Antistasin is a 119-amino acid protein initially isolated from salivary glands of the Mexican leech,Haementeria officinalis, that exhibits potent anticoagulant properties resulting from selective inhibition of blood coagulation factor Xa. The comparative antithrombotic efficacies of recombinant antistasin (rATS), standard heparin (Hep), and aspirin (ASA) administered adjunctly with recombinant tissue-type plasminogen activator (tPA) on thrombolytic reperfusion and reocclusion were determined in a canine model of femoral arterial thrombosis. An occlusive thrombus was formed by insertion of a thrombogenic copper coil into the femoral artery, and blood flow velocity was monitored directly and continuously by Doppler flowmetry. Sixty minutes after occlusion, dogs received an intravenous infusion of either saline (vehicle) or rATS (0.31, 1.25, or 2.5 μg/kg/min), intravenous boluses of Hep (100 units/kg+50 units/kg/hr or 200 units/kg+150 units/kg/hr), or a single intravenous bolus of ASA (2.0 mg/kg), followed 45 minutes later by tPA (0.8 mg/kg i.v. over 90 minutes). The saline and rATS infusions were discontinued 60 minutes after termination of tPA, and the last Hep boluses were given 105 minutes after termination of tPA. All dogs achieved reperfusion. The time to reperfusion in the ASA group was similar to that in the vehicle group (50±9 versus 50±6 minutes, respectively). Reperfusion times were slightly decreased by the low and high doses of Hep (34±6 and 31±4 minutes, respectively) and the rATS doses of 0.31 and 1.25 μg/kg/min (37±4 and 36±5 minutes, respectively). However, the time to reperfusion was dramatically reduced with the 2.5 μg/kg/min rATS dose (15±3 minutes,p<0.05). After termination of the tPA, the femoral arteries of all vehicle-treated dogs reoccluded within 24±2 minutes. Reocclusion was unaffected by ASA (30±6 minutes) and slightly delayed with low-dose Hep (52±12 minutes). High-dose Hep decreased the incidence of (four of eight) and prolonged the time to (83±24 minutes) reocclusion with an associated elevation in the activated partial thromboplastin time to 8.3-fold control. In stark contrast, reocclusion was prevented in all dogs both during rATS infusions and 2 hours after they were terminated. rATS caused a dose-dependent elevation in the activated partial thromboplastin time, resulting in clotting times 1.8-, 2.9-, and 3.9-fold control at doses of 0.31, 1.25, and 2.5 μg/kg/min, respectively. Slight elevations in bleeding time (≤1.6-fold) were observed with high-dose Hep and rATS at doses of 1.25 and 2.5 μg/kg/min. Thus, in this model of arterial thrombosis, rATS as compared with Hep and ASA significantly accelerated reperfusion and completely prevented acute reocclusion, suggesting that specific factor Xa inhibition represents an effective pharmacological approach to adjunctive thrombolytic therapy.