Comparative pharmacodynamic analysis of imidazoline compounds using rat model of ocular mydriasis with a test of quantitative structure–activity relationships

Highlights

• Imidazol(in)e agents were studied with regard to their pharmacodynamic activity on rat mydriasis model.

• Activity parameters were mutually compared and molecular mechanisms of receptor activation have been discussed.

• The structure of the agents was described in terms of molecular descriptors.

• Quantitative structure–activity relationships (QSAR) were derived.

• It was found that imidazol(in)e agents activity depends on their specific polar interactions with alpha2-adrenergic receptors.

Abstract

Imidazol(in)e derivatives, having the chemical structure similar to clonidine, exert diverse pharmacological activities connected with their interactions with alpha2-adrenergic receptors, e.g. hypotension, bradycardia, sedation as well as antinociceptive, anxiolytic, antiarrhythmic, muscle relaxant and mydriatic effects. The mechanism of pupillary dilation observed after systemic administration of imidazol(in)es to rats, mice and cats depends on the stimulation of postsynaptic alpha2-adrenoceptors within the brain. It was proved that the central nervous system (CNS)-localized I1-imidazoline receptors are not engaged in those effects. It appeared interesting to analyze the CNS-mediated pharmacodynamics of imidazole(in)e agents in terms of their chromatographic and calculation chemistry-derived parameters.

In the present study a systematic determination and comparative pharmacometric analysis of mydriatic effects in rats were performed on a series of 20 imidazol(in)e agents, composed of the well-known drugs and of the substances used in experimental pharmacology. The eye pupil dilatory activities of the compounds were assessed in anesthetized Wistar rats according to the established Koss method. Among twenty imidazol(in)e derivatives studied, 18 produced diverse dose-dependent mydriatic effects. In the quantitative structure–activity relationships (QSAR) analysis, the pharmacological data (half maximum mydriatic effect – ED₅₀ in μmol/kg) were considered along with the structural parameters of the agents from molecular modeling. The theoretically calculated lipophilicity parameters, CLOGP, of imidazol(in)es, as well as their lipophilicity parameters from HPLC, log k_w, were also considered. The attempts to derive statistically significant QSAR equations for a full series of the agents under study were unsuccessful. However, for a subgroup of eight apparently structurally related imidazol(in)es a significant relationship between log(1/ED₅₀) and log k_w values was obtained. The lack of “predictive” QSAR for the whole series of the structurally diverse agents is probably due to a complex mechanism of the ligand–alpha2-adrenergic receptor interactions, which are predominantly of a highly structurally specific polar nature. Such interactions are difficult to quantify with the established chemical structural descriptors, contrary to the less specific, molecular bulkiness-related interactions.

Introduction

Compounds containing imidazoline or imidazole moiety exert diverse pharmacological actions. Blood pressure reducing and heart rate lowering activities of imidazolines, known as “clonidine-like” drugs of the first generation, i.e. clonidine, lofexidine, guanfacine, are mainly due to the stimulation of alpha2-adrenergic receptors in the brain [1]. Second generation hypotensives, as moxonidine and rilmenidine, having higher affinity to the central imidazoline I₁- than to alpha2-adrenergic-receptors, showed less pronounced side effects, e.g. sedation and dry mouth [2]. Imidazoline alpha2-adrenoceptor agonists, as brimonidine and para-aminoclonidine, due to the decrease of aqueous humor secretion, increase uveoscleral outflow in the eye and exert neuroprotective effects on retina and are thus beneficial in glaucoma [3]. The other beneficial properties of imidazolines, associated with activation of central alpha2-adrenoceptors, include sedative and hypnotic effects. Therefore, detomidine and medetomidine are used in veterinary medicine [4]. Also prevention of peri- or postoperative heart ischemia after administration of mivazerol or clonidine was reported [5], [6]. Centrally acting imidazoline derivatives, like lofexidine or clonidine, were applied as agents relieving opioids withdrawal symptoms [7]. Another imidazoline derivative tizanidine, stimulating alpha2-adrenoceptors at both the brain stem and the spinal cord, exerts antinociceptive and muscle relaxant effects. Therefore it could be used in the treatment of chronic tension-type headache and spasticity, associated with multiple sclerosis and spinal cord injury [8]. In turn, alpha2-adrenoceptor agonist fadolmidine, having fewer CNS-mediated side effects, may have therapeutic value for targeted spinal analgesia in postoperative and chronic pain [9]. Similarly, dexmedetomidine, an imidazole analog stimulating alpha2-adrenergic receptors and exerting antinociceptive effects, accompanied by limited penetration through blood brain barrier, is used in intensive care units [10]. Recently, some new compounds of imidazoline structure were synthesized, having hypotensive and bradycardic properties, like marsanidine, selectively stimulating brain alpha2-adrenergic receptors, and its 7-methyl analog, having mixed alpha2/imidazoline I1 receptor agonistic properties [11]. Another derivative, PT-31, chemically related to 3-benzyl-imidazolidine, that acts via alpha2A-adrenoceptor activation in the CNS, produces antinociceptive effects and seems to be an interesting substance for pain therapy [12]. Interestingly, rilmenidine was recently found to have cytotoxic effects on cultured cancer cell lines and novel rilmenidine-related compounds with anticancer activity, but devoid of α₂-adrenoceptor effects, were identified [13].

The quantitative structure-activity (QSAR), structure-activity (SAR), or quantitative structure-property (QSPR) analysis in a group of imidazol(in)e compounds were recently a subject of interesting publications. Nikolic et al. [14] carried out a QSAR study of twelve imidazoline derivatives, using regression method, and considering their imidazoline-I1 and alpha2-adrenergic receptors binding affinities in human platelets. A statistically significant three parameter equation was derived to describe ligands binding affinity to I1-receptors (log 1/K_i) in relation to parameters reflecting their lipophilicity (log D_pH7.4), electronic properties (partial atomic charges on nitrogen in the heterocyclic ring) and molecular bulkiness (molar refraction).

The aim of the work of Sączewski et al. [11], [15] was to investigate, whether the α₁-adrenergic antagonistic properties of previously described hypotensive imidazoline analogs, containing in their molecules an indazole moiety, were transferable to newly synthesized compounds of similar structure but having bioisosteric indole instead of indazole ring. The results obtained show that target-based SAR information from in vitro assayed binding affinity to alpha1-adrenoceptors could be transferred from 1-[(imidazoli-2-yl)methyl]indazoles to corresponding indole derivatives. However, when screened in vivo in anesthetized rats, the new 1-[(imidazolin-2-yl)methyl]indole derivatives showed different hemodynamic profiles and different side effects than expected and no correlation was found between in vitro binding to the alpha-adrenergic receptors and the cardiovascular activities of the agents. The conclusion has been drawn, that in a search for new imidazoline drugs acting via α-adrenoceptors, the target-based SAR information, obtained from radioligand binding experiments, has not always been reliable.

The results of theoretical study, with the use of 3D-QSAR, informational spectrum method (ISM) and virtual docking techniques, on evaluation of I1-imidazoline agonistic/antagonistic activity of novel ligands having imidazol(in)e structure, designed as potential anticancer drugs, were also reported [16].

In a comprehensive review, Nikolic and Agbaba [17] paid particular attention to chemical diversity of the imidazol(in)e derivatives and proposed pharmacophore features of both the well-known and the recently synthesized selective ligands of particular (mainly I1 and I2) imidazoline receptors. The authors described the development of the compounds, which have in their molecules imidazoline, oxazoline and pyrroline rings and are characterized by diverse affinity to imidazoline I1 and I2 receptor subtypes. The role of 2D-QSAR in studies reported by many research groups on design and synthesis of new, more selective I1 and I2 imidazoline receptor ligands, based on radioligand binding constants and molecular descriptors of imidazol(in)es, was underlined. Moreover, the physicochemical features resulting in the developed 2D-QSAR models of imidazolines, essential for their affinity for particular imidazoline receptors, were presented. The data on other approaches (e.g. 3D-QSAR, CoMFA), applied for the selection of the combination of the electrostatic and steric fields as the most significant molecular interaction fields (MIFs) parameters for the affinity of newly obtained imidazol(in)e analogs to I1 and I2 receptor subtypes, were also reported.

The QSPR analysis was performed in a group of guanidine/imidazoline derivatives, exerting antihypertensive and diuretic activities, due to the interaction with I₁-imidazoline receptor [14]. Significant correlation was found between the capillary electrophoresis migration times of the complexes of the analytes with β-cyclodextrin (BCD) and the theoretically calculated structural parameters, reflecting constitutional, physicochemical and electronic properties of the agents studied.

Vucicevic et al. [18] studied the relationships between the blood–brain barrier (BBB) permeability and chemical structure of 40 alpha2-adrenergic/imidazoline receptor ligands of potential centrally-mediated hypotensive activity, considered as drug candidates for treatment of neurological diseases. The obtained QSPR equations, connecting effective BBB permeability, determined experimentally with use of PAMPA, with the structural descriptors of the agents (lipophilicity, bulkiness and electronic properties), could be useful as fast screening models of brain penetration of the analyzed compounds. A similar group of alpha-adrenergic/imidazoline receptor ligands, composed of 29 compounds, was examined in a QSPR study by Nikolic et al. [19]. The data considered were: rate of brain penetration, (logPS), brain/plasma equilibration rate (logPS-brain) and extent of blood–brain barrier permeation, along with HPLC retention data, CE migration parameters as well as the calculated molecular descriptors of the agents. The results of multivariate processing of the obtained data with the use of principal component analysis (PCA) and hierarchical clustering analysis (HCA) confirmed the suitability of these statistical tools for evaluation of blood–brain barrier permeation of novel alpha2-adrenergic/imidazoline receptor ligands.

The retention on an alpha1-acid glycoprotein (AGP) HPLC column of a series of selected imidazolines, containing known drugs and ligands currently used in experimental pharmacology, was determined by Filipic et al. [20]. The obtained retention factors (log k, log k_w), along with theoretically calculated molecular descriptors of the compounds, were subjected to QSRR analysis by use of Partial Least Squares (PLS) and Multiple Linear Regression (MLR) methods. The equations derived allowed to describe AGP retention mechanism of analytes at various chromatographic conditions. Obtained linear solvation energy relationships (LSER) confirmed, that for all the examined systems, the retention parameters on the AGP stationary phase depended mainly on basicity, McGowan volume and excess molar refraction of the imidazoline derivatives studied.

The overview of recent publications on imidazol(in)e derivatives shows, that results of QSAR (SAR) as well as QSPR investigations in that group of compounds could be of importance not only for design and prediction of alpha-adrenergic and/or I1/I2-imidazoline receptor affinities of newly obtained agents with drug-like properties, but also could be used as time and cost efficient screening methods for evaluation of pharmacokinetic properties of potential drugs.

The physiological effects produced by imidazoline analogs acting on the alpha2-adrenergic and on the imidazoline receptors are often not easy to distinguish. Because these compounds could bind to both the α2-adrenoceptors and the I1/I2-imidazoline receptors, it could be difficult to demonstrate, which type of receptors is engaged in particular reaction observed in a living system, like hypotension, blood platelet aggregation and depression of OUN. The results of radioligand binding studies, widely applied in pharmacological investigations of potential ligands of alpha-adrenergic/imidazoline receptors, having imidazol(in)e moiety, could only demonstrate affinity of a given compound to alpha2-adrenergic or I1/I2-imidazoline receptors and are not sufficient to demonstrate, whether the given compound behaves as receptor agonist or antagonist. Because of that, the model of rat eye mydriasis, according to Koss [21], [22], arises as a simple method in vivo not only allowing to test on whole animals the interactions of imidazol(in)es with brain alpha2-adrenoceptors, but also to assess both agonistic as well as antagonistic properties of these compounds. Therefore we applied this model in the present work. The comparative pharmacological analysis data were determined on a series of 20 imidazol(in)e agents, composed of the well-known drugs and of the substances used in experimental pharmacology. The pharmacometric parameters of the agents under study, along with their molecular descriptors, were used in QSAR analysis.