to its antiestrogenic activity, fulvestrant is an agonist of the G protein-coupled estrogen receptor (GPER), albeit with relatively low affinity (10–100 nM, relative to 3–6 nM for estradiol). Pharmacokinetics Fulvestrant after an intramuscular injection is slowly absorbed and maximal levels (Cmax) are reached after 5 days on average with a range of 2 to 19 days. The elimination half-life of fulvestrant with intramuscular injection is 40 to 50 days. This is 40 times longer than the half-life of fulvestrant by intravenous injection, indicating that its long half-life with intramuscular injection is due to slow absorption from the injection site. Levels of fulvestrant with 500 mg/month by intramuscular injection (and a single additional 500 mg loading dose on day 15 of therapy) in postmenopausal women with advanced breast cancer were 25.1 ng/mL (25,100 pg/mL) at peak and 28.0 ng/mL (28,000 pg/mL) at trough with a single dose and 28.0 ng/mL (28,000 pg/mL) at peak and 12.2 ng/mL (12,200 pg/mL) at trough after multiple doses at steady state. Fulvestrant does not cross the blood–brain barrier in animals and may not in humans as well. Accordingly, no effects of fulvestrant on brain function have been observed in preclinical or clinical research. Fulvestrant is highly (99%) bound to plasma proteins. It is bound to very low density lipoprotein, low density lipoprotein, and high density lipoprotein, but not to sex hormone-binding globulin. Fulvestrant appears to be metabolized along similar pathways as endogenous steroids; CYP3A4 may be involved, but non-cytochrome P450 routes appear to be more important. It does not inhibit any cytochrome P450 enzymes. Elimination is almost all via feces. Fulvestrant can form colloidal aggregates at certain concentration ranges and this can limit its activity as well as produce bell-shaped concentration–response curves. Chemistry Fulvestrant, also known as 7α-[9-[(4,4,5,5,5-pentafluoropentyl)sulfinyl]nonyl]estradiol, is a synthetic estrane steroid and a derivative of estradiol. An alkyl-sulfinyl moiety was added to the endogenous estrogen receptor ligand. It was discovered through rational drug design, but was selected for further development via phenotypic screening. History Fulvestrant was the first selective estrogen receptor degrader to be approved. It was approved in the United States in 2002 and in Europe in 2004. Society and culture NICE evaluation The U.K. National Institute for Health and Clinical Excellence (NICE) said in 2011 that it found no evidence Faslodex was significantly better than existing treatments, so its widespread use would not be a good use of resources for the