1H-NMR studies of sarmesin and [des1]sarmesin conformation in dimethylsulfoxide by nuclear Overhauser effect (NOE) enhancement spectroscopy: folding of the N- and C-terminal domains

An efficient synthesis of a rationally designed 1,5 disubstituted imidazole AT(1) angiotensin II receptor antagonist: reorientation of imidazole pharmacophore groups in losartan reserves high receptor affinity and confirms docking studies

A new 1,5 disubstituted imidazole AT(1) Angiotensin II (AII) receptor antagonist related to losartan with reversion of butyl and hydroxymethyl groups at the 2-, 5-positions of the imidazole ring was synthesized and evaluated for its antagonist activity (V8). In vitro results indicated that the reorientation of butyl and hydroxymethyl groups on the imidazole template of losartan retained high binding affinity to the AT(1) receptor concluding that the spacing of the substituents at the 2,5- positions is of primary importance. The docking studies are confirmed by binding assay results which clearly show a comparable binding score of the designed compound V8 with that of the prototype losartan. An efficient, regioselective and cost effective synthesis renders the new compound as an attractive candidate for advanced toxicological evaluation and a drug against hypertension.

A Review of the Structural and Functional Features of Olmesartan Medoxomil, An Angiotensin Receptor Blocker

The angiotensin II (A-II) type 1 (AT1) receptor-mediated effects of A-II play a key role in the pathophysiology of hypertension. Effective inhibition of A-II is provided by the latest class of antihypertensive medications, the AT1 receptor blockers (ARBs). These orally available agents were developed around a common imidazole-based structural core. The most recent member of this drug class to be approved by the Food and Drug Administration, olmesartan medoxomil, contains unique features that may explain its clinical efficacy. Key structural elements of olmesartan medoxomil include a hydroxyalkyl substituent at the imidazole 4-position and a hydrolyzable ester group at the imidazole 5-position. Inter- and intramolecular hydrogen bonding involving these groups may contribute to the potentiation of antagonist activity. After oral administration, olmesartan medoxomil is deesterified in the intestinal tract to produce the active metabolite olmesartan, which undergoes no additional metabolic change. The marked antihypertensive efficacy of olmesartan medoxomil may result from a unique pharmacological interaction of the drug with the AT1 receptor, resulting in a potent, long-lasting, dose-dependent blockade of A-II. This review article characterizes the structural features of olmesartan that may be responsible for its clinical efficacy. Inferential pharmacological studies compare and contrast the effects of olmesartan to those of other ARBs in comparable preclinical animal models.

Synthesis and study of a cyclic angiotensin II antagonist analogue reveals the role of pi*--pi* interactions in the C-terminal aromatic residue for agonist activity and its structure resemblance with AT(1) non-peptide antagonists

The novel amide linked Angiotensin II (ANG II) cyclic analogue cyclo(3, 5) -[Sar(1)-Lys(3)-Glu(5)-Ile(8)] ANG II (18) has been designed, synthesized and bioassayed in anesthetized rabbits. The constrained cyclic analogue with a lactam amide bridge linking a Lys-Glu pair at positions 3 and 5 and possessing Ile at position 8, was synthesized by solution procedure using the maximum protection strategy. This analogue was found to be inhibitor of Angiotensin II. NMR spectroscopy coupled with computational analysis showed clustering between the side chains of the key aminoacids Tyr(4)-His(6)-Ile(8) similar to that observed with ANG II. The obtained data show that only pi*--pi* interactions observed in ANG II or its superagonist Sar(1) [ANG II] are missing. Therefore, it can be concluded that these interactions are essential for agonist activity. Conformational analysis comparisons between AT(1) antagonists losartan, eprosartan and irbesartan with C-terminal segment of cyclic compound 18 revealed structural similarities.

Conformational analysis of the thrombin receptor agonist peptides SFLLR and SFLLR-NH2 by NMR: evidence for a cyclic bioactive conformation

The conformational properties of the pentapeptide Ser-Phe-Leu-Leu-Arg (P5), a human thrombin receptor-derived sequence forming part of a tethered ligand which activates the thrombin receptor, and its more active amide derivative Ser-Phe-Leu-Leu-Arg-NH2 (P5-NH2), have been studied by proton NMR spectroscopy in dimethylsulfoxide. Measurements of nuclear Overhauser effects, performed using two-dimensional rotating frame nuclear Overhauser (ROESY) and one-dimensional nuclear Overhauser enhancement (NOE) spectroscopy, revealed that P5 exists mainly in an extended conformation. However, proton-proton 1D-NOEs between Phe C alpha H and Ser C alpha H Leu3 C alpha H and Leu3 NH, and Leu4 C alpha H and Leu4 NH, as well as between the Ser and Arg sidechains, also implicated a minor conformer for P5 having a curved backbone and a near-cyclic structure. In contrast to P5, measurements of NOEs and ROEs for P5-NH2 revealed a more stabilized cyclic structure which may account for its higher biological potency. Thus strong interresidue sequential NH (i)-NH (i + 1) interactions, as well as C-terminal carboxamide to N-terminal side-chain interactions, i.e., Arg CONH2 to Phe ring and Arg CONH2 to Ser C alpha/C beta beta', observed at lower levels of the ROESY spectrum, supported a curved backbone structure for SFLLR-NH2. Since the higher potency P5-NH2 analogue adopts predominantly a cyclic structure, a cyclic bioactive conformation for thrombin receptor agonist peptides is suggested.

Receptor interactions of the position 4 side chains of angiotensin II analogues: importance of aromatic ring quadrupole

A triad of interacting groups (TyrOH-His-O2C) in angiotensin II (ANG II) has been postulated to create the tyrosinate anion pharmacophore (tyanophore) responsible for receptor activation/triggering (Biochim. Biophys. Acta 1991, 1065, 21). In the present study we investigated the effects on bioactivity of substituting the Tyr4 residue in [Sar1]ANG II with other anionic or electronegative amino acids, and with a number of aromatic amino acids lacking a hydroxyl group. [Sar1 Nva(delta-OH)4]ANG II, [Sar1 Nva(delta-OCH3)4]ANG II, [Sar1 Met4]ANG II, [Sar1 Gln4]ANG II, [Sar1 Glu4]ANG II and [Sar1 DL-Alg]ANG II had agonist activities in the rat isolated uterus assay of 4, 3, 19, 10, < 0.1 and < 0.1%, respectively, of that of ANG II. [Sar1 Nal4]ANG II, [Sar1 Pal4]ANG II, [Sar1 DL-Phg(4'-F)4]ANG II, [Sar1 Phe(4'-F)4]ANG II, [Sar1 Phe(F5)4]ANG II and [Sar1 His4]ANG II had agonist activities of 4.5, 7, < 0.1, 0.2, 1 and 0.6%, respectively. All peptides investigated were devoid of measurable antagonist activity except [Sar1 Phe(4'-F)4ANG II (pA2 = 7.7). These findings illustrate that anionic or electronegative aliphatic side chains replacing tyrosinate at position 4 can partially activate the angiotensin receptor. For ANG II analogues containing an aromatic amino acid other than Tyr at position 4, ligand binding and agonist activity are not dependent on the electronegativity or dipole moment of the aromatic ring, or on the ability of the 4' ring substituent to accept a proton.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of methylation of the histidine ring of [Sar1]angiotensin II on conformation and biological activity

[Sar1His(1-Me)6]ANG II and [Sar1His(3-Me)6]ANG II were synthesized by the solid-phase method and purified by reversed-phase HPLC. 1H-NMR spectroscopy at 400 MHz demonstrated the presence of two major conformers for both peptides in DMSO, representing cis and trans isomers (ratio 1:3 and 1:4, respectively) due to restricted rotation at the His-Pro bond. The contractile activities of these peptides in the rat isolated uterus assay were < 0.1 and 27% of that of ANG II, respectively. The bioactivities of these analogues were mirrored in their NMR spectra: the inactive analogue [Sar1His(1-Me)6]ANG II showed perturbations of the Sar and His residues which were not present for [Sar1His(3-Me)6]ANG II and the reference agonist [Sar1]ANG II. The high activity of the analogue methylated at His N3 suggests that an ionizable imidazole proton is not an absolute requirement for expression of biological activity by angiotensin ligands, and that the imidazole group in the molecule may function in an ion dipole-based mechanism when an intramolecular proton transfer (charge relay) mechanism is not available. The biological activity of the ligand appears to depend on the degree of proton transfer from Tyr-OH to the imidazole acceptor, wherein complete (formal) proton transfer represents 100% activity.

Influence of polyfluorination of the phenylalanine ring of angiotensin II on conformation and biological activity

[Phe(F5)8]angiotensin II was synthesized by the solid phase method and purified by reverse-phase HPLC. In rat uterus and rabbit aorta bioassays the analogue had 10 and 50%, respectively, of the contractile activity of angiotensin II and demonstrated antagonist properties. These findings illustrate that inversion of the Phe8 ring quadrupole moment in angiotensin II decreases agonist activity and invokes antagonist properties. 1H-NMR studies at 400 MHz in DMSO-d6 demonstrated the presence of cis and trans isomers in the ratio 1:3 due to restricted rotation of the His-Pro bond. Downfield shifts of the His C2 and C4 protons in [Phe(F5)]ANG II compared to ANG II suggest that the Phe(F5) residue may be involved in a parallel-plate ring pairing interaction with the imidazole group. However heteronuclear NOE studies, carried out by measuring the proton difference spectrum before and after saturation of the fluorine resonances, showed the absence of any NOE enhancement illustrating that electrostatic influences of the Phe(F5) ring occur at relatively long range.