Factor XII full and partial null in rat confers robust antithrombotic efficacy with no bleeding.

: This report aims at exploring quantitatively the relationship between FXII inhibition and thromboprotection. FXII full and partial null in rats were established via zinc finger nuclease-mediated knockout and siRNA-mediated knockdown, respectively. The rats were subsequently characterized in thrombosis and hemostasis models. Knockout rats exhibited complete thromboprotection in both the arteriovenous shunt model (∼100% clot weight reduction) and the FeCl3-induced arterial thrombosis model (no reduction in blood flow), without any increase in cuticle bleeding time compared with wild-type control rats. Ex-vivo aPTT and the ellagic acid-triggered thrombin generation assay (TGA) exhibited anticoagulant changes. In contrast, ex-vivo PT or high tissue factor-triggered TGA was indistinguishable from control. Rats receiving single doses (0, 0.01, 0.03, 0.1, 0.3, 1 mg/kg) of FXII siRNA exhibited dose-dependent knockdown in liver FXII mRNA and plasma FXII protein (95 and 99%, respectively, at 1 mg/kg) at day 7 post dosing. FXII knockdown was associated with dose-dependent thromboprotection (maximal efficacy achieved with 1 mg/kg in both models) and negligible change in cuticle bleeding times. Ex-vivo TGA triggered with low-level (0.5 μmol/l) ellagic acid tracked best with the knockdown levels and efficacy. Our findings confirm and extend literature reports of an attractive benefit-to-risk profile of targeting FXII for antithrombotic therapies. Titrating of FXII is instructive for its pharmacological inhibition. The knockout rat is valuable for evaluating both mechanism-based safety concerns and off-target effects of FXII(a) inhibitors. Detailed TGA analyses will inform on optimal trigger conditions in studying pharmacodynamic effects of FXII(a) inhibition.