Remissions of Multiple Myeloma during a First-in-Humans Clinical Trial of T Cells Expressing an Anti-B-Cell Maturation Antigen Chimeric Antigen Receptor

B-cell maturation antigen (BCMA) is a protein expressed by normal and malignant plasma cells. We are conducting a phase I clinical trial of an anti-BCMA chimeric antigen receptor (CAR-BCMA) that incorporates an anti-BCMA single-chain variable fragment, a CD28 domain, and a CD3-zeta T-cell activation domain (Carpenter et al. Clinical Cancer Research 2013). Autologous T cells are genetically modified to express the CAR with a gamma-retroviral vector. Patients receive a single infusion of CAR-BCMA T cells. Before the CAR T-cell infusions, patients receive a chemotherapy regimen of 300 mg/m² of cyclophosphamide and 30 mg/m² of fludarabine with each chemotherapy agent given daily for 3 days. The purpose of the chemotherapy is to enhance activity of the CAR T cells by depleting endogenous leukocytes. Twelve patients have been enrolled, and 11 patients have been treated on one of 4 dose levels, 0.3x10⁶, 1x10⁶, 3x10⁶, and 9x10⁶CAR+ T cells/kg of bodyweight. Patients had advanced multiple myeloma (MM) with a median of 7 prior lines of therapy.

Of the 6 patients treated on the lowest 2 dose levels, one patient had a transient partial remission (PR) of 2 weeks duration; the other 5 patients had responses of stable disease (SD). On the 3^rddose level, 2 patients obtained responses of stable disease, and one patient obtained a response of very good PR (VGPR) with complete elimination of MM bone disease on positron emission tomography (PET) scan, normalization of serum free light chains, and clearance of bone marrow plasma cells. Toxicity among patients on the first 3 dose levels was mild and included cytopenias attributable to chemotherapy, fever in 3 patients, and signs of cytokine release syndrome including tachycardia and hypotension in Patient 8 who had a VGPR.

Two patients have been treated on the highest dose level of 9x10⁶CAR+ T cells/kg. The first patient on this dose level, Patient 10, had MM making up 90% of total bone marrow cells before treatment. Starting 4 hours after infusion of CAR T cells, Patient 10 exhibited signs of cytokine release syndrome including fever, tachycardia, dyspnea, acute kidney injury, coagulopathy, hypotension requiring vasopressor support, and muscle damage manifesting as an elevated creatine kinase level and weakness. His neutrophil count was less than 500/µL before the CAR-BCMA T-cell infusion and remained below 500/µL for 40 days after the CAR T-cell infusion before recovering. He also experienced prolonged thrombocytopenia. Patient 10’s myeloma was rapidly eliminated after CAR-BCMA T-cell infusion. By immunohistochemistry staining for CD138, bone marrow plasma cells decreased from 90% before treatment to 0% one month after the CAR T-cell infusion. The serum M-protein decreased from 1.6 g/dL before treatment to undetectable 2 months after treatment. The serum and urine immunofixation electrophoresis tests were negative 2 months after the CAR T-cell infusion. Patient 10’s current myeloma response is stringent complete remission.

The second patient treated on the 9x10⁶CAR+ T cells/kg dose level, Patient 11, had IgG lambda MM with 80% bone marrow plasma cells before treatment. Patient 11 experienced signs of cytokine release syndrome with toxicities including fever, tachycardia, hypotension, delirium, hypoxia, and coagulopathy. Patient 11’s M-protein decreased from 3.6 g/dL before treatment to 0.8 g/dL 4 weeks after treatment. His serum lambda free light chain decreased from 95.9 mg/dL before treatment to 0.15 mg/dL 4 weeks after treatment. Four weeks after CAR T-cell infusion, bone marrow plasma cells were undetectable.

T cells containing the CAR-BCMA gene were detected in the blood of all 10 patients evaluated with peak levels of 0.04 to 18.2% of blood mononuclear cells. Patient 10 had the highest peak absolute number of blood CAR T cells with 51 CAR+ T cells/µL. Blood levels of IL-6 and other inflammatory cytokines were highest in patients with clinical signs of cytokine release syndrome, and the 3 patients with the highest serum IL-6 levels also had the most impressive anti-myeloma responses. Before treatment, the mean serum BCMA level of treated patients was 243 ng/mL. In responding patients, serum BCMA levels decreased after treatment. Toxicities in patients receiving CAR-BCMA T cells were similar to toxicities in leukemia patients treated with anti-CD19 CAR T cells. Our findings demonstrate strong anti-myeloma activity in the first clinical trial of a CAR targeting BCMA.