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Commentary
Thrombosis, Inflammation, and Our Evolving Understanding
of Vascular Biology during Interventional Therapy
January 2006
Peripheral arterial disease (PAD) is a problem of substantial public health importance. The disorder is estimated to affect 27 million people in Europe and North America alone.1 Recent data have consistently revealed a sharp increase in PAD prevalence with age and generally somewhat higher rates in men than in women.2,3 In addition, most studies have indicated significant associations between PAD and major cardiovascular disease risk factors. Diabetes and cigarette smoking typically show the strongest risk factor associations with PAD, with hypertension and dyslipidemia correlating as well.2,3 The generalized systemic process underlying PAD poses a much greater threat to the cardiovascular health of patients than it does to their limbs. Approximately 50% of patients are asymptomatic, which may explain why this disease is often underdiagnosed and thus undertreated.4 However, PAD is associated with significant morbidity and mortality. Patients with symptomatic advanced PAD have a particularly poor long-term prognosis, with a 15-fold increased mortality at 10 years.5 Percutaneous peripheral revascularization procedures in this patient population are often technically more complex than coronary interventions, placing these patients at an increased risk for ischemic and hemorrhagic complications.
Atherothrombosis refers to the formation of thrombus superimposed on preexisting atherosclerosis. This common pathophysiologic process results in morbid or fatal clinical ischemic events affecting the cerebral, coronary or peripheral arterial circulation. Because the platelet is a pivotal mediator in the initiation and propagation of thrombus formation, antiplatelet drugs have emerged as key agents for the prevention of recurrent ischemic events.6 However, controversy regarding the choice of antiplatelet therapy in patients with vascular disease is reflected in the uncertainty of clinical practice: neurologists prefer to use aspirin combined with dipyridamole for patients with transient ischemic attacks (TIA) or ischemic strokes; cardiologists use aspirin, clopidogrel, or their combination, for patients with coronary artery disease (CAD); and there are few data regarding the optimal antiplatelet regimen in patients with PAD.6 Relatively new information revealing the important link between the processes of thrombosis and inflammation has influenced drug selection, both at the time of revascularization procedures and for secondary prevention.7
Circulating markers of systemic inflammation have been shown to predict future cardiovascular disease such as myocardial infarction (MI), stroke (CVA), and PAD.8–10 These markers of systemic inflammation include C-reactive protein (CRP), proinflammatory cytokines such as interleukin-6 (IL-6), soluble adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), and P- and E-selectins. CRP has received the most attention and, along with IL-6, appears to be a consistent predictor of future cardiovascular events in large, prospective studies.11–14 The recently published Edinburgh Artery Study of 1,592 patients demonstrated an association between circulating levels of CRP, IL-6, and ICAM-1 with the subsequent risk of progressive peripheral atherosclerosis as measured by the ankle-brachial index (ABI).14 In addition, elevated CRP levels have been associated with the complexity of carotid atherosclerotic plaques by duplex ultrasound and have provided additional prognostic value to ABI measurements in predicting cardiovascular events.15,16 Soluble CD40 ligand (sCD40L), a marker of platelet activation, has garnered great interest as a potential biomarker for cardiovascular disease. However, the distinct processes of expression, shedding and clearance of this ligand are complex and require further elucidation in patients with cardiovascular disease.17 Finally, a recent large meta-analysis demonstrated a moderately strong association between plasma fibrinogen level and the risk of coronary heart disease.18
In this issue of the Journal, Shammas et al. present their findings of a single-center, randomized, open-label study of the anti-inflammatory effect of the glycoprotein (GP) IIb/IIIa inhibitor eptifibatide in patients undergoing peripheral vascular intervention (PVI).19 Forty-two patients undergoing iliac or infrainguinal PVI were randomized to either eptifibatide (180 µg/kg bolus x 2, 10 minutes apart; then 2 µg/kg/min infusion for 18 hours) plus low-dose (60 U/kg) unfractionated heparin (UFH), versus high-dose (100 U/kg) UFH only. Baseline demographics demonstrated a high percentage of current or former smokers (71.5%), diabetes mellitus in 47.6%, with the majority (61.9%) presenting with Fontaine Class IIb claudication (ABIs at rest: right 0.73 / left 0.78). Aspirin use preprocedure was 90.5%, clopidogrel 38.1%, and importantly, lipid-lowering agents in 71.4%, with resultant levels of total and LDL cholesterol of 173.5 and 94.4, respectively. Mean platelet inhibition measured by the Accumetrics Rapid Platelet Function Assay (RPFA) with eptifibatide was 98% at 10 minutes following the second bolus, with considerable variability in ACT values in both groups.
The primary endpoints of the study were inflammatory markers (sCD40L, CRP, IL-6), thrombin generation (Fragment 1.2 [F1.2]), and fibrinogen levels measured post-randomization at baseline, 30 minutes, 2, 18, 48 hours and 7 days post-PVI. After adjusting for baseline values, there was no significant difference in sCD40L, CRP or F1.2 between the two groups. IL-6 was more detectable and fibrinogen had significantly higher mean levels at 7 days in the eptifibatide/low-dose UFH group. IL-6 values increased early (within hours), and CRP peaked at about 48–72 hours in both groups. CRP and fibrinogen significantly increased with PVI and did not return to baseline by day 7 in either group. Major adverse events (MAE) defined as: death, limb loss, unplanned urgent revascularization, vascular complications, major bleeding, embolic stroke or distal embolization occurred in 3 patients (all major bleeds) in the eptifibatide/ low-dose UFH group, and in 1 patient (death) in the high-dose UFH group. The authors conclude that the addition of eptifibatide to UFH for periprocedural adjunctive pharmacotherapy does not reduce the acute inflammatory response following PVI.
Several aspects of this study are interesting and deserve discussion. First, as pointed out by the authors, the study is limited by a small sample size and single-center design. The patient demographics are consistent with the usual cohort of PAD patients undergoing PVI. Interestingly, preprocedure lipid-lowering therapy (presumably mainly statin therapy) was quite high (71.4%), with resultant cholesterol levels mainly within current guidelines. This most certainly had an impact on reducing some (if not all) of the inflammatory mediators at baseline and following intervention.20–22 In addition, the two groups had different UFH doses that may confound the results of the analysis. UFH is known to activate platelets and has variable nonspecific protein binding that may have interfered with the interpretation of results.23,24 The variable pharmacokinetics of UFH and difficulty maintaining stable anticoagulation are reflected in the highly variable ACT results reported. Anticoagulation was also administered prior to obtaining baseline markers, potentially affecting these results as described by the authors.
The rise in IL-6 immediately following PVI is consistent with what has been demonstrated following PCI.25 As the main stimulant for hepatic production of CRP, IL-6 has been demonstrated to be proinflammatory and proatherogenic, but values have varied widely across studies in selected populations.26–28 The observed difference in this study (with higher IL-6 values in the eptifibatide/low-dose UFH group) is difficult to interpret, and may be due to detection limits of the assay or just a play of chance in a small sample size. The acute-phase reactant CRP, one of the most studied inflammatory markers, is a powerful predictor of cardiovascular events.11,12,14,20,29 Recent data have demonstrated the prognostic value of CRP in patients with PAD, especially when combined with objective measures of disease severity (i.e., ABI measurements).16 The demonstrated rise in CRP following PVI (48–72 hours post) is also consistent with what has been demonstrated following PCI.30 No significant difference was demonstrated in this study between the two groups, with values still elevated above baseline at 7 days post-PVI. Statin therapy significantly lowers CRP and results in improved clinical outcomes.20 The administration of lipid-lowering therapy in 71.4% of these patients preprocedure certainly affected the CRP results, and may have “blunted” any potential difference between groups.
The cytokine CD40L is a transmembrane protein expressed on cells of the immune system and activated platelets. CD40L is stored in platelets and translocates to the plasma membrane on activation. CD40L binding to the CD40 receptor on endothelial cells induces inflammation, and by proteolytic cleavage, creates the soluble protein (sCD40L) that is detected.17 This early marker of platelet activation provides a direct link between the processes of thrombosis and inflammation.7 There are many uncertainties surrounding the complex biology of sCD40L with distinct processes of expression, shedding and clearance that make interpretation of values difficult. In addition, no standardized, optimal method for measurement has been determined. In the current study by Shammas et al., there was no significant difference in sCD40L between the two groups. The reduction seen in both groups at 18 hours is interesting and may be due to decreased shedding of the ligand or due to other yet undefined mechanisms.
After injury to a vessel wall, tissue factor (TF) is exposed on the surface of the damaged endothelium. The interaction of TF with plasma factor VII activates the coagulation cascade, producing thrombin by stepwise activation of a series of proenzymes.31 Thrombin generation is central in the clotting process, converts fibrinogen to fibrin and activates platelets. Markers of thrombin generation include thrombin-antithrombin III complexes (TAT), D-dimer, and thrombin fragment 1+2 (F1+2 or F1.2).31,32
Antiplatelet agents, anticoagulants and direct thrombin inhibitors have demonstrated the ability to reduce these markers of thrombin generation with the potential for improved clinical outcomes.32 In the current study, no significant difference in F1.2 was found between the two groups. There was considerable variability in measured values, with an overall increase in both groups at day 7. This marker is affected by the administration of heparins. The different doses of UFH in the current study, as well as the demonstrated variable anticoagulation response (i.e., ACT values reported), may have played a significant role in the values obtained. No conclusions can be drawn from the F1.2 values presented.
Finally, many prospective studies have reported positive associations between the risk of CAD and plasma fibrinogen levels.18 Fibrinogen is the major coagulation protein in blood by mass, the precursor of fibrin and an important determinant of blood viscosity and platelet aggregation. Recent interest has focused on the possibility that measurement (or modification) of fibrinogen levels may help in disease prediction. One of the issues with this biomarker is that lifestyle factors (regular exercise, smoking cessation, alcohol intake) can reduce levels considerably, making comparisons across groups difficult.18 Mean fibrinogen levels were significantly elevated compared to baseline in the current study, with significant differences only at day 7. Due to the small sample size, considerable variability in measured values, and the inability to control for the above-mentioned lifestyle factors, interpretation of this marker in the current study is inconclusive.
Overall, the current study by Shammas et al. demonstrated no significant anti-inflammatory effect of the GP IIb/IIIa inhibitor eptifibatide in patients undergoing PVI using the above markers. Considering the limitations of the study as outlined, no definitive statements can be made regarding the potential anti-inflammatory effect of eptifibatide in patients undergoing PVI. This study and recent literature raise interesting and important questions about the similarities and potentially important differences across vascular beds. While inflammation and thrombosis clearly play a role in all ischemic atherosclerotic disease processes, the balance between inflammation and thrombosis may vary by anatomic location. For example, recent evidence suggests that inflammation within the coronary bed is largely unrelated to thrombus burden and that markers of thrombin generation are considerably higher in patients with PAD.33 This may be due to the overall increased burden of atherothrombotic disease in patients with PAD, but other recent data support differing underlying biology.28,34 The inflammatory/thrombotic balance may lean more towards inflammation within the coronary and carotid vasculature (relatively high-flow states), and more towards thrombosis in the infrainguinal vessels (relatively low-flow states). This has direct impact on periprocedural as well as long-term adjunctive pharmacotherapy. In addition, in our current biomechanical device-based world, the potential differences in the underlying vascular biology across arterial beds directly impacts device design, the therapeutic agent to be delivered by the device, the release kinetics of that agent, and ultimate long-term durability. With the recent demonstration of the ability to select the therapeutic agent for device delivery at the time of intervention, there may be a role in the near future for patient- or disease-targeted interventional therapy.35 However, as mentioned by Shammas et al., larger studies are needed to further elucidate the underlying biology in patients with PAD so that these therapeutic strategies can be tailored to the disease process, and perhaps even to specific anatomic location.
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