤0.008/0.016 mg/L), Pleurostomophora richardsiae (MEC, 0.06 mg/L), Sarocladium kiliense (MEC range, 0.016-0.12 mg/L), and Scedosporium spp. (MEC50/90, 0.03/0.06 mg/L). Manogepix demonstrated potent activity against a majority of the infrequently encountered yeast and mold isolates tested including strains with elevated MIC/MEC values to other drug classes. Additional clinical development of manogepix (fosmanogepix) in difficult-to-treat, resistant fungal infections is warranted.

Update on therapeutic approaches for invasive fungal infections in adults

Invasive fungal infections are increasingly encountered with the expansion of iatrogenic immunosuppression, including not only solid organ and hematopoietic stem cell transplant recipients but also patients with malignancies or autoimmune diseases receiving immunomodulatory therapies, such as Bruton Tyrosine Kinase (BTK) inhibitor. Their attributable mortality remains elevated, part of which is a contribution from globally emerging resistance in both molds and yeasts. Because antifungal susceptibility test results are often unavailable or delayed, empiric and tailored antifungal approaches including choice of agent(s) and use of combination therapy are heterogeneous and often based on clinician experience with knowledge of host's net state of immunosuppression, prior antifungal exposure, antifungal side effects and interaction profile, clinical severity of disease including site(s) of infection and local resistance data. In this review, we aim to summarize previous recommendations and most recent literature on treatment of invasive mold and yeast infections in adults to guide optimal evidence-based therapeutic approaches. We review the recent data that support use of available antifungal agents, including the different triazoles that have now been studied in comparison to previously preferred agents. We discuss management of complex infections with specific emerging fungi such as Scedosporium spp., Fusarium spp., Trichosporon asahii, and Candida auris. We briefly explore newer antifungal agents or formulations that are now being investigated to overcome therapeutic pitfalls, including but not limited to olorofim, rezafungin, fosmanogepix, and encochleated Amphotericin B. We discuss the role of surgical resection or debridement, duration of treatment, follow-up modalities, and need for secondary prophylaxis, all of which remain challenging, especially in patients chronically immunocompromised or awaiting more immunosuppressive therapies.

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.

The combination of manogepix and itraconazole is synergistic and inhibits the growth of Madurella mycetomatis in vitro but not in vivo

Mycetoma is a neglected tropical disease commonly caused by the fungus Madurella mycetomatis. Standard treatment consists of extensive treatment with itraconazole in combination with surgical excision of the infected tissue, but has a low success rate. To improve treatment outcomes, novel treatment strategies are needed. Here, we determined the potential of manogepix, a novel antifungal agent that targets the GPI-anchor biosynthesis pathway by inhibition of the GWT1 enzyme. Manogepix was evaluated by determining the minimal inhibitory concentrations (MICs) according to the CLSI-based in vitro susceptibility assay for 22 M. mycetomatis strains and by in silico protein comparison of the target protein. The synergy between manogepix and itraconazole was determined using a checkerboard assay. The efficacy of clinically relevant dosages was assessed in an in vivo grain model in Galleria mellonella larvae. MICs for manogepix ranged from <0.008 to >8 mg/l and 16/22 M. mycetomatis strains had an MIC ≥4 mg/ml. Differences in MICs were not related to differences observed in the GWT1 protein sequence. For 70% of the tested isolates, synergism was found between manogepix and itraconazole in vitro. In vivo, enhanced survival was not observed upon admission of 8.6 mg/kg manogepix, nor in combination treatment with 5.7 mg/kg itraconazole. MICs of manogepix were high, but the in vitro antifungal activity of itraconazole was enhanced in combination therapy. However, no efficacy of manogepix was found in an in vivo grain model using clinically relevant dosages. Therefore, the therapeutic potential of manogepix in mycetoma caused by M. mycetomatis seems limited.

Itraconazole Loaded Biosurfactin Micelles with Enhanced Antifungal Activity: Fabrication, Evaluation and Molecular Simulation

Itraconazole (ITZ) is a broad-spectrum antifungal for superficial subcutaneous and systemic fungal infections. This study aimed to enhance the antifungal activity of ITZ using surfactin A (SA), a cyclic lipopeptide produced by the SA-producing Bacillus strain NH-100, possessing strong antifungal activity. SA was extracted, and ITZ-loaded SA micelles formulations were prepared via a single-pot rinsing method and characterized by particle size, zeta potential, and infrared spectroscopy. In vitro dissolution at pH 1.2, as well as hemolysis studies, was also carried out. The fabricated formulations were stable and non-spherical in shape, with an average size of about 179 nm, and FTIR spectra depicted no chemical interaction among formulation components. ITZ-loaded micelles showed decreased hemolysis activity in comparison to pure ITZ. The drug released followed the Korsmeyer-Peppas model, having R2 0.98 with the diffusion release mechanism. In an acidic buffer, drug release of all prepared formulations was in the range of 73-89% in 2 h. The molecular simulation showed the outstanding binding and stability profile of the ITZ-SA complex. The aromatic ring of the ITZ mediates a π-alkyl contact with a side chain in the SA. It can be concluded that ITZ-loaded micelles, owing to significant enhanced antifungal activity up to 6-fold due to the synergistic effect of SA, can be a promising drug delivery platform for delivery of poorly soluble ITZ.

Invasive Candida infection: epidemiology, clinical and therapeutic aspects of an evolving disease and the role of rezafungin

INTRODUCTION

Invasive Candida Infections (ICIs) have undergone a series of significant epidemiological, pathophysiological, and clinical changes during the last decades, with a shift toward non-albicans species, an increase in the rate of exogenous infections and clinical manifestations ranging from candidemia to an array of highly invasive and life-threatening clinical syndromes. The long-acting echinocandin rezafungin exhibits potent in-vitro activity against most wild-type and azole-resistant Candida spp. including C.auris.

AREAS COVERED

The following topics regarding candidemia only and ICIs were reviewed and addressed: i) pathogenesis; ii) epidemiology and temporal evolution of Candida species; iii) clinical approach; iv) potential role of the novel long-acting rezafungin in the treatment of ICIs.

EXPERT OPINION

Authors' expert opinion focused on considering the potential role of rezafungin in the evolving context of ICIs. Rezafungin, which combines a potent in-vitro activity against Candida species, including azole-resistant strains and C.auris, with a low likelihood of drug-drug interactions and a good safety profile, may revolutionize the treatment of candidemia/ICI. Indeed, it may shorten the length of hospital stays when clinical conditions allow and extend outpatient access to treatment of invasive candidiasis, especially when prolonged treatment duration is expected.

Novel Antifungals and Aspergillus Section Terrei with Potpourri Susceptibility Profiles to Conventional Antifungals

The epidemiology of invasive fungal infections (IFIs) is currently changing, driven by aggressive immunosuppressive therapy, leading to an expanded spectrum of patients at risk of IFIs. Aspergillosis is a leading cause of IFIs, which usually affects immunocompromised patients. There are a limited number of antifungal medications available for treating IFIs, and their effectiveness is often hindered by rising resistance rates and practical limitations. Consequently, new antifungals, especially those with novel mechanisms of action, are increasingly required. This study assessed the activity of four novel antifungal agents with different mechanisms of activity, namely, manogepix, rezafungin, ibrexafungerp, and olorofim, against 100 isolates of Aspergillus section Terrei, containing amphotericin-B (AmB)-wildtype/non-wildtype and azole-susceptible/-resistant strains, according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) method. In general, all tested agents showed potent and consistent activity against the tested isolates, exhibiting geometric mean (GM) and minimum effective concentration (MEC)/minimum inhibitory concentration (MIC) ranges, respectively, as follows: manogepix (0.048 mg/L, 0.032-0.5 mg/L), rezafungin (0.020 mg/L, 0.016-0.5 mg/L), ibrexafungerp (0.071 mg/L, 0.032-2 mg/L), and olorofim (0.008 mg/L, 0.008-0.032 mg/L). In terms of MIC90/MEC90, olorofim had the lowest values (0.008 mg/L), followed by rezafungin (0.032 mg/L), manogepix (0.125 mg/L), and ibrexafungerp (0.25 mg/L). All the antifungals tested demonstrated promising in vitro activity against Aspergillus section Terrei, including A. terreus as well as azole-resistant and AmB-non-wildtype cryptic species.

Fungal Drug Response and Antimicrobial Resistance

Antifungal resistance is a growing concern as it poses a significant threat to public health. Fungal infections are a significant cause of morbidity and mortality, especially in immunocompromised individuals. The limited number of antifungal agents and the emergence of resistance have led to a critical need to understand the mechanisms of antifungal drug resistance. This review provides an overview of the importance of antifungal resistance, the classes of antifungal agents, and their mode of action. It highlights the molecular mechanisms of antifungal drug resistance, including alterations in drug modification, activation, and availability. In addition, the review discusses the response to drugs via the regulation of multidrug efflux systems and antifungal drug-target interactions. We emphasize the importance of understanding the molecular mechanisms of antifungal drug resistance to develop effective strategies to combat the emergence of resistance and highlight the need for continued research to identify new targets for antifungal drug development and explore alternative therapeutic options to overcome resistance. Overall, an understanding of antifungal drug resistance and its mechanisms will be indispensable for the field of antifungal drug development and clinical management of fungal infections.

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

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

Architecture of the dynamic fungal cell wall

The fungal cell wall is essential for growth and survival, and is a key target for antifungal drugs and the immune system. The cell wall must be robust but flexible, protective and shielding yet porous to nutrients and membrane vesicles and receptive to exogenous signals. Most fungi have a common inner wall skeleton of chitin and β-glucans that functions as a flexible viscoelastic frame to which a more diverse set of outer cell wall polymers and glycosylated proteins are attached. Whereas the inner wall largely determines shape and strength, the outer wall confers properties of hydrophobicity, adhesiveness, and chemical and immunological heterogeneity. The spatial organization and dynamic regulation of the wall in response to prevailing growth conditions enable fungi to thrive within changing, diverse and often hostile environments. Understanding this architecture provides opportunities to develop diagnostics and drugs to combat life-threatening fungal infections.

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

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

Recent Antifungal Pipeline Developments against Candida auris: A Systematic Review

The alarming spread and impact of multidrug-resistant Candida auris infections alongside the limited therapeutic options have prompted the development of new antifungals. These promising agents are currently in different stages of development, offering novel dosing regimens and mechanisms of action. A systematic search in MEDLINE, EMBASE, Web of Science, and Scopus up to 27 June 2022 was conducted to find relevant articles reporting data of in vitro activity and in vivo efficacy of investigational antifungals against C. auris. These included new additions to existing antifungal classes (rezafungin and opelconazole), first-in-class drugs such as ibrexafungerp, manogepix/fosmanogepix, olorofim and tetrazoles (quilseconazole, oteseconazole and VT-1598), as well as other innovative agents like ATI-2307, MGCD290 and VL-2397. From 592 articles retrieved in the primary search, 27 met the eligibility criteria. The most studied agent was manogepix/fosmanogepix (overall MIC₉₀: 0.03 mg/L), followed by ibrexafungerp (overall MIC₉₀: 1 mg/L) and rezafungin (overall MIC mode: 0.25 mg/L), while VT-1598 and ATI-2307 were the least explored drugs against C. auris. All these compounds demonstrated significant improvements in survival and reduction in tissue fungal burden on neutropenic animal models of candidemia due to C. auris. Continual efforts towards the discovery of new treatments against this multidrug-resistant fungus are essential.

Activities of Manogepix and Comparators against 1,435 Recent Fungal Isolates Collected during an International Surveillance Program (2020)

We evaluated the in vitro activity of manogepix and comparator agents against 1,435 contemporary fungal isolates collected worldwide from 73 medical centers in North America, Europe, the Asia-Pacific region, and Latin America during 2020. Of the isolates tested, 74.7% were Candida spp.; 3.7% were non- Candida yeasts, including 27 Cryptococcus neoformans var. grubii (1.9%); 17.1% were Aspergillus spp.; and 4.5% were other molds.

Investigational Antifungal Agents for Invasive Mycoses: A Clinical Perspective

Treatment of invasive fungal infections (IFIs) remains challenging, because of the limitations of the current antifungal agents (ie, mode of administration, toxicity, and drug-drug interactions) and the emergence of resistant fungal pathogens. Therefore, there is an urgent need to expand our antifungal armamentarium. Several compounds are reaching the stage of phase II or III clinical assessment. These include new drugs within the existing antifungal classes or displaying similar mechanism of activity with improved pharmacologic properties (rezafungin and ibrexafungerp) or first-in-class drugs with novel mechanisms of action (olorofim and fosmanogepix). Although critical information regarding the performance of these agents in heavily immunosuppressed patients is pending, they may provide useful additions to current therapies in some clinical scenarios, including IFIs caused by azole-resistant Aspergillus or multiresistant fungal pathogens (eg, Candida auris, Lomentospora prolificans). However, their limited activity against Mucorales and some other opportunistic molds (eg, some Fusarium spp.) persists as a major unmet need.

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 avenues for identification of new antifungal drugs and current challenges

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Invasive candidiasis: Investigational drugs in the clinical development pipeline and mechanisms of action

INTRODUCTION

The epidemiology of invasive Candida infections is evolving. Infections caused by non-albicans Candida spp. are increasing; however, the antifungal pipeline is more promising than ever and is enriched with repurposed drugs and agents that have new mechanisms of action. Despite progress, unmet needs in the treatment of invasive candidiasis remain and there are still too few antifungals that can be administered orally or that have CNS penetration.

AREAS COVERED

The authors shed light on those antifungal agents active against Candida that are in late-stage clinical development. Mechanisms of action and key pharmacokinetic and pharmacodynamic properties are discussed. Insights are offered on the potential future roles of the investigational agents MAT-2203, oteseconazole, ATI-2307, VL-2397, NP-339, and the repurposed drug miltefosine.

EXPERT OPINION

Ibrexafungerp and fosmanogepix have novel mechanisms of action and will provide effective options for the treatment of Candida infections (including those caused by multiresistant Candida spp). Rezafungin, an echinocandin with an extended half-life allowing for once weekly administration, will be particularly valuable for outpatient treatment and prophylaxis. Despite this, there is an urgent need to garner clinical data on investigational drugs, especially in the current rise of azole-resistant and multi-drug resistant Candida spp.

Clinical utility of antifungal susceptibility testing

Invasive fungal diseases cause significant morbidity and mortality, in particular affecting immunocompromised patients. Resistant organisms are of increasing importance, yet there are many notable differences in the ability to both perform and interpret antifungal susceptibility testing compared with bacteria. In this review, we will highlight the strengths and limitations of resistance data of pathogenic yeasts and moulds that may be used to guide treatment and predict clinical outcomes.

In Vitro Antifungal Activity of Manogepix and Other Antifungal Agents against South African Candida auris Isolates from Bloodstream Infections

We determined the susceptibility of South African Candida auris bloodstream surveillance isolates to manogepix, a novel antifungal, and several registered antifungal agents. C. auris isolates were submitted to a reference laboratory between 2016 and 2017. Species identification was confirmed by phenotypic methods. We determined MICs for amphotericin B, anidulafungin, caspofungin, micafungin, itraconazole, posaconazole, voriconazole, fluconazole, and flucytosine using Sensititre YeastOne and manogepix using a modified Clinical and Laboratory Standards Institute broth microdilution method. Clade distribution was determined for a subset of isolates using whole-genome sequencing. Of 394 tested isolates, 357 were resistant to at least 1 antifungal class. The manogepix MIC range was 0.002 to 0.06 μg/mL for 335 isolates with fluconazole monoresistance. Nineteen isolates were resistant to both fluconazole and amphotericin B yet still had low manogepix MICs (range, 0.004 to 0.03 μg/mL). Two isolates from the same patient were panresistant but had manogepix MICs of 0.004 μg/mL and 0.008 μg/mL. Comparing MIC₅₀ values, manogepix was >3-fold more potent than azoles, 4-fold more potent than echinocandins, and 9-fold more potent than amphotericin B. Of 84 sequenced isolates, the manogepix MIC range for 70 clade III isolates was 0.002 to 0.031 μg/mL, for 13 clade I isolates was 0.008 to 0.031 μg/mL, and for one clade IV isolate, 0.016 μg/mL. Manogepix exhibited potent activity against all isolates, including those resistant to more than one antifungal agent and in three different clades. These data support manogepix as a promising candidate for treatment of C. auris infections. IMPORTANCE Since C. auris was first detected in South Africa in 2012, health care-associated transmission events and large outbreaks have led to this pathogen accounting for more than 1 in 10 cases of candidemia. A large proportion of South African C. auris isolates are highly resistant to fluconazole but variably resistant to amphotericin B and echinocandins. There is also an emergence of pandrug-resistant C. auris isolates, limiting treatment options. Therefore, the development of new antifungal agents such as fosmanogepix or the use of new combinations of antifungal agents is imperative to the continued effective treatment of C. auris infections. Manogepix, the active moiety of fosmanogepix, has shown excellent activity against C. auris isolates. With the emergence of C. auris isolates that are pandrug-resistant in South Africa, our in vitro susceptibility data support manogepix as a promising new drug candidate for treatment of C. auris and difficult-to-treat C. auris infections.

Candidemia: Evolution of Drug Resistance and Novel Therapeutic Approaches

Candidemia and invasive candidiasis are the most common healthcare-associated invasive fungal infections, with a crude mortality rate of 25–50%. Candida albicans remains the most frequent etiology, followed by C. glabrata, C. parapsilosis and C. tropicalis. With the exception of a limited number of species (ie: C. krusei, C. glabrata and rare Candida species), resistance to fluconazole and other triazoles are quite uncommon. However, recently fluconazole-resistant C. parapsilosis, echinocandin-resistant C. glabrata and the multidrug resistant C. auris have emerged. Resistance to amphotericin B is even more rare due to the reduced fitness of resistant isolates. The mechanisms of antifungal resistance in Candida (altered drug-target interactions, reduced cellular drug concentrations, and physical barriers associated with biofilms) are analyzed. The choice of the antifungal therapy for candidemia must take into account several factors such as type of patient, presence of devices, severity of illness, recent exposure to antifungals, local epidemiology, organs involvement, and Candida species. The first-line therapy in non-neutropenic critical patient is an echinocandin switching to fluconazole in clinically stable patients with negative blood cultures and azole susceptible isolate. Similarly, an echinocandin is the drug of choice also in neutropenic patients. The treatment duration is 14 days after the first negative blood culture or longer in cases of organ involvement. An early removal of vascular catheter improves the outcome. The promising results of new antifungal molecules, such as the terpenoid derivative ibrexafungerp, the novel echinocandin with an enhanced half-life rezafungin, oteseconazole and fosmanogepix, representative of new classes of antifungals, are discussed.

The Antifungal Pipeline: Fosmanogepix, Ibrexafungerp, Olorofim, Opelconazole, and Rezafungin

The epidemiology of invasive fungal infections is changing, with new populations at risk and the emergence of resistance caused by the selective pressure from increased usage of antifungal agents in prophylaxis, empiric therapy, and agriculture. Limited antifungal therapeutic options are further challenged by drug-drug interactions, toxicity, and constraints in administration routes. Despite the need for more antifungal drug options, no new classes of antifungal drugs have become available over the last 2 decades, and only one single new agent from a known antifungal class has been approved in the last decade. Nevertheless, there is hope on the horizon, with a number of new antifungal classes in late-stage clinical development. In this review, we describe the mechanisms of drug resistance employed by fungi and extensively discuss the most promising drugs in development, including fosmanogepix (a novel Gwt1 enzyme inhibitor), ibrexafungerp (a first-in-class triterpenoid), olorofim (a novel dihyroorotate dehydrogenase enzyme inhibitor), opelconazole (a novel triazole optimized for inhalation), and rezafungin (an echinocandin designed to be dosed once weekly). We focus on the mechanism of action and pharmacokinetics, as well as the spectrum of activity and stages of clinical development. We also highlight the potential future role of these drugs and unmet needs.