The Anti-Factor XII Antibody AB052 Is Antithrombotic without Hemostatic Impairment in a Primate Model of Extracorporeal Membrane Oxygenation

In standard clinical practice, accumulation of contact initiated thrombi within extracorporeal membrane oxygenation (ECMO) systems is generally controlled by systemic anticoagulation using unfractionated heparin (UFH). However, the efficacy of UFH is limited by dose-restricting hemorrhagic safety concerns, and thus oxygenator replacement and thrombo-embolic complications are frequent during sustained oxygenation. Since coagulation factor XII (FXII) is essential in contact initiated blood coagulation, but has no known contribution to hemostasis, we sought to further explore the roles of FXII in hemostasis and thrombosis in a baboon ECMO model utilizing the anti-FXII monoclonal antibody AB052. In vitro, using human plasma, AB052 is 30-times more potent at inhibiting FXII activity than corn trypsin inhibitor, as assessed by a doubling of the activated partial thromboplastin time (aPTT). In baboon ECMO experiments, a Terumo CAPIOX RX5 baby oxygenator was inserted into a chronic arteriovenous shunt with blood flow regulated to 100 ml/min. An intravenous bolus of AB052 (5 mg/kg), UFH (20 U/kg), or the combination was given between 15 and 30 min prior to each experiment. ACT times immediately prior to experiments were 382±9 sec (AB052), 271±11 sec (UFH), 473±26 sec (combination), compared to pre-dosing values (207±8 sec). Control experiments without any anticoagulation were also performed, and resulted in high platelet accumulation rates of 1.72x10⁹ min^-1 and total deposition of 72±9x10⁹ platelets (n=2), with one device completely occluding during the 60 min study period. Heparinization resulted in a total deposition of 52±11x10⁹ platelets and an accumulation rate of 1.20x10⁹ platelets min^-1 over 60 min (n=4). AB052 alone (n=3) or in combination with UFH (n=3) significantly reduced total platelet deposition (28±3x10⁹ and 24±5x10⁹, respectively) as well as the platelet accumulation rates (0.60x10⁹ min^-1 and 0.56x10⁹ min^-1, respectively) compared to heparinization alone. Fibrin content within the ECMO device was also significantly lower in the AB052 treated groups when compared with either UFH alone or non-anticoagulated controls (45±22 mg (UFH), 15±3 mg (AB052), 15±9 mg (AB052 plus UFH), and 60±11 mg (non-anticoagulated). Furthermore, measurement of systemic thrombin anti-thrombin (TAT) complexes at the end of the experiments showed a marked 63% decrease in TAT levels in the AB052 treatment group and a 67% decrease in the combination treatment group compared to heparinization alone. Importantly, template bleeding time tests (Surgicutt) that were used to assess hemostasis impairment during the experiments showed no prolongation associated with AB052 administration. In summary, our data suggest that selective inhibition of FXII activation may be effective and safe when used alone or in combination with UFH for pharmacological thromboprophylaxis during contact initiated thrombosis.