Abstract 818: A logic-gated chimeric antigen receptor T cell (CAR T) for post-transplant acute myeloid leukemia (AML) that distinguishes donor and recipient hematopoiesis

Abstract Background: AML relapse after a stem cell transplant is associated with dismal outcomes and no current FDA-approved therapeutic options. Cell therapy for AML has been challenging due to a lack of antigens distinguishing malignant from normal hematopoiesis, the potential for antigen escape due to intratumor antigen heterogeneity, and poor T cell function in lymphocytes from AML patients. To overcome these problems, we focus on opportunities in patients who have received human leukocyte antigen (HLA)-A*02 mismatched transplants. We have developed a Tmod HLA-A*02 NOT gate (blocker) that prevents CAR T killing of HLA-A*02 positive cells. In patients who are HLA-A*02 negative and receive an HLA-A*02 positive stem cell transplant, the blocker distinguishes tumor (HLA-A*02 negative) and normal (HLA-A*02 positive) hematopoiesis. A similar Tmod NOT gate has been safe in an ongoing solid tumor clinical trial (NCT05736731) (Grierson, et al. SITC. 2024). The blocker is paired with a dual CD33/CLL-1 activator to prevent antigen escape in donor-derived HLA-A*02 positive T cells to mitigate AML patient T-cell dysfunction. Additional blockers targeting HLA-A*03, etc can expand therapeutic opportunity to patients who have received other HLA-mismatch transplants. Methods: CAR constructs were designed and screened in Jurkat and primary T cell assays. The 50% effective and inhibitory concentration (EC50 and IC50) results were compared to monospecific CAR constructs using benchmark activators based on single-chain variable fragments (scFvs) from M26 (CLL-1) and lintuzumab (CD33) using wild-type A*02-negative and A*02-positive engineered AML cell lines. Results: 111 tandem CD33/CLL-1 activator CAR constructs were evaluated in Jurkat cells and 23 in conjunction with HLA-A*02 blocker. Six tandem constructs were prioritized for evaluation in primary T cells, identifying one (20I21-1A4 mBAAsh) with IC50 potency similar to monospecific CAR constructs, and selectivity observed between HLA-A*02 positive vs. HLA-A*02 negative cells. Selective protection also was observed in HLA-A*02 positive primary hematopoietic stem cells compared with CAR constructs without the blocker. Conclusions: We have developed a cell therapy strategy to leverage HLA mismatched stem cell transplants using a Tmod HLA-A*02 blocker. This approach could be applied to patients who have relapsed after a mismatched transplant or integrated into a stem cell transplant to prevent relapse. Pre-clinical data of this design demonstrates selectivity between HLA-A*02 positive and HLA-A*02 negative cells, and dual antigen activity addresses antigen escape. Future studies will interrogate the impact of membrane-tethered cytokines to augment potency and xenografts studies to further define activity. Citation Format: Diane Manry, Breanna L. Luna, Aaron G. Winters, Aaron D. Flynn, Tanveer K. Gill, Michele Mcelvain, Alaia Sima, Jason Wang, Alexandre Zampieri, Mark Daris, Jingli Zhang, Sara Martire, Breanna DiAndreth, Chawita Netirojjanakul, Talar Tokatlian, Alexander Kamb, John S. Welch. A2 Biotherapeutics, Inc, Agoura Hills, CA. A logic-gated chimeric antigen receptor T cell (CAR T) for post-transplant acute myeloid leukemia (AML) that distinguishes donor and recipient hematopoiesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 818.