Design, Synthesis, Molecular Modeling Studies, and Calpain Inhibitory Activity of Novel α-Ketoamides Incorporating Polar Residues at the P1‘-Position

A series of novel alpha-ketoamides incorporating stereoisomeric residues with different electronic properties at the P(1)'-position were synthesized to study the electronic requirements for inhibitor binding to the S(1)'-subsite of calpain I. The results of the study suggested the presence of an acidic amino acid residue at the S(1)'-subsite of calpain I. For example, ester 1a (Cbz-l-Leu-l-Phe-CO-d-Phe-OMe) was over 450-fold more potent than its carboxylic acid derivative 2a (Cbz-l-Leu-l-Phe-CO-d-Phe-OH). Additionally, amidino derivative 3a (Cbz-l-Leu-l-Phe-CONH-d-CH[C(NH)NH(2)]Bn) was about 6000-fold more potent than 2a. Furthermore, 4a (Cbz-l-Leu-l-Phe-CONHCH(2)Bn) was 12-fold less potent than its aza analogue 4b (Cbz-l-Leu-l-Phe-CONHNHBn). The results are consistent with the presence of an acidic amino acid residue at the S(1)'-subsite of calpain I. The acidic amino acid residue was found to be Glu261 via molecular modeling studies.