Study of Tucatinib and Doxil in Participants With Human Epidermal Growth Factor Receptor 2 Positive (HER2+) Metastatic Breast Cancer

This study is testing the combination of tucatinib (a targeted therapy) and liposomal doxorubicin (a chemotherapy) in patients with HER2-positive metastatic breast cancer — a subtype where cancer cells produce too much of the HER2 protein. Researchers want to see if this combination is safe and effective. **You may be eligible if...** - You are 18 or older - You have confirmed HER2-positive locally advanced or metastatic breast cancer - You have had at least 1 prior treatment with anti-HER2 therapy for advanced disease, or relapsed within 6 months of completing adjuvant HER2 therapy - You have at least one measurable tumor on scans - Your general health level is good (ECOG 0 or 1) **You may NOT be eligible if...** - You have HER2-negative breast cancer - You have active or untreated brain metastases that are symptomatic - You have significant heart problems - You are pregnant or breastfeeding - Your liver or blood counts do not meet the required levels Talk to your doctor to see if this trial is right for you.