Neprilysin Inhibition Increases Glucagon Levels in Humans and Mice with Potential Effects on Amino Acid Metabolism

Context Inhibitors of the protease neprilysin (NEP) are used for treating heart failure, but are also linked to improvements in metabolism. NEP may cleave proglucagon-derived peptides, including the glucose and amino acid (AA)-regulating hormone glucagon. Studies investigating NEP inhibition on glucagon metabolism are warranted. Objective To investigate whether NEP inhibition increases glucagon levels. Subjects and methods Plasma concentrations of glucagon and AAs were measured in eight healthy men during a mixed meal with and without a single dose of the NEP inhibitor/angiotensin II type 1 receptor antagonist, sacubitril/valsartan (194 mg/206 mg). Long-term effects of sacubitril/valsartan (eight weeks) were investigated in individuals with obesity (n=7). Mass-spectrometry was used to investigate NEP-induced glucagon degradation, and the derived glucagon fragments were tested pharmacologically in cells transfected with the glucagon receptor (GCGR). Genetic deletion or pharmacological inhibition of NEP with or without concomitant GCGR antagonism was tested in mice to evaluate effects on AA metabolism. Results In healthy men, a single dose of sacubitril/valsartan significantly increased postprandial concentrations of glucagon by 228%, concomitantly lowering concentrations of AAs including glucagonotropic AAs. Eight weeks sacubitril/valsartan treatment increased fasting glucagon concentrations in individuals with obesity. NEP cleaved glucagon into five inactive fragments (in vitro). Pharmacological NEP inhibition protected both exogenous and endogenous glucagon in mice after an AA challenge, while NEP-deficient mice showed elevated fasting and AA-stimulated plasma concentrations of glucagon and urea compared to controls. Conclusion NEP cleaves glucagon, and inhibitors of NEP result in hyperglucagonemia and may increase postprandial AA catabolism without affecting glycemia.

Combined RAAS and NEP Inhibition

The neurohormonal model of HF has provided the rationale for the use of drug classes blocking the effectors of both the RAAS and SNS at different sites, including angiotensin-converting enzyme inhibitors (ACEI), angiotensin receptor blockers (ARB), mineralocorticoid receptor antagonists (MRA), and beta-blockers. Combined NEP and ACE blockade although unsuccessful with omapatrilat in the OVERTURE trial, found success with sacubitril/valsartan in the Paradigm-HF trial. The results of PARADIGM-HF trial represent one of the most significant breakthroughs in the management of HF of the last decade, representing a shift from neurohomonal antagonism to neurohormonal modulation.

P1661The myocardial tissue Renin-Angiotensin-System (RAS) of the failing heart

Background Prognosis of patients with HFrEF remains poor despite recent advances in pharmacologic therapy as the introduction of the angiotensin-receptor neprilysin-inhibitor (ARNI). The Renin-Angiotensin-System (RAS) is dysregulated in HF with elevated AngII levels as a central driver of disease progression. The myocardium is capable of synthesizing all RAS components resulting in tissue specific angiotensin levels. Neprilysin (NEP) catalyzes the generation of Ang1–7 which counteracts the deleterious effects of AngII. Myocardial tissue angiotensins of the failing heart and the role of long-lasting RAS-inhibitor therapy and particularly NEP inhibition on tissue RAS have not been investigated yet. Methods Concentrations of AngI, AngII, Ang1–7, AngIII, Ang1–5 and AngIV (RAS-fingerprints) were investigated in myocardial samples of end-stage HFrEF patients undergoing heart transplantation with a mass-spectrometry based method. Patients were stratified according to background therapy with RAS-inhibitors and variables were compared by a non-parametrical test. Results A total of 30 patients were included (n=6 without RAS-blockade, n=16 with ACE-I, n=6 with ARB and n=2 with ARNI). Median age was 55 (IQR 45–63) years and 87% of patients were male. 40% of patients had an ischemic etiology of HF, median NT-proBNP levels were 3498pg/ml (IQR 1761–8400). Tissue RAS patterns were visually similar between all groups (Figure 1). Myocardial AngI, Ang1–7, Ang1–5 and AngIV levels were below the detection limit for all samples. Median tissue AngII and AngIII concentrations across all samples were 83.1pg/ml (IQR 29.3–196.6) and 26.4pg/ml (IQR 5.0–64.5). Despite different background RAS-inhibitor therapy, AngII and AngIII levels were comparable between all groups [median (IQR) in pg/ml – AngII: 51.5 (41.5–123.8) vs. 72.4 (28.5–177.6) vs. 176.1 (22.4–286.8) vs. 266.0 (108.2–423.8); p=ns and 26.4 (5.0–89.2) vs. 23.2 (5.0–59.3) vs. 39.4 (5.0–94.3) vs. 105.9 (46.5–165.3); p=ns for no therapy, ACE-I, ARB and ARNI respectively]. Figure 1. RAS-fingerprints of the failing heart according to RAS-inhibiton. Numbers in brackets indicate the specific angiotensin peptides. Side of spheres and numbers beside represent absolute concentrations of angiotensins (pg/ml, median value). Conclusions Although in the plasma of HFrEF patients only AngI and AngII are detectable at substantial concentrations, the predominant angiotensins of the failing heart are AngII and AngIII. AngII levels are high in the failing heart supporting the hypothesis that excess AngII is involved in disease progression. AngIII similarly increases cardiac sympathetic activity assumedly potentiating further deteoriation. The modality of long established RAS-inhibitor therapy in end-stage HF, particularly the inhibition of NEP, seems to have no (more) influence on myocardial tissue RAS regulation. The impact of NEP inhibition by ARNI on tissue RAS enzymes and mechanism of action need to be further investigated.

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.

Localization of 99mTc-GRP Analogs in GRPR-Expressing Tumors: Effects of Peptide Length and Neprilysin Inhibition on Biological Responses

The overexpression of gastrin-releasing peptide receptors (GRPRs) in frequently occurring human tumors has provided the opportunity to use bombesin (BBN) analogs as radionuclide carriers to cancer sites for diagnostic and therapeutic purposes. We have been alternatively exploring human GRP motifs of higher GRPR selectivity compared to frog BBN sequences aiming to improve pharmacokinetic profiles. In the present study, we compared two differently truncated human endogenous GRP motifs: GRP(14–27) and GRP(18–27). An acyclic tetraamine was coupled at the N-terminus to allow for stable binding of the SPECT radionuclide 99mTc. Their biological profiles were compared in PC-3 cells and in mice without or with coinjection of phosphoramidon (PA) to induce transient neprilysin (NEP) inhibition in vivo. The two 99mTc-N4-GRP(14/18–27) radioligands displayed similar biological behavior in mice. Coinjection of PA exerted a profound effect on in vivo stability and translated into notably improved radiolabel localization in PC-3 experimental tumors. Hence, this study has shown that promising 99mTc-radiotracers for SPECT imaging may indeed derive from human GRP sequences. Radiotracer bioavailability was found to be of major significance. It could be improved during in situ NEP inhibition resulting in drastically enhanced uptake in GRPR-expressing lesions.

Neutral endopeptidase inhibitors blunt kidney fibrosis by reducing myofibroblast formation

Kidney fibrosis is the common pathophysiological mechanism in end-stage renal disease characterized by excessive accumulation of myofibroblast-derived extracellular matrix. Natriuretic peptides have been demonstrated to have cyclic guanosine monophosphate (cGMP)-dependent anti-fibrotic properties likely due to interference with pro-fibrotic tissue growth factor β (TGF-β) signaling. However, in vivo, natriuretic peptides are rapidly degraded by neutral endopeptidases (NEP). In a unilateral ureteral obstruction (UUO) mouse model for kidney fibrosis we assessed the anti-fibrotic effects of SOL1, an orally active compound that inhibits NEP and endothelin-converting enzyme (ECE). Mice (n=10 per group) subjected to UUO were treated for 1 week with either solvent, NEP-/ECE-inhibitor SOL1 (two doses), reference NEP-inhibitor candoxatril or the angiotensin II receptor type 1 (AT1)-antagonist losartan. While NEP-inhibitors had no significant effect on blood pressure, they did increase urinary cGMP levels as well as endothelin-1 (ET-1) levels. Immunohistochemical staining revealed a marked decrease in renal collagen (∼55% reduction, P<0.05) and α-smooth muscle actin (α-SMA; ∼40% reduction, P<0.05). Moreover, the number of α-SMA positive cells in the kidneys of SOL1-treated groups inversely correlated with cGMP levels consistent with a NEP-dependent anti-fibrotic effect. To dissect the molecular mechanisms associated with the anti-fibrotic effects of NEP inhibition, we performed a ‘deep serial analysis of gene expression (Deep SAGE)’ transcriptome and targeted metabolomics analysis of total kidneys of all treatment groups. Pathway analyses linked increased cGMP and ET-1 levels with decreased nuclear receptor signaling (peroxisome proliferator-activated receptor [PPAR] and liver X receptor/retinoid X receptor [LXR/RXR] signaling) and actin cytoskeleton organization. Taken together, although our transcriptome and metabolome data indicate metabolic dysregulation, our data support the therapeutic potential of NEP inhibition in the treatment of kidney fibrosis via cGMP elevation and reduced myofibroblast formation.

Biological and In silico Studies on Synthetic Analogues of Tyrosine Betaine as Inhibitors of Neprilysin - A Drug Target for the Treatment of Heart Failure

Background: Fungal secondary metabolites are important sources for the discovery of new pharmaceuticals, as exemplified by penicillin, lovastatin and cyclosporine. Searching for secondary metabolites of the fungi Metarhizium spp., we previously identified tyrosine betaine as a major constituent. Methods: Because of the structural similarity with other inhibitors of neprilysin (NEP), an enzyme explored for the treatment of heart failure, we devised the synthesis of tyrosine betaine and three analogues to be subjected to in vitro NEP inhibition assays and to molecular modeling studies. Results: In spite of the similar binding modes with other NEP inhibitors, these compounds only displayed moderate inhibitory activities (IC50 ranging from 170.0 to 52.9 µM). However, they enclose structural features required to hinder passive blood brain barrier permeation (BBB). Conclusions: Tyrosine betaine remains as a starting point for the development of NEP inhibitors because of the low probability of BBB permeation and, consequently, of NEP inhibition at the Central Nervous System, which is associated to an increment in the Aβ levels and, accordingly, with a higher risk for the onset of Alzheimer's disease.