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

The LM609 antibody specifically recognizes alpha(V)beta(3) integrin and inhibits angiogenesis, bone resorption, and viral infections in an arginine-glycine-aspartatein-dependent manner. LM609 entered phase II clinical trials for the treatment of several cancers and was also used for alpha(V)beta(3)-targeted radioimmunotherapy. To elucidate the mechanisms of recognition and inhibition of alpha(V)beta(3) integrin, we solved the structure of the LM609 antigen-binding fragment by X-ray crystallography and determined its binding affinity for alpha(V)beta(3). Using single-particle electron microscopy, we show that LM609 binds at the interface between the beta-propeller domain of the alpha(V) chain and the beta I domain of the beta(3) 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.