Phase II/III Study of SPH4336 Combined With Letrozole vs Placebo Combined With Letrozole in First-line Treatment of Breast Cancer

This study is testing a new drug called SPH4336 combined with letrozole (a hormone therapy) as a first treatment for advanced breast cancer. Researchers want to find out if adding SPH4336 works better than letrozole alone. **You may be eligible if...** - You are a woman aged 18–75 - You have locally advanced or metastatic breast cancer that is hormone receptor-positive (HR+) and HER2-negative - You have not yet received any systemic therapy for your advanced or metastatic disease - You have at least one measurable tumor on scans - You are post-menopausal, or pre-menopausal and willing to take a hormone-suppressing injection (goserelin) during the study **You may NOT be eligible if...** - You have previously been treated with a CDK4/6 inhibitor (e.g., palbociclib, ribociclib, abemaciclib) - You have inflammatory breast cancer - You have cancer that has spread to the brain - You have significant heart problems (heart failure, recent heart attack, irregular heartbeat) - You are pregnant or breastfeeding - You have active, uncontrolled hepatitis B, hepatitis C, or HIV Talk to your doctor to see if this trial is right for you.