PB1344 - Impact of spleen tyrosine kinase inhibition on vaccine-induced thrombotic thrombocytopenia antibody-induced procoagulant platelet and thrombus formation

Background: Vaccine-induced thrombotic thrombocytopenia (VITT) is a rare but life-threatening prothrombotic syndrome observed in individuals following vaccination with vector-based SARS-CoV-2 (ChAdOx1 CoV-19) vaccine. The pathomechanism of VITT resembles the pathophysiology of autoimmune-mediated heparin-induced thrombocytopenia, and is associated with platelet (PLT)-activating antibodies against PLT factor 4 (PF4) that harbor the potential to drive a multicellular-driven prothrombotic coagulation disorder with the onset of thrombosis at unusual sites.

Aims: In this study we investigated whether the inhibition of PLT spleen tyrosine kinase (Syk) prevents VITT Ab-induced procoagulant PLT and increased thrombus formation ex vivo.

Methods: VITT Ab-induced changes in the expression levels of P-selectin (CD62p) and externalization of the negatively charged membrane phospholipid phosphatidylserine (PS) were analyzed using a flow cytometry-based procoagulant PLT assay. Via the use of an ex vivo thrombosis model, the impact of different Syk inhibitors on VITT Ab-mediated thrombus formation was assessed.

Results: VITT patient sera induced significant changes in healthy PLTs, namely the formation of a procoagulant PLT (CD62p/PS double positive) phenotype compared to controls (p=0.0006). These changes were observed in the presence or absence of exogenous PF4 (p=0.0582) and most importantly were markedly reduced in the presence of different Syk inhibitors. Interestingly, the pretreatment of PLTs with the clinically approved Syk inhibitor Fostamatinib prior to VITT serum incubation resulted only in a slight reduction of VITT Ab-induced procoagulant PLT formation (p=0.0625). Contrary, the Syk inhibitors Entospletinib as well as Lanraplenib showed a high potential to prevent from VITT Ab-induced procoagulant PLT formation (p=0.0014 and p=0.0011, respectively). Most importantly, similar preventive effects were observed in our ex vivo thrombosis model, where PLT Syk inhibition with Entospletinib and Lanraplenib showed marked potential to rescue from VITT Ab-induced thrombus formation.

Conclusion(s): Taken together, our data indicate that Syk inhibition might be a potential therapeutic approach to prevent thromboembolic events at unusual sites in VITT patients.