Definition
Hormone receptors (HR) in breast cancer refer to the estrogen receptor (ER) and progesterone receptor (PR). These are nuclear receptors that, when activated by their respective hormones, promote transcription of genes driving tumor cell growth and survival.
Testing for ER and PR expression helps determine if a tumor is hormone receptor–positive (HR+) or hormone receptor–negative (HR−), which is critical for guiding therapy.
Clinical Relevance
- HR-positive breast cancer:
- Defined as ER and/or PR expression in ≥1% of tumor cells (via IHC).
- Represents ~70% of all breast cancers.
- Generally has a better prognosis and responds to endocrine therapy.
- HR-negative breast cancer:
- Lacks ER and PR expression.
- Does not respond to endocrine therapy.
- Prognosis depends on HER2 or triple-negative status.
Therapeutic Implications
| Subtype | Implication | Pharmacist-Relevant Therapy |
|---|---|---|
| HR+ (ER+ and/or PR+) | Sensitive to endocrine therapy | Tamoxifen, Aromatase Inhibitors (letrozole, anastrozole, exemestane), Fulvestrant; may combine with CDK4/6 inhibitors in advanced disease |
| HR− (ER−, PR−) | Endocrine therapy ineffective | Treat according to HER2 or triple-negative status (chemo, HER2-targeted agents, immunotherapy) |
Pharmacist Considerations
- Endocrine therapy adherence: Often given for 5–10 years in early breast cancer.
- Adverse effects:
- Tamoxifen → hot flashes, thromboembolism risk, endometrial cancer risk.
- Aromatase inhibitors → bone loss, arthralgia, cardiovascular risk.
- Fulvestrant → injection site pain, hot flashes.
- Drug interactions:
- Tamoxifen → avoid strong CYP2D6 inhibitors (e.g., paroxetine, fluoxetine).
- Combination therapy: In advanced/metastatic HR+/HER2− disease, HR therapy is combined with CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib), PI3K inhibitors (alpelisib in PIK3CA-mutated disease), or mTOR inhibitors (everolimus).