Synthesis and pharmacology of a noncompetitive antagonist of angiotensin-induced contractions of vascular smooth muscle. [Sarcosyl]1-[cysteinyl (s-methyl)]8-angiotensin II

Synthesis and antagonistic activity of four new 2-alkyl-N-biphenyl fused imidazoles on angiotensin II receptors

In the current study, four new 2-alkyl-N-biphenyl fused imidazoles were synthesized and the pharmacological properties of these compounds as angiotensin II antagonists were studied. First, the potency of the synthesized compounds on guinea-pig ileum was evaluated and the vasopressor effect of the most potent compound 6a was compared with losartan on isolated perfused rat kidney. The antagonistic activity of compound 6a (sodium 2-propyl-5-carbomethoxy-1-[(biphenyl-4-yl)methyl]pyrrolo[3,2-d]imidazole-2'-carboxylate) on angiotensin II receptors was greater than the other synthesized compounds and in isolated perfused rat kidney was similar to losartan.

Functional evidence that the central renin-angiotensin system plays a role in the pressor response induced by central injection of carbachol

We investigated the effects of losartan, an AT1-receptor blocker, and ramipril, a converting enzyme inhibitor, on the pressor response induced by angiotensin II (ANG II) and carbachol (a cholinergic receptor agonist). Male Holtzman rats (250-300 g) with a stainless steel cannula implanted into the lateral ventricle (LV) were used. The injection of losartan (50 nmol/1 microliter) into the LV blocked the pressor response induced by ANG II (12 ng/1 microliter) and carbachol (2 nmol/1 microliter). After injection of ANG II and carbachol into the LV, mean arterial pressure (MAP) increased to 31 +/- 1 and 28 +/- 2 mmHg, respectively. Previous injection of losartan abolished the increase in MAP induced by ANG II and carbachol into the LV (2 +/- 1 and 5 +/- 2 mmHg, respectively). The injection of ramipril (12 ng/1 microliter) prior to carbachol blocked the pressor effect of carbachol to 7 +/- 3 mmHg. These results suggest an interaction between central cholinergic pathways and the angiotensinergic system in the regulation of arterial blood pressure.

Modulation of tachyphylaxis to angiotensin II in rabbit isolated aorta by the angiotensin AT1 receptor antagonist, losartan

In rabbit aortic strip preparations, angiotensin II (AII) concentration-contractile response curves (0.3-100 nM) were highly reproducible. However, tachyphylaxis to the contractile response elicited by AII could be induced by repeated exposure to a supramaximal concentration (10 microM) of the peptide. In contrast, a correspondingly supramaximal concentration of the alpha 1-adrenoceptor agonist, phenylephrine (1 mM), did not cause the tissue to become tachyphylactic to phenylephrine. Furthermore, phenylephrine responses were unaffected in tissues previously made tachyphylactic to AII. When the non-peptide, competitive angiotensin AT1 receptor antagonist, losartan (300 nM), was administered before each supramaximal AII challenge, tachyphylaxis did not subsequently occur. Additionally, in tissues made tachyphylactic to AII, subsequent incubation with losartan (300 nM) reversed the AII tachyphylaxis. Thus, losartan may prevent the loss of contractility by preventing AII from interacting with its receptor in a manner which induces tachyphylaxis. However, since losartan can also completely reverse the loss of contractility, it appears capable of restoring the AT1 receptor to a state which allows subsequently administered AII to fully activate the contractile pathway.

Pharmacokinetics and biochemical efficacy after single and multiple oral administration of losartan, an orally active nonpeptide angiotensin II receptor antagonist, in humans

1. The pharmacokinetics and biochemical efficacy of losartan, an orally active nonpeptide angiotensin II (AII) receptor antagonist, were evaluated in healthy male volunteers after single and multiple oral administration. 2. Plasma and urinary concentrations of losartan and its active metabolite, E-3174, were determined by a specific high performance liquid chromatographic (h.p.l.c.) method. 3. Plasma concentrations of losartan were proportional to dose over the range of 25 to 200 mg and the terminal half-lives (t1/2,z) ranged from 1.5 to 2.5 h. The mean values of Cmax and AUC0-infinity increased in a dose-dependent manner. 4. Plasma concentrations of E-3174 were higher than those of losartan at all dose levels. The values of Cmax and AUC0-infinity for E-3174 were approximately 2 and 5-8 times higher than those for losartan, respectively. Also the value of t1/2,z was 2 times longer than that of losartan. 5. After multiple dosing for 7 days, the pharmacokinetics of losartan and E-3174 each did not change significantly between day 1 and day 7. 6. Plasma renin activity (PRA) and plasma concentrations of AII increased markedly at all dose levels. Plasma aldosterone levels were slightly reduced, but a similar decrease was also observed with placebo. 7. No clinically significant adverse reaction was observed in any of the volunteers during either study. Blood counts, routine laboratory tests, urine analyses, and electrocardiograms were also not modified by losartan. 8. Losartan appears to be a potent orally active angiotensin II antagonist with a relatively long duration of action.

Pharmacological profile of GR117289 in vitro: a novel, potent and specific non-peptide angiotensin AT1 receptor antagonist

1. This paper describes the effects of GR117289 (1-[[3-bromo-2-[2-(1H-tetrazol-5-yl)phenyl]-5-benzo-furanyl]methyl ]-2-butyl-4-chloro-1H-imidazole-5-carboxylic acid) at angiotensin receptors and binding sites in rabbit aorta, rat liver and bovine cerebellum preparations in vitro. 2. In rabbit isolated aortic strips, GR117289 (0.3, 1 and 3 nM) caused a concentration-related, insurmountable suppression of the concentration-response curve to angiotensin II (AII). When the contact time was increased, a greater degree of antagonism of AII was observed, suggesting that GR117289 is slow to reach equilibrium. A pKB of 9.8 +/- 0.1 was calculated for GR117289 after 3 h incubation. GR117289 (1 microM) did not affect contractile responses to phenylephrine or 5-hydroxytryptamine (5-HT) in the rabbit aorta. 3. GR117289 (1 nM) alone caused a marked suppression and a slight rightward displacement of the AII concentration-response curve. Co-incubation with the competitive, surmountable AT1 receptor antagonist, losartan (10 nM, 100 nM and 1 microM), resulted in a concentration-related upward and rightward displacement of the concentration-response curve to subsequently administered AII. In separate experiments in which preparations were pre-incubated with GR117289 (1 nM), subsequent addition of losartan (1 microM) for 2, 15 or 45 min caused a further, but similar, rightward displacement of the concentration-response curve to subsequently administered AII with a time-dependent increase in the maximum response.4. Suppression of All-induced contractile responses, caused by superfusion with GRI17289 (0.3, 1 or 3 nM) was not reversed by continuously washing the tissues for 3 h; in fact, the potency of GRI 17289 was slightly enhanced after this period.5. In rat liver membranes, GRI17289 was a potent competitor with [3H]-AII for AT, binding sites(pKi = 8.7 +/- 0.1) but in bovine cerebellum membranes, it was a very weak competitor for AT2 binding sites (pKi<6). Pre-incubation of rat liver membranes with GRI17289 had little effect on its affinity(pKi = 9.1 +/- 0.21), but increasing the concentration of bovine serum albumen in the assay buffer from 0.001% to 0.1% w/v decreased affinity (pKi= 7.5 +/- 0.1).6. In saturation binding experiments in rat liver membranes, GRI 17289 (12 nM) increased the Kd of[3H]-AII from 0.28 +/- 0.06 nM to 0.37 +/- 0.02 nM, and decreased Bm. from 10.0 +/- 0.1 to 5.6 +/-0.3 fmol mg' tissue. In other experiments, GR1 17289 (1 jIM) did not alter the rate of dissociation of[3H]-AII from AT1 binding sites, following addition of excess unlabelled All.7. In rabbit aorta vascular smooth muscle membranes, GR1 17289 competed with ['25I]-Sar'1le8 All for binding to AT, binding sites. In the presence of 0.1% w/v bovine serum albumen, a pIC50 of 7.6 +/- 0.1 was calculated. Under the same conditions, but with rat liver membranes, a pIC50 of 7.8 +/- 0.1 was determined.8. Taken together, these results show that GRI17289 is a potent, specific, selective and insurmountable antagonist at angiotensin AT, receptors. Its profile in the rabbit aorta is consistent with the proposalthat GRI17289 is a slowly reversible (pseudo-irreversible) antagonist at these receptors.

Effect of extracellular calcium concentration on angiotensin AT1 receptor-mediated smooth muscle contraction and antagonism in rabbit aorta

Angiotensin-II (AII) stimulates smooth muscle contraction by activating angiotensin AT1 receptor which induces intracellular Ca2+ release and Ca2+ influx from extracellular space. In this study, effect of extracellular Ca2+ concentration ([Ca2+]0) on angiotensin AT1 receptor-mediated contractile response to AII has been examined in the absence and presence of [Sar1,Ala8]AII in rabbit aorta. A decrease in agonist potency and an increase in antagonist potency for depression were observed in low [Ca2+]0. Data were interpreted by applying an explanatory model developed previously. The result indicates that [Ca2+]0 is linked to the efficacy expression of AII at angiotensin AT1 receptor and this prompted speculation about the underlying mechanism.

Effects of peptidase inhibition on angiotensin receptor agonist and antagonist potency in rabbit isolated thoracic aorta

1. Experiments were performed with peptidase inhibitors on rabbit aortic strip preparations, to determine whether endogenous peptidase activity can influence the potency estimates for angiotensin receptor agonists and antagonists in this tissue. 2. Angiotensin II (A II) and angiotensin III (A III) both induced concentration-related contractions of rabbit aortic strip preparations. A III was approximately 38 fold less potent than A II, and the gradient of the A III concentration-response curve (1.00 +/- 0.04) was significantly more shallow than that (1.76 +/- 0.05) of the A II curve. 3. Neither the aminopeptidase-A and -M inhibitor, amastatin, nor the aminopeptidase-B and -M inhibitor, bestatin, affected the potency of, or the maximum response to, A II. In contrast, the potency of A III was increased by both amastatin and bestatin. Amastatin had the most marked effect and at 10 microM caused approximately a 12 fold increase in the potency of A III (EC50 values, 102 nM and 8.6 nM in the absence and presence of amastatin, respectively), and also significantly steepened the gradient of the A III concentration-response curve. Amastatin did not affect the position or shape of the concentration-response curve to the alpha 1-adrenoceptor agonist, phenylephrine. Finally, the carboxypeptidase-N inhibitor, D-L-mercaptomethyl-3-guanidine-ethylpropanoic acid (MERGETPA) did not change the position or shape of the concentration-response curves to either A II or A III.4. In the presence of amastatin, the potency of the peptide angiotensin receptor antagonist, Ile7-A III (100nM-l microM ), was increased approximately 13 fold (pA2, with A II as the agonist, 7.0 +/- 0.1 and 8.1 +/- 0.1, in the absence and presence of amastatin, respectively). However, the potency of the nonpeptide angiotensin receptor antagonist, DuP 753 (30-300 nM), was little affected by amastatin (pA2, 8.2 +/- 0.1 and 8.1 +/- 0.1 in the absence and presence of amastatin, respectively).5. The results of this study suggest that endogenous aminopeptidase activity in the rabbit thoracic aorta can profoundly affect estimates of the potency of peptide angiotensin receptor agonists and antagonists.A suitable aminopeptidase inhibitor should therefore be included in studies, using this tissue, which aim to classify angiotensin receptor subtype(s) based on the rank order of peptide angiotensin receptor agonist and/or antagonist potencies.

Evidence that the apparent complexity of receptor antagonism by angiotensin II analogues is due to a reversible and syntopic action

1. The interactions between angiotensin II (AII), two non-peptide antagonists DuP 753 and IMI, and eight peptide analogues of AII were investigated on the rabbit isolated aorta assay. DuP 753 and IMI behaved as simple competitive antagonists (pKB values 8.4 and 6.8, respectively). To different degrees, all the AII-peptide analogue interactions failed to meet the basic criteria for simple competition. In addition to rightward shift, the most significant feature was a concentration-dependent saturable depression of the upper asymptote of the AII concentration-effect curves. 2. 'Washout' and combined dose-ratio analysis experiments, in which DuP 753 was used as a reference antagonist, indicated that the profile of peptide antagonism was solely due to a reversible and syntopic action at the AII receptor. 3. By use of an operational model of agonism (Black & Leff, 1983) as a starting point, it was possible to account for the data with a new model which describes reversible receptor occupancy and occupied receptor-determined, saturable reduction in the efficacy of AII. Model-fitting gave estimates of pKB values for the peptide analogues and agonist affinity and efficacy parameters for AII. 4. The model was successfully tested by applying it to qualitatively similar results obtained in a cross-tissue analysis on guinea-pig aorta, ileum and stomach. 5. A 'molecular' interpretation of the efficacy changes, based on the concepts of receptor internalisation and expression, is offered.

DuP 532: a second generation of nonpeptide angiotensin II receptor antagonists

DuP 532 is a novel nonpeptide angiotensin II (AII) receptor antagonist under development for the treatment of hypertension. DuP 532 is a more potent antihypertensive agent in renal hypertensive rats (ED30 = 0.042 mg/kg, i.v.) and displays a similar or longer duration of action than the previously described AII antagonist, DuP 753. DuP 532, in contrast to DuP 753, is a noncompetitive antagonist of AII-induced contractions of rabbit aortic strips (KB = 1.1 x 10(-10) M). However, the inhibition of AII binding by DuP 532 in rat adrenal cortex does not correlate with either the aortic contractile response or with the hypotensive response. Assay conditions were evaluated and the presence or absence of BSA was shown to markedly affect the apparent binding affinity of DuP 532 and other 5-carboxylic acid derivatives. DuP 753 and other compounds were much less affected. The IC50 for DuP 532 was 4.7 x 10(-6) M with and 3 x 10(-9) M without BSA. The IC50s for DuP 753 were 1.7 x 10(-8) M with and 5 x -9 M without BSA. Both compounds with or without BSA did not completely inhibit AII binding which is characteristic of AT1 selectivity. BSA also reduced the effect of DuP 532 on the AII-induced contractions of rat main pulmonary artery preparations and the AII-induced Ca2+ mobilization in rat aortic smooth muscle cells. DuP 532 was very specific for AT1 receptors and did not interfere with receptors associated with neurotensin, prazosin, bradykinin, nitrendipine, or vasopressin. It is concluded that DuP 532 represents a new class of specific, but noncompetitive. AII receptor antagonists whose binding characteristics may provide new insight into AII receptor function.

Angiotensin II receptor subtypes in rat brain

1. Angiotensin II (AII) receptor binding was localized in the rat brain by in vitro autoradiography using the antagonist analogue, 125I-[Sar1, Ile8] AII. AII receptor binding was then differentiated into type I and type II subtypes by displacement with unlabelled non-peptide antagonists specific for AII subtypes. 2. Type I binding was determined as that inhibited by Dup753 (10 mumol/L) and type II binding as that inhibited by XD329-1 (10 mumol/L). The reducing agent dithiothreitol (DTT) decreased the binding to type I receptors and enhanced the binding to type II receptors. 3. Structures such as the vascular organ of the lamina terminalis, subfornical organ, median preoptic nucleus, area postrema, nucleus of the solitary tract, which are known to be related to some central actions of AII, contain exclusively type I AII receptors. 4. In contrast, the locus coeruleus, ventral and dorsal parts of lateral septum, superior colliculus, subthalamic nucleus, some nuclei of the thalamus, and the nuclei of the inferior olive contain predominantly type II AII receptors. 5. These results reveal important pharmacological heterogeneity of brain AII receptors which suggest different regional functions and are relevant to the central actions of emerging classes of new non-peptide AII receptor antagonists.

Sarmesin is a partial agonist of angiotensin-II receptors in rabbit, but not in rat, aortic rings

Sarmesin, [Sar1, Tyr(Me)4]angiotensinII], has been reported to be a competitive angiotensin II (AII) receptor antagonist in rat smooth muscle preparations (Scanlon et al., (1984), Life Science 34, 317-321). In the present study, sarmesin displaced AII from its binding sites in rat aortic smooth muscle cells and in a rabbit aorta membrane preparation (IC50 5 and 6 nM resp.; Ki 4.1 and 5.3 resp.) In rabbit aortic rings, sarmesin (0.003-3 microM) produced concentration-dependent contractions (ED50 89 nM) and this effect was inhibited by saralasin. No contraction was observed in the rat aorta up to 100 microM. In rabbit aortic rings, sarmesin, at the same concentrations that produced contraction, inhibited contractions induced by AII in a competitive manner (pA2 7, 26). These results indicate that, in rabbit aortic rings sarmesin is a partial agonist of AII receptors.

Identification of angiotensin II receptor subtypes

We have demonstrated the existence of two distinct subtypes of the angiotensin II receptor in the rat adrenal gland using radioligand binding and tissue section autoradiography. The identification of the subtypes was made possible by the discovery of two structurally dissimilar, nonpeptide compounds, DuP 753 and EXP655, that show reciprocal selectivity for the two subtypes. In the rat adrenal cortex, DuP 753 inhibited 80% of the total AII binding with an IC50 value on the sensitive sites of 2 x 10(-8) M, while EXP655 displaced only 20%. In the rat adrenal medulla, EXP655 gave 90% inhibition of AII binding with an IC50 value of 3.0 x 10(-8) M, while DuP 753 was essentially inactive. The combination of the two compounds completely inhibited AII binding in both tissues.

Non-peptide angiotensin II receptor antagonists. II. Pharmacology of S-8308

2-Butyl-4-chloro-1-(2-nitrobenzyl)imidazole-5-acetic acid, sodium salt (S-8308), inhibited the specific binding of labeled angiotensin II (AII) to its receptor sites in rat adrenal cortical microsomes and in cultured aortic smooth muscle cells with IC50S of 15 and 4.5 microM, respectively. In the presence of S-8308 (15 microM) the dissociation constant for AII was increased 2-fold and the total number of binding sites was unaltered. In a concentration-dependent manner S-8308 blocked the 45Ca2+ influx induced by AII (3 X 10(-8) M) in rat aortic rings (IC50 7 microM) and the contractile response in rabbit aorta was competitively inhibited (pA2 = 5.74). This agent was highly specific for AII: it showed no affinity for alpha 1-adrenoceptors or Ca2+ channels and in addition, it did not alter the contractile responses to norepinephrine (10(-7) M) or KCl (55 mM). In conscious renal artery-ligated rats, S-8308 (30 mg/kg i.v.) elicited a rapid decrease of mean arterial pressure with a duration of about 30 min. The results demonstrate that S-8308 is a weak, but specific and competitive, non-peptide antagonist of AII exerting its inhibitory action at the receptor level.

Angiotensin III-induced modulation of neurogenic responses in the rabbit vas deferens and portal vein

The effects of prostaglandin synthesis inhibitors on the presynaptic actions of angiotensin (ang) II and III were examined in isolated rabbit vasa deferentia and portal veins. Ang II caused dose-dependent potentiation of low frequency, nerve stimulation in vasa deferentia and portal vein. Indomethacin (26 microM) enhanced the electrically-induced contractions in the vasa deferentia only but did not alter the potency of ang II in either preparation. In contrast, ang III decreased contractions in vasa deferentia induced by nerve stimulation by up to 36% (10(-6) M) and potentiated these contractions at concentrations higher than 10(-6) M. The inhibitory action of ang III on vasa deferentia was converted to potentiation by pretreatment with indomethacin or mepacrine. Exogenous PGE2 blocked low frequency nerve stimulation and not responses to norepinephrine. This prostaglandin appeared to mimic ang III in the vas deferens. No effects of ang III were observed if the contractions were induced by exogenous alpha adrenergic agonists. [Sar1, Ala8] ang II antagonized all responses to the angiotensins, whereas [Sar1, Cys-CH3(8)] ang II selectively antagonized the angiotensin-induced potentiation. Responses in field-stimulated portal veins were potentiated by ang III and this response was unaffected by indomethacin. This investigation strongly suggests the existence of at least two ang receptors in the vas deferens and demonstrates for the first time selective responses to the octa- and heptapeptides in the same effector organ.

Role of the two N-terminal residues of angiotensin II in the production of tachyphylaxis

The structural requirements for the production of angiotensin tachyphylaxis in the guinea-pig ileum were studied by analyzing the tachyphylactic properties of the following synthetic analogues of angiotensin II (AII): [1-sarcosine]AII, [1-betaine]AII; [1-guanidinoacetic]AII; betainyl-AII; [2-lysine]AII; [2-ornithine]AII. In the non-atropinized ileum, no tachyphylaxis was observed with any of the following analogues: [2-lysine]AII, [2-ornithine]AII, [2-ornithine]AII, [1-betaine]AII and betainyl-AII. [1-Guanidinoacetic]AII induced tachyphylaxis, but to a smaller degree than AII, while [1-sarcosine]AII was significantly more tachyphylactic than AII. Similar results were obtained in the atropinized ileum, except that moderate tachyphylaxis was also observed with betainyl-AII and [1-betaine]AII. The analogues with lysine or ornithine residues in position 2 did not induce tachyphylaxis under any of the conditions studied. It is concluded that, besides the protonated N-terminal amino group, the guanidino group of the Arg2 side-chain is essential for the manifestation of angiotensin tachyphylaxis in the guinea-pig ileum.

Effect of three angiotensin II antagonists, [Sar1, Thr8]-, [Sar1, Ile8]- and [Sar1, Ala8]angiotensin II on blood pressure and endocrine factors in normal subjects

The biological effects of 1-Sarcosine, 8-Threonine angiotensin II ([Sar1, Thr8]ANG II) on blood pressure, plasma aldosterone concentration (PAC) and plasma renin activity (PRA) were investigated in six normal subjects on an unrestricted diet, and compared with those of 1-Sarcosine, 8-Isoleucine ANG II ([Sar1, Ile8]ANG II) and 1-Sarcosine, 8-Alanine ANG II ([Sar1, Ala8]ANG II). All three ANG II analogues (AIIA) showed agonistic pressor activity, that of [Sar1, Ile8]ANG II being greater than that of [Sar1, Thr8]ANG II or [Sar1, Ala8]ANG II. The antagonistic effect of [Sar1, Thr8]ANG II on blood pressure was less than [Sar1, I1e8]ANG II or [Sar1, Ala8]ANG II. Both [Sar1, Ile8]ANG II and [Sar1, Ala8]ANG II increased PAC and blocked the steroidogenic action of ANG II, while [Sar1, Thr8]ANG II showed little effect on PAC. All three AIIA caused similar suppression of PRA and showed no inhibitory effect on the decrease in PRA produced by ANG II. These results indicate that [Sar1, Thr8]ANG II is an AIIA with weak agonistic pressor action and that it has vascular selective properties. It is also suggested that ANG II receptors in a variety of target organs are heterogeneous.

Organ selective angiotensin antagonists: sarcosyl1-cysteinyl(S-methyl)8-angiotensin I

An angiotensin antagonist, Sarcosyl1-Cysteinyl(S-Methyl)8-angiotensin I [Sar1-Cys(Me)8-ANG I] was synthesized and its pharmacological properties evaluated in vivo (rat blood pressure assay) and in vitro (rabbit aortic strips, guinea-pig ileum and rat uterus assays). It was found to be an extremely potent angiotensin II (ANG II) antagonist in the rat pressor assay (dose ratio for ANG II of 1300 during infusion of 5.0 micrograms/kg/min Sar1-Cys(Me)8-ANG I) and a moderately effective antagonist in guinea-pig ileum (pA2 congruent to 8.2). Moderate antagonism was also seen in the rabbit aortic strip preparation (pA2 congruent to 8.1) while the analog was inactive in the rat uterus assay. In each of the preparations where antagonist activity was observed there was evidence of non-competitive antagonism. Most striking was the inability of extremely high doses ( up to 125 micrograms ANG II/kg) of ANG II to overcome the Sar1-Cys(Me)8-ANG I blockade. In both the rat pressor and guinea-pig ileum assays the Sar1-Cys(Me)8-ANG I antagonism is completely abolished in the presence of the converting enzyme inhibitor SQ14225 (Captopril-Squibb). Organ selectivity of this analog is discussed in terms of the inherent activity of the active principle (i.e. the Sar1-Cys(Me)8-angiotensin II [Sar1-Cys(Me)8-ANG II] released by the action of converting enzyme) and the availability of converting enzyme in each bioassay.