Fulvestrant
A selective estrogen receptor degrader (SERD) given by intramuscular injection for ER-positive breast cancer. Unlike SERMs it has no agonist activity — it binds, blocks and degrades the oestrogen receptor outright.
01 Overview
Fulvestrant (Faslodex) is a steroidal antiestrogen that acts as a pure oestrogen receptor antagonist and degrader. Administered as a monthly intramuscular depot, it downregulates receptor levels rather than merely blocking them, making it mechanistically distinct from tamoxifen-type SERMs.
It is a mainstream oncology drug with robust clinical data. It is not a practical PED ancillary due to injectable-only dosing and cost, but it is the definitive example of a SERD in anti-estrogen pharmacology.
02 Mechanism
Selective estrogen receptor degrader (SERD): binds the oestrogen receptor with high affinity, blocks dimerisation and accelerates receptor degradation, producing pure antagonism with no agonist activity.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Breast cancer | 500 mg/wk | IM | 500 mg on days 1, 15, 29 then monthly (given as two 250 mg injections). |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Complete estrogen receptor blockadeDegrades the oestrogen receptor, eliminating oestrogen signalling in target tissue. | Receptor downregulation | Clinical | |
| No agonist activityUnlike SERMs it exerts no partial oestrogenic effect on any tissue, avoiding agonist-driven stimulation. | Pure antagonism | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Injection site reactionsPain and inflammation from the large-volume intramuscular depot. | Mild | Very common | Clinical | |
| Hot flashesVasomotor symptoms from oestrogen blockade. | Mild | Common | Clinical | |
| Elevated liver enzymesTransaminase rises reported in trials. | Moderate | Uncommon | Clinical |