Clarifying busulfan metabolism and drug interactions to support new therapeutic drug monitoring strategies: a comprehensive review

Introduction: Busulfan (Bu) is an alkylating agent with a limited therapeutic margin and exhibits inter-patient variability in pharmacokinetics (PK). Despite decades of use, mechanisms of Bu PK-based drug-drug interactions (DDIs), as well as the negative downstream effects of these DDIs, have not been fully characterized. Areas covered: This article provides an overview of Bu PK, with a primary focus on how known and potentially unknown drug metabolism pathways influence Bu-associated DDIs. In addition, pharmaco-genomics of Bu chemotherapy and Bu-related DDIs observed in the stem cell transplant clinic (SCT) are summarized. Finally the increasing importance of Bu therapeutic drug monitoring is highlighted. Expert opinion: Mechanistic studies of Bu metabolism have shown that in addition to GST isoenzymes, other oxidative enzymes (CYP, FMO) and ABC/MDR drug transporters likely contribute to the overall clearance of Bu. Despite many insights, results from clinical studies, especially in polypharmacy settings and between pediatric and adult patients, remain conflicting. Further basic science and clinical investigative efforts are required to fully understand the key factors determining Bu PK characteristics and its effects on complications after SCT. Improved TDM strategies are promising components to further investigate, for instance DDI mechanisms and patient outcomes, in the highly complex SCT treatment setting.