ATG exposure in T replete transplant for hematological malignancies: Real world analysis from BMT CTN 1202

Traditional weight-based dosing results in variable rabbit anti-thymocyte globulin (rATG) clearance, delaying CD4+Tcell-immune-reconstitution (CD4 + IR) and impacting outcomes. In a retrospective pharmacokinetic/pharmacodynamic analysis of patients undergoing first T-replete hematopoietic cell transplantation (HCT), enrolled on BMT CTN 1202, we estimated post-HCT rATG-exposures as area under the curve (arbitrary unit per day/milliliter [AUxd/mL]) using a validated pharmacokinetic-model. Results were compared to patients who did not receive rATG. Previously defined post-HCT rATG-exposure groups: A: < 30, B:30-55 and C: ≥ 55 AUxd/mL were correlated with outcomes of interest using cox-proportional-hazard and cause-specific-hazards models. 325 patients (median age 51 years) were included: 228 received rATG. Median post-HCT rATG-exposure was 44.1 (Range 6.2-125.0). Among patients who received rATG, higher exposure correlated with worse five-year overall survival (OS) (no-ATG:51%; group A:67%; B:49%; C:34%, p = 0.01), higher five-year relapse incidence (no-ATG:31%; A:27%; B: 42%; C: 58%, p < 0.001) and lower CD4 + IR (no-ATG:64%; A:73%; B:51%; C:19%, p = 0.001). Grade 2-4 acute graft versus host disease (GVHD) rates were: no-ATG:40% ; A:26%, B:39%, C:35%, (p = 0.44). Any rATG-exposure was associated with lower moderate/severe chronic GVHD (HR:0.63, p = 0.025). Low post-HCT rATG-exposure ( < 30AUxd/mL) but not absent correlated with higher OS, lower relapse, and lower GVHD related deaths. Model-based-dosing could be used to target optimal post-HCT rATG-exposure and improve HCT outcomes.