The Therapeutic Antibody LM609 Selectively Inhibits Ligand Binding to Human αVβ3 Integrin via Steric Hindrance

The LM609 antibody specifically recognizes αVβ3 integrin and inhibits angiogenesis, bone resorption, and viral infections in an arginine-glycine-aspartate-independent manner. LM609 entered phase II clinical trials for the treatment of several cancers and was also used for αVβ3-targeted radioimmunotherapy. To elucidate the mechanisms of recognition and inhibition of αVβ3 integrin, we solved the structure of the LM609 antigen-binding fragment by X-ray crystallography and determined its binding affinity for αVβ3. Using single-particle electron microscopy, we show that LM609 binds at the interface between the β-propeller domain of the αV chain and the βI domain of the β3 chain, near the RGD-binding site, of all observed integrin conformational states. Integrating these data with fluorescence size-exclusion chromatography, we demonstrate that LM609 sterically hinders access of large ligands to the RGD-binding pocket, without obstructing it. This work provides a structural framework to expedite future efforts utilizing LM609 as a diagnostic or therapeutic tool. [Display omitted] •Crystal structure of the LM609 antigen-binding fragment•LM609 binds to the headpiece region in all αVβ3 integrin conformations•The LM609 epitope does not overlap with the RGD ligand-binding pocket•LM609 sterically hinders access of large ligands to the αVβ3 RGD-binding site The LM609 antibody specifically recognizes αVβ3 integrin and has been employed in numerous clinical and academic research applications. In this article, Borst et al. shed light on the mode of action of a widely used integrin-targeting antibody.