Pharmacological properties of angiotensin II receptor antagonists.

The availability of selective, potent, orally active and long acting nonpeptide angiotensin II type 1 (AT1) receptor antagonists has provided the opportunity to obtain the benefits of selectively blocking the renin-angiotensin-aldosterone system at the level of the AT1 receptor that mediates most, if not all, of the important actions of angiotensin II, and avoid the nonspecificity of the angiotensin I converting enzyme inhibitors. Losartan was the first, but by no means remained the only, AT1 receptor antagonist. Numerous other 'sartans' have emerged in the past several years and successfully completed clinical development. With the exception of eprosartan, all others, ie, candesartan, irbesartan, saprisartan, tasosartan, telmisartan, valsartan and zolasartan, are based on medications of losartan's prototypical chemical structure. Among the current AT1 receptor antagonists, the rank order of the relative binding affinities (highest affinity = 1) is: candesartan 1, telmisartan 10, E3174 (the active metabolite of losartan) 10, tasosartan 20, losartan 50, eprosartan 100 and the prodrug candesartan cilexetil 280. The mode of (functional) AT1 receptor antagonism has been characterized as surmountable/noncompetitive (losartan, tasosartan, eprosartan) or insurmountable/noncompetitive (candesartan, saprisartan, zolasartan, irbesartan, valsartan, telmisartan, E3174). It is very likely that slow dissociation kinetics from the AT1 receptor underlie insurmountable antagonism. However, competitive or noncompetitive antagonism does not determine the antihypertensive efficacy, but the slow off-rate may extend the occupancy of the AT1 receptor and thereby lengthen the duration of the antihypertensive effect.

Angiotensin II receptor antagonists: an emerging new class of cardiovascular therapeutics.

Acceptance of the notion that physiologically specific interruption of the renin-angiotensin-aldosterone system (RAAS) is of considerable therapeutic benefit in the treatment of hypertension and congestive heart failure has generated great interest in the search for novel pharmacological inhibitors. The RAAS is expressed at the whole body, organ/tissue and cellular level through the action of the octapeptide angiotensin II (Ang II), the primary effector molecule of the RAAS. The availability of selective, potent, orally active and long-acting nonpeptide Ang II type 1 (AT1) receptor antagonists provided the opportunity to obtain the benefits of selectively blocking the RAAS at the level of the AT1 receptor that mediates most, if not all, of the important actions of Ang II, and avoid the nonspecificity of the Ang I converting enzyme (ACE) inhibitors. Losartan was the first, but by no means remained the only nonpeptide AT1 receptor antagonist. Numerous other "sartans" have emerged in the past several years and successfully completed clinical development. With the exception of Eprosartan, all others, i.e. Candesartan, Irbesartan, Saprisartan, Tasosartan, Telmisartan, Valsartan and Zolasartan, are based on modifications of Losartan's prototypic chemical structure. AT1 receptor antagonists represent the newest addition to the arsenal of cardiovascular therapeutics. The predominant role of the AT1 receptor in mediating the pathophysiological role of Ang II underlies the effectiveness of this novel class of agents to lower arterial blood pressure, reduce pre- and afterload, inhibit sympathetic nervous system activity and prevent cardiovascular hypertrophy and cardiac failure induced by inappropriate control of the RAAS.

Selective, covalent modification of beta-tubulin residue Cys-239 by T138067, an antitumor agent with in vivo efficacy against multidrug-resistant tumors.

Microtubules are linear polymers of alpha- and beta-tubulin heterodimers and are the major constituents of mitotic spindles, which are essential for the separation of chromosomes during mitosis. Here we describe a synthetic compound, 2-fluoro-1-methoxy-4-pentafluorophenylsulfonamidobenzene (T138067), which covalently and selectively modifies the beta1, beta2, and beta4 isotypes of beta-tubulin at a conserved cysteine residue, thereby disrupting microtubule polymerization. Cells exposed to T138067 become altered in shape, indicating a collapse of the cytoskeleton, and show an increase in chromosomal ploidy. Subsequently, these cells undergo apoptosis. Furthermore, T138067 exhibits cytotoxicity against tumor cell lines that exhibit substantial resistance to vinblastine, paclitaxel, doxorubicin, and actinomycin D. T138067 is also equally efficacious in inhibiting the growth of sensitive and multidrug-resistant human tumor xenografts in athymic nude mice. These observations suggest that T138067 may be clinically useful for the treatment of multidrug-resistant tumors.

Rationale, background, and design of the randomized angiotensin receptor antagonist--angiotensin-converting enzyme inhibitor study (RAAS).

The randomized angiotensin receptor antagonist--angiotensin converting enzyme (ACE)--Inhibitor Study (RAAS) was designed to test the hypothesis that the addition of an angiotensin II type 1 receptor blocking agent, losartan 50 mg/day, to an ACE-inhibitor, enalapril 10 mg twice a day (group 1), will be more effective than standard-dose enalapril 10 mg twice a day (group 2) or high-dose enalapril alone 20 mg twice a day (group 3), in blocking the activation of the renin angiotensin aldosterone system in patients with heart failure and left ventricular systolic dysfunction. The addition of an angiotensin II type 1 receptor blocking agent to an ACE inhibitor would theoretically block ACE as well as non-ACE-dependent angiotensin II formation while maintaining the potential beneficial effect of ACE inhibitor-induced bradykinin formation. One hundred twenty patients with left ventricular systolic dysfunction and moderate to severe heart failure despite treatment with an ACE inhibitor will be randomized to 1 of the 3 groups and followed for 6 weeks, with an optional long-term week extension to determine the safety and tolerability of the combination of losaratan and enalapril, its effectiveness in preventing rest and exercise-induced neurohumoral activation (plasma norepinephrine, N-terminal proatrial natriuretic factor, angiotensin II, and aldosterone), as well as quality of life and exercise performance (6-minute walk test).

Historical development of losartan (DuP 753) and angiotensin II receptor subtypes.

Angiotensin II is an active peptide of the renin-angiotensin system, whose biological actions are mediated through cell surface receptors. It is an important hormone in the regulation of blood pressure, fluid and electrolyte balance. This report describes the historical development of non-peptide angiotensin II receptor antagonists and angiotensin II receptor subtypes. Previous receptor antagonists for angiotensin II are angiotensin-like peptides with limitations of short duration, lack of oral bioavailability and partial agonistic activity. Recently, we have developed the first long-acting and orally-active non-peptide angiotensin II receptor antagonist losartan (DuP 753) which does not have agonistic activities. The discovery of losartan provides a potentially important therapy for the treatment of hypertension and congestive heart failure. Moreover, the use of losartan and other angiotensin II receptor antagonists in research has rapidly expanded our understanding of the physiological and pathophysiological roles of angiotensin II and led to the identification of receptor subtypes, AT1 and AT2. All the known physiological effects of angiotensin II including vasoconstriction and aldosterone release, which can be inhibited by losartan, are attributable to the AT1 receptor subtype. Whether the AT2 receptors serve any important physiological functions remain to be determined.

The diversified pharmacology of angiotensin II-receptor blockade.

The discovery of orally active nonpeptide angiotensin II (A II)-receptor antagonists has initiated a growing understanding of the physiologic and pathophysiologic roles of A II. Losartan is the first of the new class of antagonists that block all the well-known effects of A II, including vasoconstriction, aldosterone release, renin release (negative feedback), and the stimulation of thirst. A II-receptor subtypes have been described, with losartan antagonism defining the AT1 subtype and with PD123319 antagonism defining the AT2 subtype. The AT1 receptor is G-protein-coupled, involving PLC, PLA2, PLD, or adenylate cyclase and the release of intracellular calcium. The receptor-response coupling of the AT2 site remains elusive but may involve protein tyrosine phosphatase and subserve an antiproliferative role. Losartan as the prototype of an AT1-selective antagonist: i) inhibits A II binding, ii) antagonizes effects of A II in vivo and in vitro, and iii) lowers blood pressure in models of A II-dependent hypertension A II stimulates growth in vitro (DNA and protein synthesis) and in vivo (cardiac and vascular hypertrophy), and these effects are blocked by losartan. Losartan, like angiotensin-converting enzyme inhibitors, has significant renal, cardiac, and cerebral protective effects in models of renal failure, cardiac failure, and stroke, confirming the pathologic role of A II in these models. The pioneering studies in experimental animals are being confirmed by a growing number of other AT1-selective blockers and provide the basis of use of losartan for hypertension and its clinical trial in other disease states.

Pharmacology and pharmacokinetics of a novel nonpeptide angiotensin II receptor antagonist--DMP 811.

DMP 811 exhibited high binding affinity for the angiotensin II subtype receptor AT1 in rat adrenal tissues with an IC50 of 6 nM, but not for the subtype receptor AT2. In the isolated rabbit aorta, DMP 811 inhibited the contractile response to angiotensin II selectively and noncompetitively with a KB value of 0.1 nM. In conscious renal hypertensive rats, DMP 811 decreased blood pressure with i.v. and p.o. ED30s of 0.005 and 0.03 mg/kg, respectively (p.o. ED30 for losartan = 0.59 mg/kg). In conscious furosemide-treated dogs, DMP 811 given either at 0.3 or 1 mg/kg p.o. decreased blood pressure. DMP 811 has oral bioavailabilities of 7 and 29% in rats and dogs, respectively, after a solution dose and 8 and 13%, respectively, after a suspension or capsule dosing. Our study indicates that DMP 811 is a selective and insurmountable AT1 receptor antagonist and is a 20-fold more potent orally-active antihypertensive agent than losartan.

Discovery of losartan, the first angiotensin II receptor antagonist.

The 'discovery' of losartan represents three separate discoveries: (1) losartan as the unique biphenyltetrazole molecule and the first of a new chemical class; (2) losartan as a tool to identify AT1-subtype receptors; and (3) losartan as a specific probe for exploring the multiple roles of angiotensin II (Ang II) in normal physiology and pathologic states. Losartan is the first nonpeptide orally active Ang II receptor antagonist to reach clinical trials. Losartan was selected for its affinity for Ang II receptors, functional antagonism of Ang II, lack of agonist properties, and oral anti-hypertensive effects. Losartan has been widely used to define the distribution and function of AT receptor subtypes. Although possible roles of the AT2 subtype have been reported, virtually all of the known effects of Ang II are blocked by losartan. Specific AT1 receptor blockade has been broadly compared with ACE inhibition. Possible differences on the basis of AT1 selectivity, bradykinin potentiating effects and Ang II formed by non-ACE pathways are discussed. Losartan blocks the vascular constrictor effect of Ang II, the Ang II-induced aldosterone synthesis and/or release, and the Ang II-induced cardiovascular 'growth' in vitro and in vivo. In various models of experimental hypertension, losartan prevents or reverses the elevated blood pressure and the associated cardiovascular hypertrophy similar to ACE inhibitors. Likewise, in models of renal failure (for example reduced renal mass, puromycin, ochratoxin), losartan, like ACE inhibition, markedly reduced the elevation in blood pressure, proteinuria or sclerosis. In aortocaval shunt, coronary ligation and ventricular pacing models of heart failure, losartan demonstrated a pathological role for Ang II by reversing the associated haemodynamic findings. In SHR-stroke prone, losartan dramatically increased survival while having a limited effect on blood pressure, suggesting a non-pressure dependent effect of Ang II. These collective data show that Ang II exerts complex pathological effects in experimental models of vascular, cardiac, renal and cerebral disease. The effectiveness of losartan in experimental models of heart failure supports its evaluation in clinical trials with patients with heart failure.

Angiotensin II receptor antagonists in heart failure: rationale and design of the evaluation of losartan in the elderly (ELITE) trial.

Angiotensin-converting enzyme inhibitors (ACE-I) have been proven to be effective in reducing morbidity and mortality in patients with heart failure or post-myocardial infarction left ventricular dysfunction. Despite evidence from several large-scale randomized trials, the use of ACE-I in patients with heart failure remains relatively low. In part, the failure to achieve more widespread use of ACE-I in patients with heart failure may be due to physician's perceptions of the side effects associated with ACE-I, such as angioedema, renal dysfunction, cough, and hypotension. Many of these side effects are thought to be due to ACE-I-induced bradykinin accumulation. It is possible to inhibit the effect of angiotensin II without increasing bradykinin levels using an angiotensin II type I blocking agent such as losartan. How effective losartan is compared with an ACE-I is uncertain, however. Some of the beneficial effects of ACE-I have been attributed to bradykinin accumulation, and therefore ACE-I might have an advantage compared with an angiotensin II type I receptor antagonist such as losartan. On the other hand, angiotensin II may be produced by non-ACE-I-dependent mechanisms, which would suggest that an angiotensin II type I receptor blocking agent would be advantageous. To determine the relative safety and efficacy of an ACE-I, which results in bradykinin accumulation and inhibitors of angiotensin II, versus an angiotensin II type I receptor blocking agent, which does not result in bradykinin accumulation, we have begun the Evaluation of Losartan In The Elderly (ELITE) trial, which will compare the safety and efficacy of captopril and losartan in elderly patients with heart failure.

The preclinical basis of the therapeutic evaluation of losartan.

ANGIOTENSIN II RECEPTOR ANTAGONISTS: Losartan (DuP 753, MK-954) is the prototype of a new class of orally active, non-peptide angiotensin II receptor antagonists able to inhibit the renin-angiotensin system specifically and selectively without the agonistic effects of the peptide receptor antagonists, e.g. saralasin, or the bradykinin-potentiating effects of the angiotensin converting enzyme (ACE) inhibitors. PRECLINICAL PHARMACOLOGY: The preclinical pharmacology of angiotensin II receptor blockade is exemplified by the experience with losartan. Over 1200 abstracts and papers have been published from studies in which losartan has been used to explore the role of angiotensin II in a wide range of normal and pathological states. Losartan has also proved useful in further defining the heterogeneity of angiotensin II receptors. According to current nomenclature, losartan represents the prototype antagonist of the angiotensin II type 1 (AT1) receptor family (AT1a and AT1b) and does not possess significant affinity for the so-called AT2 receptor. Virtually all of the known actions of angiotensin II, e.g. those defined by angiotensin II itself, saralasin, ACE inhibitors or renin inhibitors, are blocked by losartan, emphasizing the major role of AT1 receptors in mediating the responses of angiotensin II. Although the AT2 receptor has now been cloned, the function of this receptor remains poorly understood. PRECLINICAL STUDIES WITH LOSARTAN: Preclinical studies with losartan have suggested that this agent produces inhibition of the renin-angiotensin system comparable to that of ACE (and renin) inhibitors, without the bradykinin-potentiating effects. In several models of experimental and genetic hypertension losartan has proved to be an orally effective antihypertensive agent with a long duration of action and similar efficacy to that of ACE and renin inhibitors. In animal models of renal disease losartan significantly decreases proteinuria, provides protection against diabetic glomerulopathy and increases survival in stroke-prone spontaneously hypertensive rats. A growing number of experimental studies have also shown that losartan inhibits neointimal proliferation and markedly reduces or prevents cardiovascular hypertrophy/remodeling and cardiac failure mediated by activation of the renin-angiotensin system. Non-peptide AT1 receptor antagonists have added another dimension to the arsenal of drugs manipulating the renin-angiotensin system. These agents do not have the experimental limitations of the peptide antagonists and ACE inhibitors. CONCLUSIONS: Losartan, the first potent and specific AT1 receptor antagonist, is orally active with a long duration of action and therefore has potential for treatment of chronic diseases, such as hypertension and heart failure.

Balanced AT1/AT2 receptor antagonists. 4. XR510 and related 5-(3-amidopropanoyl)imidazoles possessing equal affinity for the AT1 and AT2 receptors.

The identification of the AT1 and AT2 receptor subtypes has stimulated interest in developing balanced angiotensin II receptor antagonists. A series of 5-(3-amidopropanoyl)imidazoles has been prepared which possess balanced affinity for the AT1 and AT2 receptors. XR510 (1), 1-[[2'-[[(isopentoxycarbonyl)amino]sulfonyl]-3- fluoro(1,1'-biphenyl)-4-yl]methyl]-5-[3-(N-pyridin-3- ylbutanamido)propanoyl]-4-ethyl-2-propyl-1H-imidazole, potassium salt, exhibits subnanomolar affinity for both receptor sites. XR510 is very active in lowering blood pressure in renal hypertensive rats and furosemide-treated dogs following oral administration.

A new class of therapeutic agents: the angiotensin II receptor antagonists.

Angiotensin II (Ang II) is the primary mediator of the renin-angiotensin system (RAS). Inappropriate control of the RAS is critically involved in the development and maintenance of hypertension and congestive heart failure. The actions of Ang II are thought to be mediated by specific surface receptors on the various target organs. At present, two receptors for Ang II have been firmly established in mammals, including man. According to current nomenclature, losartan represents the prototype antagonist of the Ang II type 1 (AT1) receptor and does not possess significant affinity for the so-called AT2 receptor. Losartan is the first of a new class of orally active, nonpeptide Ang II receptor antagonists able to very specifically and selectively inhibit the RAS while lacking the agonistic effects of the peptide receptor antagonists, e.g. sarlasin, or the bradykinin potentiating effects of the angiotensin converting enzyme (ACE) inhibitors. Virtually all of the known actions of Ang II, e.g. those defined by Ang II itself, saralasin, ACE or renin-inhibitors are blocked by losartan, emphasizing the major role of this distinct Ang II receptor subtype in mediating the responses of Ang II. The functional correlate of the AT2 receptor remains poorly understood. In several models of experimental and genetic hypertension, AT1 receptor antagonists are effective antihypertensive agents with similar efficacy to that of ACE and renin-inhibitors. In animal models of renal disease, AT1 receptor antagonists significantly decrease proteinuria, protect against diabetic glomerulopathy and increase survival in stroke-prone spontaneously hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II receptor subtypes: selective antagonists and functional correlates.

Angiotensin II (Ang II) receptor heterogeneity is currently defined by the new subtype-selective agents, losartan (AT1) and PD123177 (AT2). Although both subtypes have been cloned and sequenced, only the AT1 receptor has been shown to have an important physiological or pathophysiological role. AT1 and AT2 receptors are found in both normal and failing cardiac tissue. They are found on myocytes, endothelial cells, fibroblasts, coronary arterial smooth muscle cells, and peripheral sympathetic nerves. The AT1 receptors mediate virtually all of the effects of Ang II in myocytes even though cardiac tissue may contain over 50% AT2 sites. In endothelial cells, functional responses are predominately AT1. In fibroblasts, preliminary data suggest that AT2 receptors may be involved in collagen synthesis. In isolated tissue, Ang II has a limited positive inotropic effect in atrial, but not in ventricular tissue, which is blocked by losartan. Ang II may also have a tonic effect on coronary artery resistance as angiotensin inhibitors can increase coronary flow. Both ACE (Ang II synthesis) inhibitors and Ang II receptor antagonists produce beneficial effects in experimental models of heart failure, suggesting Ang II is an important mediator of heart failure. Because ACE inhibitors also potentiate bradykinin and are non-specific inhibitors of Ang II synthesis (availability of Ang II to both receptor subtypes) some differences can be anticipated. At the present time, however, the beneficial role of bradykinin is controversial and the predominant functional Ang II receptor in the heart and other tissues is the AT1 subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nifedipine on phasic and tonic vasoconstrictor responses to Sgd 101/75 in the isolated perfused rabbit ear artery.

The relative importance of extracellular Ca2+ influx and intracellular Ca2+ release for the vasoconstrictor response to Sgd 101/75 (4(2-imidazoline-amino)-2-methyl-indazol-chlorhydrate), an alpha 1-adrenoceptor agonist, was examined in perfused rabbit ear arteries. Sdg 101/75 induced a phasic and a tonic vasoconstrictor response in a concentration-dependent manner. It is a full agonist on the tonic response but a partial agonist on the phasic response. Nifedipine preferentially inhibited the tonic but not the phasic response to Sgd 101/75. Therefore, Sgd 101/75 appears to require both the intracellular Ca2+ release and extracellular Ca2+ influx for its phasic and tonic responses in rabbit ear arteries, respectively.

EXP597, a nonpeptide angiotensin II receptor antagonist with high affinities for the angiotensin AT1 and AT2 receptor subtypes.

AT1 and AT2 are the two major receptor subtypes for angiotensin II that have been pharmacologically defined by using the selective ligands losartan and PD123177, respectively. EXP597 (4-[(5-(2-benzoyl)benzyloxycarbonyl-4-ethyl-2-n-propylimidazole-1- yl)methyl]-3-fluoro-2'-isoamyloxycarbonylaminosulfonyl-[1,1']-biph enyl, potassium salt) is a nonpeptide angiotensin II receptor ligand which in the rat adrenal exhibits binding affinities (IC50) of 0.5 and 0.7 nM for angiotensin AT1 and AT2 receptor subtypes, respectively. Further, EXP597 is an insurmountable angiotensin II receptor antagonist in the isolated rabbit aorta and lowers blood pressure in renal hypertensive rats with i.v. and p.o. ED30 values of 0.05 and 0.9 mg/kg, respectively.

Human angiotensin receptor subtypes.

The rapid expansion of our knowledge of angiotensin receptors has been led by the development of subtype-specific angiotensin II receptor antagonists and by the cloning and sequencing of the AT1 receptor in angiotensin II. Although some actions of angiotensin II have been attributed to AT2 subtype receptors, the importance of these binding sites remains elusive. Nonpeptide, AT1-selective antagonists, such as losartan, block virtually all of the well-known effects of angiotensin II in mammalian cells. The effects of losartan are now being confirmed by the rapidly developing class of nonpeptide, AT1-selective angiotensin II receptor antagonists. In rodents, subtypes of the AT1 receptor have been cloned and designated AT1A and AT1B, but they have not been functionally distinguished. Such isoforms have not been identified for human AT1 receptors. Importantly, it now appears that the AT2 receptor has been cloned. The angiotensin receptors undergoing intense study are those in fetal tissue, brain, endothelial cells, and fibroblasts. Angiotensin II-induced growth and cardiovascular hypertrophy are the focus of broad-based research efforts. The clinical relevance of angiotensin II receptor subtypes is unknown, but there is growing evidence that AT1-selective agents are effective inhibitors of the angiotensin system in humans.

A novel series of selective, non-peptide inhibitors of angiotensin II binding to the AT2 site.

The availability of peptide and non-peptide Ang II receptor antagonists has permitted the study of Ang II receptor heterogeneity. It is now widely recognized that there are at least two distinct Ang II receptor subtypes. AT1 receptors are selective in their recognition of agents such as losartan, DuP 532, L-158,809, SK&F108566, and similar non-peptides. To date, all of the well-known actions of Ang II in mammals are blocked by the AT1 selective antagonists such as losartan and are thus designated as being mediated by the AT1 receptor. Although there have been reports of functional activity mediated through AT2 sites, the pharmacological role for the AT2 receptor has not yet been elucidated. Herein, we report the chemistry and SAR on a novel series of 1,2,3,4-tetrahydrosioquinoline-3-carboxylic acids which have selective affinity for AT2 receptors. The most potent of which (19) has an IC50 of 30 nM for the AT2 receptor in the rat adrenal radioligand binding assay.

EXP063, a nonpeptide angiotensin II and beta adrenergic receptor antagonist.

EXP063 (4-[[4-[[3-(N-isopropylamino)-2-hydroxypropyl]oxy]- indole-2-carboxamido]methyl]-2-propyl-1-[(2'-(1H-tetrazol-5-yl)bip henyl-4 - yl)methyl]imidazole-5-carboxylic acid) was designed to possess both the angiotensin II (Ang II) and beta adrenergic receptor antagonistic properties. EXP063 inhibited the specific binding of [125I]Sar1,lle8-Ang II in rat adrenal membranes with Ki values of 3.9 +/- 0.6 nM for the Ang II type 1 and > 1 microM for the Ang II type 2 receptor binding sites. It displaced [3H]dihydroalprenolol in the rat cerebral frontal cortex with a Ki of 80 +/- 13 nM. EXP063 antagonized the contractile effect of Ang II competitively (pA2 = 8.9 +/- 0.1) and selectively in rabbit aorta and guinea pig ileum. EXP063 appears to be a partial beta adrenoceptor agonist as it increased heart rate in vitro and in vivo. At 1 and 10 microM, it inhibited the positive inotropic effect of isoproterenol in guinea pig atria. In pithed rats, EXP063 was more potent in blocking the pressor effect of Ang II than the positive chronotropic effect of isoproterenol. In renal hypertensive rats, EXP063 given i.v. produced a long-lasting decrease in blood pressure for at least 6 hr with an ED30 of 0.53 mg/kg. In summary, this study demonstrates that EXP063 is a novel chemical entity possessing both the Ang II and beta adrenergic receptor blocking properties and, thus, represents a promising agent for the treatment of hypertension and congestive heart failure.