A phase I/II trial was conducted to explore the safety and activity of the addition of bortezomib on days -6, -3, and +1 relative to the day of autologous stem cell transplantation (ASCT) to a conditioning regimen with busulfan and melphalan (BuMel; 3.2 mg/kg/day busulfan on days -5 to -3 and 140 mg/m2/day melphalan on day -2) in patients with multiple myeloma (MM) following bortezomib-based induction chemotherapy. In phase I, doses of bortezomib (.7, 1.0, and 1.3 mg/m2) with BuMel were administered to groups of 3 patients each. No dose-limiting toxicities were observed. The maximum tolerated dose of bortezomib was 1.3 mg/m2/day. A subsequent cohort with 41 patients was analyzed in a phase II trial to identify safety and efficacy. The phase II trial showed a 75% response rate, including very good partial response (VGPR) or better, and a 55% rate of complete response (CR) at 3 months; For post-transplantation best response, an 83% rate of VGPR or better (68% CR) was observed. With a median follow-up of 31.4 months, the median progression-free survival (PFS) was 26.8 months. The probability of 2 year-PFS was 56.5%, and median overall survival (OS) could not calculated. Specifically, high-risk cytogenetics were associated with adverse survival outcomes compared with standard-risk cytogenetics (median PFS, 12.2 months versus 35.7 months, P = .039; median OS, 26.7 months versus 73.3 months; P = .086). With a median of 11 days to neutrophil engraftment and 10 days for platelet engraftment, no graft failure or delayed engrafting were observed. The most common grade 3 or severe nonhematologic adverse events included neutropenic fever (73.2%) and stomatitis (14.6%). Except for 3 patients with transplantation-related mortality due to sepsis, other adverse events were manageable. These findings demonstrate that bortezomib is safe and has a potential role in conditioning regimens in combination with BuMel for patients with transplantation-eligible MM.
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