Age-Dependent Pharmacokinetic Profile of Single Daily Dose Intravenous Busulfan in Children Undergoing Reduced-Intensity Conditioning Stem Cell Transplantation

Busulfan, an important agent used in conditioning regimens before hematopoietic stem/progenitor cell (HSPC) transplantation, has a “narrow” therapeutic index. Targeted dosing of busulfan based on its pharmacokinetic (PK) profile is often used. We studied the age-dependent PK profile of single daily dose intravenous Bu (IV Bu) in pediatric patients and determined a target range of Bu exposure that is appropriate for children undergoing reduced-intensity conditioning (RIC) HSPC transplantation. The RIC regimen included: fludarabine 30 mg/m² per day for 6 days (days –10 to –5); single daily treatment doses of IV Bu given over 3 hours each day for 2 days (days –5 and –4), with individualized Bu dosing based on the PK results of a test dose; and rabbit ATG (2 mg/kg/day) or equine ATG (40 mg/kg/day) for 4 days (days –4, –3, –2 and –1). In 2 variations of the regimen, ATG was either eliminated or replaced with extracorporal photopheresis. Allogeneic HSPC were infused on day 0. Cyclosporin A and mycophenolate mofetil were used for graft-versus-host disease prophylaxis. An initial cohort of 52 patients was studied. The median age of the cohort was 7.7 years (range 0.06–17.6), with 19 patients less than 4 years of age (group 1) and 33 patients older than 4 years (group 2). Patients had either malignant or non-malignant diseases (immunodeficiency (n=14), metabolic disease (2), marrow failure (6), histiocytosis (1), ALL (9), AML (5), CML (4), MDS (2), lymphoma (5)), neuroblastoma (4)). Patients received a test dose of IV Bu (0.8 mg/kg) five days before the treatment doses. After the test dose, the median AUC attained was 886 μM·min in group 1 (range 439–1828) and 965 μM·min in group 2 (range 579–1970). Only 59% of the patients in both groups attained a plasma concentration-time “area-under-the-curve” (AUC) within the expected range of 800–1200 μM·min. Based on the test dose PK, patients received individualized treatment doses adjusted to target an AUC of 4000 μM·min per day for 2 days (range 3200–4800 μM·min). The median daily treatment dose of IV Bu given was 3.45 mg/kg in group 1 (range 0.8–7.2) and 3.20 mg/kg in group 2 (range 0.9–5.4), with a distinct and linear inverse-relationship between the weight-normalized treatment dose required and the weight of the patient (treatment dose = 4.0 mg/kg - 0.01*weight in kg, p=0.001). After the adjusted treatment dose, patients who achieved AUC within the targeted range increased to 67% in group 1 and 84% in group 2. Despite the dose adjustment, group 1 attained a lower median treatment dose AUC (3568 μM·min) as compared to group 2 (4035 μM·min) (p=0.001). All patients in this cohort tolerated the conditioning regimen. No patient developed seizures or hepatic veno-occlusive disease. Stable donor chimerism, however, was achieved in only 56% of patients in group 1 and 79% in group 2. Eight patients received a second transplantation because of primary or secondary graft failure. Eight patients died of transplantation-related causes. Because of the concern that a low AUC correlated with an adverse clinical outcome, a second cohort of 23 patients followed a modified protocol in which a treatment dose AUC of 5000 μM·min was targeted (range 4200–5800 μM·min). The median age of the second cohort was 2.1 years (range 0.3–21), with 13 patients less than 4 years and 10 patients older than 4 years. The median daily treatment dose given in this cohort was 4.5 mg/kg (range 2.6–6.5). All patients tolerated the higher Bu treatment dose with minimal regimen-related toxicity. After the treatment dose, patients attained a higher median AUC of 4825 μM·min (range 3719–5900), with over 90% of the patients achieving AUC within the targeted range of 4200–5800 μM·min. The number of patients who achieved stable donor chimerism also increased to 91%. One patient died of a transplantation-related cause. In conclusion, our results showed that RIC regimens utilizing 2 single daily doses of IV Bu were effective and well tolerated in children. An age-dependent variability in the PK profile of IV Bu could have contributed to a higher rate of graft failure in the younger patients after they received the Bu doses targeted to achieve an AUC of 4000 μM·min per day. Since there appears to be a wide safety margin in the upper limit of the Bu therapeutic range in RIC transplantation, a targeted AUC of 5000 μM·min per day for two days is recommended in pediatric patients, especially those less than 4 years of age.