Background: Current therapies targeting HER2 overexpressing cancers, such as Herceptin (trastuzumab) and Kadcyla (T-DM1), have proven beneficial but therapeutic benefit is limited by many resistance mechanisms. Checkpoint inhibition therapies demonstrate the potential of mobilizing T cell activities to elicit anti-tumor responses, but these T cell tumor-specific immune responses are highly immune contexture-dependent. Using Glenmark’s BEAT platform, we developed GBR 1302, a T cell redirecting antibody targeting CD3 and HER2, as an alternative way of leveraging T cell potency against tumor cells, independently of existing tumor immune response. Methods: In vitro cytotoxic assays. In vivo tumor models. Ex vivo assay recreating native TME, including immune compartment, stroma and vasculature. Results: Preclinical pharmacology studies demonstrated that GBR 1302 can trigger a potent killing of HER2 positive (IHC3+) as well as HER2 equivocal (IHC2+) cancer cells while maintaining an acceptable therapeutic window on cells expressing normal levels of HER2. In vitro assays, as well as in vivo tumor models comparing the potency of GBR 1302 to trastuzumab or T-DM1 demonstrated a superior cytotoxic potential for GBR 1302 against a variety of tumor cells and that GBR1302 is effective in trastuzumab resistant tumors in vitro and in vivo. To further translate these observations into a clinically relevant human context, we studied the effects of GBR1302, as a single agent and combination partner, in a patient derived tumor microenvironment matched ex vivo assay with co-culture of autologous immune system and tumor tissue from 50 subjects with varying levels of HER2 expression ranging from 3+ to 1+. GBR 1302 treatment arm was compared to trastuzumab and to a combination of GBR 1302 + a PD-1 inhibitor on metastatic breast, gastric and gastro-esophageal cancers. GBR 1302 is currently in a phase 1 dose escalation clinical trial in HER2 positive and equivocal cancers. Preliminary data from peripheral blood biomarkers indicate that GBR 1302 triggers relevant T cell activation and cytokine production.