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

Ann Oncol. 2021 Oct;32(10):1286-1293. doi: 10.1016/j.annonc.2021.07.005. Epub 2021 Jul 18.


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.

Keywords: CYP2D6 genotype; breast cancer; mammographic density; tamoxifen; treatment discontinuation.

Publication types

  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antineoplastic Agents, Hormonal / therapeutic use
  • Breast Neoplasms* / drug therapy
  • Breast Neoplasms* / genetics
  • Cytochrome P-450 CYP2D6 / genetics
  • Female
  • Genotype
  • Humans
  • Pharmaceutical Preparations*
  • Tamoxifen


  • Antineoplastic Agents, Hormonal
  • Pharmaceutical Preparations
  • Tamoxifen
  • Cytochrome P-450 CYP2D6