Observational Comparison of Overall Survival between Phase 1b Orca-T and Registry-Based Post-Transplant Cyclophosphamide Patients.

Orca-T is a cellular immunotherapy comprising purified donor hematopoietic progenitor stem cells, regulatory T cells (Tregs), and conventional T cells (Tcons). Both Orca-T and post-transplant cyclophosphamide (PTCy) have demonstrated superior graft-versus-host disease (GVHD) control compared to tacrolimus and methotrexate in randomized clinical trials. Orca-T achieves immune tolerance through high-purity regulatory T cells in contrast to broad pharmacological immunosuppression with PTCy regimens. The principal goal of this study was to evaluate the long-term overall survival (OS) of Orca-T compared to PTCy. In this retrospective analysis, long-term survival follow-up was collected from a multicenter Phase 1b study of Orca-T that was initiated in 2019. OS outcomes were evaluated against a cohort of patients receiving PTCy-based GVHD prophylaxis, using a registry dataset obtained from the Center for International Blood and Marrow Transplant Research/NMDP. To ensure comparability, inclusion criteria were aligned with Orca-T Ph3 eligibility, specifically age ≤65 yr, diagnosis of intermediate- or high-risk acute myeloid leukemia or acute lymphoblastic leukemia in complete remission or myelodysplastic syndrome, myeloablative conditioning, and an 8/8 HLA-matched donor. The analysis included 76 Orca-T patients and 360 PTCy patients. Orca-T was associated with significantly higher OS over a 3-yr follow-up period compared to PTCy (hazard ratio [HR] = 0.41; log-rank P = .003). OS at 1, 2, and 3 yr for Orca-T was 96% (95% CI: 88% to 99%), 86% (76% to 92%), and 83% (73% to 90%), respectively. For the PTCy cohort, OS at 1, 2, and 3 yr was 81% (77% to 85%), 72% (67% to 77%), and 66% (60% to 71%), respectively. A propensity score-matched analysis (n = 76/group) confirmed the primary findings (HR = 0.40; log-rank test P = .010), as did a multivariable Cox model adjusted for recipient age, sex, disease, disease risk index, HCT-CI, and donor type (adjusted HR = 0.38; 95% CI 0.21 to 0.71; P = .002). The advantage persisted in sensitivity analyses excluding bone marrow grafts from the comparator and restricting both arms to the overlapping 2019 to 2021 transplant era (HR range across all specifications, 0.36 to 0.48). Three-yr nonrelapse mortality was lower with Orca-T (3.1% versus 10%; Gray's P = .0497), while relapse did not differ significantly; within the PTCy cohort, outcomes did not differ by mycophenolate mofetil use. The OS advantage for Orca-T was consistently observed across age, disease type, and other clinical subgroups. Within the limitations of this retrospective analysis, these results suggest that survival following allogeneic hematopoietic stem cell transplantation may be improved with Orca-T relative to PTCy.