Cyclophosphamide followed by Intravenous Targeted Busulfan for Allogeneic Hematopoietic Cell Transplantation: Pharmacokinetics and Clinical Outcomes

Targeted busulfan (TBU) and cyclophosphamide (CY) for allogeneic hematopoietic cell transplantation carries a high risk of sinusoidal obstruction syndrome (SOS) in patients undergoing transplantation for myelofibrosis. We tested the hypothesis that reversing the sequence of administration (from TBU/CY to CY/TBU) would reduce SOS and day +100 nonrelapse mortality. We enrolled 51 patients with myelofibrosis (n = 20), acute myelogenous leukemia (n = 20), or myelodysplastic syndrome (n = 11) in a prospective trial of CY/TBU conditioning for allogeneic hematopoietic cell transplantation. CY 60 mg/kg/day i.v. for 2 days was followed by daily i.v. BU for 4 days, targeted to a concentration at steady state (Css) of 800-900 ng/mL. Compared with TBU/CY-conditioned patients, CY/TBU-conditioned patients had greater exposure to CY (P < .0001) and less exposure to 4-hydroxycyclophosphamide (P < .0001). Clinical outcomes were compared between cases and controls (n = 271) conditioned with TBU/CY for the same indications. In patients with myelofibrosis, CY/TBU conditioning was associated with a significantly reduced incidence of SOS (0% versus 30% after TBU/CY; P = .006), whereas the incidence of SOS was low in both cohorts with acute myelogenous leukemia/myelodysplastic syndrome. Day +100 mortality was significantly lower in the CY/TBU cohort (2% versus 13%; P = .01). CY/TBU conditioning had a marked affect on the pharmacokinetics of CY and was associated with significantly lower incidence of SOS and day +100 mortality, suggesting that CY/TBU is superior to TBU/CY as conditioning for patients with myelofibrosis.