CONJUNCTIVE ENHANCEMENT OF ENZYMATIC THROMBOLYSIS AND PREVENTION OF THROMBOTIC REOCCLUSION WITH THE SELECTIVE FACTOR-XA INHIBITOR, TICK ANTICOAGULANT PEPTIDE - COMPARISON TO HIRUDIN AND HEPARIN IN A CANINE MODEL OF ACUTE CORONARY-ARTERY THROMBOSIS

Background. Effective thrombolytic recanalization of an occluded coronary vessel is often limited by acute thrombotic reocclusion, which has galvanized the search for effective adjunctive or conjunctive antithrombotic agents. Methods and Results. Recombinant versions of tick anticoagulant peptide (rTAP) and hirudin (rHIR) are highly selective and potent polypeptide inhibitors of factor Xa and thrombin, respectively. The comparative antithrombotic efficacies of rTAP, rHIR, and heparin, administered conjunctively with recombinant tissue-type plasminogen activator (rt-PA), on thrombolytic reperfusion and reocclusion, were determined in a canine model of occlusive coronary artery thrombosis with a superimposed critical stenosis. In this model, a platelet-rich occlusive thrombus was formed after damage to the intimal surface of the left circumflex coronary artery induced by electrolytic injury. Fifteen minutes after occlusion, the dogs received a systemic intravenous administration of either saline (control), heparin (200 units/kg bolus + 2 min, heparin (HEP) 200 or 100 units/kg bolus + 1 unit/kg/min, HEP 100), rHIR (50 or 100 units/kg/mu-g/kg/min, rHIR 50 or 100, respectively), or rTAP (100-mu-g/kg/min, rTAP 100) followed 15 minutes later by rt-PA (100-mu-g/kg bolus + 10-mu-g/kg/min over 90 minutes). Infusions of the conjunctive agents were discontinued 60 minutes after termination of rt-PA. The incidence and time (mean +/- SEM) to thrombolytic reperfusion were determined for control (five of 12; 68.0 +/- 7.8 minutes), HEP 100 (six of eight; 40.1 +/- 8.3 minutes), HEP 200 (six of eight; 39.8 +/- 9.5 minutes), rHIR 50 (six of eight; 51.7 +/- 14.6 minutes), rHIR 100 (eight of eight; 19.5 +/- 4.2 minutes), and rTAP 100 (eight of eight; 22.8 +/- 10.0 minutes). The incidence and time to reocclusion after rt-PA were determined for control (four of five; 45.7 +/- 12.5 minutes), HEP 100 (four of six; 18.2 +/- 10.7 minutes), HEP 200 (five of six; 26.2 +/- 20.7 minutes), rHIR 50 (four of six; 47.3 +/- 21.6 minutes), rHIR 100 (six of eight; 89.8 +/- 5.9 minutes), and rTAP 100 (three of eight; 54.0 +/- 16.3 minutes). All of the dogs that reoccluded in the rHIR 100 group did so after termination of the inhibitor infusion, whereas two of the three dogs in the rTAP 100 group that reoccluded did so during the inhibitor infusion. Coronary artery blood flow was characterized by intermittent periods of reocclusion and recanalization in all groups except rTAP 100. Conclusions. The potent antithrombotic effects of rTAP in this model directly implicate de novo thrombin formation as a major source of thrombin activity within the highly thrombogenic residual thrombus. These findings suggest that direct inhibition of prothrombinase activity may be an effective strategy in the development of a new class of conjunctive agents.