ABSTRACT Acute myeloid leukemia (AML) is a malignant neoplasm of myeloid lineage cells arising in the bone marrow and outgrowing normal hematopoietic elements. In the US, ~3,500 AML patients receive hematopoietic stem cell transplant (HSCT) every year, however relapse after HSCT remains a major cause of mortality, leading to poor 1-year survival and low complete remission rates. Although AML has been shown to be sensitive to immune-based interventions (e.g., donor lymphocyte infusion or CAR-T cells), these are limited in AML because of: 1) lack of one antigen with sufficient tumor specificity, 2) tumor immune escape, 3) requirement for lymphodepletion, preventing engagement of the endogenous immune system (epitope spreading), and 4) risk of graft-versus-host disease (GVHD) and other adverse effects. Marker is proposing a novel T cell-based therapy that targets multiple tumor-associated antigens (mTAAs) simultaneously, thereby minimizing tumor escape. Specifically, the product to be tested here, MT-401, targets 4 antigens which are highly expressed in AML but are absent or expressed at low levels in healthy tissues. Manufactured from allogeneic apheresis material from an HSCT donor, MT-401 recognizes the target cells via the native T cell receptors (TCRs), by interacting with both class I and II MHCs, leading to killing of cells expressing any of these antigens, as well activation of other immune cells. Pre-clinically, MT-401 T cells exhibited specific killing of HLA-matched leukemia cells expressing these antigens. Such mTAA-specific T cells were shown to be clinically safe in >150 patients with various kinds of cancer. In a heavily pretreated AML population with active disease post-HSCT, this therapy demonstrated objective clinical evidence resulting in complete (CR) or partial (PR) responses in some patients, while adjuvant patients remained in remission longer than expected. Additionally, a patient with measurable residual disease (MRD) showed a relatively steady decline in MRD levels post-treatment. Importantly, epitope spreading was observed due to the lack lymphodepletion, leading to more durable responses compared to other cellular therapies. This grant proposes a Phase 2 clinical study of MT-401, an innovative allogeneic T cell product for the treatment of patients with AML who have received their first allogeneic HSCT. Within the portion of the study covered by the proposed grant, 40 AML patients who are MRD+ following HSCT will be enrolled in the study. Specific Aim 1 will include execution of the clinical trial: enrolling, treating and following subjects until study completion. Specific Aim 2 will include evaluation of the primary and secondary efficacy and safety endpoints. Specific Aim 3 will include evaluation of patient samples for biomarker analysis, including expansion, persistence, clonality, anti-tumor immune effects of MT-401, and epitope spreading, as determined by exploratory objectives.