Kinetic Studies of Potent Thiosemicarbazone Inhibitors of Cruzain

Abstract

Cruzain, a cathepsin L‐ and B‐like enzyme, is the major cysteine protease from Trypanosoma cruzi, the causative agent of Chagas' disease which is a major public health problem in Latin America. Previously, we reported a small library of thiosemicarbazone derivatives as inhibitors of cruzain. (Siles, R.; Chen, S‐E.; Zhou, M.; Pinney, K. G.; Trawick, M. L. Bioorg. & Med. Chem. Lett. 2006, 16, –4409). Kinetic data from the two most active compounds in this series, a bromo substitutued α‐tetralone (IC50 17 nM), and a dibromobenzophenone (IC50 24 nM), fit the Williams‐Morrison equation which applies to reversible covalent inhibition as well as slow, tight binding. The Ki values for the two inhibitors are 1.5 and 2.2 nM respectively. Modeling studies support the kinetic results and suggest that, in addition to favorable interaction energies resulting from binding of the tetralone and benzene rings in the S2 pocket of the enzyme, the thiosemicarbazone moiety is in close proximity to the active site Cys25 thiolate of cruzain in the best inhibitors of this series. Several of the thiosemicarbazone compounds from the library inhibit cathepsin B but with a distinct specificity from that of cruzain and cathepsin L. The authors thank the Vice‐Provost for Research of Baylor University (MLT) and the Welch Foundation (Grant No. AA‐1278, KGP) for financial support.