Introduction: Balancing GVHD Prevention and Immune Reconstitution
The landscape of allogeneic hematopoietic cell transplantation (HCT) has been defined by a precarious balance between preventing graft-versus-host disease (GVHD) and ensuring rapid immune recovery. Ex vivo CD34+ selection, a process that physically removes T-cells from the donor graft, has emerged as a powerful technique to minimize the risk of chronic GVHD without the need for prolonged immunosuppression. However, this method has historically been hampered by delayed immune reconstitution (IR), specifically the slow recovery of CD4+ T-cells, which increases the patient’s vulnerability to opportunistic infections and non-relapse mortality (NRM).
The Role of Antithymocyte Globulin
Antithymocyte globulin (ATG) is frequently used in the conditioning phase of CD34+ selected HCT to prevent graft rejection and further reduce GVHD risk. However, ATG has a long half-life, and its persistence in the recipient’s circulation post-transplant can lead to the depletion of the very donor T-cells that are essential for early immune defense. Traditional weight-based dosing of ATG often leads to highly variable exposure, frequently resulting in over-exposure that stifles the fledgling immune system.
The Pharmacokinetic Rationale for Precision Dosing
Recognizing the limitations of weight-based dosing, researchers at Memorial Sloan Kettering Cancer Center (MSKCC) sought to apply a pharmacokinetic (PK) model-based approach to ATG administration. The hypothesis was simple yet profound: by targeting a specific low exposure of ATG in the post-transplant period, it would be possible to maintain the benefits of the drug while allowing donor CD4+ T-cells to thrive.
Defining the Target Exposure
Prior retrospective analyses have suggested that high post-HCT ATG exposure—specifically levels exceeding 20 AU × d/mL—is strongly associated with delayed CD4+ T-cell recovery. The current Phase 2 study (NCT04872595) was designed to test whether prospectively targeting a post-HCT exposure of less than 20 AU × d/mL using a validated PK model could improve clinical outcomes in patients undergoing myeloablative conditioning (MAC).
Study Design and Methodology
This single-center, single-arm, Phase 2 trial enrolled 59 participants with various hematological malignancies between June 2021 and November 2023. The study population was diverse, with a median age of 55 years (IQR 30–63). The majority (79%) had myeloid malignancies, such as acute myeloid leukemia (AML) or myelodysplastic syndromes (MDS).
Conditioning Regimens and ATG Administration
Participants received one of two myeloablative conditioning regimens based on physician discretion: a chemotherapy-only regimen (busulfan, melphalan, and fludarabine) or a total-body irradiation (TBI)-based regimen (TBI, thiotepa, and cyclophosphamide). The model-based ATG dosing was calculated based on individual patient characteristics, including body weight and absolute lymphocyte counts, to ensure the target exposure was met.
Primary Endpoint
The primary objective was the achievement of robust CD4+ immune reconstitution, defined as reaching a CD4+ T-cell count of >50 cells per μL at two consecutive timepoints by day +100 post-transplant. Historically, achieving this milestone in 32% of the population was considered the benchmark for success in this setting.
Key Findings: Robust CD4+ Recovery and Pharmacokinetic Precision
The study’s results were remarkably positive, significantly exceeding the pre-specified primary endpoint. Among the 56 evaluable participants, 39 (70%) achieved the target CD4+ immune reconstitution by day +100. This represents a more than twofold increase over the historical expectation of 32%.
Pharmacokinetic Accuracy
The median estimated ATG exposure after HCT was 10 AU × d/mL (IQR 9–11), well within the targeted window of <20 AU × d/mL. This precision highlights the reliability of the pharmacokinetic model in a clinical setting, suggesting that individualized dosing can effectively eliminate the 'guesswork' associated with traditional weight-based protocols.
Clinical Outcomes and GVHD
By optimizing the ATG exposure, the study maintained the core benefits of CD34+ selection. The incidence of severe acute GVHD and chronic GVHD remained low, consistent with the advantages of T-cell depletion, while the accelerated immune recovery provided a potential buffer against early infectious complications.
Safety Profile and Adverse Events
While the study successfully met its primary endpoint, the intensive nature of myeloablative HCT means that safety monitoring remains paramount. The most common grade 3 or worse adverse events were infections (40% of all events) and gastrointestinal issues (17%). These findings are typical for this high-risk population undergoing MAC.
Treatment-Related Mortality
The study reported four treatment-related deaths among participants, including cases of secondary graft failure and multi-organ failure. These events underscore the continued complexity of allogeneic HCT, even with advanced precision dosing strategies. However, the overall improvement in CD4+ recovery is expected to contribute to a long-term reduction in NRM by decreasing the window of extreme vulnerability to pathogens.
Expert Commentary: Toward an Individualized Conditioning Paradigm
The findings of this Phase 2 trial represent a significant step forward in the field of transplant medicine. For decades, conditioning regimens have been largely ‘one-size-fits-all,’ despite known variability in drug metabolism and clearance among patients.
Translational Implications
By demonstrating that model-based ATG dosing can be successfully implemented in a real-world clinical workflow, this study paves the way for broader adoption of precision conditioning. Experts suggest that this approach should not be limited to CD34+ selected grafts but could potentially benefit other transplant platforms, including haploidentical transplants and those using post-transplant cyclophosphamide (PTCy).
Study Limitations
As a single-center, single-arm study, the results must be interpreted with caution. While the comparison to historical benchmarks is favorable, multi-center randomized controlled trials will be necessary to confirm these findings and establish model-based ATG dosing as a standard of care. Additionally, longer-term follow-up is needed to determine the impact on overall survival and relapse-free survival.
Conclusion: A New Standard for Immune Recovery
The Scordo et al. study provides compelling evidence that pharmacokinetically guided ATG dosing promotes robust and timely CD4+ immune reconstitution after ex vivo CD34+ selected allogeneic HCT. By addressing the ‘Achilles’ heel’ of T-cell depleted transplantation—delayed immune recovery—this strategy enhances the safety and viability of a technique known for its superior GVHD prevention. As medicine moves toward more personalized interventions, the integration of pharmacokinetic modeling into transplant conditioning represents a major leap toward optimizing outcomes for patients with hematological malignancies.
Funding and ClinicalTrials.gov
This study was funded by the US National Cancer Institute and Memorial Sloan Kettering Cancer Center. ClinicalTrials.gov Identifier: NCT04872595.
References
1. Scordo M, et al. Model-based antithymocyte globulin dosing in ex vivo CD34+ selected allogeneic haematopoietic cell transplantation: a single-centre, single-arm, phase 2 study. Lancet Haematol. 2025;12(12):e956-e965.
2. Bosch M, et al. Post-transplant immune reconstitution and its impact on outcomes. Nature Reviews Clinical Oncology. 2020;17:684-700.
3. Boelens JJ, et al. Outcomes of transplantation using various ATG dosing strategies. Blood. 2014;124(12):2001-2008.
