Targeted‐dose of busulfan: Higher risk of sinusoidal obstructive syndrome observed with systemic exposure dose above 5000 µMol⸱min. A historically controlled clinical trial

Busulfan is given in the conditioning regimens preceding hematopoietic stem cell transplantation (HSCT), and plasma levels can be monitored. A targeted, individualized systemic exposure (SE) dose can be achieved by calculating the area under the plasma concentration versus time curve (AUC). The objective of this study was to determine a cutoff value for safety for the AUC for busulfan plasma levels in patients undergoing HSCT. A total of 149 consecutive HSCT patients were studied. After an oral test dose of busulfan, we set target doses of 4000, 5000, or 6000 µMol⸱min/day, and analyzed the AUC of oral or intravenous Bu. These patients were compared with 53 historical control subjects who had received myeloablative conditioning regimen without busulfan pharmacokinetic monitoring. Using a test dose and the administration route had no impact on the sinusoidal obstructive syndrome (SOS) incidence, transplant-related mortality or 1-year overall survival. However, patients receiving busulfan at doses set up at AUC > 5000 had an increased risk to develop SOS after HSCT (hazard ratio 3.39, p = 0.034, 95% CI 1.09-10.52). Adjusting the busulfan dose according to SE levels target dose during conditioning is associated with lower rates of oral severe mucositis and SOS. A cutoff of 5000 µMol⸱min is safe and does not impair survival.