Immunologic Targeting of ESR1 Receptor for Hormone Receptor Expressing Metastatic Breast Cancer

This study is testing a vaccine-based immunotherapy that targets the estrogen receptor (ESR1) — a protein that fuels the growth of many hormone receptor-positive breast cancers — in women with metastatic hormone receptor-positive, HER2-negative breast cancer. The goal is to train the immune system to attack cancer cells that rely on this receptor. **You may be eligible if...** - You have been diagnosed with metastatic (spread) hormone receptor-positive, HER2-negative breast cancer confirmed by biopsy - Your cancer expresses estrogen receptor or progesterone receptor (at least 1% on testing) - You have received prior standard treatments including CDK4/6 inhibitors and at least one hormonal therapy **You may NOT be eligible if...** - Your cancer is HER2-positive - You have not received prior standard hormone therapy - You have active autoimmune disease or are on immunosuppressant medications - You have untreated brain metastases - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.