ALBERT

Description: Abstract 2250 It has recently been demonstrated that the intrinsic pathway is an important contributor to pathologic intravascular thrombosis, suggesting that targeting this pathway may yield effective antithrombotic agents. Furthermore, FXI deficiency is not associated with spontaneous bleeding in humans, and deficiency of a contact factor does not cause abnormal bleeding. We have previously shown that treatment with FXI Antisense Oligonucleotides (ASOs) produces potent, dose-dependent antithrombotic activity in various venous and arterial murine thrombosis models. In the current study we characterize the antithrombotic effects of FXI ASOs in a non-human primate model. Subcutaneous administration of FXI ASOs in cynomologus monkeys resulted in a dose-and time-dependant decrease in FXI mRNA levels in liver (up to 90%), decreased FXI protein and activity levels measured in plasma, and prolonged activated partial thromboplastin times (aPTT). Importantly, FXI depletion in monkeys was not associated with an increased risk of bleeding. Recent studies demonstrated that complete inhibition (〉99%) of FXI by a monoclonal antibody reduced thrombin generation and prevented vascular occlusion in a collagen-coated graft inserted into a baboon arteriovenous shunt (Tucker et al., Blood 2009). In the present study, we set out to determine the relationship between FXI levels and antithrombotic activity, and in particular, the minimal level of FXI reduction required to produce an antithrombotic effect in a baboon thrombosis model. Using the FXI monoclonal antibody, we demonstrate that ∼50% inhibition of FXI levels produces antithrombotic activity in baboons. Next we applied baboon-specific FXI ASOs and demonstrated reductions in FXI protein levels and activity, with corresponding increases in aPTT levels. Baboons were then treated with FXI ASOs in a manner that would produce ∼50% reduction of FXI protein levels and measured anti-thrombotic activity. Similar to the antibody approach, ASO-mediated reduction of FXI plasma levels ≥ 50% resulted in a potent antithrombotic effect. Furthermore, reductions in FXI levels do not increase bleeding times in baboons. These results further support the development of FXI ASOs for antithrombotic therapy with the potential for a superior safety profile compared to currently available anticoagulants. Disclosures: Crosby: Isis Pharmaceuticals: Employment. Zhao:Isis Pharmaceuticals: Employment. Gao:Isis Pharmaceuticals: Employment. Revenko:Isis Pharmaceuticals: Employment. MacLeod:Isis Pharmaceuticals: Employment. Gruber:Aronora, LLC: Consultancy, Equity Ownership. Monia:Isis Pharmaceuticals: Employment.

Description: Recent studies indicate that the plasma contact system plays an important role in thrombosis, despite being dispensable for hemostasis. For example, mice deficient in coagulation factor XII (fXII) are protected from arterial thrombosis and cerebral ischemia-reperfusion injury. We demonstrate that selective reduction of prekallikrein (PKK), another member of the contact system, using antisense oligonucleotide (ASO) technology results in an antithrombotic phenotype in mice. The effects of PKK deficiency were compared with those of fXII deficiency produced by specific ASO-mediated reduction of fXII. Mice with reduced PKK had ∼ 3-fold higher plasma levels of fXII, and reduced levels of fXIIa-serpin complexes, consistent with fXII being a substrate for activated PKK in vivo. PKK or fXII deficiency reduced thrombus formation in both arterial and venous thrombosis models, without an apparent effect on hemostasis. The amount of reduction of PKK and fXII required to produce an antithrombotic effect differed between venous and arterial models, suggesting that these factors may regulate thrombus formation by distinct mechanisms. Our results support the concept that fXII and PKK play important and perhaps nonredundant roles in pathogenic thrombus propagation, and highlight a novel, specific and safe pharmaceutical approach to target these contact system proteases.

Description: 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 2nd 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.