Biomarkers for cardiovascular risk assessment

Medical Laboratory Observer, May, 2009 by Robert L. Wolfert

Research into inflammatory biomarkers has opened up a new era in the assessment of risk in patients with cardiovascular disease (CVD). Of the dozens of candidate biomarkers, there are two that have accumulated sufficient published evidence to support their utility in clinical practice: high-sensitivity C-reactive protein (hs-CRP) and lipoprotein-associated phospholipase [A.sub.2] ([Lp-PLA.sub.2]).

Clinical review of [Lp-PLA.sub.2]

[Lp-PLA.sub.2] is produced predominantly by macrophages and is strongly associated with rupture-prone plaque. Because it is produced by macrophages in atherosclerotic lesions in the arterial intima, it is a more vascular-specific marker than hs-CRP or other acute phase reactant inflammatory markers, many of which are produced in the liver.(1) [Lp-PLA.sub.2] is potentially linked to the causal pathway of plaque inflammation, instability, and eventual rupture; found at high levels in thin fibrous cap atheroma; and can be lowered by lipid-modifying medications (statins, fibrates, niacin, ezetimibe, and omega-3 fish oil).

An elevated [Lp-PLA.sub.2] result may indicate a need for more aggressive therapy, including treatment to lower low-density lipoprotein cholesterol (LDL-C) goals. Lipid-lowering therapies, including statins, are proven to reduce cardiovascular events, regardless of baseline LDL-C levels. In multiple clinical studies, [Lp-PLA.sub.2] has been shown to be a predictor of unstable plaque, myocardial infarction (MI), and ischemic stroke.(2) Since low-density lipoprotein has proven not to be a reliable predictor of stroke, measuring levels of [Lp-PLA.sub.2] addresses this unmet clinical need.

[Lp-PLA.sub.2] resides mainly on and travels with LDL particles in plasma via apolipoprotein B binding, although it is also associated with high-density lipoprotein, or HDL, particles, lipoprotein (a), and remnant lipoproteins. [Lp-PLA.sub.2] is highly upregulated in atherosclerotic plaque; and through hydrolysis of oxidized LDL, this enzyme generates two pro-inflammatory mediators, lysophosphatidylcholine and non-esterified oxidized fatty acid. In pre-clinical animal studies, inhibition of the enzyme attenuates the inflammatory process and slows atherosclerotic-disease progression. A Phase II study sponsored by GlaxoSmithKline showed that a direct [Lp-PLA.sub.2] inhibitor (darapladib), in addition to standard-of-care treatment, prevented expansion of the necrotic core, a region within coronary plaque associated with a high risk of rupture.(3)

The [Lp-PLA.sub.2] difference

Numerous peer-reviewed publications have confirmed that elevated plasma levels of [Lp-PLA.sub.2] are independently associated with risk of coronary heart disease (CHD) and ischemic stroke. The Atherosclerosis Risk in Communities, or ARIC, study showed that in individuals with normal LDL, elevated [Lp-PLA.sub.2] levels were strongly associated with heart disease and ischemic stroke, independent of traditional risk factors and hs-CRP.(4,5) Elevated levels of both inflammatory markers conferred an even higher risk of MI and stroke. Individuals with elevated [Lp-PLA.sub.2] and hs-CRP levels had greater than a fourfold increase in risk for heart attacks, and more than an elevenfold increase in risk for ischemic stroke. Additionally, increased levels of [Lp-PLA.sub.2] doubled the risk of ischemic stroke at every level of systolic blood pressure, while individuals with the highest levels of [Lp-PLA.sub.2] and elevated blood pressure had nearly a sevenfold increase in risk of suffering an ischemic stroke.(6) In the KAROLA study, high-risk patients followed for four to six years showed a significantly lower incidence of cardiovascular events if their [Lp-PLA.sub.2] levels were <223 ng/mL.(7)

Lab measurement of [Lp-PLA.sub.2]

Testing for [Lp-PLA.sub.2] in the laboratory is available in an ELISA test format or as an automated format. Two highly specific monoclonal antibodies are used in the assay, and it is calibrated to a well-characterized recombinant [Lp-PLA.sub.2] standard to increase the accuracy of the test. The automated assay employs immunoturbidimetric technology and can be run on the Hitachi, Roche Modular P and Olympus analyzers. Additional applications are in development. The [Lp-PLA.sub.2] protein in serum is generally .stable (i.e., the protein itself does not degrade), but it is highly recommended that the serum and plasma samples be collected and stored according to the Recommended Specimen Collection and Storage procedures.

Acknowledging the limitations of traditional risk factors to precisely assess cardiovascular risk across the general population, the National Cholesterol Education Program Adult Treatment Panel, or NCEP ATP III, report recognized the potential of inflammatory markers to help refine cardiovascular risk assessment. As [Lp-PLA.sub.2] evaluates vascular inflammation specifically, persons with elevated levels of [Lp-PLA.sub.2] could potentially be classified into a higher risk category, prompting the need to further intensify lifestyle and medication therapy in direct proportion to the degree of determined risk.(8-11)