New hormonal blockade strategies in cardiovascular disease

Combined inhibition of neutral endopeptidase with angiotensin converting enzyme or endothelin converting enzyme in experimental diabetes

OBJECTIVE

The effects of combined inhibition of neutral endopeptidase (NEP) with either angiotensin-converting enzyme (ACE), or endothelin-converting enzyme (ECE) on blood pressure, urinary albumin excretion and heart weight were explored in experimental diabetes.

DESIGN

Streptozotocin-induced diabetic Sprague-Dawley rats were treated with vehicle, the NEP/ACE inhibitor S 21402, the NEP/ECE inhibitor CGS 26303, the NEP inhibitor SCH 42495, the ACE inhibitor captopril or the endothelin receptor antagonist bosentan for 4 weeks.

METHODS

Blood pressure was measured by tail-cuff method and radiotelemetry. Albuminuria, plasma renin activity and plasma atrial natriuretic peptide (ANP) were determined by radioimmunoassay. NEP binding was assessed by in vitro quantitative autoradiography. Metabolic and biochemistry parameters including food intake, 24-h urine volume, plasma glucose, glycated hemoglobin, glomerular filtration rate (GFR) and urinary sodium excretion were also determined.

RESULTS

Mean blood pressure over the 4-week study period after commencement of treatment was reduced to a similar extent by a range of treatments including the ACE inhibitor, NEP/ACE inhibitor, endothelin receptor antagonist, NEP/ECE inhibitor, but not the NEP inhibitor, compared with vehicle-treated diabetic rats. Heart to body weight ratio in diabetic rats was only reduced by the NEP/ACE and the NEP/ECE inhibitor. Increased albuminuria in diabetic rats (1.1 times/divided by 1.2 mg/day) was reduced by the NEP/ACE (0.6 times/divided by 1.2 mg/day) and the NEP/ECE inhibitors (0.4 times/divided by 1.2 mg/day). Renal NEP was reduced by the NEP/ACE inhibitor (35 +/- 4%) or NEP/ECE inhibitor (38 +/- 4%) as well as by the pure NEP inhibitor (27 +/- 4%) compared with the untreated diabetic group. Other abnormal metabolic and biochemical parameters in diabetic rats were not influenced by any drug treatment.

CONCLUSIONS

Combined inhibition of NEP/ACE or NEP/ECE confers beneficial effects on blood pressure, albuminuria and heart to body weight ratio in experimental diabetes.

Evidence for cardioprotective, renoprotective, and vasculoprotective effects of vasopeptidase inhibitors in disease

In both the natriuretic peptide and renin-angiotensin systems, peptidases play an important role in the inactivation or activation of the system. Angiotensin-converting enzyme is responsible for the conversion of angiotensin I to angiotensin II, while neutral endopeptidase is one of the pathways involved in the degradation of the natriuretic peptides. The vasopeptidase inhibitors, which simultaneously inhibit neutral endopeptidase and angiotensin-converting enzyme, appear to offer distinct therapeutic advantages in treating hypertension, heart failure, and endothelial dysfunction.

Oxidative stress and vascular smooth muscle cell function in liver disease

Reactive oxygen species and reactive nitroxy species are now being recognized as regulatory molecules in signaling pathways influencing contractile and noncontractile functions of healthy vascular smooth muscle cells. In liver disease, oxidative stress is a systemic phenomenon, whose extent correlates with the severity of disease. A role for oxidative stress in the development of the hyperdynamic circulation in portal hypertension has been proposed. Evaluation of the limited available data indicates that it is premature to conclude that oxidative stress per se impacts on vascular smooth muscle cell function in liver disease.

Beneficial renal and cardiac effects of vasopeptidase inhibition with S21402 in heart failure

S21402 is a vasopeptidase inhibitor that simultaneously inhibits neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE). This study determined whether chronic treatment with S21402 produced different effects on sodium and water excretion, hormonal parameters, and cardiovascular structure compared with selective inhibition of ACE and NEP in a rat model of myocardial infarction-induced congestive heart failure (CHF). CHF rats received the vasopeptidase inhibitor (S21402, 100 mg. kg(-1). d(-1)), an ACE inhibitor (captopril, 50 mg. kg(-1). d(-1)), a NEP inhibitor (SCH42495, 60 mg. kg(-1). d(-1)), or vehicle for 4 weeks. S21402 alone caused a diuresis and natriuresis (P<0.01) in CHF. After 4 weeks, blood pressure was lowered by captopril but not other treatments (P<0.01). Both S21402 and captopril increased plasma renin activity (P<0.01), all treatment lowered plasma aldosterone (P<0.05) and plasma natriuretic peptide levels were unchanged. In the kidney, S21402 inhibited NEP and ACE (P<0.01), SCH42495 inhibited NEP (P<0.01), and captopril inhibited ACE (P<0.01). Heart mass was reduced by all active treatments; captopril reduced left ventricular mass (P<0.01), SCH42495 reduced right ventricular mass (P<0.01), and S21402 decreased left (P<0.05) and right ventricular mass (P<0.01), atrial mass (P<0.05), and lung mass (P<0.01). In CHF, vasopeptidase inhibition with S21402 produces effects that differ from those of selective NEP or ACE inhibition. S21402 improved sodium and water excretion, reduced pulmonary congestion, and attenuated both right and left ventricular remodeling. These effects, which occurred in the absence of any hypotensive action, suggest that S21402 may offer several advantages over ACE inhibition alone in the treatment of heart failure.

Antihypertensive and antihypertrophic effects of omapatrilat in SHR

Vasopeptidase inhibitors, such as omapatrilat are single molecules that simultaneously inhibit neutral endopeptidase (NEP) and angiotensin converting enzyme (ACE). In normotensive rats, a single dose of oral omapatrilat (10 mg/kg) and 1 mg/kg inhibited plasma ACE (P < .01) for 24 h and increased plasma renin activity for 8 h (P < .01). In vitro autoradiography using the specific NEP inhibitor radioligand 125I-RB104 and the specific ACE inhibitor radioligand 125I-MK351A showed omapatrilat (10 mg/kg) caused rapid and potent inhibition of renal NEP and ACE, respectively, for 24 h (P < .01). In spontaneously hypertensive rats, 10 days of oral omapatrilat (40 mg/kg/day) reduced blood pressure (vehicle 237 +/- 4 mm Hg; omapatrilat, 10 mg/kg, 212 +/- 4 mm Hg; omapatrilat 40 mg/kg, 197 +/- 4 mm Hg, P < .01) in a dose-dependent manner (10 v 40 mg/kg, P < .01). Left ventricular hypertrophy was significantly reduced by high-dose omapatrilat (vehicle 2.76 +/- 0.03 mg/g body weight; omapatrilat, 10 mg/kg, 2.71 +/- 0.02 mg/g; omapatrilat 40 mg/kg, 2.55 +/- 0.02 mg/g, P < .01) and omapatrilat also increased kidney weight compared to vehicle (both doses, P < .01). Omapatrilat caused significant inhibition of plasma ACE and increased plasma renin activity (both doses, P < .01), and in vitro autoradiographic studies indicated sustained inhibition of renal ACE and NEP (both doses, P < .01). Omapatrilat is a potent vasopeptidase inhibitor, and its antihypertensive effects are associated with inhibition of NEP and ACE at the tissue level and beneficial effects on cardiovascular structure. Relating the degree of tissue inhibition to physiologic responses may allow further definition of the role of local renin angiotensin and natriuretic peptide systems in the beneficial effects of vasopeptidase inhibitors.

Reversal of cardiac hypertrophy and fibrosis by S21402, a dual inhibitor of neutral endopeptidase and angiotensin converting enzyme in SHRs

OBJECTIVE

The major advantage of dual inhibitors of neutral endopeptidase (NEP) and angiotensin converting enzyme (ACE) is their ability to lower blood pressure irrespective of renin or volume status. The aim of this study was to determine whether dual NEP/ACE inhibition produces different effects on cardiovascular structure and fibrosis, hormonal parameters and inhibition of tissue enzymes compared with selective inhibition of ACE and NEP in the spontaneously hypertensive rat (SHR).

METHODS

Male SHRs received the dual NEP/ACE inhibitor (S21402, 100 mg/kg per day), the ACE inhibitor (captopril, 50 mg/kg per day), the NEP inhibitor (SCH42495, 60 mg/kg per day) or vehicle for 2 weeks.

RESULTS

S21402 produced equivalent blood pressure lowering effects to captopril (vehicle, 220 +/- 1 mmHg; S21402, 189 +/- 2 mmHg; captopril, 187 +/- 3 mmHg), but was a more effective antihypertensive agent than SCH42495 (214 +/- 2 mmHg, P< 0.01). All treatments reduced left ventricular mass (P< 0.05) and cardiac fibrosis (P< 0.01). S21402 inhibited renal NEP and ACE (P< 0.01), SCH42495 inhibited renal NEP (P < 0.01), and captopril inhibited renal ACE (P< 0.01). Captopril and S21402 increased plasma renin activity (P< 0.05), but the rise with S21402 was attenuated compared with that caused by captopril (P< 0.01). All treatments reduced plasma aldosterone levels (P< 0.01), and NEP inhibition with SCH42495 and S21402 increased plasma atrial natriuretic peptide (ANP; P< 0.05).

CONCLUSIONS

These results indicate that selective NEP inhibition has major benefits in the regression of cardiac hypertrophy and reduction of fibrosis but has limited antihypertensive effects. The dual NEP/ACE inhibitor S21402 offered no advantage over the selective ACE inhibitor in terms of blood pressure reduction, or attenuation of cardiac hypertrophy and fibrosis, but did increase plasma ANP and blunted the reactive rise in renin with ACE inhibition. Further studies are needed to determine whether more complete blockade of the renin-angiotensin system with dual NEP/ACE inhibition results in additional benefits in terms of morbidity and mortality in cardiovascular disease.

Vasopressin receptor antagonism--a therapeutic option in heart failure and hypertension

The precise role of vasopressin in the pathophysiology of cardiovascular disease is controversial, but this peptide hormone is important for several reasons. Firstly, circulating concentrations of vasopressin are elevated in heart failure and some forms of hypertension. Secondly, there is evidence that vasopressin is synthesized not only in the hypophysial-pituitary axis but also in peripheral tissues including the heart where it acts as a paracrine hormone. Thirdly, vasopressin has vasoconstrictor, mitogenic, hyperplastic and renal fluid retaining properties which, by analogy with angiotensin II, may have deleterious effects when present in chronic excess. Finally, the availability of orally active non-peptide vasopressin receptor antagonists allows vasopressin receptor antagonism to be considered as a therapeutic option in cardiovascular disease.

Dual inhibition of neutral endopeptidase and angiotensin-converting enzyme in rats with hypertension and diabetes mellitus

It has been suggested that combined inhibition of angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP) may lower blood pressure more effectively than either treatment alone, independent of the degree of salt and volume status or the activity of the renin-angiotensin system. The effects of NEP inhibition in hypertension associated with diabetes mellitus are largely unknown. We therefore compared ACE inhibition, NEP inhibition, and dual NEP/ACE inhibition in diabetic hypertensive rats. Spontaneously hypertensive rats (SHR) aged 9 to 10 weeks were injected with either streptozotocin (45 mg/kg) or citrate buffer and randomized to receive either the ACE inhibitor captopril (25 mg/kg BID), the NEP inhibitor SCH 42495 (30 mg/kg BID), the dual NEP/ACE inhibitor S 21402 (25 or 50 mg/kg BID), or vehicle by gavage for 4 weeks. A group of diabetic SHR was also allocated to receive the combination of SCH 42495 (30 mg/kg BID) and captopril (25 mg/kg BID). The degree of renal NEP inhibition was determined by autoradiography, and plasma renin activity (PRA) was determined by radioimmunoassay. In diabetic SHR, the dual NEP/ACE inhibitor (50 mg/kg BID), as well as the combination of the NEP inhibitor and the ACE inhibitor, reduced systolic blood pressure more effectively than the ACE inhibitor (P<0.001) or the NEP inhibitor (P<0.001) alone. In nondiabetic SHR, the dual NEP/ACE inhibitor and the ACE inhibitor were equally effective, while the NEP inhibitor had only slight blood pressure lowering effects. Relative heart weight decreased in parallel to the changes in blood pressure. Renal NEP was clearly inhibited (70% to 92%; P<0.001) by both the NEP inhibitor and the dual NEP/ACE inhibitor. Both the ACE inhibitor and the dual NEP/ACE inhibitor increased PRA, but the stimulating effect of dual NEP/ACE inhibition on PRA was less than that observed with ACE inhibition alone (P<0.05). Albuminuria in diabetic SHR was lower during treatment with both the dual NEP/ACE inhibitor (50 mg/kg BID) and the combination of NEP inhibition and ACE inhibition compared with vehicle treatment (P<0.05). In conclusion, the present study shows that hypertension in SHR with streptozotocin-induced diabetes is modulated by natriuretic peptides and thus is sensitive to NEP inhibition. The increased efficacy of dual NEP/ACE inhibition on blood pressure in diabetic SHR, compared with ACE or NEP inhibition alone, suggests that this therapeutic approach may prove beneficial in the treatment of hypertension associated with diabetes mellitus and other forms of volume-dependent hypertension.