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.