Preliminary Clinical Results of a Phase 1 Study Evaluating the Safety and Anti-Tumor Activity of ACTR707 in Combination with Rituximab in Subjects with Relapsed or Refractory CD20+ B-Cell Lymphoma

Background: Recent regulatory approvals of two CD19-targeted chimeric antigen receptor (CAR)-expressing autologous T lymphocyte therapies provide compelling evidence of the clinical potential of re-engineering T cells to specifically attack tumor cells, but the broader applicability of these therapies is constrained by safety considerations and target specificity. A universal approach to T cell therapy that enables flexibility in tumor target selection has been demonstrated by engineering autologous T cells to express an antibody-coupled T cell receptor (ACTR) composed of the ectodomain of the CD16 Fc receptor fused to costimulatory and CD3ζ signaling domains. Thus, the ACTR platform couples T cell anti-tumor effector functions, including cytotoxicity, cytokine production, and T cell proliferation, to target-specific therapeutic antibodies.

Here we present the preliminary clinical findings of the ongoing, multicenter Phase 1 study, ATTCK-20-03 (NCT03189836), of ACTR707, a CD28-containing ACTR chimeric receptor, in combination with rituximab in subjects with relapsed or refractory CD20+ B cell lymphoma.

Methods: The primary objectives of this first-in-human, dose escalation study are to evaluate the safety of the combination of ACTR707 and rituximab and to determine a maximum tolerated dose (MTD) and a proposed recommended phase 2 dose (RP2D). Other objectives include evaluation of antitumor activity, and assessment of ACTR T cell persistence, cytokine levels, and rituximab pharmacokinetics. Eligible subjects must have histologically confirmed relapsed or refractory CD20+ non-Hodgkin lymphoma and have received prior anti-CD20 mAb in combination with chemotherapy. Subjects received lymphodepleting chemotherapy (cyclophosphamide 400 mg/m² and fludarabine 30 mg/m²) for 3 days, followed by rituximab (375 mg/m²) and ACTR707. Additional doses of rituximab were administered, one dose every 3 weeks in the absence of disease progression. The study is separated into 2 sequential phases, a dose escalation and a safety expansion phase. During the dose escalation phase, ACTR707 is being tested at increasing doses in combination with rituximab.

Results: Six subjects were enrolled and received ACTR707 at the first dose level in combination with rituximab: 5 diagnosed with diffuse large B cell lymphoma (83%) and one with follicular lymphoma, Grade 3b (17%). Median age was 61 years (range: 57-76), 83% were male, 50% were treated with ≥3 lines of prior therapy, and 67% had no response to or relapse within 6 months from immediate prior therapy. ACTR707 was successfully manufactured for all subjects and demonstrated post-infusion expansion in the peripheral blood. ACTR+ T cells were detectable at Day 28 post-infusion for all subjects tested. No dose-limiting toxicities (DLTs) were observed at the first dose level in 4 DLT-evaluable subjects (2 subjects experienced disease progression during the DLT evaluation period). There were no cytokine release syndrome (CRS) or autoimmune adverse events (AEs), serious or severe (≥Gr3) neurotoxicity AEs, or deaths on treatment. AEs (all grades) reported in >1 subject included neutropenia (n=3), anemia, decreased appetite, febrile neutropenia, and thrombocytopenia (each in 2 subjects); the 2 events of febrile neutropenia were considered serious. Investigator-reported complete responses were observed in 3 of 6 subjects. These complete responses (duration of response range: 47+ to 81+ days) are ongoing as of the data cut-off. Enrollment into the second dose level is ongoing.

Conclusions: ACTR707 in combination with rituximab induced complete responses in 3 of 6 subjects with relapsed or refractory aggressive CD20+ B cell lymphoma treated at the first dose level with ACTR707 in combination with rituximab, with no CRS, serious or severe (≥Gr3) neurotoxicity, or AEs leading to treatment discontinuation. ACTR+ T cells were detectable in all subjects and persisted. These results support the continued dose escalation of ACTR707 in combination with rituximab. Updated data, inclusive of preliminary dose level 2 and correlative biomarkers, will be presented.