The future of proteasome inhibitors in relapsed/refractory multiple myeloma.

The ubiquitin-proteasome pathway was first validated as a target for cancer therapy with the demonstration of the activity of the boronic acid proteasome inhibitor (PI) bortezomib (Velcade) against relapsed and relapsed/refractory multiple myeloma. Another generation of PIs is now entering the clinical arena; this includes intravenous agents such as carfilzomib, CEP-18770, and marizomib, and oral drugs such as MLN9708 and ONX 0912. These novel agents will likely first be used for patients with disease that has either relapsed or been refractory to prior therapy (including bortezomib-based regimens) because of their ability to overcome drug resistance, or will be used in patients who are intolerant of, or are not candidates for bortezomib. Preclinical studies also suggest that PIs may act synergistically with other conventional and novel agents, or even with one another in rationally designed combination regimens. In addition, other inhibitors that selectively target only the immunoproteasome and not the constitutive proteasome, as well as agents that bind to noncatalytic proteasome subunits, are emerging as potential drug candidates. Taken together, it seems likely that we have only begun to appreciate the full potential of inhibition of the proteasome. This article extrapolates our current knowledge into an algorithm for the future use of these inhibitors against multiple myeloma.