BSI-128 (AK2024) enhances trastuzumab function and potentiates trastuzumab-deruxtecan activity across HER2-positive preclinical models

Background: HER2-targeted therapy continues to evolve with the development of synergistic antibody combinations and novel payload-coupled modalities. Pertuzumab, the first approved trastuzumab-efficacy booster, demonstrates clinical benefit when combined with trastuzumab or trastuzumab-deruxtecan (T-Dxd) in HER2-positive breast cancer. BSI-128 (AK2024), a HER2-targeting monoclonal antibody identified through trastuzumab-synergy-based functional screening, has been characterized as a differentiated trastuzumab efficacy enhancer. Here, we further evaluate the synergistic cytotoxicity and anti-tumor activity of BSI-128 in combination with T-Dxd across HER2-positive in vitro and in vivo models. Methods: The binding epitope of BSI-128 was compared with trastuzumab, pertuzumab, and a competing antibody of similar epitope class HLX22 analog (synthetic antibody generated from published Henlius sequence). Trastuzumab-mediated cell binding, HER2 internalization, and antibody-dependent cell-mediated cytotoxicity (ADCC) in the presence of BSI-128 were assessed by flow cytometry, pH-sensitive internalization probes, and ADCC reporter assay. In vitro cytotoxicity assays for BSI-128 ± T-Dxd were performed using a 1:1 antibody ratio. In vivo synergy studies evaluated BSI-128 with sub-efficacious doses of T-Dxd in HER2-positive xenograft models. The efficacy comparisons were performed against pertuzumab and HLX22 analog. Results: BSI-128 engaged a HER2 epitope distinct from trastuzumab and pertuzumab and partially overlapping with the HLX22 analog. Despite epitope proximity, BSI-128 did not block trastuzumab binding, unlike the HLX22 analog. When combined at equal molar ratios, BSI-128 plus trastuzumab induced an approximate 2-fold increase in HER2 internalization compared with trastuzumab alone, and this enhancement exceeded that observed with pertuzumab or HLX22 analog combinations. Trastuzumab-mediated ADCC activity was maintained despite a 50% reduction in trastuzumab concentration when BSI-128 was added. Superior synergy of BSI-128 with trastuzumab relative to pertuzumab and HLX22 analog was confirmed in multiple HER2-positive in vitro and in vivo models. The presence of BSI-128 enhanced T-Dxd in vitro cytotoxicity by 2-fold in HER2-positive cells, whereas pertuzumab did not augment T-Dxd activity. BSI-128 also significantly potentiated the anti-tumor activity of T-Dxd in HER2-positive animal models. Conclusion: These data robustly support BSI-128 as a differentiated trastuzumab-synergy antibody with improved performance relative to pertuzumab and the competing synergy antibody when combined with T-Dxd. These findings support the clinical development of BSI-128 plus T-Dxd for HER2-positive gastric cancer and other T-Dxd-approved indications. BSI-128 is currently being evaluated in a Phase 1 clinical trial.