Carbon Dioxide Potentiates Flucytosine Susceptibility in Cryptococcus neoformans

Rapid in vitro evolution of flucytosine resistance in Candida auris

The pan-antifungal-resistant pathogen Candida auris has been causing high-mortality infection outbreaks in hospitals and healthcare settings. The prodrug 5-fluorocytosine (5FC) is one of four chemical entities, but its clinical use as an antifungal drug has been limited owing to pronounced resistance. However, antifungal combination therapy with 5FC appears as a promising strategy for treating C. auris infections. Here, we show that a C. auris clinical isolate can rapidly acquire genetic mutations to mount 5FC resistance after only one to two passages under drug selection. We exploit a new bioinformatics workflow to identify genetic polymorphisms from RNA-seq data. Strikingly, we identify several mutations in the FUR1 gene encoding the 5-fluorouracil convertase that normally generates the active drug. A single nonsense mutation truncates the enzyme at residue Q30*, leading to 5FC resistance due to inactive Fur1. Whole-genome sequencing analysis revealed that an indel mutation in FCY2 also contributes to 5FC resistance. Furthermore, at least one out of seven adapted strains acquired enhanced 5FC tolerance without mutations in the 5FC conversion pathway. Thus, we demonstrate that FUR1 mutations are critical drivers of 5FC resistance in C. auris.IMPORTANCECandida auris is a high-priority human fungal pathogen, causing infection outbreaks of high mortality in healthcare settings. Antifungal combination therapy with 5-fluorocytosine (5FC) is one of the emerging approaches in treatment. However, acquired 5FC resistance traits have been a matter of concern. 5FC is taken up by fungal cells via a cytosine permease and further metabolized by a cytosine deaminase to 5-fluorouracil (5FU). 5FU is then converted by the Fur1 uracil phosphoribosyltransferase into a toxic antimetabolite that disrupts fungal RNA and DNA syntheses. Mutations in these proteins are commonly associated with 5FC resistance in fungal species. Here, we show that C. auris can rapidly develop resistance under 5FC selective stress owing to mutational inactivation of Fur1 function. Moreover, other mechanisms that bypass mutations in the 5FC conversion pathway may also contribute to 5FC resistance traits. Finally, we have developed a tailored bioinformatics workflow that facilitates the identification of polymorphisms associated with 5FC resistance in clinical isolates.

Genome restructuring and lineage diversification of Cryptococcus neoformans during chronic infection of human hosts

Classified as a critical public health threat by the World Health Organization, Cryptococcus neo-formans infections with significant morbidity and mortality. Reports of cryptococcosis persistence, relapse, and reinfection date back to the 1950s, yet the factors driving chronic infections remain poorly understood. A major challenge is the scarcity of serial patient specimens and detailed medical records to study the simultaneous evolution of the pathogen and host health status. This study provides the first genomic and phenotypic analysis of in-host evolution of C. neoformans during chronic infections lasting over a year in six immunocompromised patients. We find fungal genome evolution during persistent infection is characterized by large-scale genome restructuring and increasing genomic heterogeneity. Phenotypic changes show diversification in virulence traits and antifungal susceptibility. Genotypically and phenotypically distinct sub-lineages arise and co-persist within the same tissues, consistent with a model of diversifying selection and niche partitioning in the complex environment of human hosts.

Unsaturated fatty acid perturbation combats emerging triazole antifungal resistance in the human fungal pathogen Aspergillus fumigatus

Contemporary antifungal therapies utilized to treat filamentous fungal infections are inhibited by intrinsic and emerging drug resistance. Consequently, there is an urgent need to develop novel antifungal compounds that are effective against drug-resistant filamentous fungi. Here, we utilized an Aspergillus fumigatus cell-based high-throughput screen to identify small molecules with antifungal activity that also potentiated triazole activity. The screen identified 16 hits with promising activity against A. fumigatus. A nonspirocyclic piperidine, herein named MBX-7591, exhibited synergy with triazole antifungal drugs and activity against pan-azole-resistant A. fumigatus isolates. MBX-7591 has additional potent activity against Rhizopus species and CO2-dependent activity against Cryptococcus neoformans. Chemical, genetic, and biochemical mode of action analyses revealed that MBX-7591 increases cell membrane saturation by decreasing oleic acid content. MBX-7591 has low toxicity in vivo and shows good efficacy in decreasing fungal burden in a murine model of invasive pulmonary aspergillosis. Taken together, our results suggest MBX-7591 is a promising hit with a novel mode of action for further antifungal drug development to combat the rising incidence of triazole-resistant filamentous fungal infections.IMPORTANCEThe incidence of infections caused by fungi continues to increase with advances in medical therapies. Unfortunately, antifungal drug development has not kept pace with the incidence and importance of fungal infections, with only three major classes of antifungal drugs currently available for use in the clinic. Filamentous fungi, also called molds, are particularly recalcitrant to contemporary antifungal therapies. Here, a recently developed Aspergillus fumigatus cell reporter strain was utilized to conduct a high-throughput screen to identify small molecules with antifungal activity. An emphasis was placed on small molecules that potentiated the activity of contemporary triazole antifungals and led to the discovery of MBX-7591. MBX-7591 potentiates triazole activity against drug-resistant molds such as A. fumigatus and has activity against Mucorales fungi. MBX-7591's mode of action involves inhibiting the conversion of saturated to unsaturated fatty acids, thereby impacting fungal membrane integrity. MBX-7591 is a novel small molecule with antifungal activity poised for lead development.

Effects of CO2 in fungi

Carbon dioxide supplies carbon for photosynthetic species and is a major product of respiration for all life forms. Inside the human body where CO2 is a by-product of the tricarboxylic acid cycle, its level reaches 5% or higher. In the ambient atmosphere, ∼.04% of the air is CO2. Different organisms can tolerate different CO2 levels to various degrees, and experiencing higher CO2 is toxic and can lead to death. The fungal kingdom shows great variations in response to CO2 that has been documented by different researchers at different time periods. This literature review aims to connect these studies, highlight mechanisms underlying tolerance to high levels of CO2, and emphasize the effects of CO2 on fungal metabolism and morphogenesis.

5-Flucytosine Longitudinal Antifungal Susceptibility Testing of Cryptococcus neoformans: A Substudy of the EnACT Trial Testing Oral Amphotericin

Background

The EnACT trial was a phase 2 randomized clinical trial conducted in Uganda, which evaluated a novel orally delivered lipid nanocrystal (LNC) amphotericin B in combination with flucytosine for the treatment of cryptococcal meningitis. When flucytosine (5FC) is used as monotherapy in cryptococcosis, 5FC can induce resistant Cryptococcus mutants. Oral amphotericin B uses a novel drug delivery mechanism, and we assessed whether resistance to 5FC develops during oral LNC-amphotericin B therapy.

Methods

We enrolled Ugandans with HIV diagnosed with cryptococcal meningitis and who were randomized to receive 5FC and either standard intravenous (IV) amphotericin B or oral LNC-amphotericin B. We used broth microdilution to measure the minimum inhibitory concentration (MIC) of the first and last cryptococcal isolates in each participant. Breakpoints are inferred from 5FC in Candida albicans. We measured cerebral spinal fluid (CSF) 5FC concentrations by liquid chromatography and tandem mass spectrometry.

Results

Cryptococcus 5FC MIC50 was 4 µg/mL, and MIC90 was 8 µg/mL. After 2 weeks of therapy, there was no evidence of 5FC resistance developing, defined as a >4-fold change in susceptibility in any Cryptococcus isolate tested. The median CSF 5FC concentration to MIC ratio (interquartile range) was 3.0 (1.7-5.5) µg/mL. There was no association between 5FC/MIC ratio and early fungicidal activity of the quantitative rate of CSF yeast clearance (R2 = 0.004; P = .63).

Conclusions

There is no evidence of baseline resistance to 5FC or incident resistance during combination therapy with oral or IV amphotericin B in Uganda. Oral amphotericin B can safely be used in combination with 5FC.