Project Summary Thromboinflammatory diseases, including atherothrombosis, stroke and peripheral vasculitis, result in >30% of all deaths globally. Underlying the pathology of thromboinflammation is increased adhesiveness of platelets and leukocytes. Although many antiplatelet and anti-inflammatory therapies have been used for disease treatment, these drugs increase the risk of major bleeding or impair immune responses. Using protein disulfide isomerase (PDI) conditional knockout (CKO) mice and inhibitors, we and others have shown that extracellular PDI is crucial for platelet thrombus formation in arterial thrombosis and neutrophil recruitment to inflamed endothelium in vascular inflammation. However, inhibition of extracellular PDI with a function-blocking antibody prolongs tail bleeding times in mice, raising a concern that specific inhibition of PDI may perturb hemostatic function. Our recent studies have demonstrated that platelet-released PDI promotes the ligand-binding function of glycoprotein Ibα (GPIbα) and enhances GPIbα-mediated platelet adhesiveness, platelet-neutrophil aggregation and vascular occlusion under thromboinflammatory conditions. These results suggest that inhibitors blocking the PDI-GPIbα signaling axis may be a novel antithromboinflammatory drug. Using high throughput screening, we have identified one compound that specifically inhibits PDI-GPIbα binding and GPIbα-mediated platelet aggregation. We have found that iv injection of the compound into mice abolishes platelet-neutrophil interactions and improves blood flow rates in microvessels under thromboinflammatory conditions. Unlike a conventional inhibitor of PDI or GPIbα, treatment with our compound does not prolong tail bleeding times in mice. These results have provided evidence for the feasibility of identifying small-molecule inhibitors targeting a specific PDI signaling pathway. In Aim 1, using the computer-aided drug design technique and a series of in vitro studies, we will identify small- molecule compounds that specifically block PDI-GPIbα signaling and GPIbα-mediated platelet functions. In Aim 2, we will synthesize derivatives of hits, test them in animal studies and examine DMPK profiles of the selected compounds. The proposed study will prove that compared to conventional inhibition of PDI or GPIbα, specific inhibition of the PDI-GPIbα signaling axis might be a safer and effective therapeutic strategy for treating thromboinflammatory disease.