Comparison of busulfan pharmacokinetics between four-times-daily and once-daily administration in pediatric patients: a preliminary prospective observational trial.

Therapeutic drug monitoring (TDM) of busulfan (BU) is useful for achieving the target area under the curve (AUC) because its effective plasma-concentration range is narrow. This preliminary observational study evaluated the pharmacokinetic (PK) parameters of BU administered four times daily or once daily to pediatric patients. The plasma concentrations were measured at both the test dose and first dose, and the doses on day 1 and days 2-4 were determined based on each TDM. A comparison of PK parameters between four-times-daily and once-daily administration was performed for both the test dose and first dose of BU. Of the 11 patients, five received four-times-daily BU and six received once-daily BU. The Vd for once-daily administration was higher than that for four-times-daily administration for the first dose but not for the test dose. The ratio of actual AUC for the first dose to estimated AUC guided by the test dose was lower with once-daily administration than with four-times-daily administration. These results indicate that the PK parameters of BU administered once daily are challenging to predict based on the TDM of the test dose. TDM should be considered on day 1 to achieve the target AUC, especially with once-daily administration.