A Randomised, Open-label, Multicentre Phase III Clinical Study to Evaluate the Efficacy and Safety of JS105 Combined With Dalpiciclib and Fulvestrant Compared With Dalpiciclib and Fulvestrant in Patients With PIK3CA-mutated, HR-positive, HER2-negative Recurrent or Metastatic Breast Cancer.

This study is testing whether adding a drug called JS105 (a PI3K inhibitor) to a standard hormone-blocking treatment combination (dalpiciclib plus fulvestrant) can improve outcomes for women and men with advanced breast cancer that has a specific genetic mutation (PIK3CA mutation), is hormone receptor-positive, and HER2-negative. **You may be eligible if...** - You are 18–75 years old with confirmed advanced or metastatic breast cancer - Your cancer is hormone receptor-positive, HER2-negative, and has a PIK3CA mutation (confirmed by testing) - You have measurable tumor(s) or bone-only metastases - Your overall health is rated good (ECOG 0 or 1) - Your organ function meets required levels - You are willing to provide a tumor or blood sample for mutation testing **You may NOT be eligible if...** - You have previously been treated with fulvestrant or drugs targeting PI3K, AKT, or mTOR pathways - You have active or untreated brain metastases - You have poorly controlled pleural fluid, abdominal fluid, or fluid around the heart - You have uncontrolled diabetes (Type I or uncontrolled Type II) - You are pregnant or breastfeeding - You have another active cancer Talk to your doctor to see if this trial is right for you.