Different Susceptibility of B-Type Natriuretic Peptide (BNP) and BNP Precursor (proBNP) to Cleavage by Neprilysin: The N-Terminal Part Does Matter

Inhibition of Fap Promotes Cardiac Repair by Stabilizing BNP

BACKGROUND

Myocardial infarction (MI) elicits cardiac fibroblast activation and extracellular matrix (ECM) deposition to maintain the structural integrity of the heart. Recent studies demonstrate that Fap (fibroblast activation protein)-a prolyl-specific serine protease-is an important marker of activated cardiac fibroblasts after MI.

METHODS

Left ventricle and plasma samples from patients and healthy donors were used to analyze the expression level of FAP and its prognostic value. Echocardiography and histological analysis of heart sections were used to analyze cardiac functions, scar formation, ECM deposition and angiogenesis after MI. RNA-Sequencing, biochemical analysis, cardiac fibroblasts (CFs) and endothelial cells co-culture were used to reveal the molecular and cellular mechanisms by which Fap regulates angiogenesis.

RESULTS

We found that Fap is upregulated in patient cardiac fibroblasts after cardiac injuries, while plasma Fap is downregulated and functions as a prognostic marker for cardiac repair. Genetic or pharmacological inhibition of Fap in mice significantly improved cardiac function after MI. Histological and transcriptomic analyses showed that Fap inhibition leads to increased angiogenesis in the peri-infarct zone, which promotes ECM deposition and alignment by cardiac fibroblasts and prevents their overactivation, thereby limiting scar expansion. Mechanistically, we found that BNP (brain natriuretic peptide) is a novel substrate of Fap that mediates postischemic angiogenesis. Fap degrades BNP to inhibit vascular endothelial cell migration and tube formation. Pharmacological inhibition of Fap in Nppb (encoding pre-proBNP) or Npr1 (encoding the BNP receptor)-deficient mice showed no cardioprotective effects, suggesting that BNP is a physiological substrate of Fap.

CONCLUSIONS

This study identifies Fap as a negative regulator of cardiac repair and a potential drug target to treat MI. Inhibition of Fap stabilizes BNP to promote angiogenesis and cardiac repair.

Comparative Effect of Angiotensin Receptor Neprilysin Inhibition on B-type Natriuretic Peptide Levels Measured by Three Different Assays: The PROVE-HF Study

Background

Several different B-type natriuretic peptide (BNP) assays are used clinically for diagnostic and prognostic evaluation of heart failure (HF). BNP binds weakly to neprilysin and is cleaved in multiple areas adjacent to the binding sites for the antibodies used in these immunoassays. We assessed the changes in BNP following neprilysin inhibition as measured by 3 immunoassays that recognize different epitopes.

Methods

Among 130 participants with HF with reduced ejection fraction, blood was collected prior to treatment with sacubitril/valsartan (sac/val) and then repeatedly measured through 52 weeks of treatment. BNP concentrations were measured with 3 widely used BNP assays (Siemens, Abbott, and Quidel).

Results

Study participants had a mean age of 65 ± 13 years and 76% were men. The median BNP concentration at baseline was 133 ng/L by the Siemens assay, 127 ng/L by the Abbott assay, and 141 ng/L by the Quidel assay. Following initiation of sac/val, there were significantly greater declines in BNP measured by Quidel and Abbott (P = 0.009 and P < 0.001), respectively (both with N-terminal capture antibodies), compared to Siemens (with C-terminal capture antibodies). The difference from baseline was not statistically significant until after week 12 (mean –10.1% for Quidel and –14.3% for Abbott) compared to non-significant differences before 12 weeks (mean –4.5% for Quidel and –6.0% for Abbott).

Conclusions

Following initiation of sac/val, BNP measurements may modestly differ depending on the assay method used, particularly after a few months of treatment. Whether these differences relate to neprilysin-mediated degradation of antibody binding sites deserves further study.

Study registration

PROVE-HF ClinicalTrials.gov Identifier: NCT02887183.

B-Type Natriuretic Peptide (BNP) Revisited—Is BNP Still a Biomarker for Heart Failure in the Angiotensin Receptor/Neprilysin Inhibitor Era?

Simple Summary

Active BNP-32, less active proBNP-108, and inactive N-terminal proBNP-76 all circulate in the blood. The circulating protease neprilysin has lower substrate specificity for BNP than ANP, while proBNP and N-terminal proBNP are not degraded by neprilysin. Currently available BNP immunoassays react with both mature BNP and proBNP; therefore, measured plasma BNP is mature BNP + proBNP. Because ARNI administration increases mature BNP, measured plasma BNP initially increases with ARNI administration by the amount of the increase in mature BNP. Later, ARNI administration reduces myocardial wall stress, and the resultant reduction in BNP production more than offsets the increase of mature BNP due to inhibition of degradation by neprilysin, resulting in lower plasma BNP levels. In the ARNI era, BNP remains a useful biomarker for heart failure, though mild increases early during ARNI administration should be taken into consideration.

Abstract

Myocardial wall stress, cytokines, hormones, and ischemia all stimulate B-type (or brain) natriuretic peptide (BNP) gene expression. Within the myocardium, ProBNP-108, a BNP precursor, undergoes glycosylation, after which a portion is cleaved by furin into mature BNP-32 and N-terminal proBNP-76, depending on the glycosylation status. As a result, active BNP, less active proBNP, and inactive N-terminal proBNP all circulate in the blood. There are three major pathways for BNP clearance: (1) cellular internalization via natriuretic peptide receptor (NPR)-A and NPR-C; (2) degradation by proteases in the blood, including neprilysin, dipeptidyl-peptidase-IV, insulin degrading enzyme, etc.; and (3) excretion in the urine. Because neprilysin has lower substrate specificity for BNP than atrial natriuretic peptide (ANP), the increase in plasma BNP after angiotensin receptor neprilysin inhibitor (ARNI) administration is much smaller than the increase in plasma ANP. Currently available BNP immunoassays react with both mature BNP and proBNP. Therefore, BNP measured with an immunoassay is mature BNP + proBNP. ARNI administration increases mature BNP but not proBNP, as the latter is not degraded by neprilysin. Consequently, measured plasma BNP initially increases with ARNI administration by the amount of the increase in mature BNP. Later, ARNI reduces myocardial wall stress, and the resultant reduction in BNP production more than offsets the increase in mature BNP mediated by inhibiting degradation by neprilysin, which lowers plasma BNP levels. These results suggest that even in the ARNI era, BNP can be used for diagnosis and assessment of the pathophysiology and prognosis of heart failure, though the mild increases early during ARNI administration should be taken into consideration.

Active B-Type Natriuretic Peptide Measured by Mass Spectrometry and Response to Sacubitril/Valsartan

BACKGROUND

B-type natriuretic peptide (BNP) immunoassays (BNP_ia) do not differentiate active and inactive forms. Inactive NT-proBNP is used to track heart failure (HF) during treatment with sacubitril/valsartan, which inhibits BNP degradation. Mass spectrometry (MS) may better assess effects of HF treatment on biologically active BNP1-32.

METHODS AND RESULTS

We developed a MS assay with immediate protease inhibition to quantify BNP1-32 over a linear range, using labeled recombinant BNP standard. In 4 healthy volunteers, BNP1-32 by MS (BNP_MS) increased from below the 5 pg/mL detection limit to 228 pg/mL after nesiritide. In patients with HF, BNP_MS was measured in parallel with BNP and NT-proBNP immunoassays before and during sacubitril/valsartan treatment. BNP_MS was 4.4-fold lower than BNP_ia in patients with HF. Among patients not taking sacubitril/valsartan and without end-stage renal disease, BNP_MS correlated with BNP_ia (r_s = 0.77, P < .001) and NT-proBNP (r_s = 0.74, P < .001). After a median of 8 weeks on sacubitril/valsartan, active BNP_MS levels decreased by 50% (interquartile range -98.3% to 41.7%, n = 22, P = .048) and correlated with NT-proBNP (r_s = 0.64, P < .001), but not with BNP_ia (r_s = 0.46, P = .057).

CONCLUSIONS

Active BNP measured by MS accounts for only a small amount of BNP measured by immunoassays. Although decreased BNP production was anticipated to be masked by inhibition of degradation, levels of active BNP decreased during chronic sacubitril/valsartan treatment.

Exposing the High Heterogeneity of Circulating Pro B-Type Natriuretic Peptide Fragments in Healthy Individuals and Heart Failure Patients

BACKGROUND

The high molecular complexity of variably O-glycosylated and degraded pro B-type natriuretic peptide (proBNP) derived molecular forms challenges current immunoassays. Antibodies used show pronounced differences in cross-reactivities with these circulating fragments, which still need to be better characterized on a molecular level. To pave the way for advanced quantitative assays in the future, it is critical to fully understand these circulating forms.

METHODS

Plasma samples were collected from 8 heart failure (HF) patients and 2 healthy controls. NT-proBNP and proBNP were purified by immunoprecipitation and analyzed by nano-flow liquid chromatography coupled to high-resolution mass spectrometry. Fragments formed during proteolysis in solution digestion were distinguished from naturally occurring peptides by using an 18O stable isotope labeling strategy.

RESULTS

We detected 16 previously unknown circulating fragments of proBNP peptides (9 of which are located in the N-terminal and 7 in the C-terminal region), revealing a more advanced state of degradation than previously known. Two of these fragments are indicative of either unidentified processing modes or a far-reaching C-terminal degradation (or a combination thereof) of the precursor proBNP.

CONCLUSIONS

Our results further restrict ideal target epitopes for immunoassay antibodies and expand the current thinking of diversity, degradation, and processing of proBNP, as well as the distribution of circulating forms.

Effect of Neprilysin Inhibition on Various Natriuretic Peptide Assays

BACKGROUND

With sacubitril/valsartan treatment, B-type natriuretic peptide (BNP) concentrations increase; it remains unclear whether change in BNP concentrations is similar across all assays for its measurement. Effects of sacubitril/valsartan on atrial natriuretic peptide (ANP) concentrations in patients are unknown. Lastly, the impact of neprilysin inhibition on mid-regional pro-ANP (MR-proANP), N-terminal pro-BNP (NT-proBNP), proBNP_1-108, or C-type natriuretic peptide (CNP) is not well understood.

OBJECTIVES

This study sought to examine the effects of sacubitril/valsartan on results from different natriuretic peptide assays.

METHODS

Twenty-three consecutive stable patients with heart failure and reduced ejection fraction were initiated and titrated on sacubitril/valsartan. Change in ANP, MR-proANP, BNP (using 5 assays), NT-proBNP (3 assays), proBNP_1-108, and CNP were measured over 3 visits.

RESULTS

Average time to 3 follow-up visits was 22, 46, and 84 days. ANP rapidly and substantially increased with initiation and titration of sacubitril/valsartan, more than doubling by the first follow-up visit (+105.8%). Magnitude of ANP increase was greatest in those with concentrations above the median at baseline (+188%) compared with those with lower baseline concentrations (+44%); ANP increases were sustained. Treatment with sacubitril/valsartan led to inconsistent changes in BNP, which varied across methods assessed. Concentrations of MR-proANP, NT-proBNP, and proBNP_1-108 variably declined after treatment; whereas CNP concentrations showed no consistent change.

CONCLUSIONS

Initiation and titration of sacubitril/valsartan led to variable changes in concentrations of multiple natriuretic peptides. These results provide important insights into the effects of sacubitril/valsartan treatment on individual patient results, and further suggest the benefit of neprilysin inhibition may be partially mediated by increased ANP concentrations.

Neutral Endopeptidase (Neprilysin) in Therapy and Diagnostics: Yin and Yang

Neprilysin (NEP) is a zinc-dependent metalloproteinase that exists in organisms in both transmembrane and soluble forms. NEP substrates are involved in regulating the cardiovascular and nervous systems. In this review, we discuss some of the biochemical characteristics and physiological functions of this enzyme with special emphasis on the use of NEP as a therapeutic target. The history and various physiological aspects of applying NEP inhibitors for treating heart failure and attempts to increase NEP activity when treating Alzheimer's disease using gene and cell therapies are described. Another important issue discussed is the role of NEP as a potential marker for predicting the risk of cardiovascular disease complications. The diagnostic and prognostic performance of soluble NEP in various types of heart failure is analyzed and presented. We also discuss the methods and approaches for measuring NEP activity for prognosis and diagnosis, as well as a possible new role of natriuretic peptides (NEP substrates) in cardiovascular diagnostics.

Detection of Neprilysin-Derived BNP Fragments in the Circulation: Possible Insights for Targeted Neprilysin Inhibition Therapy for Heart Failure

BACKGROUND

Entresto™ is a new heart failure (HF) therapy that includes the neprilysin (NEP) inhibitor sacubitril. One of the NEP substrates is B-type natriuretic peptide (BNP); its augmentation by NEP inhibition is considered as a possible mechanism for the positive effects of Entresto. We hypothesized that the circulating products of BNP proteolysis by NEP might reflect NEP impact on the metabolism of active BNP. We suggest that NEP-based BNP cleavage at position 17–18 results in BNP ring opening and formation of a novel epitope with C-terminal Arg-17 (BNP-neo17 form). In this study, we use a specific immunoassay to explore BNP-neo17 in a rat model and HF patient plasma.

METHODS

We injected BNP into rats, with or without NEP inhibition with sacubitril. BNP-neo17 in plasma samples at different time points was measured with a specific immunoassay with neglectable cross-reactivity to intact forms. BNP-neo17 and total BNP were measured in EDTA plasma samples of HF patients.

RESULTS

BNP-neo17 generation in rat circulation was prevented by NEP inhibition. The maximum 13.2-fold difference in BNP-neo17 concentrations with and without sacubitril was observed at 2 min after injection. BNP-neo17 concentrations in 32 HF patient EDTA plasma samples ranged from 0 to 37 pg/mL (median, 5.4; interquartile range, 0–9.1). BNP-neo17/total BNP had no correlation with total BNP concentration (with r = −0.175, P = 0.680) and showed variability among individuals.

CONCLUSIONS

BNP-neo17 formation is NEP dependent. Considering that BNP-neo17 is generated from the active form of BNP by NEP, we speculate that BNP-neo17 may reflect both the NEP activity and natriuretic potential and serve for HF therapy guidance.

Future Considerations in Nocturia and Nocturnal Polyuria

Nocturnal polyuria (NP), the most common etiology of nocturia, can be caused by various medical conditions, including cardiovascular disease, obstructive sleep apnea, renal tubular dysfunction, as well as medications (eg, diuretics) and/or behavioral patterns. NP in the absence of underlying medical conditions has been described as NP syndrome and is thought be the result of impaired circadian release of endogenous arginine vasopressin. Desmopressin, a synthetic arginine vasopressin analog, has been shown to be an effective replacement therapy in adults with nocturia due to NP. Further studies on the subset of patients with NP syndrome are warranted to maximize benefit from antidiuretic treatment. In addition, a connection between the pathophysiological mechanisms underlying NP and essential hypertension has been suggested, and hypertension has been shown to be a significant risk factor for nocturia, while an association between NP and brain natriuretic peptide levels has also been reported in patients with nocturia. Hypertension is now viewed as a disorder of blood vessels and treatment is directed at the vasculature rather than the blood pressure, with the latter currently serving as a biomarker for arterial injury. Nocturia is thought to be associated with the beginning of this cardiovascular continuum as studies have reported a link between coronary heart disease and nocturia. Therefore, there is an increasing need to elucidate the complex mechanisms implicated in the association between nocturia and hypertension to promote the development of more individualized therapies for the treatment of nocturia.

Analytical performance of a single epitope B-type natriuretic peptide sandwich immunoassay on the Minicare platform for point-of-care diagnostics

Point-of-care B-type natriuretic peptide (BNP) testing with adequate analytical performance has the potential to improve patient flow and provide primary care givers with easy-to-use advanced diagnostic tools in the management of heart failure. We present the analytical evaluation of the Minicare BNP immunoassay under development on the Minicare I-20 platform for point-of-care testing. Analytical performance was evaluated using EDTA venous whole blood, EDTA plasma and capillary whole blood. Method comparison with a lab-testing system was performed using samples from 187 patients. Normal values were determined based on 160 healthy adults, aging from 19 to 70 years. Limit of blank (LoB), limit of detection (LoD) were determined to be 3.3 ng/L, 5.8 ng/L. Limit of quantitation (LoQ) in whole blood at 20% and 10% coefficient of variation (CV) was found < 9 ng/L and <30 ng/L respectively without significant differences between EDTA whole blood and EDTA plasma. Total CV was found to be from 6.7% to 9.7% for BNP concentrations between 92.6 and 3984 ng/L. The sample type comparison study demonstrated correlation coefficients between 0.97 and 0.99 with slopes between 1.03 and 1.09 between the different samples. Method comparison between Minicare BNP and Siemens ADVIA Centaur BNP demonstrated a correlation coefficient of 0.92 with a slope of 1.06. The 97.5% URL of a healthy population was calculated to be 72.6 ng/L. The Minicare BNP assay is a robust, easy-to-use and sensitive test for rapid determination of BNP concentrations that can be used in a near-patient setting.

B-Type Natriuretic Peptide During Treatment With Sacubitril/Valsartan: The PARADIGM-HF Trial

BACKGROUND

Natriuretic peptides are substrates of neprilysin; hence, B-type natriuretic peptide (BNP) concentrations rise with neprilysin inhibition. Thus, the clinical validity of measuring BNP in sacubitril/valsartan-treated patients has been questioned, and use of N-terminal pro-B-type natriuretic peptides (NT-proBNP) has been preferred and recommended.

OBJECTIVES

The purpose of this study was to determine the prognostic performance of BNP measurements before and during treatment with sacubitril/valsartan.

METHODS

BNP and NT-proBNP were measured before and after 4 to 6 weeks, 8 to 10 weeks, and 9 months of treatment with sacubitril/valsartan in the PARADIGM-HF (Prospective Comparison of ARNI with ACEI to Determine Impact on Global Mortality and Morbidity in Heart Failure) trial. We assessed the association of levels of these natriuretic peptides with the subsequent risk of cardiovascular death or hospitalization for HF.

RESULTS

Median BNP concentration (before treatment: 202 ng/l [Q1 to Q3: 126 to 335 ng/l]) increased to 235 ng/l (Q1 to Q3: 128 to 422 ng/l) after 8 to 10 weeks of treatment. BNP concentrations doubled in 141 (18%) patients and tripled in 49 (6%) patients during the first 8 to 10 weeks of sacubitril/valsartan. In contrast, such striking increases in NT-proBNP following the use of the neprilysin inhibitor were extremely rare. Treatment with sacubitril/valsartan caused a rightward shift in the distribution of BNP when compared with NT-proBNP, but both peptides retained their prognostic accuracy (C-statistics of 63% to 67% for BNP and C-statistics of 64% to 70% for NT-proBNP) with no difference between the 2 biomarkers. Increases in both BNP and NT-proBNP during 8 to 10 weeks of sacubitril/valsartan were associated with worse outcomes (p = 0.003 and p = 0.005, respectively).

CONCLUSIONS

Circulating levels of BNP may increase meaningfully early after initiation of sacubitril/valsartan. In comparison, NT-proBNP is not a substrate of neprilysin inhibition, and thus may lead to less clinical confusion when measured within 8 to 10 weeks of drug initiation. However, during treatment, either biomarker predicts the risk of major adverse outcomes in patients treated with angiotensin receptor-neprilysin inhibitors. (Prospective Comparison of ARNI with ACEI to Determine Impact on Global Mortality and Morbidity in Heart Failure [PARADIGM-HF]; NCT01035255).

Natriuretic peptides in human heart: Novel insight into their molecular forms, functions, and diagnostic use

Among the three natriuretic peptides, atrial/A-type natriuretic peptide (ANP) and brain/B-type natriuretic peptide (BNP) are primarily produced by, and secreted from, heart tissue. They maintain cardiovascular homeostasis by binding to natriuretic peptide receptor-A. Since plasma ANP and BNP concentrations, as well as expression, are elevated in response to increased body fluid volume and pressure load on the heart wall, these peptides are widely utilized as diagnostic biomarkers for evaluating heart failure. Regardless of their high utility, differences in their molecular forms between healthy and diseased subjects and how these relate to pathophysiology have not well been examined. Recent studies have shown that the circulating molecular forms of ANP and BNP are not uniform; bioactive α-ANP is the major ANP form, whereas the weakly active proBNP is the major BNP form. The relative ratios of the different molecular forms are altered under different pathophysiological conditions. These facts indicate that detailed measurements of each form may provide useful information on the pathophysiological state of heart tissue. Here, we revisit the relationship between the molecular forms of, and pathophysiological alterations in, human ANP and BNP and discuss the possible utility of the measurement of each of the molecular forms. The third peptide, C-type natriuretic peptide, activates natriuretic peptide receptor-B, but little is known about its production and function in the heart because of its extremely low levels. However, through recent studies, its role in the heart is gradually becoming clear. Here, we summarize its molecular forms, assay systems, and functions in the heart.

The plasma peptides of ovarian cancer

Background

It may be possible to discover new diagnostic or therapeutic peptides or proteins from blood plasma by using liquid chromatography and tandem mass spectrometry to identify, quantify and compare the peptides cleaved ex vivo from different clinical populations. The endogenous tryptic peptides of ovarian cancer plasma were compared to breast cancer and female cancer normal controls, other diseases with their matched or normal controls, plus ice cold plasma to control for pre-analytical variation.

Methods

The endogenous tryptic peptides or tryptic phospho peptides (i.e. without exogenous digestion) were analyzed from 200 μl of EDTA plasma. The plasma peptides were extracted by a step gradient of organic/water with differential centrifugation, dried, and collected over C18 for analytical HPLC nano electrospray ionization and tandem mass spectrometry (LC–ESI–MS/MS) with a linear quadrupole ion trap. The endogenous peptides of ovarian cancer were compared to multiple disease and normal samples from different institutions alongside ice cold controls. Peptides were randomly and independently sampled by LC–ESI–MS/MS. Precursor ions from peptides > E4 counts were identified by the SEQUEST and X!TANDEM algorithms, filtered in SQL Server, before testing of frequency counts by Chi Square (χ²), for analysis with the STRING algorithm, and comparison of precursor intensity by ANOVA in the R statistical system with the Tukey-Kramer Honestly Significant Difference (HSD) test.

Results

Peptides and/or phosphopeptides of common plasma proteins such as HPR, HP, HPX, and SERPINA1 showed increased observation frequency and/or precursor intensity in ovarian cancer. Many cellular proteins showed large changes in frequency by Chi Square (χ² > 60, p < 0.0001) in the ovarian cancer samples such as ZNF91, ZNF254, F13A1, LOC102723511, ZNF253, QSER1, P4HA1, GPC6, LMNB2, PYGB, NBR1, CCNI2, LOC101930455, TRPM5, IGSF1, ITGB1, CHD6, SIRT1, NEFM, SKOR2, SUPT20HL1, PLCE1, CCDC148, CPSF3, MORN3, NMI, XTP11, LOC101927572, SMC5, SEMA6B, LOXL3, SEZ6L2, and DHCR24. The protein gene symbols with large Chi Square values were significantly enriched in proteins that showed a complex set of previously established functional and structural relationships by STRING analysis. Analysis of the frequently observed proteins by ANOVA confirmed increases in mean precursor intensity in ZFN91, TRPM5, SIRT1, CHD6, RIMS1, LOC101930455 (XP_005275896), CCDC37 and GIMAP4 between ovarian cancer versus normal female and other diseases or controls by the Tukey–Kramer HSD test.

Conclusion

Here we show that separation of endogenous peptides with a step gradient of organic/water and differential centrifugation followed by random and independent sampling by LC–ESI–MS/MS with analysis of peptide frequency and intensity by SQL Server and R revealed significant difference in the ex vivo cleavage of peptides between ovarian cancer and other clinical treatments. There was striking agreement between the proteins discovered from cancer plasma versus previous biomarkers discovered in tumors by genetic or biochemical methods. The results indicate that variation in plasma proteins from ovarian cancer may be directly discovered by LC–ESI–MS/MS that will be a powerful tool for clinical research.

Electronic supplementary material

The online version of this article (10.1186/s12014-018-9215-z) contains supplementary material, which is available to authorized users.

Analytical barriers in clinical B-type natriuretic peptide measurement and the promising analytical methods based on mass spectrometry technology

B-type natriuretic peptide (BNP) is a circulating biomarker that is mainly applied in heart failure (HF) diagnosis and to monitor disease progression. Because some identical amino acid sequences occur in the precursor and metabolites of BNP, undesirable cross-reactions are common in immunoassays. This review first summarizes current analytical methods, such as immunoassay- and mass spectrometry (MS)-based approaches, including the accuracy of measurement and the inconsistency of the results. Second, the review presents some promising approaches to resolve the current barriers in clinical BNP measurement, such as how to decrease cross-reactions and increase the measurement consistency. Specific approaches include research on novel BNP assays with higher-specificity chemical antibodies, the development of International System of Units (SI)-traceable reference materials, and the development of structure characterization methods based on state-of-the-art ambient and ion mobility MS technologies. The factors that could affect MS analysis are also discussed, such as biological sample cleanup and peptide ionization efficiency. The purpose of this review is to explore and identify the main problems in BNP clinical measurement and to present three types of approaches to resolve these problems, namely, materials, methods and instruments. Although novel approaches are proposed here, in practice, it is worth noting that the BNP-related peptides including unprocessed proBNP were all measured in clinical BNP assays. Therefore, approaches that aimed to measure a specific BNP or proBNP might be an effective way for the standardization of a particular BNP form measurement, instead of the standardization of “total” immunoreactive BNP assays in clinical at present.

Brain Natriuretic Peptide and Its Biochemical, Analytical, and Clinical Issues in Heart Failure: A Narrative Review

Heart failure (HF) is a primary cause of morbidity and mortality worldwide. As the most widely studied and commonly applied natriuretic peptide (NP), B-type natriuretic peptide (BNP) has the effects of diuresis, natriuresis, vasodilation, anti-hypertrophy, and anti-fibrosis and it inhibits the renin-angiotensin-aldosterone and sympathetic nervous systems to maintain cardiorenal homeostasis and counteract the effects of HF. Both BNP and N-terminal pro B-type natriuretic peptide (NT-proBNP) are applied as diagnostic, managing, and prognostic tools for HF. However, due to the complexity of BNP system, the diversity of BNP forms and the heterogeneity of HF status, there are biochemical, analytical, and clinical issues on BNP not fully understood. Current immunoassays cross-react to varying degrees with pro B-type natriuretic peptide (proBNP), NT-proBNP and various BNP forms and cannot effectively differentiate between these forms. Moreover, current immunoassays have different results and may not accurately reflect cardiac function. It is essential to design assays that can recognize specific forms of BNP, NT-proBNP, and proBNP to obtain more clinical information. Not only the processing of proBNP (corin/furin) and BNP (neprilysin), but also the effects of glycosylation on proBNP processing and BNP assays, should be targeted in future studies to enhance their diagnostic, therapeutic, and prognostic values.

Standardization of BNP and NT-proBNP Immunoassays in Light of the Diverse and Complex Nature of Circulating BNP-Related Peptides

Brain natriuretic peptide (BNP) and the N-terminal fragment of the BNP precursor (NT-proBNP) are widely used as heart failure (HF) biomarkers. Since the discovery of BNP in 1988, much effort has been allocated to the precise detection of BNP and NT-proBNP levels for reliable HF diagnostics. As a result, measurements of these biomarkers are globally accepted and used in clinical practice for the diagnosis of acute and chronic HF, risk stratification, and monitoring response to therapy. Several immunoassays specific for BNP and NT-proBNP are currently commercially available. Recent comparative studies show that there are marked differences between different BNP and NT-proBNP assays and platforms, and the results of measurements are not comparable enough. The lack of equivalence between the assays complicates the interpretation of the results and renders the cut-off points for diagnostic decisions to be method dependent. Presently, there is no agreement on what kind of BNP or NT-proBNP standard should be used for calibration, and a certified reference material as well as reference measurement procedures are lacking. The aim of this chapter is to summarize the available data on the complex nature of BNP-related peptides, specificity for existing BNP and NT-proBNP immunoassays, and to discuss potential approaches for standardization of BNP and NT-proBNP measurements.

Interpretation of B-type natriuretic peptides in the era of angiotensin receptor-neprilysin inhibitors

Assessment of serum levels of natriuretic peptides, especially the amino-terminal portion (NT-proBNP) and the carboxy-terminal portion (BNP) of pro-B-type natriuretic peptide, has had a highly significant clinical impact on the diagnosis and prognostic stratification of patients with heart failure (HF). They are now an instrument with recognized value in this context and several studies have demonstrated their value in tailoring therapy for these patients. Following the recent advent of angiotensin receptor-neprilysin inhibitors (ARNIs), there is a need to review how these two biomarkers are interpreted in HF. The use of ARNIs is associated with a reduction in NT-proBNP but an increase in BNP levels. The authors of this concise article review the interpretation of natriuretic peptide levels in the light of the most recent evidence.

Natriuretic Peptides, 6-Min Walk Test, and Quality-of-Life Questionnaires as Clinically Meaningful Endpoints in HF Trials

The Expedited Access for Premarket Approval and De Novo Medical Devices Intended for Unmet Medical Need for Life Threatening or Irreversibly Debilitating Diseases or Conditions document was issued as a guidance for industry and for the Food and Drug Administration. The Expedited Access Pathway was designed as a new program for medical devices that demonstrated the potential to address unmet medical needs for life threatening or irreversibly debilitating conditions. The Food and Drug Administration would consider assessments of a device's effect on intermediate endpoints that, when improving in a congruent fashion, are reasonably likely to predict clinical benefit. The purpose of this review is to provide evidence to support the use of 3 such intermediate endpoints: natriuretic peptides, such as N-terminal pro-B-type natriuretic peptide/B-type natriuretic peptide, the 6-min walk test distance, and health-related quality of life in heart failure.

Potential Expanded Indications for Neprilysin Inhibitors

PURPOSE OF REVIEW

The goal of this article is to review potential expanded indications for neprilysin inhibitors. This article reviews the rationale and design for ongoing and future trials of sacubitril/valsartan in cardiovascular and non-cardiovascular disease.

RECENT FINDINGS

Randomized trial data are lacking for use of sacubitril/valsartan in acute heart failure and advanced heart failure. Mechanistic data from animal studies suggest a role for neprilysin inhibition in the treatment of post-myocardial infarction systolic dysfunction and heart failure with preserved ejection fraction. Beyond the cardiovascular system, renal and neurological function may be impacted by neprilysin inhibition. Forthcoming randomized trials will address the clinical impact of sacubitril/valsartan on these conditions. Neprilysin inhibition with sacubitril/valsartan offers a new therapeutic strategy with a broad range of potential therapeutic actions. In PARADIGM-HF, the combination of neprilysin and RAAS inhibition was proven to be superior to enalapril for patients with stable NYHA class II-III heart failure and reduced left ventricular ejection fraction. Preliminary data suggests it may also have a role in other cardiovascular and non-cardiovascular disease. Several ongoing and planned studies will determine the extent of its benefit for these other indications.

Peptide Antibodies in Clinical Laboratory Diagnostics

Peptide antibodies, with their high specificities and affinities, are invaluable reagents for peptide and protein recognition in biological specimens. Depending on the application and the assay, in which the peptide antibody is to used, several factors influence successful antibody production, including peptide selection and antibody screening. Peptide antibodies have been used in clinical laboratory diagnostics with great success for decades, primarily because they can be produced to multiple targets, recognizing native wildtype proteins, denatured proteins, and newly generated epitopes. Especially mutation-specific peptide antibodies have become important as diagnostic tools in the detection of various cancers. In addition to their use as diagnostic tools in malignant and premalignant conditions, peptide antibodies are applied in all other areas of clinical laboratory diagnostics, including endocrinology, hematology, neurodegenerative diseases, cardiovascular diseases, infectious diseases, and amyloidoses.

Novel Biomarkers of Heart Failure

Although substantial improvements have been made in majority of cardiac disorders, heart failure (HF) remains a major health problem, with both increasing incidence and prevalence over the past decades. For that reason, the number of potential biomarkers that could contribute to diagnosis and treatment of HF patients is, almost exponentially, increasing over the recent years. The biomarkers that are, at the moment, more or less ready for use in everyday clinical practice, reflect different pathophysiological processes present in HF. In this review, seven groups of biomarkers associated to myocardial stretch (mid-regional proatrial natriuretic peptide, MR-proANP), myocyte injury (high-sensitive troponins, hs-cTn; heart-type fatty acid-binding protein, H-FABP; glutathione transferase P1, GSTP1), matrix remodeling (galectin-3; soluble isoform of suppression of tumorigenicity 2, sST2), inflammation (growth differentiation factor-15, GDF-15), renal dysfunction (neutrophil gelatinase-associated lipocalin, NGAL; kidney injury molecule-1, KIM-1), neurohumoral activation (adrenomedullin, MR-proADM; copeptin), and oxidative stress (ceruloplasmin; myeloperoxidase, MPO; 8-hydroxy-2'-deoxyguanosine, 8-OHdG; thioredoxin 1, Trx1) in HF will be overviewed. It is important to note that clinical value of individual biomarkers within the single time points in both diagnosis and outcome prediction in HF is limited. Hence, the future of biomarker application in HF lies in the multimarker panel strategy, which would include specific combination of biomarkers that reflect different pathophysiological processes underlying HF.

Natriuretic Peptides and Analytical Barriers

BACKGROUND

The natriuretic peptide system is an endocrine, autocrine and paracrine system that plays an important role in the maintenance of cardiovascular homeostasis. Biomarkers based on these peptides are important diagnostic and prognostic tools for myocardial function.

CONTENT

Although natriuretic peptides were discovered more than 2 decades ago, their intricate and complex biology is associated with important questions not yet elucidated. The diversity of circulating forms of natriuretic peptides, the distinct expression of these forms in particular patients, and the heterogeneity of heart failure forms, along with specific assay-related and preanalytic issues, cause assays to be poorly harmonized.

SUMMARY

This review presents the relevant issues related to the biology of natriuretic peptides and differences between assays with immediate implications for clinical practice.

Natriuretic peptides as biomarkers of cardiac endocrine function in heart failure: new challenges and perspectives

Several studies indicated that B-type natriuretic peptide (BNP) assay is able to detect patients even in the early phases of heart failure (HF), when the myocardial remodeling process may be still reversible. BNP assay may assist the physician to initiate appropriate and prompt pharmacological treatments. However, clinical relevance and result interpretation of BNP assay for the guide of therapy or in particular clinical conditions, such as renal failure or treatment with inhibitors of enzymes degrading BNP in HF patients, are still debated. The aim of this article is to discuss some still controversial issues concerning the clinical use of measurement of cardiac natriuretic peptides, and also to provide a general overview and some perspectives related to pathophysiological mechanisms of HF.

Monitoring B-type natriuretic peptide in patients undergoing therapy with neprilysin inhibitors. An emerging challenge?

B-type natriuretic peptide (BNP) is primarily synthesized by the ventricles of the heart as a 108-amino acid polypeptide precursor (i.e., proBNP), which is then cleaved into a 76-amino acid biologically inert N-terminal fragment (NT-proBNP) and a biologically active 32-amino acid peptide (BNP). The generation of BNP is considerably enhanced in response to high ventricular filling pressures, so that the measurement of either the active hormone or NT-proBNP has become a mainstay in patients with congestive heart failure. Recent evidence was brought that the enzyme neprilysin efficiently degrades circulating BNP in vivo, whereas proBNP and NT-proBNP are virtually resistant to enzymatic cleavage. Increasing emphasis is currently placed on the fact that that measuring BNP in patients taking the novel and promising neprilysin inhibitors such as LCZ696 may not reliably reflect cardiac dysfunction. Since laboratory monitoring in patients with heart failure should be aimed to define the role of BNP in modulating fluid hemostasis and cardiac remodeling, but natriuretic peptides should also serve as reliable biomarkers of cardiac function and treatment response in these patients, the assessment of neither BNP nor NT-proBNP alone provides a comprehensive biological and clinical picture. Therefore, it seems reasonable to suggest both BNP and the neprilysin-resistant peptide NT-proBNP should be concomitantly assessed in patients with heart failure who take neprilysin inhibitors, so allowing to concomitantly monitor the progression of heart failure and to assess the actual cardiorenal potency of circulating BNP.

Biomarkers of Heart Failure with Preserved and Reduced Ejection Fraction

Biomarkers are increaingly being used in the management of heart failure not only for the purpose of screening, diagnosis, and risk stratification, but also as a guide to evaluate the response to treatment in the individual patient and as an entry criterion and/or a surrogate marker of efficacy in clinical trials testing novel drugs. In this chapter, we review the role of established biomarkers for heart failure management, according to the main classification of HF phenotypes, based on the measurement of left ventricular ejection fraction, including heart failure with reduced (<40%), preserved (≥50%), and, as recently proposed, mid-range (40-49%) ejection fraction.