CYP2D6 genotype predicts tamoxifen discontinuation and drug response: a secondary analysis of the KARISMA trial

W. He, M. Eriksson, E. Eliasson, F. Grassmann, M. Bäcklund, M. Gabrielson, M. Hammarström, S. Margolin, L. Thorén, Y. Wengström, S. Borgquist, P. Hall*, K. Czene

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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Background: Guidelines regarding whether tamoxifen should be prescribed based on women's cytochrome P450 2D6 (CYP2D6) genotypes are conflicting and have caused confusion. This study aims to investigate if CYP2D6 metabolizer status is associated with tamoxifen-related endocrine symptoms, tamoxifen discontinuation, and mammographic density change. Patients and methods: We used data from 1440 healthy women who participated the KARISMA dose determination trial. Endocrine symptoms were measured using a modified Functional Assessment of Cancer Therapy – Endocrine Symptoms (FACT-ES) questionnaire. Change in mammographic density was measured and used as a proxy for tamoxifen response. Participants were genotyped and categorized as poor, intermediate, normal, or ultrarapid CYP2D6 metabolizers. Results: The median endoxifen level per mg oral tamoxifen among poor, intermediate, normal and ultrarapid CYP2D6 metabolizers were 0.18 ng/ml, 0.38 ng/ml, 0.56 ng/ml and 0.67 ng/ml, respectively. Ultrarapid CYP2D6 metabolizers were more likely than other groups to report a clinically relevant change in cold sweats, hot flash, mood swings, being irritable, as well as the overall modified FACT-ES score, after taking tamoxifen. The 6-month tamoxifen discontinuation rates among poor, intermediate, normal, and ultrarapid CYP2D6 metabolizers were 25.7%, 23.6%, 28.6%, and 44.4%, respectively. Among those who continued and finished the 6-month tamoxifen intervention, the mean change in dense area among poor, intermediate, normal, and ultrarapid CYP2D6 metabolizers were −0.8 cm2, −4.5 cm2, −4.1 cm2, and −8.0 cm2 respectively. Conclusions: Poor CYP2D6 metabolizers are likely to experience an impaired response to tamoxifen, measured through mammographic density reduction. In contrast, ultrarapid CYP2D6 metabolizers are at risk for exaggerated response with pronounced adverse effects that may lead to treatment discontinuation.

Original languageEnglish
Pages (from-to)1286-1293
Number of pages8
JournalAnnals of Oncology
Issue number10
Early online date18 Jul 2021
Publication statusPublished - 1 Oct 2021

Bibliographical note

Funding Information:
This work was supported by Märit and Hans Rausing's Initiative Against Breast Cancer (no grant number); the Kamprad Foundation [grant number: 20150052 ]; and Stockholm County Council [grant number: 4-2645/2015 ]; the Swedish Research Council [grant number 2018-02547 ]; the Swedish Cancer Society [grant number 19 0266 ]; and Swedish Research Council for Health, Working Life and Welfare (FORTE) [grant number 2016-00081 , 2018-00877 ]. WH is supported by Zhejiang University through ‘Hundred Talents Program’ (no grant number). The funding source had no role in study design, collection, analysis, or interpretation of the data, the writing of the manuscript, or the decision to approve publication of the finished manuscript.


  • breast cancer
  • CYP2D6 genotype
  • mammographic density
  • tamoxifen
  • treatment discontinuation


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