Comparative effects of three different potent renin inhibitors in primates

Hydrogen Sulfide in Pharmacotherapy, Beyond the Hydrogen Sulfide-Donors

Hydrogen sulfide (H₂S) is one of the important biological mediators involved in physiological and pathological processes in mammals. Recently developed H₂S donors show promising effects against several pathological processes in preclinical and early clinical studies. For example, H₂S donors have been found to be effective in the prevention of gastrointestinal ulcers during anti-inflammatory treatment. Notably, there are well-established medicines used for the treatment of a variety of diseases, whose chemical structure contains sulfur moieties and may release H₂S. Hence, the therapeutic effect of these drugs may be partly the result of the release of H₂S occurring during drug metabolism and/or the effect of these drugs on the production of endogenous hydrogen sulfide. In this work, we review data regarding sulfur drugs commonly used in clinical practice that can support the hypothesis about H₂S-dependent pharmacotherapeutic effects of these drugs.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Oral renin inhibitors

Use of drugs that inhibit the renin-angiotensin system is an effective way to intervene in the pathogenesis of cardiovascular and renal disorders. The idea of blocking the renin system at its origin by inhibition of renin has existed for more than 30 years. Renin inhibition suppresses the generation of the active peptide angiotensin II. The first generation of orally active renin inhibitors were never used clinically because of low bioavailability and weak blood-pressure-lowering activity. At present, aliskiren is the first non-peptide orally active renin inhibitor to progress to phase-III clinical trials. It might become the first renin inhibitor with indications for the treatment of hypertension and cardiovascular and renal disorders. Novel compounds with improved oral bioavailability, specificity, and efficacy are now in preclinical development. This Review summarises the development of oral renin inhibitors and their pharmacokinetic and pharmacodynamic properties, with a focus on aliskiren.

Aliskiren, a human renin inhibitor, ameliorates cardiac and renal damage in double-transgenic rats

We tested the hypothesis that the renin inhibitor aliskiren ameliorates organ damage in rats transgenic for human renin and angiotensinogen genes (double transgenic rat [dTGR]). Six-week-old dTGR were matched by albuminuria (2 mg per day) and divided into 5 groups. Untreated dTGR were compared with aliskiren (3 and 0.3 mg/kg per day)-treated and valsartan (Val; 10 and 1 mg/kg per day)-treated rats. Treatment was from week 6 through week 9. At week 6, all groups had elevated systolic blood pressure (BP). Untreated dTGR showed increased BP (202+/-4 mm Hg), serum creatinine, and albuminuria (34+/-5.7 mg per day) at week 7. At week 9, both doses of aliskiren lowered BP (115+/-6 and 139+/-5 mm Hg) and albuminuria (0.4+/-0.1 and 1.6+/-0.6 mg per day) and normalized serum creatinine. Although high-dose Val lowered BP (148+/-4 mm Hg) and albuminuria (2.1+/-0.7 mg per day), low-dose Val reduced BP (182+/-3 mm Hg) and albuminuria (24+/-3.8 mg per day) to a lesser extent. Mortality was 100% in untreated dTGR and 26% in Val (1 mg/kg per day) treated rats, whereas in all other groups, survival was 100%. dTGR treated with low-dose Val had cardiac hypertrophy (4.4+/-0.1 mg/g), increased left ventricular (LV) wall thickness, and diastolic dysfunction. LV atrial natriuretic peptide and beta-myosin heavy chain mRNA, albuminuria, fibrosis, and cell infiltration were also increased. In contrast, both aliskiren doses and the high-dose Val lowered BP to a similar extent and more effectively than low-dose Val. We conclude that in dTGR, equieffective antihypertensive doses of Val or aliskiren attenuated end-organ damage. Thus, renin inhibition compares favorably to angiotensin receptor blockade in reversing organ damage in dTGR.

Renal and systemic effects of continued treatment with renin inhibitor remikiren in hypertensive patients with normal and impaired renal function

BACKGROUND

Remikiren is an orally active renin inhibitor with established antihypertensive efficacy. As a single dose it induces renal vasodilatation, suggesting specific renal actions. Data on the renal effects of continued treatment by renin inhibition are not available, either in subjects with normal, or in subjects with impaired renal function.

METHODS

The effect of 8 days of treatment with remikiren 600 mg o.i.d. on blood pressure, renal haemodynamics, and proteinuria was studied in 14 hypertensive patients with normal or impaired renal function.The study was conducted on an ambulatory in-hospital basis and was designed in a single-blind, longitudinal order.

RESULTS

Remikiren induced a significant peak fall in mean arterial pressure of 11.2+/-0.8%, with corresponding trough values of -6+/-0.8%. This fall was somewhat more pronounced in the patients with renal function impairment (-13.3 vs -9.6%; P<0.01). Glomerular filtration rate remained stable, whereas effective renal plasma flow increased from 301+/-35 to 330+/-36 ml/min/1.73 m(2) (P<0.05). Filtration fraction and renal vascular resistance fell by 10+/-2% and 15+/-2% respectively (both P<0.01). Remikiren induced a cumulated sodium loss of -82+/-22 mmol and a positive potassium balance of 49+/-9 mmol (both P<0.01). During remikiren, proteinuria fell by 27% (range -18 to -38%; P<0.01) in the patients with overt proteinuria at onset (n=6). In the remainder of the patients albuminuria fell by 20% (range -1 to -61%, P<0.05). No side-effects were observed.

CONCLUSIONS

Continued treatment with remikiren induced a sustained fall in blood pressure, renal vasodilatation, negative sodium balance, and a reduction in glomerular protein leakage. These data are consistent with a renoprotective potential of renin inhibition.

Cloning and characterization of marmoset renin: comparison with human renin

The poor interspecies conservation of the renin-angiotensin system prevents the use of nonprimate in vivo models to test renin inhibitors. Thus the small New-World monkey marmoset is used in many instances as a model. However, large differences between the potencies of renin inhibitors as measured in human and marmoset plasma were observed. To understand this phenomenon, we cloned marmoset renin and angiotensinogen. They were highly homologous to their human counterparts, except for a six-residue deletion in the marmoset renin propeptide. Human and marmoset recombinant renins were found in vitro to display comparable activities, suggesting that the observed differences in plasma apparent affinity of inhibitors could be due to different plasma protein binding of the inhibitors.

Renin inhibition: a novel therapy for cardiovascular disease

The renin-angiotensin system (RAS) is a major contributor in the regulation of blood pressure, and pharmacologic manipulation of this system has resulted in a beneficial class of therapeutic agents, which include angiotensin-converting enzyme (ACE) inhibitors. However, ACE inhibitors are not specific for RAS, and in addition, they can affect bradykinin and prostaglandin, which can also cause changes in vascular tone. Under development are renin inhibitors that are specific for angiotensinogen and act at the initial, rate-determining step of the RAS cascade. The various pharmacologic approaches to renin inhibition include specific renin antibodies, synthetic derivatives of the prosegment of renin precursor, pepstatin analogs, and angiotensinogen analogs. The last approach is the most promising for patient therapy. Multiple studies have shown the effectiveness of the renin inhibitors in both primates and human beings. Further research is now directed toward the development of an agent with good oral bioavailability for patient treatment and as a biologic probe for helping to understand the role of the RAS in control of blood pressure and blood volume.

Pharmacokinetics of remikiren, a potent orally active inhibitor of human renin, in rat, dog and primates

1. An hplc method with fluorescence derivatization was developed for the quantification of remikiren in plasma (limit of quantification 2 ng/ml). This was used to determine the pharmacokinetics in various species of primate, in which the compound is a potent inhibitor of renin, as well as in the rat and dog in which it is less active. 2. After intravenous administration the mean residence time was < or = 1.5 h in all species, and the plasma clearance approached the corresponding hepatic blood flows. 3. Studies in the bile-duct cannulated rat and dog demonstrated that the high clearance was due to a combination of rapid metabolism, plus biliary and renal excretion of intact drug. 4. Consistent with the high hepatic clearance, oral bioavailability was low ( < or = 6%) in each species. However, all of the species tested absorbed a small proportion of an oral dose extremely rapidly, to give peak concentrations generally within 5 min of administration. 5. 'Simultaneous' collection of blood samples from the hepatic portal vein and aorta of rat confirmed that shortly after oral dosing the intact drug did cross the liver; however, the later collections contained predominantly more polar metabolites. 6. The rapid absorption of intact remikiren is consistent with the transient blockade of plasma renin activity, previously observed in primates after oral administration. However, the high clearance appears inconsistent with the subsequent prolonged decrease in blood pressure, suggesting that the latter effect is mediated through a 'tissue' compartment.

Distribution of remikiren, a potent orally active inhibitor of human renin, in laboratory animals

1. Whole-body autoradiography was used to compare the distribution of remikiren in the squirrel monkey, in which the compound is a potent inhibitor of renin, with the rat and the guinea-pig in which it is less active. 2. Following intravenous administration, drug-related material was rapidly and extensively taken up by the tissues of all three species. Consistent with rapid biliary elimination, high levels of radioactivity were found in the bile duct/gall bladder/intestinal contents. Of the remaining organs, the kidney consistently showed the highest concentrations of drug-related material. 3. Radio-hplc analysis of the kidney samples demonstrated that the majority of the retained material was present as intact remikiren, even at 24 h after administration. A similar degree of retention by the kidney was also found after oral dosing. 4. Uptake of remikiren by the kidney may act as a reservoir for the drug, resulting in the prolonged duration of pharmacological activity, which, despite the high plasma clearance of the drug, has previously been observed in primates.

Assessment of the role of the renin-angiotensin system in cardiac contractility utilizing the renin inhibitor remikiren

1. The role of the renin-angiotensin system in the regulation of myocardial contractility is still debated. In order to investigate whether renin inhibition affects myocardial contractility and whether this action depends on intracardiac rather than circulating angiotensin II, the regional myocardial effects of systemic (i.v.) and intracoronary (i.c.) infusions of the renin inhibitor remikiren, were compared and related to the effects on systemic haemodynamics and circulating angiotensin II in open-chest anaesthetized pigs (25-30 kg). The specificity of the remikiren-induced effects was tested (1) by studying its i.c. effects after administration of the AT1-receptor antagonist L-158,809 and (2) by measuring its effects on contractile force of porcine isolated cardiac trabeculae. 2. Consecutive 10 min i.v. infusions of remikiren were given at 2, 5, 10 and 20 mg min-1. Mean arterial pressure (MAP), cardiac output (CO), heart rate (HR), systemic vascular resistance (SVR), myocardial oxygen consumption (MVO2) and left ventricular (LV) dP/dtmax were not affected by remikiren at 2 and 5 mg min-1, and were lowered at higher doses. At the highest dose, MAP decreased by 48%, CO by 13%, HR by 14%, SVR by 40%, MVO2 by 28% and LV dp/dtmax by 52% (mean values; P < 0.05 for difference from baseline, n = 5). The decrease in MVO2 was accompanied by a decrease in myocardial work (MAP x CO), but the larger decline in work (55% vs. 28%; P < 0.05) implies a reduced myocardial efficiency ((MAP x CO)/MVO2). 3. Consecutive 10 min i.c. infusions of remikiren were given at 0.2, 0.5, 1, 2, 5 and 10 mg min-1. MAP, CO, MVO2 and LV dP/dtmax were not affected by remikiren at 0.2, 0.5 and 1 mg min-1, and were reduced at higher doses. At the highest dose, MAP decreased by 31%, CO by 26%, MVO2 by 46% and LV dP/dtmax by 43% (mean values; P < 0.05 for difference from baseline, n = 6). HR and SVR did not change at any dose. 4. Thirty minutes after a 10 min i.v. infusion of the AT1 receptor antagonist, L-158,809 at 1 mg min-1, consecutive 10 min i.c. infusions (n = 5) of remikiren at 2, 5 and 10 mg min-1 no longer affected CO and MVO2, and decreased LV dP/dtmax by maximally 27% (P < 0.05) and MAP by 14% (P < 0.05), which was less than without AT1-receptor blockade (P < 0.05). HR and SVR remained unaffected. 5. Plasma renin activity and angiotensin I and II were reduced to levels at or below the detection limit at doses of remikiren that were not high enough to affect systemic haemodynamics or regional myocardial function, both after i.v. and i.c. infusion. 6. Remikiren (10(-10) to 10(-4) M) did not affect contractile force of porcine isolated cardiac trabeculae precontracted with noradrenaline. In trabeculae that were not precontracted no decrease in baseline contractility was observed with remikiren in concentrations up to 10(-5) M, whereas at 10(-4) M baseline contractility decreased by 19% (P < 0.05). 7. Results show that with remikiren i.v., at the doses we used, blood pressure was lowered primarily by vasodilation and with remikiren i.c. by cardiac depression. The blood levels of remikiren required for its vasodilator action are lower than the levels affecting cardiac contractile function. A decrease in circulating angiotensin II does not appear to be the sole explanation for these haemodynamic responses. Data support the contention that myocardial contractility is increased by renin-dependent angiotensin II formation in the heart.

Effects of human renin in the vasculature of rats transgenic for human angiotensinogen

Transgenic rats, which express the human angiotensinogen gene, provide a unique model for studying local vascular effects of human renin. We examined the cleavage of human angiotensinogen to angiotensin I (Ang I) by human renin and its inhibition by a human renin inhibitor in an isolated perfused hindlimb preparation from such rats. Perfusion resulted in the sustained release of human angiotensinogen, which decreased from 19.4 to 11.8 pmol/mL over 45 minutes. Active human renin at doses of 3, 10, and 30 ng/mL perfusate for 15 minutes increased Ang I release from undetectable levels (mean +/- SEM) to 31.9 +/- 3.3, 147.1 +/- 26.2, and 206.4 +/- 17.1 fmol/mL, respectively, by 9 minutes. In separate experiments aimed at the quantification of renin-induced vasoconstriction, captopril decreased the perfusion pressure and lowered Ang II concentrations to nondetectable levels, whereas Ang I values increased sharply. When renin (30 ng/mL) was infused for 15 minutes, renin values in the perfusate decreased to barely detectable levels within minutes after termination of the infusion. However, Ang I values remained high for at least 30 minutes thereafter. The addition of a human renin inhibitor during renin infusion caused Ang I values to promptly decrease within minutes to undetectable levels. Hindlimbs from non-transgenic control rats released no detectable amounts of Ang I, with or without human renin. Finally, by in situ hybridization we documented the presence of human angiotensinogen message in the vessels of the hindlimb. We conclude that renin acts on angiotensinogen at a site in the vascular wall. The cleavage depends on renin and not on other lysosomal proteases.(ABSTRACT TRUNCATED AT 250 WORDS)

Appraisal of a glycopeptide cloaking strategy for a therapeutic oligopeptide: glycopeptide analogs of the renin inhibitor ditekiren

Among the limitations to the practical therapeutic oligopeptide are low oral availability, indifferent aqueous solubility, and an astonishing efficient sequestration and biliary elimination by a multi-capacity liver transporter. Given the purposed use of N- and O- linked saccharides as functional appendages of eukaryotic peptides and proteins, a strategy of glycopeptide mimicry was examined for the oligopeptide renin inhibitor, ditekiren. The anticipation was that the saccharide would impart significant aqueous solubility, and might impact beneficially on the remaining two limitations. Execution of this approach was achieved by the removal of the (dimethylethoxy)carbonyl amino terminus of ditekiren, and its substitution by Boc-L-asparagine N-linked mono- and disaccharides. Potent hypotensive activity, as measured by a human renin-infused rat assay, is observed for virtually all of these structures (N-linked beta-pyranose D-N-acetyglucosaminyl, D-glucosaminyl, D-N-acetylgalactosaminyl, D-mannosyl, D-galactosyl, D-maltosyl, D-cellobiosyl, D-chitobiosyl, but not L-fucosyl). The basis for this dramatic improvement (relative to ditekiren in the same assay) is the diversion of the peptide clearance from rapid liver biliary clearance to slower urinary clearance (Fisher, J. F.; Harrison, A. W.; Wilkinson, K. F.; Rush, B. R.; Ruwart, M. J. J. Med. Chem. 1991, 34, 3140). Guided by the human renin-infused rat hypertension assay, an evaluation of the linker-saccharide pairing was made. Loss of hypotensive activity is observed upon substitution of the Boc-L-asn by Boc-D-asn, and by removal of the Boc amino terminus of the glycopeptide. Potent hypotensive activity is preserved by replacement of the Boc-L-asn linker by succinate, malate, tartrate, and adipate linkers. With the longer adipate spacer, attachment of the saccharide to the P-3 phenylalanine--with omission of the P-4 proline--retains activity. These data suggest value to the glycopeptide guise for preserving the in vivo activity, and for the beneficial manipulation of pharmacodynamics, of this renin inhibitory oligopeptide. This strategy may have general applicability.

Angiotensinergic versus nonangiotensinergic hemodynamic effects of converting enzyme inhibition in patients with chronic heart failure. Assessment by acute renin and converting enzyme inhibition

BACKGROUND

The contribution of nonangiotensinergic effects of converting enzyme inhibitors to their hemodynamic effects in patients with chronic heart failure is not clear. A comparison of the effects of renin and converting enzyme inhibition should help to clarify this issue.

METHODS AND RESULTS

Thirty-six patients with chronic heart failure (New York Heart Association class II or III) were randomly assigned to receive double-blind either intravenous placebo, the renin inhibitor remikiren, or the converting enzyme inhibitor enalaprilat followed by coinfusion of a second placebo infusion, the addition of remikiren to enalaprilat, or the addition of enalaprilat to remikiren, respectively. Systemic hemodynamics (Swan-Ganz and radial artery catheters) were measured before (rest and submaximal recumbent bicycle ergometry), during (rest), and at the end (rest and exercise) of each 45-minute single- or combination-infusion period. Placebo did not change hemodynamics or renin activity. Effective inhibition of the renin-angiotensin system by remikiren and enalaprilat was indicated by increases of plasma immunoreactive renin together with rapid and complete inhibition of renin activity after remikiren and an increase after enalaprilat (all P < or = .05). Remikiren and enalaprilat rapidly and to a similar extent reduced resting blood pressure through a reduction of systemic vascular resistance, and these changes were significantly correlated to baseline plasma renin activity. Both compounds also decreased pulmonary artery, pulmonary capillary wedge, and right atrial pressures to a similar extent (P < .05). During exercise, pulmonary capillary wedge and right atrial pressures were equally reduced and stroke volume index was increased with remikiren and enalaprilat (P < .05) for both). The combination of converting enzyme with renin inhibition or vice versa did not cause additional hemodynamic changes.

CONCLUSIONS

Specific renin inhibition in patients with chronic heart failure produces short-term hemodynamic effects that are almost indistinguishable from those of converting enzyme inhibition. This finding and the lack of additional effects of converting enzyme inhibition added to renin inhibition suggest that nonangiotensinergic effects of converting enzyme inhibitors do not play a significant role in their short-term hemodynamic effects in patients with chronic heart failure.

Ciprokiren (Ro 44-9375). A renin inhibitor with increasing effects on chronic treatment

The present study characterizes the new transition-state renin inhibitor ciprokiren (Ro 44-9375) in squirrel monkeys. Arterial blood pressure was monitored by telemetry in freely moving, chronically instrumented conscious animals. In vitro at pH 7.4, ciprokiren inhibited human renin in buffer and human plasma with an IC50 of 0.07 and 0.65 nmol/L, respectively. It was equipotent against primate plasma renin and also inhibited plasma renin from dog and guinea pig in the nanomolar range (IC50, 29 and 65 nmol/L, respectively). After acute oral administration it reduced arterial blood pressure dose dependently in normotensive sodium-depleted and cyclosporin-induced hypertensive squirrel monkeys, starting with the minimal oral dose of 3 micrograms/kg. Daily oral doses of 1 microgram/kg showed a progressive blood pressure decrease, with a maximal response reached after 1 week. The drug could also be applied transdermally with similar hemodynamic effects without any decrease of plasma renin activity or plasma immunoreactive angiotensin II. Thus, ciprokiren is characterized in squirrel monkeys as a renin inhibitor with high in vivo potency that might act mainly in the tissular compartment.

Major role of the renin angiotensin system in the neointima formation after vascular injury in guinea pigs

ACE inhibition has been shown to prevent neointima formation after vascular injury. However, it is not known if this effect is due to a specific inhibition of the renin angiotensin system or to another mechanism such as the accumulation of bradykinin. In order to answer this question we compared the effects of maximal effective doses of cilazapril, an angiotensin converting enzyme (ACE) inhibitor, and ciprokiren, a new renin inhibitor, in guinea pigs. Vascular injury was induced by endothelial denudation of the right carotid artery of guinea pigs treated either by saline (control group), cilazapril (30 mg/kg/day) or ciprokiren (24 mg/kg/day). Twelve days after the ballooning, the guinea pigs were sacrificed, the carotid arteries were perfused fixed and neointima formation was evaluated by quantitative morphometry. Both, ciprokiren and cilazapril prevented neointima formation to the same extent (inhibition by 42 and 49%, respectively, p < 0.05). These results suggest that, in guinea pigs, renin inhibition prevents neointima formation to a similar extent as ACE inhibition. Therefore, ACE inhibitors seem to act in this model by inhibiting the renin angiotensin system and not by other effects such as accumulation of bradykinin.