Substrate/propeptide‐derived endo‐epoxysuccinyl peptides as highly potent and selective cathepsin B inhibitors

Abstract
Based on recent information about the anti‐substrate binding mode of the propeptide portion of procathepsin B and the well established substrate‐like binding of epoxysuccinyl‐dipeptide carboxylates to the S′ subsites of cathepsin B a new endo‐trans‐epoxysuccinyl peptide was synthesized that contains the dipeptide moiety Leu‐Pro‐OH for the P1′–P2′ substrate positions and the tripeptide moiety Leu‐Gly‐Gly‐OMe (sequence portion 46–48 of the propeptide) for the P2–P4 positions in anti‐substrate orientation. With an unequivocal (2S,3S) configuration this new trans‐epoxysuccinyl peptide derivative was found to inhibit cathepsin B with an apparent second‐order rate constant of 1 520 000 M−1 s−1 which represents so far the most potent inhibitor among E‐64‐derived compounds. Conversely, the (2R,3R) diastereomer exhibited a significantly lower inhibition potency. This observation fully agrees with our previous findings that inhibitor/enzyme interactions at the S subsites are favored by the (2S,3S) and reverse interactions at the S′ subsites by the (2R,3R) configuration of the trans‐epoxysuccinyl moiety.