Structure elucidation and conformational properties of eprosartan, a non peptide angiotensin II AT1 antagonist

On the Rational Drug Design for Hypertension through NMR Spectroscopy

Antagonists of the AT1receptor (AT1R) are beneficial molecules that can prevent the peptide hormone angiotensin II from binding and activating the specific receptor causing hypertension in pathological states. This review article summarizes the multifaced applications of solid and liquid state high resolution nuclear magnetic resonance (NMR) spectroscopy in antihypertensive commercial drugs that act as AT1R antagonists. The 3D architecture of these compounds is explored through 2D NOESY spectroscopy and their interactions with micelles and lipid bilayers are described using solid state 13CP/MAS, 31P and 2H static solid state NMR spectroscopy. Due to their hydrophobic character, AT1R antagonists do not exert their optimum profile on the AT1R. Therefore, various vehicles are explored so as to effectively deliver these molecules to the site of action and to enhance their pharmaceutical efficacy. Cyclodextrins and polymers comprise successful examples of effective drug delivery vehicles, widely used for the delivery of hydrophobic drugs to the active site of the receptor. High resolution NMR spectroscopy provides valuable information on the physical-chemical forces that govern these drug:vehicle interactions, knowledge required to get a deeper understanding on the stability of the formed complexes and therefore the appropriateness and usefulness of the drug delivery system. In addition, it provides valuable information on the rational design towards the synthesis of more stable and efficient drug formulations.

Insights into AT1 receptor activation through AngII binding studies

This study investigates the binding of angiotensin II (AngII) to the angiotensin II type 1 receptor (AT1R), taking into consideration several known activation elements that have been observed for G-protein-coupled receptors (GPCRs). In order to determine the crucial interactions of AngII upon binding, several MD simulations were implemented using AngII conformations derived from experimental data (NMR ROEs) and in silico flexible docking methodologies. An additional goal was to simulate the induced activation mechanism and examine the already known structural rearrangements of GPCRs upon activation. Performing MD simulations to the AT1R - AngII - lipids complex, a series of dynamic changes in the topology of AngII and the intracellular part of the receptor were observed. Overall, the present study proposes a complete binding profile of AngII to the AT1R, as well as the key transitional elements of the receptor and the agonist peptide upon activation through NMR and in silico studies.

A systematic comparison of the properties of clinically used angiotensin II type 1 receptor antagonists

Angiotensin II type 1 receptor antagonists (ARBs) have become an important drug class in the treatment of hypertension and heart failure and the protection from diabetic nephropathy. Eight ARBs are clinically available [azilsartan, candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan, valsartan]. Azilsartan (in some countries), candesartan, and olmesartan are orally administered as prodrugs, whereas the blocking action of some is mediated through active metabolites. On the basis of their chemical structures, ARBs use different binding pockets in the receptor, which are associated with differences in dissociation times and, in most cases, apparently insurmountable antagonism. The physicochemical differences between ARBs also manifest in different tissue penetration, including passage through the blood-brain barrier. Differences in binding mode and tissue penetration are also associated with differences in pharmacokinetic profile, particularly duration of action. Although generally highly specific for angiotensin II type 1 receptors, some ARBs, particularly telmisartan, are partial agonists at peroxisome proliferator-activated receptor-γ. All of these properties are comprehensively reviewed in this article. Although there is general consensus that a continuous receptor blockade over a 24-hour period is desirable, the clinical relevance of other pharmacological differences between individual ARBs remains to be assessed.

Rational design, efficient syntheses and biological evaluation of N,N'-symmetrically bis-substituted butylimidazole analogs as a new class of potent Angiotensin II receptor blockers

A series of symmetrically bis-substituted imidazole analogs bearing at the N-1 and N-3 two biphenyl moieties ortho substituted either with tetrazole or carboxylate functional groups was designed based on docking studies and utilizing for the first time an extra hydrophobic binding cleft of AT1 receptor. The synthesized analogs were evaluated for their in vitro antagonistic activities (pA2 values) and binding affinities (-logIC50 values) to the Angiotensin II AT1 receptor. Among them, the potassium (-logIC50 = 9.04) and the sodium (-logIC50 = 8.54) salts of 4-butyl-N,N'-bis{[2'-(2H-tetrazol-5-yl)biphenyl-4-yl]methyl}imidazolium bromide (12a and 12b, respectively) as well as its free acid 11 (-logIC50 = 9.46) and the 4-butyl-2-hydroxymethyl-N,N'-bis{[2'-(2H-tetrazol-5-yl)biphenyl-4-yl]methyl}imidazolium bromide (14) (-logIC50 = 8.37, pA2 = 8.58) showed high binding affinity to the AT1 receptor and high antagonistic activity (potency). The potency was similar or even superior to that of Losartan (-logIC50 = 8.25, pA2 = 8.25). On the contrary, 2-butyl-N,N'-bis{[2'-[2H-tetrazol-5-yl)]biphenyl-4-yl]methyl}imidazolium bromide (27) (-logIC50 = 5.77) and 2-butyl-4-chloro-5-hydroxymethyl-N,N'-bis{[2'-[2H-tetrazol-5-yl)]biphenyl-4-yl]methyl}imidazolium bromide (30) (-logIC50 = 6.38) displayed very low binding affinity indicating that the orientation of the n-butyl group is of primary importance. Docking studies of the representative highly active 12b clearly showed that this molecule has an extra hydrophobic binding feature compared to prototype drug Losartan and it fits to the extra hydrophobic cavity. These results may contribute to the discovery and development of a new class of biologically active molecules through bis-alkylation of the imidazole ring by a convenient and cost effective synthetic strategy.

Losartan's affinity to fluid bilayers modulates lipid-cholesterol interactions

Losartan is an angiotensin II receptor antagonist mainly used for the regulation of high blood pressure. Since it was anticipated that losartan reaches the receptor site via membrane diffusion, the impact of losartan on model membranes has been investigated by small angle X-ray scattering. For this purpose 2-20 mol% losartan was incorporated into dimyristoyl-phosphatidylcholine (DMPC) and palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers and into their binary mixtures with cholesterol in the concentration range of 0 to 40 mol%. Effects of losartan on single component bilayers are alike. Partitioning of losartan into the membranes confers a negative charge to the lipid bilayers that causes the formation of unilamellar vesicles and a reduction of the bilayer thickness by 3-4%. Analysis of the structural data resulted in an estimate for the partial area of losartan, A(Los) ≈ 40 Å(2). In the presence of cholesterol, differences between the effects of losartan on POPC and DMPC are striking. Membrane condensation by cholesterol is retarded by losartan in POPC. This contrasts with DMPC, where an increase of the cholesterol content shifts the partitioning equilibrium of losartan towards the aqueous phase, such that losartan gets depleted from the bilayers from 20 mol% cholesterol onwards. This indicates (i) a chain-saturation dependent competition of losartan with lipid-cholesterol interactions, and (ii) the insolubility of losartan in the liquid ordered phase of PCs. Consequently, losartan's action is more likely to take place in fluid plasma membrane patches rather than in domains rich in cholesterol and saturated lipid species such as in membrane rafts.

Docking and Molecular Dynamics Calculations of Pyrrolidinone Analog MMK16 Bound to COX and LOX Enzymes

The new molecule 4-[(2S)-2-(1H-imidazol-1-ylmethyl)-5-oxotetrahydro-1H-pyrrol-1-yl]methylbenzenecarboxylic acid (MMK16) was found to have promising anti-inflammatory activity. This biological behavior of MMK16 triggered our interest to study its binding affinity using NMR spectroscopy in LOX and its docking and molecular dynamics (MD) properties in LOX and COX enzymes. The present NMR and docking binding studies not only rationalize the obtained biological results since in all three receptors MMK16 shows high affinity and scoring but also make it a potential dual LOX-5/COX-2 inhibitor. Thus, this class of molecules must be further investigated for discovering compounds possessing better biological activity and more lasting biological effect.

Development of a CP 31P NMR broadline simulation methodology for studying the interactions of antihypertensive AT1 antagonist losartan with phospholipid bilayers

A cross-polarization (CP) (31)P NMR broadline simulation methodology was developed for studying the effects of drugs in phospholipids bilayers. Based on seven-parameter fittings, this methodology provided information concerning the conformational changes and dynamics effects of losartan in the polar region of the dipalmitoylphosphatidylcholine bilayers. The test molecule for this study was losartan, an antihypertensive drug known to exert its effect on AT(1) transmembrane receptors. The results were complemented and compared with those of differential scanning calorimetry, solid-state (13)C NMR spectroscopy, Raman spectroscopy, and electron spin resonance. More specifically, these physical chemical methodologies indicated that the amphipathic losartan molecule interacts with the hydrophilic-head zone of the lipid bilayers. The CP (31)P NMR broadline simulations showed that the lipid molecules in the bilayers containing losartan displayed greater collective tilt compared to the tilt displayed by the load-free bilayers, indicating improved packing. The Raman results displayed a decrease in the trans/gauche ratio and increased intermolecular interactions of the acyl chains in the liquid crystalline phase. Additional evidence, suggesting that losartan possibly anchors in the realm of the headgroup, was derived from upfield shift of the average chemical shift sigma(iso) of the (31)P signal in the presence of losartan and from shift of the observed peak at 715 cm(-1) attributed to C-N stretching in the Raman spectra.

Dynamic NMR study and theoretical calculations on the conformational exchange of valsartan and related compounds

Valsartan (1), an antihypertensive drug of the sartan family, and three related compounds, 3-methyl-2-((2'-(1-methyl-1H-tetrazol-5-yl)biphenyl-4-ylmethyl) pentanoylamino)butyric acid (2), 3-isopropyl-6-propyl-4-(2'-(1H-tetrazol-5-yl)biphenyl-4-ylmethyl) morpholine-2,5-dione (3), and 3-isopropyl-6-propyl-4-(4'-(1H-tetrazol-5-yl)biphenyl4-ylmethyl) morpholine-2,5-dione (4), were studied by nuclear magnetic resonance (NMR) spectroscopy. Assignment of (1)H and (13)C NMR resonances for the compounds were completed using COSY, HSQC and HMBC techniques. It was found that each of the compounds 1, 2, and 4 had two sets of (1)H and (13)C resonances, suggesting the presence of two conformers in solution. Based on NOESY experiments at different temperatures, thermodynamic parameters of the conformational exchange process were deduced for these compounds. The exchange barrier was found to be 17.9 +/- 0.7, 18.5 +/- 0.8, and 17.7 +/- 0.6 kcal mol(-1) with the corresponding free energy difference (DeltaG) of 0.32 +/- 0.04, 0.23 +/- 0.01, and 0.13 +/- 0.04 kcal mol(-1) for 1, 2, and 4, respectively, at 298 K. Two conformations of valsartan were elucidated by NMR spectroscopy and quantum chemistry calculation. The results showed that two conformers of valsartan interchange via rotation about the C(O)--N bond.

Structure elucidation and conformational study of V8

AT(1) antagonists constitute the most recent class of antihypertensive drugs which act through the Renin Angiotensin System (RAS). In an effort to comprehend their stereoelectronic features, a study was initiated to compare the conformational properties of drugs already marketed for the treatment of hypertension with synthetic ones, possessing common structural characteristics. In this study, the synthetic AT(1) antagonist V8 is structurally elucidated and its conformational properties are studied through a combination of NMR spectroscopy and computational analysis. Its conformational properties are compared with those of the structurally similar prototype AT(1) antagonist losartan.

Conformational and biological studies for a pair of novel synthetic AT1 antagonists: stereoelectronic requirements for antihypertensive efficacy

One of the major systems which interferes with the disease of hypertension, is the Renin Angiotensin Aldosterone System (RAS). The octapeptide hormone angiotensin II is the active product of RAS which causes vasoconstriction when binds to the AT(1) receptor. In the last years, there has been a development of drugs which block the Angiotensin II from binding the AT(1) receptor and are called AT(1) antagonists. In an effort to comprehend their stereoelectronic features, a study has been initiated to compare the conformational properties of drugs already marketed for the treatment of hypertension and others which are designed and synthesized in our laboratory possessing structural characteristics necessary for antihypertensive activity. In this study, two synthetic non-peptide AT(1) antagonists, are structurally elucidated and their conformational properties and bioactivity are compared to the prototype and first approved drug of this category in the market; losartan (trade name: COZAAR).

Design, synthesis and biological evaluation of cyclic angiotensin II analogues with 3,5 side-chain bridges. Role of C-terminal aromatic residue and ring cluster for activity and implications in the drug design of AT1 non-peptide antagonists

The novel amide linked angiotensin II (ANG II) cyclic analogues: gamma, epsilon -cyclo(3, 5)-[Sar(1)-Glu(3)-Lys(5)-Ile(8)] ANG II (I) and gamma, epsilon -cyclo(3, 5)-[Sar(1)-Glu(3)-Lys(5)-Phe(8)] ANG II (II) have been designed, synthesized and bioassayed in anesthetized rabbits in order to unravel structural ring cluster characteristics important for receptor activation. Analogue I with Ile at position 8 was an inhibitor of Angiotensin II while analogue II with Phe at position 8 was found to be an agonist. Similar results were reported for cyclic compounds that have reversed the linking between positions 3 and 5. The overall results show that positions 3 and 5 do not govern the biological activity of the synthetic analogues. It also appears that the aromatic ring cluster (Tyr-His-Phe) in agonist peptides is an essential stereo-electronic feature for Angiotensin II to exert its biological activity. A non-peptide mimetic of ANG II, 1-[2'-[(N-benzyl)tetrazol-5-yl]biphenyl-4-yl]methyl]-2-hydroxymethylbenzimidazole (BZI8) has been designed and synthesized. This molecule is more rigid and much less active than AT(1) non-peptide mimetic losartan probably because it lacks to mimic the orientation of tetrazole and the pharmacophore segments of butyl chain and imidazole ring.