A phase I study of samarium lexidronam/bortezomib combination therapy for the treatment of relapsed or refractory multiple myeloma
Twenty-four patients were enrolled. Median values for age, time since diagnosis, and number of prior treatments were 63 years, 29 months, and three regimens, respectively. The most common toxicities were hematologic; during the first cycle, median neutrophil and platelet nadirs were 1,000/mm3 and 98,500/mm3, respectively, and observed generally 3 to 4 weeks post-treatment. The incidences of grade 4 neutropenia and thrombocytopenia were 12.5% and 8.3%, respectively, during treatment cycle 1. Dose-limiting toxicity, reached in cohort 6 as a result of hematologic toxicity, defined the maximum tolerated dose as 0.5 mCi/kg 153Sm-lexidronam in combination with 1.3 mg/m2 bortezomib. The maximum tolerated dose for 153Sm-lexidronam in combination with the 1.0 mg/m2 bortezomib was not reached. No nonhematologic dose-limiting toxicities were observed; both the incidence and the severity of peripheral neuropathy were low. Responses occurred in 5 (21%) patients, including 3 (12.5%) complete and 2 (8.3%) minimal responses.