Efficacy of nikkomycin Z in the treatment of murine histoplasmosis

Chitin Biosynthesis in Aspergillus Species

The fungal cell wall (FCW) is a dynamic structure responsible for the maintenance of cellular homeostasis, and is essential for modulating the interaction of the fungus with its environment. It is composed of proteins, lipids, pigments and polysaccharides, including chitin. Chitin synthesis is catalyzed by chitin synthases (CS), and up to eight CS-encoding genes can be found in Aspergillus species. This review discusses in detail the chitin synthesis and regulation in Aspergillus species, and how manipulation of chitin synthesis pathways can modulate fungal growth, enzyme production, virulence and susceptibility to antifungal agents. More specifically, the metabolic steps involved in chitin biosynthesis are described with an emphasis on how the initiation of chitin biosynthesis remains unknown. A description of the classification, localization and transport of CS was also made. Chitin biosynthesis is shown to underlie a complex regulatory network, with extensive cross-talks existing between the different signaling pathways. Furthermore, pathways and recently identified regulators of chitin biosynthesis during the caspofungin paradoxical effect (CPE) are described. The effect of a chitin on the mammalian immune system is also discussed. Lastly, interference with chitin biosynthesis may also be beneficial for biotechnological applications. Even after more than 30 years of research, chitin biosynthesis remains a topic of current interest in mycology.

Emergomycosis, an Emerging Systemic Mycosis in Immunocompromised Patients: Current Trends and Future Prospects

Recently, the global emergence of emergomycosis, a systemic fungal infection caused by a novel dimorphic fungus Emergomyces species has been observed among immunocompromised individuals. Though initially classified under the genus Emmonsia, a taxonomic revision in 2017 based on DNA sequence analyses placed five Emmonsia-like fungi under a separate genus Emergomyces. These include Emergomyces pasteurianus, Emergomyces africanus, Emergomyces canadensis, Emergomyces orientalis, and Emergomyces europaeus. Emmonsia parva was renamed as Blastomyces parvus, while Emmonsia crescens and Emmonsia sola remained within the genus Emmonsia until a taxonomic revision in 2020 placed both the species under the genus Emergomyces. However, unlike other members of the genus, Emergomyces crescens and Emergomyces sola do not cause disseminated disease. The former causes adiaspiromycosis, a granulomatous pulmonary disease, while the latter has not been associated with human disease. So far, emergomycosis has been mapped across four continents: Asia, Europe, Africa and North America. However, considering the increasing prevalence of HIV/AIDS, it is presumed that the disease must have a worldwide distribution with many cases going undetected. Diagnosis of emergomycosis remains challenging. It should be considered in the differential diagnosis of histoplasmosis as there is considerable clinical and histopathological overlap between the two entities. Sequencing the internal transcribed spacer region of ribosomal DNA is considered as the gold standard for identification, but its application is compromised in resource limited settings. Serological tests are non-specific and demonstrate cross-reactivity with Histoplasma galactomannan antigen. Therefore, an affordable, accessible, and reliable diagnostic test is the need of the hour to enable its diagnosis in endemic regions and also for epidemiological surveillance. Currently, there are no consensus guidelines for the treatment of emergomycosis. The recommended regimen consists of amphotericin B (deoxycholate or liposomal formulation) for 1–2 weeks, followed by oral itraconazole for at least 12 months. This review elaborates the taxonomic, clinical, diagnostic, and therapeutic aspects of emergomycosis. It also enumerates several novel antifungal drugs which might hold promise in the treatment of this condition and therefore, can be potential areas of future studies.

Nikkomycin Z-Ready to Meet the Promise?

Nikkomycin Z (NikZ) has fungicidal activity against some fungal species which currently requires patients to endure chronic therapy, sometimes for years. This review highlights reports of NikZ activity against fungal species for which current therapeutics are still inadequate, as a potential roadmap for continuing investigation. The possibility of faster and more complete clinical resolution by using NikZ has attracted scientific attention for decades. NikZ inhibits chitin structure formation, which is important for fungi, but not found in mammals. NikZ raised no safety concerns in a human Phase 1 trial or in extensive toxicology studies. NikZ showed strong clinical benefit in dogs with natural Coccidioides infection. NikZ has protected animals against fatal infections of Candida albicans. NikZ provides high protection in synergistic combination with several agent classes against Candida and Aspergillus species.

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.

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.

Specific enzyme functionalities of Fusarium oxysporum compared to host plants

The genus Fusarium contains some of the most studied and important species of plant pathogens that economically affect world agriculture and horticulture. Fusarium spp. are ubiquitous fungi widely distributed in soil, plants as well as in different organic substrates and are also considered as opportunistic human pathogens. The identification of specific enzymes essential to the metabolism of these fungi is expected to provide molecular targets to control the diseases they induce to their hosts. Through applications of traditional techniques of sequence homology comparison by similarity search and Markov modeling, this report describes the characterization of enzymatic functionalities associated to protein targets that could be considered for the control of root rots induced by Fusarium oxysporum. From the analysis of 318 F. graminearum enzymes, we retrieved 30 enzymes that are specific of F. oxysporum compared to 15 species of host plants. By comparing these 30 specific enzymes of F. oxysporum with the genome of Arabidopsis thaliana, Brassica rapa, Glycine max, Jatropha curcas and Ricinus communis, we found 7 key specific enzymes whose inhibition is expected to affect significantly the development of the fungus and 5 specific enzymes that were considered here to be secondary because they are inserted in pathways with alternative routes.

Antifungal therapeutics for dimorphic fungal pathogens

Dimorphic fungi cause several endemic mycoses which range from subclinical respiratory infections to life-threatening systemic disease. Pathogenic-phase cells of Histoplasma, Blastomyces, Paracoccidioides and Coccidioides escape elimination by the innate immune response with control ultimately requiring activation of cell-mediated immunity. Clinical management of disease relies primarily on antifungal compounds; however, dimorphic fungal pathogens create a number of challenges for antifungal drug therapy. In addition to the drug toxicity issues known for current antifungals, barriers to efficient drug treatment of dimorphic fungal infections include natural resistance to the echinocandins, residence of fungal cells within immune cells, the requirement for systemic delivery of drugs, prolonged treatment times, potential for latent infections, and lack of optimized standardized methodology for in vitro testing of drug susceptibilities. This review will highlight recent advances, current therapeutic options, and new compounds on the horizon for treating infections by dimorphic fungal pathogens.

Regulation of expression, activity and localization of fungal chitin synthases

The fungal cell wall represents an attractive target for pharmacologic inhibition, as many of the components are fungal-specific. Though targeted inhibition of β-glucan synthesis is effective treatment for certain fungal infections, the ability of the cell wall to dynamically compensate via the cell wall integrity pathway may limit overall efficacy. To date, chitin synthesis inhibitors have not been successfully deployed in the clinical setting. Fungal chitin synthesis is a complex and highly regulated process. Regulation of chitin synthesis occurs on multiple levels, thus targeting of these regulatory pathways may represent an exciting alternative approach. A variety of signaling pathways have been implicated in chitin synthase regulation, at both transcriptional and post-transcriptional levels. Recent research suggests that localization of chitin synthases likely represents a major regulatory mechanism. However, much of the regulatory machinery is not necessarily shared among different chitin synthases. Thus, an in-depth understanding of the precise roles of each protein in cell wall maintenance and repair will be essential to identifying the most likely therapeutic targets.

Pharmacokinetics of nikkomycin Z after single rising oral doses

Nikkomycin Z is an antifungal drug that inhibits chitin synthase. This agent is under development as an orphan product for treatment of coccidioidomycosis. Safety and pharmacokinetics of nikkomycin Z were evaluated in healthy male subjects following single, rising oral doses ranging from 250 mg to 2,000 mg. A total of 12 subjects were recruited and divided into two groups. Group 1 (n = 6) received two out of three doses of 250 mg, 1,000 mg, or 1,750 mg and a placebo randomly in place of one of the doses. Group 2 (n = 6) received two out of three doses of 500 mg, 1,500 mg, or 2,000 mg and a placebo in place of one of the doses. Subjects were confined to the study unit overnight prior to dosing, and 12 blood samples were collected over 24 h postdosing while subjects were confined. Subjects returned for additional blood samples and safety evaluations at 48 h and 72 h after each dose. There was a 2-week washout period between doses. Plasma drug concentrations were determined using a validated high-performance liquid chromatography method. Nikkomycin Z was absorbed after oral administration, reaching a maximum concentration in serum of 2.21 microg/ml at 2 h postdose and an area under the concentration-time curve from 0 h to infinity of 11.3 microg x h/ml for the 250-mg dose. Pharmacokinetics appeared linear over the range of 250 to 500 mg; however, relative bioavailability was about 62 to 70% for the 1,000-mg dose and 42 to 47% for doses between 1,500 and 2,000 mg. The mean terminal half-life ranged from 2.1 to 2.5 h and was independent of dose. No serious or dose-related adverse events were observed. This study provides a basis for pharmacokinetic simulations and continued studies of nikkomycin Z administered in multiple doses.

Combinations of antifungal agents in therapy--what value are they?

Concurrent or sequential antifungal treatment for invasive mycoses has been typically considered as an option to improve results of monotherapy. However, data on the efficacy of combination therapy are sparse and consist largely of results from studies in vitro and experimental animal models. These studies have yielded controversial results depending on the criteria used to evaluate the antifungal interaction. Several combinations that showed synergy in vitro failed to do so in animal models. Overall, apart from cryptococcal infections, combined antifungal therapy is not significantly better than monotherapy in terms of clinical efficacy. It is questionable whether combination therapy should be used in most cases as there is a lack of evidence from well-designed clinical trials. However, combination therapy could be an alternative to monotherapy for patients with invasive infections that are difficult to treat, such as those due to multi-resistant species and for those who fail to respond to standard treatment.

New and emerging antifungal agents: impact on respiratory infections

Fungal pathogens are increasingly important causes of respiratory disease, yet the number of antifungal agents available for clinical use is limited. Use of amphotericin B deoxycholate is hampered by severe toxicity. Triazole agents currently available have significant drug interactions; fluconazole has a limited spectrum of activity and itraconazole was, until recently, available only in oral formulations with limited bioavailability. The development of resistance to all three agents is increasingly being recognized and some filamentous fungi are resistant to the action of all of these agents. In the past few years, new antifungal agents and new formulations of existing agents have become available.The use of liposomal amphotericin B preparations is associated with reduced, but still substantial, rates of nephrotoxicity and infusion-related reactions. An intravenous formulation of itraconazole has been introduced, and several new triazole agents have been developed, with the view of identifying agents that have enhanced potency, broader spectra of action and improved pharmacodynamic properties. One of these, voriconazole, has completed large-scale clinical trials. In addition, caspofungin, the first of a new class of agents, the echinocandins, which inhibit cell wall glucan synthesis, was approved for use in the US in 2001 as salvage therapy for invasive aspergillosis. It is hoped that the availability of these agents will have a significant impact on the morbidity and mortality of fungal respiratory infections. However, at the present time, our ability to assess their impact is limited by the problematic nature of conducting trials for antifungal therapy.

Efficacy of ravuconazole in treatment of systemic murine histoplasmosis

Ravuconazole (RCZ) was evaluated for efficacy in comparison to fluconazole (FCZ) and itraconazole (ITZ) in murine models of disseminated histoplasmosis. All regimens tested prolonged survival (P < 0.05 to 0.0001). At equivalent doses of 50 mg/kg of body weight, RCZ and ITZ were equally effective and RCZ was more effective than FCZ (P = 0.02). Clearance of fungal burden from the livers and spleens of mice showed RCZ and ITZ at doses of 50 mg/kg to be efficacious but not curative. These data indicate that RCZ should be studied further.

Update on antifungals targeted to the cell wall: focus on beta-1,3-glucan synthase inhibitors

Currently available antifungal drugs for serious infections are either fungistatic and vulnerable to resistance (azoles) or fungicidal but toxic to the host (polyenes). Cell wall-acting antifungals are inherently selective and fungicidal, features that make them particularly attractive for clinical development. Three classes of such compounds, targeted respectively to chitin synthase (nikkomycins), beta-1,3-glucan synthase (echinocandins) and mannoproteins (pradimicins/benanomicins), have entered clinical development. While nikkomycins and pradimicins/benanomicins are no longer in development, echinocandins have emerged as potentially clinically useful and three compounds, caspofungin (MK-991, L-743,872), micafungin (FK-463) and anidulafungin (LY-303366) are in late clinical development (Phase II and III).

The role of murine models in the development of antifungal therapy for systemic mycoses

Animal testing is crucial to the development of new antifungal compounds. This review describes the role that murine and other animal models have played in the development of three classes of antifungal agents: the polyenes, the triazoles and the echinocandins and the ways in which these models have been either the positive link in the path from in vitro studies to the patient, or have foreclosed later clinical evaluation. Efficacy studies in particular mycoses are discussed, as well as studies designed to determine whether combinations of antifungal drugs may have value over single agents. Copyright 2000 Harcourt Publishers Ltd.

In vitro efficacy of nikkomycin Z against the human isolate of the microsporidian species Encephalitozoon hellem

Since 1985 microsporidia have been recognized as a cause of emerging infections in humans, mainly in immunocompromised human immunodeficiency virus-positive subjects. As chitin is a basic component of the microsporidian infective stage, the spore, we evaluated in vitro the susceptibility of a human-derived strain of Encephalitozoon hellem to nikkomycin Z, a peptide-nucleoside antibiotic known as a competitive inhibitor of chitin synthase enzymes. Transmission electron microscopy showed that this drug, at 25 microgram/ml, reduced the number of parasitic foci by about 35% +/- standard deviation after 7 days of culture (P < 0.0001) and induced cell damage of both mature and immature spores and also other sporogonic and merogonic stages. In particular, an irregular outline of the cell shape and an abnormally condensed cytoplasm in meronts and sporonts were documented. Also, the polar tubule and the polaroplast membranes appeared disarrayed in the sporoblast stage. The spore wall showed an enlarged endospore and delaminated exospore. Mature spores had a complete cytoplasmic disorganization and a swollen and delaminated cell wall. No ultrastructural cell damage was observed in uninfected control cultures treated with the drug.

New investigational antifungal agents for treating invasive fungal infections

Systemic fungal infections have been recognised as a major cause of morbidity and mortality during the last two decades. There are only a few therapeutic options for these infections. Severe toxicity, such as impairment of renal function, limits the use of amphotericin B. Flucytosine is associated with side effects and drug resistance. Fluconazole and itraconazole are safer, though emergence of resistance and innate resistance in some fungal pathogens is a concern in their use. Therefore, there is a need for developing novel drugs and/or treatment strategies to combat these infections. In recent years, increased efforts by the pharmaceutical industry and academia have led to the discovery of new re-engineered or reconsidered antifungal agents that are more efficacious, safer and have a broad spectrum of activity. Lipid formulations of polyene antifungal agents, amphotericin B and nystatin, have the advantage of improved therapeutic index. Activity against resistant fungi, high bioavailability, safety and longer half-life are the properties that encourage development of the newer triazoles (e.g., voriconazole, ravuconazole and posaconazole). Echinocandin-like lipopeptide antibiotics are among the antifungal agents with a novel mode of action. In addition to these lead investigational compounds, development of newer antifungal agents is underway.

New drugs and novel targets for treatment of invasive fungal infections in patients with cancer

Invasive fungal infections have emerged as important causes of morbidity and mortality in profoundly immunocompromised patients with cancer. Current treatment strategies for these infections are limited by antifungal resistance, toxicity, drug interactions, and expense. In order to overcome these limitations, new antifungal compounds are being developed, which may improve our therapeutic armamentarium for prevention and treatment of invasive mycoses in high-risk patients with neoplastic diseases.

Comparison of nikkomycin Z with amphotericin B and itraconazole for treatment of histoplasmosis in a murine model

Nikkomycin Z was tested both in vitro and in vivo for efficacy against Histoplasma capsulatum. Twenty clinical isolates were tested for susceptibility to nikkomycin Z in comparison to amphotericin B and itraconazole. The median MIC was 8 microg/ml with a range of 4 to 64 microg/ml for nikkomycin Z, 0.56 microg/ml with a range of 0.5 to 1.0 microg/ml for amphotericin B, and < or =0.019 microg/ml for itraconazole. Primary studies were carried out by using a clinical isolate of H. capsulatum for which the MIC of nikkomycin Z was greater than or equal to 64 microg/ml. In survival experiments, mice treated with amphotericin B at 2.0 mg/kg/dose every other day (QOD) itraconazole at 75 mg/kg/dose twice daily (BID), and nikkomycin Z at 100 mg/kg/dose BID survived to day 14, while 70% of mice receiving nikkomycin Z at 20 mg/kg/dose BID and none of the mice receiving nikkomycin Z at 5 mg/kg/dose BID survived to day 14. All vehicle control mice died by day 12. Fungal burden was assessed on survivors. Mice treated with nikkomycin Z at 20 and 100 mg/kg/dose BID had significantly higher CFUs per gram of organ weight in quantitative cultures and higher levels of Histoplasma antigen in lung and spleen homogenates than mice treated with amphotericin B at 2.0 mg/kg/dose QOD or itraconazole at 75 mg/kg/dose BID. Studies also were carried out with a clinical isolate for which the MIC of nikkomycin Z was 4 microg/ml. All mice treated with amphotericin B at 2.0 mg/kg/dose QOD; itraconazole at 75 mg/kg/dose BID; and nikkomycin Z at 100, 20, and 5 mg/kg/dose BID survived until the end of the study at day 17 postinfection, while 30% of the untreated vehicle control mice survived. Fungal burden assessed on survivors showed similar levels of Histoplasma antigen in lung and spleen homogenates of mice treated with amphotericin B at 2.0 mg/kg/dose QOD; itraconazole at 75 mg/kg/dose BID; and nikkomycin Z at 100, 20, and 5 mg/kg/dose BID. The three surviving vehicle control mice had significantly higher antigen levels in lung and spleen than other groups (P<0.05). The efficacy of nikkomycin Z at preventing mortality and reducing fungal burden correlates with in vitro susceptibility.

Novel antifungal drugs

There have been many new developments in antifungal therapy in the past few years. Some antifungal drugs have been reformulated to reduce toxicity (e.g. new lipid formulations of polyenes), and new derivatives of drugs have been developed to enhance potencies. The search for unique drug targets will be enhanced by the availability of sequencing data from whole genome sequencing projects.

The 1998 Garrod lecture. Current and future antifungal therapy: new targets for antifungal agents

Invasive fungal infections are a major problem in immunocompromised patients. The recent expansion of antifungal drug research has occurred because there is a critical need for new antifungal agents to treat these life-threatening invasive infections. The overview of the development of antifungal therapy which is provided herein reflects the increased interest in this very special area of infectious diseases. Although we have newer, less toxic, antifungal agents that are available for clinical use, their clinical efficacy in some invasive fungal infections, such as aspergillosis and fusariosis, is not optimal. Thus, intense efforts in antifungal drug discovery are still needed to develop more promising and effective antifungal agents for use in the clinical arena.