Crystallographic and Electroanalytical Analyses of Fexinidazole and Its Major Metabolites

Fexinidazole, a drug active against trypanosomiasis and leishmaniasis, is a rare example of a nitroaromatic compound approved under the contemporary drug discovery framework. In an earlier study, we showed that the nitro group is absolutely required for antileishmanial activity. The current study employed X-ray crystallography to unveil the structural intricacies of fexinidazole and its principal metabolites, as well as electroanalytical analyses to characterize the reduction properties of the aromatic nitro group. Fexinidazole showcases a predominantly planar geometry with two distinct conformers. While most metrical parameters were conserved between fexinidazole and its metabolites, differences in the methyl ether bridge and <i>S</i>-methyl tail indicated distinctive preferences in molecular arrangement: conformer I of fexinidazole closely resembles the sulfone metabolite, while conformer II aligns with the sulfoxide metabolite. On the other hand, electroanalytical analysis of fexinidazole revealed a pH-dependent, two-step nitro group reduction mechanism, involving an initial concerted transfer of an electron and a proton, followed by the uptake of three electrons and three protons to likely form a hydroxylamine species. These findings characterize the molecular architecture and reduction mechanism of fexinidazole, providing valuable insights into its structural features and activation mechanism required for anti-infective activity.