Design and synthesis of non-peptidic thiosemicarbazone derivatives as inhibitors of cathepsins L and K.

Cathepsin L, a member of the lysosome cysteine proteases, is ubiquitously expressed in tissue and is responsible for protein turnover. An overexpression of cathepsin L by certain tumor types is associated with enhanced tumor expansion through degradation of the extracellular matrix and the invasion and migration of cancer cells. Therefore, the inhibition of cathepsin L has emerged as a therapeutic strategy against metastatic cancer. Cathepsin K is involved in osteoclastic bone resorption because it is selectively expressed in osteoclasts and is capable of degrading bone matrix. In this project, a small library of thiosemicarbazone derivatives containing thiochromanone, 2,3-dihydroquinoline-4-one, and dibenzoylbenzene scaffolds has been successfully designed and synthesized as potential inhibitors of cathepsin L and other homologous cathepsins (B and K). Through a collaborative study, ten compounds from this library were found to be potent inhibitors (IC50 < 300 nM) of cathepsin L, and nine compounds were potent inhibitors of cathepsin K. None of these inhibitors showed activity against cathepsin B. For example, 6,7-difluorothiochromanone thiosemicarbazone 4 (IC50 = 46 nM) was the most potent inhibitor against cathepsin L from this group, while the 6-trifluoromethyl derivative 28 (IC50 = 21 nM) was the most potent inhibitor against cathepsin K. Structure activity relationship (SAR) studies centered on the thiochromanone thiosemicarbazone scaffold demonstrated that electron-withdrawing functionalities, incorporated primarily at the 6-position of the thiochromanone scaffold, showed good inhibition against cathepsin L in comparison to related analogues bearing electron-donating groups. Collectively, these results expand the known SAR regarding molecular structures and their inhibitory activity against cathepsins L and K.