Selective Btk inhibition by PRN1008/PRN473 blocks human CLEC-2 & PRN473 reduces venous thrombosis formation in mice.

• PRN473 blocks CLEC-2 mediated platelet function in vitro, ex vivo and reduces thrombosis in two mouse models of venous thrombosis.

• These data and lack of bleeding in PRN1008 / rilzabrutininb clinical trials suggest a future use in immune-mediated thrombosis.

Platelet CLEC-2 is a hemITAM-containing receptor which has a critical role in venous thrombosis, but minimal involvement in haemostasis. CLEC-2 can be blocked by Btk inhibitors. Treatment with ibrutinib is associated with increased bleeding due to off-target inhibition of Src family kinases (SFKs). Patients with X-linked agammaglobulinemia (XLA) who lack Btk however do not bleed, suggesting selective Btk inhibition is a viable antithrombotic strategy. We assessed the effects of selective Btk inhibitors PRN1008 (rilzabrutinib) and PRN473 on platelet signalling and function mediated by CLEC-2 and GPVI. We used healthy donor and XLA platelets to determine off-target inhibitor effects. Inferior vena cava (IVC) stenosis and Salmonella infection mouse models were used to assess antithrombotic effects of PRN473 in vivo. PRN1008 and PRN473 potently inhibited CLEC-2-mediated platelet activation to rhodocytin. No off-target inhibition of SFKs was seen. PRN1008 treatment of Btk-deficient platelets resulted in minor additional inhibition of aggregation and tyrosine phosphorylation, likely reflecting inhibition of Tec. No effect on GPCR-mediated platelet function was observed. PRN473 significantly reduced the number of thrombi in podoplanin positive vessels following Salmonella infection and the presence of IVC thrombosis following vein stenosis. The potent inhibition of human platelet CLEC-2, and reduced thrombosis in in vivo models, together with the lack of off-target SFK inhibition and absence of bleeding reported in rilzabrutinib treated immune thrombocytopenia patients, suggest Btk inhibition as a promising antithrombotic strategy.