Evaluating the Efficacy and Safety of Fulvestrant Plus DNA Damage Repair Inhibitors After a CDK4/6 Inhibitor

This study is testing combinations of fulvestrant (a hormone-blocking drug) with DNA damage repair inhibitors for women with hormone receptor-positive, HER2-negative breast cancer that has spread (metastatic) and progressed after prior hormone therapy and CDK4/6 inhibitor treatment. Researchers are exploring whether targeting DNA repair pathways can overcome treatment resistance. **You may be eligible if:** - You are 19 years or older - You have HR+/HER2-negative metastatic or inoperable breast cancer - Your cancer progressed after treatment with hormone therapy AND a CDK4/6 inhibitor (such as palbociclib, ribociclib, or abemaciclib) - You have adequate organ function (heart, liver, kidneys) - You are willing to provide tumor tissue for testing **You may NOT be eligible if:** - You have received more than a certain number of prior lines of therapy (per protocol limits) - You have active brain metastases - You have certain blood disorders or poor organ function - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.