What is the mechanism of action of tamoxifen in breast cancer treatment?

Tamoxifen is primarily used in the treatment of estrogen receptor-positive breast cancer. Its mechanism of action involves selectively binding to estrogen receptors in breast tissue, which leads to inhibition of estrogen-mediated effects that promote the growth and proliferation of breast cancer cells. By blocking these receptors, tamoxifen effectively reduces the stimulatory effect of estrogen on cancer cells, thereby slowing the progression of the disease.

Notably, while tamoxifen functions as an estrogen receptor antagonist in breast tissue, it can also act as an estrogen agonist in other tissues, such as the uterus and bone. This dual action is important, as it can have beneficial effects, such as reducing the risk of osteoporosis, while potentially increasing the risk of endometrial cancer due to estrogenic stimulation in the uterus.

The other choices involve mechanisms not related to the action of tamoxifen. It does not interact with testosterone or androgen receptors, nor does it inhibit estrogen production. Instead, its antitumor effects in breast cancer are specifically through blocking the estrogen receptors, validating the chosen answer.