Abstract 2335: Structure of the anti-MET antibody, onartuzumab (MetMAb) with MET demonstrates the primary mechanism of action is inhibition of hepatocyte growth factor (HGF) α-chain binding.
Authors: Maun Henry R. , Ma Xiaolei , Merchant Mark , Lazarus Robert A. , Starovasnik Melissa A. 
Published: August 2014

Abstract
Binding of proteolytically activated HGF to the receptor tyrosine kinase MET causes receptor oligomerization and initiation of cell signaling that leads to cell proliferation, scattering, migration, invasion and branching morphogenesis, collectively described as invasive growth. HGF/MET signaling has been implicated in the oncogenic process of multiple cancers. Onartuzumab is a ligand blocking anti-MET monovalent antibody that has shown early promise in clinical studies in combination with the epidermal growth factor receptor inhibitor erlotinib. As a second- or third-line treatment for advanced or metastatic non-small-cell lung cancer, the combination of onartuzumab and erlotinib significantly prolonged progression-free and overall survival versus erlotinib plus placebo in a phase II trial. We investigated the molecular mechanism by which onartuzumab inhibits HGF binding to MET. Biochemical binding studies with individual fragments of HGF showed that onartuzumab blocks the high affinity binding of the HGF α-chain to MET, as well as the smaller NK1 fragment, but not the binding of the HGF β-chain. We resolved a 2.8 Å crystal structure of a ternary complex comprising onartuzumab Fab, MET Sema-PSI and HGF β-chain to elucidate in detail the binding site for onartuzumab on MET. The structure revealed that HGF β-chain and onartuzumab Fab bind simultaneously to adjacent sites on the same side of the 7-bladed β-propeller of the Sema domain. These structural data, together with the finding that onartuzumab and HGF α-chain binding for MET are mutually exclusive, allow us to suggest a likely binding site for the HGF α-chain on MET. Furthermore, these data support the necessity for onartuzumab to be engineered as a monovalent antibody, since a bivalent antibody would act as a potent HGF mimetic, consistent with the derivation of onartuzumab from an agonistic antibody. In addition, the structural data identify key interaction residues for onartuzumab on MET that explain why onartuzumab only binds to human MET and not to murine MET, despite having 87% sequence homology between the species. We were able to show that mutation of key binding residues in human MET to the murine residues resulted in ablation of onartuzumab binding. Taken together, these data explain how onartuzumab inhibits HGF-dependent MET signaling by blocking a unique site in the MET Sema domain in a 1:1 stoichiometry that occludes binding of the high affinity HGF α-chain, thereby acting as a pure antagonist on the MET receptor.
[Xiaolei Ma is now affiliated with Novartis Institutes for Biomedical Research, 4560 Horton Street, Emeryville, CA 94608.]
Citation Format: Henry R. Maun, Xiaolei Ma, Mark Merchant, Robert A. Lazarus, Melissa A. Starovasnik. Structure of the anti-MET antibody, onartuzumab (MetMAb) with MET demonstrates the primary mechanism of action is inhibition of hepatocyte growth factor (HGF) α-chain binding. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2335. doi:10.1158/1538-7445.AM2013-2335