Comparative mechanisms of action of proteasome inhibitors

The proteasome has emerged as an important target for therapeutic intervention. In preclinical studies, proteasome inhibitors (PIs) induced apoptosis and inhibited tumor growth, supporting their potential role in the treatment of various tumor types, especially hematologic malignancies. Bortezomib (Velcade), the first clinically validated PI, reversibly binds to the chymotrypsin-like (ChT-L) active sites in the 20S proteasome and potently inhibits cell growth and proliferation in human tumor cell lines and in multiple myeloma (MM) and mantle cell lymphoma. However, the adverse event profile and intravenous administration of bortezomib have underscored the need for the development of PIs with selective actions on different proteasome subunits, which would have different binding kinetics and routes of administration. The most advanced next-generation PI is carfilzomib, an epoxyketone that differs structurally and mechanistically from bortezomib. In preclinical studies, carfilzomib demonstrates sustained inhibition of proteasomal ChT-L activity and greater selectivity than bortezomib. It is thought that the selectivity of carfilzomib for the β5 subunit contributes to its greater cytotoxic response and improved tolerability profile relative to bortezomib. Furthermore, in preclinical studies, carfilzomib did not exhibit the same magnitude of off-target activity against non-proteasomal proteases that is observed with bortezomib. Variations in the binding profiles of some of the next-generation PIs may translate into key differences in pharmacokinetic and toxicity profiles, and thus may be clinically relevant in the treatment of MM.