Protein C activator AB002 rapidly interrupts thrombus development in baboons and appears safe in a first-in-human trial.

While thrombin is a key enzyme in the coagulation cascade and is required for both normal hemostasis and pathologic thrombogenesis, it also participates in its own negative feedback via activation of protein C, which down-regulates thrombin generation by enzymatically inactivating factors Va and VIIIa. Our group and others have previously shown that thrombin's procoagulant and anticoagulant activities can be effectively disassociated to varying extents through site-directed mutagenesis. The thrombin mutant W215A/E217A (WE thrombin) has been one of the best characterized constructs with selective activity toward protein C. While animal studies have demonstrated that WE thrombin acts as an anticoagulant through activated protein C (APC) generation, the observed limited systemic anticoagulation does not fully explain the antithrombotic potency of this or other thrombin mutants. AB002 (E-WE thrombin) is an investigational protein C activator thrombin analog in phase 2 clinical development (clinicaltrials.gov, NCT03963895). Here we demonstrate that this molecule is a potent enzyme that is able to rapidly interrupt arterial-type thrombus propagation at exceedingly low doses (<2 µg/kg, IV), yet without substantial systemic anticoagulation in baboons. We demonstrate that AB002 produces APC on platelet aggregates and competitively inhibits TAFI (carboxypeptidase B2) activation, in vitro, which may contribute to the observed in vivo efficacy. We also describe its safety and activity in a phase 1 first-in-human clinical trial (www.clinicaltrials.gov; NCT03453060). Together, these results support further clinical evaluation of AB002 as a potentially safe and effective new approach for treating or preventing acute thrombotic and thromboembolic conditions.