Fulvestrant S enantiomer is primarily used in the treatment of hormone receptor-positive breast cancer in postmenopausal women. It functions as a selective estrogen receptor degrader (SERD), effectively blocking the estrogen receptor and reducing the growth of cancer cells that depend on estrogen. This compound is particularly beneficial for patients who have not responded well to other hormonal therapies, such as tamoxifen or aromatase inhibitors. It is administered via intramuscular injection, typically on a monthly basis, to ensure sustained therapeutic levels in the body. Its ability to degrade estrogen receptors makes it a valuable option in managing advanced or metastatic breast cancer, offering a mechanism to slow disease progression and improve patient outcomes.