Objectives Calcineurin inhibitor (CNI) use for acute graft‐versus‐host disease (aGVHD) prophylaxis in allogeneic hematopoietic cell transplantation (allo‐HCT) recipients has been associated with toxicities. Toxicities may be managed by converting CNI to sirolimus as often done in solid organ transplantation. This study aimed to characterize allo‐HCT patients who completely transitioned from tacrolimus to sirolimus and evaluate the incidence of aGVHD within 100 days post‐transition, overall survival (OS), and incidence of relapse. Methods Safety and efficacy data were collected at baseline and at day 30 and 90 post‐transition from tacrolimus to sirolimus and at one‐year post‐HCT. Results Most patients who transitioned had acute leukemia, received a matched unrelated donor allo‐HCT, and transitioned due to nephrotoxicity or neurotoxicity. The resolution rate was 83% and 48% in the nephrotoxicity group, 78% and 61% in the neurotoxicity group, 33% and 33% in the group that developed both nephrotoxicity and transplant‐associated thrombotic microangiopathy at 30 and 90 days of assessments, respectively. Patients who transitioned before day 55 post‐allo‐HCT were more likely to develop new or worsening aGVHD. The one‐year OS and relapse rates were 37% and 20%, respectively. Conclusions The conversion from tacrolimus to sirolimus demonstrates promising resolution of acute toxicities; however, overall mortality remains high.

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