| Drug(s) | Mechanism of Action |
| Letrozole, Anastrozole | Inhibits cell proliferation and survival by blocking production of estrogen |
| Tamoxifen, Fulvestrant | Inhibits cell proliferation and survival by interfering with estrogen action |
| Medroxyprogesterone acetate (MPA) | Inhibits cell proliferation by inducing apoptosis and growth inhibition |
- Estrogen levels are increased by the conversion of circulating androgen (testosterone) to 17b-estradiol by aromatase produced by the tumor, surrounding stromal tissue, or by other tissues (e.g., adipose).
- Estrogen binding to its receptor, ER (estrogen receptor), induces the ER transcriptional activity leading to increased expression of genes involved in cell growth and DNA synthesis.
- One of the genes induced by the estrogen-bound ER is the progesterone receptor (PR). Expression of this protein is an indicator of the activity of the ER in the tumor cell.
- Anti-estrogens compete with estrogen for binding to the ER and block its full activation, thereby reducing the transcriptional effects of estrogen.
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Green squares, sensitivity markers. Red squares, resistance markers