Human African trypanosomiasis (HAT), a major health concern in sub-Saharan Africa, is caused by the protozoan parasite Trypanosoma brucei. Recent studies have shown that a cathepsin B like protease, TbcatB, is essential to the survival of T. brucei in vitro (Mackey, Z. B.; O'Brien, T. C.; Greenbaum, D. C.; Blank, R. B.; McKerrow, J. H. J. Biol. Chem. 2004, 279, 48426-48433). Herein, we describe the first inhibitors of TbcatB, a series of purine nitriles. The compounds are potent trypanocides, killing the parasite with a high degree of selectivity over a panel of three human cell lines. In addition, a predictive model of trypanocidal activity was developed on the basis of potency against TbcatB and various calculated physical property descriptors.