Aza-peptide Michael acceptors and epoxides with the general structure of YCO-Ala-Ala-AAsn-trans-CH horizontal lineCHCOR and YCO-Ala-Ala-AAsn-EP-COR, respectively, are shown to be potent inhibitors of asparaginyl endopeptidases (legumains) from the bloodfluke, Schistosoma mansoni (SmAE), and the hard tick, Ixodes ricinus (IrAE). Structure-activity relationships (SARs) were determined for a set of 41 aza-peptide Michael acceptors and eight aza-peptide epoxides. Both enzymes prefer disubstituted amides to monosubstituted amides in the P1' position, and potency increased as we increased the hydrophobicity of the inhibitor in this position. Extending the inhibitor to P5 resulted in increased potency, especially against IrAE, and both enzymes prefer small over large hydrophobic residues at P2. Aza-peptide Michael acceptor inhibitors are more potent than aza-peptide epoxide inhibitors, and for some of these compounds, second-order inhibiton rate constants are the fastest yet discovered. Given the central functions of these enzymes in both parasites, the data presented here may facilitate the eventual design of selective antiparasitic drugs.

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 bloodbrain barrier. Two of these compounds (Cbz-Leu-D,L-Abu-CONH-(CH 2 ) 3 -adenin-9-yl and CbzLeu-D,L-Abu-CONH-(CH 2 ) 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-(CH 2 ) 3 -2-methoxyadenin-9-yl (K i = 23 nM) and the best inhibitor for calpain II was CbzLeu-D,L-Phe-CONH-(CH 2 ) 3 -adenin-9-yl (K i = 68 nM). Based on 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.