A selective AT2 receptor ligand with a gamma-turn-like mimetic replacing the amino acid residues 4-5 of angiotensin II

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.

Efficient Synthesis of Polysubstituted 1,5-Benzodiazepinone Dipeptide Mimetics via an Ugi-4CR-Ullmann Condensation Sequence

An efficient three-step synthesis towards 3-amino-1,4-benzodiazepin-2-one derivatives is presented. The versatile Ugi-4-component reaction (Ugi-4CR) and Boc deprotection is followed by a ligand-free Ullmann condensation. This protocol allows the rapid construction of a diverse array of substituted 1,5-benzodiazepinones. Since Ugi-based products are typically limited by their ‘inert’ C-terminal amides, the use of a convertible (‘cleavable’) isocyanide was envisaged and resulted in building blocks that can be made SPPS compatible. To demonstrate the potential of this novel synthetic route, the design and preparation of novel phenylurea-1,5-benzodiazepin-4(5H)-one dipeptide mimetics with potential CCK2-antagonist properties is reported.

Recent Advances in the Synthesis of Heterocycles by the Aza-Wittig Reaction

The formation of the C=N bond in recent studies on heterocyclic compounds via the aza-Wittig reaction is reviewed. Furthermore, two different strategies for the formation of heterocyclic compounds, including intermolecular and intramolecular aza-Wittig reactions are described. The primary aim of this review is to provide up-to-date information on the application of the aza-Wittig reaction in the synthesis of a wide range of N-containing heterocyclic compounds in the chemical literature since 2010.

1 Introduction

2 Mechanism of the Staudinger and Aza-Wittig Reactions

3 Intramolecular Aza-Wittig Reaction

4 Intermolecular Aza-Wittig Reaction

5 Conclusion

Update on Angiotensin II Subtype 2 Receptor: Focus on Peptide and Nonpeptide Agonists

Angiotensin II (Ang II) is the most dominant effector component of the renin-angiotensin system (RAS) that generally acts through binding to two main classes of G protein-coupled receptors, namely Ang II subtype 1 receptor (AT1R) and angiotensin II subtype 2 receptor (AT2R). Despite some controversial reports, the activation of AT2R generally antagonizes the effects of Ang II binding on AT1R. Studying AT2R signaling, function, and its specific ligands in cell culture or animal studies has confirmed its beneficial effects throughout the body. These characteristics classify AT2R as part of the protective arm of the RAS that, along with functions of Ang (1-7) through Mas receptor signaling, modulates the harmful effects of Ang II on AT1R in the activated classic arm of the RAS. Although Ang II is the primary ligand for AT2R, we have summarized other natural or synthetic peptide and nonpeptide agonists with critical evaluation of their structure, mechanism of action, and biologic activity. SIGNIFICANCE STATEMENT: AT2R is one of the main components of the RAS and has a significant prospective for mediating the beneficial action of the RAS through its protective arm on the body's homeostasis. Targeting AT2R offers substantial clinical application possibilities for modulating various pathological conditions. This review provided concise information regarding the AT2R peptide and nonpeptide agonists and their potential clinical applications for various diseases.

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.

Mechanism of Hormone Peptide Activation of a GPCR: Angiotensin II Activated State of AT1R Initiated by van der Waals Attraction

We present a succession of structural changes involved in hormone peptide activation of a prototypical GPCR. Microsecond molecular dynamics simulation generated conformational ensembles reveal propagation of structural changes through key "microswitches" within human AT₁R bound to native hormone. The endocrine octa-peptide angiotensin II (AngII) activates AT₁R signaling in our bodies which maintains physiological blood pressure, electrolyte balance, and cardiovascular homeostasis. Excessive AT₁R activation is associated with pathogenesis of hypertension and cardiovascular diseases which are treated by sartan drugs. The mechanism of AT₁R inhibition by sartans has been elucidated by 2.8 Å X-ray structures, mutagenesis, and computational analyses. Yet, the mechanism of AT₁R activation by AngII is unclear. The current study delineates an activation scheme initiated by AngII binding. A van der Waals "grasp" interaction between Phe8^AngII with Ile288^7.39 in AT₁R induced mechanical strain pulling Tyr292^7.43 and breakage of critical interhelical H-bonds, first between Tyr292^7.43 and Val108^3.32 and second between Asn111^3.35 and Asn295^7.46. Subsequently changes are observed in conserved microswitches DRY^TM3, Yx7K(R)^TM5, CWxP^TM6, and NPxxY^TM7 in AT₁R. Activating the microswitches in the intracellular region of AT₁R may trigger formation of the G-protein binding pocket as well as exposure of helix-8 to cytoplasm. Thus, the active-like conformation of AT₁R is initiated by the van der Waals interaction of Phe8^AngII with Ile288^7.39, followed by systematic reorganization of critical interhelical H-bonds and activation of microswitches.

Angiotensin II analogs comprised of Pro-Amb (γ-turn scaffold) as angiotensin II type 2 (AT2) receptor agonists

We describe herein the design, synthesis and conformational investigation of Pro-Amb (proline-3-amino-2-methoxybenzoic acid) incorporated Angiotensin II and its truncated analogues.

Structural determinants for binding to angiotensin converting enzyme 2 (ACE2) and angiotensin receptors 1 and 2

Angiotensin converting enzyme 2 (ACE2) is a zinc carboxypeptidase involved in the renin–angiotensin system (RAS) and inactivates the potent vasopressive peptide angiotensin II (Ang II) by removing the C-terminal phenylalanine residue to yield Ang1–7. This conversion inactivates the vasoconstrictive action of Ang II and yields a peptide that acts as a vasodilatory molecule at the Mas receptor and potentially other receptors. Given the growing complexity of RAS and level of cross-talk between ligands and their corresponding enzymes and receptors, the design of molecules with selectivity for the major RAS binding partners to control cardiovascular tone is an on-going challenge. In previous studies we used single β-amino acid substitutions to modulate the structure of Ang II and its selectivity for ACE2, AT₁R, and angiotensin type 2 (AT₂R) receptor. We showed that modification at the C-terminus of Ang II generally resulted in more pronounced changes to secondary structure and ligand binding, and here, we further explore this region for the potential to modulate ligand specificity. In this study, (1) a library of 47 peptides derived from the C-terminal tetrapeptide sequence (-IHPF) of Ang II was synthesized and assessed for ACE2 binding, (2) the terminal group requirements for high affinity ACE2 binding were explored by and N- and C-terminal modification, (3) high affinity ACE2 binding chimeric AngII analogs were then synthesized and assessed, (4) the structure of the full-length Ang II analogs were assessed by circular dichroism, and (5) the Ang II analogs were assessed for AT₁R/AT₂R selectivity by cell-based assays. Studies on the C-terminus of Ang II demonstrated varied specificity at different residue positions for ACE2 binding and four Ang II chimeric peptides were identified as selective ligands for the AT₂ receptor. Overall, these results provide insight into the residue and structural requirements for ACE2 binding and angiotensin receptor selectivity.

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.

Synthesis and evaluation of isoleucine derived angiotensin II AT(2) receptor ligands

Sixteen new C-terminally modified analogues of 2, a previously described potent and selective AT2R ligand, were designed, synthesized and evaluated for their affinity to the AT2R receptor. The introduction of large, hydrophobic substituents was shown to be beneficial and the most active compound (17, Ki=8.5 μM) was over 12-times more potent than the lead compound 2.

Update on the angiotensin AT(2) receptor

It is quite well established that activation of the AT(2) receptor (AT(2)R) provides a counter-regulatory role to AT(1)R overactivity, particularly during pathological conditions. Indeed, a potential therapeutic role for the AT(2)R is currently being promulgated with the introduction of novel AT(2)R ligands such as compound 21 (C21). In this brief review, we will focus on recent evidence to suggest that AT(2)R exhibits promising organ protection in the context of the heart, kidney and brain, with inflammation and gender influencing outcome. However, this field is not without controversy since the 'flagship' ligand C21 has also come under scrutiny, although it is safe to say there is much evidence to support a potentially important role of AT(2)R in a number of cardiovascular diseases. This report updates recent data in this field.

ANP and BNP responses to dehydration in the one-humped camel and effects of blocking the renin-angiotensin system

The objectives of this study were to investigate and compare the responses of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) in the circulation of hydrated, dehydrated, and dehydrated losartan - treated camels; and to document the cardiac storage form of B-type natriuretic peptide in the camel heart. Eighteen male camels were used in the study: control or hydrated camels (n = 6), dehydrated camels (n = 6) and dehydrated losartan-treated camels (n = 6) which were dehydrated and received the angiotensin II (Ang II) AT-1 receptor blocker, losartan, at a dose of 5 mg/kg body weight intravenously for 20 days. Control animals were supplied with feed and water ad-libitum while both dehydrated and dehydrated-losartan treated groups were supplied with feed ad-libitum but no water for 20 days. Compared with time-matched controls, dehydrated camels exhibited a significant decrease in plasma levels of both ANP and BNP. Losartan-treated camels also exhibited a significant decline in ANP and BNP levels across 20 days of dehydration but the changes were not different from those seen with dehydration alone. Size exclusion high performance liquid chromatography of extracts of camel heart indicated that proB-type natriuretic peptide is the storage form of the peptide. We conclude first, that dehydration in the camel induces vigorous decrements in circulating levels of ANP and BNP; second, blockade of the renin-angiotensin system has little or no modulatory effect on the ANP and BNP responses to dehydration; third, proB-type natriuretic peptide is the storage form of this hormone in the heart of the one-humped camel.

Responses to dehydration in the one-humped camel and effects of blocking the renin-angiotensin system

Our objectives were to compare the levels of circulating electrolytes, hormones, and renal function during 20 days of dehydration in camels versus the level in non-dehydrated camels and to record the effect of blocking angiotensin II AT1 receptors with losartan during dehydration. Dehydration induced significant increments in serum sodium, creatinine, urea, a substantial fall in body weight, and a doubling in plasma arginine vasopressin (AVP) levels. Plasma aldosterone, however, was unaltered compared with time-matched controls. Losartan significantly enhanced the effect of dehydration to reduce body weight and increase serum levels of creatinine and urea, whilst also impairing the rise in plasma AVP and reducing aldosterone levels. We conclude that dehydration in the camel induces substantial increments in serum sodium, creatinine, urea and AVP levels; that aldosterone levels are altered little by dehydration; that blockade of angiotensin II type 1 receptors enhances the dehydration-induced fall in body weight and increase in serum creatinine and urea levels whilst reducing aldosterone and attenuating the rise in plasma AVP.

Classification of scaffold-hopping approaches

The general goal of drug discovery is to identify novel compounds that are active against a preselected biological target with acceptable pharmacological properties defined by marketed drugs. Scaffold hopping has been widely applied by medicinal chemists to discover equipotent compounds with novel backbones that have improved properties. In this article we classify scaffold hopping into four major categories, namely heterocycle replacements, ring opening or closure, peptidomimetics and topology-based hopping. We review the structural diversity of original and final scaffolds with respect to each category. We discuss the advantages and limitations of small, medium and large-step scaffold hopping. Finally, we summarize software that is frequently used to facilitate different kinds of scaffold-hopping methods.

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.

An ab initio study of AT2 antagonists

In this study we report ab initio molecular orbital calculations on the natural hormone angiotensin II, which induces activity at AT1/AT2 receptor subtypes leading to vasoconstriction and subsequent hypertension, and AT2 antagonists. Pharmacophoric features of AT2 antagonists have been studied. A model of AT2 receptor has been made, and angiotensin II as well as antagonists has been systematically docked and their interactions with the receptor analyzed. Calculated ligand-receptor interaction energies have been correlated with experimentally observed biological potency data. Our studies indicate that antagonists retain sufficient interactions to block the receptor but may not be adequate to induce activity at the receptor. A poor antagonist is, therefore, proposed as a close mimic of angiotensin II in terms of interacting with the receptor. These studies further explore the mechanistic aspects of this important class of drugs.

Synthesis of a new class of druglike angiotensin II C-terminal mimics with affinity for the AT2 receptor

Four tripeptides corresponding to the C-terminal region of angiotensin II were synthesized. One of these peptides (Ac-His-Pro-Ile) showed moderate binding affinity for the AT2 receptor. Two aromatic histidine-related scaffolds were synthesized and introduced in the tripeptides to give eight new peptidomimetic structures. Three of the new peptide-derived druglike molecules exhibited selective, nanomolar affinity for the AT2 receptor. These ligands may become lead compounds in the future development of novel classes of selective AT2 receptor agonists.

Design, Synthesis, and Incorporation of a β-Turn Mimetic in Angiotensin II Forming Novel Pseudopeptides with Affinity for AT1 and AT2 Receptors

A benzodiazepine-based beta-turn mimetic has been designed, synthesized, and incorporated into angiotensin II. Comparison of the mimetic with beta-turns in crystallized proteins showed that it most closely resembles a type II beta-turn. The compounds exhibited high to moderate binding affinity for the AT2 receptor, and one also displayed high affinity for the AT1 receptor. Molecular modeling showed that the high-affinity compounds could be incorporated into a previously derived model of AT2 receptor ligands.

Short pseudopeptides containing turn scaffolds with high AT2 receptor affinity

Two pentapeptides, Ac-Tyr-Ile-His-Pro-Phe/Ile, were synthesized and shown to have angiotensin II AT2 receptor affinity and agonistic activity. Based on these peptides, a new series of 13 pseudopeptides was synthesized via introduction of five different turn scaffolds replacing the Tyr-Ile amino acid residues. Pharmacological evaluation disclosed subnanomolar affinities for some of these compounds at the AT2 receptor. Substitution of Phe by Ile in this series of ligands enhanced the AT2 receptor affinity of all compounds. These results suggest that the C-terminal amino acid residues can be elaborated on to enhance the AT2 receptor affinity in truncated Ang II analogues.

Angiotensin II pseudopeptides containing 1,3,5-trisubstituted benzene scaffolds with high AT2 receptor affinity

Two 1,3,5-trisubstituted aromatic scaffolds intended to serve as gamma-turn mimetics have been synthesized and incorporated in five pseudopeptide analogues of angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe), replacing Val-Tyr-Ile, Val-Tyr, or Tyr-Ile. All the tested compounds exhibited nanomolar affinity for the AT2 receptor with the best compound (3) having a K(i) of 1.85 nM. Four pseudopeptides were AT2 selective, while one (5) also exhibited good affinity for the AT1 receptor (K(i) = 30.3 nM). This pseudopeptide exerted full agonistic activity in an AT2 receptor induced neurite outgrowth assay but displayed no agonistic effect in an AT1 receptor functional assay. Molecular modeling, using the program DISCOtech, showed that the high-affinity ligands could interact similarly with the AT2 receptor as other ligands with high affinity for this receptor. A tentative agonist model is proposed for AT2 receptor activation by angiotensin II analogues. We conclude that the 1,3,5-trisubstituted benzene rings can be conveniently prepared and are suitable as gamma-turn mimics.

New selective AT2 receptor ligands encompassing a gamma-turn mimetic replacing the amino acid residues 4-5 of angiotensin II act as agonists

New benzodiazepine-based gamma-turn mimetics with one or two amino acid side chains were synthesized. The gamma-turn mimetics were incorporated into angiotensin II (Ang II) replacing the Val(3)-Tyr(4)-Ile(5) or Tyr(4)-Ile(5) peptide segments. All of the resulting pseudopeptides displayed high AT(2)/AT(1) receptor selectivity and exhibited AT(2) receptor affinity in the low nanomolar range. Molecular modeling was used to investigate whether the compounds binding to the AT(2) receptor could position important structural elements in common areas. A previously described benzodiazepine-based gamma-turn mimetic with high affinity for the AT(2) receptor was also included in the modeling. It was found that the molecules, although being structurally quite different, could adopt the same binding mode/interaction pattern in agreement with the model hypothesis. The pseudopeptides selected for agonist studies were shown to act as AT(2) receptor agonists being able to induce outgrowth of neurite cells, stimulate p42/p44(mapk), and suppress proliferation of PC12 cells.

AT2-selective angiotensin II analogues containing tyrosine-functionalized 5,5-bicyclic thiazabicycloalkane dipeptide mimetics

This paper reports the synthesis of two angiotensin II analogues with tyrosine-functionalized 5,5-bicyclic thiazabicycloalkane dipeptide mimetics replacing the Tyr(4)-Ile(5) residues. The preparation of these analogues relies on the synthesis and incorporation of an alpha,alpha-disubstituted chimeric amino acid derivative and on-resin bicyclization to a cysteine residue. The synthesized analogues both displayed high angiotensin AT(2)/AT(1) receptor binding preferences and had AT(2) receptor affinities in the same low nanomolar range as angiotensin II itself. Conformational analysis, using experimental constraints derived from NMR studies, indicated that the Tyr(4) and His(6) residues in one of the angiotensin II analogues were in close proximity to each other.

Synthesis and AT2 receptor-binding properties of angiotensin II analogues

The present study investigates the importance of the amino acid side chains in the octapeptide angiotensin II (Ang II) for binding to the AT2 receptor. A Gly scan was performed where each amino acid in Ang II was substituted one-by-one with glycine. The resulting set of peptides was tested for affinity to the AT2 receptor (porcine myometrial membranes). For a comparison, the peptides were also tested for affinity to the AT1 receptor (rat liver membranes). Only the substitution of Arg2 reduced affinity to the AT2 receptor considerably (92-fold when compared with Ang II). For the other Gly-substituted analogues the affinity to the AT2 receptor was only moderately affected. To further investigate the role of the Arg2 side chain for receptor binding, we synthesized some N-terminally modified Ang II analogues. According to these studies a positive charge in the N-terminal end of angiotensin III [Ang II (2-8)] is not required for high AT2 receptor affinity but seems to be more important in Ang II. With respect to the AT1 receptor, [Gly2]Ang II and [Gly8]Ang II lacked binding affinity (Ki > 10 microM). Replacement of the Val3 or Ile5 residues with Gly produced only a slight decrease in affinity. Interestingly, substitution of Tyr4 or His6, which are known to be very important for AT1 receptor binding, resulted in only 48 and 14 times reduction in affinity, respectively.