Novel aza peptide inhibitors and active-site probes of papain-family cysteine proteases.

Recent characterization of multiple classes of functionalized azapeptides as effective covalent inhibitors of cysteine proteases prompted us to investigate O-acyl hydroxamates and their azapeptide analogues for use as activity-based probes (ABPs). We report here a new class of azaglycine-containing O-acylhydroxamates that form stable covalent adducts with target proteases. This allows them to be used as ABPs for papain family cysteine proteases. A second class of related analogues containing a novel O-acyl hydroxyurea warhead was found to function as covalent inhibitors of papain-like proteases. These inhibitors can be easily synthesized on solid support, which allows rapid optimization of compounds with improved selectivity and potency for a given target enzyme. We present here one such optimized inhibitor that showed selective inhibition of falcipain 1, a protease of the malaria-causing parasite, Plasmodium falciparum.