A new UPLC-MS/MS method for the determination of irinotecan and 7-ethyl-10-hydroxycamptothecin (SN-38) in mice: application to plasma and brain pharmacokinetics.

A sensitive and accurate liquid chromatography method with mass spectrometry detection using MRM in positive ion mode was developed and validated for the simultaneous quantification of irinotecan (CPT-11) and 7-ethyl-10-hydroxycamptothecin (SN-38) in mouse plasma and brain. Camptothecin (CPT) was used as internal standard. A single step protein precipitation was used for plasma sample preparation, and a liquid-liquid extraction was needed for brain sample preparation. The method was validated with respect to selectivity, extraction recovery, linearity, intra- and inter-day precision and accuracy, limit of quantification and stability. Limits of quantification were 5 ng/mL for CPT-11 and SN-38 in plasma samples and 1.25 ng/g in brain. Linear calibration curves were obtained over concentration ranges of 5-5000 ng/mL in plasma and 1.25-1250 ng/g in brain for CPT-11 and SN-38. The intra-day and inter-day variation (relative standard deviation, R.S.D.) found to be less than 15% for both substances in both media. Stability studies showed that plasma carboxylesterase had to be inactivated in order to prevent in vitro conversion of CPT-11 into SN-38. Zinc sulfate (1 M) was used to inactivate the enzyme before sample storage. Brain samples did not require enzyme inactivation. This method was successfully applied to perform brain and plasma pharmacokinetic studies of CPT-11 and SN-38 in mice after intraperitoneal administration.