Targeted busulfan/cyclophosphamide (TBU/CY) for allogeneic hematopoietic cell transplantation (HCT) carries a high risk of sinusoidal obstruction syndrome (SOS) in patients transplanted for myelofibrosis. followed by daily IV BU for four days targeted to a concentration at steady state (Css) of 800-900 ng/mL. CY/TBU-conditioned patients had higher exposure to CY (p<0.0001) and lower exposure to 4-hydroxyCY (p<0.0001) compared to TBU/CY-conditioned patients. Clinical outcomes were compared with 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% vs. 30% after GAP-134 TBU/CY p=0.006) while SOS incidence was low in both cohorts with AML/MDS. Day +100 mortality was significantly lower in the CY/TBU cohort (2% vs. 13% p=0.01). CY/TBU conditioning markedly impacted CY pharmacokinetics and was associated with significantly lower incidences of SOS and day +100 mortality suggesting that CY/TBU is usually superior to TBU/CY as conditioning for patients with myelofibrosis. INTRODUCTION Busulfan followed by cyclophosphamide (BU/CY) is usually a commonly used high-dose conditioning regimen in allogeneic hematopoietic cell transplantation (HCT). Regimen-related toxicity graft rejection and relapse in patients conditioned with BU/CY have been reduced by individualized dosing of BU to a target steady-state concentration (targeted BU/CY TBU/CY) [1 2 However neither BU GAP-134 dose-targeting nor the introduction of intravenous TBU has GAP-134 eliminated hepatic sinusoidal obstruction syndrome (SOS) as a cause of morbidity and mortality [3 4 BU is not inherently harmful to GAP-134 hepatocytes or to sinusoidal endothelial cells whereas metabolites of CY generated within hepatocytes and transported into hepatic sinusoids are highly harmful to sinusoidal endothelial cells [5-7]. It follows that CY metabolites are the prime cause of regimen-related liver toxicity following the TBU/CY regimen. There are several possible approaches to minimizing regimen-related toxicity caused by the combination of TBU and CY. GAP-134 One approach is usually to eliminate CY altogether for example by using a regimen of fludarabine and BU (FLU/TBU) [8 9 A second approach is usually to eliminate variability in CY exposure with pharmacokinetic targeting of CY doses which is usually feasible and effective in reducing toxicity . A third simpler approach is usually to reverse the order of administration giving CY first followed by IV TBU (CY/TBU). The pharmacologic rationale for any CY/ TBU regimen rests on the following observations: 1) BU depletes hepatic glutathione NESP (GSH) and at high concentrations induces oxidative stress in murine hepatocytes ; 2) glutathione is usually important in both the detoxification of the CY metabolite 4-hydroxycyclophosphamide (HCY) through conversion to glutathionyl-CY and in the removal of the harmful CY metabolite acrolein [5 11 3 restoration of hepatic and sinusoidal endothelial cell GSH levels prevents injury to hepatic sinusoids in several different animal models of harmful liver injury ; and 4) studies in patients receiving high-dose conditioning regimens have suggested a reduced risk of hepatotoxicity when BU was given after rather than before other conditioning agents [13-15]. Thus giving BU first appears to potentiate CY toxicity providing the basis for administering these drugs in reverse order (CY/ TBU) to reduce toxicity. Here we statement the results of a prospective clinical trial designed to test the hypothesis that reversed-sequence (CY/TBU) conditioning reduces the frequency and severity of hepatotoxicity compared to the standard sequence of BU followed by CY (TBU/CY). Additionally we collected pharmacokinetic data to test whether altering the sequence of conditioning brokers led to measurable changes in CY metabolism and exposure to CY metabolites. We enrolled two cohorts of patients one at high risk for harmful sinusoidal GAP-134 liver injury (patients with myelofibrosis)  and one at standard risk (patients with myelodysplastic syndrome [MDS] or acute myeloid leukemia [AML]). We compared liver toxicity and outcomes with those in concurrent and historical control patients who received TBU/CY and.