A Phase 1/1b Safety Study of Prgn-3006 Ultracar-T™ in Patients with Relapsed or Refractory CD33-Positive Acute Myeloid Leukemia and Higher Risk Myelodysplastic Syndrome

Background: Therapeutic options for relapse or refractory (r/r) acute myeloid leukemia (AML) and hypomethylating agent (HMA) failure higher risk myelodysplastic syndrome (MDS) pts are limited with median overall survival of < 6 months. Consequently, novel therapies are urgently needed. CD33 is highly expressed on most myeloid leukemia stem cells with lesser expression on normal hematopoietic stem cell populations and minimal non-hematopoietic expression making CD33 a leading target in chimeric antigen receptor therapy (CAR-T) development for myeloid malignancies. However, additional barriers of CAR-T development in myeloid malignancies include long manufacturing period, expansion of CAR-T cells and potential toxicity related to on-target, off-tumor toxicity.

Scientific Rationale:

Current CAR-T cells utilize viral vectors for gene transfer and subsequent lengthy ex vivo expansion at centralized manufacturing facilities, which is costly and leads to cell product that is exhausted and short lived in vivo. Time is of the essence for pts with rapidly progressing disease such as r/r AML and the prolonged interval between apheresis to product infusion with current CAR-T cell therapies can be a disadvantage. Although allogeneic "off-the-shelf" products allow for rapid administration, challenges remain with rapid rejection. Precigen has developed UltraCAR-T platform to overcome these limitations by utilizing an advanced non-viral gene delivery system and a rapid, decentralized manufacturing process. UltraCAR-T cells are manufactured overnight at medical center's cGMP facility using patient's autologous T cells and administered back to patient only one day after gene transfer with no need for ex vivo expansion. PRGN-3006 UltraCAR-T cells co-express CD33 CAR, membrane bound IL-15 (mbIL15) and a kill switch. Preclinical studies have demonstrated that the expression of the mbIL15 on UltraCAR-T cells leads to maintenance of preferred stem-like memory phenotype (TSCM). Superior efficacy of UltraCAR-T cells was demonstrated in an aggressive murine xenograft model of AML where a single administration of PRGN-3006, only one day after gene transfer, showed significantly higher expansion and persistence; effectively eliminated tumor burden; and significantly improved overall survival compared to traditional CD33 CAR-T cells lacking mbIL15 expression (Blood (2019) 134(S1): 2660).

Study Design: The PRGN-3006 UltraCAR-T cells are currently being evaluated in a Phase 1/1b first-in-human dose escalation/dose expansion clinical trial (NCT03927261). The study population includes adult pts (≥ 18 years) with relapsed or refractory AML and HMA failure higher risk MDS or chronic myelomonocytic leukemia (CMML) with ≥ 5% blasts. Pts who have relapsed post allogeneic stem cell transplant are allowed if > 3 months out from transplant without evidence of active graft versus host disease and off immunosuppression for 6 weeks. Key inclusion criteria include an absolute lymphocyte count ≥ 0.2k/µL, KPS > 60%, absence of other active malignancy within 1 year of study entry, daily corticosteroid dose < 10mg of prednisone daily, adequate organ function and a backup allogeneic donor should bone marrow aplasia occur. Hydroxyurea is allowed for cytoreduction with cessation 3 days prior to apheresis/infusion but can be reinitiated post-infusion.

To test the hypothesis that expression of mbIL15 on PRGN-3006 cells is sufficient to promote CAR-T cell expansion and persistence, study subjects will receive PRGN-3006 infusion either without prior lymphodepletion (Cohort 1) or following lymphodepleting chemotherapy (Cohort 2 with fludarabine 30mg/m2 and cyclophosphamide 500mg/m2 days -5 to -3). Up to 5 dose levels are planned in dose escalation. All subjects will be followed for adverse events, CAR-T-related toxicities, disease response and PRGN-3006 cell expansion and persistence in blood and bone marrow compartments. In addition, the mechanisms of safety and effectiveness of PRGN-3006 cells will be evaluated with correlative assays of specific immune response pathways. Currently, the study is in the dose escalation phase and has cleared the lower dose level while demonstrating successful manufacturing of UltraCAR-T cells. Additionally, multi-center expansion of the trial is in progress.