EBM Tools for Practice: Best Biomarkers for Inflammation

Atherosclerosis is widely recognized as a chronic inflammatory disorder. Therapies with anti-inflammatory effects in patients with an increased inflammatory status can reduce cardiovascular events, as the CANTOS study showed.(1) Inflammatory markers are an essential tool for evaluating a patient’s inflammatory state. The question is: which biomarker is best?

A significant body of evidence involves the interleukin-Iβ/interleukin-6 (IL-Iβ/IL-6) axis, with epidemiologic studies mainly focusing on downstream markers of inflammation (e.g. C-reactive protein [CRP] and fibrinogen). Upstream markers of inflammation, such as IL-6, have had less focus.(Figure 1)

In addition, other biomarkers that involve different pathways, such as lipoproteinassociated phospholipase A2 (Lp-PLA2) and myeloperoxidase (MPO) have been studied with promising results.

Interleukin-6 (IL-6)

Primary risk prediction: In a metaanalysis of patients without known coronary artery disease (CAD), IL-6 elevation showed a relative risk of 1.25 for non-fatal myocardial infarction (MI) or coronary heart disease (CHD) death (p=0.001).(2) Matrix metalloproteinase-9 (MMP-9), soluble CD40-ligand (sCD40L), and tumor necrosis factor alpha (TNF- ) —all upstream markers — had no significant association.

In the Physicians’ Health Study, IL-6 and high-sensitivity C-reactive protein (hsCRP) had similar predictive risk for future myocardial infarction.(3)

Secondary risk prediction: In the Stabilization of Atherosclerotic Plaque by Initiation of Darapladib Therapy Trial (STABILITY) and the Stabilization of Plaques using Darapladib — Thrombosis in Myocardial Infarction 52 (SOLIDTIMI 52) trial, IL-6 was significantly and independently associated with incident cardiovascular (CV) events, including CV death, MI and heart failure.(4,5) There are no studies showing IL-6 as a predictive marker of the success of therapy.

High-Sensitivity C-Reactive

ProteinPrimary risk prediction: Multiple studies have shown that elevated levels of hs-CRP among healthy individuals is a strong predictor of future cardiovascular events, with a two to four-fold increase in risk comparing top versus bottom tertile, independently of age, gender and cholesterol levels.(6) The addition of hs-CRP to the Reynolds Risk Score has shown improved ability to estimate 10-year cardiovascular risk.(7) In a direct comparison of a panel of inflammatory markers — including serum amyloid A, IL-6 and soluble intercellular adhesion molecule type 1 (sICAM-1) — and various lipid markers, hs-CRP surpassed all of them as a predictor of cardiovascular events.(8) In the Physicians’ Health Study, those in the highest quartile of CRP had three times the risk of an MI, and twice the risk of an ischemic stroke.(9) A cohort study of patients with chronic inflammatory conditions revealed that the incidence of developing diabetes or coronary heart disease was greatest in those with the highest tertile of CRP. (10) A study of diabetics demonstrated that baseline and second-year CRP levels provided cardiovascular risk prediction independently of standard risk factors and glycemic control.(11) In the Anglo Scandinavian Cardiac Outcomes Trial (ASCOT), baseline levels of CRP and lowdensity lipoprotein cholesterol (LDL-C) were comparable in identifying subsets at risk of future events.(12) The 2013 ACC/AHA guidelines include hs-CRP as a recommended marker for measurement when the treatment decision is uncertain based on global risk scoring.

Secondary risk prediction: The baseline level of hs-CRP in the Further Cardiovascular Outcomes Research with PCSK9 Inhibition in Subjects with Elevated Risk Trial (FOURIER) predicted those at greatest risk for cardiovascular events.(13)

On-treatment risk prediction: Achieved level of hs-CRP on treatment is a strong predictor of subsequent outcomes(14) as several trials have shown:

1. Aggrastat-to-Zocor Trial (A to Z). The achieved levels of hs-CRP at 30 days and four months were independently associated with long-term survival and risk of MI.(15)
2. Reversing Atherosclerosis with Aggressive Lipid Lowering Trial (REVERSAL) reported that regression of vascular disease occurred only in the group of patients who achieved low levels of both LDL-C and hs-CRP. (16)
3. Improved Reduction of Outcomes: Vytorin Efficacy International Trial (IMPROVE-IT) and Pravastatin or Atorvastatin Evaluation and Infection Therapy Trial (PROVE-IT) showed that patients that met both targets — LDL-C <70 mg/dL and hs-CRP <2 mg/L — had the best outcomes. (17,18)
4. Justification for the Use of Statins in Prevention: An Intervention Trial Evaluating Rosuvastatin (JUPITER) patients had the least CV events when achieving both an LDL-C <70 mg/dL and an hs-CRP <2 mg/L.(19)
5. In ASCOT, on-treatment hs-CRP following statin treatment predicted subsequent risk reductions.(12)
6. The Safety and Cardiovascular Event Efficacy of Bococizumab in High-Risk Patients (SPIRES) Trial showed that persistent elevation of hs-CRP at 14 weeks was associated with increased future CV events even after low levels of LDL-C were achieved.(20)
7. Canakinumab Anti-inflammatory Thrombosis Outcome Trial (CANTOS). Despite no change in LDL-C, lowering hs-CRP below 2 mg/L resulted in a 25% reduction in major adverse cardiovascular events (p<0.001) and 31% reduction in cardiovascular mortality (p<0.0001).(21)

Lipoprotein-Associated Phospholipase A2 (Lp-PLA2 activity)
Lp-PLA2 is an enzyme produced by lymphocytes and activated macrophages that binds primarily to low-density lipoprotein (LDL) in the serum. Reductions in LDL cause a decline in Lp-PLA2 activity, limiting some of its predictive value in patients on lipid-lowering therapy.(22,23)

Multiple studies have shown that elevated Lp-PLA2 levels are predictive for cardiovascular events in primary prevention and are associated with roughly a doubling of risk(24), however, since pharmacologic inhibition of Lp-PLA2 does not reduce event rates(4,5), it is neither a goal of therapy nor a U.S. Food and Drug Administration (FDA)-approved or health insurance-reimbursed test.(25)

Lp-PLA2 has shown an additive effect with CRP on CAD risk prediction.(26) Compared with high CRP/low Lp-PLA2 levels, high CRP/high Lp-PLA2 levels predict a significant risk increase for CAD death (p=0.048).(27)

Importantly, there are two available assays to measure Lp-PLA2 protein with equally predictive value. However, direct measurement of protein concentration can be dramatically affected by pre-analytic handling conditions. Therefore, most instead favor measuring the protein’s enzymatic activity, which possesses much less variability.

Myeloperoxidase (MPO)
Myeloperoxidase (MPO), released from activated neutrophils, produces reactive oxygen species that can cause endothelial dysfunction, high-density lipoprotein (HDL) dysfunction and plaque instability.

Prospective studies of healthy individuals and patients with stable and unstable CAD have shown an association between elevated MPO levels and future CV event risk, although other studies were less predictive.(28,29)

In a study of patients with CAD, hs-CRP and MPO each was predictive for CV mortality with additive value and a 4.33-fold increase in risk when both were elevated.(30) There are no studies showing MPO as a predictive marker of success of therapy.

Conclusions:

1. Hs-CRP has the most data regarding prediction of CV risk and is the only inflammatory marker able to predict therapeutic success. It appears to be the biomarker of choice at this time and the only one recommended for measurement by the 2013 ACC/AHA guidelines.
2. IL-6 has potential, but in head-to-head comparison with hs-CRP, it was not as predictive. It also is not as accessible as hs-CRP for the practicing clinician.
3. Lp-PLA2 can be used to evaluate risk in primary prevention in statinnaïve patients. It may be helpful as a complementary marker of risk with hs-CRP. It is not designed as a goal of therapy nor to be predictive of risk in secondary prevention.
4. MPO could be helpful as a complementary marker of risk with hsCRP, but it is less effective as a single marker.

Disclosure statement: Dr. Moran has received honoraria from Sanofi/Regeneron and Amgen.