Releasing the Brakes on the Fibrinolytic System in Pulmonary Emboli: Unique Effects of Plasminogen Activation and α2-Antiplasmin Inactivation
Background—In patients with hemodynamically significant pulmonary embolism, physiologic fibrinolysis fails to dissolve thrombi acutely and recombinant tissue plasminogen activator (r-tPA) therapy may be required, despite its bleeding risk. To examine potential mechanisms, we analyzed the expression of key fibrinolytic molecules in experimental pulmonary emboli, assessed the contribution of α2-antiplasmin to fibrinolytic failure and compared the effects of plasminogen activation and α2-antiplasmin-inactivation on experimental thrombus dissolution and bleeding.
Methods—Pulmonary embolism was induced by jugular vein infusion of 125I-fibrin or FITC-fibrin-labeled emboli in anesthetized mice. Thrombus site expression of key fibrinolytic molecules was determined by immunofluorescence staining. The effects of r-tPA and α2-antiplasmin-inactivation on fibrinolysis and bleeding were examined in a humanized model of pulmonary embolism.
Results—The plasminogen activation and plasmin inhibition system assembled at the site of acute pulmonary emboli in vivo. Thrombus dissolution was markedly accelerated in mice with normal α2-antiplasmin levels treated with an α2-antiplasmin-inactivating antibody (p<0.0001). Dissolution of pulmonary emboli by α2-antiplasmin-inactivation alone was comparable to 3 mg/kg r-tPA. Low dose r-tPA alone did not dissolve emboli, but was synergistic with α2-antiplasmin-inactivation, causing more embolus dissolution than clinical dose r-tPA alone (p<0.001) or α2-antiplasmin-inactivation alone (p<0.001). Despite greater thrombus dissolution, α2-antiplasmin-inactivation alone, or in combination with low dose r-tPA, did not lead to fibrinogen degradation, did not cause bleeding (vs. controls) and caused less bleeding than clinical dose r-tPA (p<0.001).
Conclusions—Although the fibrinolytic system assembles at the site of pulmonary emboli, thrombus dissolution is halted by α2-antiplasmin. Inactivation of α2-antiplasmin was comparable to pharmacologic r-tPA for dissolving thrombi. However, α2-antiplasmin-inactivation showed a unique pattern of thrombus specificity, because unlike r-tPA, it did not degrade fibrinogen or enhance experimental bleeding. This suggests that modifying the activity of a key regulator of the fibrinolytic system, like α2-antiplasmin, may have unique therapeutic value in pulmonary embolism.
- plasminogen activator inhibitor-1
- tissue plasminogen activator
- pulmonary embolism
- Received July 9, 2016.
- Revision received November 28, 2016.
- Accepted December 14, 2016.