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BACKGROUND. CAR T cells are a promising therapy for hematologic malignancies. B cell maturation antigen (BCMA) is a rational target in multiple myeloma (MM). METHODS. We conducted a phase I study of autologous T cells lentivirally transduced with a fully human, BCMA-specific CAR containing CD3ζ and 4-1BB signaling domains (CART-BCMA), in subjects with relapsed/refractory MM. Twenty-five subjects were treated in 3 cohorts as follows: cohort 1, 1 × 108 to 5 × 108 CART-BCMA cells alone; cohort 2, cyclophosphamide (Cy) 1.5 g/m2 plus 1 × 107 to 5 × 107 CART-BCMA cells; cohort 3, Cy 1.5 g/m2 plus 1 × 108 to 5 × 108 CART-BCMA cells. No prespecified BCMA expression level was required. RESULTS. CART-BCMA cells were manufactured and expanded in all subjects. Toxicities included cytokine release syndrome and neurotoxicity, which were grade 3–4 in 8 (32%) and 3 (12%) subjects, respectively, and reversible. One subject died at day 24 from candidemia and progressive myeloma, following treatment for severe cytokine release syndrome and encephalopathy. Responses (based on treated subjects) were seen in 4 of 9 (44%) in cohort 1, 1 of 5 (20%) in cohort 2, and 7 of 11 (64%) in cohort 3, including 5 partial, 5 very good partial, and 2 complete responses, 3 of which were ongoing at 11, 14, and 32 months. Decreased BCMA expression on residual MM cells was noted in responders; expression increased at progression in most. Responses and CART-BCMA expansion were associated with CD4/CD8 T cell ratio and frequency of CD45RO–CD27+CD8+ T cells in the premanufacturing leukapheresis product. CONCLUSION. CART-BCMA infusions with or without lymphodepleting chemotherapy are clinically active in heavily pretreated patients with MM. TRIAL REGISTRATION. {"type":"clinical-trial","attrs":{"text":"NCT02546167","term_id":"NCT02546167"}}NCT02546167. FUNDING. University of Pennsylvania-Novartis Alliance and NIH.