The Underrated Risks of Tamoxifen Drug Interactions

Tamoxifen is a prodrug, and most of the therapeutic effect in treating breast cancer stems from its metabolite, endoxifen. Since cytochrome P450 (CYP) 2D6 is the most important enzyme in the production of endoxifen, drugs that inhibit CYP2D6 would be expected to reduce tamoxifen efficacy. In addition to drug–drug interactions (DDI) involving CYP2D6, there is growing evidence that enzyme inducers can substantially alter the disposition of endoxifen, reducing tamoxifen efficacy. Although the clinical evidence on the impact of CYP2D6 inhibitors on tamoxifen efficacy is mixed, there were serious flaws in many of the studies. Thus, there is a reasonable chance that CYP2D6 inhibitors do in fact inhibit tamoxifen efficacy. Tamoxifen has extraordinarily complex pharmacokinetics, with more than a dozen drug-metabolizing enzymes and transporters involved in its disposition. Enzyme inducers may increase the activity of several of these pathways, including phase II enzymes, ABC transporters, and various CYP enzymes other than CYP2D6. Based on current clinical evidence, one could argue that enzyme inducers are potentially more dangerous than CYP2D6 inhibitors in patients taking tamoxifen. Moreover, early evidence suggests that the combination of CYP2D6 inhibitors plus enzyme inducers may produce catastrophic inhibition of tamoxifen efficacy. One could argue that, given the available evidence, an agnostic “wait and see” position on tamoxifen DDI is ethically untenable, and that many women with breast cancer are currently being subjected to an unnecessary risk of cancer recurrence. Specific recommendations to reduce the risk of adverse tamoxifen DDI are offered for consideration.