Phase Ib/II Clinical Study of QLS1304 Combined With Endocrine Therapy in the Treatment of ER+/HER2- Breast Cancer Patients

This study is testing a new drug called QLS1304 combined with hormone-blocking (endocrine) therapy in people with a type of breast cancer that is fueled by estrogen (ER+) but does not overexpress the HER2 protein, when the cancer has come back or spread after prior hormone therapy. **You may be eligible if...** - You are 18 or older - You have been confirmed to have ER-positive, HER2-negative breast cancer that has spread or come back - You are in reasonably good health and able to carry out daily activities - At least one prior hormone (endocrine) therapy has not worked for you - You have at least one measurable tumor **You may NOT be eligible if...** - You have never received any endocrine (hormone-blocking) therapy - You have significant organ function problems - You are pregnant or planning to become pregnant Talk to your doctor to see if this trial is right for you.