Synthesis of peptide aldehyde derivatives as selective inhibitors of human cathepsin L and their inhibitory effect on bone resorption

Cathepsin L, a lysosomal cysteine protease, is secreted by osteoclasts and participates in bone collagen degradation. In a search for cathepsin L inhibitors as antiosteoporotic agents, a series of peptide aldehyde derivatives were prepared by two synthetic approaches, DMSO oxidation of the corresponding alcohol derivatives and DIBAL-H reduction of the corresponding N,O-dimethylhydroxylamide derivatives, and evaluated for inhibitory activity against human cathepsin L and for inhibitory effects on bone resorption. Some of the peptide aldehyde derivatives including α-acylamino aldehyde derivatives showed potent activities. Among these compounds, N-(1-naphthalenylsulfonyl-L-isoleucyl-L-tryptophanal (12) was selected as a candidate for further investigation. Compound 12, a potent, selective, and reversible inhibitor of human cathepsin L with an IC 50 of 1.9 nM, inhibited the release of Ca 2+ and hydroxyproline from bone in in vitro bone culture system and also prevented bone loss in ovariectomized mice at an oral dose of 50 mg/kg.