Which drug is primarily used as a selective estrogen receptor modulator in ER+ metastatic breast cancer?

• A. Doxorubicin
• B. Tamoxifen
• C. Vincristine
• D. Carmustine

Answer: (B)

Tamoxifen is a classic selective estrogen receptor modulator used in ER-positive breast cancer. It works by binding to the estrogen receptor in breast tissue and blocking estrogen from activating gene expression that drives tumor cell growth. This makes it effective in slowing or stopping progression of ER-positive cancers in metastatic settings. The other drugs are cytotoxic chemotherapies: doxorubicin intercalates DNA, vincristine disrupts microtubules, and carmustine alkylates DNA. They don’t modulate the estrogen receptor, so they’re not SERMs. Tamoxifen’s action as an ER antagonist in breast tissue (though it can have agonist effects elsewhere) specifically targets estrogen-driven proliferation, which is why it’s the best choice for this scenario.