Quantitative structure-activity relationships of 1,3,4-thiadiazol-2(3H)-ones and 1,3,4-oxadiazol-2(3H)-ones as human protoporphyrinogen oxidase inhibitors

Protoporphyrinogen oxidase (Protox, EC 1.3.3.4) has attracted great interest during the last decades due to its unique biochemical characteristics and biomedical significance. As a continuation of our research work on the development of new PPO inhibitors, 23 new 1,3,4-thiadiazol-2(3H)-ones bearing benzothiazole substructure were designed and synthesized. The in vitro assay indicated that the newly synthesized compounds 1a-w displayed good inhibition activity against human PPO (hPPO) with Ki values ranging from 0.04 μM to 245 μM. To the knowledge, compound 1a, O-ethyl S-(5-(5-(tert-butyl)-2-oxo-1,3,4-thiadiazol-3(2H)-yl)-6-fluorobenzothiazol-2-yl)carbonothioate, with the Ki value of 40 nM, is so far known as the most potent inhibitor against hPPO. Based on the molecular docking and modified molecular mechanics/Poisson-Boltzmann surface area (MM-PBSA) calculations, the quantitative structure-activity relationships of 1,3,4-thiadiazol-2(3H)-ones and 1,3,4-oxadiazol-2(3H)-one derivatives were established with excellent correlation relationships (r2 = 0.81) between the calculated and experimental binding free energies. Some important insights were also concluded for guiding the future rational design of new hPPO inhibitors with improved potency.