In this issue of the Journal of Cardiovascular Pharmacology, Li et al1 show that cardiac adriamycin-responsive protein (CARP) might serve as an informative biomarker of cardiotoxicity in patients treated with daunorubicin, member of the anthracycline family that has been used for some 40 years now to treat a broad spectrum of human tumors.

As any other anthracycline approved for clinical use, daunorubicin can be viewed as a sort of double-edged sword, causing excellent oncologic responses but introducing also a measurable risk of acute or chronic cardiotoxicity. Acute cardiotoxicity manifests early during the course of anthracycline treatment, usually in the form of arrhythmias and blood pressure instability, whereas chronic cardiotoxicity may manifest months or years after treatment completion, usually but not exclusively in the form of heart failure (HF) with reduced ejection fraction.2 The two forms of cardiotoxicity should not be viewed as separate pathophysiologic entities. Cardiotoxicity is a continuum that begins with the first infusion3 and then proceeds according to patient characteristics such as competing cardiovascular risk factors and concomitant or sequential oncologic treatments.4 The cumulative dose of anthracycline is of paramount importance, with each anthracycline showing a threshold dose above which HF risk increases disproportionately.5 When examined in this context, measurements of cardiac biomarkers during the anthracycline treatment may help to identify patients with acute cardiotoxicity.6

To illuminate strengths and weaknesses of the work by Li et al,1 two questions must first be answered: what is CARP and in which clinical settings was its role characterized? CARP is the acronym of cardiac adriamycin (doxorubicin)-responsive protein but one may also read it as the acronym of cardiac ankyrin repeat protein. The original report of an adriamycin suppressible 40 kDa protein of neonatal rat heart did in fact characterize its homology to a 36 kDa human protein expressed in endothelial cells and known to contain ankyrin-repeat motifs. The newly discovered CARP contained as many as 5 ankyrin-like repeats but also displayed 4-residue potential nuclear localization signals, 5 putative protein kinase C phosphorylation sites, 1 putative cAMP-dependent protein kinase phosphorylation site, and 4 presumed casein kinase II phosphorylation sites.7 The new CARP was expressed in cardiomyocytes but much less so in endocardial cushions and valves. Given the role of ankyrin-repeat proteins in assisting the attachment of integral membrane proteins to cytoskeletal proteins, and considering also the abundance of regulatory and nuclear translocation domains, the new CARP was considered as a regulator of cardiomyocyte structure and gene expression, and its downregulation by adriamycin was intuitively reconciled with earlier evidence for a broad effect of anthracyclines in disregulating cardiac-specific gene expression programs.7,8 As to the second question, Li et al1 focused on the circulating levels of CARP in children diagnosed with acute lymphoblastic leukemia (ALL) and treated with daunorubicin, a cogener of adriamycin. This was a good setting to explore, given that long-term survivors of childhood ALL and other cancers are plagued with an increased incidence of HF.9 Changes of CARP levels during chemotherapy might thus help to perceive the risk of late HF in this special population.

Li et al1 monitored 55 children, of whom as many as 43 were diagnosed with daunorubicin-attributable adverse events (AE), usually in the form of grade ≤2 palpitation, chest tightness, or QTc prolongation. Only 4 children experienced grade ≥3 AE. CARP decreased in children with AEs but showed a modest trend to increasing in patients without AEs. Overall, CARP levels inversely correlated with AE grade, and receiver operator characteristic curves showed that CARP was a more sensitive and specific biomarker of acute cardiotoxicity than other popular biomarkers such as high sensitivity troponin or cardiac-specific creatine kinase. Given the limited sample size and the type of AE reported, not much more can be said at this point in time.

Having summarized the main findings reported by Li et al,1 what are their strengths and weaknesses? We believe that the most remarkable strength of this work actually coincides with its most disappointing limitation. From our reading of this work, it would in fact appear that children were monitored for AE and biomarkers just after 1 daunorubicin infusion. Keeping this in mind, we cannot escape the conclusion that it did not take more than a few hours to see daunorubicin downregulate CARP expression in children with ALL, which is precisely the same time frame as that observed when isolated cardiomyocytes were treated with daunorubicin or adriamycin. Thus, CARP downregulation, and the consequent dysregulation of cardiac-specific gene expression programs, seems to fingerprint the very early trajectory of anthracycline cardiotoxicity, which makes CARP a very attractive biomarker. Such a remarkable strength is nonetheless counterbalanced by a number of concerns. First, the AE described by Li et al are almost unanimously considered as benign changes that need to be confirmed and diagnosed as reversible or progressive during the entire course of anthracycline treatment. Second, and likewise, biomarkers should be measured over the course of multiple anthracycline infusions, so as to approximate cause-and-effect relations between repeat anthracycline injury and the likelihood and magnitude of changes of one biomarker or another. Lessons from troponin actually show that just one peak is less predictive of cardiac dysfunction than a gradual increase over the entire chemotherapy period.10 Third, we do not know how precisely some children responded to daunorubicin by a trend toward increased rather than decreased CARP levels. Was it due to myocyte necrosis releasing CARP from preformed intracellular pools, or did it reflect some compensatory overexpression of unknown significance? Would upregulation and downregulation of CARP levels also occur after the second or third daunorubicin infusion?

The aforesaid questions embrace a number of biological and clinical implications. For example, overexpression of CARP in cardiomyocytes was shown to suppress atrial natriuretic factor transcription, and cotransfection experiments in HeLa cells helped to decipher that CARP inhibited Nkx2.5-dependent transactivation of the promoter region of atrial natriuretic factor gene.7 Anthracycline cardiotoxicity has been traditionally associated with elevations of atrial or B-type natriuretic peptides, and recent cardio-oncology guidelines of the European Society of Cardiology identify natriuretic peptides as reliable tools to diagnose a patient with anthracycline-related cardiotoxicity.6 Li et al1 did not characterize if and how a decrease of CARP was accompanied by a concomitant increase in atrial natriuretic peptide, but this should be put in the bucket list for future investigations.

In sum, Li et al1 should be congratulated for bringing CARP in the arena of anthracycline cardiotoxicity. It goes without saying that this is just the beginning of a potentially long story, at least as long as the trajectories of anthracycline cardiotoxicity can be. We hope to see studies of CARP in patients monitored over the entire chemotherapy cycle, and we also hope to see studies empowered with at least 1 or 2 years of follow-up, so as to intercept possible manifestations of chronic cardiotoxicity and establish correlations with CARP changes during chemotherapy. Combining patient clinical assessment with both biomarkers and echocardiographic imaging would be mandatory for such studies to be sufficiently informative (Fig. 1). For the time being, we welcome CARP as a potential new marker of anthracycline cardiotoxicity and a promising driver of translational research.

FIGURE 1.:

Research tasks for understanding the role of CARP in anthracycline cardiotoxicity.

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