Abstract 957: Zenocutuzumab: An antibody that can overcome HER3 mediated HRG signalling in tumor cells by docking on HER2
Authors: Geuijen Cecile , Maussang-Detaille David , Gallenne Tristan , Hendriks Linda , Kramer Arjen , Gerlach Jan , Throsby Mark , de Kruif John 
Published: July 2021

Abstract
HER2-driven cancers require phosphatidylinositide-3 kinase (PI3K)/Akt signaling through HER3 to promote tumor growth and survival. The HER2/HER3 complex is most active in the presence of heregulin (HRG) and upregulation of HRG is one of the key resistance mechanisms after TKI treatment. Zenocutuzumab is an HER2/HER3 bispecific antibody that specifically and potently blocks HRG-driven signaling of the heterodimeric complex. Zenocutuzumab has demonstrated clinical activity in patients with advanced cancer where the HRG-driven HER2/HER3 pathway is active [Ref].Zenocutuzumab was discovered through a large unbiased screen with HER2×HER3 Biclonics® (bAbs). Zenocutuzumab specifically blocks HRG-induced signaling of HER2:HER3 but not of HER2:HER4, EGFR:HER2 or EGFR:HER3 heterodimers. Zenocutuzumab demonstrates an in vitro potency superior to other high affinity anti-HER2 and anti-HER3 antibodies in cells stimulated with high concentrations of HRG, thereby overcoming one of the resistance mechanisms of current HER2 therapies. The potent activity of Zenocutuzumab is based in-part on its binding to domain I of HER2. Docking of Zenocutuzumab in this position increases the effective affinity of the anti-HER3 Fab arm. As a result, Zenocutuzumab displaces HRG binding to HER3, even at high ligand concentrations, keeping HER3 in an inactive state and blocking HRG-mediated HER2/HER3 signaling. In binding experiments on cells expressing different levels of HER2 and HER3, HER2:HER3 ratios point to the anti-HER2 Fab binding arm being the main driver of Zenocutuzumab avidity. In an in vivo orthotopic breast cancer xenograft model that used Micro-PET imaging, the tumor penetration of a variant of Zenocutuzumab is HER2-dependent despite the high subnanomolar affinity of the HER3 Fab arm for its receptor. Gamma-counter quantification of radioactivity confirmed that antibody levels in the tumors with the HER2xHER3 bAb were 2.5-fold higher than for the parental HER3 mAb. Zenocutuzumab reduced tumor growth in an in vivo orthotopic pancreatic cancer xenograft model that expresses HRG. The brain is one of the organs with high HRG concentrations to which many tumors metastasize. Mice bearing intracranial HER2-amplified breast cancer cells presented at the end of the treatment period 100% survival when treated with MCLA-128, in contrast to 38% survival when treated with HER2 antibody drug conjugate T-DM1.Conclusion: Zenocutuzumab uses a Dock & Block™ mechanism for potent and specific tumor targeting and simultaneous inhibition of HRG-driven proliferation of HER2-amplified tumors, even at supramaximal concentrations of HRG.
Citation Format: Cecile Geuijen, David Maussang-Detaille, Tristan Gallenne, Linda Hendriks, Arjen Kramer, Jan Gerlach, Mark Throsby, John de Kruif. Zenocutuzumab: An antibody that can overcome HER3 mediated HRG signalling in tumor cells by docking on HER2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 957.