Variability in the pharmacokinetics of intravenous busulphan given as a single daily dose to paediatric blood or marrow transplant recipients

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Abstract
To examine inter- and intrapatient variability in the pharmacokinetics of intravenous (i.v.) busulphan given as a single daily dose to children with malignant (n = 19) and nonmalignant (n = 21) disease. Busulphan (120 mg m(-2), 130 mg m(-2) or 3.2 mg kg(-1)) was administered over median 2.1 h. Blood samples (4-10) were collected after the first dose, busulphan concentrations were measured and pharmacokinetic parameters, including clearance (CL) and area under the concentration-time curve (AUC), were determined using the Kinetica software (Innaphase). Interpatient variability was assessed as percent coefficient of variation (% CV). Intrapatient variability was assessed by calculating percent differences between observed full dose AUC and AUC predicted from an initial 65 mg m(-2) dose in 13 children who had busulphan pharmacokinetic monitoring. Clearance of i.v. busulphan in 40 children was 4.78 +/- 2.93 l h(-1) (% CV 61%), 0.23 +/- 0.08 l h(-1) kg(-1) (% CV 35%) and 5.79 +/- 1.59 l h(-1) m(-2) (% CV 27%). Age correlated significantly (p < 0.001) with CL (l h(-1)) and CL (l h(-1) kg(-1)), but not with CL (l h(-1) m(-2)). AUC normalized to the 130 mg m(-2) dose ranged from 14.1 to 56.3 mg l(-1) x h (% CV 37%) and also did not correlate with age. Interpatient variability in CL (l h(-1) m(-2)) was highest in six children with immune deficiencies (60%) and lowest in seven children with solid tumours (14%). Intrapatient variability was <13% for nine (of 13) children, but between 20 and 44% for four children. There is considerable inter- and intrapatient variability in i.v. busulphan CL (l h(-1) m(-2)) and exposure that is unrelated to age, especially in children with immune deficiencies. These results suggest that monitoring of i.v. busulphan pharmacokinetics is required.

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