Abstract LB-261: Mechanism of action of MCLA-128, a humanized bispecific IgG1 antibody targeting the HER2:HER3 heterodimer

Background: MCLA-128 is an ADCC-enhanced humanized common light chain bispecific IgG1 antibody that targets the HER2:HER3 dimer with nanomolar affinity, potently inhibiting tumor growth in vitro and in vivo . MCLA-128 shows superior activity to the combination trastuzumab/ pertuzumab and HER3 targeting monoclonal antibodies and is currently being evaluated in a Phase I clinical trial. This study investigated the mechanism of action of MCLA-128. Methods: Phosphorylation of HER receptors and downstream signaling molecules was studied in vitro and in vivo on HER2-amplified cancer cell lines by Pathscan arrays, luminex beads and Western blot analysis. Inhibition of MCLA-128 cell growth in combination with tyrosine kinase inhibitors and small molecules targeting the MAPK and PI3 kinase/Akt pathway was determined by proliferation inhibition and high content imaging assays. The potential effect of MCLA-128 on primary cardiomyocytes in the presence of Doxorubicin was analyzed by measuring ATP. Binding of MCLA-128 to a panel of cell lines in comparison to HER2 and HER3 antibodies was determined by FACS. Results: In contrast to other HER2 and HER3 targeted agents, only MCLA-128 inhibited phosphorylation of HER3 and downstream Akt and ERK in HER2 amplified cell lines cultured with high concentrations of heregulin in vitro . In xenograft studies, growth inhibition of the trastuzumab-resistant cell line JIMT-1 by MCLA-128 was correlated with reduced HER2:HER3 dimerization and a profound inhibition of the PI3K pathway. Synergistic growth inhibition i n vitro was observed when tyrosine kinase inhibitors or inhibitors of the PI3K pathway were added to HER2 amplified cancer cells in the presence of MCLA-128. MCLA-128 did not show any evidence of cardiotoxicity in vitro in contrast to trastuzumab. MCLA-128 binds and coats breast cancer cell lines with differing levels of HER2 expression more efficiently in comparison to monospecific HER2 or HER3 monoclonal antibodies. Conclusions: The unique simultaneous targeting of MCLA-128 to HER2 and HER3 on HER2-overexpressing breast cancer cells leads to severe impairment of PI3K signaling and reduced cell growth whereas proliferation of primary cardiomyocytes is unaffected. The enhanced coating effect of MCLA-128 also supports its ADCC activity. Citation Format: Cecile Geuijen, Eric Rovers, Tristan Gallenne, David Maussang-Detaille, Arjen Kramer, Nellie Nieuwenhuizen, Carina Clements, Katinka van Zoest, Roy Nijhuis, Therese Visser, Renate Den Blanken-Smit, Willem Bartelink, Vanessa Zondag-van der Zande, Linda Kaldenberg, Pieter-Fokko van Loo, Rob Roovers, Robert Doornbos, Leo Price, Stefan Braam, Setareh van Driel, Lex Bakker, Ton Logtenberg, John de Kruif, Mark Throsby. Mechanism of action of MCLA-128, a humanized bispecific IgG1 antibody targeting the HER2:HER3 heterodimer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-261. doi:10.1158/1538-7445.AM2015-LB-261