Discovery of potent calpain inhibitors based on the azolo-imidazolidenone scaffold.

A series of new azolopyrimidine-peptide hybrids and indolomethylideneimidazolones were obtained and evaluated as calpain inhibitors. The hybrid compounds were inactive, whereas some members of the initial azolomethylideneimidazolone series showed interesting calpain inhibitory activity. By using 4b as a hit compound, a new series of analogs were synthesized by an efficient synthetic procedure based on a multicomponent reaction followed by an unprecedented reaction at the methylene position of the molecule. The best inhibitor found for calpain I (IC50 = 20 nM) was about 20 times more potent than the hit compound. Studies on 4b showed that its inhibition is consistent with an uncompetitive inhibition mode. This compound did not exhibit cellular toxicity at any of the doses tested (0.1-10 µM) and further studies indicated that it was capable of blockading chemical ischemia induction of apoptosis by preventing sodium azide-dependent calpain activation in intact human kidney tubular epithelial cells. The results of molecular modeling studies rationalized the inhibitory activity found for this series and account, from a structural point of view, for the most active compound identified (4j).