Characterization of BIBS 39 and BIBS 222: two new nonpeptide angiotensin II receptor antagonists

Development of selective non-peptide angiotensin II type 2 receptor agonists

The development of the first drug-like selective angiotensin II type 2 (AT(2)) receptor agonist (22) derived from the non-selective angiotensin II type 1 (AT( 1)) receptor/AT(2) receptor agonist L-162,313 is presented. Compound 22 with a K(i) value of 0.4 nM for the AT( 2) receptor and a K(i) > 10 microM for the AT(1) receptor induces outgrowth of neurite cells, stimulates p42/p44( mapk), enhances in vivo duodenal alkaline secretion in Sprague-Dawley rats and lowers the mean arterial blood pressure in anaesthetised spontaneously hypertensive rats. Thus, the peptidomimetic 22 exerts a similar biological response as the endogenous peptide angiotensin II after selective activation of the AT(2) receptor. In addition, Compound 22 has a bioavailability of 20-30% after oral administration and a half-life estimated to four hours in the rat. Compound 22 will therefore serve as a valuable research tool enabling studies of the function of the AT(2) receptor in more detail.

An update on non-peptide angiotensin receptor antagonists and related RAAS modulators

The renin-angiotensin-aldosterone-system (RAAS) is an important regulator of blood pressure and fluid-electrolyte homeostasis. RAAS has been implicated in pathogenesis of hypertension, congestive heart failure, and chronic renal failure. Aliskiren is the first non-peptide orally active renin inhibitor approved by FDA. Angiotensin Converting Enzyme (ACE) Inhibitors are associated with frequent side effects such as cough and angio-oedema. Recently, the role of ACE2 and neutral endopeptidase (NEP) in the formation of an important active metabolite/mediator of RAAS, ang 1-7, has initiated attempts towards development of ACE2 inhibitors and combined ACE/NEP inhibitors. Furukawa and colleagues developed a series of low molecular weight nonpeptide imidazole analogues that possess weak but selective, competitive AT1 receptor blocking property. Till date, many compounds have exhibited promising AT1 blocking activity which cause a more complete RAAS blockade than ACE inhibitors. Many have reached the market for alternative treatment of hypertension, heart failure and diabetic nephropathy in ACE inhibitor intolerant patients and still more are waiting in the queue. But, the hallmark of this area of drug research is marked by a progress in understanding molecular interaction of these blockers at the AT1 receptor and unraveling the enigmatic influence of AT2 receptors on growth/anti-growth, differentiation and the regeneration of neuronal tissue. Different modeling strategies are underway to develop tailor made molecules with the best of properties like Dual Action (Angiotensin And Endothelin) Receptor Antagonists (DARA), ACE/NEP inhibitors, triple inhibitors, AT2 agonists, AT1/TxA2 antagonists, balanced AT1/AT2 antagonists, and nonpeptide renin inhibitors. This abstract gives an overview of these various angiotensin receptor antagonists.

Design and synthesis of new tetrazolyl- and carboxy-biphenylylmethyl-quinazolin-4-one derivatives as angiotensin II AT1 receptor antagonists

A series of novel quinazolin-4-ones was designed and their molecular modeling simulation fitting to a new HipHop 3D pharmacophore model using CATALYST was examined. Several compounds showed significant high simulation fit values. The designed compounds were synthesized and eight of them were biologically evaluated in vitro using an AT1 receptor binding assay, where compound XX competed weakly against radiolabeled Sar1Ile8-angiotensin II (Ang II) binding, compounds XIV and XXII showed moderate competition, and compound XXV showed almost equal ability to displace radiolabeled Sar1Ile8-Ang II binding to AT1 receptors as losartan. In vivo biological evaluation study of compounds XIV, XXII, and XXV on both normotensive and hypertensive rats revealed that compound XXV demonstrated higher hypotensive and antihypertensive activity than the reference compound losartan. To obtain a highly active compound from a candidate set of only eight tested compounds illustrates the power and utility of our pharmacophore model.

Angiotensin II receptor antagonists - antihypertensive agents

Blockade of the renin-angiotensin system (RAS) is now recognised as an effective approach for the treatment of hypertension and congestive heart failure (CHF). Today, it is possible to antagonise the effects of angiotensin II more specifically by blocking its receptors using non-peptide receptor antagonists. These compounds, which at first were used to identify the various subtypes of angiotensin II receptors, are now available clinically. Some of them have recently been launched on the market and several others are preregistered for the treatment of hypertension. These new molecules are as effective as angiotensin converting enzyme (ACE) inhibitors at lowering blood pressure in hypertensive patients, and appear to have similar systemic and renal haemodynamic properties in patients with CHF and renal diseases. Large-scale clinical trials such as the LIFE, the ELITE and the RENAAL studies are now underway to investigate the long-term benefits of one of these agents in hypertension, heart failure and Type II diabetic nephropathy. The major clinical advantage of AT1 receptor antagonists is that, in contrast to ACE inhibitors, they do not induce cough. With the more widespread use of AT1 receptor antagonists, two unresolved questions remains unanswered: what is the role of AT2 receptors? Are the unblocked effects of angiotensin II on AT2 receptor sites of any clinical relevance to the safety profile or efficacy of AT1 receptor antagonists? Another interesting question is whether the combination of an ACE inhibitor with an AT1 receptor antagonist is advantageous. Studies attempting to answer these questions are underway and will certainly enable researchers to define more precisely the role and the advantages of these new specific non-peptide AT1 receptor antagonists in the treatment of hypertension and heart failure.

3-D QSAR studies of triazolinone based balanced AT1/AT2 receptor antagonists

Essential structural and physicochemical requirements in terms of common biophoric sites (pharmacophore) and secondary sites for binding and interacting with AT1 and AT2 receptors have been identified using APEX-3-D expert system on 16 N2- aryl triazolinone biphenyl sulphonamides. Among several biophoric 3-D QSAR models two models (Nos. 1 and 2) having R2 > 0.7, chance <0.05 and match >0.5 and two models (Nos. 3 and 4) having R2 > 0.89, chance <0.03 and match >0.5 with three biophoric sites and two secondary sites (except model No. 4 with three secondary sites) describe the variation in AT1 and AT2 antagonistic activities, respectively.

New developments in drug therapy of hypertension

The explosion of new knowledge about the complex mechanisms mediating high blood pressure is providing new targets for drug therapy of hypertension and other cardiovascular disorders. This article reviews the current status of several new approaches in the management of hypertension, including vasopressin antagonists, natriuretic peptide clearance inhibitors, endothelin antagonists, renin inhibitors, angiotensin receptor antagonists, and selective T-type calcium ion channel antagonists.

Angiotensin II receptor antagonists: the prototype losartan

OBJECTIVE

To describe a new class of antihypertensive agents, the angiotensin II receptor antagonists, with emphasis on the prototype losartan. Pharmacokinetic data and clinical trials are reviewed, as well as adverse reactions, drug interactions, and dosing guidelines.

DATA SOURCES

A MEDLINE search of English-language literature published from 1966 through 1995 was performed. In addition, Merck and Co. provided bibliographic data on file for losartan.

STUDY SELECTION

Emphasis was placed on clinical and pharmacokinetic studies in humans. Controlled, double-blind studies were evaluated to assess the efficacy and adverse effect profile of losartan.

DATA SYNTHESIS

Losartan is a nonpeptide, competitive antagonist of the type I angiotensin II receptor. In comparative clinical trials, losartan appears to have antihypertensive efficacy similar to that of the angiotensin-converting enzyme (ACE) inhibitors. Losartan is well tolerated, with an adverse effect profile similar to that of placebo and a reduced incidence of cough versus that with ACE inhibitors. A combination product consisting of losartan 50 mg and hydrochlorothiazide 12.5 mg has also received approval for the treatment of hypertension. The combination product is not indicated for initial therapy, but is recommended for patients who do not respond adequately to losartan monotherapy. The angiotensin II receptor antagonists are also being investigated for beneficial effects in patients with ventricular hypertrophy, renal disease, and heart failure.

CONCLUSIONS

Losartan, the first angiotensin II receptor antagonist to receive approval for use in the US, appears to be an effective new antihypertensive agent with an adverse effect profile similar to that of placebo. Losartan may be an alternative for patients who cannot tolerate ACE inhibitors. However, the effect of losartan on mortality remains to be evaluated. The role of the angiotensin II receptor antagonists in areas such as ventricular hypertrophy, renal function, and heart failure has yet to be determined.

Effects of irbesartan (SR47436/BMS-186295) on angiotensin II-induced pressor responses in the pithed rat: potential mechanisms of action

The effects of two new non-peptide angiotensin receptor antagonists, irbesartan (SR 47436/BMS-186295, (2-n-butyl-4-spirocyclopentane-1-[((2'-tetrazol-5-yl)bipheny l-4-yl)methyl]2 - imidazolin-5-one) and SR 47155A (2-n-butyl-4-spirocyclopentane-1-[((2'-carboxy)biphenyl-4-yl)methy l]2- imidazolin-5-one, trifluoroacetate), on angiotensin II-induced pressor responses were studied in the pithed rat in comparison to losartan, EXP 3174 and [Sar1,Val5,Ala8]angiotensin II. SR 47155A (1-10 mg/kg i.v.) and losartan (1-10 mg/kg i.v.) shifted dose dependently the dose-response curve of angiotensin II to the right without affecting the maximal response. SR 47436 (0.3-10 mg/kg i.v.), EXP 3174 (0.03-1 mg/kg i.v.) and [Sar1,Val5,Ala8]angiotensin II (0.03-1 mg/kg i.v.) induced, at least at high doses, a non-parallel shift to the right of the angiotensin II dose-response curve and this was associated with a reduction of the maximal response. During a 70 min period, the effect of [Sar1,Val5,Ala8]angiotensin II (1 mg/kg i.v.) on the angiotensin II (0.3 microgram/kg i.v.)-induced pressor response was shown to be reversible, the effect of SR 47155A (10 mg/kg i.v.) was partially reversible and the effect of SR 47436 (3 mg/kg i.v.), EXP 3174 (1 mg/kg i.v.) or losartan (6 mg/kg i.v.) was not reversed at the end of this 70 min period. Administration of SR 47155A (10 mg/kg i.v.) before SR 47436 (1-10 mg/kg i.v.) reversed the reduced angiotensin II-maximal response induced by SR 47436. Administration of SR 47436 (10 mg/kg i.v.) before SR 47155A (1-10 mg/kg i.v.) prevented the full development of the pressor response as observed in the absence of SR 47436. In the pithed rat, SR 47436 (30 mg/kg i.v.) and losartan (30 mg/kg i.v.) reduced the change in diastolic blood pressure induced by electrical stimulation of the spinal cord only at low stimulation rates. Taken together these results indicate that SR 47436, under in vivo conditions, is a potent non-peptide angiotensin receptor antagonist. The type of antagonism (partially insurmountable but selective) can be explained by different theoretical models which are discussed.

Molecular pharmacology of LR-B/081, a new non-peptide angiotensin AT1 receptor antagonist

This report describes the molecular pharmacological properties of LR-B/081 (methyl 2-[[4-butyl-2-methyl-6-oxo-5-[[2'-(1H-tetrazol-5- yl) [1,1'-biphenyl]-4-yl]methyl]-1 (6H)-pyrimidinyl]methyl]- 3-thiophenecarboxilate), a novel non-peptide angiotensin II receptor antagonist. This compound potently displaced [3H]angiotensin II from angiotensin AT1 (Ki = 1.4 nM, rat adrenal cortex), but not from angiotensin AT2 (Ki > 1 microM, bovine cerebellar cortex) receptors and did not show affinity for other receptor systems (Ki > 10 microM). In saturation studies, LR-B/081 both increased KD and decreased Bmax values in a dose-dependent fashion. The rate of dissociation of [3H]angiotenin II from angiotensin AT1 receptors was not affected by the presence of 1 microM LR-B/081 and the association rate of [3H]angiotensin II was not decreased by the presence of 1 or 30 nM LR-B/081, indicating that the Bmax reduction was not due to an allosteric interaction or to a delay in reaching the steady-state conditions. These data underline the complexity of the antagonistic nature of LR-B/081, presenting features of both competitive and noncompetitive antagonism.

In vitro pharmacological characterization of UP 269-6, a novel nonpeptide angiotensin II receptor antagonist

f1p4in vitro pharmacology of UP 269-6, a novel nonpeptide angiotensin II antagonist, was examined in radioligand binding and functional isolated tissue assays. UP 269-6 bound selectively to AT1 receptors as evidenced by the inhibition of specific [125I] Sar1, Ile8-AII binding in rat adrenal membranes (IC50 = 35.8 nM) and in cultured vascular smooth muscle cells (IC50 = 23.8 nM). UP 269-6 displayed a very high selectivity for the AT1 compared to the AT2 receptor subtype (IC50 > 10,000 nM). UP 269-6 inhibited the AII-induced contraction of isolated rabbit aortic strips. The pattern of AII antagonism suggested competitive antagonism at low concentrations (10(-10), 3 x 10(-10), 10(-9) M) of UP 269-6 and insurmountable antagonism at higher concentrations (3 x 10(-9), 10(-8), 3 x 10(-8) M). Based on the calculated pA2 values, UP 269-6 (9.86 +/- 0.25) was an angiotensin II receptor antagonist as potent as L-158,809 (9.82 +/- 0.37) and much more potent than losartan (7.96 +/- 0.38). UP 269-6 was devoid of affinity (IC50 > 10,000 nM) for many other receptors, ion channels and uptake sites, demonstrating its high specificity for AII receptors. Furthermore, this compound did not affect the contractile response to KCl or phenylephrine in rabbit aorta and exhibited no effect on angiotensin converting enzyme activity. These data demonstrate that UP 269-6 is a highly potent, selective and specific AT1 receptor antagonist.

Differentiation between binding sites for angiotensin II and nonpeptide antagonists on the angiotensin II type 1 receptors

To characterize binding sites for nonpeptide angiotensin antagonists on the human angiotensin II receptor type 1 (AT1 receptor) we have systematically exchanged segments of the human receptor with corresponding segments from a homologous Xenopus laevis receptor, which does not bind the nonpeptide compounds. Substitution of transmembrane segment VII of the human AT1 receptor dramatically reduced the binding affinity of all of the 11 nonpeptide antagonists tested (55- to > 2000-fold) with no effect on the binding of angiotensin. The affinity for the nonpeptide compounds decreased additionally one order of magnitude when transmembrane segment VI and the connecting extracellular loop 3 from the Xenopus receptor were also introduced into the human AT1 receptor. Exchanges of smaller segments and single residues in transmembrane segments VI and VII and extracellular loop 3 revealed that the binding of nonpeptide antagonists was dependent on nonconserved residues located deep within the transmembrane segments VI and VII, in particular Asn295 in transmembrane segment VII. Surprisingly, all exchanges in transmembrane segment VII, including the Asn295 to Ser substitution, had a more pronounced effect on the binding of the competitive antagonists relative to the insurmountable antagonists. It is concluded that the binding mode for peptide and nonpeptide ligands on the AT1 receptor is rather different and that competitive and insurmountable antagonists presumably bind to overlapping but distinct sites located in transmembrane segments VI and VII.

Inhibitory effect of dithiothreitol on angiotensin II-induced contractions mediated by AT1-receptors in rat portal vein and rabbit aorta

The disulfide-reducing agent dithiothreitol (DTT) has been shown to reduce angiotensin II (Ang II) subtype 1 receptor (AT1) binding sites in various tissues. Its effect on Ang II-induced contractions was studied in the rat portal vein and rabbit aorta. In the isolated rat portal vein, DTT shifted the concentration-response curve for Ang II to the right (DTT 0.5-3 mmol/l) and depressed the maximal response (DTT 1-3 mmol/l). DTT 5 mmol/l almost abolished the effect of Ang II. In the isolated rabbit aorta, the inhibitory effect of DTT was more pronounced and its pattern of effect was different, since DTT 0.3 and 0.5 mmol/l caused a progressive flattening of the concentration-response curve of Ang II. DTT (1 mmol/l) fully suppressed the effect of Ang II. A biphasic curve consisting of a high sensitivity component and a component of low sensitivity for Ang II was observed after pretreatment with DTT 1 mmol/l in the rat portal vein but not in the rabbit aorta. In the presence of DTT 1 mmol/l, the AT1-receptor antagonist losartan antagonized the high sensitivity response to Ang II in a competitive manner with a pA2 value very similar to that obtained in the absence of DTT, suggesting that this response to Ang II is mediated by those AT1-receptors which were not inactivated by DTT. The biphasic curve may be explained by the occurrence of a single AT1-receptor subtype existing in two different states. Another possibility might be the involvement of two AT1-receptor subpopulations.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II receptor antagonists: a new approach to blockade of the renin-angiotensin system

A-II exerts its activity on various target tissues by binding to its receptors. The discovery of local RASs and A-II receptors within various tissues has generated interest in the clinical usefulness of RAS inhibition by directly blocking the action of A-II at the receptor level. Different A-II receptor subtypes have been identified and subsequently termed AT1 and AT2. AT1-receptor subtypes are the predominant receptor subtypes existing in most organs and, by coupling to a transmembrane G protein, seem to be the main subtypes participating in the vasoactive responses of A-II. Saralasin, a peptide with specific A-II receptor-antagonistic activity, had limited practical long-term usefulness as a result of its short half-life, significant agonistic properties, and lack of oral bioavailability. The discovery of simple benzyl-substituted imidazoles, which possess weak but highly selective A-II receptor antagonistic properties, led to the development of losartan (DuP 753). Losartan is a potent, orally active, specific, competitive nonpeptide A-II receptor antagonist that appears to be an effective antihypertensive agent both in animal studies and in preliminary clinical trials. The therapeutic usefulness of losartan, however, is not limited to its antihypertensive effects. The potential benefits of A-II receptor antagonists include roles in postmyocardial infarction therapy, slowing A-II-induced cardiac hypertrophy, 154, 155 slowing the progression of heart failure, preventing postangioplasty restenosis, and in slowing the progression of renal disease. Furthermore, losartan, a selective A-II type 1 (AT1) receptor antagonist, has also been a valuable pharmacologic probe for studying the mechanism of A-II stimulation of its receptors. A-II receptor antagonism appears to be as effective as ACE inhibition in the treatment of hypertension and other pathologic processes that involve the RAS and may offer an alternative to those patients who cannot tolerate ACE inhibitors because of their side effects.

The renin-angiotensin system in the brain: an update 1993

The renin-angiotensin system is considered to be one of the most important hormonal systems in the regulation of blood pressure and body fluid homeostasis. Ever since this system has been demonstrated to be present also in the brain, vast efforts have been made in investigating its central impact and function. The last few years, and especially the development of non-peptidic angiotensin II receptor subtype specific antagonists and the subsequent pharmacological characterization of these subtypes, brought this field of research a large step forward. This progress also might have opened up new avenues of developing highly specific anti-hypertensive drugs and thereby new ways of treating hypertension. This paper intends to provide a summary of the knowledge about the brain renin-angiotensin system accumulated during recent years; an update 1993.

Cholera toxin but not pertussis toxin inhibits angiotensin II-enhanced contractions in the rat portal vein

Angiotensin II (Ang II)-enhanced phasic contractions in the rat portal vein were concentration dependently inhibited by cholera toxin (0.1-10 micrograms/ml) and dibutyryl cyclic AMP (0.1-1 mM), but not by pertussis toxin (1 micrograms/ml), which suggests that Gi is not involved in the Ang II signal transduction pathway. It also seems likely that the effect of cholera toxin is due to its ability to increase cyclic AMP production through Gs.