Background: To test the safety and efficacy of the CAR T cell modality in relapsed/refractory multiple myeloma (MM), we have designed a second-generation CAR construct targeting B cell maturation antigen (BCMA) to redirect T cells to MM. bb2121 consists of autologous T cells transduced with a lentiviral vector encoding a novel CAR incorporating an anti-BCMA scFv, a 4-1BB costimulatory motif and a CD3-zeta T cell activation domain. We will report updated safety and efficacy following initial results (Berdeja et al, ENA 2016). Methods: CRB-401 (NCT02658929) is a multi-center phase 1 dose escalation trial of bb2121 in patients with relapsed and/or refractory MM who have received ≥ 3 prior regimens, including a proteasome inhibitor and an immunomodulatory agent, or are double-refractory, and have ≥ 50% BCMA expression on plasma cells. Peripheral blood mononuclear cells are collected via leukapheresis. Patients undergo lymphodepletion with Flu (30 mg/m²) / Cy (300 mg/m²) daily for 3 days then receive 1 infusion of bb2121. The study follows a standard 3+3 design with planned dose levels of 5, 15, 45, 80 and 120 x 10⁷ CAR+ T cells. Results: As of November 18, 2016, 11 patients had been infused with bb2121 in the first 4 dose cohorts, and 9 patients had reached at least 1 month of follow-up. As of data cut-off, no dose-limiting toxicities and no > Grade 2 neurotoxicities or cytokine release syndrome (CRS) had been observed. Grade 1-2 CRS had been reported in 8/11 (73%) treated patients. All patients treated with doses of 15.0 x 10⁷or higher remained on study and the overall response rate (ORR) in the 6 evaluable patients at these doses was 100%, including 2 sCRs and 2 MRD-negative responses (1 sC, 1 VGPR). CAR+ T cell expansion has been demonstrated consistently. An additional 6 months of follow up on previously reported results and initial data from an additional ~10 patients will be presented. Conclusions: bb2121 shows promising efficacy at dose levels above 5 x 10⁷ CAR+ T cells, including 2 sCRs and ongoing clinical responses at 6 months, with mild and manageable CRS to date. These initial data support the potential of CAR T therapy with bb2121 as a new treatment paradigm in MM. Study sponsored by bluebird bio. Clinical trial information: NCT02658929