Selective angiotensin II AT2 receptor agonists devoid of the imidazole ring system

N-(Heteroaryl)thiophene sulfonamides as angiotensin AT2 receptor ligands

Two series of N-(heteroaryl)thiophene sulfonamides, encompassing either a methylene imidazole group or a tert-butylimidazolylacetyl group in the meta position of the benzene ring, have been synthesized. An AT2R selective ligand with a Ki of 42 nM was identified in the first series and in the second series, six AT2R selective ligands with significantly improved binding affinities and Ki values of <5 nM were discovered. The binding modes to AT2R were explored by docking calculations combined with molecular dynamics simulations. Although some of the high affinity ligands exhibited fair stability in human liver microsomes, comparable to that observed with C21 undergoing clinical trials, most ligands displayed a very low metabolic stability with t½ of less than 10 min in human liver microsomes. The most promising ligand, with an AT2R Ki value of 4.9 nM and with intermediate stability in human hepatocytes (t½ = 77 min) caused a concentration-dependent vasorelaxation of pre-contracted mouse aorta.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

2-Alkyl substituted benzimidazoles as a new class of selective AT2 receptor ligands

Ligands comprising a benzimidazole rather than the imidazole ring that is common in AT₂R ligands e.g. in the AT₂R agonist C21, can provide both high affinity and receptor selectivity. In particular, compounds encompassing benzimidazoles, substituted in the 2-position with small bulky groups such as an isopropyl (K_i = 4.0 nM) or a tert-butyl (K_i = 5.3 nM) or alternatively a thiazole heterocycle (K_i = 5.1 nM) demonstrate high affinity and AT₂R selectivity. An n-butyl chain, as found in the AT₁R selective sartans, makes the ligand less receptor selective. The isobutyl group on the biaryl scaffold present in most AT₂R selective ligands reported so far was originally derived from the nonselective potent AT₁R/AT₂R ligand L-162,313. Notably, in all ligands discussed herein, the isobutyl group was substituted by an n-propyl group and ligands with high affinity to AT₂R were provided and in addition the majority of them demonstrate a favorable AT₂R/AT₁R selectivity. The introduction of fluoro atoms in various positions had no pronounced effect on the affinity data. Ligands with a thiazole or a tert-butyl group attached to the 2-position and with a terminal trifluoromethyl butoxycarbonyl sidechain exhibited a similar stability as C21 in human liver microsomes, while other ligands examined were less stable in the microsome assay.

Angiotensin II AT2 receptor ligands with phenylthiazole scaffolds

The syntheses and the AT₁R and AT₂R binding data of a series of new small molecule ligands are reported. These ligands comprise a phenylthiazole scaffold rather than the biphenyl or phenylthiophene scaffolds found in essentially all of the previously described ligands originating from the nonselective AT₁R/AT₂R ligand L-162,313 and the AT₂R selective agonist C21, the latter now in Phase II/III clinical trials. A phenylthiazole rather than the phenylthiophene scaffold that is present in the AT₂R selective agonist C21 and in the AT₂R selective antagonist C38 had a deleterious effect on the affinity to AT₂R. Nevertheless, a significant improvement could be accomplished by introduction of a small bulky alkyl group in the 2-position of the imidazole ring attached through a methylene group bridge to the phenylthiazole scaffold. Hence, a combination of a 2-tert-butyl or a 2-isopropyl group and a butoxycarbonyl furnished potent AT₂R selective ligands. Furthermore, a high affinity ligand derived from L-162,313 and exhibiting a > 35 fold selectivity for AT₁R was identified (10). The ligand 21 with the 2-tert-butyl group and ∼ 35 fold selectivity for AT₂R, demonstrated high stability in human, rat and mouse liver microsomes and a very attractive profile with regard to the inhibition of common drug-metabolizing CYP enzymes. Thus, very low levels of inhibition of CYP 3A (5%), 2D6 (12%), 2C8 (26%), 2C9 (23%) and 2B6 (24%) were observed with the 2-tert-butyl derivative comprising the methoxycarbonyl sulfonamide function, levels that are significantly lower than those obtained with C21 under the same experimental conditions.

N-(Methyloxycarbonyl)thiophene sulfonamides as high affinity AT2 receptor ligands

A series of meta-substituted acetophenone derivatives, encompassing N-(alkyloxycarbonyl)thiophene sulfonamide fragments have been synthesized. Several selective AT2 receptor ligands were identified, among those a tert-butylimidazole derivative (20) with a K_i of 9.3 nM, that demonstrates a high stability in human liver microsomes (t_½ = 62 min) and in human hepatocytes (t_½ = 194 min). This methyloxycarbonylthiophene sulfonamide is a 20-fold more potent binder to the AT2 receptor and is considerably more stable in human liver microsomes, than a previously reported and broadly studied structurally related AT₂R prototype antagonist 3 (C38). Ligand 20 acts as an AT₂R agonist and caused an AT₂R mediated concentration-dependent vasorelaxation of pre-contracted mouse aorta. Furthermore, in contrast to imidazole derivative C38, the tert-butylimidazole derivative 20 is a poor inhibitor of CYP3A4, CYP2D6 and CYP2C9. It is demonstrated herein that smaller alkyloxycarbonyl groups make the ligands in this series of AT₂R selective compounds less prone to degradation and that a high AT2 receptor affinity can be retained after truncation of the alkyloxycarbonyl group. Binding modes of the most potent AT₂R ligands were explored by docking calculations combined with molecular dynamics simulations.

Evolution of Angiotensin Peptides and Peptidomimetics as Angiotensin II Receptor Type 2 (AT2) Receptor Agonists

Angiotensin II receptor type 1 and 2 (AT1R and AT2R) are two G-protein coupled receptors that mediate most biological functions of the octapeptide Angiotensin II (Ang II). AT2R is upregulated upon tissue damage and its activation by selective AT2R agonists has become a promising approach in the search for new classes of pharmaceutical agents. We herein analyzed the chemical evolution of AT2R agonists starting from octapeptides, through shorter peptides and peptidomimetics to the first drug-like AT2R-selective agonist, C21, which is in Phase II clinical trials and aimed for idiopathic pulmonary fibrosis. Based on the recent crystal structures of AT1R and AT2R in complex with sarile, we identified a common binding model for a series of 11 selected AT2R agonists, consisting of peptides and peptidomimetics of different length, affinity towards AT2R and selectivity versus AT1R. Subsequent molecular dynamics simulations and free energy perturbation (FEP) calculations of binding affinities allowed the identification of the bioactive conformation and common pharmacophoric points, responsible for the key interactions with the receptor, which are maintained by the drug-like agonists. The results of this study should be helpful and facilitate the search for improved and even more potent AT2R-selective drug-like agonists.

The Other Angiotensin II Receptor: AT2R as a Therapeutic Target

The active hormone of the renin-angiotensin system (RAS), angiotensin II (Ang II), is involved in several human diseases, driving the development and clinical use of several therapeutic drugs, mostly angiotensin I converting enzyme (ACE) inhibitors and angiotensin receptor type I (AT₁R) antagonists. However, angiotensin peptides can also bind to receptors different from AT₁R, in particular, angiotensin receptor type II (AT₂R), resulting in biological and physiological effects different, and sometimes antagonistic, of their binding to AT₁R. In the present Perspective, the components of the RAS and the therapeutic tools developed to control it will be reviewed. In particular, the characteristics of AT₂R and tools to modulate its functions will be discussed. Agonists or antagonists to AT₂R are potential therapeutics in cardiovascular diseases, for agonists, and in the control of pain, for antagonists, respectively. However, controlling their binding properties and their targeting to the target tissues must be optimized.

High affinity rigidified AT2 receptor ligands with indane scaffolds

Rigidification of the isobutyl side chain of drug-like AT2 receptor agonists and antagonists that are structurally related to the first reported selective AT2 receptor agonist 1 (C21) delivered bioactive indane derivatives. Four enantiomer pairs were synthesized and the enantiomers were isolated in an optical purity >99%. The enantiomers 7a, 7b, 8a, 8b, 9a, 9b, 10a and 10b bind to the AT2 receptor with moderate (K i = 54-223 nM) to high affinity (K i = 2.2-7.0 nM). The enantiomer with positive optical rotation (+) exhibited the highest affinity at the receptor. The indane derivatives 7b and 10a are among the most potent AT2 receptor antagonists reported so far. As illustrated by the enantiomer pairs 7a/b and 10a/b, an alteration at the stereogenic center has a pronounced impact on the activation process of the AT2 receptor, and can convert agonists to antagonists and vice versa.

Neuroprotective effect of angiotensin II type 2 receptor stimulation in vincristine-induced mechanical allodynia

Peripheral neuropathy is the major dose-limiting side effect of many currently used chemotherapies, such as vincristine (VCR). We recently demonstrated that candesartan, an angiotensin II type 1 receptor antagonist, was neuroprotective against resiniferatoxin-induced sensory neuropathy, and that this effect is mediated by stimulation of the angiotensin II type 2 receptor (AT2R). Thus, we evaluated the effect of preventive treatment with candesartan and a specific AT2R agonist, C21, on a mouse model of VCR-induced neuropathy. Vincristine was administered daily for 7 days to male Swiss mice. Treatment with candesartan and C21 was started on day 1, before VCR treatment, and continued until day 7. We evaluated the development of VCR-induced neuropathy and the effect of treatment by functional tests, immunohistochemical analyses of intraepidermal nerve fibers and dorsal root ganglia neurons, and ultrastructural analysis of the sciatic nerve. Mice treated with VCR showed high mechanical allodynia but no modifications of motor performance or mechanical/thermal nociception. Treatment with candesartan and C21 completely restored normal tactile sensitivity of VCR-treated mice. Both drugs prevented VCR-induced nonpeptidergic intraepidermal nerve fiber loss. Only C21 displayed neuroprotective effects against VCR-induced loss and enlargement of myelinated nerve fibers in the sciatic nerve. Our finding that candesartan and C21 are protective against VCR-induced neuropathic pain through AT2R stimulation favors evaluation of its therapeutic potential in patients receiving chemotherapy.

A Series of Analogues to the AT2R Prototype Antagonist C38 Allow Fine Tuning of the Previously Reported Antagonist Binding Mode

We here report on our continued studies of ligands binding to the promising drug target angiotensin II type 2 receptor (AT2R). Two series of compounds were synthesized and investigated. The first series explored the effects of adding small substituents to the phenyl ring of the known selective nonpeptide AT2R antagonist C38, generating small but significant shifts in AT2R affinity. One compound in the first series was equipotent to C38 and showed similar kinetic solubility, and stability in both human and mouse liver microsomes. The second series was comprised of new bicyclic derivatives, amongst which one ligand exhibited a five-fold improved affinity to AT2R as compared to C38. The majority of the compounds in the second series, including the most potent ligand, were inferior to C38 with regard to stability in both human and mouse microsomes. In contrast to our previously reported findings, ligands with shorter carbamate alkyl chains only demonstrated slightly improved stability in microsomes. Based on data presented herein, a more adequate, tentative model of the binding modes of ligand analogues to the prototype AT2R antagonist C38 is proposed, as deduced from docking redefined by molecular dynamic simulations.

A convenient transesterification method for synthesis of AT2 receptor ligands with improved stability in human liver microsomes

A series of AT₂R ligands have been synthesized applying a quick, simple, and safe transesterification-type reaction whereby the sulfonyl carbamate alkyl tail of the selective AT₂R antagonist C38 was varied. Furthermore, a limited number of compounds where acyl sulfonamides and sulfonyl ureas served as carboxylic acid bioisosteres were synthesized and evaluated. By reducing the size of the alkyl chain of the sulfonyl carbamates, ligands 7a and 7b were identified with significantly improved in vitro metabolic stability in both human and mouse liver microsomes as compared to C38 while retaining the AT₂R binding affinity and AT₂R/AT₁R selectivity. Eight of the compounds synthesized exhibit an improved stability in human microsomes as compared to C38.

Structural determinants of subtype selectivity and functional activity of angiotensin II receptors

Agonists of the angiotensin II receptor type 2 (AT2), a G-protein coupled receptor, promote tissue protective effects in cardiovascular and renal diseases, while antagonists reduce neuropathic pain. We here report detailed molecular models that explain the AT2 receptor selectivity of our recent series of non-peptide ligands. In addition, minor structural changes of these ligands that provoke different functional activity are rationalized at a molecular level, and related to the selectivity for the different receptor conformations. These findings should pave the way to structure based drug discovery of AT2 receptor ligands.

Angiotensin type 2 receptor (AT2R) and receptor Mas: a complex liaison

The angiotensin type 2 receptor (AT2R) and the receptor Mas are components of the protective arms of the renin-angiotensin system (RAS), i.e. they both mediate tissue protective and regenerative actions. The spectrum of actions of these two receptors and their signalling mechanisms display striking similarities. Moreover, in some instances, antagonists for one receptor are able to inhibit the action of agonists for the respective other receptor. These observations suggest that there may be a functional or even physical interaction of both receptors. This article discusses potential mechanisms underlying the phenomenon of blockade of angiotensin-(1-7) [Ang-(1-7)] actions by AT2R antagonists and vice versa. Such mechanisms may comprise dimerization of the receptors or dimerization-independent mechanisms such as lack of specificity of the receptor ligands used in the experiments or involvement of the Ang-(1-7) metabolite alamandine and its receptor MrgD in the observed effects. We conclude that evidence for a functional interaction of both receptors is strong, but that such an interaction may be species- and/or tissue-specific and that elucidation of the precise nature of the interaction is only at the very beginning.

Interconversion of Functional Activity by Minor Structural Alterations in Nonpeptide AT2 Receptor Ligands

Migration of the methylene imidazole side chain in the first reported selective drug-like AT2 receptor agonist C21/M024 (1) delivered the AT2 receptor antagonist C38/M132 (2). We now report that the AT2 receptor antagonist compound 4, a biphenyl derivative that is structurally related to 2, is transformed to the agonist 6 by migration of the isobutyl group. The importance of the relative position of the methylene imidazole and the isobutyl substituent is highlighted herein.

Saralasin and Sarile Are AT2 Receptor Agonists

Saralasin and sarile, extensively studied over the past 40 years as angiotensin II (Ang II) receptor blockers, induce neurite outgrowth in a NG108-15 cell assay to a similar extent as the endogenous Ang II. In their undifferentiated state, these cells express mainly the AT2 receptor. The neurite outgrowth was inhibited by preincubation with the AT2 receptor selective antagonist PD 123,319, which suggests that the observed outgrowth was mediated by the AT2 receptor. Neither saralasin nor sarile reduced the neurite outgrowth induced by Ang II proving that the two octapeptides do not act as antagonists at the AT2 receptor and may be considered as AT2 receptor agonists.

N-Aryl Isoleucine Derivatives as Angiotensin II AT2 Receptor Ligands

A novel series of ligands for the recombinant human AT2 receptor has been synthesized utilizing a fast and efficient palladium-catalyzed procedure for aminocarbonylation as the key reaction. Molybdenum hexacarbonyl [Mo(CO)6] was employed as the carbon monoxide source, and controlled microwave heating was applied. The prepared N-aryl isoleucine derivatives, encompassing a variety of amide groups attached to the aromatic system, exhibit binding affinities at best with K i values in the low micromolar range versus the recombinant human AT2 receptor. Some of the new nonpeptidic isoleucine derivatives may serve as starting points for further structural optimization. The presented data emphasize the importance of using human receptors in drug discovery programs.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

Discovery of inhibitors of insulin-regulated aminopeptidase as cognitive enhancers

The hexapeptide angiotensin IV (Ang IV) is a metabolite of angiotensin II (Ang II) and plays a central role in the brain. It was reported more than two decades ago that intracerebroventricular injection of Ang IV improved memory and learning in the rat. Several hypotheses have been put forward to explain the positive effects of Ang IV and related analogues on cognition. It has been proposed that the insulin-regulated aminopeptidase (IRAP) is the main target of Ang IV. This paper discusses progress in the discovery of inhibitors of IRAP as potential enhancers of cognitive functions. Very potent inhibitors of the protease have been synthesised, but pharmacokinetic issues (including problems associated with crossing the blood-brain barrier) remain to be solved. The paper also briefly presents an overview of the status in the discovery of inhibitors of ACE and renin, and of AT1R antagonists and AT2R agonists, in order to enable other discovery processes within the RAS system to be compared. The paper focuses on the relationship between binding affinities/inhibition capacity and the structures of the ligands that interact with the target proteins.

From the first selective non-peptide AT(2) receptor agonist to structurally related antagonists

A para substitution pattern of the phenyl ring is a characteristic feature of the first reported selective AT(2) receptor agonist M024/C21 (1) and all the nonpeptidic AT(2) receptor agonists described so far. Two series of compounds structurally related to 1 but with a meta substitution pattern have now been synthesized and biologically evaluated for their affinity to the AT(1) and AT(2) receptors. A high AT(2)/AT(1) receptor selectivity was obtained with all 41 compounds synthesized, and the majority exhibited K(i) ranging from 2 to 100 nM. Five compounds were evaluated for their functional activity at the AT(2) receptor, applying a neurite outgrowth assay in NG108-15 cells. Notably, four of the five compounds, with representatives from both series, acted as potent AT(2) receptor antagonists. These compounds were found to be considerably more effective than PD 123,319, the standard AT(2) receptor antagonist used in most laboratories. No AT(2) receptor antagonists were previously reported among the derivatives with a para substitution pattern. Hence, by a minor modification of the agonist 1 it could be transformed into the antagonist, compound 38. These compounds should serve as valuable tools in the assessment of the role of the AT(2) receptor in more complex physiological models.

Effects of captopril and angiotensin II receptor blockers (AT1, AT2) on myocardial ischemia-reperfusion induced infarct size

The renin-angiotensin system (RAS) plays a major role in regulating the cardiovascular system, and disorders of the RAS contribute largely to the cardiac pathophysiology, including myocardial ischemia-reperfusion (MI/R) injury. Two subtypes of angiotensin II (Ang II) receptors have been defined on the basis of their differential pharmacological properties. The current study was undertaken to address the question as to whether the inhibition of the angiotensin converting enzyme (ACE) by captopril and the AT1 and AT2 receptor blockers losartan and PD123319 modulate MI/R-induced infarct size in an in vivo rat model. To produce necrosis, a branch of the descending left coronary artery was occluded for 30 min followed by two hours of reperfusion. ECG changes, blood pressure, and heart rate were measured during the experiment. Captopril (3 mg/kg), losartan (2 mg/kg), and PD123319 (20 μg/kg/min) were given in an IV 10 min before ischemia and were continued during the ischemic period. The infarcted area was measured by TTC staining. The volume of infarct and the risk zone was determined by planimetry. Compared to the control group (55.62±4.00%) both captopril and losartan significantly reduced the myocardial infarct size (30.50±3.26% and 37.75±4.44%), whereas neither PD123319 nor PD123319+losartan affected the infarct size volume (46.50±3.72% and 54.62±2.43%). Our data indicates that captopril and losartan exert cardioprotective activity after an MI/R injury. Also, infarct size reduction by losartan was halted by a blockade of the AT2 receptor. Therefore, the activation of AT2 receptors may be potentially protective and appear to oppose the effects mediated by the AT1 receptors.

Angiotensin II, a Neuropeptide at the Frontier between Endocrinology and Neuroscience: Is There a Link between the Angiotensin II Type 2 Receptor and Alzheimer's Disease?

Amyloid-β peptide deposition, abnormal hyperphosphorylation of tau, as well as inflammation and vascular damage, are associated with the development of Alzheimer's disease (AD). Angiotensin II (Ang II) is a peripheral hormone, as well as a neuropeptide, which binds two major receptors, namely the Ang II type 1 receptor (AT1R) and the type 2 receptor (AT2R). Activation of the AT2R counteracts most of the AT1R-mediated actions, promoting vasodilation, decreasing the expression of pro-inflammatory cytokines, both in the brain and in the cardiovascular system. There is evidence that treatment with AT1R blockers (ARBs) attenuates learning and memory deficits. Studies suggest that the therapeutic effects of ARBs may reflect this unopposed activation of the AT2R in addition to the inhibition of the AT1R. Within the context of AD, modulation of AT2R signaling could improve cognitive performance not only through its action on blood flow/brain microcirculation but also through more specific effects on neurons. This review summarizes the current state of knowledge and potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT2R activation by non-peptide and highly selective agonists, acting on neuronal plasticity, could represent new pharmacological tools that may help improve impaired cognitive performance in AD and other neurological cognitive disorders.

Selective angiotensin II AT2 receptor agonists: Benzamide structure-activity relationships

In the investigation of the structure-activity relationship of nonpeptide AT(2) receptor agonists, a series of substituted benzamide analogues of the selective nonpeptide AT(2) receptor agonist M024 have been synthesised. In a second series, the biphenyl scaffold was compared to the thienylphenyl scaffold and the impact of the isobutyl substituent and its position on AT(1)/AT(2) receptor selectivity was also investigated. Both series included several compounds with high affinity and selectivity for the AT(2) receptor. Three of the compounds were also proven to function as agonists at the AT(2) receptor, as deduced from a neurite outgrowth assay, conducted in NG108-15 cells.