Rational Design and Synthesis of AT1R Antagonists.

Hypertension is one of the most common diseases nowadays and is still the major cause of premature death despite of the continuous discovery of novel therapeutics. The discovery of the Renin Angiotensin System (RAS) unveiled a path to develop efficient drugs to fruitfully combat hypertension. Several compounds that prevent the Angiotensin II hormone from binding and activating the AT1R, named sartans, have been developed. Herein, we report a comprehensive review of the synthetic paths followed for the development of different sartans since the discovery of the first sartan, Losartan.

Electrospun orally disintegrating film formulation of telmisartan.

Telmisartan (TEL) is an antihypertensive BCS class II drug with low solubility at physiological pH. However, the solubility of TEL increases with the presence of an alkalizer. Electrospinning is one of the most recent techniques for the solubility enhancement studies. In this study, an electrospun orally disintegrating film (ODF) formulation of TEL was developed with L-arginine and polyvinylpyrrolidone K90 (PVP), and its characterization studies were performed. Preformulation studies were performed to investigate possible incompatibilities in the components of formulation with differential scanning calorimetry (DSC) and Fourier transform infrared spectrometer (FT-IR) analyses. ODFs were characterized in terms of drug content and uniformity, mechanical properties, fiber shape and diameter and in vitro dissolution profile. Smooth nanofibers without any beads were obtained. The dissolution rate of the TEL significantly increased. The chosen formulation had acceptable mechanical properties with much faster dissolution compared to the commercially available product. Developed ODF and marketed product were compared with a dissolution study in phosphate-buffered solution (pH 7.4). ODF and marketed product both reached 100% release in the 45th minute, and ODF results showed that ODF had much faster release than marketed product. In this study, TEL ODF formulation was successfully produced and characterized.

Design, synthesis, and pharmacological evaluation of 5-oxo-1,2,4-oxadiazole derivatives as AT1 antagonists with antihypertension activities.

A series of new 5-oxo-1,2,4-oxadiazole derivatives with 1, 4-disubsituted or 1, 5-disubsituted indole group was designed, synthesized, and pharmacologically evaluated. These derivatives displayed high affinities to the AT1 receptor at the same order of magnitude to losartan. The methyl ester with 1, 4-disubsituted indole group, 1 (5.01 ± 1.67 nM) showed high antihypertension activity on spontaneously hypertensive rats (SHRs). Its maximal response lowered 30 mmHg of mean blood pressure (MBP) at 10 mg/kg after oral administration, which was better than irbesartan, and the antihypertensive effect lasted beyond 24 h. These results made 1 deserve further investigation.

Synthesis and Physicochemical Characterization of the Process-Related Impurities of Olmesartan Medoxomil. Do 5-(Biphenyl-2-yl)-1-triphenylmethyltetrazole Intermediates in Sartan Syntheses Exist?

During the process development for multigram-scale synthesis of olmesartan medoxomil (OM), two principal regioisomeric process-related impurities were observed along with the final active pharmaceutical ingredient (API). The impurities were identified as N-1- and N-2-(5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl derivatives of OM. Both compounds, of which N-2 isomer of olmesartan dimedoxomil is a novel impurity of OM, were synthesized and fully characterized by differential scanning calorimetry (DSC), infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR) and high-resolution mass spectrometry/electrospray ionization (HRMS/ESI). Their ¹H, (13)C and (15)N nuclear magnetic resonance signals were fully assigned. The molecular structures of N-triphenylmethylolmesartan ethyl (N-tritylolmesartan ethyl) and N-tritylolmesartan medoxomil, the key intermediates in OM synthesis, were solved and refined using single-crystal X-ray diffraction (SCXRD). The SCXRD study revealed that N-tritylated intermediates of OM exist exclusively as one of the two possible regioisomers. In molecular structures of these regioisomers, the trityl substituent is attached to the N-2 nitrogen atom of the tetrazole ring, and not to the N-1 nitrogen, as has been widely reported up to the present. This finding indicates that the reported structural formula of N-tritylolmesartan ethyl and N-tritylolmesartan medoxomil, as well as their systematic chemical names, must be revised. The careful analysis of literature spectroscopic data for other sartan intermediates and their analogs with 5-(biphenyl-2-yl)tetrazole moiety showed that they also exist exclusively as N-2-trityl regioisomers.

Synthesis and evaluation of the novel 2-[¹8F]fluoro-3-propoxy-triazole-pyridine-substituted losartan for imaging AT1 receptors.

The 2-[(18)F]fluoro-3-pent-4-yn-1-yloxypyridine ([(18)F]FPyKYNE) analog of the potent non-peptide angiotensin II type 1 receptor (AT1R) blocker losartan was produced via click chemistry linking [(18)F]FPyKYNE to azide-modified tetrazole-protected losartan followed by TFA deprotection. Preliminary small animal imaging with positron emission tomography (PET) in rats displayed high uptake in the kidneys with good contrast to surrounding tissue. Rat metabolism displayed the presence of 23% unchanged tracer in plasma at 30 min. Upon co-administration with AT1R blocker candesartan (2.5, 5 and 10 mg/kg), a dose-dependent reduction (47-65%) in tracer uptake was observed in the kidney, while no difference was observed following AT2R blocker PD123,319 (5 mg/kg), indicating binding selectivity for AT1R over AT2R and potential for imaging AT1R using PET.

Discovery of olmesartan hexetil: a new potential prodrug of olmesartan.

Synthesis of a new ester prodrug of olmesartan, olmesartan hexetil (1), is described. It is in vitro stabilities and in vivo pharmacokinetics (PK) were evaluated. It showed high stability in simulated gastric juice, and was rapidly hydrolyzed to olmesartan in rat liver microsomes and rat plasma in vitro. C(max) and AUC(last) for olmesartan were significantly increased in case of hexetil prodrug, compared with olmesartan medoxomil. Olmesartan hexetil is proposed to be an efficient prodrug of olmesartan with markedly increased oral bioavailability.

Design, synthesis, and evaluation of imidazo[4,5-c]pyridin-4-one derivatives with dual activity at angiotensin II type 1 receptor and peroxisome proliferator-activated receptor-γ.

Identification of a series of imidazo[4,5-c]pyridin-4-one derivatives that act as dual angiotensin II type 1 (AT1) receptor antagonists and peroxisome proliferator-activated receptor-γ (PPARγ) partial agonists is described. Starting from a known AT1 antagonist template, conformational restriction was introduced by incorporation of an indane ring that when combined with appropriate substitution at the imidazo[4,5-c]pyridin-4-one provided novel series 5 possessing the desired dual activity. The mode of interaction of this series with PPARγ was corroborated through the X-ray crystal structure of 12b bound to the human PPARγ ligand binding domain. Modulation of activity at both receptors through substitution at the pyridone nitrogen led to the identification of potent dual AT1 antagonists/PPARγ partial agonists. Among them, 21b was identified possessing potent dual pharmacology (AT1 IC(50) = 7 nM; PPARγ EC(50) = 295 nM, 27% max) and good ADME properties.

Facile and efficient syntheses of a series of N-benzyl and N-biphenylmethyl substituted imidazole derivatives based on (E)-urocanic acid, as angiotensin II AT1 receptor blockers.

In the present work, a facile and efficient route for the synthesis of a series of N-substituted imidazole derivatives is described. Docking studies have revealed that N-substituted imidazole derivatives based on (E)-urocanic acid may be potential antihypertensive leads. Therefore, new AT1 receptor blockers bearing either the benzyl or the biphenylmethyl moiety at the N-1 or N-3 position, either the (E)-acrylate or the propanoate fragment and their related acids at the C-4 position as well as a halogen atom at the C-5 position of the imidazole ring, were synthesized. The newly synthesized analogues were evaluated for binding to human AT1 receptor. The biological results showed that this class of molecules possesses moderate or no activity, thus not always confirming high docking scores. Nonetheless, important conclusions can be derived for their molecular basis of their mode of action and help medicinal chemists to design and synthesize more potent ones. An aliphatic group as in losartan seems to be important for enhancing binding affinity and activity.

Synthesis of telmisartan impurity B.

Telmisartan is an antihypertensive drug and is a specific angiotensin II receptor (AT1) antagonist. According to European Pharmacopoeia 7 Edition 2008 telmisartan quality standard, there are seven impurities in telmisartan. Impurity B which is not available commercially and no synthetic method is published so far. We report herein the first synthesis of impurity B. The structure of impurity B was confirmed by 1H NMR, 13C NMR and MS data. These findings should be important for quality control purposes in the manufacture and quality control of telmisartan.

Synthesis and antihypertensive activity of pyrimidin-4(3H)-one derivatives as losartan analogue for new angiotensin II receptor type 1 (AT1) antagonists.

The discovery, in vitro and in vivo studies of the highly potent AT(1) antagonist 12a (BR-A-657, Fimasartan) antagonists are presented. A series of pyrimidin-4(3H)-one derivatives as losartan analogue were synthesized and evaluated for a novel class of AT(1) receptor antagonists. Among them, 12a containing thioamido moiety displayed both high in vitro functional antagonism and binding affinity [IC(50)=0.42 and 0.13 nM, respectively] and inhibited strongly in vivo AngII-induced pressor response in pithed rats with an ED(50) of 0.018 mg/kg. Moreover, in vivo evaluation in furosemide-treated rat and conscious renal hypertensive rat models and the pharmacokinetic study showed that 12a is a highly potent and orally active AT(1) selective antagonist having stronger in vivo potency than losartan.

Synthesis and biological evaluation of novel potent angiotensin II receptor antagonists with anti-hypertension effect.

A series of novel angiotensin II type 1 receptor antagonists were prepared. Radioligand binding assay suggested that compounds 1b and 1c could be recognized by the AT(1) receptor with an IC(50) value of 1.6 ± 0.09 nM and 2.64 ± 0.7 nM, respectively. In vivo anti-hypertension experiments showed that compounds (1a, 1b, 1c, 1e) elicited a significant decrease in SBP and DBP of spontaneous hypertensive rats (SHRs). The antihypertensive effects maintained for 10 h, which indicated that these compounds had a favorable blood pressure-lowering effect. Acute toxicity testing suggested that the LD(50) value of compound 1b was 2316.8 mg/kg which was lower than valsartan (LD(50)=307.50 mg/kg) but higher than losartan (LD(50)=2248 mg/kg). So they could be considered as novel anti-hypertension candidates and deserved for further investigation.

Design, synthesis and biological activity of 6-substituted carbamoyl benzimidazoles as new nonpeptidic angiotensin II AT1 receptor antagonists.

A series of 6-substituted carbamoyl benzimidazoles were designed and synthesised as new nonpeptidic angiotensin II AT(1) receptor antagonists. The preliminary pharmacological evaluation revealed a nanomolar AT(1) receptor binding affinity for all compounds in the series, and a potent antagonistic activity in an isolated rabbit aortic strip functional assay for compounds 6f, 6g, 6h and 6k was also demonstrated. Furthermore, evaluation in spontaneous hypertensive rats and a preliminary toxicity evaluation showed that compound 6g is an orally active AT(1) receptor antagonist with low toxicity.

Synthesis, pharmacokinetics, and pharmacodynamics studies of valsartan peptide derivatives.

In order to increase the intestinal permeability of valsartan, 14 esters and peptide derivatives of valsartan were chemically synthesized and their absorption characteristics were described. All derivatives were stable and could be better absorbed into the small intestine than valsartan. There are two barriers for the absorption of valsartan derivatives. The elongated half-life (t(1/2)) and stable blood concentrations for compound 4 due to the hydrolysis of the ester group in the second barrier were demonstrated in pharmacokinetic experiments. Furthermore, compound 4 also displayed modest anti-hypertension activity in vivo, which makes structural modification of valsartan, especially for the purpose of improvement of its intestinal permeability, valuable for further studies.

Computer--based drug design, synthesis and biological evaluation of new pyrimidinone derivatives linked to arylpiperazine and 2'-carbethoxy-biphenylylmethyl moeities as alpha1-adrenoceptor antagonists and angiotensin II AT1 receptor antagonists.

Two new series of pyrimidinone derivatives linked to arylpiperazine moieties and 2'-carbethoxy-biphenylmethyl moieties were designed, synthesized and biologically evaluated for their in vivo hypotensive activities. The design of arylpiperazine analogues (IIa-f, IIIa-c, VIIa.b, IX) was based upon structural modification of the newly discovered selective alpha1-AR antagonist drug; Urapidil. Compare/fit studies of these molecules with the previously generated and validated alpha1-AR antagonist hypothesis showed that these molecules have comparable affinities for the alpha1-AR antagonist hypothesis while compound IIIc had the highest fitting value. The in vivo biological evaluation of these compounds for their effects on blood pressure of normotensive cats in comparison to the lead compound prazosin, was consistent with the results of molecular modeling fit values. As expected, compound IIIc exhibited the highest hypotensive activity among the test set compounds. Meanwhile, the design of 2'-carbethoxy-biphenylylmethyl analogues (XIa,b) was based upon the molecular modeling simulation fitting of their carboxylic acid bioprecursors with the previously generated and validated Ang II receptor antagonist hypothesis. Such compare/fit studies predicted that the designed compounds (XIa,b) showed comparable fitting affinities between their de-esterified analogues and the Ang II antagonist pharmacophore. In vivo biological evaluation of these compounds for their effects on blood pressure of normotensive cats showed that compound Xla exhibited hypotensive activity more or less similar to losartan.

Novel O-[(11)C]methylated derivatives of candesartan as angiotensin II AT(1) receptor imaging ligands: radiosynthesis and ex vivo evaluation in rats.

[(11)C]Methyl-candesartan and its desethyl derivative ([(11)C]TH4) were developed as potential radiotracers for imaging angiotensin II (Ang II) type 1 (AT(1)) receptors. These compounds were synthesized via methylation of tetrazole-protected candesartan using [(11)C]methyl iodide followed by deprotection through HCl hydrolysis at 65 degrees C to produce [(11)C]methyl-candesartan, and 90 degrees C for [(11)C]TH4. Ex vivo biodistribution and competition studies were carried out for both [(11)C]methyl-candesartan and [(11)C]TH4 to assess tissue retention time course and binding selectivity. Besides the liver, [(11)C]methyl-candesartan and [(11)C]TH4 displayed highest tissue retention in the AT(1) receptor-rich renal cortex and outer medulla. At tracer doses 15 min post-injection, [(11)C]methyl-candesartan demonstrated higher specific binding proportion for AT(1) receptors, and selectivity for AT(1) over Ang II AT(2), Mas, beta-adrenergic, and alpha(2)-adrenergic receptors in rat kidneys compared to [(11)C]TH4. This study indicates that [(11)C]methyl-candesartan has potential for in vivo imaging renal AT(1) receptors selectively using positron emission tomography.

Design, synthesis and biological evaluation of AT1 receptor blockers derived from 6-substituted aminocarbonyl benzimidazoles.

A series of new 6-substituted aminocarbonyl benzimidazole derivatives with 1, 4-disubsituted or 1, 5-disubsituted indole moiety and benzoic acid moiety were designed, synthesized and pharmacologically evaluated. Most of the synthesized compounds could bind to the AT1 receptor and decrease blood pressure significantly. Notably, 2e and 1h could obviously decrease MBP in a dose dependent manner. The maximal response lowered 57.9 ±â€¯2.3 mmHg (2e) and 57.6 ±â€¯1.9 mmHg (1h) of MBP at 10 mg/kg after oral administration, and the antihypertensive effect lasted beyond 24 h, which performed better than Losartan (Fig. 1). These results indicate that 2e and 1h are effective and long-lasting anti-hypertension drug candidates and deserve further investigation for therapeutic application.

Design, synthesis, and biological evaluation of AT1 angiotensin II receptor antagonists based on the pyrazolo[3,4-b]pyridine and related heteroaromatic bicyclic systems.

Novel AT 1 receptor antagonists bearing the pyrazolo[3,4- b]pyridine bicyclic heteroaromatic system (or structurally related moieties) were designed and synthesized as the final step of a large program devoted to the development of new antihypertensive agents and to the understanding of the molecular basis of their pharmacodynamic and pharmacokinetic properties. The preliminary pharmacological characterization revealed nanomolar AT 1 receptor affinity for several compounds of the series and a potent antagonistic activity in isolated rabbit aortic strip functional assay for 7c and 8a. These results stimulated the study of the biopharmaceutical properties of some selected compounds, which were found to be characterized by a permeability from medium to high. Remarkably, the least permeable 7c showed both permeability and oral bioavailability (80%) higher than losartan, but its terminal half-life was shorter. These results suggest that the permeability is not a limiting factor in the pharmacokinetics of these AT 1 receptor antagonists.

Selenosartans: novel selenophene analogues of milfasartan and eprosartan.

A series of selenophene analogues of the thiophene-containing antihypertensives milfasartan and eprosartan were prepared and tested for AT(1) receptor antagonist properties. All four selenophene compounds proved to be potent AT(1) receptor antagonists, with pK(B) estimates indicating that these selenides are at least as effective as the thiophene parent compounds at blocking AT(1) receptor mediated responses. These results reveal that replacement of sulfur with selenium in thiophene-containing sartans does not interfere with sartan activity.

Enzyme-assisted synthesis and structure characterization of glucuronic acid conjugates of losartan, candesartan, and zolarsartan.

Three angiotensin II receptor antagonists--losartan, candesartan, and zolarsartan--were investigated. All the compounds, which are structural analogues, are metabolized via conjugation to glucuronic acid. Interestingly, both O- and N-glucuronidation take place, so that regioisomers are formed. One ether O-glucuronide, two acyl O-glucuronides, and five tetrazole-N-glucuronides were biosynthesized, in milligram scale, from the three sartan aglycones. Liver microsomes from bovine, moose, rat, and pig and recombinant human UDP-glucuronosyltransferases were used as catalysts. The synthesized compounds were identified as sartan glucuronides by mass spectrometry, while the sites of glucuronidation were determined by nuclear magnetic resonance spectroscopy. Drug metabolites are needed as standards for pharmaceutical research and, as the present study shows, they can easily be produced with enzymes as catalyst.

Angiotensin II--AT1 receptor antagonists: design, synthesis and evaluation of substituted carboxamido benzimidazole derivatives.

A series of 5-(alkyl and aryl)carboxamido benzimidazole derivatives had been designed, synthesized and evaluated for in vitro angiotensin II--AT1 receptor antagonism and in vivo antihypertensive activities. The pharmacological activities were inversely related to the size of alkyl and aryl substituents. It can be suggested that compounds with lower alkyl groups at 5-position of benzimidazole nucleus demonstrated potent antihypertensive activity.