The invention provides compounds and methods for inhibiting proteases. One aspect of the invention features pro-soft inhibitors which react with an activating protease to release an active inhibitor moiety in proximity to a target protease. In certain instances, compounds inhibit proteasomes and/or post-proline cleaving enzymes (PPCE), such as dipeptidyl peptidase IV. The compounds of the invention provide a better therapeutic index, owing in part to reduced toxicity and/or improved specificity for the targeted protease.