Treatment of Patients with T Cells Expressing a Fully-Human Anti-BCMA CAR with a Heavy-Chain Antigen-Recognition Domain Caused High Rates of Sustained Complete Responses and Relatively Mild Toxicity

Multiple myeloma (MM) is a malignancy of plasma cells that is nearly always incurable. T cells expressing chimeric antigen receptors (CAR) that target B-cell maturation antigen (BCMA) can recognize and eliminate MM. The murine or other non-human sequences in the single-chain variable fragments (scFv) of many anti-BCMA CARs can elicit recipient immune responses against CAR T cells. We constructed a CAR incorporating an anti-BCMA fully-human heavy-chain variable domain designated FHVH33. FHVH33 lacks the light chain, the artificial linker sequence, and the 2 linker-associated junctions of a scFv, so FHVH33 is smaller than a scFv and is likely to be less immunogenic. The FHVH33-containing CAR utilized in this clinical trial also incorporated a CD8a hinge and transmembrane domain, a 4-1BB domain, and a CD3z domain. The CAR was designated FHVH33-CD8BBZ and was encoded by a gamma-retroviral vector. T cells expressing FHVH33-CD8BBZ were designated FHVH33-T. The FHVH33-T production process was initiated with unsorted peripheral blood mononuclear cells and took 7 days. The treatment protocol was 300 mg/m 2 of cyclophosphamide and 30 mg/m 2 of fludarabine on days -5 to -3 followed by infusion of FHVH33-T on day 0. Twenty-five patients received FHVH33-T infusions. Median age of the treated patients was 62 (range 39-73). Patients received a median of 6 prior lines of therapy (range 3-10). Five dose levels were assessed (Table). Dose level 4, 6x10 6 CAR + T cells/kg was identified as the maximum feasible dose after considering efficacy and manufacturing factors. Twenty-three of 25 patients (92%) obtained objective responses (OR) of partial response (PR) or better. Seventeen patients (68%) attained a best response of stringent complete response (sCR) or very good partial response (VGPR). Thirteen patients have ongoing responses. To date, the median duration of response is 50 weeks for the highest two dose levels. At present, the overall median progression free survival (PFS) is 78 weeks; as responses are ongoing in 13 patients (52%), PFS will likely improve. Nine of 25 patients had extramedullary plasmacytomas at baseline; patients with extramedullary plasmacytomas at baseline were less likely to achieve sCR (P=0.011). All 25 treated patients were evaluable for toxicity. Eighteen patients had grade 1 or 2 cytokine-release syndrome (CRS), and 6 patients had grade 3 CRS. One patient had no CRS. No patients had grade 4 CRS. Five patients received tocilizumab and 4 patients