The
alternative oxidase (AOX) is a ubiquitous terminal
oxidase of plants and many fungi, catalyzing the four-electron reduction of
oxygen to water alongside the
cytochrome-based electron transfer chain. Unlike the classical electron transfer chain, however, the activity of AOX does not generate
adenosine triphosphate but has functions such as thermogenesis and stress response. As it lacks a mammalian counterpart, it has been investigated intensely in pathogenic fungi. However, it is in African trypanosomes, which lack
cytochrome-based respiration in their infective stages, that trypanosome
alternative oxidase (
TAO) plays the central and essential role in their energy metabolism.
TAO was validated as a
drug target decades ago and among the first inhibitors to be identified was
salicylhydroxamic acid (
SHAM), which produced the expected trypanocidal effects, especially when potentiated by coadministration with
glycerol to inhibit anaerobic energy metabolism as well. However, the efficacy of this combination was too low to be of practical clinical use. The
antibiotic ascofuranone (AF) proved a much stronger
TAO inhibitor and was able to cure Trypanosoma vivax
infections in mice without
glycerol and at much lower doses, providing an important proof of concept milestone. Systematic efforts to improve the
SHAM and AF scaffolds, aided with the elucidation of the
TAO crystal structure, provided detailed structure-activity relationship information and reinvigorated the
drug discovery effort. Recently, the coupling of mitochondrion-targeting lipophilic
cations to
TAO inhibitors has dramatically improved drug targeting and trypanocidal activity while retaining target
protein potency. These developments appear to have finally signposted the way to preclinical development of
TAO inhibitors.