Antithrombotic Effects of a FXI Antisense Oligonucleotide in Mice

FXI, a component of the intrinsic pathway of coagulation, contributes to the generation of thrombin and is also activated by thrombin on the surface of platelets, resulting in further amplification of the coagulation process. Reports have shown that FXI deficiency in mice or in humans is generally not associated with major bleeding yet is associated with reduced risk for thromboembolic disease. To determine whether targeting FXI with antisense oligonucleotides (ASOs) can produce antithrombotic activity without increasing risk of bleeding, a series of 2^nd Generation 2′-O-methoxyethyl modified ASOs were identified that specifically reduce levels of FXI in mice and were evaluated for antithrombotic activity and bleeding potential. Mice were dosed with FXI ASO (ISIS 404071) subcutaneously for three weeks. FXI ASO treatment resulted in a dosedependent and specific reduction in FXI mRNA levels in liver and FXI protein levels in plasma with an ED50 of 3.2 mg/kg. These reductions in FXI levels correlated well with a prolongation of aPTT with no effects on PT prolongation. The anticoagulant effect of FXI ASO treatment also correlated well with antithrombotic activity in a 10% FeCl3 induced IVC thrombosis mouse model across a wide dose range. Furthermore, FXI ASO treatment was generally well tolerated and no prolongation of tail bleeding time was observed at any dose tested, indicating a broad safety margin for FXI ASO targeting. Additional studies using intravital microscopy to study the effects of FXI ASO treatment on thrombosis formation in the mesenteric vein following 10% FeCl3 injury demonstrated that FXI ASO treatment significantly impeded platelet aggregation at the site of vessel injury, thrombus growth and vessel occlusion. Studies to further evaluate the safety of FXI targeting were conducted using a mouse model of intracerebral hemorrhage induced by intracranial injection of collagenase. In this model, FXI ASO treatment had no effect on hemorrhagic size, neurologic deficit score, or mortality at doses well above that needed to produce maximal antithrombotic activity. These results strongly suggest that FXI is an effective and safe target for antithrombotic therapy and that antisense drug technology could be an attractive approach to specifically target FXI for the treatment of thrombotic disorders.