Pharmacokinetics and Pharmacodynamics of Ibrexafungerp

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.

Amphotericin B in the Era of New Antifungals: Where Will It Stand?

Amphotericin B (AmB) has long stood as a cornerstone in the treatment of invasive fungal infections (IFIs), especially among immunocompromised patients. However, the landscape of antifungal therapy is evolving. New antifungal agents, boasting novel mechanisms of action and better safety profiles, are entering the scene, presenting alternatives to AmB's traditional dominance. This shift, prompted by an increase in the incidence of IFIs, the growing demographic of immunocompromised individuals, and changing patterns of fungal resistance, underscores the continuous need for effective treatments. Despite these challenges, AmB's broad efficacy and low resistance rates maintain its essential status in antifungal therapy. Innovations in AmB formulations, such as lipid complexes and liposomal delivery systems, have significantly mitigated its notorious nephrotoxicity and infusion-related reactions, thereby enhancing its clinical utility. Moreover, AmB's efficacy in treating severe and rare fungal infections and its pivotal role as prophylaxis in high-risk settings highlight its value and ongoing relevance. This review examines AmB's standing amidst the ever-changing antifungal landscape, focusing on its enduring significance in current clinical practice and exploring its potential future therapeutic adaptations.

Safety, tolerability, and pharmacokinetics of ibrexafungerp in healthy Chinese subjects: a randomized, double-blind, placebo-controlled phase 1 trial

Ibrexafungerp (code name in China: HS-10366) is a first-in-class and orally active triterpenoid antifungal agent with broad antifungal activity against Candida spp., Aspergillus spp., and other fungal pathogens. It was approved by the U.S. Food and Drug Administration for the treatment of vulvovaginal candidiasis. The study aimed to evaluate the safety, tolerability, and pharmacokinetic (PK) characteristics of oral ibrexafungerp in healthy Chinese adults. A single-center, randomized, double-blind, placebo-controlled single ascending dose (SAD, n = 42), and multiple ascending dose (MAD, n = 28) study was conducted in healthy Chinese subjects from March to October 2022. There were three cohorts in the SAD stage (300, 600, and 1,500 mg) and two cohorts in the MAD stage [450 mg once daily (QD) for 7 days; a loading dose of 750 mg twice daily (BID) for the first 2 days followed by a maintenance dose of 750 mg QD for consecutive 5 days]. Eligible participants in each cohort were randomly assigned in a 6:1 ratio to receive either ibrexafungerp or placebo orally. The primary objectives were to evaluate the safety and tolerability. The secondary objective was to evaluate PK parameters, including Cmax, AUC, and t1/2. A total of 70 healthy Chinese subjects were enrolled in the study. The mean (SD) age was 29.0 (6.32), and 55.7% were male. All treatment-emergent adverse events (TEAEs) were mild or moderate. There were no serious adverse events, and no subjects were discontinued from the study due to TEAEs. All TEAEs were recovered or resolved. The most common TEAEs were diarrhea, abdominal pain, and nausea. In the SAD stage, Cmax, and AUC increased in an approximately dose-proportional manner in the dose range of 300-1,500 mg. The mean t1/2 was within 18.29-21.30 hours. In the MAD stage, an accumulation of exposure (Cmax and AUC) was observed following multiple doses. This phase 1 study demonstrates a favorable safety, tolerability, and PK profile of ibrexafungerp in healthy Chinese subjects.

Treatment of invasive candidiasis in the era of Candida resistance

PURPOSE OF REVIEW

The increasing incidence of drug-resistant Candida brings a new challenge to the treatment of invasive candidiasis. Although cross-resistance among azoles and echinocandins was generally uncommon, reports of multidrug-resistant (MDR) Candida markedly increased in the last decade. The purpose of this review is to understand mechanisms and risk factors for resistance and how to tackle antifungal resistance.

RECENT FINDINGS

The paper describes the action of the three main classes of antifungals - azoles, echinocandins and polyenes - and Candida's mechanisms of resistance. The current evolution from cross-resistance to multiresistance among Candida explains the modern glossary - multidrug-resistant (MDR), extensively drug-resistant (XDR), and pandrug-resistant (PDR) - imported from bacteria. MDR Candida most commonly involves acquired resistance in species with intrinsic resistance, therefore it mostly involves C. glabrata, C. parapsilosis, C. krusei, C guilliermondii or C. auris , which is intrinsically multidrug resistant. Finally, strategies to tackle antifungal resistance became clearer, ideally implemented through antifungal stewardship.

SUMMARY

Avoiding antifungal's overuse and selecting the best drug, dose and duration, when they are needed, is fundamental. Knowledge of risk factors for resistance, microbiological diagnosis to the species, use of susceptibility test supported by antifungal stewardship programs help attaining effective therapy and sustaining the effectiveness of the current antifungal armamentarium.

Structure of a fungal 1,3-β-glucan synthase

1,3-β-Glucan serves as the primary component of the fungal cell wall and is produced by 1,3-β-glucan synthase located in the plasma membrane. This synthase is a molecular target for antifungal drugs such as echinocandins and the triterpenoid ibrexafungerp. In this study, we present the cryo-electron microscopy structure of Saccharomyces cerevisiae 1,3-β-glucan synthase (Fks1) at 2.47-Å resolution. The structure reveals a central catalytic region adopting a cellulose synthase fold with a cytosolic conserved GT-A-type glycosyltransferase domain and a closed transmembrane channel responsible for glucan transportation. Two extracellular disulfide bonds are found to be crucial for Fks1 enzymatic activity. Through structural comparative analysis with cellulose synthases and structure-guided mutagenesis studies, we gain previously unknown insights into the molecular mechanisms of fungal 1,3-β-glucan synthase.

Advances in the treatment of invasive fungal disease

With over 300 million severe cases and 1.5 million deaths annually, invasive fungal diseases (IFDs) are a major medical burden and source of global morbidity and mortality. The World Health Organization (WHO) recently released the first-ever fungal priority pathogens list including 19 fungal pathogens, considering the perceived public health importance. Most of the pathogenic fungi are opportunistic and cause diseases in patients under immunocompromised conditions such as HIV infection, cancer, chemotherapy, transplantation, and immune suppressive drug therapy. Worryingly, the morbidity and mortality caused by IFDs are continuously on the rise due to the limited available antifungal therapies, the emergence of drug resistance, and the increase of population that is vulnerable to IFDs. Moreover, the COVID-19 pandemic worsened IFDs as a globe health threat as it predisposes the patients to secondary life-threatening fungi. In this mini-review, we provide a perspective on the advances and strategies for combating IFDs with antifungal therapies.

N-Myristoyltransferase, a Potential Antifungal Candidate Drug-Target for Aspergillus flavus

The filamentous fungus Aspergillus flavus causes devastating diseases not only to cash crops but also to humans by secreting a series of secondary metabolites called aflatoxins. In the cotranslational or posttranslational process, N-myristoyltransferase (Nmt) is a crucial enzyme that catalyzes the myristate group from myristoyl-coenzyme A (myristoyl-CoA) to the N terminus or internal glycine residue of a protein by forming a covalent bond. Members of the Nmt family execute a diverse range of biological functions across a broad range of fungi. However, the underlying mechanism of AflNmt action in the A. flavus life cycle is unclear, particularly during the growth, development, and secondary metabolic synthesis stages. In the present study, AlfNmt was found to be essential for the development of spore and sclerotia, based on the regulation of the xylose-inducible promoter. AflNmt, located in the cytoplasm of A. flavus, is also involved in modulating aflatoxin (AFB1) in A. flavus, which has not previously been reported in Aspergillus spp. In addition, we purified, characterized, and crystallized the recombinant AflNmt protein (rAflNmt) from the Escherichia coli expression system. Interestingly, the crystal structure of rAlfNmt is moderately different from the models predicted by AlphaFold2 in the N-terminal region, indicating the limitations of machine-learning prediction. In conclusion, these results provide a molecular basis for the functional role of AflNmt in A. flavus and structural insights concerning protein prediction. IMPORTANCE As an opportunistic pathogen, A. flavus causes crop loss due to fungal growth and mycotoxin contamination. Investigating the role of virulence factors during infection and searching for novel drug targets have been popular scientific topics in the field of fungal control. Nmt has become a potential target in some organisms. However, whether Nmt is involved in the developmental stages of A. flavus and aflatoxin synthesis, and whether AlfNmt is an ideal target for structure-based drug design, remains unclear. This study systematically explored and identified the role of AlfNmt in the development of spore and sclerotia, especially in aflatoxin biosynthesis. Moreover, although there is not much difference between the AflNmt model predicted using the AlphaFold2 technique and the structure determined using the X-ray method, current AI prediction models may not be suitable for structure-based drug development. There is still room for further improvements in protein prediction.

Antifungal chemotherapies and immunotherapies for the future

Human fungal pathogens cause a broad plethora of infections, spanning cutaneous dermatophytoses to invasive infections in immunocompromised hosts. As eukaryotic pathogens are capable of morphotype switching, they present unique challenges both for drug development and the immunological response. Whilst current antifungal therapies are limited to the orally available triazoles, intravenous echonocandins and polyenes, and flucytosine and terbinafine, there has been recent significant progress in the antifungal armamentorium with ibrexafungerp, a novel orally available terpanoid that inhibits 1,3-beta-D-glucan-approved by Food and Drug Administration in 2021, and fosmanogepix, an orally available pro-drug of manogepix, which targets glycosylphosphatidylinositol-anchored protein maturation entering Phase 3 studies for candidaemia. A number of further candidates are in development. There has been significant use of existing immunotherapies such as recombinant interferon-γ and G-CSF for fungal disease in immunocompromised patients, and there are emerging opportunities for monoclonal antibodies targeting TH2 inflammation. Omalizumab, an anti-IgE monoclonal antibody in asthma, is now used routinely for the treatment of allergic bronchopulmonary aspergillosis, and further agents targeting IL-4 and IL-5 are being evaluated. In addition, T-cell CAR therapy is showing early promise for fungal disease. Thus, we are likely to see rapid advances to our approach to the management of fungal disease in the near future.

Biosynthetic characterization of the antifungal fernane-type triterpenoid polytolypin for generation of new analogues via combinatorial biosynthesis

Fernane-type triterpenoids are a small group of natural products mainly found in plants and fungi with a wide range of biological activities. Polytolypin is a representative fernane-type triterpenoid from fungi and possesses potent antifungal activity. So far, biosynthesis of fungal-derived fernane-type triterpenoids has not been characterized, which hinders the expansion of their structural diversity using biosynthetic approaches. Herein, we identified the biosynthetic gene cluster of polytolypin and elucidated its biosynthetic pathway through heterologous expression in Aspergillus oryzae NSAR1, which involves a new triterpene cyclase for the biosynthesis of the hydrocarbon skeleton motiol, followed by multiple oxidations via three P450 enzymes. Moreover, two new triterpene cyclases for the biosynthesis of two other fernane-type skeletons isomotiol and fernenol were identified from fungi, and were individually co-expressed with the three P450 enzymes involved in polytolypin biosynthesis. These studies led to the generation of 13 fernane-type triterpenoids including eight new compounds, and two of them showed stronger antifungal activity towards Candida albicans FIM709 than polytolypin.

Ibrexafungerp, a Novel Triterpenoid Antifungal in Development for the Treatment of Mold Infections

Molds are ubiquitous in the environment, and immunocompromised patients are at substantial risk of morbidity and mortality due to their underlying disease and the resistance of pathogenic molds to currently recommended antifungal therapies. This combination of weakened-host defense, with limited antifungal treatment options, and the opportunism of environmental molds renders patients at risk and especially vulnerable to invasive mold infections such as Aspergillus and members of the Order Mucorales. Currently, available antifungal drugs such as azoles and echinocandins, as well as combinations of the same, offer some degree of efficacy in the prevention and treatment of invasive mold infections, but their use is often limited by drug resistance mechanisms, toxicity, drug-drug interactions, and the relative paucity of oral treatment options. Clearly, there is a need for agents that are of a new class that provides adequate tissue penetration, can be administered orally, and have broad-spectrum efficacy against fungal infections, including those caused by invasive mold organisms. Ibrexafungerp, an orally bioavailable glucan synthase inhibitor, is the first in a new class of triterpenoid antifungals and shares a similar target to the well-established echinocandins. Ibrexafungerp has a very favorable pharmacokinetic profile for the treatment of fungal infections with excellent tissue penetration in organs targeted by molds, such as the lungs, liver, and skin. Ibrexafungerp has demonstrated in vitro activity against Aspergillus spp. as well as efficacy in animal models of invasive aspergillosis and mucormycosis. Furthermore, ibrexafungerp is approved for use in the USA for the treatment of women with vulvovaginal candidiasis. Ibrexafungerp is currently being evaluated in clinical trials as monotherapy or in combination with other antifungals for treating invasive fungal infections caused by yeasts and molds. Thus, ibrexafungerp offers promise as a new addition to the clinician's armamentarium against these difficult-to-treat infections.

Updates on Histoplasmosis in Solid Organ Transplantation

Purpose of Review

Recent Findings

Summary

Postantifungal Effect of Antifungal Drugs against Candida: What Do We Know and How Can We Apply This Knowledge in the Clinical Setting?

The study of the pharmacological properties of an antifungal agent integrates the drug pharmacokinetics, the fungal growth inhibition, the fungicidal effect and the postantifungal activity, laying the basis to guide optimal dosing regimen selection. The current manuscript reviews concepts regarding the postantifungal effect (PAFE) of the main classes of drugs used to treat Candida infections or candidiasis. The existence of PAFE and its magnitude are highly dependent on both the fungal species and the class of the antifungal agent. Therefore, the aim of this article was to compile the information described in the literature concerning the PAFE of polyenes, azoles and echinocandins against the Candida species of medical interest. In addition, the mechanisms involved in these phenomena, methods of study, and finally, the clinical applicability of these studies relating to the design of dosing regimens were reviewed and discussed. Additionally, different factors that could determine the variability in the PAFE were described. Most PAFE studies were conducted in vitro, and a scarcity of PAFE studies in animal models was observed. It can be stated that the echinocandins cause the most prolonged PAFE, followed by polyenes and azoles. In the case of the triazoles, it is worth noting the inconsistency found between in vitro and in vivo studies.

Antifungal Resistance and the Role of New Therapeutic Agents

Purpose of Review

Advances in health care over time have led to an evolution in the epidemiology of invasive fungal infections. There is an increasing concern for antifungal resistance and emergence of less common fungal species for which optimal therapies are not well defined. The purpose of this review is to describe mechanisms of antifungal resistance and to evaluate the modern role of new and investigational antifungals.

Recent Findings

Isavuconazole and ibrexafungerp represent the two newest antifungal agents. Evidence from in vivo and in vitro studies has been published recently to help define their place in therapy and potential roles in treating resistant fungi. Isavuconazole is a broad-spectrum triazole antifungal with evidence to support its use in invasive aspergillosis and mucormycosis. Its utility in treating voriconazole-resistant Candida should be confirmed with susceptibility testing if available. Ibrexafungerp is an oral glucan synthase inhibitor with little cross-resistance among currently available antifungals, including echinocandins. It is a promising new agent for invasive candidiasis, including azole-resistant Candida species, and in combination therapy with voriconazole for aspergillosis. Multiple antifungals, some with novel mechanisms, are in development, including rezafungin, oteseconazole, olorofim, fosmanogepix, and opelconazole.

Summary

Both isavuconazole and ibrexafungerp are welcome additions to the arsenal of antifungals, and the prospect of more antifungal options in the future is encouraging. Such an array of antifungals will be important as antifungal resistance continues to expand alongside evolving medical practices. However, managing resistant fungal infections will grow in complexity as the unique role of each new agent is defined.