How often did Belgian physicians co-prescribe tamoxifen with strong CYP2D6 inhibitors over the last 6 years?

Acta clinica Belgica 02/2014; 69(1):47-52. DOI: 10.1179/0001551213Z.00000000017
Source: PubMed


Tamoxifen is widely used in the treatment of breast cancer. It is a pro-drug metabolized to the more active endoxifen through CYP2D6. Concomitant intake of CYP2D6 inhibitors results in lower endoxifen levels and could influence efficacy. The objective of this study was to evaluate the evolution of co-prescription of tamoxifen and CYP2D6 inhibitors in Belgium.

Data were retrieved from the Pharmanet database of the National Institute for Health and Disability Insurance for the period January 2006-December 2009. For the analysis of the evolution of the co-prescription, the period was divided in subperiods of 2 months. The category tamoxifen+CYP2D6 inhibitor was defined as women who were delivered tamoxifen and a CYP2D6 inhibitor in that subperiod. The results were validated on the period December 2011-May 2012.

The percentage of co-prescription decreased over time for the strong CYP2D6 inhibitors and increased for the weak CYP2D6 inhibitor, with these trends persisting in 2012. Tamoxifen and CYP2D6 inhibitors were mostly prescribed by general practitioners and gynaecologists and by general practitioners and psychiatrists, respectively.

This study shows that a proportion of women taking tamoxifen in Belgium are prescribed a strong CYP2D6 inhibitor, which could affect tamoxifen efficacy. Over time, the concomitant intake decreased. Paroxetine was the most prescribed strong CYP2D6 inhibitor. Venlafaxine, a weak CYP2D6 inhibitor, was prescribed more often. This study also shows that tamoxifen and the CYP2D6 inhibitors are not only prescribed by physicians specialized in breast cancer; therefore, all physicians should be aware of this interaction.

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    ABSTRACT: Clinical response to tamoxifen varies widely among women treated with this drug for hormone receptor-positive breast cancer. The principal active metabolite – endoxifen – is generated through hepatic metabolism of tamoxifen, with key roles for cytochrome P450 (CYP) CYP2D6 and CYP3A. By influencing endoxifen formation, genetic variants of CYP2D6 may affect response to tamoxifen. After a decade of research, examining the effects of CYP2D6 genetic variants on tamoxifen efficacy, there is still no agreement on the clinical utility of CYP2D6 genotype as biomarker for the prediction of breast cancer outcome, because studies revealed conflicting results. However, tamoxifen metabolism is complex and involves several other drug-metabolizing enzymes. Genetic variants of other CYP enzymes, including CYP3A4 and CYP2C9/19, as well as co-medication interfering with the metabolic activity of CYP2D6 and CYP3A4 have been shown to affect endoxifen concentrations and may also contribute to the variability in response to tamoxifen. Phenotyping strategies can predict endoxifen exposure more accurately than CYP2D6 genotype, but do not take into account all factors influencing endoxifen exposure. Therapeutic drug monitoring (TDM) is likely to be the optimal strategy for individualization of tamoxifen treatment. According to a growing amount of literature, endoxifen concentration seems to be a predictor of clinical outcome. The relationship between endoxifen levels and breast cancer outcomes has to be replicated and confirmed and the value of TDM should be evaluated in prospective clinical trials. Caution is advised regarding the concomitant use of medications which could interact with tamoxifen, including inhibitors and inducers of CYP enzymes.
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