High-performance liquid chromatographic assay for glucuronidation activity of 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan (CPT-11), in human liver microsomes

A simple and sensitive assay for glucuronidation activity of 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan (CPT-11), in human liver microsomes by high-performance liquid chromatography (HPLC) with fluorescence detection is reported. The method was validated for the determination of SN-38 glucuronide (SN-38G) with respect to specificity, linearity, recovery, stability, precision, accuracy, and limits of detection and quantitation. There was no interference from matrix and non-enzymatic reactions. The calibration curve for SN-38G was linear from 5 to 500 nM. Average recoveries ranged from 98 to 100% in spiked human liver microsome samples, and the SN-38G was stable at 4 degrees C for at least 72 h. The newly developed method was found to be more sensitive and selective than previous methods using thin layer chromatography and HPLC. The limit of quantitation for SN-38G was 5 nM (2.5 pmol/assay). The intra- and inter-day precision and accuracy were less than 7 and 4%, respectively. The intra- and inter-day precision of enzyme assay for UDP-glucuronosyltransferase (UGT) activity toward SN-38 in human liver microsomes was less than 4%. With this improved sensitivity, the kinetics of SN-38 glucuronidation in human liver microsomes could be determined more precisely. Therefore, this method is applicable to in vitro study on the side effects and drug interactions of CPT-11 using small amounts of biological sample.