Bosutinib is a second-generation tyrosine kinase inhibitor indicated for treatment of chronic myeloid leukemia (CML) in adult patients. The safety and efficacy of bosutinib in patients younger than 18 years of age have not been established. We here report the case of a 4-year-old male with CML who was treated with bosutinib during coordination of human leukocyte antigen–matched unrelated bone-marrow transplantation because of insufficient responses to imatinib and dasatinib. The patient achieved a complete cytogenetic response immediately after starting bosutinib at 180 mg/day (290 mg/m2/day). Because toxicity was tolerable, the dose was increased to 200 mg/day (330 mg/m2/day). A complete cytogenetic response was maintained, but a major molecular response was not achieved 6 months after initiation of treatment with bosutinib. At steady state, maximum plasma concentration, minimum plasma concentration, and area under the plasma concentration-time curve were 89.2 ng/mL, 16.7 ng/mL, and 1017.4 ng·hr/mL, respectively, at 290 mg/m2/day; and 141.1 ng/mL, 18.9 ng/mL, and 1278.5 ng·hr/mL, respectively, at 330 mg/m2/day. To the best of our knowledge, this is the first case report to show the pharmacokinetics of bosutinib with efficacy and safety in a pediatric patient with CML. This rare case in a very young child with CML can also be valuable reference for clinical practice.

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