Inhibition of concomitant thrombin-mediated fibrin formation enhances clot lysis in whole blood

To characterize potential mechanisms for enhanced thrombolysis in the presence of thrombin inhibitors, we measured lysis of ¹²⁵I-fibrin(ogen)-labelled clots after incubation in rotating tubes with whole blood containing tissue-type plasminogen activator (t-PA, 1500 ng/ml) and either saline or increasing concentrations of recombinant desulphatohirudin (hirudin), or heparin. Thrombin activity in washed clots was negligible, but incubation of clots with t-PA in non-anticoagulated blood resulted in marked thrombin activity within 5 min as measured by fibrinopeptide A generation. Incubation with hirudin over a range of concentrations increased clot lysis compared with t-PA alone (control) from 132 ± 18% of control with 0.5 μg/ml (n = 4) to 216 ± 48% of control with 10 μg/ml (n = 4). Incubation with less than 0.5 U/ml of heparin attenuated clot lysis (35 ± 22% of control with 0.08 U/ml, n = 3) while concentrations ≥ 0.5 U/ml increased lysis (178 ± 13% of control with 1.7 U/ml, n = 4). Similar results were obtained for incubations in recalcified platelet-poor plasma indicating that platelets are not required for the enhancement of clot lysis induced by thrombin inhibitors. However, incubations in recalcified plasma from a patient with afibrinogenaemia abolished the increased lysis observed with 10 μg/ml of hirudin and addition of physiological concentrations of fibrin monomer (400-800 nM) or fibrinogen degradation products (500 nM) to the mixture of whole blood, t-PA and hirudin blunted the extent of clot lysis. Thus, inhibition of thrombin activity induced by exposure of clots to t-PA prevents concomitant formation of fibrin that attenuates fibrinolysis. Conjunctive administration of potent thrombin inhibitors during pharmacological thrombolysis should be useful to accelerate recanalization and contribute to sustained vessel patency.