Background: Given data showing efficacy of bendamustine in treating multiple myeloma (MM), with the known toxicity profile significant for myelosuppression, we hypothesized that the addition of bendamustine to autologous stem cell transplant (ASCT) conditioning would enhance response without significant toxicity change. We now report the results of a phase 1 study of bendamustine + melphalan conditioning ASCT in MM.
Methods: 21 patients were enrolled onto a 3+3 phase 1 study of bendamustine added to melphalan 200mg/m2 split on subsequent days at the following dose levels on the indicated days of melphalan: 1) 30 mg/m2 given on day 2; 2) 60 mg/m2 on day 2; 3) 90 mg/m2 on day 2; 4) 60mg/m2 on days 1 and 2; 5) 90 mg/m2 on day 1 and 60mg/m2 on day 2; 6) 125 mg/m2 on day 1 and 100mg/m2 on day 2. Stem cell infusion of > 2 million x 106 CD34+ cells/kg was performed at 24-48 hours after final chemotherapy. Patients received G-CSF with standard supportive care until engraftment, defined as absolute neutrophil count > 500/ml and a platelet count > 20,000/ml. Bone marrow biopsy and skeletal imaging were prior to and 100 days after ASCT to assess baseline and post-ASCT disease status. Disease response evaluation was performed monthly with immunoelectrophoresis and free light chain analysis in accordance with international uniform myeloma reporting criteria.
Results: Twenty-one patients completed ASCT over 6 cohorts: 3 in dose level 1, 6 at dose level 2, and 3 each in dose levels 3, 4, 5, and 6. Median number of days (range) to wbc engraftment and platelet engraftment was 11 (9-13) and 12.5 (10-22), respectively. Toxicities were graded according to the Bearman scale. There was only one Grade 3 toxicity: a patient with respiratory decompensation in the setting of neutropenic fever in cohort 2 (a pulmonary dose limiting toxicity-DLT). Other toxicities noted in patients (%) (Grade 1 / Grade 2 respectively) were Cardiac: 6 (29%) / 2 (10%); Pulmonary 1 (5%) / 1 (5%); CNS 0 / 1 (5%); Stomatitis 15 (71%) / 0; GI 10 (48%) / 1 (5%). The CNS toxicity was narcotic-related psychosis. Eight patients had neutropenic fever, with 3 cases of bacteremia. No transplant-related mortality occurred. At present, there are 18 evaluable Day +100 myeloma responses: disease progression in 5 (24%), stable disease in 1 (6%), partial response 1 (6%), very good partial response in 2 (11%), and complete response in 9 (50%). Four patients (19%) died from MM, all with disease progression at 100 days post-ASCT, and all with pre-existing extramedullary disease.
Summary: Bendamustine added to ASCT conditioning in MM does not exacerbate expected toxicities. The maximum tolerated dose of bendamustine in combination with melphalan 200mg/m2 was not found after a series of 6 treatment cohorts. The relatively high complete response rate and good regimen tolerability has prompted an extension phase 2 trial at the cohort 6 dosing to further evaluate regimen efficacy.