UC San Diego

The LM609 antibody specifically recognizes α_Vβ₃ 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 α_Vβ₃-targeted radioimmunotherapy. To elucidate the mechanisms of recognition and inhibition of α_Vβ₃ integrin, we solved the structure of the LM609 antigen-binding fragment by X-ray crystallography and determined its binding affinity for α_Vβ₃. Using single-particle electron microscopy, we show that LM609 binds at the interface between the β-propeller domain of the α_V 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.