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Clin Pharmacol Ther. 2012 Jul;92(1):62-7. doi: 10.1038/clpt.2011.372. Epub 2012 May 23.

Effects of CYP induction by rifampicin on tamoxifen exposure.

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Department of Medical Oncology, Erasmus MC-Daniel den Hoed Cancer Center, University Medical Center, Rotterdam, The Netherlands.


Tamoxifen undergoes biotransformation into several metabolites, including endoxifen. Differences in metabolism contribute to the interindividual variability in endoxifen concentrations, potentially affecting treatment efficacy. We evaluated the effects of cytochrome P450 (CYP) induction by rifampicin on the exposure levels of tamoxifen and its metabolites and found that coadministration of rifampicin resulted in markedly reduced (up to 86%, P ≤ 0.040) concentrations of tamoxifen and its metabolites. Given the extensive metabolism undergone by tamoxifen, several factors may have contributed to this effect. Similar drug-drug interactions may exist between tamoxifen and other strong CYP inducers.

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