Effects of omeprazole on the pharmacokinetics and toxicities of irinotecan in cancer patients: A prospective open-label cross-over drug-interaction study

Abstract

2502 Background: The proton pump inhibitor omeprazole (Losec) is one of the most extensively prescribed medications worldwide and within its class, omeprazole is most frequently associated with drug interactions. In vitro studies have shown that omeprazole can alter the function of metabolic enzymes and efflux transporters that are involved in the metabolism of irinotecan, such as UGT1A1, CYP3A, and ABCC2. 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 one course (C1) of single agent irinotecan (600 mg i.v., 90 min) followed three weeks later by a second course (C2) with concurrent use of omeprazole 40 mg once daily, which was started 2 weeks prior to C2. Plasma samples were obtained up to 55 hours after infusion and analyzed for irinotecan, and its metabolites SN-38, SN-38 glucuronide (SN-38G), NPC, and APC by reversed-phase high-performance liquid chromatography with fluorescence detection. Non-compartmental modelling of pharmacokinetic data was performed with WinNonLin. Toxicities were monitored during both courses and graded according to the CTCAE criteria v3.0. Paired statistical tests were performed with SPSS. Results: The mean AUCs of irinotecan and all metabolites were not significantly different between both courses (p>.151; see table). In addition, no differences were seen in Cmax and clearance of irinotecan and all metabolites between C1 and C2 (p>.072). The nadir ANC and WBC and the percentage decrease in ANC and WBC from baseline were not different between both courses (p>.529). Neither were there significant differences in the severity of neutropenia, febrile neutropenia, diarrhea, nausea, and vomiting. Conclusions: 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. [Table: see text] No significant financial relationships to disclose.