The riboflavin biosynthetic pathway as a novel target for antifungal drugs against Candida species

Abstract

In recent decades, there has been an increase in the occurrence of fungal infections, yet the arsenal of drugs available to fight invasive infections remains very limited. The development of new antifungal agents is hindered by the restricted number of molecular targets that can be exploited, given the shared eukaryotic nature of fungi and their hosts which often leads to host toxicity. In this paper, we examine the riboflavin biosynthetic pathway as a potential novel drug target. Riboflavin is an essential nutrient for all living organisms. Its biosynthetic pathway does not exist in humans, who obtain riboflavin through their diet. Our findings demonstrate that all enzymes in the pathway are essential for Candida albicans, Candida glabrata, and Saccharomyces cerevisiae. Among these enzymes, Rib1 and Rib3 are the most promising targets. Auxotrophic strains, which mimic a drug targeting the biosynthesis pathway, experience rapid mortality in the absence of supplemented riboflavin. Nevertheless, the cells can still take up external riboflavin when supplemented. We identified Orf19.4337 as the riboflavin importer in C. albicans and named it Rut1. We found that Rut1 only facilitates growth at external riboflavin concentrations that exceed the physiological concentrations in the human body, making it unlikely that riboflavin uptake to act as a potential resistance mechanism for a drug targeting the biosynthesis pathway. Interestingly, the uptake system in S. cerevisiae is more effective than in C. albicans and C. glabrata, enabling an auxotrophic S. cerevisiae strain to outcompete an auxotrophic C. albicans strain in lower riboflavin concentrations. ImportanceCandida species are a common cause of invasive fungal infections. Candida albicans, in particular, poses a significant threat to immunocompromised individuals. This opportunistic pathogen typically lives as a commensal on mucosal surfaces of healthy individuals, but it can also cause invasive infections associated with high morbidity and mortality. Currently, there are only three major classes of antifungal drugs available to treat these infections. Additionally, the efficacy of these antifungal agents is restricted by host toxicity, suboptimal pharmacokinetics, a narrow spectrum of activity, intrinsic resistance of fungal species, such as Candida glabrata, to certain drugs, and the acquisition of resistance over time. Therefore, it is crucial to identify new antifungal drug targets with novel modes of action to add to the limited armamentarium.