Conditioning Regimen for Hematopoietic Stem Cell Transplant

HSCT remains a cornerstone in the management of a variety of hematological malignancies, bone marrow failure syndromes, benign and select inherited disorders. Conditioning serves a multifaceted role—eradicating residual malignant cells, immunosuppressing the host to prevent graft rejection, and creating space within the marrow microenvironment to facilitate engraftment of donor hematopoietic stem cells (Appelbaum, N Engl J Med 357(15):1472–1475, 2007). Over the decades, conditioning regimens have evolved from high-intensity, MAC protocols to more nuanced approaches such as RIC and NMA, tailored to disease biology, patient age, comorbidities, and transplant platform. This evolution is driven by a growing recognition of the importance of balancing RRT with the need for effective disease control and long-term immune reconstitution (Copelan, N Engl J Med 354(17):1813–1826, 2006). MAC, typically involving high-dose alkylating agents such as busulfan or TBI, continues to be the standard for younger, fit patients with high-risk disease. However, these regimens are associated with substantial toxicity, including sinusoidal obstruction syndrome, interstitial pneumonitis, and long-term organ dysfunction, which limit their applicability in older or medically frail populations (Carreras and Diaz-Ricart, Bone Marrow Transplant. 46:1495–1502, 2011). Conversely, RIC regimens, which emerged in the late 1990s, rely more on the GVT effect than on cytoreduction and thus aim to minimize regimen-related mortality while maintaining therapeutic efficacy. Agents such as fludarabine, melphalan, and low-dose TBI are frequently employed in these protocols, and they enable transplants in MAC-ineligible patients (Deeg and Sandmaier, Blood 116(23):4762–4770, 2010). Nonetheless, RIC may increase the risk of disease relapse, particularly in aggressive or chemo-resistant malignancies, necessitating careful patient and disease stratification. Furthermore, the increasing application of novel transplant platforms—such as haploidentical transplantation with PTCy, and cord blood transplantation—has catalyzed further refinements in conditioning strategies. These newer modalities often require individualized conditioning approaches that integrate both immunologic considerations and the unique engraftment kinetics of the graft source (Luznik et al., Biol Blood Marrow Transplant J Am Soc Blood Marrow Transplant 14:641–650, 2008). In parallel, advances in pharmacokinetics-guided dosing, particularly with agents like busulfan, and the incorporation of targeted radiotherapeutics, such as radioimmunotherapy, have introduced a level of precision previously unattainable in conditioning regimen design.