Novel Immunoassay for Quantification of Brain Natriuretic Peptide and Its Precursor in Human Blood

Diagnostic utility of total NT-proBNP testing by immunoassay based on antibodies targeting glycosylation-free regions of NT-proBNP

OBJECTIVES

The N-terminal fragment of pro-B-type natriuretic peptide (NT-proBNP) is a widely used heart failure (HF) biomarker. Commercial NT-proBNP immunoassays detect only a subfraction of endogenous NT-proBNP, as the antibodies target a region of NT-proBNP that could be glycosylated at Ser44. The diagnostic utility of immunoassays measuring total NT-proBNP remains unclear.

METHODS

NT-proBNP was measured in 183 HF and 200 non-HF patients diagnosed by two independent cardiologists blinded to NT-proBNP results. Plasma samples either non-treated or treated with a mixture of glycosidases were analyzed by the Elecsys proBNP II assay (Roche Diagnostics, based on antibodies targeting a glycosylated region of NT-proBNP) and the SuperFlex NT-proBNP assay (PerkinElmer, based on antibodies targeting regions of NT-proBNP that are free of O-glycans). The diagnostic accuracy of the two assays was analyzed by comparison of ROC curves.

RESULTS

The ROC-AUC for the proBNP II assay was 0.943 (95% CI 0.922-0.964) for NT-proBNP measured in untreated samples and 0.935 (0.913-0.958) for NT-proBNP measured in glycosidase-treated samples. The SuperFlex NT-proBNP assay in untreated samples gave a ROC-AUC of 0.930 (95% CI 0.907-0.954). The median percentage of non-glycosylated NT-proBNP to total NT-proBNP was 1.5-1.6-fold lower in the non-HF group compared to that in the HF group.

CONCLUSIONS

The clinical value of total NT-proBNP for HF diagnosis was similar to the subfraction of NT-proBNP that was non-glycosylated at Ser44. The lower percentage of non-glycosylated NT-proBNP to total NT-proBNP in non-HF patients suggests that total NT-proBNP might be more sensitive in individuals without current or prior symptoms of HF.

Development, validation, and application of an UHPLC-MS/MS method for quantification of the adiponectin-derived active peptide ADP355 in rat plasma

An UHPLC-MS/MS method for quantification of ADP355, an adiponectin-derived active peptide, was developed and validated. The extraction method employed simple protein precipitation using methanol and the chromatographic separation was achieved on the Accucore™ RP-MS C18 column (100 × 2.1mm, 2.6 μm, 80 Å), using 0.1% formic acid in both water and acetonitrile with gradient elution at the flow rate of 400 μL/min within 4.0 min. Detections were performed under positive ion mode with MRM ion transitions m/z 1109.2→309.8 and 871.4→310.1 for ADP355 and Jt003 respectively at unit resolution. The linearity range of the calibration curve was 2-1000 ng/mL with lower limit detection of 0.5 ng/mL. Selectivity, linearity, precision, accuracy, recovery, matrix effect, and stability were validated, and all items met the requirement of FDA guidance. This method has been successfully applied to an intravenous pharmacokinetic study of ADP355 in rats and the in-vitro stability in rat serum, plasma, and whole blood was also assessed.

Recent advances in cardiac biomarkers detection: From commercial devices to emerging technologies

Although cardiac pathologies are the major cause of death in the world, it remains difficult to provide a reliable diagnosis to prevent heart attacks. Rapid patient care and management in emergencies are critical to prevent dramatic consequences. Thus, relevant biomarkers such as cardiac troponin and natriuretic peptides are currently targeted by commercialized Point-Of-Care immunoassays. Key points still to be addressed concern cost, lack of standardization, and poor specificity, which could limit the reliability of the assays. Consequently, alternatives are emerging to address these issues. New probe molecules such as aptamers or molecularly imprinted polymers should allow a reduction in cost of the assays and an increase in reproducibility. In addition, the assay specificity and reliability could be improved by enabling multiplexing through the detection of several molecular targets in a single device.

B-Type Natriuretic Peptide as a Significant Brain Biomarker for Stroke Triaging Using a Bedside Point-of-Care Monitoring Biosensor

Stroke is a widespread condition that causes 7 million deaths globally. Survivors suffer from a range of disabilities that affect their everyday life. It is a complex condition and there is a need to monitor the different signals that are associated with it. Stroke patients need to be rapidly diagnosed in the emergency department in order to allow the admission of the time-limited treatment of tissue plasminogen activator (tPA). Stroke diagnostics show the use of sophisticated technologies; however, they still contain limitations. The hidden information and technological advancements behind the utilization of biomarkers for stroke triaging are significant. Stroke biomarkers can revolutionize the way stroke patients are diagnosed, monitored, and how they recover. Different biomarkers indicate different cascades and exhibit unique expression patterns which are connected to certain pathologies in the human body. Over the past decades, B-type natriuretic peptide (BNP) and its derivative N-terminal fragment (NT-proBNP) have been increasingly investigated and highlighted as significant cardiovascular biomarkers. This work reviews the recent studies that have reported on the usefulness of BNP and NT-proBNP for stroke triaging. Their classification association is also presented, with increased mortality in stroke, correlation with cardioembolic stroke, and an indication of a second stroke recurrence. Moreover, recent scientific efforts conducted for the technological advancement of a bedside point-of-care (POC) device for BNP and NT-proBNP measurements are discussed. The conclusions presented in this review may hopefully assist in the major efforts that are currently being conducted in order to improve the care of stroke patients.

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.

B-Type Natriuretic Peptides and Cardiac Troponins for Diagnosis and Risk-Stratification of Syncope

BACKGROUND

The utility of BNP (B-type natriuretic peptide), NT-proBNP (N-terminal proBNP), and hs-cTn (high-sensitivity cardiac troponin) concentrations for diagnosis and risk-stratification of syncope is incompletely understood.

METHODS

We evaluated the diagnostic and prognostic accuracy of BNP, NT-proBNP, hs-cTnT, and hs-cTnI concentrations, alone and against those of clinical assessments, in patients >45-years old presenting with syncope to the emergency department in a prospective diagnostic multicenter study. BNP, NT-proBNP, hs-cTnT and hs-cTnI concentrations were measured in a blinded fashion. Cardiac syncope, as adjudicated by 2 physicians based on all information available including cardiac work-up and 1-year follow-up, was the diagnostic end point. EGSYS (Evaluation of Guidelines in Syncope Study), a syncope-specific diagnostic score, served as the diagnostic comparator. Death and major adverse cardiac events at 30 and 720 days were the prognostic end points. Major adverse cardiac events were defined as death, cardiopulmonary resuscitation, life-threatening arrhythmia, implantation of pacemaker/implantable cardioverter defibrillator, acute myocardial infarction, pulmonary embolism, stroke/transient ischemic attack, intracranial bleeding, or valvular surgery. ROSE (Risk Stratification of Syncope in the Emergency Department), OESIL (Osservatorio Epidemiologico della Sincope nel Lazio), SFSR (San Fransisco Syncope Rule), and CSRS (Canadian Syncope Risk Score) served as the prognostic comparators.

RESULTS

Among 1538 patients eligible for diagnostic assessment, cardiac syncope was the adjudicated diagnosis in 234 patients (15.2%). BNP, NT-proBNP, hs-cTnT, and hs-cTnI were significantly higher in cardiac syncope versus other causes (P<0.01). The diagnostic accuracy for cardiac syncope, as quantified by the area under the curve, was 0.77 to 0.78 (95% CI, 0.74-0.81) for all 4 biomarkers, and superior to EGSYS (area under the curve, 0.68 [95%-CI 0.65-0.71], P<0.001). Combining BNP/NT-proBNP with hs-cTnT/hs-cTnI further improved diagnostic accuracy to an area under the curve of 0.81 (P<0.01). BNP, NT-proBNP, hs-cTnT, and hs-cTnI cut-offs, achieving predefined thresholds for sensitivity and specificity (95%), allowed for rule-in or rule-out of ≈30% of all patients. A total of 450 major adverse cardiac events occurred during follow-up. The prognostic accuracy of BNP, NT-proBNP, hs-cTnI, and hs-cTnT for major adverse cardiac events was moderate-to-good (area under the curve, 0.75-0.79), superior to ROSE, OESIL, and SFSR, and inferior to CSRS.

CONCLUSIONS

BNP, NT-proBNP, hs-cTnT, and hs-cTnI concentrations provide useful diagnostic and prognostic information in emergency department patients with syncope.

CLINICAL TRIAL REGISTRATION

URL: https://www.

CLINICALTRIALS

gov. Unique identifier: NCT01548352.

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.

Combining high-sensitivity cardiac troponin and B-type natriuretic peptide in the detection of inducible myocardial ischemia

BACKGROUND

Single biomarker approaches provide only moderate accuracy in the non-invasive detection of exercise-induced myocardial ischemia. We therefore assessed the combination of the two most promising single biomarkers: high-sensitivity cardiac troponin I (hs-cTnI) and B-type natriuretic peptide (BNP).

METHODS

Consecutive patients with suspected myocardial ischemia referred to stress myocardial perfusion single-photon emission tomography imaging (MPI) were enrolled. Clinical judgment (CJ) of the treating cardiologist regarding myocardial ischemia, quantified using a visual analogue scale, and blood concentrations of hs-cTnI and BNP were determined before and after stress. The presence of myocardial ischemia was adjudicated by independent cardiologists using MPI, blinded to biomarker measurements. Death and acute myocardial infarction (AMI) during follow-up were the prognostic endpoints.

RESULTS

Among 1142 consecutive patients inducible myocardial ischemia was found in 456 (40%) of all patients. For the detection of inducible myocardial ischemia, CJ before exercise stress testing (CJb) showed an area under the receiver-operating-characteristics curve (AUC) of 0.66 (95%CI 0.63-0.69), hs-cTnI 0.70 (95%CI 0.67-0.73, p=0.07 vs CJb), and BNP 0.66 (95%CI 0.62-0.69, p=0.98). The use of a dual-biomarker strategy combining hs-cTnI and BNP with CJb did not provide a significant advantage over the combination of hs-cTnI alone and CJb (AUC 0.74, 95%CI 0.72-0.77 vs AUC 0.74, 95%CI 0.71-0.77, p=0.16). Hs-cTnI showed good prognostic value for AMI (HR 1.6, 95%CI 1.3-1.9), and BNP for death (HR 1.6, 95%CI 1.3-2.1).

CONCLUSION

A dual-biomarker strategy combing BNP and hs-cTnI does not further increase diagnostic accuracy on top of clinical judgment and hs-cTnI alone.

SUMMARY AND HIGHLIGHTS

We included 1142 consecutive patients with suspected inducible ischemia, and evaluated the added value of the biomarkers high-sensitivity cardiac troponin (hs-cTn) and B-type natriuretic peptide (BNP), alone and in combination, on top of clinical judgment.

CLINICAL TRIAL REGISTRATION

Biochemical and Electrocardiographic Signatures in the Detection of Exercise-induced Myocardial Ischemia (BASEL VIII), NCT01838148, https://clinicaltrials.gov/ct2/show/NCT01838148.

A candidate liquid chromatography mass spectrometry reference method for the quantification of the cardiac marker 1-32 B-type natriuretic peptide

BACKGROUND

B-type natriuretic peptide (BNP) is a 32 amino acid cardiac hormone routinely measured by immunoassays to diagnose heart failure. While it is reported that immunoassay results can vary up to 45%, no attempt of standardization and/or harmonization through the development of certified reference materials (CRMs) or reference measurement procedures (RMPs) has yet been carried out.

METHODS

B-type natriuretic peptide primary calibrator was quantified traceably to the International System of Units (SI) by both amino acid analysis and tryptic digestion. A method for the stabilization of BNP in plasma followed by protein precipitation, solid phase extraction (SPE) and liquid chromatography (LC) mass spectrometry (MS) was then developed and validated for the quantification of BNP at clinically relevant concentrations (15-150 fmol/g).

RESULTS

The candidate reference method was applied to the quantification of BNP in a number of samples from the UK NEQAS Cardiac Markers Scheme to demonstrate its applicability to generate reference values and to preliminary evaluate the commutability of a potential CRM. The results from the reference method were consistently lower than the immunoassay results and discrepancy between the immunoassays was observed confirming previous data.

CONCLUSIONS

The application of the liquid chromatography-mass spectrometry (LC-MS) method to the UK NEQAS samples and the correlation of the results with the immunoassay results shows the potential of the method to support external quality assessment schemes, to improve understanding of the bias of the assays and to establish RMPs for BNP measurements. Furthermore, the method has the potential to be multiplexed for monitoring circulating truncated forms of BNP.

Searching for a BNP standard: Glycosylated proBNP as a common calibrator enables improved comparability of commercial BNP immunoassays

BACKGROUND

Circulating B-type natriuretic peptide (BNP) is widely accepted as a diagnostic and risk assessment biomarker of cardiac function. Studies suggest that there are significant differences in measured concentrations among different commercial BNP immunoassays. The purpose of our study was to compare BNP-related proteins to determine a form that could be used as a common calibrator to improve the comparability of commercial BNP immunoassay results.

METHODS

BNP was measured in 40 EDTA-plasma samples from acute and chronic heart failure patients using five commercial BNP assays: Alere Triage, Siemens Centaur XP, Abbott I-STAT, Beckman Access2 and ET Healthcare Pylon. In parallel with internal calibrators from each manufacturer, six preparations containing BNP 1-32 motif a) synthetic BNP, b) recombinant BNP (E. coli), c) recombinant nonglycosylated proBNP (E. coli), d) recombinant His-tagged (N-terminal) nonglycosylated proBNP (E. coli), e) recombinant glycosylated proBNP (HEK cells), and f) recombinant glycosylated proBNP (CHO cells) were also used as external calibrators for each assay.

RESULTS

Using the internal standards provided by manufacturers and for five of six external calibrators, up to 3.6-fold differences (mean 1.9-fold) were observed between BNP immunoassays (mean between-assay CV 24.5-47.2%). A marked reduction of the between-assay variability was achieved, when glycosylated proBNP expressed in HEK cells was used as the common calibrator for all assays (mean between-assay CV 14.8%).

CONCLUSIONS

Our data suggest that recombinant glycosylated proBNP could serve as a common calibrator for BNP immunoassays to reduce between-assay variability and achieve better comparability of BNP concentrations of commercial BNP immunoassays.

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

BACKGROUND

Protease neprilysin is known to be responsible for the degradation of natriuretic peptides. A recent heart failure (HF) drug, LCZ696 (EntrestoTM), that combines a neprilysin inhibitor and an angiotensin II receptor inhibitor was suggested to augment circulating B-type natriuretic peptide (BNP) concentrations, making the results of BNP measurements diagnostically ambiguous. Because the main form of measured BNP in HF patients is represented by its uncleaved precursor, proBNP, it is important to know the susceptibility of proBNP to cleavage by neprilysin.

METHODS

BNP 1–32 and nonglycosylated and glycosylated forms of proBNP 1–108 were incubated with neprilysin for different time periods. BNP immunoreactivity was analyzed using 2 sandwich immunoassays: one utilizing monoclonal antibody (mAb) KY-BNP-II (epitope 14–21) as capture with mAb 50E1 (epitope 26–32) for detection and a single-epitope sandwich BNP (SES-BNP) immunoassay specific to the epitope 11–17. Mass-spectrometry was applied to determine the sites of BNP cleavage.

RESULTS

In contrast to BNP, both forms of proBNP were resistant to degradation by neprilysin. The SES-BNP assay was much less susceptible to the BNP cleavage by neprilysin compared with the immunoassay utilizing antibodies specific to the region 14–21, comprising the site Arg17-Ile18, known as the site of BNP cleavage by neprilysin.

CONCLUSIONS

These findings suggest that modulation of neprilysin activity by specific inhibitors may not greatly influence the circulating concentrations of immunoreactive BNP, mostly represented in HF by proBNP, which is not susceptible to neprilysin. The different susceptibility of the BNP regions to neprilysin-dependent degradation highlights the importance of the choice of epitopes for reliable BNP immunodetection.

Characterization of endogenously circulating IGFBP-4 fragments-Novel biomarkers for cardiac risk assessment

BACKGROUND

Recent findings show that circulating N- and C-terminal fragments of IGF-binding protein-4 (NT-IGFBP-4 and CT-IGFBP-4) can be utilized as biomarkers for cardiac risk assessment in acute coronary syndrome (ACS) patients. The fragments are thought to be the products of pregnancy-associated plasma protein A (PAPP-A)-dependent proteolysis. Two immunoassays for the measurement of IGFBP-4 fragments have been proposed. However, properties of the endogenous IGFBP-4 fragments that could influence the performance of the immunoassays were still not investigated.

METHODS

NT- and CT-IGFBP-4 were extracted from pooled ACS plasma using affinity purification, and their concentrations were measured using sandwich immunoassays utilizing antibodies specific to their proteolytic neo-epitopes or internal epitopes. The extracted fragments were characterized by Western blots (WB) and mass-spectrometry. ACS plasma samples were analyzed by size exclusion chromatography (SEC).

RESULTS

Immunoassays utilizing the neo-epitope-specific and the internal epitope-specific antibodies measured equal concentrations of the analyte in the endogenous IGFBP-4 fragments preparations. Only the 18 kDa NT-IGFBP-4 and 14 kDa CT-IGFBP-4 were detected in the WB analysis. Using mass-spectrometry, peaks corresponding to intact non-truncated and non-modified NT-IGFBP-4 (14626 Da) and CT-IGFBP-4 (11346 Da) were observed. The absence of complexed forms of IGFBP-4 in patients' plasma was demonstrated using SEC.

CONCLUSIONS

Endogenous NT- and CT-IGFBP-4 from ACS patients' plasma correspond to the PAPP-A-derived IGFBP-4 fragments and do not undergo any truncation, modification, or complex formation in the patients' blood. Because of the demonstrated intact state of the circulating IGFBP-4 fragments, the neo-epitope-specific immunoassays perform reliably, allowing further clinical validation of these novel biomarkers.

State of the art of immunoassay methods for B-type natriuretic peptides: An update

The aim of this review article is to give an update on the state of the art of the immunoassay methods for the measurement of B-type natriuretic peptide (BNP) and its related peptides. Using chromatographic procedures, several studies reported an increasing number of circulating peptides related to BNP in human plasma of patients with heart failure. These peptides may have reduced or even no biological activity. Furthermore, other studies have suggested that, using immunoassays that are considered specific for BNP, the precursor of the peptide hormone, proBNP, constitutes a major portion of the peptide measured in plasma of patients with heart failure. Because BNP immunoassay methods show large (up to 50%) systematic differences in values, the use of identical decision values for all immunoassay methods, as suggested by the most recent international guidelines, seems unreasonable. Since proBNP significantly cross-reacts with all commercial immunoassay methods considered specific for BNP, manufacturers should test and clearly declare the degree of cross-reactivity of glycosylated and non-glycosylated proBNP in their BNP immunoassay methods. Clinicians should take into account that there are large systematic differences between methods when they compare results from different laboratories that use different BNP immunoassays. On the other hand, clinical laboratories should take part in external quality assessment (EQA) programs to evaluate the bias of their method in comparison to other BNP methods. Finally, the authors believe that the development of more specific methods for the active peptide, BNP1-32, should reduce the systematic differences between methods and result in better harmonization of results.

Value of proBNP1-108 testing for the risk stratification of patients with systolic heart failure

The study objectives were to determine the circulating levels of proBNP1-108, the precursor of B-type natriuretic peptide (BNP) and amino-terminal pro-BNP (NT-proBNP), in patients with systolic heart failure (HF) and to assess their prognosis value for cardiovascular (CV) death over a long-term follow-up. Seventy-three patients with systolic HF and 68 healthy volunteers were included. ProBNP1-108, BNP and NT-proBNP levels were measured with automated immunoassays and their predictive value for long-term survival was assessed through an 8 years follow-up. ProBNP1-108 levels were markedly increased in patients with systolic HF in comparison to healthy volunteers. In univariate proportional hazard model, survival was related to proBNP1-108, BNP, NT-proBNP, age, EF and glomerular filtration rate (eGFR). Kaplan-Meier survival curves according to proBNP tertiles diverged significantly, and the highest proBNP levels were related to patients with the highest risk of CV death. In a multivariate analysis including age, EF, proBNP1-108, BNP, NT-proBNP, and eGFR levels, NT-proBNP was the strongest predictor of long term CV death. Our study therefore demonstrated that high levels of proBNP1-108, measured with an assay with enhanced analytical specificity, are related to the long-term risk of cardiovascular death in systolic heart failure.

A novel fluorescence reaction for N-terminal Ser-containing peptides and its application to assay caspase activity

Caspases are the key regulatory factors of apoptosis and are also found to be involved in inflammatory cytokinesis. Sensitive and selective determination of caspases has significant importance in evaluation of apoptosis, disease diagnosis, and drug development. Here, we developed an assay method for the determination of caspase activity. This method is based on a novel fluorescence (FL) reaction selective for N-terminal Ser-containing peptides. FL derivatization of peptides requires heating in the presence of catechol, HEPES buffer (pH 7.5), and sodium periodate. Under optimized conditions, the reaction showed a unique sequence preference for N-terminal Ser-containing peptides, and a lower detection limit (signal/noise [S/N] = 3) of approximately 0.1 μM was obtained for SKTS and SSNSF. Acetylated substrates were enzymatically cleaved to produce N-terminal Ser-containing peptides, which were selectively converted to FL compounds. The enzyme activities were simultaneously determined as low as 2 U (4.3 nM) caspase-3 and 2.5 U (3.3 nM) caspase-8 by high-performance liquid chromatography (HPLC) with FL detection. The proposed assay method does not require any labeled substrates and can be applied to evaluate cell-based apoptosis and also to study apoptosis inhibitors or inducers.

Circulating forms of the b-type natriuretic peptide prohormone: pathophysiologic and clinical considerations

Recent studies reported that many different biochemical forms of B-type-related peptides circulate in human blood. In particular, a significant amount of the prohormone peptide (i.e., proBNP108) can be detected in plasma of patients with heart failure. These data indicate that the posttranslational maturation processing of the B-type natriuretic peptide (BNP) precursor may not be efficient in heart failure. The aim of this chapter is to describe the biochemical pathways of proBNP108 maturation and to discuss the pathophysiological relevance of alteration of the posttranslational maturation mechanisms in heart failure. An impaired cardiac endocrine function was proposed to explain the altered electrolyte and fluid homeostasis occurring in chronic heart failure. Recent studies demonstrated that a great part of BNPs assayed by immunoassay methods in healthy subjects and in patients with cardiovascular disease is devoid of biological activity. These findings suggest that an alteration in posttranslational maturation of BNP precursor may promote the resistance to biological action of BNP in patients with heart failure at a prereceptor level. These studies also open a new and more complex scenario regarding the circulating BNPs. The active hormone (i.e., BNP1-32) may be produced even in vivo from the circulating precursor proBNP108 by plasma enzyme degradation, such as the soluble form of corin, possibly able to process the circulating intact precursor of natriuretic hormones. As a future perspective, the simultaneous measurement of the proBNP1-108 and the active peptide BNP1-32 with more specific methods could allow a more accurate estimation of both production/secretion of B-type-related peptides from cardiomyocytes and the true activity of the cardiac endocrine function.

Diagnosing destabilized heart failure in the emergency setting: current and future biomarker tests

Acute or destabilized heart failure (DHF) is characterized by new or worsening signs and symptoms of heart failure leading to admission to an emergency department. Biomarkers may support the diagnosis, the prognosis and the management of DHF patients. The aim of this review article is to discuss and evaluate the clinical usefulness of both recognized and potential new biomarker tests for use in heart failure.

Human pro-B-type natriuretic peptide is processed in the circulation in a rat model

BACKGROUND

The appearance of B-type natriuretic peptide (BNP) in the blood is ultimately caused by proteolytic processing of its precursor, proBNP. The mechanisms leading to the high plasma concentration of unprocessed proBNP are still poorly understood. The goals of the present study were to examine whether processing of proBNP takes place in the circulation and to evaluate the clearance rate of proBNP and proBNP-derived peptides.

METHODS

We studied the processing of human proBNP in the circulation and the clearance rate of proBNP and proBNP-derived peptides (BNP and N-terminal fragment of proBNP, NT-proBNP) in rats by injecting the corresponding peptides and analyzing immunoreactivity at specific time points. Glycosylated and nonglycosylated proBNP and NT-proBNP were used in the experiments. We applied immunoassays, gel filtration, and mass spectrometry (MS) techniques to analyze the circulation-mediated processing of proBNP.

RESULTS

ProBNP was effectively processed in the circulation into BNP (1-32) and various truncated BNP forms as confirmed by gel filtration and MS analysis. Glycosylation of proBNP close to the cleavage-site region suppressed its processing in the circulation. The terminal half-life for human glycosylated proBNP was 9.0 (0.5) min compared with 6.4 (0.5) min for BNP. For NT-proBNP, the terminal half-lives were 15.7 (1.4) min and 15.5 (1.3) min for glycosylated and nonglycosylated forms, respectively.

CONCLUSIONS

In rats, processing of human proBNP to active BNP occurs in the circulation. The clearance rate of proBNP is quite similar to that of BNP. These observations suggest that peripheral proBNP processing may be an important regulatory step rather than mere degradation.

Processing of pro-B-type natriuretic peptide: furin and corin as candidate convertases

BACKGROUND

B-type natriuretic peptide (BNP) and its N-terminal fragment (NT-proBNP) are the products of the enzyme-mediated cleavage of their precursor molecule, proBNP. The clinical significance of proBNP-derived peptides as biomarkers of heart failure has been explored thoroughly, whereas little is known about the mechanisms of proBNP processing. We investigated the role of 2 candidate convertases, furin and corin, in human proBNP processing.

METHODS

We measured proBNP expression in HEK 293 and furin-deficient LoVo cells. We used a furin inhibitor and a furin-specific small interfering RNA (siRNA) to explore the implication of furin in proBNP processing. Recombinant proBNPs were incubated with HEK 293 cells transfected with the corin-expressing plasmid. We applied mass spectrometry to analyze the products of furin- and corin-mediated cleavage.

RESULTS

Reduction of furin activity significantly impaired proBNP processing in HEK 293 cells. Furin-deficient LoVo cells were unable to process proBNP, whereas coexpression with furin resulted in effective proBNP processing. Mass spectrometric analysis revealed that the furin-mediated cleavage of proBNP resulted in BNP 1-32, whereas corin-mediated cleavage led to the production of BNP 4-32. Some portion of proBNP in the plasma of heart failure patients was not glycosylated in the cleavage site region and was susceptible to furin-mediated cleavage.

CONCLUSIONS

Both furin and corin are involved in the proBNP processing pathway, giving rise to distinct BNP forms. The significance of the presence of unprocessed proBNP in circulation that could be cleaved by the endogenous convertases should be further investigated for better understanding BNP physiology.

Processing of Pro–Brain Natriuretic Peptide Is Suppressed by O-Glycosylation in the Region Close to the Cleavage Site

Background: Processing of the brain natriuretic peptide (BNP) precursor, proBNP, is a convertase-dependent reaction that produces 2 molecules—the active BNP hormone and the N-terminal part of proBNP (NT-proBNP). Although proBNP was first described more than 15 years ago, very little is known about the cellular mechanism of its processing. The study of proBNP processing mechanisms is important, because processing impairments could be associated with the development of heart failure (HF).

Methods: The biochemical properties of recombinant proBNP and NT-proBNP and the same molecules derived from the blood of HF patients were analyzed by gel-filtration chromatography, site-directed mutagenesis, and different immunochemical methods with a panel of monoclonal antibodies (MAbs).

Results: Part of the proBNP molecule (amino acid residues 61–76) located near the cleavage site was inaccessible to specific MAbs because of the presence of O-glycans, whereas the same region in NT-proBNP was completely accessible. We demonstrated that a convertase (furin) could effectively cleave deglycosylated (but not intact) proBNP. Of several mutant proBNP forms produced in a HEK 293 cell line, only the T71A variant was effectively processed in the cell.

Conclusions: Only proBNP that was not glycosylated in the region of the cleavage site could effectively be processed into BNP and NT-proBNP. Site-directed mutagenesis enabled us to ascertain the unique suppressing role of T71-bound O-glycan in proBNP processing.