Purpose: To study the possible pharmacokinetic and pharmacodynamic interactions between irinotecan and methimazole. Methods: A patient treated for colorectal cancer with single agent irinotecan received methimazole co-medication for Graves' disease. Irinotecan pharmacokinetics and side effects were followed during a total of four courses (two courses with and two courses without methimazole). Results: Plasma concentrations of the active irinotecan metabolite SN-38 and its inactive metabolite SN-38-Glucuronide were both higher (a mean increase of 14 and 67%, respectively) with methimazole co-medication, compared to irinotecan monotherapy. As a result, the mean SN-38 glucuronidation rate increased with 47% during concurrent treatment. Other possible confounding factors did not change over time. Specific adverse events due to methimazole co-treatment were not seen. Conclusions: Additional in vitro experiments suggest that these results can be explained by induction of UGT1A1 by methimazole, leading to higher SN-38G concentrations. The prescribed combination of these drugs may lead to highly toxic intestinal SN-38 levels. We therefore advise physicians to be very careful in combining methimazole with regular irinotecan doses, especially in patients who are prone to irinotecan toxicity.

Additional Metadata
Keywords CYP3A, Diarrhea, Drug interaction, Irinotecan, Methimazole, SN-38, UGT1A
Persistent URL dx.doi.org/10.1007/s00280-010-1414-x, hdl.handle.net/1765/33889
Journal Cancer Chemotherapy and Pharmacology
van der Bol, J.M, Visser, T.J, Loos, W.J, de Jonge, M.J.A, Wiemer, E.A.C, van Aken, M.O, … Mathijssen, A.H.J. (2011). Effects of methimazole on the elimination of irinotecan. Cancer Chemotherapy and Pharmacology, 67(1), 231–236. doi:10.1007/s00280-010-1414-x