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JACC CardioOncol
JACC CardioOncol
JACC: CardioOncology
2666-0873
Elsevier

S2666-0873(24)00222-9
10.1016/j.jaccao.2024.06.002
Viewpoint
Circulating Macrotroponin Complexes and Their Impact on Cardiac Troponin Measurements
Potential Implications for Cardio-Oncology
Nardi-Agmon Inbar MD, MPH a
Di Meo Ashley PhD bc
Lam Leo MBChB d
Kyle Campbell MBChB, PhD d
Abdel-Qadir Husam MD, PhD ae
Amir Eitan MD, PhD f
Thavendiranathan Paaladinesh MD, SM dinesh.thavendiranathan@uhn.ca
@dineshpmcc1
a∗
a Department of Medicine, Division of Cardiology, Ted Rogers Program in Cardiotoxicity Prevention, Peter Munk Cardiac Center, Toronto General Hospital, University Health Network, University of Toronto, Toronto, Ontario, Canada
b Division of Clinical Biochemistry, Laboratory Medicine Program, Toronto General Hospital, Toronto, Canada
c Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Canada
d Department of Pathology and Laboratory Medicine (LabPlus), Grafton, Auckland, New Zealand
e Women’s College Hospital, Toronto, Ontario, Canada
f Department of Medicine, Division of Medical Oncology and Hematology, Princess Margaret Cancer Center, University Health Network, University of Toronto, Toronto, Ontario, Canada
∗ Address for correspondence: Dr Paaladinesh Thavendiranathan, Division of Cardiology, Peter Munk Cardiac Center, Ted Rogers Program in Cardiotoxicity Prevention, Toronto General Hospital, 4N-490, 200 Elizabeth Street, Toronto, Ontario M5G 2C4, Canada. dinesh.thavendiranathan@uhn.ca@dineshpmcc1
20 8 2024
8 2024
20 8 2024
6 4 608611
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Graphical Abstract

Key Words

cardiac biomarkers
cardio-oncology
macrotroponin
troponin antibody
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pmcCase 1

An otherwise healthy 63-year-old women with HER2+ breast cancer scheduled to receive sequential therapy with anthracycline and trastuzumab had an elevated pre-anthracycline high-sensitivity troponin I (hsTnI) of 69 ng/L (sex-specific upper reference limit [URL], 16 ng/L [Alinity ci, Abbott Diagnostics]). She had no cardiac history or symptoms; had a normal electrocardiogram; and had B-type natriuretic peptide of 31 pg/mL, echocardiography 3-dimensional left ventricular ejection fraction of 60%, and global longitudinal strain of −21.1%. Cardiac magnetic resonance was normal without signs of edema or inflammation. Given the unexplained hsTnI elevation, the laboratory performed further investigations upon the request of the cardio-oncology team and identified the presence of macrotroponin (troponin I–immunoglobulin complex) with undetectable free troponin I after immunoglobulin depletion and normal troponin T (7 ng/L; sex-specific URL, 9 ng/L). Her elevated hsTnI was considered noncontributory for acute myocardial injury. The patient continued and completed her cancer therapy without cardiotoxicity.

Case 2

A 54-year-old-man with metastatic melanoma receiving nivolumab and ipilimumab developed thyroiditis, hepatitis, and hypophysitis and was treated with oral steroids. Troponin was not measured at that time; however, at the 6-week follow-up, his hsTnI was 51 ng/L (sex-specific URL, 26 ng/L). He was asymptomatic and electrocardiography was normal, but cardiac magnetic resonance demonstrated a left ventricular ejection fraction of 53% with mild midwall and subepicardial late gadolinium enhancement without edema, suggesting prior myocarditis. On analytical investigation, free troponin I was undetectable after immunoglobulin depletion, indicating the presence of macrotroponin. Over the 1-year follow-up, his hsTnI remained elevated between 30 and 50 ng/L and without deterioration in cardiac function. His hsTnI elevation was attributed to the presence of macrotroponin, likely related to prior immune checkpoint inhibitor–related myocardial injury or chronic autoantibodies.

Troponins play a fundamental role in the function of skeletal and cardiac muscles. Cardiac troponin I (cTnI) and cardiac troponin T (cTnT) are isoforms entirely specific to the heart, apart from instances of troponin T re-expression in muscle such as in chronic muscle injury.1 Along with troponin C, they control the calcium-mediated interaction between actin and myosin required for cardiac contraction. Cardiac troponins (cTns) may be released into the circulation because of cardiomyocyte necrosis and/or apoptosis during acute or chronic injury, severe cardiac stress, increased myocyte membrane permeability, or increased cardiomyocyte turnover or renewal.2 Elevated circulating troponin above the 99th percentile URL from a healthy population currently defines the presence of myocardial injury under the universal definition of myocardial infarction and has prognostic implications.3 The development of high-sensitivity cardiac troponin assays has enabled the detection of very low concentrations in the blood to within healthy population limits, improving diagnostic accuracy and allowing for more rapid triage of patients with symptoms suspicious of acute coronary syndrome. These high-sensitivity cardiac troponin assays consist of 2 or more (capture and detection) antibodies that recognize unique epitopes on the cTn peptide to generate a signal.4

In patients receiving potentially cardiotoxic cancer therapy (particularly anthracyclines and/or immunotherapy), the European Society of Cardiology cardio-oncology guidelines recommend using cTn for pretreatment risk stratification and surveillance for cardiotoxicity during cancer treatment and diagnosis of acute cardiac events such as myocarditis or infarction. This approach continues to be debated because mild cTn elevation has been shown to be frequent during cancer treatment and does not necessarily carry a prognostic implication.5 Given the potential impact of cTn elevation on ongoing cancer therapy, in this viewpoint, other than cardiac injury, we raise the importance of considering sources of analytical variation in cTn results with a focus on macrotroponin.

First, there are substantial discrepancies in cTn measurements between various available commercial troponin assays, attributed to differences in reagent antibody cross-reactivity to different fragments and to macrotroponin.4 This emphasizes the importance of sequential follow-up with the same cTn assay during cardiotoxicity surveillance unless the assay is shown to be affected by analytical interference. Second, it is important to recognize that, in a small proportion of patients, low-level elevations in cTn can be observed in the absence of clinically evident cardiovascular disease because of release into the circulation in various conditions (eg, sepsis, significant blood pressure elevation, and prolonged and strenuous exercise).2

In addition, consideration should be given to false-positive (or, less commonly, negative) cTn measurement because of interferences affecting cTn assays. The false-positive measurements can be caused by cross-linking antibodies (ie, heterophile antibodies), fibrin clots, cross-reactivity with troponin T that is re-expressed and released from skeletal muscle in inflammatory muscle diseases, or antibody-mediated assay interference because of long-lived cTn-immunoglobulin complexes called macrotroponin.6, 7, 8

Alongside circulating cTn, endogenous cTn-specific autoantibodies (immunoglobulin G) can be present in the circulation. The origin of these autoantibodies is thought to include molecular mimicry from environmental or infectious antigens, immune provocation from cTn released into the bloodstream from previous myocardial injury (eg, viral infection, strenuous exercise, or extreme stress), or homology to troponin released from skeletal muscle damage. Their mechanism of development remains poorly understood. Troponin autoantibodies can form against all troponin subunits, both cTnI and cTnT in isolation, and to cTnI-TnT complexes, although their impact is most frequent and problematic with cTnI assays.4,7,9

When these autoantibodies bind to circulating cTn in the blood, they form a complex called macrotroponin (Figure 1). The prevalence of macrotroponin complexes in patients with elevated cTn values (above the URL) has been variable in the literature. Circulating cTn autoantibodies have been reported in up to 12.7% of samples when troponin was measured as part of medical investigation in an ambulatory setting, with a higher prevalence in those with certain autoimmune conditions (eg, rheumatoid arthritis) and in those with established cardiac disease (up to 27.9%).9 More recent estimates were as high as 55% of elevated cTnI results in a community population where troponin was requested as a “rule out” test in a low- to moderate-risk clinical setting.4Figure 1 A Suggested Approach to Assessing Elevated cTn Values During Cancer Therapy

The flow diagram provides a potential approach to patients who are undergoing surveillance with cardiac troponin (cTn) during cancer therapy with identification of elevated cTn measurements. IFCC = International Federation of Clinical Chemistry; PEG = polyethylene glycol.

Macrotroponin is recognized by both capture and detection antibodies of the cTn assay, generating a false-positive result. Compared to unbound cTn, which can be detected in the circulation for a few hours to days/weeks depending on the pathology, macrotroponin has a longer half-life, reflecting the slower clearance of immunoglobulins.10 Therefore, patients with macrotroponin can have prolonged elevation in cTn measurements,10 well beyond the period of acute troponin release. In the early phase of injury, it is possible that both free cTn and macrotroponin can be identified, although the dynamics of troponin rise and fall may be affected with a different contribution from the two as the event evolves. However, it is important to highlight that even in healthy individuals, the heart releases small amounts of cTn (because of turnover or renewal), which can form a macrotroponin complex with circulating autoantibodies, resulting in persistently elevated cTn above the population URL in stable asymptomatic individuals.

Another described hypothesis is that these autoantibodies themselves are cardiotoxic, causing further myocardial injury and cTn spillage into the circulation.9,10 However, patients with macrotroponin have favorable overall survival compared with those with troponin elevation in the absence of macrotroponin, possibly reflecting a lower degree of cardiac injury and myocyte loss for the same degree of measured troponin elevation.10 Furthermore, although by far the most common impact of endogenously occurring autoantibodies is unexpected elevated concentrations on current commercial assays, it is also possible that it may lead to a falsely low result by blocking the assay antibody binding to cTn.7

There are multiple approaches to detecting macrotroponin; the available methods include immunoglobulin depletion (protein A/G/L and polyethylene glycol [PEG] precipitation), size-exclusion chromatography, and sucrose gradient separation.7 Because these more complex methods have limited availability and can be time-consuming, when an cTn elevation is unexpected or seems clinically inconsistent, it is reasonable to perform initial screening by testing the same specimen on a different troponin assay platform (troponin I and/or troponin T). A gross discrepancy can be taken as an indicator of potential macrotroponin interference, prompting further analytical and clinical evaluation. Clinical vigilance and close communication with the laboratory are keys to the success of this process. In the 2 cases presented previously, plasma samples were subjected to protein G and PEG treatment and tested with the Abbott high-sensitivity cTnI assay to detect the presence of macrotroponin (macrotroponin considered present if cTnI recovery is <40% using protein G immunodepletion and/or recovery <20% using PEG precipitation).8

The 2 clinical cases described reflect the important clinical implications of cTn measurement interference in cardio-oncology. In the first case, elevated pretreatment cTnI was associated with a substantial number of additional cardiac investigations and a delay in the initiation of cancer therapy. Given normal cardiac imaging, the absence of cardiac risk factors, recent illnesses, or symptoms and the absence of free cTnI, the original trigger for the presence of macrotroponin was unclear, but it was believed to not signify acute myocardial pathology, and the patient completed cancer therapy safely. In the second case, cTnI measurements were not obtained during the acute phase of the immune checkpoint inhibitor–related adverse events. In the presence of cardiac magnetic resonance findings suspicious of prior myocarditis and the clinical history, the detection of myocardial injury was possibly missed. It is possible that the patient had chronic autoantibodies or developed it after the current injury, resulting in the macrotroponin. The elevated cTnI was unlikely to represent an ongoing myocardial injury. This is further supported by the observation that the patient had a persistently elevated cTnI assay over the one year of follow-up after the acute event despite the lack of continuation of immunotherapy and the absence of symptoms.

In summary, when cTn levels are inconsistent with the clinical picture (ie, unexpectedly high, low, or persistently elevated), it should promote a dialogue with the laboratory to determine if further analysis is warranted, including the consideration of the presence of macrotroponin (Figure 1). With the recognized and frequent elevation of cTn measurements in patients undergoing potentially cardiotoxic chemotherapy, the clinical significance of cTn autoantibodies and the presence of macrotroponin complexes is unclear and warrants further investigation. However, clinicians using cTn measurements in screening, diagnosing disease, initiating cardioprotective therapies, or prognosticating patients receiving cancer therapy should be familiar with the concept of macrotroponin. If there is a plan to use cTn for surveillance during cancer treatment, we strongly recommend a baseline measurement so that future positive measurements can be better contextualized. The clinical imperative is that clinical and laboratory findings must be carefully considered together. In case of a suspected discrepancy, the laboratory should be contacted to consider repeat measurements using different troponin assays or additional laboratory evaluation for confounders, such as macrotroponin.

Funding Support and Author Disclosures

Dr Nardi-Agmon is supported by the Hold’em for life research grant. Dr Thavendiranathan is supported by a Canada Research Chair in Cardiooncology (grant CRC-2019-00097) and the Canadian Cancer Society/Canadian Institutes of Health Research’s W. David Hargraft grant. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.

The authors attest they are in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the Author Center.
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