Question: How do antihormonal agents help treat breast cancer in patients who have failed prior therapies?

Context:
For women with hormone receptor-positive advanced breast cancer, endocrine therapies, including the selective estrogen receptor modulator tamoxifen, the aromatase inhibitors anastrozole, letrozole, and exemestane, and the selective estrogen receptor degrader fulvestrant, are recommended in clinical guidelines.
Here we describe and characterize a novel, orally bioavailable small-molecule selective estrogen receptor degrader, RAD1901, and evaluate its therapeutic potential for the treatment of breast cancer.
Identification of GDC-0810 (ARN-810), an Orally Bioavailable Selective Estrogen Receptor Degrader (SERD) that Demonstrates Robust Activity in Tamoxifen-Resistant Breast Cancer Xenografts.
Fulvestrant is a steroid-based, selective estrogen receptor degrader (SERD) that both antagonizes and degrades ER-α and is active in patients who have progressed on antihormonal agents. 
Evaluation of the pharmacological activities of RAD1901, a selective estrogen receptor degrader.
However, being the only SERD currently known to access the brain, RAD1901 merits evaluation as a targeted therapy for the treatment of breast cancer brain metastases.
Tamoxifen, toremifene, and the selective estrogen receptor degrader fulvestrant are used to treat breast cancer, and tamoxifen and raloxifene protect against breast cancer in high-risk women.
The turnover of estrogen receptor α by the selective estrogen receptor degrader (SERD) fulvestrant is a saturable process that is not required for antagonist efficacy.
Currently, ICI 182,780 (ICI, fulvestrant) is the only SERD approved for the treatment of breast cancer. 
Fulvestrant is a steroid-based, selective estrogen receptor degrader (SERD) that both antagonizes and degrades ER-á and is active in patients who have progressed on antihormonal agents.
Fulvestrant is a steroid-based, selective estrogen receptor degrader (SERD) that both antagonizes and degrades ER-α and is active in patients who have progressed on antihormonal agents
RAD1901: a novel, orally bioavailable selective estrogen receptor degrader that demonstrates antitumor activity in breast cancer xenograft models.
RAD1901 selectively binds to and degrades the ER and is a potent antagonist of ER-positive breast cancer cell proliferation. 
The selective estrogen receptor degrader, fulvestrant, significantly reduced the anchorage-independent growth of ESR1 mutant-expressing cells, while combination treatments with the mTOR inhibitor everolimus, or an inhibitor blocking IGF1R, and the insulin receptor significantly enhanced anti-proliferative responses.
SUM-44 cells do not require exogenous estrogen for growth in vitro; however, they are dependent on ERα expression, as ESR1 knock-down or exposure to the selective estrogen receptor degrader fulvestrant resulted in growth inhibition.
Evaluation of the pharmacological activities of RAD1901, a selective estrogen receptor degrader.
Fulvestrant is a steroid-based, selective estrogen receptor degrader (SERD) that both antagonizes and degrades ER-α and is active in patients who have progressed on antihormonal agents.

Answer: Antihormonal agents, such as the selective estrogen receptor degrader fulvestrant, are used to treat breast cancer in patients who have failed prior therapies by antagonizing and degrading ER-α, which is active in patients who have progressed on antihormonal agents.