DP303c Versus Trastuzumab Emtansine in in Patients With HER2-positive Advanced Breast Cancer

This study compares a new drug called DP303c against the standard treatment T-DM1 (trastuzumab emtansine) for patients with HER2-positive advanced breast cancer. HER2-positive means the cancer cells have too many copies of a protein that drives growth, and both drugs target that protein. **You may be eligible if...** - You are 18 or older with locally advanced or metastatic breast cancer (cannot be surgically removed) - Your cancer has been confirmed HER2-positive by lab testing - You have previously received both trastuzumab (Herceptin) and a taxane chemotherapy for advanced disease - You have reasonable physical function (ECOG 0–2) and adequate organ function - Your life expectancy is at least 3 months - You agree to use effective contraception during the study **You may NOT be eligible if...** - You have previously received a drug that works by releasing a toxin from a tubulin-targeting compound (antibody-drug conjugate with tubulin inhibitor payload) - You have active brain metastases that are symptomatic or untreated Talk to your doctor to see if this trial is right for you.