Pharmacokinetics-directed Intravenous Busulfan Combined With High-dose Melphalan and Bortezomib as a Conditioning Regimen for Patients With Multiple Myeloma

Background: High-dose chemotherapy followed by autologous stem cell transplantation (ASCT) has a well-established role in the treatment of patients with multiple myeloma. Melphalan 200 mg/m² (Mel200) is the most commonly used preparative regimen. Several studies have provided evidence for potential synergism and safety when combining bortezomib (Btz) or busulfan (Bu) with melphalan (Mel). Patients and Methods: We conducted a prospective phase II study to investigate the safety and efficacy of conditioning with pharmacokinetics (PK)-directed intravenous (IV) Bu with Btz and Mel. Bu dosing was adjusted to target a total area under the curve (AUC) of 20,000 μM × min. Patients received Btz (1 mg/m² × 4 doses) and Mel (140 mg/m²). Results: A total of 19 subjects were enrolled. Their median age was 55 years, and the median follow-up period was 23.7 months. PK testing resulted in 86% of patients achieving an estimated total AUC of 20,000 ± 2500 μM × min. The overall response rate (ORR) at day +100 after ASCT was 100% in the evaluable patients, with 11% of patients achieving a complete response. The 2-year progression-free survival rate was 57.9% (95% confidence interval [CI], 38%-89%), and the 2-year overall survival rate was 88.5% (95% CI, 76%-100%). The most common grade 3 and 4 toxicities were febrile neutropenia, dysphagia/odynophagia, and oral mucositis. No case of hepatic sinusoidal obstruction syndrome developed. One treatment-related mortality occurred before day +100. Conclusion: A preparative regimen of PK-directed IV Bu with Btz and Mel led to an ORR of 100% with acceptable toxicity and should be considered for direct comparison with the Mel200 regimen in future trials.