A Phase 1 Study of Two Investigational Agents, ACTR087, an Autologous T Cell Product Expressing an Antibody-Coupled T Cell Receptor, in Combination with SEA-BCMA, a Novel Non-Fucosylated Monoclonal Antibody, in Subjects with Relapsed or Refractory Multiple Myeloma

Background: B cell maturation antigen (BCMA) has recently emerged as a promising candidate antigen for therapeutic targeting in multiple myeloma (MM), with several targeted agents in clinical studies including antibody-drug conjugates and bispecific T cell engagers as well as CAR T cells. The Antibody-Coupled T cell Receptor (ACTR) platform is a universal, engineered autologous T cell therapy developed to mediate anti-tumor activity in combination with tumor-targeting antibodies. The ACTR construct is composed of the ectodomain of CD16 fused to intracellular co-stimulatory and CD3ζ signaling domains (Kudo et al., Cancer Res. 2014), which allow ACTR T cells to exert antibody-dependent cell-mediated cytotoxicity, a function otherwise physiologically limited to CD16-expressing natural killer cells and macrophages.

ACTR087 expresses a 4-1BB-containing receptor and has been evaluated in combination with rituximab in subjects with relapsed or refractory (R/R) CD20+ B cell lymphoma as previously reported (Akard et al., Blood 2017). SEA-BCMA is a novel, humanized non-fucosylated anti-BCMA IgG1 antibody that has been demonstrated pre-clinically to bind to ACTR087 T cells to mediate ACTR T cell activation, cytotoxicity, cytokine release, and proliferation in the presence of BCMA-expressing MM cell lines. These functional activities were demonstrated to be BCMA-specific and SEA-BCMA dose-dependent (Cheema et al., AACR 2017). Here we present preliminary findings from the first 2 single-subject cohorts of the ATTCK-17-01 study (NCT03266692), an ongoing Phase 1 study of ACTR087 in combination with the first-in-human administration of SEA-BCMA.

Methods: ATTCK-17-01 is a multicenter, Phase 1, dose-escalation study of ACTR087 in combination with SEA-BCMA. The primary objectives are to characterize the safety and to determine the recommended Phase 2 dose of ACTR087 in combination with SEA-BCMA in subjects with R/R MM. The secondary objectives include evaluation of anti-myeloma activity, ACTR T cell persistence, cytokines, and SEA-BCMA pharmacokinetics (PK); exploratory objectives include the anti-myeloma activity of SEA-BCMA alone. Subjects must have measurable disease and must have received at least 3 prior lines of therapy including treatment with a proteasome inhibitor and an immunomodulatory agent, and hematopoietic stem cell transplant (HSCT) for HSCT-eligible subjects. BCMA expression on MM cells was not a condition of eligibility. Dose escalation of the 2 investigational agents, ACTR087 and SEA-BCMA, is determined according to adaptive design principles.

After study enrollment and leukapheresis, subjects receive SEA-BCMA by IV infusion once every 3 weeks until disease progression or treatment discontinuation. After the third dose of SEA-BCMA and lymphodepleting chemotherapy (cyclophosphamide 300 mg/m² and fludarabine 30 mg/m², each daily for 3 days), subjects receive a single dose of ACTR087.

Results: Two subjects enrolled and received ACTR087 at the first dose level in combination with the first 2 dose levels of SEA-BCMA. First-in-human dosing of SEA-BCMA was well tolerated, with no reported SEA-BCMA-related adverse events (AEs) or dose-limiting toxicities (DLTs).

Following ACTR087 infusion, ACTR+ T cells were detectable in the peripheral blood and demonstrated expansion post-infusion. No DLTs were observed with the combination of ACTR087 and SEA-BCMA in the first 2 cohorts. Grade 3 or higher treatment-emergent AEs experienced by at least 1 subject, regardless of causality assessment, include cytopenias, increased ALT, and bone pain.

Conclusions: ACTR087 in combination with SEA-BCMA was well tolerated in the first 2 subjects treated, with no DLTs or AEs leading to treatment discontinuation. These results support the continued dose escalation of ACTR087 in combination with SEA-BCMA. Enrollment in Cohort 3 is ongoing. Updated data, including SEA-BCMA PK, biomarkers, and preliminary Cohort 3 data, will be presented.