[125I]2-(2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline (LNP 911), a high-affinity radioligand selective for I1 imidazoline receptors

Nischarin Is Not the Functional I1 Imidazoline Receptor Involved in Blood Pressure Regulation

ABSTRACT

Imidazoline receptor antisera selected/Nischarin was proposed several years ago as the functional entity for the I1 medullary receptors (I1Rs) targeted, together with α2-adrenoceptors, by the centrally acting antihypertensive drugs, such as clonidine. The objective of this study was to test this assumption using a pyrroline analog of clonidine, LNP599, which, unlike clonidine and related compounds, displays high selectivity toward I1Rs. Cardiovascular effects of LNP599 (3 mg/kg intravenous) were evaluated in anesthetized, artificially ventilated nischarin mutant rats expressing a truncated form of nischarin lacking the putative imidazoline binding site. LNP599 induced a rapid and pronounced fall in arterial blood pressure in wild-type animals (-42.7% ± 11.0% after 15 minutes), associated with a ≈30% heart rate reduction. Similar effects were obtained in homozygous and heterozygous nischarin mutant rats. The observation that the hypotensive response to I1R activation is not affected by the absence of the putative imidazoline binding site on nischarin strongly suggests that nischarin cannot be regarded as the functional I1R. Carbohydrate regulation was improved in nischarin mutant rats, further supporting the conclusion that nischarin and I1R are 2 distinct molecular entities.

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

New 2-Aminothiazoline derivatives lower blood pressure of spontaneously hypertensive rats (SHR) via I1-imidazoline and alpha-2 adrenergic receptors activation

2-Aminothiazolines share an isosteric relationship with imidazolines and oxazolines with antihypertensive activity mainly mediated by the imidazoline I₁-receptor. In the present work, we have prepared five aminothiazolines, following a previously described synthetic pathway. Aminothiazolines derived from dicyclopropylmethylamine (ATZ1) and cyclohexylamine (3) are unprecedented in the literature. Competitive radioligand assay was carried out with all synthetic compounds, and the I₁ receptor affinity in comparison to rilmenidine in PC12 cells was determined. Surprisingly, the rilmenidine isoster (ATZ1) showed no I₁-receptor interaction. Diethyl (ATZ4) and 2-ethyl-hexylamine (ATZ5) derivatives bind to the receptor with 11.98 and 10.94nmol/l, respectively. These compounds were selected for in vivo experiments. Both compounds reduced the blood pressure of spontaneously hypertensive rats (SHR). The hypotensive effect of these compounds was abrogated in the presence of α₂ adrenergic (yohimbine) and I₁ (efaroxan) receptor antagonists suggesting that both aminothiazolines bind to the adrenergic and imidazoline receptors. Lipinski's descriptors of the synthesized aminothiazolines were calculated and are similar to the known imidazoline I₁ receptor ligands. 3D-Similarity between ATZ5 and agmatine, the natural imidazoline receptor ligand, was also observed.

Imidazoline antihypertensive drugs: selective i(1) -imidazoline receptors activation

Involvement of imidazoline receptors (IR) in the regulation of vasomotor tone as well as in the mechanism of action of some centrally acting antihypertensives has received tremendous attention. To date, pharmacological studies have allowed the characterization of three main imidazoline receptor classes, the I(1) -imidazoline receptor which is involved in central inhibition of sympathetic tone to lower blood pressure, the I(2) -imidazoline receptor which is an allosteric binding site of monoamine oxidase B (MAO-B), and the I(3) -imidazoline receptor which regulates insulin secretion from pancreatic β-cells. All three imidazoline receptors represent important targets for cardiovascular research. The hypotensive effect of clonidine-like centrally acting antihypertensives was attributed both to α(2) -adrenergic receptors and nonadrenergic I(1) -imidazoline receptors, whereas their sedative action involves activation of only α(2) -adrenergic receptors located in the locus coeruleus. Since more selective I(1) -imidazoline receptors ligands reduced incidence of typical side effects of other centrally acting antihypertensives, there is significant interest in developing new agents with higher selectivity and affinity for I(1) -imidazoline receptors. The selective imidazoline receptors agents are also more effective in regulation of body fat, neuroprotection, inflammation, cell proliferation, epilepsy, depression, stress, cell adhesion, and pain. New agonists and antagonists with high selectivity for imidazoline receptor subtypes have been recently developed. In the present review we provide a brief update to the field of imidazoline research, highlighting some of the chemical diversity and progress made in the theoretical studies of imidazoline receptor ligands.

QSAR study of selective I1-imidazoline receptor ligands

Selective imidazoline(1)-receptor (I(1)-R) ligands are used clinically to reduce blood pressure. Thus, there is significant interest in developing new imidazoline analogs with high selectivity and affinity for I(1) receptors. A quantitative structure-activity relationship (QSAR) study was carried out on 11 potent I(1)-R ligands (derivatives of imidazoline, oxazoline and pyrroline) using a multiple linear regression (MLR) procedure. The selected compounds have been studied using B3LYP/3-21G(d, p) and B3LYP/6-31G(d, p) methods. Among the 42 descriptors that were considered in generating the QSAR model, three descriptors (partial atomic charges of nitrogen in the heterocyclic moiety (N-2 charge), log D and the dipole moment of the ligands) resulted in a statistically significant model with r(2) > 0.874 and [image omitted] > 0.802. The QSAR models were validated through cross-validation and external test set prediction. The aim of the developed MLR models for the I(1)-R ligands was to link the structures to their reported I(1)-R binding affinity log(1/Ki). The proposed QSAR models indicate that an increase in log D and the dipole moment value and a decrease in N-2 charge in the heterocyclic moiety are predictors of better selectivity and affinity for I(1) receptors. The developed QSAR model is intended to predict the I(1)-R binding affinity of related compounds and aid in the rational design of new potent and selective I(1)-R ligands.

G-protein inwardly rectifying potassium channels are involved in the hypotensive effect of I1-imidazoline receptor selective ligands

OBJECTIVE

The present study examined the role of G-protein inwardly rectifying potassium (GIRK) channels in the depressor responses elicited by intracisternal injections of imidazoline-like drugs in anesthetized rabbits.

METHODS AND RESULTS

Intracisternal injections of the I1-imidazoline receptor (I1R) selective ligands LNP509 (30 microg/kg) and LNP640 (2 microg/kg) (subthreshold doses), and of the GIRK channel opener flupirtine (30 microg/kg) did not affect mean arterial blood pressure (MAP). LNP509 and LNP640, however, elicited substantial depressor responses in rabbits pretreated with flupirtine (-17 +/- 2 and -18 +/- 1 mmHg, respectively, P < 0.05). Injection of higher doses of LNP509 (200 microg/kg) or LNP640 (10 microg/kg) elicited substantial reductions in MAP (-45 +/- 3 and -39 +/- 2 mmHg, respectively, P < 0.05) in naive rabbits. The depressor responses elicited by the higher doses of LNP509 or LNP640 were markedly diminished by pretreatment with the GIRK channel blocker tertiapin-Q (10 microg/kg) (-23 +/- 3 and -26 +/- 2 mmHg, respectively, P < 0.05 compared with nonpretreated rabbits), whereas tertiapin-Q (10 microg/kg) did not affect MAP by itself. Maximal-specific binding (Bmax) of the I1R ligand [I]LNP911 to PC12 cell membranes (296 +/- 59 fmol/mg protein) was enhanced by flupirtine pretreatment whereas it was reduced by tertiapin-Q pretreatment (687 +/- 122 and 68 +/- 21 fmol/mg protein, respectively, P < 0.05 vs. control binding).

CONCLUSION

These findings demonstrate that the modulation of GIRK channels affects I1R's function and raise the possibility that GIRK channels, and I1Rs are parts of a single proteic complex.

S1P-receptors in PC12 and transfected HEK293 cells: molecular targets of hypotensive imidazoline I(1) receptor ligands

The present study aimed at elucidating the molecular identity of the proposed "I(1)-imidazoline receptors", i.e. non-adrenoceptor recognition sites via which the centrally acting imidazolines clonidine and moxonidine mediate a major part of their effects. In radioligand binding experiments with [(3)H]clonidine and [(3)H]lysophosphatidic acid on intact, alpha(2)-adrenoceptor-deficient PC12 cells, moxonidine, clonidine, lysophosphatidic acid and sphingosine-1-phosphate (S1P) competed for the specific binding sites of both radioligands with similar affinities. RNA interference with the rat S1P(1)-, S1P(2)- or S1P(3)-receptor abolished specific [(3)H]lysophosphatidic acid binding. [(3)H]Clonidine binding was markedly decreased by siRNA targeting S1P(1)- and S1P(3)-receptors but not by siRNA against S1P(2)-receptors. Finally, in HEK293 cells transiently expressing human S1P(3)-receptors, sphingosine-1-phosphate, clonidine and moxonidine induced increases in intracellular calcium concentration, moxonidine being more potent than clonidine; this is in agreement with the known properties of the "I(1)-imidazoline receptors". The present results indicate that the "I(1)-imidazoline receptors" mediating effects of clonidine and moxonidine in PC12 and the transfected HEK293 cells belong to the S1P-receptor family; in particular, the data obtained in PC12 cells suggest that the I(1) imidazoline receptors represent a mixture of S1P(1)- and S1P(3)-receptors and/or hetero-dimers of both.

Theoretical study of structure, pKa, lipophilicity, solubility, absorption, and polar surface area of some centrally acting antihypertensives

The methods of theoretical chemistry have been used to elucidate the molecular properties of the substituted imidazoline and oxazoline structures, a class of potent agonists and antagonists of imidazoline receptors. The geometries of various tautomers and isomers of 2-[2,6-dichlorophenylimino]imidazolidine (clonidine), 1-(N-dicyclopropylmethyl)amino-2-oxazoline (rilmenidine), 4-chloro-N-(4,5-dihydro-1H-imidazol-2yl)-6-methoxy-2-methyl-5-pyrimidinamine (moxonidine), N-(dicyclopropylmethyl)-4,5-dihydro-1H-pyrrol-2-amine (aminopyrroline), N-dicyclopropylmethyl-4,5-dihydrothiazol-2-amine (aminothiazoline), 4,5-dihydro-2-(2-methoxyphenyl)-1H-imidazole (compound_6), 4,5-dihydro-2-(3-methylthiophen-2-yl)-1H-imidazole (compound_7), N-(2-chloro-4-iodophenyl)-4,5-dihydro-5-methyl-3H-pyrrol-2-amine (LNP_911), N-amidino-3,5-diamino-6-chloropyrazine-carboxamide (amiloride), 2-(1,4-benzodioxan-2-yl)-2-imidazoline (idazoxan), (+/-)-2-(2-ethyl-2,3-dihydro-2-benzofuranyl)-2-imidazoline (efaroxan), (4-aminobutyl)guaninine (agmatine), and 1-methyl-9H-pyrido[3,4-b]indole (harmane) have been studied using Becke3LYP/6-31+G(d,p) and BP86/TZ2P DFT methods. The optimized geometries indicate that these molecules show a distinctly nonplanar configuration of the imidazoline and oxazoline moieties. In the gas-phase, rilmenidine and aminothiazoline exist in two forms (amino and imino), the amino tautomers being more stable by about 6 kJ/mol. The calculations showed, in agreement with experiments, that clonidine, moxonidine, and LNP_911 exist in a more stable imino tautomer. The tautomer containing the amino group is by about 30 kJ/mol less stable. Computations that include the effect of solvation indicated that also in water the relative stability order of individual tautomers (amino and imino forms) is preserved. The computed pKa values varied between 6.7 and 9.0, and correlate well with the available experimental pKa's found in the literature. Among the clinically useful antihypertensives moxonidine exhibits the lowest basicity in water. At pH = 7.4 only about 50% of this drug exists in ionized form. The available experimental partition coefficients of compounds investigated are best reproduced by the CLOGP method. The computed partition coefficients varied between -1.80 (agmatine) and 5.35 (LNP_911) (CLOGP). Clonidine, moxonidine, and rilmenidine are moderately lipophilic compounds with lipophilicities between these two extreme values. The computed solubilities (about 0.1-4 g/L) show that the imidazoline and oxazoline derivatives studied have very low water solubility. The analysis of molecular descriptors defined by Lipinski has shown that most of the compounds studied obey 'rule of five'. Amiloride and agmatine 'outlets' exhibit also the lowest absorption. Therefore, in the early stages of the design of ligands acting on imidazoline binding sites, it is becoming more important to determine the pKa, lipophilicity, water solubility, polar surface area, absorption, and other physicochemical properties associated with a drug, before synthetic work is undertaken, with the aim of avoiding the synthesis of compounds that are predicted to have poor biopharmaceutical characteristics.

LNP 906, the first high-affinity photoaffinity ligand selective for I1 imidazoline receptors

1 The hypotensive effect of imidazoline-like drugs, such as clonidine, was attributed both to alpha2-adrenergic receptors and nonadrenergic imidazoline receptors, which are divided into I1, I2 and I3 subtypes. 2 We have recently synthesized a derivative of (2-(2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline (LNP 911), the first high-affinity and selective ligand for I1 receptors (I1R), with a photoactivable function (LNP 906). 3 This work aims to test whether this derivative retained the binding properties of LNP 911 and bound irreversibly to I1R. 4 Binding studies showed that LNP 906 exhibited nanomolar affinity for I1R and was selective for I1R over I2 receptors and alpha2-adrenergic receptors (alpha2Ars). 5 Upon exposure to u.v. light, LNP 906 irreversibly blocked the binding of [125I]-paraiodoclonidine (PIC) to I1R, time- and dose-dependently, on PC12 cell membranes and interacted with I1R in a reversible and competitive manner in the absence of light. Pharmacological studies showed that this blockade was prevented by the concomitant presence of rilmenidine (a well-known I1 agonist), but not by rauwolscine (an alpha2 antagonist). 6 Finally, LNP 906 clearly antagonized the decrease in forskolin-stimulated cAMP level induced by rilmenidine, but not by melatonin. 7 These results indicate that LNP 906 is the first high-affinity and selective photoaffinity ligand for I1R and that it behaves as an I1R antagonist.

Presynaptic effects of moxonidine in isolated buffer perfused rat hearts: role of imidazoline-1 receptors and alpha2-adrenoceptors

Numerous studies support the concept that centrally acting antihypertensive drugs, such as imidazolines, mediate sympathoinhibition not only via activation of central nervous alpha2-adrenoceptors (alpha2-AR) but also via imidazoline-1 receptors (I1-R). An additional presynaptic involvement in sympathetic neurotransmission of imidazolines, via I1-R independent of alpha2-AR, is still controversial and remains to be clarified in the heart. Concentration response curves on endogenous norepinephrine (NE) overflow evoked by stimulation of epicardial postganglionic sympathetic nerves in isolated buffer-perfused rat hearts were performed for brimonidine, moxonidine, rauwolscine, 2-endo-amino-3-exo-isopropylbicyclo[2.2.1]heptane (AGN192403), and efaroxan. To unmask an I1-R-mediated effect of moxonidine, hearts were pre-exposed in additional experiments with brimonidine or rauwolscine with or without AGN192403 or efaroxan, respectively. Moxonidine reduced stimulated NE overflow (log EC50: -6.15 +/- 0.14). AGN192403, a selective ligand at I1-R, had no influence on the dose-response curve of moxonidine (log EC50: -6.01 +/- 0.25). After pre-exposure to brimonidine [ stimulation 1 (S1) + stimulation 2 (S2); 10(-5) M], the inhibitory action of moxonidine was potentiated compared with control (32.0 +/- 2.8 versus 73.13 +/- 3.0%) and completely abolished with AGN192403 or efaroxan. This effect was also totally inhibited by pre-exposure with indomethacin (10(-7) M) and tricyclodecan-9-yl-xanthogenate (D-609), an inhibitor of phosphatidylcholine-selective phospholipase C (PC-PLC; 10(-7) M). Conversely, moxonidine was without modulating efficacy under alpha2-AR-blockade by rauwolscine. In summary, we demonstrate that moxonidine reduces NE release independently of I1-R, demonstrating the prominent effect of alpha2-AR in cardiac tissue under basal conditions. We also demonstrate that I1-R are involved in NE release under conditions of alpha2-AR-stimulation involving both a pathway of prostaglandins and PC-PLC.