Post-Transplant Bendamustine as a Platform for Immune Modulation After Allogeneic Hematopoietic Cell Transplantation

Post-transplant cyclophosphamide (PT-CY) remains the standard of care for graft-versus-host disease (GvHD) prophylaxis in haploidentical hematopoietic cell transplantation (HCT), yet relapse, delayed immune reconstitution, infections, and organ toxicity represent persistent and clinically meaningful limitations. Post-transplant bendamustine (PT-BEN) has been investigated as a biologically distinct alternative. In stringent haploidentical murine models, PT-BEN uncoupled GvHD from graft-versus-leukemia (GvL) effects through mechanisms centered on myeloid modulation and preservation of innate immunity rather than regulatory T-cell expansion. These findings were extended in humanized xenogeneic models, where PT-BEN combined with calcineurin inhibition demonstrated superior survival compared to PT-CY. Clinical translation began with a phase I partial-substitution platform (PT-CY day +3/PT-BEN day +4), demonstrating safety, accelerated hematopoietic recovery, reduced chronic GvHD, and improved GvHD-free relapse-free survival in a small non-randomized cohort. Immune profiling revealed earlier CD4⁺ reconstitution, restrained CD8⁺ effector skewing, and preserved T-cell receptor diversity. Independent studies from multiple international centers have provided preliminary corroboration of these signals and clarified a dose- and platform-dependent cytokine release syndrome as the principal emerging toxicity. The accumulating preclinical consistency and early multi-institutional clinical signals now provide sufficient biological plausibility to justify formal randomized evaluation of PT-BEN against the PT-CY standard. PT-BEN appears to be a mechanistically distinct strategy with immunomodulatory properties that requires randomized validation before platform-level conclusions can be drawn. A multicenter randomized trial incorporating a reduced-dose PT-CY comparator arm will be essential to establish PT-BEN's independent contribution and define its role in improving long-term HCT outcomes.