A rapid & sensitive liquid chromatography- tandem mass spectrometry method for the quantitation of busulfan levels in plasma & application for routine therapeutic monitoring in haematopoietic stem cell transplantation

BACKGROUND & OBJECTIVES Busulfan (Bu) in combination with cyclophosphamide is widely used in myeloablative conditioning regimen prior to haematopoietic stem cell transplantation (HSCT). Its narrow therapeutic range and toxic side effects at high systemic exposure and graft rejection at low exposure emphasize the need for busulfan dose optimization using targeted dose adjustment prior to HSCT. We report here a rapid and sensitive method to quantitate busulfan plasma levels in patients receiving busulfan as part of pre-transplant conditioning. METHODS The method involves simple protein precipitation of the plasma followed by analysis using a high performance liquid chromatography (HPLC) with tandem mass spectrometry - electrospray ionization technique (LC-ESI MS/MS) in positive ionization mode and quantified using multiple reaction monitoring (MRM). Deuterated busulfan (d₈-busulf`an) was used as the internal standard. RESULTS The method was linear for the concentration ranging from 0 to 4000 ng/ml of busulfan with a limit of detection of 2 ng/ml and limit of quantitation of 5 ng/ml. The assay was accurate for serial concentrations of Bu in plasma for five consecutive days and the CV was less than 10 per cent. CONCLUSION Using this rapid and sensitive method, busulfan levels were targeted and subsequent doses adjusted at our center in 26 patients receiving high dose busulfan in combination with cyclophosphamide or fludarabine.