1,2,4-thiadiazole: a novel Cathepsin B inhibitor

A novel class of Cathepsin B inhibitors has been developed with a 1,2,4-thiadiazole heterocycle as the thiol trapping pharmacophore. Several compounds with different dipeptide recognition sequence (i.e., P1′–P2′=Leu-Pro-OH or P2–P1=Cbz-Phe-Ala) at the C5 position and with different substituents (i.e., OMe, Ph, or COOH) at the C3 position of the 1,2,4-thiadiazole ring have been synthesized and tested for their inhibitory activities. The substituted thiadiazoles 3a–h inhibit Cat B in a time dependent, irreversible manner. A mechanism based on active-site directed inactivation of the enzyme by disulfide bond formation between the active site cysteine thiol and the sulfur atom of the heterocycle is proposed. Compound 3a (K_i=2.6 μM, k_i/K_i=5630 M^?1 s^?1) with a C3 methoxy moiety and a Leu-Pro-OH dipeptide recognition sequence, is found to be the most potent inhibitor in this series. The enhanced inhibitory potency of 3a is a consequence of its increased enzyme binding affinity (lower K_i) rather than its increased intrinsic reactivity (higher k_i). In addition, 3a is inactive against Cathepsin S, is a poor inhibitor of Cathepsin H and is >100-fold more selective for Cat B over papain.