How Does Neprilysin Inhibition in Heart Failure Work? (2019)

This article refers to ‘Effects of sacubitril/valsartan on neprilysin targets and the metabolism of natriuretic peptides in chronic heart failure: a mechanistic clinical study’ by H. Nougué et al., published in this issue on pages 598-605. A compellingly positive, large randomized controlled therapeutic trial, augmented by many ancillary studies and sub-analyses, indicates therapy based on combined angiotensin II type 1 receptor blockade and neprilysin inhibition (ARNI) confers an approximate 20% benefit for all major clinical endpoints in chronic heart failure (HF) compared to treatment based on angiotensin-converting enzyme inhibition (ACEI).1 Patients across a broad range of age, co-morbidities and concurrent treatments enjoy net benefit from ARNI over periods of at least 1–2 years, which seemingly constitutes the greatest advance in the pharmacotherapy of HF made in the last 25 years. The treatment has received full endorsement from authoritative international clinical guidelines and will eventually be prescribed to millions of patients worldwide in expectation of major benefit.2, 3 Neprilysin cleaves cardiac natriuretic peptides and it is generally assumed augmentation of intact bioactive natriuretic peptides in plasma and tissues underlies the beneficial effects of neprilysin inhibition (NEPi). It is important we understand the mechanisms underpinning the biological effects of ARNI in order to anticipate both advantageous and adverse effects and to potentially improve on the current ARNI options and garner further benefits from later drug modifications. The B-type cardiac peptides, B-type natriuretic peptide (BNP) and N-terminal pro BNP (NT-proBNP), are the best known natriuretic peptides in the clinical community having had long-term exposure as well-established diagnostic and prognostic biomarkers and as therapy in the form of human recombinant BNP (nesiritide). Subsequently BNP has been invoked as mediating important effects in ARNI and early neurohormonal sub-studies in ARNI therapy have focused upon these peptides.4 However, as emphasized by Nougué et al.5 in the current issue of the Journal, BNP is not the key natriuretic peptide affected by, or mediating the effects of, NEPi and natriuretic peptides are not the sole set of potentially important neprilysin substrates involved. Nougué et al.5 studied 73 patients with chronic HF undergoing conversion from ACEI-based to ARNI-based treatment with sacubitril/valsartan. Reduction in New York Heart Association class and improved echocardiographic findings observed over 90 days reflected clinical benefit from the new treatment. A dose-dependent fall in plasma neprilysin activity was associated with increased plasma concentrations of substance P, atrial natriuretic peptide (ANP), and glucagon-like peptide-1 (GLP-1) and decreased fructosamine. Soluble ST2 and cardiac troponin also fell. Whilst ANP rose several-fold, and in contradistinction to previous report,4 no change in plasma BNP and only a minor decrease in NT-proBNP were observed. The ratio of BNP to NT-proBNP rose and was correlated to glycosylation of proBNP, the common precursor to both BNP and NT-proBNP. The authors suggest these results challenge the assumed role of BNP in ARNI treatment and attribute greater importance to ANP. Neprilysin has over 40 substrates (Table 1) in human biology, which act within the cardiovascular, renal, central nervous, gastrointestinal and respiratory systems. Many have vasoactive actions and others mediate metabolic and inflammatory pathways. Dissecting the main drivers of benefit (or harm) from NEPi is therefore complex.6 It is long established that BNP is relatively resistant to degradation by neprilysin compared to ANP and C-type natriuretic peptide (CNP).6, 7 In both pre-clinical settings and in early studies in humans, NEPi has resulted in far greater augmentation of plasma ANP than BNP. The data on plasma ANP from Nougué et al. would have been enhanced by assessment of proportions of cleaved to intact ANP before and after NEPi.8 The authors contend their observation of stable or raised BNP with concurrent falls in NT-proBNP follows from enhanced glycosylation of proBNP, which is then rendered less amenable to cleavage to produce BNP and NT-proBNP with the sustained BNP level posited to be an artefact reflecting cross-reactivity of BNP assays with proBNP. They point to a measured index of proBNP glycosylation which increases with NEPi and use an indirect in vitro method suggesting NEPi produced no change in BNP-generated cGMP. The implication is that authentic BNP is not raised and is not responsible for any NEPi effect. These findings, although in line with previous work by these investigators suggesting a major role for post-translational processing in the biology of circulating BNP in HF, do not provide a universally applicable description or explanation of changes in plasma BNP with administration of NEPi.9, 10 Administration of NEPi to animals with experimental HF produced clear and immediate elevations in both plasma ANP and BNP in a setting where glycosylation is not known to occur.11 Single-dose intravenous NEPi given to patients undergoing cardiac catheterization induced several-fold elevations in plasma ANP and a doubling of plasma BNP in less than 1 h, a temporal profile inconsistent with major influence from glycosylation.12 Finally, in normal volunteers receiving steady low-dose infusions of BNP, the introduction of NEPi induced clear cut increments in achieved plasma BNP levels.13 In each setting, plasma cGMP (second messenger to the natriuretic peptides) rose. The impact of NEPi upon plasma ANP certainly exceeded that for BNP but elevations in immunoreactive plasma BNP, which cannot be readily ascribed to any glycosylation process, were consistently observed. Whilst the impact upon ANP of NEPi and the subsequent increased natriuretic peptide bioactivity attributable to ANP clearly exceed those related to BNP, at least some augmentation of, and added bioactivity from, BNP appears likely to occur. Several reports indicate even relatively low-dose single or co-infusion of natriuretic peptides (ANP and/or BNP and/or CNP) in humans results in augmented plasma concentrations of both the infused and non-infused natriuretic peptides, suggesting competition for clearance which is mediated by the natriuretic peptide clearance receptor as well as by enzymatic degradation.14 It is likely this mechanism (competition for clearance with ANP) plays a role in augmenting plasma BNP when neprilysin is inhibited. Further work using assays specific for BNP without cross-reactivity to proBNP and experiments in settings with and without glycosylation will elucidate these points.15 The indirect evidence the authors present to suggest absence of augmentation of BNP-induced cGMP with NEPi requires corroboration. Plasma samples obtained from a limited sample of HF patients before and after introduction of NEPi, compared to without prior B peptide depletion, did not produce any difference in release of cGMP from cultured cells. However, the minor proportional contribution of BNP to cGMP production compared to that from ANP at both baseline and (especially) after a major increase in ANP induced by NEPi, coupled with the inevitable inherent variations in cell response and assay analytic variation make it plausible this experimental approach suffered from inadequate signal to noise ratio. An important aspect of the paper from Nougué et al. is consideration of some non-natriuretic peptide neprilysin substrates. Substance P and GLP-1 were enhanced. In view of pre-clinical evidence, the former effect may exacerbate inflammatory conditions.6 However, no clear clinical signal of this has been reported to date. Increased GLP-1 has been invoked as contributing to improved glycaemic control in HF cases receiving ARNI. However, whether this will always confer unalloyed benefit is not clear. Overall, in type 2 diabetes GLP-1 receptor agonists decrease adverse cardiovascular events including cardiovascular death, myocardial infarction and stroke. However, long-acting GLP-1 agonists given to diabetic patients with reduced ejection fraction have been associated with increased rates of clinical HF and therefore their co-administration with NEPi in diabetic patients with chronic HF carries uncertain risks. The role of neprilysin in degradation of amyloid beta further illustrates the need to consider a wide array of neprilysin substrates. Pre-clinical data from rodents, although not universally concordant, point to NEPi promoting accumulation of amyloid beta in the brain along with induction of behavioural abnormalities, and restoration of intra-cerebral neprilysin reverses these changes.6 This raises the spectre of long-term NEPi inducing or accelerating Alzheimer's disease. It must be emphasized that these threats are theoretical and to date are underpinned by pre-clinical experimental data only. Observations in humans administered with NEPi have not highlighted any sentinel findings regarding exacerbation of inflammatory pathologies or accelerated dementia. Notably, trial inclusion criteria inevitably select patients with a lower burden of co-morbidities than in ‘real-life’ populations. To be certain of long-term safety in chronic conditions including Alzheimer's disease, a sustained period of monitoring of a large number of elderly patients is required. Table 1 of neprilysin substrates highlights the incomplete nature of our knowledge of the spectrum of acute and chronic consequences of chronic NEPi in the clinical setting. The full picture will emerge from ongoing mechanistic studies in parallel with observations from long-term exposure of ARNI therapy in chronic HF. Nougué et al. have reported novel findings and pointed the way to further interrogation of both the partially realized promise as well as the potential threats of ARNI therapy. Conflict of interest: none declared.