Mechanism of action of potent boron-containing antifungals

Background: Boron is unusual to organic chemists, yet boron interacts greatly with organic biochemicals and has considerable bioactivity, especially as an antifungal and insecticide. The best-known bioactive boron compounds are boric acid, its salt borax, and the closely related boronic acids. A newcomer is tavaborole (trade name Kerydin), recently developed and approved in 2014 for topical treatment of onychomycosis, a fungal infection of nails and the nail bed. It is timely to review the literature and explore the way in which these compounds may work. Methods: The focus of this review is to examine peer-reviewed literature relating to boric acid, boronic acid and tavaborole, the most bioactive boron-containing compounds, and the evidence for their pro-posed mechanism of antifungal action. In parallel with the literature, we have examined the fungistatic effects of boric acid on yeast. Results: All three compounds are reported to inhibit protein synthesis but their mechanism of action may differ. Chemistry studies indicate an interaction of boric acid with ribose and ribose-containing moieties such as NAD. In this review, we discuss the activity of boric acid and use both tavaborole and the boronic acids to exemplify the similar underlying mechanisms used. As there is a push to develop new antimicrobials, we demonstrate that boric acid’s fungistatic effect is alleviated with ribose, NAD and tryptophan. Conclusion: We speculate that boric acid inhibits yeast growth by disrupting tryptophan synthesis as well as downstream NAD, a rate limiting co-enzyme, essential for cellular function.