Fast liquid chromatography-tandem mass spectrometry method for routine assessment of irinotecan metabolic phenotype

Irinotecan (CPT-11) is an anticancer drug with a complex in vivo metabolism widely used in the treatment of colon cancer. The assessment of the CPT-11 metabolic phenotype is an important clinical component for personalizing the administered dose. In this study, we report the development of a rapid and sensitive liquid chromatography-tandem mass spectroscopy method for the simultaneous quantitation of CPT-11 and its major metabolites generated by hydrolysis (SN-38, active metabolite) or through oxidative (NPC and APC) and glucuronidation (SN-38G) pathways. The method used a small volume of plasma and is based on simple protein plasma precipitation with two volumes of acetonitrile containing camptothecine as an internal standard. Analytes extracted in the supernatant fraction were converted to the lactone form and further separated by an acetonitrile gradient on a Kinetex C18 (50 × 2 mm) column in the presence of 0.01% formic acid. The quantitative measurement was performed by an API 4000/Qtrap operating in the triple-quadruple mode. The Multiple Reaction Monitoring transitions were: m/z 587→167 for CPT-11, m/z 393→349 for SN-38, m/z 619→393 for APC, m/z 519→393 for NPC, and m/z 569→393 for SN-38G. The mean overall recovery in plasma was in the range of 78% to 86% with a low matrix suppression effect (9.0% or less). The lower limit of quantitation was 0.2 ng/mL for NPC and SN-38 and 0.5 ng/mL for SN-38G, APC, and CPT-11. Linearity was checked up to 2000 ng/mL, whereas the intra- and interday accuracy and precision for both the parent drug and its metabolites was -3.7% to 13.8% and 3.7% to 13.1%, respectively. The method proved to be robust and suitable for therapeutic drug monitoring as well as for pharmacogenetic- pharmacokinetic correlation studies.