American Heart Journal
Volume 146, Issue 2 , Pages 199-202, August 2003

Inflammation in atherosclerosis: causal or casual? The need for randomized trials

  • Karen Meir, MD

      Affiliations

    • Department of Pathology, Hadassah University Hospital, Jerusalem, Israel
    • Center for Research, Prevention, and Treatment of Atherosclerosis, Hadassah University Hospital, Jerusalem, Israel
    • ,
  • Eran Leitersdorf, MD

      Affiliations

    • Center for Research, Prevention, and Treatment of Atherosclerosis, Hadassah University Hospital, Jerusalem, Israel
    • Department of Medicine B, Hadassah University Hospital, Jerusalem, Israel
    • Corresponding Author InformationReprint requests: Eran Leitersdorf, MD, Department of Medicine B, Hadassah University Hospital, Kiryat Hadassah, PO Box 12221, Jerusalem 91120, Israel.
    • ,
  • Charles H. Hennekens, MD

      Affiliations

    • Mount Sinai Medical Center—Miami Heart Institute and Departments of Medicine & Epidemiology and Public Health, University of Miami School of Medicine, Miami, Fla, USA

Article Outline

 

Advances in medical knowledge proceed on several fronts, optimally simultaneously. Basic researchers provide biologic mechanisms to answer the crucial question of why an agent reduces premature death. Health care providers confer enormous benefits to individual patients by applications of advances in diagnosis and treatment and formulate hypotheses from their case reports and series. Clinical investigators test the relevance of basic research to healthy individuals and patients. Epidemiologists and biostatisticians formulate hypotheses from descriptive studies and test hypotheses in observational analytic studies, (case-control and cohort) and, when necessary, randomized trials to detect reliably small to moderate but clinically worthwhile effects. Each discipline contributes importantly relevant and complementary information to a totality of evidence. When the totality of evidence becomes sufficient, it is possible to make rational clinical judgments for individual patients and policy statements for the health of the general public.1

What then, is the totality of evidence regarding the role of inflammation in atherosclerosis? The view that atherosclerosis is an inflammatory disease is now commonly accepted.2, 3 Several believe the atherosclerotic plaque to be the result of an inflammatory response to endothelial and vessel wall injury caused by various factors such as oxidized low-density lipoprotein, free radicals, or infectious agents.3, 4 There is evidence that inflammatory mediators are related to clinical events. For example, nuclear factor-KappaB (NF-κB), which transcriptionally activates several inflammatory mediators has been found to be elevated in patients with unstable angina.5 In patients with chronic stable angina, proinflammatory factors such as macrophage colongy-stimulating factor (MCSF), interleukin (IL)-1b, IL-6, and C-reactive protein levels are increased and, according to 1 study, short-term treatment with aspirin reduces these levels.6 Moreover, according to prospective data from 2 large cohorts, 1 in men7 and the other in women,8 an elevated baseline level of C-reactive protein is associated with increased risks of subsequent cardiovascular disease (CVD) events.

What role does inflammation play in atherogenesis? The evidence from basic research that favors atherogenesis as a primarily inflammatory process derive mainly from animal models and in vitro studies. Although adhesion molecules, such as vascular cell adhesion molecule-1, are not present in normal mice, apolipoprotein (Apo)-E deficient mice in which atherosclerosis spontaneously develops, express intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule-1 at lesional sites.3 Transfer of the anti-inflammatory cytokine IL-10 to IL-10 deficient C57Bl/6 mice caused a dramatic reduction in atherosclerosis susceptibility.9 There is evidence to support the role of adaptive immunity in atherogenesis.10 It has recently been shown that atherogenesis may be inhibited in New Zealand white rabbits immunized with the mycobacterial heat shock protein, hsp65, by immunosuppression with an anti-CD3 monoclonal antibody plus prednisolone.11 Moreover, double knockout immunodeficient apo E -/- x scid/scid female mice exhibited 70% reduced atherosclerosis when compared with their immunocompetent littermates. This reduction was reversed with passive transfer of CD4+ T cells from immunocompetent mice to immunodeficient mice.12

In secondary prevention trials, statin drugs clearly reduced myocardial infarction, stroke, and cardiovascular and total mortality rates.13 Differences in CVD end points were observed as early as 6 to 12 months after randomization, leading to the speculation that these drugs may influence vascular biology by mechanisms other than plasma cholesterol lowering. The pleiotropic effects of statin drugs, some of which are anti-inflammatory, have been extensively reviewed.14 In vitro, statin drugs have been shown to selectively block lymphocyte function-associated antigen-1-mediated adhesion and costimulation of lymphocytes.15 Oral simvastatin was found to be comparable to indomethacin in its ability to reduce carrageenan-induced foot pad edema in apoE knockout mice.16 In the recent large-scale randomized, double-blind placebo-controlled trial of the effect of pravastatin on C-reactive protein (CRP) levels (PRINCE), statin therapy reduced CRP by 17% independent of any effects on LDL.17 The totality of evidence on CRP, a sensitive marker of inflammation, and subsequent risks of CVD, is extremely promising but not yet proven. Although it has been recently suggested that CRP is a better predictor of CVD than LDL,18 it should be noted that for LDL reduction, randomized data are now available on approximately 60,000 patients in secondary and primary prevention. These data consistently demonstrate clear benefits of LDL reduction on myocardial infarction, stroke, cardiovascular death, and total mortality. In contrast, there are no randomized data with clinical outcomes on CRP lowering.

If inflammation causes atherosclerosis, it is tempting to speculate that agents specifically designed to decrease inflammation (not cholesterol), should reduce risks of CVD events due to atherosclerosis. What about aspirin? In secondary prevention in survivors of a wide range of occlusive vascular disease events and during acute myocardial infarction, aspirin reduces the risk of subsequent myocardial infarction, stroke, and vascular death. In primary prevention, aspirin clearly and conclusively reduces the risk of a first myocardial infarction. These well-documented benefits of aspirin are similar across a dose range from as low as 75 mg a day to >1.5 grams a day.19 The beneficial effects of low-dose aspirin are most likely related to complete and irreversible inhibition of platelet-dependent cyclooxygenase. At low-doses, aspirin certainly produces a clinically meaningful antiplatelet effect.

Can aspirin inhibit atherogenesis in vivo? A study in apoE deficient mice on a high-fat diet has shown that continuous administration of aspirin at a dose equivalent to 1.5 g/day in humans attenuates plaque formation in the aortic sinus.20 More recently, a study in LDL-receptor knockout mice showed reduced whole aortic plaque area in mice treated with low-dose aspirin, and reduced NF-κB activation in the aortas of these mice.21 Whether aspirin can inhibit plaque formation and lipid accumulation in human coronary arteries is unknown.

What about other non-steroidal anti-inflammatory agents (NSAIDs)? The NSAID, ibuprofen, a non-selective cyclooxygenase inhibitor has been shown to inhibit neutrophil adherence to endothelial cells.22 In an ex vivo study, 2 weeks of ibuprofen therapy reduced the adhesiveness of monocytes to endothelium and raised high-density lipoprotein levels in smokers and non-smokers.23 What is the mechanism of ibuprofen action? Recent evidence shows that ibuprofen, like aspirin, acts via regulation of NF-κB.24 The benefit of these agents may be related to findings that support the notion that endothelial dysfunction in atherosclerosis may reflect gene expression that is transciptionally controlled by NF-κB,25 although, as is often the case, this is likely to be a gross oversimplification. Indomethacin, another nonselective cyclooxygenase inhibitor, does not appear to modulate NF-κB activity,26 yet it has been shown to reduce thoracic aortic and carotid artery cholesterol content in New Zealand white rabbits27 and aortic atherosclerotic plaque area in LDL-receptor deficient mice.28

In humans, important relevant information about moderate to large effects of NSAIDs on prevention of vascular morbidity and mortality could derive from case-control and observational cohort studies of patients with rheumatoid arthritis treated with these agents. In an early long-term follow-up study on such patients, the reductions in vascular morbidity and mortality rates were not significantly different from expected values for the general population.29 An observational study of the effect of NSAIDs on prognosis after myocardial infarction showed a possible but non-significant beneficial trend.30 A more recent retrospective analysis revealed no additive 1-year survival benefit of NSAID therapy as compared with aspirin in patients after myocardial infarction.31 Finally, in women who are postmenopausal, the use of non-selective NSAIDs does not appear to be associated with a reduced risk of first myocardial infarction.32

Recently, the inflammatory isoform of cyclooxygenase, cyclooxygenase-2 (COX-2), has been localized to atherosclerotic plaques.33 Because of this observation, it might have been logical to hypothesize that agents that selectively inhibit COX-2 (coxibs) would have beneficial cardiovascular effects. However, in the recent and well-publicized, large-scale, randomized trial of the selective COX-2 inhibitor, rofecoxib, versus the non-selective cyclooxygenase inhibitor naproxen (the VIGOR trial), although cardiovascular and total mortality rates were similar in the 2 groups, the rate of myocardial infarction was 4-times higher in the rofecoxib group (0.4% and 0.1%, respectively).34 The trial excluded patients taking aspirin at the time of randomization, and although only 4% of patients met the Food and Drug Administration criteria for aspirin use, these patients accounted for 38% of myocardial infarction cases. Recent studies suggest that COX-2 is a major source of prostacyclin (PGI2) in humans.35 PGI2 is believed to modulate platelet-vascular interactions in vivo, and to exert a protective effect by limiting the response to platelet Thromboxane-A2 (TXA2).36 Hence COX-2 inhibition would be expected to depress vascular prostacyclin levels, without depressing TxA2. So, although coxibs would not be expected to afford cardioprotection, and might even aggravate a thrombotic tendency in a thrombosis-prone subpopulation of patients, naproxen may be cardioprotective. Is this because of sustained platelet COX-1 inhibition?37 Is it because of an unknown influence on the atherogenetic process? Is it a true effect at all? In the wake of the VIGOR study, 1 study designed to examine the interaction between aspirin, NSAIDs, and coxibs did not feature naproxen.38 A pooled analysis of cardiovascular events in 23 clinical trials of rofecoxib versus other NSAIDs showed that although rofecoxib was as safe as non-naproxen NSAIDs, naproxen was associated with a decreased risk of cardiovascular events compared with rofecoxib.39 Naproxen, like indomethacin, does not suppress NF-κB activation,40 yet unlike indomethacin, an early study in rabbits demonstrated no anti-atherogenic effect of naproxen.41

In summary, at present, the totality of evidence is both confusing, and more importantly, incomplete because of a lack of reliable data from large scale randomized trials to demonstrate conclusively that anti-inflammatory agents decrease cardiovascular morbidity or mortality rates. Thus, it still remains plausible that inflammation is not causal but a casual or non-causal marker for CVD.

A direct test of the effects of suppression of inflammation on clinical CVD events is neither simple nor straightforward. It will be difficult to isolate anti-inflammatory effects of interventions because of the pathophysiological interrelationships of inflammation with thrombososis and antiplatelet and anti-inflammatory effects of NSAIDs and aspirin.42, 43 Further, the most plausible benefits are small to moderate in size, so to test whether inflammation is causal or casual will require large scale randomized trials that should include anti-inflammatory agents. The cardiovascular benefits of anti-inflammatory therapy, if any, are likely to be greatest in individuals with greater vascular inflammation (presumably with elevated CRP levels). One possible agent to test would be naproxen, because it has already been suggested to have cardioprotective properties. Among non-aspirin NSAIDs, naproxen is the strongest inhibitor of cyclooxygenase-1.43 Such a trial, with appropriate design features including a sufficiently large sample size, could test whether the anti-inflammatory effects of naproxen confer clinical benefits greater than that of the antiplatelet effects of low-dose aspirin. Ibuprofen, because of its regulatory effects on NF-κB, may also be a promising agent to test. If so, it will be necessary to consider the possible but unproven inhibition of clinical benefits of aspirin by ibuprofen. Such a difficulty could be overcome by giving aspirin at least 2 hours before the ibuprofen.44 A third possibility might be meloxicam, a preferential cox-2 inhibitor, which in a recent pilot study was shown to reduce adverse outcomes in patients with unstable angina treated with heparin and aspirin.45 Such trials should exclude patients requiring long-term NSAID treatment for pre-existing conditions such as rheumatoid arthritis and patients with known peptic ulcer disease or bleeding disorders. End points should include death because of a cardiovascular cause, myocardial infarction, stroke, any adverse coronary event requiring urgent revascularization, and/or angiographic (or magnetic resonance imaging) progression of disease. Inflammatory biomarkers such as CRP should be monitored at the start of treatment and at regular intervals, but should not replace reliance on clinical CVD end points. The availability of definitively positive data from randomized trials would have major clinical and public health implications.46, 47 This would include clinical strategies for prevention and early detection.48 Until then, the question should remain a research challenge and not yet another unmet clinical and public health challenge.

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PII: S0002-8703(03)00226-6

doi:10.1016/S0002-8703(03)00226-6

American Heart Journal
Volume 146, Issue 2 , Pages 199-202, August 2003

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