The Food and Drug Administration (FDA) approved the first proteasome inhibitor (bortezomib; Velcade®) in 2003 for the treatment of patients with multiple myeloma (MM) and subsequently for treatment of patients with mantle cell lymphoma in 2006. Although this compound has demonstrated efficacy in both of those indications, resistance and toxicity develop with continued therapy. Resistance may result from a variety of mechanisms. Bortezomib toxicity (primarily neurological with peripheral neuropathy and neuralgia, and also thrombocytopenia and neutropenia) can result in treatment discontinuation (about 25% of patients in a clinical trial conducted in patients at time of first relapse required cessation of therapy due to adverse events).