Definition
Progesterone Receptor (PR) is a nuclear hormone receptor that is expressed in some breast cancer cells and is regulated by estrogen signaling. PR status is used alongside ER (estrogen receptor) to better characterize hormone receptor (HR)-positive breast cancer and predict response to endocrine therapy.
Clinical Relevance
- PR-positive (PR+) tumors: Usually indicate functional ER signaling, suggesting better response to endocrine therapy.
- PR-negative (PR-) tumors: May indicate less robust ER signaling, potentially lower endocrine sensitivity, especially if ER expression is low.
- PR status modulates prognosis in HR-positive breast cancer: PR+ generally has better outcomes.
Testing
- Determined by immunohistochemistry (IHC).
- Positive if ≥1% of tumor cells show nuclear staining.
- Usually reported together with ER and HER2 status.
Treatment Implications
| PR Status | Therapeutic Considerations |
|---|---|
| PR-positive | Endocrine therapy is highly effective: tamoxifen, aromatase inhibitors, fulvestrant; often combined with CDK4/6 inhibitors in advanced disease |
| PR-negative | Endocrine therapy may be less effective if ER expression is low; consider more aggressive therapy or addition of chemotherapy depending on tumor grade and Ki-67 |
Pharmacist Considerations
- Monitor endocrine therapy adherence and side effects.
- PR+ tumors are more likely to benefit from long-term hormonal therapy.
- Check drug interactions for agents like tamoxifen (CYP2D6 inhibitors).
- Educate patients about the importance of biomarker-driven therapy.