Plaque Biology: Interesting Science or Pharmacological Treasure Trove?

Introduction 

The ‘vulnerable plaque’ concept 

Atherosclerosis begins in childhood, but it takes decades to evolve into the mature plaques responsible for the onset of ischaemic symptoms. While plaque growth due to smooth muscle cell proliferation, matrix synthesis and lipid accumulation may narrow the arterial lumen and ultimately limit blood flow, uncomplicated atherosclerosis is largely a benign disease. The final clinical outcome depends on whether a plaque becomes unstable, leading to acute disruption of the surface and exposure of the thrombogenic core to luminal blood flow (Figure 1, Figure 2). The concept of the ‘vulnerable plaque’ was initially described in 1990 and is now largely accepted.1

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Figure 1 Potentially unstable carotid plaque demonstrating thin fibrous cap and lipid rich core.

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Figure 2 Unstable carotid plaque with evidence of ulceration, cap rupture and thrombus formation.

The vital question in plaque pathogenesis is why, after years of indolent growth, life-threatening disruption and subsequent thrombosis should suddenly occur. Plaque stabilisation may prove to be an important clinical strategy for preventing the development of complications. By identifying ‘vulnerable’ plaques we can direct pharmacotherapy to those most likely to benefit, and by understanding the mechanisms of plaque rupture, strive to develop new treatments aimed at prevention.

Plaque disruption: from inflammation to biomarkers 

Basic research in plaque biology over the last 20 years has identified a large number of factors linked to the progression of atherosclerosis, and particularly associated with acute plaque disruption (Fig. 3). Our understanding of the pathogenesis of atherosclerotic lesions has improved dramatically (Fig. 4). In particular it is now well established that inflammation is a key feature in all stages of the disease, especially in the destabilisation of the plaque which leads to plaque rupture.2 Several cell types within the plaque, in particular monocyte derived macrophages and T-lymphocytes, are known to produce a large range of factors including cytokines, chemokines, proteolytic enzymes, growth factors and disintegrins which perpetuate a cycle of endothelial cell activation, smooth muscle cell proliferation, lipid deposition, inflammation and lesion progression, culminating in acute plaque destabilisation. Many of these factors are also detectable systemically and have been hypothesised as markers of atherosclerosis and plaque instability. While some of these factors appear to be independent of traditional risk factors, and some may ultimately prove to be useful as biomarkers for risk of disease progression and clinical events, they do not necessarily represent targets for pharmacotherapy. Numerous associations have been made, and targets suggested for pharmacotherapy, but few have demonstrated real promise in terms of disease prevention or regression. Strategies such as vaccination against antigens promoting atherogenesis, cyclooxygenase inhibitors, statins and ACE inhibitors may reduce the levels of inflammatory markers and other potential targets, but their impact on overall cardiovascular risk is largely unknown.3

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Figure 3 Histological sections of carotid plaques revealing features of plaque instability. H&E stained (a) carotid plaque with intraplaque haemorrhage (IPH), and EVG stained (b) plaque rupture (PR), (c) plaque cap thinning (PCT), and (D) plaque necrosis (PN). Magnification × 10.

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Figure 4 The stages of plaque provide numerous potential targets for pharmacotherapy. Endothelial damage allows passage of inflammatory cells and LDL into the vessel intima; free radicals are responsible for oxidation of the deposited LDL; oxidized-LDL promotes cytokine and protease release from macrophages; proteases degrade the fibrous cap causing disruption; exposure of the thrombogenic core to the blood results in clot formation; local thrombotic and fibrinolytic activity determine the degree of thrombus progression or dissolution.

A recent review has highlighted the pressure on the pharmaceutical industry to accelerate the rate of bringing new cardiovascular therapeutic agents onto the marketplace, and the subsequent interest in biomarkers of disease severity.4 The accepted definition of a biomarker is ‘a characteristic that is objectively measured and evaluated as an indicator of normal or pathogenic processes or as a physiological response to a therapeutic intervention’.5 Biomarkers such as carotid-intima thickness, LDL-cholesterol and CRP have recently been put under the spotlight with the disappointing results of the ENHANCE study of combined simvastatin/ezetemibe, and the halting of the JUPITER trial of rosuvastatin in patients with normal LDL levels but raised CRP.4 This raises the issue of which- if any- of the established and proposed biomarkers of cardiovascular disease, are valuable surrogate endpoints.

Targets for Plaque Modification 

A variety of intrinsic and extrinsic factors predispose a plaque to instability and acute disruption. Intrinsic features that characterize a plaque as vulnerable are lipid content, increased inflammatory cell infiltration (particularly macrophages), foam cells and T lymphocytes content and a reduced collagen and smooth muscle cell content associated with increased proteolysis.6, 7 Plaque rupture tends to occur at the shoulder region, associated with cap thinning and macrophage infiltration.8 The shoulder region is the area of the plaque exposed to the greatest shear stress.9 The lipid pool has poor tensile strength, decreasing load-bearing capabilities, resulting in increased stress in the overlying fibrous cap.10 Extrinsic features that predispose a plaque to rupture, particularly coronary plaques, include increased blood pressure and vasospasm. Acute myocardial infarction has been associated with trigger events, including emotional stress and physical activity, probably related to blood pressure and vasospasm.11

Potential therapeutic strategies to achieve plaque stabilization have targeted these intrinsic or extrinsic features that promote plaque rupture.12 Lipid lowering agents, antioxidants, β-adrenergic blockers and angiotensin converting enzyme inhibitors have been shown to reduce the incidence of acute coronary syndromes, presumably through plaque stabilization. Strategies promoting extra cellular matrix synthesis or preventing degradation within the plaque, as well as more novel gene therapy approaches may show promise in achieving plaque stabilization. More complex manipulation of plasma lipid levels through administration of systemic HDL is also under investigation.

One of the major advances in risk modification over recent years has been the development and widespread uptake of HMG-CoA Reductase inhibitors. Designed as lipid lowering therapy, it is now clear that they have pleiotrophic effects independent of their lipid lowering effects. The absolute degree of protection this equates to and precise plaque stabilisation effect, in addition to improvements in the lipid profile, is difficult to establish.13

Evolution of the Unstable Plaque 

In the early 1970's Ross postulated that atherogenesis was dependent upon cellular interactions in the vessel wall following endothelial injury. The premise that atherogenesis represents a multi-staged exaggerated response to injury has evolved into an attractive unifying hypothesis of vascular disease and repair. Each step and each component of the pathway represents a potential target for intervention. Vessel damage is initiated by diverse insults including infectious agents, nicotine, homocysteine and oxidised-LDL through a direct injurious effect upon the vessel endothelium.14

Endothelial activation in response to injurious agents results in surface expression of cell adhesion molecules, including VCAM-1, ICAM-1, E-selectin and P-selectin, which permit leucocyte binding. Activated endothelial cells express chemo attractant cytokines such as MCP-1, MCSF, IL-1, IL-6 and TNF-α, creating a pro-inflammatory environment.15, 16 This, in conjunction with the increased permeability of the endothelium, mediates intimal migration of inflammatory cells. Within the vessel wall, activated inflammatory cells release a potent array of cytokines and growth factors, stimulating the intimal migration and proliferation of vascular smooth muscle cells. Furthermore, marrow-derived progenitor cells migrate to the site of injury where they adapt into endothelial or smooth muscle cell phenotypes. Stimulated smooth muscle cells produce extra cellular matrix with the subsequent formation of the fibrous plaque.

More mature, and potentially unstable lesions, develop as macrophages and smooth muscle cells accumulate lipid. A large lipid core is a hallmark of the unstable plaque, though the variability in plaque composition is poorly understood. Larger proportions of disrupted coronary plaques are occupied by lipid core than stable plaques, and 90% of thrombosing aortic plaques versus 11% of intact plaques exhibit a lipid core that occupied >40% of total lesion volume.6

While this oversimplifies the myriad of positive and negative feedback loops throughout all stages of plaque development, destabilisation and rupture, it serves as a rough template from which to approach therapeutic strategies.

Inflammation: The Process Behind the Targets 

The concept that atherosclerosis is an inflammatory phenomenon is supported by both experimental and clinical observations. Examination of plaques from the coronary and carotid circulation demonstrates up regulation of an array of growth factors and cytokines.17 As mentioned previously, some of these have been suggested as serum markers of systemic inflammation and correlated with risk of clinical events.18 Much of the work regarding inflammatory markers has related to cardiac events, though there seems to be considerable overlap with carotid risk. Markers which have received particular attention include fibrinogen, serum amyloid A and C-reactive protein (CRP).15 The Cholesterol and Recurrent Events Study demonstrated a positive correlation between increased levels of serum amyloid A and CRP and recurrent coronary events.19 CRP levels are associated with increased cardiovascular risk in apparently healthy individuals.20 Increased relative risk was also identified with serum amyloid A, soluble intracellular adhesion molecule-1, interleukin-6, homocysteine, apolipoprotein B-100 and total and LDL cholesterol levels. Other potential predictors of coronary risk include phospholipase A2, myeloperoxidase and soluble CD-40 ligand. The relative importance of these markers and whether they present specific targets for therapy remains to be seen.

C - Reactive Protein: A Marker or a Target? 

CRP is a dominant acute phase protein associated with infection. Since a commercially available quantitative assay became available in the 1980's, there has been mounting evidence of an association between CRP and severity of atherosclerosis or cardiovascular risk.21, 22, 23

Several studies have demonstrated an association between CRP and acute myocardial infarction and adverse outcome. In patients with angina, a raised CRP predicts an increased risk of MI and sudden cardiac death.24 It may also be predictive of increased cardiovascular risk in asymptomatic patients. Ridker and co-workers demonstrated that baseline CRP levels in apparently healthy men, predicted future risk of developing symptomatic peripheral arterial disease.25

CRP does not only represent a marker of risk, but also a potential target for therapy. CRP enhances cell-mediated immunity, promoting phagocytosis, chemotaxis and activating the complement cascade and circulating platelets. Traditionally antiplatelet agents and statins have represented the mainstay of cardiovascular risk prevention. An evolving pharmacological paradigm links these agents to serum CRP levels. While there is considerable evidence linking aspirin with reductions in cardiac mortality, it is now apparent that the benefit of anti-platelet therapy is greatest in healthy males with high CRP levels.26 This may reflect a direct impact on endothelial dysfunction and CRP production, but the reduction in CRP may simply act as a marker of the efficacy of aspirin (as a preventative treatment) rather than having a direct impact on risk reduction through CRP per se.

Similarly, a number of studies have demonstrated an attenuation of CRP levels with statins. The Oxford Heart Protection Study demonstrated a clear benefit in terms of reducing cardiac and peripheral vascular risk which was independent of lipid levels.27 This is likely to represent lipid-independent plaque stabilising actions including anti-proteolytic and anti-inflammatory properties. Independent of lipid lowering, statins have been shown to significantly reduce CRP levels and be effective in risk reduction in patients with low cholesterol but high CRP.28, 29 Statins increase nitric oxide synthase activity30 and promote endothelial passivation which is a direct reduction in cellular adhesion molecules interfering with inflammatory cell adherence to the endothelium.31 Nitric oxide is a potent vasodilator but also causes inhibition of smooth muscle cell proliferation and platelet aggregation.

The precise mechanisms by which aspirin and statins reduce CRP remain unclear, and the ‘chicken-and-egg’ argument (once again) applies. It is unknown whether there is a direct effect on CRP production, and whether this in turn affects cardiovascular risk, or whether the observed effect is secondary to the well-documented anti-inflammatory effects. The importance of this relationship is debatable. Of all risk prediction markers, CRP may be the most useful- and (in terms of patient care), whether the effect is primary or secondary may prove irrelevant. The JUPITER trial of rosuvastatin in patients with raised CRP and normal cholesterol has yet to be published and may help us to better define the role of CRP as a surrogate endpoint for cardiovascular trials.

Cytokines and Growth Factors 

Inflammatory mediators represent a diverse group of peptides which can be classified as growth (PDGF, FGF, EGF, IGF, VEGF), chemokine (MCP, MIP, IL-8,), pro-inflammatory (IL-1,-2,-6,-12,-13, IFN, TNF) and anti-inflammatory factors (TGF, IL-4,-10-12), though many are multifunctional.32

The interrelationship between this vast array of factors is highly complex and it is overly simplistic to assume that each represents a potential target for pharmacotherapy. Various studies have attempted to manipulate the atherogenetic process, particularly in animal models (such as gene-knockout mice) with promising results. IL-10 for example, is a pluripotent cytokine which down regulates adhesion molecule expression and inhibits smooth muscle cell proliferation. In vivo studies suggest that IL-10 effectively reduces post-injury intimal hyperplasia.33 However, translation to clinical practice has been less successful. Growth factors and cytokines regulate numerous steps in atherosclerosis, often working synergistically to exert their influences on the vessel wall. Because of the functional redundancy inherent among inflammatory mediators, strategies aimed at targeting single substances seem unlikely to succeed clinically.

Broad-based anti-inflammatory agents, however, may be more promising, and the clinical effects of statins and salicylates may, at least in part, be related to a shift in the balance of pro- and anti-inflammatory factors.

Targeting the Stimuli to Atherogenesis 

As mentioned previously, salicylates and statins are inherently anti-inflammatory, effecting expression of adhesion molecules, cytokines and inflammatory transcription factors. Their effects in risk reduction may occur at many levels of the inflammatory pathway; indeed our knowledge of the inflammatory ‘response to injury’ process creates many potential targets to attenuate the progression of the disease. While the goals of interventions are sound, the clinical translation of these goals at many levels has yet to be realised.

Primary and secondary prevention trials have clearly documented the benefit of traditional risk factor modification which is beyond the realms of this review. However, there are a number of areas which deserve attention, in particular the role of lipids, infectious agents and homocysteine.

Lipid Modification: Looking Beyond Statins 

The effects of statins on LDL and HDL levels are well documented. Traditional therapy has been aimed at reducing LDL deposition and augmenting LDL removal, but other targets may prove clinically valuable. Oxidation of LDL within the plaque induces monocytes and foam cell formation with subsequent apoptosis and free radical release. Oxidised LDL and reactive oxygen species are directly injurious to vascular cells including the endothelium; they promote platelet aggregation and inflammatory cell adhesion and stimulate smooth muscle cell proliferation.34 Antioxidants inhibit oxidation of LDL; they counteract the effect of reactive oxygen species and may promote plaque stabilisation through a reduction in matrix degradation. In animal models, antioxidants have been shown to decrease the expression of MMP-9, one of the major proteolytic enzymes implicated in acute plaque disruption, and prevent intimal hyperplasia after vessel balloon injury.35, 36

Epidemiological studies of antioxidant vitamins have also suggested a benefit, particularly for Vitamin E. In large cohort studies, people with a high dietary intake of Vitamin E demonstrated a lower long term risk of clinical coronary disease.37, 38 However, clinical trials in patients with proven coronary disease have demonstrated conflicting results.39, 40

More recently antioxidant gene therapy has been trialled to augment antioxidant defence therapies, again with conflicting results.41 It may be that such therapy must be directed very early in the disease process to maximise anti-atherogenic potential.

HDL therapy may also offer benefit, both in terms of long term prevention and also in the latter stages of atherosclerotic disease, particularly in acute plaque stabilisation. Current approaches to therapy include acute HDL infusion therapy in acute coronary syndrome42 and acute carotid instability, and long term oral therapy.43 In coronary disease, short term HDL infusions have been demonstrated to alter plaque characterisation and severity of disease on angiography. Increased circulating HDL also has a profound effect on macrophage number and activity. This may well be multifactorial and further work is required to elucidate the short and long term benefit of such therapy.

Gene therapy strategies have included increasing HDL and decreasing LDL levels. Animal models demonstrate effective gene transfer with reductions in circulating cholesterol and increase in serum HDL respectively. However, studies in humans again are at an early stage and results thus far are less promising.12

Homocysteine 

Homocysteine is an amino acid formed from methionine in animal and plant proteins. Levels rise in certain genetic conditions or vitamin deficiencies. Homocysteine is itself not harmful, but is rapidly oxidised in plasma and tissue macrophages releasing reactive oxygen species and initiating, or promoting, inflammation. Hyperhomocysteinaemia is associated with increased kevels of platelet aggregation, abnormalities of fibrinolysis, endothelial cell dysfunction, LDL oxidation and smooth muscle cell proliferation. Numerous observational studies have demonstrated a link with atherosclerosis.44

Folic acid, vitamins B12 and B6, and pyridoxine are important cofactors involved in the processing of homocysteine and food supplementation has become routine. However, reductions in homocysteine in clinical trials have failed to show benefits in term of reducing cardiovascular risk. Recent studies have shown that statins have a direct effect on homocysteine-induced endothelial adhesiveness through inhibition of vascular cell adhesion molecule-1, though again the clinical relevance of this is unclear.45

Infectious Agents: An Association or Causation? 

The role of infectious agents in atherosclerosis remains controversial. Definitive proof of a causal relationship is lacking, although studies have reported associations between plaque development and a variety of agents including Chlamydia pneumoniae, Helicobacter pylori and cytomegalovirus.46, 47, 48

Certain infectious agents can evoke cellular and molecular changes supportive of a role in atherogenesis. Work has shown that chlamydial interaction with monocytes results in up-regulation of TNF-α and IL-β, both of which are associated with plaque development.49, 50 Chlamydial production of HSP60 antigen activates human vascular endothelium, and increases TNF-α and MMP expression in macrophages.51

There remain doubts about the methods employed for Chlamydia detection and the overall prevalence of the organism makes it difficult to determine its true role. Eradication trials have further muddied the waters because it is apparent that antibiotics may exert influences independent of their anti-microbial effects. The STAMINA trial demonstrated that eradication therapy (amoxicillin/azithromycin, metronidazole and omeprazole) administered for 1 week after an acute coronary syndrome, significantly reduced death and acute coronary syndrome readmission rates in the following 12 months.52 These effects were unrelated to seropositivity rate for both Chlamydia and Helicobacter. The WIZARD Trial (azithromycin versus placebo for 12 weeks) demonstrated no clinical benefit from Chlamydia eradication in a large cohort of 7747 adults in terms of the clinical sequelae of coronary heart disease.53

There is little evidence therefore to suggest a true causal relationship between the infectious agents linked through association studies and atherosclerosis, and no indication to routinely treat patients at risk with antimicrobial therapy. There may, however, be some role for antibiotics in plaque modification independent of antimicrobial activity such as MMP inhibition. This will be discussed further later in the review.

Proteolysis- Targeting the Matrix Metalloproteinases 

Macrophages control many of the inflammatory processes within the plaque and are responsible for the production of proteolytic enzymes capable of degrading the extra cellular matrix.2, 54 The predominant proteolytic enzymes involved in plaque disruption are the matrix metalloproteinases (MMPs).55

MMPs are a family of enzymes characterised by binding of zinc to active sites. All degrade components of the extra cellular matrix and are divided into four main classes based on their substrate specificity. While essential in healthy individuals, playing a key role in processes such as wound healing, there is considerable evidence linking MMPs to a spectrum of disease states where matrix degradation occurs, including atherosclerosis. MMPs have been implicated in a number of vascular diseases, with particular interest in aneurysm formation and plaque rupture.55, 56 MMP9 levels in recently symptomatic plaques are significantly higher in plaques from patients with recent symptoms (Figure 5, Figure 6).

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Figure 5 The level of MMP9 in carotid plaques from patients with thrombo-embolic symptoms within 4 weeks of surgery (group 4) was significantly higher the asymptomatic (group 1), and patients with symprtoms >6 months and 1–6 months before surgery (groups 2 and 3 respectively. MMP9 represents a potential target to prevent plaque destabilisation.

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Figure 6 Immunostain demonstrating intense staining for MMP9 in the shoulder of an unstable carotid plaque, in an area of intense inflammatory infiltration.

This has provoked considerable interest in targeting MMPs with selective and non-selective pharmacotherapy to alter the balance of matrix accumulation and degradation. Early interest focussed on the potential for treating periodontal disease, rheumatoid arthritis and malignancy. Unfortunately, clinical results of MMP inhibition have not matched their early promise.

MMP activity is controlled at a number of levels, providing opportunities for therapeutic manipulation (Fig. 7). Firstly, MMP expression is determined at a transcriptional level by various cytokines and growth factors.57 In a number of tissue types, factors including IL-, TNF-α and PDGF stimulate MMP expression, while other factors are inhibitory, such as TGF-β. Steroids and heparin have also been shown to reduce MMP expression.58

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Figure 7 Levels of control of MMP activity providing numerous potential targets for pharmacotherapy to alter the balance between proteolysis and matrix synthesis.

MMPs are secreted as inactive proenzymes, the activation of which represents a second level of control. The major activator of MMPs is plasmin but many other factors are involved including reactive oxygen species.59 Thirdly, the occurrence of naturally occurring ‘tissue inhibitors of MMPs’ (TIMPs) provides a further level of control, with overall proteolytic effect depending on the balance between MMP and TIMP activity.60

Exogenously administered TIMPs are rapidly denatured and tissue penetration is limited. Up-regulation of TIMP production through cytokine administration or manipulation may be feasible but the clinical application of this approach seems likely to be hindered by the potential for significant side effects. In the longer term, a direct approach to TIMP expression through gene transfection techniques has been suggested but this is currently limited to animal models.12

Synthetic MMP inhibitors include tetracycline derived antibiotics, anthracyclines and synthetic peptides. Results in animal models, particularly of cancer and arthritis, have shown promise but clinical results in humans have been less impressive. In vascular disease, doxycycline has been shown to penetrate the carotid plaque with some reduction in MMP activity.61 Similarly, long term treatment in patients with small aneurysms has been shown to reduce plasma MMP9 levels but clinical benefit is unproven.62

Retinoids and steroids down regulate MMP transcription but have proven to be ineffective in reducing coronary restenosis following angioplasty. It has become clear that matrix degradation within the unstable plaque is more complex than initially perceived and many other factors are involved than the MMP family. Furthermore, the longer term effects of MMP inhibition deserve attention since MMPs form a vital part of many normal physiological processes such as wound healing.

Thrombomodulatory Factors 

The role of thrombomodulatory factors in plaque instability is complex and the cause/effect relationship unclear. During the transient period of plaque instability, inflammation and proteolysis may be influenced, at least in part, by up-regulation of thrombomodulatory factors. Acute plaque disruption itself then leads to further activation of the coagulation cascade and thrombus formation. Intraplaque haemorrhage will also directly affect the expression of thrombomodulatory factors. Conversely, thrombomodulatory factors also influence smooth muscle proliferation and migration which is important in the healing process leading into a quiescent phase following the period of instability.

The expression of thrombomodulatory factors has been investigated in unstable coronary plaque, but the relationship in carotid plaques is less well documented.63, 64, 65

Work in our unit has recently demonstrated that expression of thrombomodulatory genes is elevated following a clinical event, but that within one month levels of expression are indistinguishable from asymptomatic patients.

The most widely studied thrombomodulatory factor is tissue factor, which represents the most potent pro-thrombogenic factor identified in plaques. Tissue factor is a glycoprotein, the expression of which is strongly induced in activated macrophages and T-lymphocytes. Through binding to factor VIIa, tissue factor directly activates the coagulation cascade and has been suggested as a candidate molecule for the link between plaque inflammation, instability and thrombo-embolic phenomena.64 Tissue factor pathway inhibitor is the major down-regulator of the procoagulant activity of the TF-factor V11a complex.66 Previous studies have demonstrated increased expression of tissue factor at sites of carotid and coronary plaque inflammation.63, 65 The level of tissue factor in coronary plaques from patients with unstable angina is more than twice the level in plaques from patients with stable angina. Similarly, TFPI expression is up-regulated in plaques, and is expressed by a variety of cell types including macrophages and smooth muscle cells.

Fibrinolysis is activated by the 2 physiological plasminogen activators, t-PA and u-PA. Tissue-type plasminogen activator (t-PA) is a key enzyme mediating plasminogen to plasmin conversion. Levels of t-PA and u-PA have been demonstrated to be elevated in advanced atherosclerosis and localised to areas in the fibrous cap populated by macrophages, and areas lateral to the cap populated by migrating smooth muscle cells.67 Inhibition of the fibrinolytic system occurs through PAI-1, which is also up-regulated in atherosclerosis compared to normal tissue.68 The exact role of PAI-1 and t-PA in the atherosclerotic lesion is unknown though is likely to be multi-factorial. The balance of activation and inhibition of the fibrinolytic system is likely to shift throughout the period of plaque instability and subsequent stabilisation. This balance, or imbalance, may contribute to plaque destabilisation and disruption particularly through MMP activation and increased proteolysis, but conversely increased u-PA and t-PA activity may counteract mural fibrin deposition and facilitate plaque stability.

The precise roles of these individual factors remain unclear and further work is required to elucidate the effects of imbalances within the pro-coagulant and fibrinolytic systems in unstable plaques before there can be a clear target for pharmacotherapy.

Conclusion 

So; plaque biology: interesting science or pharmacological treasure trove? It would be reasonable to surmise that, at present, more of the former than the latter. Our understanding of the events surrounding plaque instability has improved dramatically over the last 2 decades, along with a clearer recognition of the potential importance of plaque modification in cardiovascular disease prevention. Numerous associations have been made between local and systemic factors perceived to be important in plaque evolution and destabilisation, creating many targets for pharmacotherapy with many promising in vitro and animal models of disease regression and prevention.

However, there has been a tendency to target single agents, or isolated aspects of a much more complex process, leading to poor clinical results in human studies. Similarly, there has been an emphasis (in recent years) on the detection of cardiovascular biomarkers, coincident with increasing pressure on the pharmaceutical industry to identify novel anti-atherosclerotic agents- again with disappointing results clinically.

Indirect evidence suggests that lipid lowering, β-blockers and ACE inhibitors may all influence plaque stability, but currently available markers of plaque stability are relatively insensitive in detecting subtle plaque-stabilising effects. Antioxidants, targeting infectious agents and anti-proteolytic agents have promised much but delivered little in terms of true clinical benefit. If successful new agents are to be developed for, what remains an enormous unmet clinical need, novel approaches based on the growing insight into plaque biology need to be integrated with novel clinical methods of detection and intervention.