Tamoxifen is used to treat certain breast tumours. In the liver, tamoxifen is converted into an active substance called endoxifen. How does endoxifen reduce the growth of these tumours?

• A. Endoxifen binds to the estrogen receptor and activates transcription.
• B. Endoxifen binds to the estrogen receptor and enhances estrogen signaling.
• C. Endoxifen binds to the estrogen receptor and prevents estrogen binding, blocking transcription.
• D. Endoxifen has no effect on estrogen signaling.

Answer: (C)

Endoxifen works by blocking estrogen-driven growth signals in breast tissue. The estrogen receptor is a transcription factor: when estrogen binds it, the receptor activates transcription of genes that promote cell proliferation. Endoxifen binds to the estrogen receptor and acts as an antagonist in breast tissue, preventing estrogen from binding effectively and blocking the receptor’s ability to drive transcription. With estrogen signaling suppressed, the expression of estrogen-responsive genes decreases and tumor cell growth slows. This is why the answer focuses on endoxifen binding to the receptor and preventing estrogen binding, thereby blocking transcription.