a class of cysteine protease inhibitors which inactivate a cysteine protease by covalently bonding to the protease and releasing a heterocyclic leaving group is presented . the cysteine protease inhibitors of the present invention comprise a first portion which targets a desired cysteine protease and positions the inhibitor near the thiolate anion portion of the active site of the protease , and a second portion which covalently bonds to the cysteine protease and irreversibly deactivates that protease by providing a carbonyl or carbonyl - equivalent which is attacked by the thiolate anion of the active site of the cysteine protease to sequentially cleave a heterocyclic leaving group . the heterocyclic leaving group of the protease inhibitor is of the formula : — o — het , where het is a heterocycle having 4 - 7 atoms in the ring , with at least one of the heterocycle atoms being n , o or s .