# Myeloperoxidase: A New Biomarker of Inflammation in Ischemic Heart Disease and Acute Coronary Syndromes

Authors: Valentina Loria; Ilaria Dato; Francesca Graziani; LuigiM. Biasucci
Journal: Mediators of Inflammation
 (2008-01-27)

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## Abstract

Myeloperoxidase (MPO) is an enzyme stored in azurophilic granules of polymorphonuclear neutrophils and macrophages and released into extracellular fluid in the setting of inflammatory process. The observation that myeloperoxidase is involved in oxidative stress and inflammation has been a leading factor to study myeloperoxidase as a possible marker of plaque instability and a useful clinical tool in the evaluation of patients with coronary heart disease. The purpose of this review is to provide an overview of the pathophysiological, analytical, and clinical characteristics of MPO and to summarize the state of art about the possible clinical use of MPO as a marker for diagnosis and risk stratification of patients with acute coronary syndrome (ACS).


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## Body

## 1. Introduction

Myeloperoxidase (MPO) is
a well-known enzyme, mainly released by activated neutrophils, characterised by
powerful pro-oxidative and proinflammatory properties. Recently, myeloperoxidase
has been proposed as a useful risk marker and diagnostic tool in acute coronary
syndromes and in patients admitted to emergency room for chest pain.


## 2. Pathophysiological Role of Myeloperoxidase in Ischemic Heart Disease

Oxidative stress and inflammation
play important roles in the pathogenesis of destabilization of coronary artery
disease (CAD) leading to acute coronary syndromes (ACS). Infiltrating
macrophages and neutrophils participate in the transformation of stable
coronary artery plaques to unstable lesions [1, 2].
Recently, there has been a renewed interest in MPO, a proinflammatory enzyme that is abundant in ruptured plaque [3]
and can be measured in peripheral blood. MPO is a hemoprotein that is stored in
azurophilic granules of polymorphonuclear neutrophils and macrophages. MPO
catalyzes the conversion of chloride and hydrogen peroxide to hypochlorite and
is secreted during inflammatory condition. It has been implicated in the
oxidation of lipids contained within LDL cholesterol. In addition, MPO consumes
endothelial-derived NO, thereby reducing NO bioavailability and impairing its
vasodilating and anti-inflammatory properties.Major evidence for MPO as enzymatic catalyst for
oxidative modification of lipoproteins in the artery wall has been suggested in
a number of studies performed with low-density lipoprotein [4].
In contrast to low-density lipoprotein, plasma levels of high-density
lipoprotein (HDL)-cholesterol and apoAI, the major apolipoprotein of HDL,
inversely correlate with the risk of developing coronary artery disease. There
is now strong evidence that HDL is a selective in vivo target for MPO-catalyzed
oxidation, that may represent a specific molecular mechanism for converting the
cardioprotective lipoprotein into a dysfunctional form, raising the possibility
that the enzyme represents a potential therapeutic target for preventing
vascular disease in humans [5].
Zhou et al. [6] showed that atorvastatin
reduced serum MPO and CRP concentrations in patients with ACS.MPO activity can be measured
in blood and tissues by spectrophotometric assays using hydrogen peroxide and o-dianisidine
dihydrochloride as substrates. In addition, MPO content can be measured in
neutrophils as an index of degranulation with the Coulter counter and flow
cytometry and circulating MPO by ELISA. Very recently, commercial methods
allowing low-cost and high-volume measurements have been proposed. The
introduction of these methods of measurement might make MPO a new and useful
cardiac biomarker.


## 3. Clinical Evidences for MPO as a Cardiac Biomarker

### 3.1. Primary Prevention

There have been a few but important clinical
studies examining the role of MPO as a marker of risk for CAD. Using an enzyme
assay, Zhang et al. [7]
showed that blood and leukocyte MPO activity were higher in patients with CAD
than angiographically verified normal controls, and that this increased
activity was significantly associated with presence of CAD (odds ratio, 11.9;
95% confidence interval (CI), 5.5–25.5). Results
were independent of the patient’s age, sex, hypertension, smoking, or diabetes
status, LDL concentration, leukocyte count, and Framingham global risk score.  More recently,
Meuwese et al. [8],
in the EPIC- (European prospective investigation into cancer and nutrition-) Norfolk
prospective population study, have evaluated the association of MPO levels with
the risk of future CAD in apparently healthy individuals. MPO was measured in
baseline samples of a case-control study nested in the prospective EPIC-Norfolk
population study: case subjects (n = 1138) were apparently healthy men and women
who developed CAD during 8 years of follow-up; control subjects (n = 2237)
matched for age, gender, and enrollment time, remained free of CAD.  The
MPO levels were significantly higher in case subjects than in control subjects
and correlated with C-reactive protein (CRP) and white blood cell count. Risk
of future CAD increased in consecutive quartiles of MPO concentration, with an
odds ratio (OR) of 1.49 in the top versus bottom quartile. After
adjustment for traditional risk factors, the OR in the top quartile
remained significant at 1.36 (95% CI 1.07 to 1.73). Of interest in this study, serum MPO levels were associated
with the risk of future development of CAD in apparently healthy individuals,
but the association was weaker than that of traditional risk factors and CRP.
However MPO, at variance from CRP, was largely independent from classical risk
factors.


### 3.2. Secondary Prevention

In ACS, MPO has been consistently found to be
associated with the presence of instability and risk of future events in the studies
that have explored these topics. Biasucci et al. [9]
first observed that circulating neutrophils in patients with acute myocardial
infarction (AMI) and unstable angina (UA) have a low MPO content, and therefore
high MPO levels in the circulation, as compared with those with chronic stable
angina and variant angina. This is indicative of a significant release of MPO
from neutrophils related to their activation. The lack of neutrophil activation
in patients with variant angina, and after stress test suggests that this
phenomenon may occur independently of ischemic episodes. Therefore, MPO is prevalently
a marker of instability and not simply a marker of oxidative stress and damage.
Furthermore, in this study MPO did not correlate with CK-MB and troponin T
release; this observation is clinically important as an extremely sensitive and
specific marker of damage already exists (troponin), but no definite markers of
instability exists so far. In this study, MPO content was determined on the
Coulter counter, which measures the neutrophil count by flow cytometry and
subsequently calculates the mean MPO content in that population.Using the same method, Buffon et al. [10]
studied 65 patients who underwent cardiac catheterization with coronary sinus
sampling. The MPO content of the leukocytes collected from the arterial
circulation and the coronary sinus effluent were compared. The authors found a
gradient of MPO across the coronary circulation in patients with ACS and this
gradient was present even when the culprit lesion involved with the ACS was in
the distribution of the right coronary artery, which does not drain into the
coronary sinus. In this study, as in the previous one, a significant
correlation was found between systemic levels of C-reactive protein and either
the aortic and coronary sinus neutrophil MPO.The potential usefulness for risk stratification of blood concentrations of MPO was
examined in 2 recent studies. In the CAPTURE trial [11],
MPO mass concentration was measured in 1090 patients with ACS. Rates of death
and myocardial infarction (MI) were determined at 6months of follow-up. An MPO cutoff of 350 μg/L was
associated with an adjusted hazard ratio was 2.25 (95% CI, 1.32–3.82). The
effects were particularly impressive in patients with undetectable cardiac
troponin T (cTnT < 0.01 μg/L), in whom the hazard ratio was 7.48 (95% CI, 1.98–28.29). Of
interest, the increase in risk was already evident after 72 hours, increasing
only slightly thereafter (Figure 1). This observation is in keeping with the
data by Biasucci et al. [9] who had shown return of MPO to baseline levels
in all patients, including those with myocardial infarction, within one week.
This point is important, as suggests a peculiar characteristic of MPO, at
variance from other inflammatory markers commonly used (as CRP, fibrinogen) and
from other proposed inflammatory markers that remain elevated for relatively
long time or have an extremely short and unreliable half-life (such as
interleukins). The predictive value
of MPO was independent by C-reactive protein and high MPO serum levels indicated increased cardiac risk both
in patients with medium C-reactive
protein serum levels (20.0% versus 5.9%; P < .001) and in those with low C-reactive
protein serum levels (17.8% versus 0%; P < .001), suggesting that recruitment
and degranulation of neutrophils is a primary event and is followed by release
of other systemic mediators and acute-phase proteins such as C-reactive
protein. At variance from CRP levels, levels of MPO were not influenced by
troponin, suggesting a prognostic role of MPO independent from troponin and
confirming that inflammation is a primary phenomenon in ACS.Figure 1
Increase in risk of death (D) plus MI (myocardial infarction) with increasing
tertiles of MPO [11].In a study of 604 consecutive patients presenting to the emergency department with
chest pain, Brennan et al. [12]
demonstrated a progressive increase in odds ratios for major adverse events at
30 days and 6 months with each quartile increase in MPO concentration in
patients with negative troponin. The 6-month outcomes were similar to the
results of the CAPTURE trial: corresponding odds ratios were 1.6 (95% CI, 1.0–2.7),
3.6 (2.2–5.8), and 4.7 (2.9–7.7) for the
second, third, and fourth quartiles, respectively (cut offs of 119, 198, and
394 pmol/L, resp.). It is of interest that, even in the absence of
myocardial necrosis and in negative cardiac troponin patients, baseline
measurements of MPO significantly enhanced the identification of patients at
risk, although this study included the “need for revascularization” within the
definition of major adverse cardiac events and used an inappropriately high
cTnT cutoff of 0.10 μg/L. Furthermore,
while troponin T takes three to six hours to rise to measurable
circulating levels after myocardial injury, MPO levels were significantly
elevated at baseline (even within two hours after the onset of symptoms) in
patients who were initially negative for troponin T. These findings suggest
that measurement of MPO levels may be useful in triage in the emergency
department and that elevated plasma MPO levels may be a marker of unstable
angina preceding myocardial necrosis and therefore, a predictor of vulnerable
plaque. Interestingly, C-reactive
protein levels predicted the risk of myocardial infarction at presentation for
the entire cohort but were not predictive of major adverse cardiac events in
the group that was negative for troponin T. In the cohort that was consistently
negative for troponin T, the areas under the curve for MPO were significantly
higher than those for troponin T, CK-MB, and C-reactive protein.Taken together, these data
suggest that CRP and MPO may be complementary and explore different fields: CRP is a marker of disease
activity and vascular inflammation, and is useful for long-term risk
stratification while MPO is a marker of plaque instability and neutrophil
activation and may be associated with short-term stratification, in particular
in patients with troponin negative levels.More recently, several studies have also
investigated the value of MPO in predicting long-term outcomes. Recently, S.-H. Li
et al. [13]
have studied 176 consecutive patients who underwent
coronary angiography. The patients were divided into four groups according to
the quartile of MPO level. They have found that: (1) ACS rate (36.2%) in the fourth quartile group of MPO level was 6
times higher than that in the first quartile group of MPO level, P < .01.
(2) Gensini score in the fourth quartile group of MPO level was significantly higher
than that in the first quartile group (P < .01). WBC in the fourth
quartile group was also significantly higher than that in the first quartile group, P < .05.
In addition, Kaplan-Meier
event rate curve showed that there was a significant difference in outcome (death,
AMI, revascularization) between the groups with MPO ≥ 62.9 AUU/L and ≤62.9 AUU/L of MPO serum level at 6-month follow-up visit (chi(2) = 13.5, P = .01).
Furthermore, Cavusoglu et al. [14]
have investigated the long-term prognostic significance of baseline MPO levels
in a well-characterized cohort of 193 men with ACS. All
patients were followed prospectively for the development of death and MI, and
follow-up data were available for all patients at 24 months. Using the median
MPO value of the entire cohort of patients (20.34 ng/mL) as a prespecified cutoff,
the MI-free survival at 24 months for the group with MPO values below cutoff   were
significantly lower than in those with values above cutoff.Mocatta et al. have investigated the relationship
between plasma MPO and clinical outcome after AMI [15, 16].
They have studied 512 AMI patients at hospital admission and have measured plasma
MPO in AMI patients and found a significant association of MPO with follow-up
events. Importantly, MPO was of incremental prognostic value on the top of ejection
fraction and BNP, a finding observed also by Khan in a similar population of
patients with STEMI [17].


## 3.1. Primary Prevention

There have been a few but important clinical
studies examining the role of MPO as a marker of risk for CAD. Using an enzyme
assay, Zhang et al. [7]
showed that blood and leukocyte MPO activity were higher in patients with CAD
than angiographically verified normal controls, and that this increased
activity was significantly associated with presence of CAD (odds ratio, 11.9;
95% confidence interval (CI), 5.5–25.5). Results
were independent of the patient’s age, sex, hypertension, smoking, or diabetes
status, LDL concentration, leukocyte count, and Framingham global risk score.  More recently,
Meuwese et al. [8],
in the EPIC- (European prospective investigation into cancer and nutrition-) Norfolk
prospective population study, have evaluated the association of MPO levels with
the risk of future CAD in apparently healthy individuals. MPO was measured in
baseline samples of a case-control study nested in the prospective EPIC-Norfolk
population study: case subjects (n = 1138) were apparently healthy men and women
who developed CAD during 8 years of follow-up; control subjects (n = 2237)
matched for age, gender, and enrollment time, remained free of CAD.  The
MPO levels were significantly higher in case subjects than in control subjects
and correlated with C-reactive protein (CRP) and white blood cell count. Risk
of future CAD increased in consecutive quartiles of MPO concentration, with an
odds ratio (OR) of 1.49 in the top versus bottom quartile. After
adjustment for traditional risk factors, the OR in the top quartile
remained significant at 1.36 (95% CI 1.07 to 1.73). Of interest in this study, serum MPO levels were associated
with the risk of future development of CAD in apparently healthy individuals,
but the association was weaker than that of traditional risk factors and CRP.
However MPO, at variance from CRP, was largely independent from classical risk
factors.


## 3.2. Secondary Prevention

In ACS, MPO has been consistently found to be
associated with the presence of instability and risk of future events in the studies
that have explored these topics. Biasucci et al. [9]
first observed that circulating neutrophils in patients with acute myocardial
infarction (AMI) and unstable angina (UA) have a low MPO content, and therefore
high MPO levels in the circulation, as compared with those with chronic stable
angina and variant angina. This is indicative of a significant release of MPO
from neutrophils related to their activation. The lack of neutrophil activation
in patients with variant angina, and after stress test suggests that this
phenomenon may occur independently of ischemic episodes. Therefore, MPO is prevalently
a marker of instability and not simply a marker of oxidative stress and damage.
Furthermore, in this study MPO did not correlate with CK-MB and troponin T
release; this observation is clinically important as an extremely sensitive and
specific marker of damage already exists (troponin), but no definite markers of
instability exists so far. In this study, MPO content was determined on the
Coulter counter, which measures the neutrophil count by flow cytometry and
subsequently calculates the mean MPO content in that population.Using the same method, Buffon et al. [10]
studied 65 patients who underwent cardiac catheterization with coronary sinus
sampling. The MPO content of the leukocytes collected from the arterial
circulation and the coronary sinus effluent were compared. The authors found a
gradient of MPO across the coronary circulation in patients with ACS and this
gradient was present even when the culprit lesion involved with the ACS was in
the distribution of the right coronary artery, which does not drain into the
coronary sinus. In this study, as in the previous one, a significant
correlation was found between systemic levels of C-reactive protein and either
the aortic and coronary sinus neutrophil MPO.The potential usefulness for risk stratification of blood concentrations of MPO was
examined in 2 recent studies. In the CAPTURE trial [11],
MPO mass concentration was measured in 1090 patients with ACS. Rates of death
and myocardial infarction (MI) were determined at 6months of follow-up. An MPO cutoff of 350 μg/L was
associated with an adjusted hazard ratio was 2.25 (95% CI, 1.32–3.82). The
effects were particularly impressive in patients with undetectable cardiac
troponin T (cTnT < 0.01 μg/L), in whom the hazard ratio was 7.48 (95% CI, 1.98–28.29). Of
interest, the increase in risk was already evident after 72 hours, increasing
only slightly thereafter (Figure 1). This observation is in keeping with the
data by Biasucci et al. [9] who had shown return of MPO to baseline levels
in all patients, including those with myocardial infarction, within one week.
This point is important, as suggests a peculiar characteristic of MPO, at
variance from other inflammatory markers commonly used (as CRP, fibrinogen) and
from other proposed inflammatory markers that remain elevated for relatively
long time or have an extremely short and unreliable half-life (such as
interleukins). The predictive value
of MPO was independent by C-reactive protein and high MPO serum levels indicated increased cardiac risk both
in patients with medium C-reactive
protein serum levels (20.0% versus 5.9%; P < .001) and in those with low C-reactive
protein serum levels (17.8% versus 0%; P < .001), suggesting that recruitment
and degranulation of neutrophils is a primary event and is followed by release
of other systemic mediators and acute-phase proteins such as C-reactive
protein. At variance from CRP levels, levels of MPO were not influenced by
troponin, suggesting a prognostic role of MPO independent from troponin and
confirming that inflammation is a primary phenomenon in ACS.Figure 1
Increase in risk of death (D) plus MI (myocardial infarction) with increasing
tertiles of MPO [11].In a study of 604 consecutive patients presenting to the emergency department with
chest pain, Brennan et al. [12]
demonstrated a progressive increase in odds ratios for major adverse events at
30 days and 6 months with each quartile increase in MPO concentration in
patients with negative troponin. The 6-month outcomes were similar to the
results of the CAPTURE trial: corresponding odds ratios were 1.6 (95% CI, 1.0–2.7),
3.6 (2.2–5.8), and 4.7 (2.9–7.7) for the
second, third, and fourth quartiles, respectively (cut offs of 119, 198, and
394 pmol/L, resp.). It is of interest that, even in the absence of
myocardial necrosis and in negative cardiac troponin patients, baseline
measurements of MPO significantly enhanced the identification of patients at
risk, although this study included the “need for revascularization” within the
definition of major adverse cardiac events and used an inappropriately high
cTnT cutoff of 0.10 μg/L. Furthermore,
while troponin T takes three to six hours to rise to measurable
circulating levels after myocardial injury, MPO levels were significantly
elevated at baseline (even within two hours after the onset of symptoms) in
patients who were initially negative for troponin T. These findings suggest
that measurement of MPO levels may be useful in triage in the emergency
department and that elevated plasma MPO levels may be a marker of unstable
angina preceding myocardial necrosis and therefore, a predictor of vulnerable
plaque. Interestingly, C-reactive
protein levels predicted the risk of myocardial infarction at presentation for
the entire cohort but were not predictive of major adverse cardiac events in
the group that was negative for troponin T. In the cohort that was consistently
negative for troponin T, the areas under the curve for MPO were significantly
higher than those for troponin T, CK-MB, and C-reactive protein.Taken together, these data
suggest that CRP and MPO may be complementary and explore different fields: CRP is a marker of disease
activity and vascular inflammation, and is useful for long-term risk
stratification while MPO is a marker of plaque instability and neutrophil
activation and may be associated with short-term stratification, in particular
in patients with troponin negative levels.More recently, several studies have also
investigated the value of MPO in predicting long-term outcomes. Recently, S.-H. Li
et al. [13]
have studied 176 consecutive patients who underwent
coronary angiography. The patients were divided into four groups according to
the quartile of MPO level. They have found that: (1) ACS rate (36.2%) in the fourth quartile group of MPO level was 6
times higher than that in the first quartile group of MPO level, P < .01.
(2) Gensini score in the fourth quartile group of MPO level was significantly higher
than that in the first quartile group (P < .01). WBC in the fourth
quartile group was also significantly higher than that in the first quartile group, P < .05.
In addition, Kaplan-Meier
event rate curve showed that there was a significant difference in outcome (death,
AMI, revascularization) between the groups with MPO ≥ 62.9 AUU/L and ≤62.9 AUU/L of MPO serum level at 6-month follow-up visit (chi(2) = 13.5, P = .01).
Furthermore, Cavusoglu et al. [14]
have investigated the long-term prognostic significance of baseline MPO levels
in a well-characterized cohort of 193 men with ACS. All
patients were followed prospectively for the development of death and MI, and
follow-up data were available for all patients at 24 months. Using the median
MPO value of the entire cohort of patients (20.34 ng/mL) as a prespecified cutoff,
the MI-free survival at 24 months for the group with MPO values below cutoff   were
significantly lower than in those with values above cutoff.Mocatta et al. have investigated the relationship
between plasma MPO and clinical outcome after AMI [15, 16].
They have studied 512 AMI patients at hospital admission and have measured plasma
MPO in AMI patients and found a significant association of MPO with follow-up
events. Importantly, MPO was of incremental prognostic value on the top of ejection
fraction and BNP, a finding observed also by Khan in a similar population of
patients with STEMI [17].


## 4. Conclusions

MPO is a marker of inflammation and oxidative stress that has been consistently demonstrated to be
elevated in patients with ACS. However, the data so far available are
relatively few; therefore, more studies are requested to precisely define the
role of MPO. In particular all studies involving MPO assessment have used
different methods, thus a standardization effort is needed. Furthermore, increased
MPO is not likely to be specific for cardiac diseases, as activation of
neutrophils and macrophages can occur in any infectious, inflammatory, or
infiltrative process, therefore, more studies should address and clarify these
points. So far, as reported in Table1, MPO seems to have, among the
inflammatory markers, a role superior to that of pregnancy-associated plasma
protein-A (PAPP-A), CD40 ligand (CD40L), and cytokines, but still inferior to
CRP.Table 1
Prognostic and diagnostic value and analytical performance of inflammatory markers in CAD:
myeloperoxidase (MPO), C reactive protein (CRP), Pregnancy-associated plasma
protein-A (PAPP-A), CD40 ligand (CD40L), and interleukins.
Inflammatory markers in CADprognosisdiagnosisAnalyt. Perform.MPO+++++++ (more?)CRP+++++/−++++PAPP-A+++−CD40L+++/−−Interleukins++++/−−



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*Source: 135625-2008-03-11.xml*