Eribulin With or Without Trastuzumab-biosimilar in Patients With HER2-overexpressed Recurrent or Stage IV Breast Cancer

This study is comparing eribulin (a chemotherapy drug) with or without trastuzumab (a HER2-targeted antibody) in people with HER2-positive breast cancer that has returned or spread to other parts of the body, and who have already received prior HER2-targeted therapies. **You may be eligible if...** - You are 18 or older with unresectable or metastatic HER2-positive breast cancer - You have been previously treated with trastuzumab, T-DM1 or T-Dxd, and a taxane chemotherapy - You have had fewer than 4 prior lines of chemotherapy or HER2-targeted therapy for metastatic disease - Your disease has shown clear progression on imaging - You are in good functional health (ECOG 0–1) with adequate blood, liver, and kidney function **You may NOT be eligible if...** - You have not previously received the required prior therapies - You have received 4 or more prior lines of treatment in the metastatic setting - You are pregnant or breastfeeding - Your organ function is not adequate for chemotherapy Talk to your doctor to see if this trial is right for you.