Liquid Chromatography-Tandem Mass Spectrometry Method for the Quantification of Plasma Busulfan

Busulfan is an alkylating agent and functions as a myeloablative and anti-leukemic chemotherapy drug. It is widely used with cyclophosphamide for conditioning patients undergoing bone marrow transplantation for myeloid leukemia. Studies have shown that the busulfan plasma concentration correlates better with clinical efficacy and toxicity than the patient's administered dosage. Low concentrations predispose to disease recurrence and even graft rejection, and higher concentrations can increase the risk of hepatic toxicity. As a result, dosing levels can vary significantly from patient to patient. Therapeutic drug monitoring (TDM) of busulfan plasma concentration guides the dosage adjustment to optimally achieve complete bone marrow ablation while minimizing the dosage-dependent toxicity. The quick and precise (precision <10%) UPLC-MS/MS method described here for monitoring plasma busulfan levels between 50 ng/mL and 5000 ng/mL involves the addition of an organic solvent and deuterated internal standard (busulfan d-8) followed by a liquid-liquid extraction, injection of the extract onto a C18 column, and analysis by multiple reaction monitoring (MRM) in ESI-positive mode.