Fulvestrant

Hormonal / PCT

Also known as: Faslodex, ICI 182780, Fulvestrant Injection

Selective Estrogen Receptor ModulatorsResearch phase: FDA-approvedRegulatory: FDA-approved for ER-positive metastatic breast cancer (2002). Available by prescription as Faslodex (intramuscular injection only).

Mechanism

Fulvestrant is technically a SERD (selective estrogen receptor degrader) rather than a SERM — it not only blocks the estrogen receptor but destroys it entirely. FDA-approved for ER-positive metastatic breast cancer, it is the most potent anti-estrogen available. In the performance community, it is sometimes discussed for severe estrogen-related side effects unresponsive to SERMs or aromatase inhibitors.

Technical detail

Fulvestrant is a steroidal 7α-alkylsulfinyl analog of estradiol that functions as a pure ER antagonist and selective ER degrader (SERD). Unlike SERMs, it has no tissue-selective agonist activity. It binds ER-alpha with affinity comparable to estradiol, blocks receptor dimerization, and accelerates proteasomal degradation of the ER protein, effectively eliminating ER signaling. The FALCON trial demonstrated superior progression-free survival versus anastrozole in first-line ER+ advanced breast cancer. Administered as monthly intramuscular injection due to poor oral bioavailability.

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