The purpose of this study is to assess the efficacy and safety of iza-bren, a bi-specific antibody-drug conjugate against EGFR and HER3 with a topoisomerase inhibitor payload versus treatment of physician's choice (TPC) (paclitaxel, nab-paclitaxel, carboplatin plus gemcitabine, and capecitabine) for the treatment of first-line metastatic triple-negative breast cancer (TNBC) or estrogen receptor (ER)-low, human epidermal growth factor receptor 2 (HER2)-negative BC patients who are not candidates for anti-PD(L)1 therapy and endocrine therapies.