Definition
Estrogen Receptor (ER) is a nuclear hormone receptor that, when bound by estrogen, activates transcription of genes promoting cell proliferation. ER expression is a key biomarker in breast cancer used to guide endocrine therapy.
Clinical Relevance
- ER-positive (ER+) breast cancer: Tumor cells express ER and typically respond to endocrine therapy.
- ER-negative (ER-) breast cancer: Tumor cells do not rely on estrogen signaling and do not benefit from endocrine therapy.
Testing
- Determined by immunohistochemistry (IHC).
- Positive if ≥1% of tumor cells show nuclear staining.
- Often reported along with PR (progesterone receptor) and HER2 status.
Treatment Implications
| ER Status | Recommended Therapy |
|---|---|
| ER-positive | Endocrine therapy: tamoxifen (pre-menopausal), aromatase inhibitors (post-menopausal), fulvestrant (advanced disease); may combine with CDK4/6 inhibitors in metastatic HR+/HER2- disease |
| ER-negative | Endocrine therapy not effective; treatment relies on chemotherapy, HER2-targeted therapy, or immunotherapy depending on subtype |
Pharmacist Considerations
- Counsel on endocrine therapy adherence and side effects (hot flashes, bone loss, thromboembolism risk with tamoxifen).
- Monitor for drug interactions (e.g., CYP2D6 inhibitors with tamoxifen).
- CDK4/6 inhibitors are indicated only for ER-positive/HER2-negative advanced disease.
- Educate patients on long-term therapy (often 5–10 years) for early-stage ER+ breast cancer.