Fulvestrant S enantiomer is the specific stereoisomer of Fulvestrant featuring the S configuration at the chiral sulfoxide center. Fulvestrant acts as a selective estrogen receptor degrader (SERD) for treating hormone receptor-positive breast cancer in postmenopausal women by blocking and degrading estrogen receptors, thereby inhibiting estrogen-dependent cancer cell growth. The commercial drug (Faslodex) contains a 1:1 mixture of R and S sulfoxide epimers administered via monthly intramuscular injection. This pure S enantiomer, available at high purity (e.g., ≥98%), is primarily used in pharmaceutical research to study stereoselective pharmacology, potency differences between epimers, and development of improved SERDs for patients resistant to tamoxifen or aromatase inhibitors.