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The Underrated Risks of Tamoxifen Drug Interactions

Philip D Hansten¹

¹Professor Emeritus, School of Pharmacy, University of Washington, Seattle, WA, 98195, USA. hansten@u.washington.edu.

European Journal of Drug Metabolism and Pharmacokinetics

|April 12, 2018

Abstract:

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.

Insights

Tamoxifen efficacy may be reduced by drug interactions. Enzyme inducers, particularly when combined with CYP2D6 inhibitors, pose a significant risk, potentially increasing breast cancer recurrence. Careful management of tamoxifen drug-drug interactions is crucial.

Area of Science:

Pharmacology
Oncology
Drug Metabolism

Background:

Tamoxifen is a prodrug for breast cancer treatment, with its metabolite endoxifen mediating therapeutic effects.
Cytochrome P450 (CYP) 2D6 is critical for endoxifen production, making CYP2D6 inhibitors potential disruptors of tamoxifen efficacy.
Complex pharmacokinetics of tamoxifen involve numerous enzymes and transporters, influencing its disposition.

Purpose of the Study:

To evaluate the impact of drug-drug interactions (DDIs) on tamoxifen efficacy, focusing on CYP2D6 inhibitors and enzyme inducers.
To assess the relative risks posed by CYP2D6 inhibitors versus enzyme inducers in tamoxifen therapy.
To highlight the potential dangers of combined CYP2D6 inhibitor and enzyme inducer use.

Main Methods:

Review of existing clinical evidence on tamoxifen pharmacokinetics and DDIs.
Analysis of studies investigating the effects of CYP2D6 inhibitors on tamoxifen efficacy.
Evaluation of the impact of enzyme inducers on tamoxifen metabolism and disposition.

Main Results:

Evidence suggests CYP2D6 inhibitors may reduce tamoxifen efficacy, despite mixed clinical data and study limitations.
Enzyme inducers can significantly alter endoxifen disposition, potentially posing a greater risk than CYP2D6 inhibitors alone.
Combined use of CYP2D6 inhibitors and enzyme inducers may lead to severe inhibition of tamoxifen efficacy.

Conclusions:

A passive approach to tamoxifen DDIs is ethically questionable, exposing patients to unnecessary risks of cancer recurrence.
Enzyme inducers represent a significant, potentially underestimated, risk in tamoxifen therapy.
Recommendations for managing tamoxifen DDIs are necessary to mitigate adverse outcomes and improve patient safety.

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