Background: Omeprazole is one of the most prescribed medications worldwide and within the class of proton pump inhibitors, it is most frequently associated with drug interactions. In vitro studies have shown that omeprazole can alter the function of metabolic enzymes and transporters that are involved in the metabolism of irinotecan, such as uridine diphosphate glucuronosyltransferase subfamily 1A1 (UGT1A1), cytochrome P-450 enzymes subfamily 3A (CYP3A) and ATP-binding cassette drug-transporter G2 (ABCG2). In this open-label cross-over study we investigated the effects of omeprazole on the pharmacokinetics and toxicities of irinotecan. Methods: Fourteen patients were treated with single agent irinotecan (600 mg i.v., 90 min) followed 3 weeks later by a second cycle with concurrent use of omeprazole 40 mg once daily, which was started 2 weeks prior to the second cycle. Plasma samples were obtained up to 55 h after infusion and analysed for irinotecan and its metabolites 7-ethyl-10- hydroxycampothecin (SN-38), SN-38-glucuronide (SN-38G), 7-ethyl-10-[4-(1- piperidino)-1-amino]-carbonyloxycamptothecin (NPC) and 7-ethyl-10-[4-N-(5- aminopentanoic acid)-1-piperidino]-carbonyloxycamptothecin (APC) by high-performance liquid chromatography (HPLC). Non-compartmental modelling was performed. Toxicities were monitored during both cycles. Paired statistical tests were performed with SPSS. Results: The exposure to irinotecan and its metabolites was not significantly different between both cycles. Neither were there significant differences in the absolute nadir and percentage decrease of WBC and ANC, nor on the incidence and severity of neutropenia, febrile neutropenia, diarrhoea, nausea and vomiting when irinotecan was combined with omeprazole. Conclusion: Omeprazole 40 mg did not alter the pharmacokinetics and toxicities of irinotecan. This widely used drug can, therefore, be safely administered during a 3-weekly single agent irinotecan schedule.

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European Journal of Cancer
Erasmus MC: University Medical Center Rotterdam

van der Bol, J., Loos, W., de Jonge, M., van Meerten, E., Konings, I., Lam, M., … Mathijssen, R. (2011). Effect of omeprazole on the pharmacokinetics and toxicities of irinotecan in cancer patients: A prospective cross-over drug-drug interaction study. European Journal of Cancer, 47(6), 831–838. doi:10.1016/j.ejca.2010.11.030