Inhibition of chitin synthesis in the cell wall of Coccidioides immitis by polyoxin D

Beyond Ergosterol: Strategies for Combatting Antifungal Resistance in Aspergillus fumigatus and Candida auris

Aspergillus fumigatus and Candida auris are historically problematic fungal pathogens responsible for systemic infections and high mortality rates, especially in immunocompromised populations. The three antifungal classes that comprise our present day armamentarium have facilitated efficacious treatment of these fungal infections in past decades, but their potency has steadily declined over the years as resistance to these compounds has accumulated. Importantly, pan-resistant strains of Candida auris have been observed in clinical settings, leaving affected patients with no treatment options and a death sentence. Many compounds in the ongoing antifungal drug discovery pipeline, similar to those within our aforementioned trinity, are predicated on the binding or inhibition of ergosterol. Recurring accounts of resistance to antifungals targeting this pathway suggest optimization of ergosterol-dependent antifungals is likely not the best solution for the long-term. This review aims to present several natural products with novel or underexplored biological targets, as well as similarly underutilized drug discovery strategies to inspire future biological investigations and medicinal chemistry campaigns.

Sources of Antifungal Drugs

Due to their eukaryotic heritage, the differences between a fungal pathogen's molecular makeup and its human host are small. Therefore, the discovery and subsequent development of novel antifungal drugs are extremely challenging. Nevertheless, since the 1940s, researchers have successfully uncovered potent candidates from natural or synthetic sources. Analogs and novel formulations of these drugs enhanced the pharmacological parameters and improved overall drug efficiency. These compounds ultimately became the founding members of novel drug classes and were successfully applied in clinical settings, offering valuable and efficient treatment of mycosis for decades. Currently, only five different antifungal drug classes exist, all characterized by a unique mode of action; these are polyenes, pyrimidine analogs, azoles, allylamines, and echinocandins. The latter, being the latest addition to the antifungal armamentarium, was introduced over two decades ago. As a result of this limited arsenal, antifungal resistance development has exponentially increased and, with it, a growing healthcare crisis. In this review, we discuss the original sources of antifungal compounds, either natural or synthetic. Additionally, we summarize the existing drug classes, potential novel candidates in the clinical pipeline, and emerging non-traditional treatment options.

Advances in Antifungal Drug Development: An Up-To-Date Mini Review

The utility of clinically available antifungals is limited by their narrow spectrum of activity, high toxicity, and emerging resistance. Antifungal drug discovery has always been a challenging area, since fungi and their human host are eukaryotes, making it difficult to identify unique targets for antifungals. Novel antifungals in clinical development include first-in-class agents, new structures for an established target, and formulation modifications to marketed antifungals, in addition to repurposed agents. Membrane interacting peptides and aromatherapy are gaining increased attention in the field. Immunotherapy is another promising treatment option, with antifungal antibodies advancing into clinical trials. Novel targets for antifungal therapy are also being discovered, allowing the design of new promising agents that may overcome the resistance issue. In this mini review, we will summarize the current status of antifungal drug pipelines in clinical stages, and the most recent advancements in preclinical antifungal drug development, with special focus on their chemistry.

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.

Proper Care and Feeding of Coccidioides: A Laboratorian's Guide to Cultivating the Dimorphic Stages of C. immitis and C. posadasii

Coccidioidomycosis ("Valley fever") is caused by Coccidioides immitis and C. posadasii. These fungi are thermally dimorphic, cycling between mycelia and arthroconidia in the environment and converting into spherules and endospores within a host. Coccidioides can cause a broad spectrum of disease that can be difficult to treat. There has been a steady increase in disease, with an estimated 350,000 new infections per year in the United States. With the increase in disease and difficulty in treatment, there is an unmet need to increase research in basic biology and identify new treatments, diagnostics, and vaccine candidates. Here, we describe protocols required in any Coccidioides laboratory, such as growing, harvesting, and storing the different stages of this dimorphic fungal pathogen. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Growth and harvest of liquid mycelia cultures for extractions Alternate Protocol 1: Large-volume growth and harvest of liquid mycelia cultures Basic Protocol 2: Mycelial growth on solid medium Alternate Protocol 2: Maintaining mycelial growth on solid medium Basic Protocol 3: Harvesting and quantification of arthroconidia Alternate Protocol 3: Long-term storage of arthroconidia Basic Protocol 4: Parasitic spherule growth and harvest Alternate Protocol 4: Obtaining endospores from spherules Basic Protocol 5: Intranasal infection of murine models.

An ATP-Dependent Ligase with Substrate Flexibility Involved in Assembly of the Peptidyl Nucleoside Antibiotic Polyoxin

Polyoxin (POL) is an unusual peptidyl nucleoside antibiotic, in which the peptidyl moiety and nucleoside skeleton are linked by an amide bond. However, their biosynthesis remains poorly understood. Here, we report the deciphering of PolG as an ATP-dependent ligase responsible for the assembly of POL. A polG mutant is capable of accumulating multiple intermediates, including the peptidyl moiety (carbamoylpolyoxamic acid [CPOAA]) and the nucleoside skeletons (POL-C and the previously overlooked thymine POL-C). We further demonstrate that PolG employs an ATP-dependent mechanism for amide bond formation and that the generation of the hybrid nucleoside antibiotic POL-N is also governed by PolG. Finally, we determined that the deduced ATP-binding sites are functionally essential for PolG and that they are highly conserved in a number of related ATP-dependent ligases. These insights have allowed us to propose a catalytic mechanism for the assembly of peptidyl nucleoside antibiotic via an acyl-phosphate intermediate and have opened the way for the combinatorial biosynthesis/pathway engineering of this group of nucleoside antibiotics.IMPORTANCE POL is well known for its remarkable antifungal bioactivities and unusual structural features. Actually, elucidation of the POL assembly logic not only provides the enzymatic basis for further biosynthetic understanding of related peptidyl nucleoside antibiotics but also contributes to the rational generation of more hybrid nucleoside antibiotics via synthetic biology strategy.

Coccidioides posadasii contains single chitin synthase genes corresponding to classes I to VII

Coccidioides posadasii is a dimorphic fungal pathogen of humans and other mammals. The switch between saprobic and parasitic growth involves synthesis of new cell walls of which chitin is a significant component. To determine whether particular subsets of chitin synthases (CHSes) are responsible for production of chitin at different stages of differentiation, we have isolated six CHS genes from this fungus. They correspond, together with another reported CHS gene, to single members of the seven defined classes of chitin synthases (classes I-VII). Using Real-Time RT-PCR we show their pattern of expression during morphogenesis. CpCHS2, CpCHS3, and CpCHS6 are preferentially expressed during the saprobic phase, while CpCHS1 and CpCHS4 are more highly expressed during the parasitic phase. CpCHS5 and CpCHS7 expression is similar in both saprobic and parasitic phases. Because C. posadasii contains single members of the seven classes of CHSes found in fungi, it is a good model to investigate the putatively different roles of these genes in fungal growth and differentiation.

Drug interaction studies of a glucan synthase inhibitor (LY 303366) and a chitin synthase inhibitor (Nikkomycin Z) for inhibition and killing of fungal pathogens

The interaction between inhibitors of components of the fungal cell wall, glucan and chitin, was studied in vitro with the respective synthase enzyme inhibitors LY 303366 and nikkomycin Z. With Aspergillus fumigatus synergy was noted for inhibition and killing, and synergistic activity was also noted for some isolates of other species presently regarded as difficult to treat.

Compounds active against cell walls of medically important fungi

A number of substances that directly or indirectly affect the cell walls of fungi have been identified. Those that actively interfere with the synthesis or degradation of polysaccharide components share the property of being produced by soil microbes as secondary metabolites. Compounds specifically interfering with chitin or beta-glucan synthesis have proven effective in studies of preclinical models of mycoses, though they appear to have a restricted spectrum of coverage. Semisynthetic derivatives of some of the natural products have offered improvements in activity, toxicology, or pharmacokinetic behavior. Compounds which act on the cell wall indirectly or by a secondary mechanism of action, such as the azoles, act against diverse fungi but are usually fungistatic in nature. Overall, these compounds are attractive candidates for further development.

The coccidioidal complement fixation and immunodiffusion-complement fixation antigen is a chitinase

Culture filtrates and autolysates of Coccidioides immitis have provided suitable crude antigens for the serodiagnosis and prognosis of coccidioidomycosis. One of these, a heat-labile antigen which participates in the immunodiffusion reaction corresponding to the complement fixation reaction (IDCF), has been characterized as a 110-kDa native protein that, when subjected to reducing conditions and heat, yields a 48-kDa component. The present report provides serologic and biochemical evidence that this antigen is a chitinase. This chitinase, isolated from 48-h culture filtrate of the spherule-endospore-phase C. immitis by affinity adsorption to chitin, formed a line of identity with the IDCF reference antigen and participated in the complement fixation reaction with human serum. It lost its enzymatic as well as antigenic activity when heated, but when not heated it retained its enzymatic activity even when precipitated with coccidiodal antibody present in human serum. This chitinase represents a significant serodiagnostic substance and may be important in the morphogenesis of C. immitis.

Evaluation of nikkomycins X and Z in murine models of coccidioidomycosis, histoplasmosis, and blastomycosis

Nikkomycins X and Z, competitive inhibitors of fungal chitin synthase, were evaluated as therapeutic agents in vitro and in mouse models of coccidioidomycosis, histoplasmosis, and blastomycosis. In vitro, the nikkomycins were found to be most effective against the highly chitinous, dimorphic fungi Coccidioides immitis and Blastomyces dermatitidis, were less effective against yeasts, and were virtually without effect on the filamentous fungus Aspergillus fumigatus. Additionally, by transmission electron microscopy, nikkomycin Z was highly disruptive to the cell wall and internal structure of the spherule-endospore phase of C. immitis in vitro. In vivo, nikkomycin Z was more effective than nikkomycin X, was also found to be superior on a milligram per milligram basis to the majority of azoles tested in the models of coccidioidomycosis and blastomycosis, and was moderately effective in histoplasmosis. A study of the pharmacokinetics in mice showed that nikkomycin Z was rapidly eliminated after intravenous infusion but that absorption after oral administration was sufficiently slow to allow inhibitory levels to persist for more than 2 h. Results of limited toxicology tests suggest that nikkomycin Z was well tolerated at the dosages employed.

The use of flow cytometry to monitor chitin synthesis in regenerating protoplasts of Candida albicans

Flow cytometry was used to monitor chitin synthesis in regenerating protoplasts of the yeast Candida albicans. Comparisons of cells stained with Calcofluor White, a fluorochrome with known affinity for chitin, and cells incubated in the presence of N-[3H]-acetylglucosamine, the precursor substrate for chitin, showed a linear relationship between fluorescence and incorporation of label over time. Changes in both the fluorescence and light scatter of regenerating protoplasts treated with inhibitors of fungal chitin synthase were also quantitated by flow cytometry.

Microtiter method for MIC testing with spherule-endospore-phase Coccidioides immitis

A method was developed for susceptibility testing with spherule-endospore-phase Coccidioides immitis by using a microtiter format. Isolated endospores were used to inoculate wells containing modified Converse medium with various concentrations of azole or nikkomycin antifungal substances which then were sealed with an acetate film. The plate was incubated at 37 degrees C with shaking for 96 h, after which the control wells had visible turbidity and endpoints were discernible. Microscopic examination revealed that both control and treatment wells maintained cells predominantly in the spherule-endospore phase of growth.

Synergistic action of nikkomycins X and Z with papulacandin B on whole cells and regenerating protoplasts of Candida albicans

Combinations of nikkomycin X (NX) or nikkomycin Z (NZ), known inhibitors of chitin synthesis in fungi, together with papulacandin B (PB), an inhibitor of beta-glucan synthesis, were tested for synergistic activity against four isolates of Candida albicans by using the broth microdilution checkerboard technique and a method to assess the regeneration of cell wall material in protoplasts. The construction of isobolograms from the data generated by the checkerboard determinations revealed a synergistic effect for the two classes of compounds against all strains. The combination of NX and PB was more effective than the combination of NZ and PB, perhaps reflecting the lower Ki value of NX. While the presence of NX and NZ reduced chitin synthesis, as determined by staining with calcofluor white and assaying with a microfluorimeter, cells treated with PB demonstrated an increased synthesis of chitin. Protoplast regeneration experiments using similar concentrations of the two classes of compounds resulted in comparable findings. The combination of NX and PB resulted in a greater inhibition of chitin synthesis than did equivalent combinations of NZ and PB. These data suggest that combinations of agents active against cell wall synthesis in fungi may prove more useful as chemotherapeutic agents than such compounds used singly.