Mixed inhibitors of angiotensin-converting enzyme (EC 3.4.15.1) and enkephalinase (EC 3.4.24.11): rational design, properties, and potential cardiovascular applications of glycopril and alatriopril

Molecular Basis for Multiple Omapatrilat Binding Sites within the ACE C-Domain: Implications for Drug Design

Omapatrilat was designed as a vasopeptidase inhibitor with dual activity against the zinc metallopeptidases angiotensin-1 converting enzyme (ACE) and neprilysin (NEP). ACE has two homologous catalytic domains (nACE and cACE), which exhibit different substrate specificities. Here, we report high-resolution crystal structures of omapatrilat in complex with nACE and cACE and show omapatrilat has subnanomolar affinity for both domains. The structures show nearly identical binding interactions for omapatrilat in each domain, explaining the lack of domain selectivity. The cACE complex structure revealed an omapatrilat dimer occupying the cavity beyond the S₂ subsite, and this dimer had low micromolar inhibition of nACE and cACE. These results highlight residues beyond the S₂ subsite that could be exploited for domain selective inhibition. In addition, it suggests the possibility of either domain specific allosteric inhibitors that bind exclusively to the nonprime cavity or the potential for targeting specific substrates rather than completely inhibiting the enzyme.

Combination of drugs acting on the natriuretic system and the renin-angiotensin system in heart failure

Conventional diuretic agents are very effective agents in relieving volume overload and congestive symptoms in chronic heart failure (CHF). However, they are associated with activation of the renin-angiotensin system (RAS) and the sympathetic nervous system and a reduction in glomerular filtration rate, all of which have been associated with adverse outcomes in CHF. Therefore, there is an increasing interest in drugs that target the natriuretic system without neurohormonal activation and deterioration of renal function. In this review, we will discuss the underlying rationale and evidence behind currently pursued strategies that target the natriuretic system. This includes the administration of natriuretic peptides (NPs) and strategies that potentiate the NP system, such as neutral endopeptidase inhibition. We will also highlight some potentially important interactions of these strategies with drugs that target the RAS.

Metal complexes and metalloproteases: targeting conformational diseases

In recent years many metalloproteases (MPs) have been shown to play important roles in the development of various pathological conditions. Although most of the literature is focused on matrix MPs (MMPs), many other MPs have been demonstrated to be involved in the degradation of peptides or proteins whose accumulation and dyshomeostasis are considered as being responsible for the development of conformational diseases, i.e., diseases where non-native protein conformations lead to protein aggregation. It seems clear that, at least in principle, it must be possible to control the levels of many aggregation-prone proteins not only by reducing their production, but also by enhancing their catabolism. Metal complexes that can perform this function were designed and tested according to at least two different strategies: (i) intervening on the endogenous MPs by directly or indirectly modulating their activity; (ii) acting as artificial MPs, replacing or synergistically functioning with endogenous MPs. These two different bioinorganic approaches are widely represented in the current literature and the aim of this review is to rationally organize and discuss both of them so as to give a critical insight into these approaches and highlighting their limitations and future perspectives.

Roles of proteolysis in regulation of GPCR function

The enzymatic activity of peptidases must be tightly regulated to prevent uncontrolled hydrolysis of peptide bonds, which could have devastating effects on biological systems. Peptidases are often generated as inactive propeptidases, secreted with endogenous inhibitors, or they are compartmentalized. Propeptidases become active after proteolytic removal of N-terminal activation peptides by other peptidases. Some peptidases only become active towards substrates only at certain pHs, thus confining activity to specific compartments or conditions. This review discusses the different roles proteolysis plays in regulating GPCRs. At the cell-surface, certain GPCRs are regulated by the hydrolytic inactivation of bioactive peptides by membrane-anchored peptidases, which prevent signalling. Conversely, cell-surface peptidases can also generate bioactive peptides, which directly activate GPCRs. Alternatively, cell-surface peptidases activated by GPCRs, can generate bioactive peptides to cause transactivation of receptor tyrosine kinases, thereby promoting signalling. Certain peptidases can signal directly to cells, by cleaving GPCR to initiate intracellular signalling cascades. Intracellular peptidases also regulate GPCRs; lysosomal peptidases destroy GPCRs in lysosomes to permanently terminate signalling and mediate down-regulation; endosomal peptidases cleave internalized peptide agonists to regulate GPCR recycling, resensitization and signalling; and soluble intracellular peptidases also participate in GPCR function by regulating the ubiquitination state of GPCRs, thereby altering GPCR signalling and fate. Although the use of peptidase inhibitors has already brought success in the treatment of diseases such as hypertension, the discovery of new regulatory mechanisms involving proteolysis that control GPCRs may provide additional targets to modulate dysregulated GPCR signalling in disease.

Novel mechanism of inhibition of human angiotensin-I-converting enzyme (ACE) by a highly specific phosphinic tripeptide

Human ACE (angiotensin-I-converting enzyme) has long been regarded as an excellent target for the treatment of hypertension and related cardiovascular diseases. Highly potent inhibitors have been developed and are extensively used in the clinic. To develop inhibitors with higher therapeutic efficacy and reduced side effects, recent efforts have been directed towards the discovery of compounds able to simultaneously block more than one zinc metallopeptidase (apart from ACE) involved in blood pressure regulation in humans, such as neprilysin and ECE-1 (endothelin-converting enzyme-1). In the present paper, we show the first structures of testis ACE [C-ACE, which is identical with the C-domain of somatic ACE and the dominant domain responsible for blood pressure regulation, at 1.97Å (1 Å=0.1 nm)] and the N-domain of somatic ACE (N-ACE, at 2.15Å) in complex with a highly potent and selective dual ACE/ECE-1 inhibitor. The structural determinants revealed unique features of the binding of two molecules of the dual inhibitor in the active site of C-ACE. In both structures, the first molecule is positioned in the obligatory binding site and has a bulky bicyclic P(1)' residue with the unusual R configuration which, surprisingly, is accommodated by the large S(2)' pocket. In the C-ACE complex, the isoxazole phenyl group of the second molecule makes strong pi-pi stacking interactions with the amino benzoyl group of the first molecule locking them in a 'hand-shake' conformation. These features, for the first time, highlight the unusual architecture and flexibility of the active site of C-ACE, which could be further utilized for structure-based design of new C-ACE or vasopeptidase inhibitors.

Monitoring enzyme reaction and screening of inhibitors of acetylcholinesterase by quantitative matrix-assisted laser desorption/ionization Fourier transform mass spectrometry

A matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS)-based assay was developed for kinetic measurements and inhibitor screening of acetylcholinesterase. Here, FTMS coupled to MALDI was applied to quantitative analysis of choline using the ratio of choline/acetylcholine without the use of additional internal standard, which simplified the experiment. The Michaelis constant (K(m)) of acetylcholinesterase (AChE) was determined to be 73.9 micromol L(-1) by this approach. For Huperzine A, the linear mixed inhibition of AChE reflected the presence of competitive and noncompetitive components. The half maximal inhibitory concentration (IC(50)) value of galantamine obtained for AChE was 2.39 micromol L(-1). Inhibitory potentials of Rhizoma Coptidis extracts were identified with the present method. In light of the results the referred extracts as a whole showed inhibitory action against AChE. The use of high-resolution FTMS largely eliminated the interference with the determination of ACh and Ch, produced by the low-mass compounds of chemical libraries for inhibitor screening. The excellent correlation with the reported kinetic parameters confirms that the MS-based assay is both accurate and precise for determining kinetic constants and for identifying enzyme inhibitors. The obvious advantages were demonstrated for quantitative analysis and also high-throughput characterization. This study offers a perspective into the utility of MALDI-FTMS as an alternate quantitative tool for inhibitor screening of AChE.

Vasopeptidase inhibitors: will they have a role in clinical practice?

The human cardiovascular system is regulated by haemodynamic, neurohumoral and structural mechanisms. The endothelium and the neurohumoral system play a key role in modulating both vascular tone and structure by producing vasoactive substances, and in the modulation of blood cell adhesion. Although the neurohormonal systems are essential in vascular homeostasis, they become maladaptive in conditions such as hypertension, coronary disease and heart failure. The clinical success of blocking the renin-angiotensin system by angiotensin converting enzyme (ACE)-inhibitors and the sympathetic nerve system by beta-blockers demonstrates the importance of neurohumoral blockade. The inadequate effect of angiotensin converting enzyme (ACE) or neutral endopeptidase (NEP) inhibitor monotherapy seen in some patients treated for hypertension or congestive heart failure, and the promising effect seen after their combination, led to the development of drugs that simultaneously inhibit both enzyme systems. Neutral endopeptidase, like ACE, is an endothelial cell surface zinc metallopeptidase with similar structure and catalytic site to ACE. NEP is the major enzymatic pathway for degradation of natriuretic peptides. The natriuretic peptide system can be viewed as the endogenous inhibitor of the renin angiotensin system. The dual metalloprotease inhibitors of ACE and NEP, called vasopeptidase inhibitors therefore represent a new and attractive therapeutic strategy for the treatment of cardiovascular disease. The ability to add incremental benefit over already proven therapy, with an acceptable side-effect profile however, is questionable in this new class of agents.

The therapeutic effect of natriuretic peptides in heart failure; differential regulation of endothelial and inducible nitric oxide synthases

The abnormal regulation of nitric oxide synthase activity represents an underlying feature of heart failure. Increased peripheral vascular resistance, and decreased renal function may be in part related to impaired endothelium-dependent nitric oxide (NO) synthesis. Paradoxically, the chronic production of NO by inducible nitric oxide synthase (iNOS) in heart failure exerts deleterious effects on ventricular contractility, and circulatory function. Consequently, pharmacologically improving endothelium-dependent NO synthesis and the concomitant inhibition of iNOS activity would be therapeutically advantageous. Interestingly, natriuretic peptides have been shown to differentially regulate endothelial NOS (eNOS) and iNOS activity. Moreover, in both patients and animal models of heart failure, pharmacologically increasing plasma natriuretic peptide levels ameliorated vascular tone, renal function, and ventricular contractility. Based on these observations, the following review will explore whether the therapeutic benefit of the natriuretic peptide system in heart failure may occur in part via the amelioration of endothelium-dependent NO synthesis, and the concomitant inhibition of cytokine-mediated iNOS expression.

Dual ACE and Neutral Endopeptidase Inhibitors

Elevated blood pressure is a risk factor for a variety of cardiovascular disorders, including coronary heart disease, peripheral vascular disease, cardiac failure and cerebrovascular disease. The prevailing view is that an elevated systolic rather than diastolic blood pressure is the major contributor in mortality and morbidity attributed to cardiovascular disorders. Isolated high systolic blood pressure, especially in the elderly, is a major risk factor and should undoubtedly be a target for drug treatment. In the general population, systolic and diastolic blood pressure are highly correlated, and thus it is difficult to dissociate the effects of these two components of the blood pressure and specifically ascribe cardiovascular risk factors to just elevated systolic blood pressure. Therefore, the goal in therapy of an individual with hypertension must be to reduce elevated systolic and diastolic blood pressure in order to reduce mortality and morbidity. ACE and neutral peptidase inhibitors are a new class of drugs that may be beneficial in the treatment of patients with hypertension and heart failure. They may also be useful in the treatment of diabetic patients with hypertension and/or heart failure. Drugs of this class are dual inhibitors of ACE and neutral endopeptidase, and are capable of affecting vascular tone and fluid balance. They are capable of producing vasodilatation by virtue of inhibiting the production of angiotensin II, degradation of natriuretic peptides and bradykinin. They also appear to promote natriuresis and diuresis by amplifying the actions of natriuretic peptidase and reducing aldosterone effects. In addition, they should also attenuate trophogenic actions of the renin angiotensin system and the sympathetic nervous system. Omapatrilat is one drug that appears to be at the advanced stages of clinical development. This drug has been shown to be quite effective in the treatment of hypertension. Evidence also seems to indicate that treatment with omapatrilat results in a higher tendency towards preventing death and worsening heart failure when compared with treatment with a pure ACE inhibitor in patients with advanced heart failure. Overall safety with omapatrilat appears to be good, but like other ACE inhibitors the incidence of cough is higher when compared with placebo. Other common adverse effects noted are headaches, facial flushing/warm sensation, dizziness, nausea and dyspnoea. Of greater concern is the occurrence of angio-oedema, the true incidence of which remains to be fully established as part of the published medical literature.

Vasopeptidase inhibitors

Vasopeptidase inhibitors are a new class of drugs that have dual inhibitory effects on two key enzymes involved in the metabolism of vasoactive peptides. Essentially, they inhibit angiotensin-converting enzyme (ACE), thereby blocking the generation of angiotensin II (Ang II); at the same time they prevent the breakdown of natriuretic peptides by the enzyme neutral endopeptidase. The combination of reduction of Ang II on a background of increased natriuretic peptide activity has several potential advantages for the treatment of cardiovascular and renal disease and in particular, hypertension and congestive heart failure (CHF). Several vasopeptidase inhibitors, such as sampatrilat, fasidotril, gemopatrilat and omapatrilat (Vanlev, the most clinically developed vasopeptidase inhibitor to date) are under intensive clinical investigation. Recent clinical trials have demonstrated effective antihypertensive activity in hypertension, independent of age, renin and salt status or ethnic origin, and have also highlighted the potential for vasopeptidase inhibition as a new therapeutic modality for the treatment of CHF. Moreover, ongoing research suggests that this new class of drugs may be an important approach, not only for the treatment of hypertension and of conditions associated with overt volume overload but also for ischaemic heart disease.

Cell-specific activity of neprilysin 2 isoforms and enzymic specificity compared with neprilysin

Neprilysin (NEP) 2 is a recently cloned glycoprotein displaying a high degree of sequence identity with neprilysin (EC 3.4.24.11), the prototypical member of the M13 subfamily of metalloproteases. Whereas NEP is involved in the metabolism of several bioactive peptides by plasma membranes of various cells, the enzymic properties and physiological functions of NEP2 are unknown. Here we characterize the cell-expression modalities and enzymic specificity of two alternatively spliced isoforms of NEP2 in Chinese hamster ovary and AtT20 cells. In the two cell lines, both isoforms are type II glycoproteins inserted in the endoplasmic reticulum as inactive precursors. Maturation detected by Western-blot analysis of glycosidase digests was cell-specific and more efficient in the endocrine cell line. The enzymic activity of both isoforms semi-purified from AtT20 cells reveals comparable specificities in terms of model substrates, pH optima and inhibitory patterns. NEP2 activity was compared with that of NEP regarding potencies of transition-state inhibitors, modes of hydrolysis, maximal hydrolysis rates and apparent affinities of bioactive peptides. Although all transition-state inhibitors of NEP inhibited NEP2 activity, albeit with different potencies, and many peptides were cleaved at the same amide bond by both peptidases, differences could be observed, i.e. in the hydrolysis of gonadotropin-releasing hormone and cholecystokinin, which occurred at different sites and more efficiently in the case of NEP2. Differences in cleavage of bioactive peptides, in cell-trafficking patterns and in tissue distribution indicate that NEP and NEP2 play distinct physiological roles in spite of their high degree of sequence identity.

Omapatrilat provides long-term control of hypertension: a randomized trial of treatment withdrawal

Omapatrilat simultaneously inhibits neutral endopeptidase and angiotensin-converting enzyme, increasing levels of vasodilatory peptides while decreasing production of angiotensin II. This study evaluated the clinical effects of withdrawal of omapatrilat after a patient's hypertension had been controlled (seated diastolic blood pressure <90 mm Hg) on omapatrilat for at least 6 months, with or without adjunctive antihypertensive medications. This double-blind study randomized 83 patients to receive either their established omapatrilat dose or placebo for 8 weeks; any concomitant antihypertensive medications were kept constant. Patients continuing on omapatrilat had no change in blood pressure. Patients whose chronic omapatrilat treatment was replaced by placebo had clinically important increases in both systolic (+16.5 mm Hg) and diastolic ((+9.6 mm Hg) blood pressures (both p<0.001). An increase in blood pressure was also seen in patients who were taking adjunctive antihypertensive medications prior to withdrawal of omapatrilat. This study demonstrates that when compared to withdrawal placebo, omapatrilat maintains clinically and statistically significant blood pressure reductions.

Vasopeptidase inhibitors, neutral endopeptidase inhibitors, and dual inhibitors of angiotensin-converting enzyme and neutral endopeptidase

Vasopeptidase inhibitors represent a new class of cardiovascular drugs. They function as a combined angiotensin-converting enzyme (ACE) inhibitor and neutral endopeptidase (NEP) inhibitor, the latter of which potentiates the actions of atrial natriuretic peptide (ANP) by minimizing its degradation in the circulation. The consequence of such dual inhibition is a synergistic reduction of vasoconstriction and enhancement of vasodilation, thereby serving to more effectively reduce blood pressure. Furthermore, inhibition of the renin-angiotensin-aldosterone system (RAAS) prevents physiologic compensatory responses in vivo seen with NEP inhibition alone. Vasopeptidase inhibitors have also shown to potentiate bradykinin and adrenomedullin, which additionally contribute to cardiovascular regulation. The most extensively researched and promising agents within the class of VP inhibitors is omapatrilat, a mercaptoacyl derivative of a bicyclic thiazepinone dipeptide. It is a single molecule with equal potency and affinity for ACE and NEP inhibition. Although ACE inhibition tends to more selectively benefit high-renin models of hypertension, vasopeptidase inhibition has been shown to be equally efficacious in low-, normal-, and high-renin models. Contrary to NEP inhibition alone, omapatrilat has also demonstrated the ability to significantly reduce blood pressure in spontaneously hypertensive rats, the equivalent of essential hypertension in humans. Studies also suggest that omapatrilat has cardioprotective properties, especially in the setting of congestive heart failure. More specifically, animal models have demonstrated omapatrilat to be more effective than ACE inhibition alone in remodeling the heart and improving its contractile function. Human studies have documented the efficacy of omapatrilat in the treatment of both hypertension and, to a lesser extent, heart failure. Safety concerns (specifically angioedema) are currently being addressed before the widespread utilization of this promising new agent.

Practical implications of current natriuretic peptide research

Since the original discovery of atrial natriuretic peptide (ANP) nearly 20 years ago and the subsequent realisation of the existence of a family of natriuretic peptides, there has been considerable progress in the elucidation of the physiological and pathophysiological significance of these peptides. This review has examined two potentially important practical aspects arising from natriuretic peptide research - the significance of measurement of plasma levels of ANP and of brain natriuretic peptide BNP for cardiovascular disease and the therapeutic potential of targeting the natriuretic peptide system. Several situations where the measurement of plasma ANP and BNP may be of benefit in the overall assessment and prognosis of cardiac disease have been discussed. The measurement of plasma levels of these peptides appears to have limited value as a specific diagnostic tool and is unlikely to replace well-established procedures to assess cardiac function. Nevertheless, given the strong negative predictive value, the value of the measurement of plasma natriuretic peptides particularly BNPs, in people with suspected heart disease, rests on the evidence that a normal value indicates a low risk of cardiac impairment. Moreover, a consistently elevated plasma level of BNP after myocardial infarction is associated with a distinctly poor prognosis. In turn, this may help to select those with high plasma levels for subsequent detailed investigation of cardiac dysfunction. This may be an important option, especially where the facilities for the more invasive cardiological procedures are not available. Intriguingly, recent research also suggests the possibility that plasma levels of natriuretic peptides may have an important role in guiding more effective therapy for heart failure. The potent cardiovascular and renal effects of ANP and BNP provide an important therapeutic potential for hypertension and for conditions associated with volume overload. A number of approaches which have been used to enhance endogenous activity of these peptides have been highlighted. The use of the native peptides ANP and BNP may well be valuable in some circumstances, such as in critically ill individuals with congestive heart failure or renal failure. However, the limitations of the use of peptides, especially for long-term treatment, are obvious. In view of this, considerable effort has been devoted to the development of orally active agents to enhance endogenous natriuretic peptides by inhibition of breakdown by neutral endopeptidase. This research has led to the development of vasopeptidase inhibitors - dual inhibitors of both endopeptidase and angiotensin-converting enzyme - to enhance endogenous natriuretic peptide function on a background of reduced angiotensin II activity. The broad spectrum of action and the potentially important target-organ protection of these inhibitors offer potential benefits which may well go beyond existing treatment of hypertension and of conditions associated with overt volume overload.

A double-blind, placebo-controlled study to assess tolerability, pharmacokinetics and preliminary pharmacodynamics of single escalating doses of Z13752A, a novel dual inhibitor of the metalloproteases ACE and NEP, in healthy volunteers

AIMS

The objective of this study was to evaluate the tolerability of a novel dual ACE-NEP inhibitor, Z13752A, after the oral administration of rising single doses in healthy volunteers. This study was also a preliminarily investigation of Z13752A pharmacodynamics (PD) and pharmacokinetics (PK).

METHODS

In this randomized, placebo-controlled, sequential study, two alternating panels of eight healthy male volunteers each (six subjects receiving the active treatment + two subjects receiving placebo) were treated with increasing oral doses of Z13752A: 10, 50, 200, and 600 mg were given to panel I and 20, 100, 400 and 800 mg were given to panel II. The study was double-blind relative to placebo or active treatment, and was open with respect to the dose levels. The same volunteer received placebo only once.

RESULTS

Single oral doses of Z13752A, as high as 800 mg, were well tolerated. Only six mild-to-moderate adverse events mainly headache, were reported and appeared to be of little clinical relevance. After administration of 200, 400, 600 and 800 mg of Z13752A, a nonsignificant fall in diastolic blood pressure was detected, in both the standing and supine position. After single oral doses of Z13752A, ACE inhibition appeared to be significant at all the doses tested, linearly correlated with the dose and was almost complete at doses > or = 100-200 mg. NEP inhibition was indicated by elevation of ANP and cGMP plasma concentrations in almost all subjects. In the 200-800 mg dose range, Z13752A produced a 50-100% increase of plasma cGMP levels and a 50-80% elevation in urinary cGMP concentrations. Detectable plasma levels of Z13752A were found in all the treated subjects. Z13752A was well and rapidly absorbed, with peak concentrations reached approximately 2.5 h after administration. The mean apparent elimination half-life from plasma was approximately 12 h. The pharmacokinetics of Z13752A after single oral doses were characterized by low intersubject variability and appeared to be dose-independent.

CONCLUSIONS

Z13752A showed a good single dose tolerability profile at doses up to 800 mg. The pharmacokinetic data indicate that Z13752A administered orally is rapidly absorbed and available to the systemic circulation in humans. The relatively slow clearance indicates that a once-a-day dose regimen could be considered for Z13752A.

In vitro and in vivo inhibition of the 2 active sites of ACE by omapatrilat, a vasopeptidase inhibitor

The vasopeptidase inhibitor omapatrilat inhibits both neutral endopeptidase and angiotensin-converting enzyme (ACE). The in vitro and in vivo inhibitory potency of omapatrilat and the specific ACE inhibitor fosinopril toward the 2 active sites of ACE (called N- and C-domains) was investigated with the use of 3 substrates: angiotensin I, which is equally cleaved by the 2 ACE domains; hippuryl-histidyl-leucine, specific synthetic substrate of the C-domain in high- salt conditions; and a newly synthesized specific substrate of the N-domain designed by acetylating the lysine residue of AcSDKP. In vitro, omapatrilat was 5 times more potent than fosinoprilat in inhibiting angiotensin I hydrolysis. Omapatrilat inhibited similarly both N- and C-domain hydrolysis, whereas fosinoprilat was slightly more specific for the N-domain. The in vivo selective inhibitory potency of single oral doses of 10 mg omapatrilat and 20 mg fosinopril were investigated in a double-blind, placebo-controlled, cross-over study in 9 mildly sodium-depleted normotensive subjects. In accordance with the in vitro results, fosinopril appeared to be more specific for the N-domain than the C-domain in vivo, since plasma and urine AcSDKP concentrations were significantly higher than those observed with omapatrilat. This study shows that it is possible to assess separately in vitro and in vivo the selectivity of ACE or ACE/neutral endopeptidase inhibitors. A differential selectivity may explain some peculiar properties observed with some ACE inhibitors.

Antihypertensive effects of fasidotril, a dual inhibitor of neprilysin and angiotensin-converting enzyme, in rats and humans

The aim of this study was to assess the antihypertensive activity of fasidotril, a dual inhibitor of neprilysin (NEP) and angiotensin I-converting enzyme (ACE), in various models of hypertension in rats (spontaneously hypertensive rats [SHR]; renovascular Goldblatt 2-kidney, 1-clip rats; and deoxycorticosterone acetate [DOCA]-salt hypertensive rats) and in patients with mild-to-moderate essential hypertension. Fasidotril treatment (100 mg/kg PO twice daily for 3 weeks) resulted in a progressive and sustained decrease in systolic blood pressure (-20 to -30 mm Hg) in SHR and Goldblatt rats compared with vehicle-treated rats and prevented the progressive rise in blood pressure in DOCA-salt hypertensive rats. After a 4-week placebo run-in period, 57 patients with essential hypertension were included in a randomized double-blind, placebo-controlled, parallel-group study and received orally either fasidotril (100 mg twice daily) or placebo for 6 weeks. Blood pressure was measured during the 6 hours after the first intake and then at trough (12 hours after the last intake) on days 7, 28, and 42. The first dose of fasidotril had no significant effect on blood pressure. After 42 days, compared with placebo, fasidotril lowered supine systolic and diastolic blood pressures by 7.4/5.4 mm Hg and standing blood pressure by 7.6/6.8 mm Hg. Fasidotril, a dual NEP/ACE inhibitor, was an effective oral antihypertensive agent during chronic treatment in high-renin renovascular rats, normal-renin SHR, and low-renin DOCA-salt hypertensive rats and in patients with essential hypertension.

Effects of a neutral endoprotease enzyme inhibitor, thiorphan, on hemodynamics and renal excretory function in four models of experimental hypertension

Thiorphan, a neutral endoprotease (NEP) enzyme inhibitor, has been shown to enhance the effects of atrial natriuretic peptide (ANP) in vivo. In this study, we examined the effects of an intravenous (iv) infusion of thiorphan on cardiovascular hemodynamics and excretion of urine volume (UV), sodium (U(Na)V) and potassium (UKV) in four different models of experimental hypertension, namely: 1) SHR, 2) two-kidney, one clip (2K1C),3) one-kidney, 1 clip (1K1C) and. 4) 70% reduced renal mass-salt (RRM-S) hypertensive rats. SHR has normal plasma renin activity, 2K1C is renin dependent, and 1K1C and RRM-S are low renin volume dependent models of hypertension. Rats were divided into experimental and control groups. Under inactin (120 mg/kg, body weight) anesthesia, rats were instrumented to record blood pressure and dP/dt (Millar catheter) and urine was collected through a suprapubic urinary bladder catheter. Experimental animals received an iv infusion of thiorphan, 0.5 mg/kg/min for 120 minutes. Control animals received vehicle only. In some animals, vascular smooth muscle cell membrane potentials (Em) was measured in vivo. In another series of experiments, using the identical protocol, cardiac output was recorded. The thiorphan infusion produced a similar progressive decrease in blood pressure in all models of hypertension. Cardiac output did not change relative to vehicle infused control animals. Thus pressure decreased because of a decrease in total peripheral resistance. The contractility index (dP/dt/P, where P = left ventricular pressure) did not change but vascular smooth muscle cells in tail arteries hyperpolarized in all four models. In spite of a significant decrease in blood pressure, thiorphan infusion either increased or produced no change in urinary volume (UV) and sodium (U(Na)V) excretion. These data show that thiorphan, an NEP inhibitor, decreases the blood pressure of hypertensive rats due to a decrease in total peripheral resistance, perhaps by hyperpolarizing vascular smooth muscle cells. These effects are independent of the mechanism of the hypertension. Increased UV and U(Na)V in the face of decreased pressure suggests a direct renal effect.

Effect on prolonged inhibition of neutral endopeptidase on cardiac hypertrophy in rats with myocardial infarction

Myocardial infarction was induced by rats by ligation of the left coronary artery. Treatment with TM1, a prodrug of SQ 28,603, an inhibitor of neutral endopeptidase (NEP, EC 3.4.24.11), was started 18-20 hours after ligation and was continued for 4 weeks (100 mg/kg, orally, twice daily). Morphological and biochemical parameters were assessed at the endo of therapy. The treatment resulted in a significant reduction of heart hypertrophy, which was restricted to the parts of myocardium hemodynamically upstream of the infarcted left ventricle. The weights of the right ventricle and atria were reduced by 15-20%, whereas the treatment had no effect on the left ventricle and septum weights. Treatment led to an almost complete inhibition of plasma NEP activity and to a slight decrease (-14%, p < 0.05) in plasma ACE activity. Plasma ANF level increased 3.8-fold after ligation, and treatment resulted in a slight ( + 29%) and nonsignificant additional increase in the ANF level. The amount of hydroxyproline in the right ventricle was enhanced by + 207% in control ligated rats and by +140% (NS) in treated rats. These data indicated that prolonged NEP inhibition exerts a favorable effect in heart failure by reducing the development of right ventricular and atrial hypertrophy. These effects may result from an improvement in hemodynamic conditions, leading to a reduction in cardiac preload.

Renal and depressor activities of inhibitors of neutral endopeptidase and angiotensin converting enzyme in monkeys infused with angiotensin II

In a previous study, the depressor activity of combined selective inhibitors of neutral endopeptidase EC 3.4.24.11 (NEP) and angiotensin-converting enzyme (ACE) depended on the level of ACE inhibition, whereas the renal responses were determined by NEP inhibition. Our study confirmed that a mixed NEP/ACE inhibitor BMS-182657 ([S-(R*,R*)]-2,3,4,5-tetrahydro-3-[(2-mercapto-1-oxo-3- phenylpropyl)amino]-2-oxo-1H-benzazepine-1-acetic acid) reduced mean arterial pressure (MAP) when renin release was reduced by a sodium load, suggesting that the depressor response did not require suppression of endogenous angiotensin II generation. Furthermore, a pressor dose of 30 ng/min of angiotensin II was required to block the depressor response to BMS-182657 in the presence or absence of exogenous human atrial natriuretic peptide (hANP 99-126). Thirty ng/min of angiotensin II also significantly enhanced the natriuresis induced by hANP 99-126 after BMS-182657 administration. In contrast, a nonpressor dose of angiotensin II (3 ng/min) reduced basal sodium excretion and the natriuretic responses to exogenous hANP 99-126 in the presence or absence of BMS-182657. The potentiation of the urinary ANP and cyclic guanosine monophosphate (cGMP) responses to hANP 99-126 by BMS-182657 was similar for all doses of angiotensin II; therefore angiotensin did not alter the effects of BMS-182657 on ANP metabolism or cGMP accumulation in the kidney. In summary, the renal responses to mixed metalloprotease inhibitors were apparently mediated by ANP potentiation and were modulated by angiotensin II. The depressor activity depended on ACE inhibition but was not mediated solely by reductions in endogenous angiotensin II levels.

In vivo pharmacology of dual neutral endopeptidase/angiotensin-converting enzyme inhibitors

The natriuretic and depressor responses to novel dual inhibitors of neutral endopeptidase (NEP) EC 3.4.24.11 and angiotensin-converting enzyme (ACE) were used to assess their activity in conscious cynomolgus monkeys. A survey of mercaptopropanoyl inhibitors revealed that compounds containing alanylproline or certain surrogates reduced blood pressure and increased sodium excretion, indicating a desirable profile of in vivo activity. Additional compound evaluation required specific in vivo assays for NEP and ACE inhibition. Accordingly, the potency of novel inhibitors against NEP and ACE were determined in conscious monkeys by the potentiation of the natriuretic activity of exogenous human atrial natriuretic peptide and inhibition of the pressor response to angiotensin I, respectively. This strategy led to the discovery that optimal in vivo activity was achieved when the mercaptopropanoyl group was replaced with mercaptoacetyl and the C-terminal alanylproline was replaced with conformationally constrained dipeptidomimetics. This work culminated in the identification of BMS-182657 as a prototypic dual NEP/ACE inhibitor with a highly desirable profile of in vivo pharmacology.

Acute hemodynamic effects of combined inhibition of neutral endopeptidase and angiotensin converting enzyme in spontaneously hypertensive rats

Neutral endopeptidase inhibitors (NEPI) potentiate the hypotensive effect of converting enzyme inhibitors (CEI) in conscious spontaneously hypertensive rats (SHR) but the mechanism of this potentiation remains unknown. The present study assesses the hemodynamic effects of a CEI (enalaprilat 1 mg/kg; n = 9), a NEPI (retrothiorphan 25 mg/kg + 25 mg/kg/h; n = 9) and the combination (CEI+NEPI; n = 9) versus a control group (n = 9) in anesthetized spontaneously hypertensive rats. CEI alone induced a significant hypotensive effect due to a decrease (-35.1%) in total peripheral resistance (TPR), with no significant increase in cardiac output (CO). NEPI alone had a slight hypotensive effect due to a small decrease in CO. CEI+NEPI decreased the mean arterial pressure to the same extent (-26.7%) as the CEI-induced hypotensive effect, decreased TPR (-44.4%) and induced an increase in CO (+38.2%) with an increase in heart rate. In summary, NEPI combined with CEI induces large decreases in blood pressure and in TPR which do not significantly differ from the CEI-induced effects. It also induces increases in heart rate and in cardiac output in anesthetized SHR.

Dual inhibition of angiotensin-converting enzyme and neutral endopeptidase by the orally active inhibitor mixanpril: a potential therapeutic approach in hypertension

In the treatment of cardiovascular disease, it could be of therapeutic interest to associate the hypotensive effects due to the inhibition of angiotensin II formation with the diuretic and natriuretic responses induced by the protection of the endogenous atrial natriuretic peptide (ANP). Investigation of this hypothesis requires an orally active compound able to simultaneously inhibit angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP), which is involved in renal ANP metabolism. Such compounds have been rationally designed by taking into account the structural characteristics of the active site of both peptidases. Among them, RB 105, N-[(2S,3R)-2-mercaptomethyl-1-oxo-3-phenylbutyl]-(S)-alanine, inhibited NEP and ACE with Ki values of 1.7 +/- 0.3 nM and 4.2 +/- 0.5 nM, respectively. Intravenous infusion of RB 105 in conscious spontaneously hypertensive rats prevented the pressor response to exogenous angiotensin I and potentiated the natriuretic response to ANP. Infusion of RB 105, at 2.5, 5, 10, 25, and 50 mg/kg per hr decreased blood pressure dose-dependently in conscious catheterized spontaneously hypertensive rats and increased diuresis and natriuresis. Infusion of RB 105 as a bolus of 25 mg/kg followed by 25 mg/kg per hr similarly decreased blood pressure and increased natriuresis in three different models of hypertension (renovascular, deoxycorticosterone acetate-salt, and spontaneously hypertensive rats). Mixanpril, a lipophilic prodrug of RB 105 (ED50 values when given orally to mice, 0.7 mg/kg for NEP; 7 mg/kg for ACE), elicited dose-dependent hypotensive effects of long duration in spontaneously hypertensive rats after oral administration [-37 mmHg for 50 mg/kg twice a day (1 mmHg = 133 Pa) and is therefore the first dual NEP/ACE inhibitor potentially useful for clinical investigations.

Inhibitors of enkephalin-degrading enzymes as potential therapeutic agents

A limited number of enzymes such as membrane metalloendopeptidase (enkephalinase) and angiotensin converting enzyme appear to be involved in deactivation and modulation of circulatory regulatory peptides. Peptides such as the enkephalins are also involved in a large number of physiological processes. This multiplicity of physiological roles has made it difficult to establish the therapeutic role of enkephalin-degrading enzyme inhibitors. Other factors such as difficulty in quantification and thus measurement of processes involved in pain and mental illness have also hindered the process of establishing any therapeutic role of enkephalin-degrading enzyme inhibitors in these conditions. However, they have proved to be useful pharmacological 'tools'. The most likely therapeutic role at present appears to be in the treatment of cardiovascular disorders. As a 'profile' of pharmacological actions of enkephalin-degrading enzymes emerges, it is becoming apparent that bioavailability rather than a high degree of specificity or inhibitory potency may be the most important factor. This may be used to an advantage in future developments by the use of less specific or combined inhibitors in the form of prodrugs, designed to be active at specific sites such as the central nervous system.

Atrial natriuretic peptide. An overview of clinical pharmacology and pharmacokinetics

The atrial natriuretic peptide (ANP) is part of a new family of cardiac hormones regulating water and salt homeostasis. Besides acting as a blood pressure-lowering agent, it also exerts potent natriuretic and diuretic effects. ANP can be considered an endogenous antagonist of the reninangiotensin-aldosterone system and the antidiuretic hormone. One of the roles of ANP is to protect the body against fluid overload: it decreases intravascular fluid volume, which in turn diminishes cardiac secretion of ANP. The pharmacokinetic parameters of ANP reported in the literature vary widely. In general, ANP rapidly disappears from plasma with a high total body clearance. This is in agreement with the short-lived effects of the hormone. The actions of ANP have led to efforts to use this peptide hormone in the treatment of various cardiovascular disorders such as hypertension and congestive heart failure. Intravenous ANP administration indeed resulted in beneficial effects in these disorders. However, the peptide nature of ANP and its rapid elimination from the circulation limit its suitability as a drug. More promising is the development of long-acting ANP analogues and inhibitors of ANP degradation. Proper understanding of ANP pharmacokinetics is essential for the clinical use of these pharmacological agents.

Inhibition by converting enzyme inhibitors of pig kidney aminopeptidase P

Several inhibitors of angiotensin converting enzyme were also found to inhibit aminopeptidase P, whereas inhibitors of other mammalian aminopeptidases were ineffective. Aminopeptidase P purified from pig kidney cortex was found to contain one atom of zinc per polypeptide chain, confirming its metalloenzyme nature. The concentrations of converting enzyme inhibitors required to cause 50% inhibition (I50) of aminopeptidase P were in the low micromolar range. The most potent converting enzyme inhibitors toward aminopeptidase P were the carboxylalkyl compounds, cilazaprilat, enalaprilat, and ramiprilat (I50 values of 3-12 microM). The sulfhydryl compounds captopril (I50 110 microM) and YS980 (I50 20 microM) were slightly less potent at inhibiting aminopeptidase P. In contrast, the carboxylalkyl compounds benazeprilat, lisinopril, and pentoprilat; the sulfhydryl compound rentiapril; and the phosphoryl compounds ceranopril and fosinoprilat had no inhibitory effect against aminopeptidase P. This compares with I50 values in the 1-6 nM range for these inhibitors with angiotensin converting enzyme. Inhibition of aminopeptidase P may account for some of the effects or side effects noted with the clinical use of converting enzyme inhibitors. These results may provide the basis for the design of more selective inhibitors of angiotensin converting enzyme or mixed inhibitors of aminopeptidase P and angiotensin converting enzyme, or both.