The small‐molecule MERTK inhibitor UNC2025 decreases platelet activation and prevents thrombosis

Abstract

Background Growth arrest-specific protein 6 signals through the TAM (TYRO-3-AXL-MERTK) receptor family, mediating platelet activation and thrombus formation via activation of the aggregate-stabilizing α β integrin. Objective To describe the antithrombotic effects mediated by UNC2025, a small-molecule MERTK tyrosine kinase inhibitor. Methods MERTK phosphorylation and downstream signaling were assessed by immunoblotting. Light transmission aggregometry, flow cytometry and microfluidic analysis were used to evaluate the impact of MERTK inhibition on platelet activation and stability of aggregates in vitro. The effects of MERTK inhibition on arterial and venous thrombosis, platelet accumulation at microvascular injury sites and tail bleeding times were determined with murine models. The effects of combined treatment with ADP-P2Y pathway antagonists and UNC2025 were also evaluated. Results and Conclusions Treatment with UNC2025 inhibited MERTK phosphorylation and downstream activation of AKT and SRC, decreased platelet activation, and protected animals from pulmonary embolism and arterial thrombosis without increasing bleeding times. The antiplatelet effect of UNC2025 was enhanced in combination with ADP-P2Y pathway antagonists, and a greater than additive effect was observed when these two agents with different mechanisms of inhibition were coadministered. TAM kinase signaling represents a potential therapeutic target, as inhibition of this axis, especially in combination with ADP-P2Y pathway antagonism, mediates decreased platelet activation, aggregate stability, and thrombus formation, with less hemorrhagic potential than current treatment strategies. The data presented here also demonstrate antithrombotic activity mediated by UNC2025, a novel translational agent, and support the development of TAM kinase inhibitors for clinical applications.

At the MERcy of platelet primers

“…The results of Branchford et al . clearly demonstrate that UNC2025 has antithrombotic characteristics both in vitro and in vivo in an arterial and venous thrombosis model, without increasing tail bleeding. These findings are in line with previous observations made in Mer − / − mice .…”

“…In this issue of the Journal of Thrombosis and Haemostasis , Branchford et al . present novel data on the role of Mer tyrosine kinase in platelet function and thrombosis. Mer forms, together with Axl and Tyro3, the TAM receptor family of growth arrest‐specific (Gas) receptors.…”

“…The combination of UNC2025 with low doses of a P2Y 12 inhibitor yielded a synergistic antiplatelet effect, both in vitro with human platelets, and in vivo in an arterial and venous thrombosis model. Intriguingly, this synergistic effect is not accompanied by an increase in tail bleeding time, such as is seen with high doses of P2Y 12 inhibitor . Whether this combined treatment affects rebleeding has not been investigated yet.…”

“…Branchford et al . thoroughly addressed this question by investigating the effects of a small‐molecule inhibitor of Mer, UNC2025, on platelet function and thrombosis. It should be noted that UNC2025 is equally potent in inhibiting fms‐related tyrosine kinase 3 (FLT3) as in inhibiting Mer.…”

TAM-ing T cells in the tumor microenvironment: implications for TAM receptor targeting

“…Lastly, absence of all three TAM receptors resulted in thrombocytopenia, severely impaired hemostasis and platelet function, and defective megakaryocytopoiesis [33]. In this regard, smallmolecule MERTK-inhibition decreased mouse platelet activation and subsequent thrombosis [34]. Taken together, TAM receptor expression by platelets is essential for regular platelet function in both human and mouse.…”