Screening and Identification of Metacaspase Inhibitors: Evaluation of Inhibition Mechanism and Trypanocidal Activity

Abstract

A common strategy to identify new antiparasitic agents is the targeting of proteases due to their essential contribution to parasite growth and development. Metacaspases (MCAs) are cysteine proteases present in fungi, protozoa and plants. These enzymes, which are associated with crucial cellular events in trypanosomes, are absent in the human host, thus arising as attractive drug targets. To find new MCA inhibitors with trypanocidal activity, we adapted a continuous fluorescent enzymatic assay to a medium-throughput format and carried out screening of different compound collections, followed by the construction of dose-response curves for the most promising hits. We used MCA5 from T. brucei (TbMCA5) as a model for the identification of inhibitors from the GlaxoSmithKline HAT and CHAGAS chemical boxes. We also assessed a third collection of 9 compounds from the Maybridge database identified by virtual screening as potential inhibitors of the cysteine peptidase falcipain-2 (Clan CA) from Plasmodium falciparum. Compound HTS01959 (from the Maybridge collection) was the most potent inhibitor with IC50 of 14.39 μM; also inhibiting other MCAs from T. brucei and T. cruzi (TbMCA2=4.14 μM, TbMCA3=5.04 μM and TcMCA5=151 μM). HTS01959 behaved as a reversible, slow binding and noncompetitive inhibitor of TbMCA2, with a mechanism of action that included redox components. Importantly, HTS01959 displayed trypanocidal activity against bloodstream forms of T. brucei and trypomastigotes forms of T. cruzi, without cytotoxic effect on VERO cells. Thus, HTS01959 is a promising starting point to develop more specific and potent chemical structures to target MCAs.