Phase 1 Study of CART-Ddbcma, a CAR-T Therapy Utilizing a Novel Synthetic Binding Domain for the Treatment of Subjects with Relapsed and Refractory Multiple Myeloma

Introduction: Chimeric Antigen Receptor (CAR) T cell therapies directed against B-cell maturation antigen (BCMA) have demonstrated compelling clinical activity and manageable safety in subjects with relapsed and refractory Multiple Myeloma (RRMM). These CAR T cells encode humanized or murine scFvs, or camelid heavy chain antibody fragments with CD3-zeta in combination with 41BB or CD28 co-stimulatory domains. In contrast, CART-ddBCMA is an anti-BCMA investigational CAR T cell therapy encoding a non-scFv, synthetic binding domain targeting BCMA with a 4-1BB costimulatory motif and CD3-zeta T cell activation domain. The binding domain is a small stable protein comprising 73 amino acids engineered to reduce the risk of immunogenicity. CART-ddBCMA is being studied as part of a Master Phase 1 Cell Therapy protocol for RRMM and is a first-in-human clinical study to assess the safety, pharmacokinetics, immunogenicity, efficacy, and duration of effect.

Methods: ARC-101 (NCT04155749), ARM 1 (CART-ddBCMA) is a Phase 1, multi-center, open label, dose escalation trial enrolling approximately 12 subjects with RRMM who have received ≥ 3 prior regimens, including a proteasome inhibitor, an immuno-modulatory agent, and a CD38 antibody or are triple-refractory. There is no prescreening or requirement for BCMA expression on tumor cells. Peripheral blood mononuclear cells are collected via leukapheresis and sent to a central facility for selection, transduction, and expansion on the CliniMACS Prodigy® system. The drug product is cryopreserved and undergoes release testing prior to being returned to the site for infusion. Subjects undergo lymphodepletion with fludarabine (30 mg/m²) and cyclophosphamide (300 mg/m²) daily for 3 days, then receive CART-ddBCMA as a single infusion. Planned dose levels are 100, 300, and 900 x 10⁶ CAR+ T cells. The primary outcome measure is incidence of adverse events (AEs), including dose-limiting toxicities (DLTs). Additional outcome measures are quality and duration of clinical response assessed according to the IMWG Uniform Response Criteria for MM, evaluation of minimal residual disease (MRD), progression-free and overall survival, and quantification of CAR+ cells in blood.

Results: As of July 1, 2020, 4 subjects (median age 73.5 [min;max 73 to 75]) were enrolled and 4 received CART-ddBCMA. Median follow-up after CART-ddBCMA infusion was 100 days (min:max 9 to 142 days), 3 subjects were evaluable for initial safety and clinical response and 1 subject was pending assessment. All subjects received a dose of 100 x 10⁶ CAR+ T cells±20%, median drug product CAR+ expression was 76% (min:max 72-78%) of total CD3+ T cells. Subjects had a median of 5 (min;max 5 to 7) prior lines of therapy and one had prior autologous stem cell transplant; one had high-risk cytogenetics. All 4 subjects had previously received Bort/Len/Car/Pom/Dara and 2 were penta-refractory. Three subjects had high tumor burden, with 95, 95, and 70% bone marrow plasma cells pre-infusion, respectively. Three subjects developed Grade 2 cytokine release syndrome (CRS) and 1 subject developed Grade 2 ICANS. These adverse effects resolved quickly after intervention; 3 subjects received tocilizumab and 2 received steroids (dexamethasone). All 3 evaluable subjects have demonstrated clinical response per IMWG criteria: currently 1 sCR (MRD-10^-4), 1 sCR, 1 sCR (MRD-10^-6). MRD negative results were obtained by next-generation sequencing (Adaptive clonoSEQ), 1 subject did not have baseline bone marrow involvement. Extramedullary disease resolved in three subjects. CAR+ T cell expansion during the first 30 days was observed in evaluable subjects by ddPCR. No post treatment ADA were detected in the first 3 subjects, through M1.

Conclusions: In the initial cohort receiving 100 x 10⁶ CAR+ T cells of CART-ddBCMA, no subjects experienced severe CRS and/or ICANs. Early efficacy results are encouraging, with all 3 evaluable subjects demonstrating deep clinical responses of sCR, with 2 MRD negative bone marrow responses at 1 month. No evidence of ADA has been detected to date. These data are encouraging in a small group of elderly subjects who did not initially receive autologous transplant following induction therapy. Subjects continue to be enrolled and treated. Additional subjects, and longer follow-up will establish whether treatment with CART-ddBCMA results in durable CAR+ T responses.