Engraftment syndrome, but not acute GVHD, younger age, CYP3A5 or MDR1 polymorphisms, increases tacrolimus clearance in pediatric hematopoietic SCT.

We investigated clinical factors that affected the clearance of tacrolimus (FK506) administered by continuous drip infusion to children who had received allogeneic hematopoietic SCT. Blood FK506 levels were measured every day in 27 patients in an attempt to adjust the dose to maintain the target range (10-15 ng/mL). Patients who developed engraftment syndrome (ES) and acute GVHD and patients less than 7 years of age showed a higher FK506 clearance calculated from body weight (BW) for 5 or more consecutive days compared with the control groups. A time-course study showed that the occurrence of ES, but not acute GVHD, was related to increased clearance of FK506. When calculated from body surface area (BSA), a significant increase in FK506 clearance was observed in patients with ES, but not in those less than 7 years of age. FK506 clearance was not influenced by CYP3A5, multidrug resistance 1 or ABCG2 genotypes. None of the clinical parameters affected blood FK506 levels. Determination of the FK506 dose on the basis of frequent monitoring of the blood concentration seems to minimize the serious adverse effects induced by the immunosuppressant. It may be more accurate to dose FK506 according to BSA rather than BW for pediatric patients.