Peptidyl α-Ketoamides with Nucleobases, Methylpiperazine, and Dimethylaminoalkyl Substituents as Calpain Inhibitors

A series of peptidyl α-ketoamides with the general structure Cbz-l-Leu-d,l-AA-CONH-R were synthesized and evaluated as inhibitors for the cysteine proteases calpain I, calpain II, and cathepsin B. Nucleobases, methylpiperazine, and dimethylaminoalkyl groups were incorporated into the primed region of the inhibitors to generate compounds that potentially cross the blood−brain barrier. Two of these compounds (Cbz-Leu-d,l-Abu-CONH-(CH₂)₃-adenin-9-yl and Cbz-Leu-d,l-Abu-CONH-(CH₂)₃-(4-methylpiperazin-1-yl) have been shown to have useful concentrations in the brain in animals. The best inhibitor for calpain I was Cbz-Leu-d,l-Abu-CONH-(CH₂)₃-2-methoxyadenin-9-yl (K_i = 23 nM), and the best inhibitor for calpain II was Cbz-Leu-d,l-Phe-CONH-(CH₂)₃-adenin-9-yl (K_i = 68 nM). On the basis of the crystal structure obtained with heterocyclic peptidyl α-ketoamides, we have improved inhibitor potency by introducing a small hydrophobic group on the adenine ring. These inhibitors have good potential to be used in the treatment of neurodegenerative diseases.