Optimal therapy for ST-segment elevation myocardial infarction: the role of residual thrombus

Current Perspectives in Facilitated Angioplasty

BACKGROUND

Ischemic heart disease represents the most important cause of mortality worldwide, and the management of patients with ST-elevation myocardial infarction (STEMI) still remains a great challenge. For a great number of patients who do not have immediate access to primary percutaneous coronary intervention (PCI), facilitated angioplasty may be a reasonable therapeutic option.

AREAS OF UNCERTAINTY

The goal of reperfusion therapy is achieving repermeabilization of the infarct-related artery. However, the restoration of normal epicardial flow is not always followed by microvascular tissue perfusion and the presence of myocardial blush. Early studies assessing the benefits of facilitated angioplasty over primary PCI encountered disappointing results, with an increased number of bleeding complications. The invasive strategy following fibrinolysis mainly consists in angiography and PCI of the infarct-related artery between 2 and 24 hours after successful fibrinolysis or rescue PCI in failed fibrinolysis, hemodynamic, electrical instability, or worsening ischemia. Currently, a strategy of routine early angiography after fibrinolysis is recommended, taking into account studies that have demonstrated a reduced rate of reinfarction and recurrent ischemia, without an increased risk of stroke or major bleeding complications.

THERAPEUTIC ADVANCES

After evaluating 1892 patients with STEMI within 3 hours after the onset of symptoms and revealing, beyond clear benefit of fibrinolysis, an increased risk of bleeding complications, the STREAM trial was the one that led to halving the tenecteplase dose for patients aged >75 years. A safety profile of adjusted-dose fibrinolytic therapy in elderly patients with STEMI will be further investigated by the ongoing STREAM-2 trial.

CONCLUSIONS

With the current increased burden of acute coronary syndromes and the lack of immediate primary PCI facilities for all patients with STEMI, facilitated angioplasty seems a feasible therapeutic option. Another benefit of facilitated angioplasty may be represented by a major contribution of thrombolytic therapy in re-establishing microvascular myocardial blood flow.

Quantitative Imaging of Cerebral Thromboemboli In Vivo

Background and Purpose—

Quantitative imaging for the noninvasive assessment of thrombolysis is needed to advance basic and clinical thrombosis-related research and tailor tissue-type plasminogen activator (tPA) treatment for stroke patients. We quantified the evolution of cerebral thromboemboli using fibrin-targeted glycol chitosan–coated gold nanoparticles and microcomputed tomography, with/without tPA therapy.

Methods—

We injected thrombi into the distal internal carotid artery in mice (n=50). Fifty-five minutes later, we injected fibrin-targeted glycol chitosan–coated gold nanoparticles, and 5 minutes after that, we treated animals with tPA or not (25 mg/kg). We acquired serial microcomputed tomography images for 24 hours posttreatment.

Results—

Thrombus burden at baseline was 784×10 3 ±59×10 3 μm 2 for the tPA group (n=42) and 655×10 3 ±103×10 3 μm 2 for the saline group (n=8; P =0.37). Thrombus shrinkage began at 0.5 to 1 hour after tPA therapy, with a maximum initial rate of change at 4603±957 μm 2 /min. The rate of change lowered to ≈61% level of the initial in hours 1 to 2, followed by ≈29% and ≈1% in hours 2 to 3 and 3 to 24, respectively. Thus, 85% of total thrombolysis over 24 hours (≈500 μm 2 , equivalent to 64% of the baseline thrombus burden) occurred within the first 3 hours of treatment. Thrombus burden at 24 hours could be predicted at around 1.5 to 2 hours. Saline treatment was not associated with significant changes in the thrombus burden. Infarct size was smaller in the tPA group versus saline group (18.1±2.3 versus 45.8±3.3 mm 2 ; P <0.01). Infarct size correlated to final thrombus burden ( r =0.71; P <0.01). Time to thrombolysis, completeness of thrombolysis, and tPA therapy were independent predictors of infarct size.

Conclusions—

Thromboembolic burden and the efficacy of tPA therapy can be assessed serially, noninvasively, and quantitatively using high-resolution microcomputed tomography and a fibrin-binding nanoparticle imaging agent.

Direct Thrombus Imaging in Stroke

There is an emergent need for imaging methods to better triage patients with acute stroke for tissue-plasminogen activator (tPA)-mediated thrombolysis or endovascular clot retrieval by directly visualizing the size and distribution of cerebral thromboemboli. Currently, magnetic resonance (MR) or computed tomography (CT) angiography visualizes the obstruction of blood flow within the vessel lumen rather than the thrombus itself. The present visualization method, which relies on observation of the dense artery sign (the appearance of cerebral thrombi on a non-enhanced CT), suffers from low sensitivity. When translated into the clinical setting, direct thrombus imaging is likely to enable individualized acute stroke therapy by allowing clinicians to detect the thrombus with high sensitivity, assess the size and nature of the thrombus more precisely, serially monitor the therapeutic effects of thrombolysis, and detect post-treatment recurrence. This review is intended to provide recent updates on stroke-related direct thrombus imaging using MR imaging, positron emission tomography, or CT.

Direct Imaging of Cerebral Thromboemboli Using Computed Tomography and Fibrin-targeted Gold Nanoparticles

Computed tomography (CT) is the current standard for time-critical decision-making in stroke patients, informing decisions on thrombolytic therapy with tissue plasminogen activator (tPA), which has a narrow therapeutic index. We aimed to develop a CT-based method to directly visualize cerebrovascular thrombi and guide thrombolytic therapy. Glycol-chitosan-coated gold nanoparticles (GC-AuNPs) were synthesized and conjugated to fibrin-targeting peptides, forming fib-GC-AuNP. This targeted imaging agent and non-targeted control agent were characterized in vitro and in vivo in C57Bl/6 mice (n = 107) with FeCl₃-induced carotid thrombosis and/or embolic ischemic stroke. Fibrin-binding capacity was superior with fib-GC-AuNPs compared to GC-AuNPs, with thrombi visualized as high density on microCT (mCT). mCT imaging using fib-GC-AuNP allowed the prompt detection and quantification of cerebral thrombi, and monitoring of tPA-mediated thrombolytic effect, which reflected histological stroke outcome. Furthermore, recurrent thrombosis could be diagnosed by mCT without further nanoparticle administration for up to 3 weeks. fib-GC-AuNP-based direct cerebral thrombus imaging greatly enhance the value and information obtainable by regular CT, has multiple uses in basic / translational vascular research, and will likely allow personalized thrombolytic therapy in clinic by a) optimizing tPA-dosing to match thrombus burden, b) enabling the rational triage of patients to more radical therapies such as endovascular clot-retrieval, and c) potentially serving as a theranostic platform for targeted delivery of concurrent thrombolysis.