Carfilzomib (formerly PR-171) is a novel epoxyketone-based irreversible proteasome inhibitor. In preclinical studies, carfilzomib demonstrated irreversible binding to the proteasome and minimal off-target inhibition of other proteases. In clinical studies carfilzomib has demonstrated substantial antitumor activity in hematologic malignancies while exhibiting a well-tolerated side-effect profile. Painful neuropathy was minimally reported, suggesting a possible advantage over other proteasome inhibitors. With single-agent carfilzomib, dose-limiting toxicity was hematologic and included thrombocytopenia and neutropenia. In patients with relapsed or refractory multiple myeloma, twice-weekly consecutive-day single-agent carfilzomib 20 mg/m(2) for 3 weeks every 28 days, escalating to 27 mg/m(2) the second cycle was associated with a 54% overall response rate in bortezomib-naive patients and a 26% overall response rate in bortezomib and immunomodulatory drug refractory patients.