Ultra-high throughput-based screening for the discovery of antiplatelet drugs affecting receptor dependent calcium signaling dynamics

Distinct platelet activation patterns are elicited by the tyrosine kinase-linked Collagen receptor Glycoprotein VI (GPVI) and the G-protein coupled protease-activated receptors (PAR1/4) for Thrombin. This is reflected in the different platelet CA²⁺ responses induced by the GPVI agonist collagen-related peptide (CRP) and the PAR1/4 agonist Thrombin. Using a 96 well-plate assay with human Calcium-6-loaded platelets and a panel of 22 pharmacological inhibitors, we assessed the cytosolic CA²⁺ signaling domains of these receptors and developed an automated CA²⁺ curve algorithm. The algorithm was used to evaluate an ultra-high throughput (UHT) based screening of 16,635 chemically diverse small molecules with orally active physicochemical properties for effects on platelets stimulated with CRP or Thrombin. Stringent agonist-specific selection criteria resulted in the identification of 151 drug-like molecules, of which three hit compounds were further characterized. The dibenzyl formamide derivative ANO61 selectively modulated thrombin-induced CA²⁺ responses, whereas the aromatic sulfonyl imidazole AF299 and the phenothiazine ethopropazine affected CRP-induced responses. Platelet functional assays confirmed selectivity of these hits. Ethopropazine retained its inhibitory potential in the presence of plasma, and suppressed collagen-dependent thrombus buildup at arterial shear rate. In conclusion, targeting of platelet CA²⁺ signaling dynamics in a screening campaign has the potential of identifying novel platelet-inhibiting molecules.