VLX-1005, but not argatroban, prevents ITAM-mediated platelet activation and heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune prothrombotic disorder characterized by the binding of platelet-activating immunoglobulin G antibodies to platelet factor 4/heparin. In platelets, this leads to cross-linking of the immunoreceptor tyrosine-based activation motif (ITAM)-bearing receptor FcγRIIa, platelet activation, and thrombocytopenia, which in combination with extensive thrombin generation significantly increases the risk of thrombosis. Our laboratory has previously demonstrated that 12-lipoxygenase (12-LOX), an oxygenase primarily expressed in platelets, plays a critical role in platelet activation through FcγRIIa. In this study, we aimed to determine the effectiveness of VLX-1005, a potent and selective inhibitor of 12-LOX, alone or in combination with argatroban, in preventing HIT. Pretreatment with VLX-1005 attenuated aggregation of human washed platelets stimulated with a HIT immune complex in vitro. VLX-1005 prevented ITAM-induced human whole-blood impedance alone or in combination with argatroban. VLX-1005 treatment impaired platelet adhesion and accumulation on a collagen-coated surface under shear stress. Mice expressing transgenic human FcγRIIa and 12-LOX experienced severe thrombocytopenia and thrombosis after a HIT-like challenge, whereas mice expressing transgenic human FcγRIIa with 12-LOX knockout were completely refractory to HIT pathology. VLX-1005 treatment did not affect coagulation or increase the risk of bleeding. This study demonstrates that inhibition of 12-LOX might be an effective intervention for preventing ITAM-regulated platelet activation, such as in HIT, and is independent of argatroban effects in blood. Importantly, VLX-1005 prevents platelet activation and does not increase the bleeding risk associated with direct thrombin inhibitors, such as argatroban.