American Heart Journal
Volume 150, Issue 2 , Pages 189-192, August 2005

The vulnerable endothelium:

Priming the vascular endothelium for thrombosis with unfractionated heparin: Biologic plausibility for observations from the Superior Yield of the New Strategy of Enoxaparin, Revascularization and GlYcoprotein IIb/IIIa Inhibitors (SYNERGY) trial

  • Richard C. Becker, MD

      Affiliations

    • Corresponding Author InformationReprint requests: Richard C. Becker, MD, Duke Clinical Research Institute, 2400 Pratt Street, Durham, NC.

Duke Cardiovascular Thrombosis Center, Duke Clinical Research Institute, Durham, NC

Received 30 November 2004; accepted 5 April 2005.

Article Outline

 

The vascular endothelium represents a complex, active, and vital organ system that ultimately determines blood flow and tissue perfusion through maintenance of vessel tone and a surface specifically designed to regulate inflammation, thrombosis, and the potential deleterious effects of metabolic byproducts. Endothelial dysfunction, an early hallmark of atherosclerotic vascular disease, is characterized by impaired vasoreactivity, inflammoresistance, and thromboresistance, leading to a heightened propensity for acute thrombotic events.

In the SYNERGY trial, 10027 high-risk patients with acute coronary syndrome (ACS) received either unfractionated heparin (UFH) or enoxaparin.1 The primary end point of death or nonfatal myocardial infarction (MI) occurred in 14.5% of patients assigned to UFH and 14.0% of those given enoxaparin (odds ratio [OR] 0.956, 95% CI 0.869-1.063). Although not anticipated, 75% of patients received some form of antithrombin therapy before randomization. When the data were analyzed for patients not pretreated, enoxaparin produced a 16% relative risk reduction in death and nonfatal MI compared with UFH. A meta-analysis of 6 trials (21946 patients) comparing enoxaparin and UFH made a similar observation.2 In patients receiving no prerandomization antithrombin therapy, there was a statistically significant reduction in the combined end point of death or MI (OR 0.819, 95% CI 0.709-0.946) and in MI alone (OR 0.776, 95% CI 0.657-0.916), favoring those treated with enoxaparin.

Upon closer scrutiny, pretreatment with UFH appeared to have a particularly strong impact on patients randomized to enoxaparin. The 30-day incidence of death or MI was 12.6% for patients not pretreated who subsequently received enoxaparin (OR 0.841, 95% CI 0.675-1.049). In contrast, pretreatment with UFH was associated with a 15.2% incidence of death or MI. Pretreatment with enoxaparin yielded rates of 13.5% and 13.1% in patients randomized to enoxaparin and UFH, respectively.

The potential harm attributable to antithrombin pretreatment in SYNERGY1 and the systematic overview2 warrant serious consideration, not only for purposes of future clinical trial design, but also for understanding the fundamental biology of anticoagulant compounds in wide-scale use. Is it biologically plausible that UFH impacts the response to other antithrombotic agents, including enoxaparin, among patients with ACS?

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Vascular endothelium 

The vascular endothelium is anatomically simple, yet functionally complex, defining the intravascular and extravascular space, serving as a selectively permeable barrier, and providing a continuous lining to the cardiovascular system. Its location is vital for biologic interactions with cells present within the circulation and the vessel wall itself.

The vascular endothelium is an active site of protein synthesis. Endothelial cells synthesize, secrete, modify, and regulate connective tissue components, vasoconstrictors, vasodilators, anticoagulants, procoagulants, prostanoids, and fibrinolytic molecules, each contributing to the maintenance of normal vasomotion, inflammoresistance, metabolic homeostasis, and physiological hemostasis.

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Attenuated inflammoresistance? 

The anti-inflammatory properties that characterize a normal vascular endothelium are governed by a variety of external signals and intracellular mediators. The operative external signals include anti-inflammatory cytokines, transforming growth factor β, interleukin (IL)–10, IL-1 receptor agonist, and high-density lipoprotein cholesterol. Laminar shear stress, through the generation of nitric oxide, is of particular importance in maintaining inflammoresistance.3, 4, 5

Heparin has been shown to preserve coronary endothelial cell function after ischemia-reperfusion by mechanisms independent of its anticoagulant properties.6 Heparin exhibits a wide variety of effects on inflammatory responses that include decreased IL-1, IL-6, and tumor necrosis factor release; attenuated C5b-9 complex assembly; decreased cytotoxic T-cell activity; and impaired neutrophil superoxide production.7, 8, 9

Despite the anti-inflammatory properties described with heparin in isolated tissue culture models and several human diseases, the responses may differ in the setting of coronary atherosclerosis and ACS, characterized by impaired endothelial cell performance, focal and systemic inflammation, relatively long periods of heparin exposure, and widely fluctuating concentrations (typical of UFH). In fact, heightened endothelial cell von Willebrand factor release, a marker of cellular injury, and elevated inflammatory cytokine (IL-6, C-reactive protein) levels have been reported in patients with ACS receiving UFH10 and dalteparin—a low-molecular-weight heparin (LMWH) preparation with a relatively low anti-Xa/anti-IIa profile (higher proportion of long-chain polysaccharides).11 Whether these properties are related to endothelial cell binding and/or metabolism (vascular/systemic) remains unclear. What is clear, however, is that proinflammatory effects occur within hours of drug administration.

Beyond the potential for eliciting a proinflammatory response, one must also consider a “rebound” increase of inflammatory mediators after the abrupt cessation of heparin. Given the close relationship between thrombin and inflammatory cytokines (best illustrated in sepsis) and heightened thrombin generation within hours of heparin termination (summarized in the section to follow), this hypothesis warrants consideration.

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Attenuated thromboresistance? 

The normal vascular endothelium contains natural anticoagulants, platelet inhibitors, and fibrinolytic proteins that function in an integrated fashion to maintain a nonthrombogenic surface (and vital organ perfusion). Because tissue factor, expressed locally at sites of vessel wall injury and from circulating monocytes,12, 13, 14 plays a pivotal role in the thrombotic phenotype which characterizes coronary atherosclerosis, an important surface protein antagonist, tissue factor pathway inhibitor (TFPI), has gained considerable attention.

Human TFPI is a modular protein composed of 3 Kunitz-type domains flanked by peptide fragments.15 The K1 domain inhibits factor (f) VII complexed to tissue factor, whereas the K2 domain inhibits fXa. TFPI is synthesized by vascular endothelial cells and, after its release, colocalizes with surface glycosaminoglycans. Vascular endothelial calls found within the heart express the largest quantity of TFPI mRNA16—a response mediated by physiological demands, activated monocytes, and thrombin generation.17 In patients with ACS, both total and free TFPI plasma levels in the coronary sinus are lower than corresponding levels measured in the aorta (with a concomitant rise in TFPI/fXa complexes), supporting an important role in modulating both fXa biology and coronary thrombosis.18

The administration of heparin in vivo causes a prompt mobilization of TFPI into the circulation, increases its specific anticoagulant activity, and prolongs the half-life of serine proteases. Although TFPI binds effectively to endothelium surface membrane proteoglycans,19 the evidence points more to cellular release (from microdomain storage sites) than displacement of surface-bound protein. Indeed, Northern blot analysis has revealed 2 mRNA transcripts (a 4- and 1.4-kb species) of TFPI expressed within 10 minutes of heparin exposure, peaking at 2 to 4 hours.20

The release of TFPI increases local thromboresistance; however, this may not persist. We performed a series of experiments using human vascular endothelial cells exposed to heparin compounds for up to 7 days. TFPI concentrations increased steadily over the initial 24 hours, but then declined to levels below those of control with continued heparin exposure.21 In addition, TFPI release in response to recombinant epidermal growth factor stimulation was attenuated, supporting either depletion of intracellular storage pools or reduced synthetic capacity. Decreased (depleted/exhausted) intracellular TFPI after UFH exposure has been reported by several investigators,22, 23 with an accompanying decline in tissue factor–mediated inhibitory potential.

Laboratory-based observations made by our group and others provide a mechanistic basis for several clinical phenomena. Patients with ACS receiving UFH have evidence of thrombin generation within several hours of its discontinuation, which correlates directly with fVII activity and inversely with TFPI concentration.24 Ischemic/thrombotic events occur with a near-identical time course.15, 16, 17, 18, 25, 26, 27, 28 Thrombin generation after LMWH treatment is less profound but may be influenced by charge density, molecular weight, antithrombin affinity, and endothelial cell binding capacity.29, 30, 31, 32, 33 Accordingly, biochemical and clinical events may differ among LMWH compounds.11, 34

In the SYNERGY trial,1 39% of patients received UFH (which for study enrollment required discontinuation) before randomization. Did pretreatment with UFH prime the vascular endothelium (or site of plaque disruption) for thrombosis? The available data support this biologically plausible hypothesis. Abrupt cessation of UFH provokes a rapid decrease in TFPI concentration, impairing vascular thromboresistance, and in the presence of heightened fX activity, a sudden burst of thrombin generation ensues. The concomitant decrease in TFPI storage pools (cellular exhaustion) and reductions (or loss of function alterations) in other important endothelial surface anticoagulants, antithrombin IIIs, and the thrombomodulin–protein C system, coupled with heparin-mediated platelet activation,35 monocytes tissue factor expression (Li, Becker, unpublished observation), and a facilitated immune response to long-chain sulfated polysaccharides in patients with ACS, create a prothrombotic environment that cannot be fully contained even with the subsequent administration of enoxaparin (Figure 1).

  • View full-size image.
  • Figure 1. 

    Atherosclerotic coronary artery disease is characterized by impaired inflammoresistance and thromboresistance. Heparin compounds, particularly long-chain saccharides with high endothelial cell binding capacity, may provoke inflammatory/immune responses and have been shown to deplete TFPI storage pools, as well as surface ATIII activity. A sudden cessation of heparin, in an existing proinflammatory and prothrombotic environment, shifts the balance further, augmenting the risk for coronary arterial thrombotic events. TF, Tissue factor; ATIII, antithrombin III).

In summary, the SYNERGY trial has raised important questions regarding the impact of antithrombin pretreatment and clinical outcomes among patients with ACS. The more fundamental question relates to heparin's effect on a dysfunctional vascular endothelium that may provoke proinflammatory and prothrombotic responses both during and immediately after drug cessation. Although further investigation and consideration in the context of future clinical trial design are clearly warranted, the available data suggest that the detrimental effects are more robust with UFH pretreatment (a priming of the vulnerable endothelium), supporting a role for molecular weight, charge density, and endothelial cell binding properties in the pathobiologic sequence of events.

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References 

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 Guest editor for this manuscript was Pierre Theroux, MD, Montreal Heart Institute, University of Montreal, Montreal, Quebec, Canada.

PII: S0002-8703(05)00382-0

doi:10.1016/j.ahj.2005.04.006

American Heart Journal
Volume 150, Issue 2 , Pages 189-192, August 2005

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