There are currently only four clinical drugs available for treating human African trypanosomiasis (HAT), three of which were developed over 60years ago. Despite years of effort, there has been relatively little progress towards identifying orally available chemotypes active against the parasite in vivo. Here, we report the lead optimization of a purine-nitrile scaffold that inhibits the essential TbcatB protease and its evaluation in murine models. A lead inhibitor that had potent activity against the trypanosomal protease TbcatB in vitro and cultured parasites ex vivo was optimized by rationally driven medicinal chemistry to an inhibitor that is orally available, penetrates the CNS, has a promising pharmacokinetic profile, and is non-toxic at 200mg/kg in a repeat dosage study. Efficacy models using oral administration of this lead inhibitor showed a significantly increased survival time in Trypanosoma brucei brucei infected mice but little effect on Trypanosoma brucei rhodesiense infected mice