Model-based antithymocyte globulin dosing in ex vivo CD34+ selected allogeneic haematopoietic cell transplantation: a single-centre, single-arm, phase 2 study.

BACKGROUND

Ex-vivo CD34+ selected allogeneic haematopoietic cell transplantation (HCT) provides favourable chronic graft-versus-host disease (GVHD)-free relapse-free survival but is limited by delayed immune reconstitution and early non-relapse mortality. High anti-thymocyte globulin (ATG) exposure after HCT has been associated with delayed CD4+ T-cell immune reconstitution, increased non-relapse mortality, and poor overall survival.

METHODS

We report the final analysis of a single-centre, phase 2 trial investigating pharmacokinetic model-based ATG (targeting <20 AU × d/mL post-HCT exposure) in participants of any age undergoing ex vivo CD34+ selected allogeneic HCT after myeloablative conditioning for haematological malignancies. Two myeloablative conditioning regimens were used at the discretion of the treating physician: the chemotherapy-based regimen (target cumulative exposure of 65 mg × h/L busulfan, 140 mg/m2 melphalan, and 150 mg/m2 fludarabine) and a high-dose total-body irradiation-based regimen (included total-body irradiation [1375 cGy], thiotepa [10 mg/kg], and cyclophosphamide [100 mg/kg]). The primary objective was an improvement in CD4+ immune reconstitution (>50 cells per μL at two consecutive timepoints by day +100) in at least 32% of the per protocol population. This study was registered with ClinicalTrials.gov (NCT04872595) and is completed.

FINDINGS: Between June 14, 2021, and Nov 28, 2023, we enrolled 59 participants with haematological malignancies. Among evaluable participants (n=56), the median age was 55 years (IQR 30-63), 34 (61%) were male, 22 (39%) were female, 44 (79%) had myeloid malignancies, and 44 (79%) had received chemotherapy-only myeloablative conditioning. The median estimated ATG exposure after HCT was 10 AU × d/mL (IQR 9-11). CD4+ immune reconstitution was reached in 39 (70%) of 56 participants, meeting the study's primary endpoint. The most common grade 3 or worse adverse events were infections (103 [40%] of 259 events) and oral or gastrointestinal events (44 [17%] of 259 events). Grade 5 adverse events occurred in three participants including secondary graft failure (n=1) and multi-organ failure (n=2), with a total of four treatment-related deaths among participants.

INTERPRETATION: These results demonstrate that model-based ATG dosing promotes robust CD4+ immune reconstitution after ex vivo CD34+ selected allogeneic HCT, underscoring the potential of pharmacokinetically guided ATG as a strategy to optimise immune recovery in myeloablative, calcineurin inhibitor-free transplantation for haematological malignancies.

FUNDING: US National Cancer Institute, Memorial Sloan Kettering Cancer Center.