Author + information
- Antoni Bayés-Genís, MD, PhD∗ ()
- Heart Failure Clinic, Cardiology Service, Hospital Universitari Germans Trias i Pujol, Badalona (Barcelona), Spain
- Department of Medicine, Universitat Autonoma de Barcelona, Barcelona, Spain
- ↵∗Reprint requests and correspondence:
Dr. Antoni Bayes-Genis, Cardiology Service, Hospital Universitari Germans Trias i Pujol, Carretera del Canyet s/n 08916, Badalona, Spain.
Neprilysin (NEP), also known as neutral endopeptidase, CD10, enkephalinase, common acute lymphoblastic leukemia antigen, and endopeptidase 24.11, is a well-known enzyme that was described several decades ago and fully characterized at the turn of the century. NEP is quite ubiquitous; it is expressed in the kidneys, lungs, endothelial cells, vascular smooth-muscle cells, cardiac myocytes, fibroblasts, neutrophils, adipocytes, testes, and brain, with the highest concentrations in the proximal tubules of nephrons. NEP is also quite “promiscuous”; in the cardiovascular system, NEP cleaves numerous vasoactive peptides, including natriuretic peptides, adrenomedullin, angiotensin I and II, bradykinin, substance P, and others (Figure 1A).
Awareness of the importance of NEP grew in the 1980s and early 1990s with the development of omapatrilat, the first drug that combined NEP inhibition with inhibition of angiotensin-converting enzyme (ACE). The disappointing results of the OVERTURE trial (Omapatrilat Versus Enalapril Randomized Trial of Utility in Reducing Events) (1), particularly the excess presence of life-threatening angioedema in the treated arm, put the development of NEP inhibitors on hold. The field’s focus on NEP in heart failure faded away for nearly 20 years. As NEP disappeared from the scene, natriuretic peptides (B-type natriuretic peptide [BNP] and N-terminal pro-BNP) were introduced into clinical practice as biomarkers for diagnosis, prognosis, and monitoring of heart failure, and eventually therapy guidance.
Interest in NEP revived in 2014 with the advent of angiotensin receptor and NEP inhibitors and the groundbreaking results of the PARADIGM-HF (Prospective Comparison of ARNI With ACEI to Determine Impact on Global Mortality and Morbidity in Heart Failure Trial) (2). This trial, which was the largest, most contemporary, and most geographically diverse mortality–morbidity trial in patients with heart failure with reduced ejection fraction (HF-REF), tested the hypothesis that administration of 200 mg LCZ696 twice daily is superior to 10 mg enalapril twice daily in reducing mortality and morbidity in patients with HF-REF (2). The study was closed early because of excess reduced mortality in the treatment arm, an observation that suggested that LCZ696 is a valuable alternative to an ACE inhibitor in patients with HF-REF. LCZ696 is a salt complex that consists of 2 active moieties in a 1:1 molar ratio: sacubitril (AHU377), a prodrug that is further metabolized to the NEP inhibitor LBQ657, and valsartan, an AT1 receptor blocker. Previous studies showed that valsartan was not superior in terms of all-cause mortality when combined with an ACE inhibitor; thus, the NEP-inhibition component of LCZ696 was key to the clinical benefit detected by PARADIGM-HF.
In this issue of JACC: Heart Failure, Vodovar et al. (3) are to be commended in their attempt to unravel the delicate balance between NEP and BNP. The authors found that excess circulating BNP in acute heart failure inhibits endogenous NEP activity. This new proposition suggests that when circulating BNP is present at >916 pg/ml, surplus BNP binds NEP to inhibit its catalytic activity, whereas a BNP subset would bind normally to its biological receptor (NPR-A) to cause vasodilation and natriuresis (Figure 1B). The higher the BNP levels, the lower the activity of circulating NEP. In light of these results, one could speculate that exogenous NEP inhibitors may not play a role in acute heart failure patients with high BNP levels. At present, this hypothesis remains an open issue, and further studies are needed to explore the real value of exogenous NEP inhibition (e.g., via LCZ696) in acute heart failure.
Vodovar et al. (3) explored the inhibitory potential of BNP toward NEP, but failed to provide insight into the counterprocess: cleavage of BNP by NEP at different BNP concentrations. BNP is partially degraded by NEP, although to a much lesser extent than other vasoactive peptides are degraded by it. BNP is initially cleaved at the long arm at site Met4/Val5, whereas cleavage of A-type natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) occurs within the ring at Cys7/Phe8. Evolutionary biology may yield additional insight into why NEP degrades the amino-acid ring of ANP and CNP in humans, but not that of BNP; this issue is beyond the scope of the study by Vodovar et al. (3). However, we now know that, at least in chronic heart failure, treatment with the exogenous NEP inhibitor LCZ696 affects BNP levels. Packer et al. (4) employed a subanalysis of the PARADIGM-HF trial to reveal that by inhibiting NEP, the levels of BNP and cGMP (a marker of the activation of natriuretic peptide receptor) (Figure 1) increased in treated patients. By contrast, the levels of N-terminal pro-BNP, which is not a substrate of NEP, exhibited a progressive decline in LCZ696-treated patients as heart failure improved (4).
In order to better understand each piece of this puzzle, it would be of great value to obtain data on NEP expression in the PARADIGM-HF cohort, thus elucidating the relationships among BNP levels, the level and activity of circulating NEP, and NEP inhibition by LCZ696. Do patients with high BNP levels and low NEP activity exhibit smaller responses to exogenous NEP inhibition by LCZ696? Should NEP levels be measured before initiating LCZ696 treatment? For the practicing clinician, it is important to understand the real value of NEP levels in helping to identify patients that may benefit the most from LCZ696 therapy and to evaluate whether measuring NEP levels assists clinicians in determining therapy effectiveness and optimizing/titrating dosing. Moreover, because NEP acts on a multitude of vasoactive peptides in heart failure, exploring the bidirectional relationships of NEP with bradykinin, ANP, endothelin-1, adrenomedullin, and other peptides is mandatory to elucidate the full picture.
Analysis of circulating soluble NEP levels and activity in heart failure is another highlight of the investigation of Vodovar et al. (3). NEP is a membrane-bound enzyme with a large extracellular catalytic domain, a single transmembrane region, and a short (27 amino acids) cytoplasmic N-terminal domain. Previous studies showed that NEP, like many other membrane-bound metalloproteases, can be released from the cell surface, yielding a non–membrane-associated form that retains catalytic activity. The production of soluble/non–membrane-associated counterparts of NEP is a consequence of ectodomain shedding, which involves proteolytic cleavage of the extracellular domain, or release of non–membrane-associated enzyme from cells via exosomes. The study of soluble NEP in heart failure is relatively new. In addition to the present report by Vodovar et al. (3), 2 recent studies showed that high levels of soluble NEP occur in the circulation of patients with chronic (5) and acute heart failure (6), and that soluble NEP concentrations are indicative of adverse outcomes for both cardiovascular mortality and morbidity. Some investigations have argued that soluble levels of NEP, by virtue of its central role in neurohormonal regulation, may reflect the status of several pathophysiological pathways involved in heart failure; NEP may eventually emerge as a novel biomarker. Nevertheless, the various assays currently used to measure NEP levels require refinement and validation. It is important to ascertain that these assays measure the 52–750 amino-acid fraction of NEP as immunogen (extracellular soluble fraction) and do not undergo cross-reactivity with other metallopeptidases.
It is undeniable that NEP is back at center stage after the greater-than-anticipated benefits of NEP inhibition in the PARADIGM-HF trial. However, much remains to be understood regarding NEP’s functions as a transmembrane receptor and as a soluble molecule, its interaction with other vasoactive peptides and metallopeptidases, and its value as a prognostic biomarker or guide for LCZ696 initiation or up-titration.
Cardiologists are limited by their inability to perform tissue biopsies of the heart to gain knowledge about its function, as other specialists do. Diagnoses and prognoses rely on imaging techniques and molecular understanding of circulating peptides and other molecules activated from the cardiac proteome or genome. The full spectrum of soluble biological substances that are true surrogates of cardiac function and/or dysfunction and can be measured via simple venipuncture may constitute a valid “liquid biopsy” of the heart. Here, Vodovar et al. (3) provided 1 more piece of this “liquid biopsy” puzzle, which may guide future research.
↵∗ Editorials published in JACC: Heart Failure reflect the views of the authors and do not necessarily represent the views of JACC: Heart Failure or the American College of Cardiology.
Dr. Bayes-Genis has reported that he has no relationships relevant to the contents of this paper to disclose.
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