This is a Phase I trial in subjects with unresectable locally advanced or metastatic HER2-low expressing (IHC 1+ or 2+) breast cancer. The study will be conducted in two parts: the first part will involve dose escalation using a 3 + 3 design, and the second part will involve the expansion of the MTD or HTD to further evaluate safety, preliminary efficacy, and immunogenicity. In the first part, 3 to 6 subjects will be sequentially enrolled starting at dose cohort 1. Subjects will be assessed for dose-limiting toxicities (DLTs).
The purpose of this study is to determine whether ETBX-021 is safe and effective in the treatment of unresectable locally advanced or metastatic HER2-low-expressing breast cancer.