The interaction of a small library of cyclic RGD (Arg-Gly-Asp) peptidomimetics with αVβ₆ integrin has been investigated by means of competitive solid phase binding assays to the isolated receptor and docking calculations in the crystal structure of the αVβ₆ binding site. To this aim, a rigid receptor-flexible ligand docking protocol has been set up and then applied to predict the binding mode of the cyclic RGD peptidomimetics to αVβ₆ integrin. Although the RGD interaction with αVβ₆ recapitulates the RGD binding mode observed in αVβ₃, differences between the integrin binding pockets can strongly affect the ligand binding ability. In general, the peptidomimetics exhibited IC50 values for integrin αVβ₆ (i.e., the concentration of compound required for 50% inhibition of biotinylated fibronectin binding to isolated αVβ₆ integrin) in the nanomolar range (77-345 nM), about 10-100 times higher than those for the related αVβ₃ receptor, with a single notable ligand displaying a low nanomolar IC50 value (2.3 nM). Insights from the properties of the binding pocket combined with the analysis of the docking poses provided a rationale for ligand recognition and selectivity.
Keywords: RGD peptidomimetics; binding assays; integrins; molecular docking.