Structure-based virtual screening of small-molecule antagonists of platelet integrin αIIbβ3 that do not prime the receptor to bind ligand.

Integrin αIIbβ3 has emerged as an important therapeutic target for thrombotic vascular diseases owing to its pivotal role in mediating platelet aggregation through interaction with adhesive ligands. In the search for effective anti-thrombotic agents that can be administered orally without inducing the high-affinity ligand binding state, we recently discovered via high-throughput screening of 33,264 compounds a novel, αIIbβ3-selective inhibitor (RUC-1) of adenosine-5'-diphosphate (ADP) -induced platelet aggregation that exhibits a different chemical scaffold and mode of binding with respect to classical Arg-Gly-Asp (RGD)-mimicking αIIbβ3 antagonists. Most importantly, RUC-1 and its higher-affinity derivative, RUC-2, do not induce major conformational changes in the protein β3 subunit or prime the receptor to bind ligand. To identify additional αIIbβ3-selective chemotypes that inhibit platelet aggregation through similar mechanisms, we screened in silico over 2.5 million commercially available, 'lead-like' small molecules based on complementarity to the predicted binding mode of RUC-2 into the RUC-1-αIIbβ3 crystal structure. This first reported structure-based virtual screening application to the αIIbβ3 integrin led to the identification of 2 αIIbβ3-selective antagonists out of 4 tested, which compares favorably with the 0.003 % "hit rate" of our previous high-throughput chemical screening study. The newly identified compounds, like RUC-1 and RUC-2, showed specificity for αIIbβ3 compared to αVβ3 and did not prime the receptor to bind ligand. They thus may hold promise as αIIbβ3 antagonist therapeutic scaffolds.