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Busulfan is used in preparative regimens before stem cell transplantation. There is significant interpatient variability in busulfan pharmacokinetics (PK) and exposure is related to outcome. Polymorphisms in genes encoding glutathione-S-transferases have been associated with busulfan PK but only explain a limited portion of the observed variability.The aim of this study is to identify additional genetic variants associated with busulfan PK by interrogating 1936 variants in 225 genes involved in drug absorption, distribution, metabolism, and excretion (ADME).In an exploratory cohort (n=65), patients who received busulfan were genotyped with the DMET array. Top SNPs and haplotypes associated with busulfan clearance were validated in an independent validation cohort (n=78).In the exploratory cohort, seven variants were identified to be associated with busulfan clearance (P<0.001). In the validation cohort, only GSTA5 (rs4715354 and rs7746993) remained significantly associated with busulfan clearance (P=0.025).This is the first study using an exploratory pharmacogenetic approach to explain the interindividual variability in busulfan PK. The role of glutathione-S-transferases was confirmed, but no additional genetic markers involved in drug ADME appear to be associated with busulfan PK. [ABSTRACT FROM AUTHOR]