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Key Points

  • Drug summary:

    • Tamoxifen is widely used antiestrogen therapy for pre- and postmenopausal women with metastatic breast cancer, for prevention of recurrence as adjuvant therapy, and for women with a high risk of developing breast cancer as chemoprevention.

    • Five years of adjuvant tamoxifen safely reduces 15-year risks of breast cancer recurrence.

    • The most frequent adverse events by tamoxifen treatment is hot flash (>60%).

  • Pharmacogenomics:

    • Endoxifen is an active metabolite of tamoxifen, and has the higher affinity to estrogen receptors (ERs) and greater potency in suppressing estrogen-dependent breast cancer cell proliferation than tamoxifen.

    • CYP2D6 plays an important role in biotransformation of tamoxifen into its active metabolites including endoxifen.

    • There is a large interindividual variability in the metabolism of drugs that can be explained largely by CYP2D6 genetic polymorphisms affecting the enzymatic activity and expression level.

    • Women with reduced CYP2D6 enzymatic activity display low endoxifen concentrations and are expected to have poor response to tamoxifen therapy.

Indication, Efficacy, and Side Effects of Tamoxifen

Indication for Tamoxifen

Tamoxifen is an antagonist of the ER in breast tissue via its active metabolite, hydroxytamoxifen or endoxifen. Tamoxifen has been proven to be effective in the adjuvant treatment and metastatic tumor of breast cancer. Tamoxifen is indicated for the treatment of patients with

  • Pre- and postmenopausal breast cancer

  • ER-positive breast cancer

Efficacy of Tamoxifen

Adjuvant tamoxifen therapy is a major endocrine treatment option, especially for women who have ovarian estrogenic activity that cannot be regulated by aromatase inhibitors. Five-year adjuvant tamoxifen treatment effectively reduces recurrence of ER-positive tumors. Tamoxifen is also approved for the chemoprevention of breast cancer for women with high risk of developing the disease.

Side Effects of Tamoxifen

Side effects caused by tamoxifen are relatively mild compared to chemotherapy, but are rarely severe enough to require discontinuation of treatment. The adverse event observed most commonly in patients treated with tamoxifen is hot flashes. Table 8-1 shows the frequency of adverse events of tamoxifen in the NSABP B-14 study which is randomized to 5-years tamoxifen or placebo following primary surgery.

Table 8-1Side Effects of Tamoxifen

Screening and Counseling


Although expression level of ER or the number of cancer cells expressing ER in breast cancer tissue are usually evaluated before initiation of tamoxifen treatment, genetic screening is not yet clinically available for selection of patients who should be treated with tamoxifen in adjuvant setting ...

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