A personal history of markers of myocyte injury [myocardial infarction]

BACKGROUND

The measurement of proteins in blood to reflect damage to the heart is one of the most successful examples of easily measured biomarkers identifying a serious major health problem. The concept of using a blood test to reflect organ or cell injury requires a substance that is very abundant in the target cell, has a means of reaching blood, a reasonable half-life in blood, and ideally a specific form reflective only of the target cell in tissue. The myocyte's major role is contraction so proteins involved in contraction or the energy to support it should be good candidate markers.

CONCLUSIONS

All the various biomarkers that have been used to detect cardiac damage are involved in contraction or energy metabolism, but the markers evolved empirically starting with transaminases in the 1950s leading to troponins in the 1990s. This history is reviewed with reflections on my experiences with developing assays for CK-MB and Troponin I.

Cardiac Biomarkers for Detection of Myocardial Infarction: Perspectives from Past to Present

Editor’s Note: With great pleasure and anticipation in recognition of Clinical Chemistry’s 50th anniversary, I have been able to arm-twist four talented scientists to document their impressive marks on the science of diagnostics in the field of cardiac biomarkers and detection of myocardial infarction. Their exciting discoveries and applications have dramatically influenced the fields of laboratory medicine and cardiology and have greatly influenced the care and management of thousands of patients suffering from coronary artery disease leading to acute myocardial infarction. As a matter of historical record, I owe a great deal of thanks to each one of the coauthors of this special report because each one has personally influenced my scientific career. I met Dr. Rosalki, during my postdoctoral training, at a national AACC meeting, where he kindly answered my numerous queries regarding creatine kinase enzymology and muscle physiology. Dr. Roberts, while serving as Director of the Coronary Care Unit at Washington University in St. Louis, generously allowed this fledgling fellow into his laboratory and shared many of his clinical and experimental findings with me. Dr. Katus, whom I first met at a scientific meeting sponsored by Boehringer Mannheim in 1986 in Bavaria, where I first became fascinated with cardiac troponin T, has remained a friend and colleague. Lastly, Dr. Ladenson, who as mentor, scientific colleague, and close friend remains ultimately responsible for both my professional growth as a clinical chemist (he was my postdoctoral fellowship advisor) and for stimulating and encouraging my goals and aspirations in the field of cardiac biomarkers. With the descriptions of the ground-breaking science described below, I am extremely excited and optimistic that the future of cardiac biomarkers is secure and open to new discoveries by the Rosalkis, Robertses, Katuses, and Ladensons of the future.

—Fred Apple

Characterisation of an antibody coated microcantilever as a potential immuno-based biosensor

In this study, we investigated the activity, stability, lifetime and re-usability of monoclonal antibodies to myoglobin covalently immobilised onto microfabricated cantilever surfaces. These sensing surfaces are of interest to us in the development of novel cantilever-based immunosensors. For such sensors the antibody layer represents the sensing element while the microcantilever acts as a mechanical transducer. A procedure for producing re-usable biological coatings has been tested with different independent techniques. An Enzyme Linked Immunosorbent Assay (ELISA) was used to determine the presence of an active antibody coating, and to monitor the lifetime and stability of the immobilised antibody. Through this analysis, the activity of the immobilised antibody layer was found to be more stable with the introduction of sucrose, as a stabilising agent. Sucrose was applied to the immobilised antibody layer after each regeneration step. The immobilised antibody was found to have a stable active lifetime for up to 7 weeks. Fluorescence microscopy was used to give information on the distribution of the coating on the gold and silicon nitride sides of the cantilever. Atomic Force Microscopy was used to determine the presence of the biological coating on the cantilever and to obtain information on the surface morphology of the biological element of the sensor. The combined results provide valuable information on the development of an optimised sensing element and demonstrate a set of methods to use for future sensor-to-sensor characterisation. Preliminary experimental results showing the antibody activity against myoglobin, detected with a microcantilever based sensor prototype confirmed the motivations and potentialities of the proposed immunosensing technique.

Synergistic effects in antigen capture ELISA using three monoclonal antibodies directed at different epitopes of the same antigen

Using a panel of monoclonal antibodies (mAb) against human myoglobin (Mb), we have shown that the sensitivity of antigen-capture enzyme-linked immunosorbent assay (ELISA) may be significantly increased by the simultaneous immobilization on a solid phase of two co-operating capture mAbs. This method ("a three-site ELISA") uses three mAbs at different epitopes of the same antigen (two capture/one tracer), unlike the traditional two-site assay, using one capture and one tracer mAbs. We established two-site and three-site ELISA assays for Mb, by varying capture and tracer mAbs. Three-site assays showed 4-6 fold increase in sensitivity, if compared with two-site assays. The model for the effect has been suggested, according to which in three-site ELISA the high-affinity cyclic configurations may be formed by an antigen, two-capture mAbs and the surface of solid phase.

Myoglobin, creatine-kinase-MB and cardiac troponin-I 60-minute ratios predict infarct-related artery patency after thrombolysis for acute myocardial infarction: results from the Thrombolysis in Myocardial Infarction study (TIMI) 10B

OBJECTIVES

We examined the diagnostic performance of serum myoglobin, creatine-kinase-MB (CK-MB) and cardiac troponin-I (cTnI) for predicting the infarct-related artery (IRA) patency in patients receiving TNK-tissue plasminogen activator (TNK-tPA) therapy for acute myocardial infarction (AMI) in the Thrombolysis in Myocardial Infarction (TIMI) 10B trial.

BACKGROUND

A reliable noninvasive serum marker of IRA patency is desired to permit early identification of patients with a patent IRA after thrombolysis.

METHODS

We measured myoglobin, CK-MB and cTnI concentrations in sera obtained just before thrombolysis (T0) and 60 min later (T60) in 442 patients given TNK-tPA and who underwent coronary angiography at 60 min.

RESULTS

Angiography at 60 min showed a patent IRA (TIMI flow grade 2, 3) in 344 and occluded IRA (TIMI flow grade 0, 1) in 98 patients. The median serum T60 concentration, the ratio of the T60 and T0 serum concentration (60-min ratio) and the slope of increase over 60 min for each serum marker were significantly higher in patients with patent arteries compared with patients with occluded arteries. The area under the receiver-operating characteristic (ROC) curve for diagnosis of occlusion was 0.71, 0.70 and 0.71 for the 60-min ratio of myoglobin, cTnI and CKMB, respectively. The 60-min ratios of > or =4.0 for myoglobin, > or =3.3 for CK-MB and > or =2.0 for cTnI yielded a probability of patency of 90%, 88% and 87%, respectively.

CONCLUSIONS

The diagnostic performance of serum myoglobin, CK-MB and cardiac troponin-I (cTnI) 60-min ratios was similar. The probability of a patent IRA was very high (90%) in patients with 60-min myoglobin ratio > or =4.0, and early invasive interventions to establish IRA patency may not be necessary in this group. Serum marker determinations at baseline and 60-min after thrombolysis may permit rapid triage of patients receiving thrombolytic therapy by ruling out IRA occlusion.

Diagnostic strategies using myoglobin measurement in myocardial infarction

Myoglobin, a low molecular-weight heme protein (17800 D) present in both cardiac and skeletal muscle, is an old test with new perspectives. Advantages and disadvantages of myoglobin determination are well known. Myoglobin is the earliest known, commercially available, biochemical marker of acute myocardial infarction (AMI) and its rapid kinetics make it an early, good marker of reperfusion. However, since myoglobin is present in both skeletal and cardiac muscle, any damage to these muscle types results in its release into blood. Serum myoglobin levels are falsely elevated in conditions unrelated to AMI as skeletal muscle and neuromuscular disorders, renal failure, intramuscular injection, strenuous exercise, and after several toxins and drugs intake. New strategies for myoglobin measurement may resolve this limitation. These strategies include both the combined measurement of myoglobin and a skeletal specific marker (carbonic anhydrase III) or a cardiac specific marker (troponin I), as well as the myoglobin evaluation on serial samples. In particular, the diagnostic algorithm based on the combined measurement of myoglobin and troponin I, assuring a satisfactory analytical turnaround time, significantly improves the diagnostic efficiency of laboratory assessment of suspected AMI patients, allowing the successive monitoring of coronary reperfusion.

Myoglobin, creatine kinase MB, and cardiac troponin-I to assess reperfusion after thrombolysis for acute myocardial infarction: results from TIMI 10A

BACKGROUND

The availability of a reliable, noninvasive serum marker of reperfusion may permit early identification of patients with occlusion after thrombolysis who might benefit from further interventions.

METHODS

We measured myoglobin, creatine kinase MB (CK-MB), and cardiac troponin-I (cTnI) concentrations in sera obtained just before thrombolysis (T0) and 60 minutes later (T60) in 30 patients given TNK-tPA for acute myocardial infarction as part of the Thrombolysis in Myocardial Infarction (TIMI) 10A trial.

RESULTS

Angiography at T60 showed reperfusion (TIMI flow grade 2 to 3; n = 19) or occlusion (TIMI flow grade 0 to 1; n = 8). The median serum T60 concentration, the ratio of the T60 and T0 serum concentration, and the slope of increase over a 60-minute period for each serum marker were significantly higher in patients with patent arteries compared with patients with occluded arteries. The areas under the receiver operator characteristics curve for diagnosis of occlusion were 0.96, 0.91, and 0.87 for the T60 concentration of myoglobin, CK-MB and cTnI, respectively. Although the T60 levels of <469 ng/ml for myoglobin, <11.5 ng/ml for CK-MB, and < 1.1 ng/ml for cTnI identified all patients with occlusion, the specificity of myoglobin (94%) was higher than that of CK-MB (61%) and cTnI (67%). Similar results were obtained for the 60-minute ratios and 60-minute slopes for each marker, with indexes for myoglobin having the highest specificity.

CONCLUSIONS

In this pilot study, noninvasive diagnosis of occlusion 60 minutes after thrombolysis was achieved with a high degree of sensitivity and specificity with the myoglobin, CK-MB, and cTnI concentrations measured at that time point. These preliminary findings may permit a new strategy for assessment of the success of reperfusion, with triage to rescue angioplasty for patients in whom the 60-minute cardiac marker values or indexes are consistent with occlusion of the infarct-related artery.

Biochemical markers of myocardial damage

OBJECTIVE

To assess various biochemical markers of myocardial damage.

METHODS AND RESULTS

Before routinely using any test as a biochemical marker of myocardial damage, the published evidence for its diagnostic utility must be critically assessed. Such assessment includes receiver operator curve (ROC) curve analyses, confidence interval estimates of claimed sensitivity and specificity values, and the effects of testing in serial and parallel modes. It is also necessary to establish the test's rule-in (high specificity) and rule-out (high sensitivity) decision thresholds that may vary with time after the onset of symptoms. The spectrum of ischemic heart disease includes acute (sudden death, non-Q- and Q-wave infarctions) and chronic (stable, unstable, and variant angina) conditions. Biochemical markers of myocardial damage are of most value in the diagnosis of acute ischemic heart disease, although increasingly some of these markers are being found to possess a prognostic value in chronic ischemic heart disease. The markers of enzymatic activity include aspartate aminotransferase, creatine kinase (together with isoenzymes and isoforms), and lactate dehydrogenase and isoenzymes. Creatine kinase isoenzyme-2 may also be measured immunologically, and this type of assay is in increasing use both because of its speed and because its blood levels rise earlier than the corresponding activities. The commercially available nonenzymatic markers are myoglobin and troponin T; troponin I is expected to become available in late 1995. While myoglobin is a nonspecific indicator of myocardial damage, its diagnostic value is due to its early appearance in blood. Troponin T is more cardiac specific, but the published data appears to suggest that the cardiac specificity of troponin I is superior. Troponin levels become abnormal at about the same time after the onset of symptoms as mass assays of creatine kinase isoenzyme-2; therefore, they are not useful as early markers of myocardial damage.

CONCLUSION

The availability of these nonenzymatic markers of myocardial damage must force a reassessment of the continued use of the enzymatic markers. Are they necessary, and if so, which ones should be retained?

Noninvasive assessment of reperfusion and reocclusion after thrombolysis in acute myocardial infarction

The clinical significance of ST-segment changes and of the time course of appearance in serum of different cardiac proteins has been reviewed for the diagnosis of coronary reperfusion and reocclusion after thrombolysis. In particular, the value of serial 12-lead electrocardiographic (ECG) studies, of Holter monitoring, and of continuous multilead computer-assisted ECG monitoring is compared. Regarding the serum proteins, the clinical significance of reperfusion indices described so far for serum creatine kinase (CK), its isoenzyme serum creatinine kinase MB, the CK isoforms, and myoglobin is reviewed. Emphasis is placed on (1) the calculation method used for deriving the reperfusion indices; (2) the sensitivity and the specificity of the reperfusion indices; (3) the minimum turn-around time needed to produce the reperfusion indices (depending on the practicability of the analytical and calculation methods and their applicability in an emergency laboratory); (4) the ability of the indices to produce reliable estimates of reperfusion efficacy of the thrombolytic agents under study; and (5) the ability of the marker proteins to detect reinfarction as well as the suitability of the markers to detect real-time necrosis.