Target Dose Adjustment of Oral Busulfan Using a Test Dose in Japanese Adults Undergoing Hematopoietic Stem Cell Transplantation.

Oral busulfan (BU) is widely used in patients undergoing hematopoietic stem cell transplantation (HST). Therapeutic effect of BU is related to the area under the plasma concentration-time curve (AUC) or the average plasma concentrations at steady state (Css). It has been shown that BU pharmacokinetics (PK) are highly variable and the dose adjustment according to the BU levels is critical to get the successful results of HST for Caucasians. BU is metabolized mainly in the liver through conjugation with glutathione by glutathione S-transferase (GST). Recent study has shown that the polymorphisms of GST genes are associated with the risk of developing hepatic veno-occlusive disease in patients undergoing HST. Ethnic variation is also demonstrated in relation to a gene deletion polymorphism of GST, suggesting that BU metabolism is influenced by the race. BU PK have been extensively investigated in Caucasians and few studies have focused on Asian people. We therefore underwent a prospective trial of adjusting BU doses depending on the individual BU PK in 36 Japanese patients aged from 16 to 64 years (median; 47 years). All patients received a busulfan-containing conditioning regimen and underwent allogeneic HST. Individual PK were studied following a 0.5 mg/kg test dose of BU administered orally. BU concentrations were measured by a high-performance liquid chromatographic method and individual PK parameters of BU were calculated with a one-compartment model by using Bayesian modeling program. The median clearance (CL/F) was 0.16 L/hr/kg (0.09 to 0.34 L/hr/kg), the median volume of distribution (Vd/F) 0.65 L/kg (range; 0.41 to 0.97 L/kg), the median elimination half-life (t_1/2) 2.9 hr (range; 1.9 to 7.0 hr), and the median absorption rate constant (k_a) 2.46 /hr (range; 0.53 to 6.03 /hr). BU doses were adjusted to achieve a target BU Css between 800 and 900 ng/mL. Twenty-six (72%) patients were required to reduce BU doses and adjusted BU doses ranged from 0.51 to 1.29 mg/kg/time (median; 0.85 mg/kg/time). After administrating the 6th dose of BU for the conditioning regimen, BU PK were analyzed. Expected Css was significantly correlated with observed Css and predictability of the test dose was 106.4±21.8%. Engraftment was successful in 34 of 36 (94%) patients. Grade 2 to 4 regimen-related toxicity except stomatitis occurred in 4 (11%) patients. These data demonstrate that BU PK vary widely from one patient to another after oral BU in Japanese and individualization of BU doses depending on the BU PK are useful in improving clinical outcome in patients undergoing HST.