Uncovering the Unusual Inhibition Mechanism of a Trypanosome Alternative Oxidase Inhibitor Displaying Broad-Spectrum Activity against African Animal Trypanosomes

The glucose-dependent respiration of bloodstream forms of the parasite Trypanosoma brucei depends on an unusual and essential mitochondrial electron-transport system, consisting of glycerol-3-phosphate dehydrogenase and the trypanosome alternative oxidase (TAO). We report here the discovery of an allosteric inhibitor of TAO that displays highly potent activity (EC50 values in the range 1–20 nM) against the important veterinary pathogens T. b. brucei, Trypanosoma evansi, Trypanosoma equiperdum, and Trypanosoma congolense, i.e., >5-fold greater potency than the standard drugs. The methylene-linked 2-methyl-4-hydroxybenzoate 2-pyridinyldiphenylphosphonium derivative (1) was the best inhibitor of recombinant TAO (IC50 = 1.3 nM) via a noncompetitive/allosteric mechanism (Ki = 3.46 nM). Remarkably, X-ray crystallography showed that 1 was bound to a site of TAO ∼25 Å from the catalytic pocket. Although 1 demonstrated good safety toward mammalian cells in vitro (selectivity index >2300), it did not fully clear parasitemia in experimental animals, attributable to a high hepatic clearance.