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J Am Coll Cardiol, 2003; 41:37-42 © 2003 by the American College of Cardiology Foundation |
* Center for Cardiovascular Disease Prevention, the Leducq Center for Cardiovascular Research, and the Cardiovascular Division, Department of Medicine, Brigham and Womens Hospital, Harvard Medical School, Boston, Massachusetts, UK
Manuscript received May 7, 2002; revised manuscript received October 11, 2002, accepted November 27, 2002.
*
Reprint requests and correspondence: Dr. Paul M. Ridker, Center for Cardiovascular Disease Prevention, Brigham and Womens Hospital, 900 Commonwealth Avenue East, Boston, Massachusetts 02215, UK.
pridker{at}partners.org
| Abstract |
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| Inflammatory markers: C-reactive protein |
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CRP as a predictor of risk
Numerous large-scale epidemiological studies among apparently healthy men and women have found that CRP is a strong independent predictor of future cardiovascular risk (1322). In the setting of ACS, a landmark study by Liuzzo et al. (23) showed that patients presenting with unstable angina who had elevated plasma levels of CRP (
3 mg/l) and serum amyloid A had a higher rate of death, acute myocardial infarction (MI), and need for revascularization compared with patients without elevated levels (Table 1).
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The exact source of elevated CRP levels among patients with unstable coronary syndromes remains unclear. Data suggest that plaque rupture per se may not be the cause but, rather, that elevated CRP levels may be a marker of the hyper-responsiveness of the inflammatory system to even small stimuli. The CRP levels do not change after balloon angioplasty in patients with stable or unstable angina who have normal pre-procedural levels, but they do increase after angioplasty in unstable patients with elevated CRP at baseline (28). Moreover, even diagnostic angiography without intervention caused an increase in CRP levels among patients with elevated levels at baseline.
Other inflammatory markers
Further data regarding upstream mediators of CRP production suggest that this pathway may reflect inflammatory processes that convey increased cardiovascular risk. Elevated levels of IL-1 receptor antagonist and IL-6 at 48 h after presentation are associated with an adverse in-hospital prognosis among patients with ACS, even without a rise in troponin T (29). A recent report from the FRISC II study group has found that circulating levels of IL-6 are a strong independent marker of increased mortality among patients with unstable coronary artery disease (CAD) and may be useful in directing subsequent care (30). For example, randomization to an early invasive strategy led to a 65% relative reduction in 12-month mortality among patients with elevated IL-6 levels. By contrast, among those with low IL-6 levels, an early invasive strategy did not confer any significant benefit over a non-invasive strategy. Furthermore, among patients randomized to the non-invasive arm, the risk associated with elevated IL-6 levels was markedly attenuated if they were assigned to therapy with dalteparin rather than placebo (30). Similar data were observed for CRP. Thus, the use of an inflammatory marker for risk stratification appears to identify patients at high risk for future events, but most importantly, it appears to identify individuals who might benefit most from targeted interventional or intensive medical therapy.
Other novel inflammatory markers have been studied in cardiovascular risk prediction. Lipoprotein-associated phospholipase A2 (Lp-PLA2) circulates in association with LDL-cholesterol and may contribute to atherogenesis by hydrolyzing oxidized phospholipids into pro-atherogenic fragments and by generating lysolecithin, which has pro-inflammatory properties. The West of Scotland study group reported that baseline levels of Lp-PLA2 were a strong independent predictor of risk for incident coronary heart disease in a cohort of high-risk hyperlipidemic men (31). Among a lower-risk cohort of normocholesterolemic women, baseline levels of Lp-PLA2 were also higher among cases than controls (32). However, in adjusted analyses, baseline levels of Lp-PLA2 were not a significant predictor of future cardiovascular risk, while CRP remained a strong predictor (32). Lp-PLA2 levels are highly correlated with LDL-cholesterol, which may in part explain these different results. The predictive value of Lp-PLA2 among patients with ACS is currently unknown.
Myeloperoxidase (MPO) levels may be elevated among individuals with CAD (33). Myeloperoxidase is an enzyme secreted by a variety of inflammatory cells, including activated neutrophils, monocytes, and certain tissue macrophages, such as those found in atherosclerotic plaque. The enzyme is not released until leukocyte activation and degranulation. Myeloperoxidase may convert LDL into a high-uptake form for macrophages, leading to foam cell formation, and may also deplete nitric oxide, contributing to endothelial dysfunction. In a recent case-control study, increasing levels of leukocyte-MPO and blood-MPO were significant predictors of the risk for CAD, such that after adjustment for white blood cell count and Framingham risk score, individuals in the highest quartile of blood-MPO had a 20-fold higher risk of CAD than individuals in the lowest quartile (33). Prospective studies are thus needed to test this interesting hypothesis directly.
Recent ACS data have also been presented for pregnancy-associated plasma protein A (PAPP-A) (34). This zinc-binding metalloproteinase enzyme is a specific activator of insulin-like growth factor I, a mediator of atherosclerosis. Among eight patients who died suddenly from cardiac causes, PAPP-A was abundantly expressed in ruptured and eroded unstable plaques, but PAPP-A was absent or minimally expressed in stable plaques. In plaques with large lipid cores and cap rupture, staining for PAPP-A revealed that the enzyme occurred mostly in the inflammatory shoulder region. In a small case-control study, circulating levels of PAPP-A were higher among patients with unstable angina or acute MI than among patients with stable angina and controls (34). Levels of CRP were also higher among those with acute MI and unstable angina than those with stable angina. Among patients with ACS, levels of PAPP-A and CRP were highly correlated (r = 0.61), but there was no association between PAPP-A and CK-MB (r = 0.07) or troponin I (r = 0.1). As with MPO, these data require assessment in larger cohorts.
Non-inflammatory markers
de Lemos et al. (35) have also recently reported data regarding the potential prognostic utility of B-type natriuretic peptide (BNP) among patients with ACS in the Orbofiban in Patients with Unstable coronary Syndromes (OPUS)-TIMI 16 study. Unlike inflammatory markers, BNP is a neurohormone synthesized in ventricular myocardium and released in response to pressure overload and ventricular dilation. Baseline levels of BNP, drawn on average 40 h after the onset of ischemic symptoms, correlated with the risk of death, heart failure, and MI at 30 days and 10 months. This association was significant across the full spectrum of ACS, including patients presenting with ST-segment elevation MI, MI without ST elevation, and unstable angina. Although it was statistically significant, the correlation between BNP and CRP was weak (r = 0.2; p < 0.001). After being adjusted for other independent predictors of risk of death, including the presence or absence of heart failure in patients, the odds ratio for death at 10 months for the top quartile of BNP compared with the lowest was 5.8, BNP also remained a significant predictor of death when analyses were restricted to an investigation of the presence or absence of elevated troponin levels. Non-CRP inflammatory and non-inflammatory biomarker results are summarized in Table 2.
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Of the inflammatory markers discussed in the previous text, CRP currently meets most, if not all, of these criteria. C-reactive protein has been shown to predict risk in a wide variety of clinical settings; it has incremental value in addition to standard lipid screening for primary prevention (18,19,38) and in addition to cardiac troponin testing among patients with ACS (2426). Furthermore, a recent analysis by Chew et al. (39) shows that CRP predicts the risk of death or MI at 30 days among patients undergoing percutaneous coronary intervention. In this setting, the risk associated with elevated CRP was independent of, but additive to, the effect of an increased American College of Cardiology/American Heart Association lesion score.
C-reactive protein levels are higher among smokers, diabetics, and obese subjects. Adipose tissue is a potent source of IL-6, the main hepatic stimulus for CRP production. Thus, intensification of dietary measures and exercise programs would seem to be appropriate for these individuals. Statin therapy may have powerful anti-inflammatory effects (40), and in recent clinical studies, statin therapy has been shown to lower CRP levels, an effect that is independent of lipid lowering (19,4144). Recent data suggest that baseline levels of CRP and IL-6 are strong independent predictors of the risk of developing type II diabetes (45). In this regard, intriguing data from the West of Scotland study suggest that pravastatin therapy, compared with placebo, reduced the risk of development of type II diabetes (46).
Further data suggest that the benefits of statin therapy may be greatest among those with elevated CRP levels, either among post-MI patients (47) or in the primary prevention setting (19). In the Cholesterol And Recurrent Events (CARE) trial population, patients with persistent low-grade vascular inflammation, as evidenced by high CRP and serum amyloid A levels, were at increased risk of recurrent events. Randomization to pravastatin therapy prevented 54% of recurrent events among those with persistent inflammation, compared with 25% among those without (47). Similarly, in the primary prevention Air Force/Texas Coronary Atherosclerosis Prevention Study (AFCAPS/TexCAPS), individuals with low LDL levels (<149 mg/dl) but high CRP levels (>0.16 mg/dl) were at high risk for future cardiovascular events, and they derived substantial benefit from lovastatin therapy (relative risk compared with placebo = 0.58; 95% confidence interval, 0.34 to 0.98) (19).
Current clinical practice should not be based on these post-hoc analyses (16,43), and there are currently no prospective data that prove that lowering CRP decreases cardiovascular events or improves survival, or that establish defined targets for treatment. Thus, although substantial gains may be made by targeting statin therapy at those with heightened vascular inflammation (48), prospective randomized trials to test these hypotheses directly are needed.
The effect of aspirin on CRP levels is controversial (49,50), but the benefit of aspirin therapy in preventing future MI appears to be greatest among those with elevated CRP levels (16). As noted above, data from the FRISC-II study suggest that the benefits of an early invasive approach may be greatest among those with evidence of a heightened inflammatory response (30). In the absence of an elevated inflammatory response, a less invasive approach may prove equally effective. Again, prospective randomized studies are required to test these hypotheses directly. The possibility of novel anti-inflammatory interventions targeted at specific mediators of vascular inflammation is also appealing.
The optimal cutoff point for defining high CRP levels among patients with ACS remains to be determined. The CAPTURE group found that a threshold of 10 mg/l maximized the predictive value of CRP (25). Several other investigators have used a cutoff point of 3 mg/l for patients with ACS, while the reference ranges for primary prevention populations are lower (16,18,19). The precise cause of these different thresholds remains unclear, but it is probably related to heightened vascular inflammation at the time of presentation with ACS.
| Conclusions |
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| Footnotes |
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| References |
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