Can We Image the “Active” Thrombus?
Thrombus formation after the disruption of an atherosclerotic plaque is fundamental to the onset of acute coronary syndromes and progression of atherosclerotic disease. The importance of thrombosis in the pathophysiology of coronary artery disease has been supported by the significant clinical benefits associated with the use of antithrombotic agents.1 Thrombin is a major agonist for platelet activation and, thus, thrombus formation. Thrombin is generated through the interaction of the tissue factor contained in the lipid-rich core of the disrupted atherosclerotic lesions and the flowing blood. The generated thrombin activates and recruits circulating platelets to the injured area. Thrombin, by converting fibrinogen into fibrin, anchors the forming thrombus onto the disrupted surface. In addition, several other processes including thrombus stabilization, embriogenesis, angiogenesis, and cell migration and proliferation are also modulated by thrombin.
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Because of the central role played by thrombin in thrombogenesis, the inhibition of thrombin activity and/or generation is the objective of several antithrombotic regimens. Despite their demonstrated clinical benefits, unfractionated and low molecular weight heparins are not capable of inhibiting thrombus-bound thrombin. Several direct and specific thrombin inhibitors have been shown to inactivate both free and clot-bound thrombin. These agents are in various phases of development, with some being approved for therapeutic use. Hirudin, bivalirudin, and argatroban are the parenteral direct thrombin inhibitors approved by US Food and Drug Administration. Despite promising results obtained in preclinical and initial human studies, the initial benefits observed were not maintained at long-term follow-up in larger, randomized clinical trials involving short-term administration of thrombin inhibitors. These observations led to the description of a potential “rebound” effect after the cessation of antithrombin therapy. Probably the explanation resides in the mechanism of action of these agents. Direct and specific thrombin inhibitors work mostly by blocking thrombin activity rather than by preventing thrombin generation. These observations indicate that the clinical benefits of these agents will depend on maintaining effective inhibition of thrombin activity until it ceases. Interestingly, a recent meta-analysis involving 11 randomized trials with up to 7 days of treatment and more than 35,970 patients showed direct thrombin inhibitors to be superior to heparin for the prevention of death or myocardial infarction.2
When bound to fibrin or fibrin degradation products, thrombin is resistant to inactivation by the heparin-antithrombin complex. In addition, it maintains its activity and continues to activate coagulation factors V, VIII, and XI, thus generating more thrombin. The presence of thrombus-bound thrombin has been postulated as the cause of re-thrombosis in patients after thrombolysis.
The possibility to locally detect thrombin activity in vivo would provide new insights into the effects of thrombin and facilitate the development of safer and more effective therapeutic regimens based on the inhibition of thrombin activity. The article by Jaffer et al3 in this issue of Arteriosclerosis, Thrombosis, and Vascular Biology highlights the potential impact of new advances in imaging technologies combined with newly developed “smart” optical contrast agents for the detection of thrombus and thrombin activity. The authors report the in vivo imaging of thrombin activity in a murine model of experimental thrombosis by using a thrombin-sensitive near-infrared molecular probe. The article demonstrates that the near-infrared probe detects the presence of thrombin both in acute and semi-acute (>12 hours old) thrombi. The probe is dependent on thrombin activity and, because the intensity of the near infra-red fluorescence signal decreases with thrombus age, this technique may be used to differentiate between biologically active (young) and inactive (old) thrombi. In this regard, we should keep in mind that thrombus responsiveness to fibrinolytic-therapy seems to decrease with age.4 The possibility of imaging thrombin activity within thrombi may have significant therapeutic implications not only by improving our knowledge of the kinetics and mechanism of thrombin activity, but also by facilitating the design of safer and more efficacious therapeutic regimens based on the inhibition of thrombin activity.
Other imaging modalities, such as the use of labeled platelets, antibodies, etc, have been used for thrombus detection, but they are unable to discriminate between fresh and old thrombus. Magnetic resonance imaging is a promising tool for the noninvasive detection of arterial thrombosis.
Measurement of signal intensity and the characteristic visual appearance of the thrombus have the potential to reveal thrombus age.5 While magnetic resonance imaging does not require the administration of a contrast agent, it fails to provide information on the “activity” of the thrombus.
Although we have focused on the applications of this imaging technique to atherothrombosis and thrombin activity, we should also mention all the other processes in which thrombin plays a modulatory role (embriogenesis, angiogenesis, cell proliferation) that will be affected by the availability of this new imaging technology.
The authors deserve congratulations for their interesting and elegant work that will certainly have a significant impact on the study of the kinetics and mechanisms of several thrombin-dependent processes, but one of several remaining questions is whether the degree of penetration of the thrombin-sensitive near-infrared probe will allow its use in other conditions not as close to the vessel surface as the ones used in the study.
- ↵Collaborative meta-analysis of randomized trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high-risk patients: antiplatelet Trialist’s Collaboration. Brit Med J. 2002; 324: 71–86.
- ↵Jaffer AF, Tung C-H, Gerszten RE, Weissleder R. In vivo imaging of thrombin activity in experimental thrombi using a thrombin-sensitive near-infrared molecular probe. Arterioscler Thromb Vasc Biol. 2002; 22: 1929–1935.
- ↵Robinson BR, Houng AK, Reed GL. Catalytic life of activated factor XIII in thrombi: implications for fibrinolytic resistance and thrombus aging. Circulation. 2000; 102: 1151–1157.