Synthesis, nicotinic acetylcholine receptor binding, and pharmacological properties of 3'-(substituted phenyl)deschloroepibatidine analogs

Competitive docking model for prediction of the human nicotinic acetylcholine receptor α7 binding of tobacco constituents

The detrimental health effects associated with tobacco use constitute a major public health concern. The addiction associated with nicotine found in tobacco products has led to difficulty in quitting among users. Nicotinic acetylcholine receptors (nAChRs) are the targets of nicotine and are responsible for addiction to tobacco products. However, it is unknown if the other >8000 tobacco constituents are addictive. Since it is time-consuming and costly to experimentally assess addictive potential of such larger number of chemicals, computationally predicting human nAChRs binding is important for in silico evaluation of addiction potential of tobacco constituents and needs structures of human nAChRs. Therefore, we constructed three-dimensional structures of the ligand binding domain of human nAChR α7 subtype and then developed a predictive model based on the constructed structures to predict human nAChR α7 binding activity of tobacco constituents. The predictive model correctly predicted 11 out of 12 test compounds to be binders of nAChR α7. The model is a useful tool for high-throughput screening of potential addictive tobacco constituents. These results could inform regulatory science research by providing a new validated predictive tool using cutting-edge computational methodology to high-throughput screen tobacco additives and constituents for their binding interaction with the human α7 nicotinic receptor. The tool represents a prediction model capable of screening thousands of chemicals found in tobacco products for addiction potential, which improves the understanding of the potential effects of additives.

Synthesis, nicotinic acetylcholine receptor binding, in vitro and in vivo pharmacology properties of 3'-(substituted pyridinyl)-deschloroepibatidine analogs

Over the last several years we have synthesized and studied the in vitro and in vivo nAChR pharmacological properties of epibatidine (4) analogs. In this study we report the synthesis, nAChR in vitro and in vivo pharmacological properties of 3'-(substituted pyridinyl)-deschloroepibatidine analogs (5a-e and 6a-e). All of the analogs had high binding affinity for α4β2(∗)-nAChRs. Several of the analogs were potent antagonists of α4β2-nAChRs in in vitro efficacy tests and were potent antagonists of nicotine-induced antinociception in the mouse tail-flick test. Compound 6b had a Ki = 0.13 nM in the binding assay, 25- and 46-fold selectivity for the α4β2(∗)-nAChR relative to the α3β4- and α7-nAChR, respectively, in the in vitro efficacy test and an AD50 = 0.13 μg/kg in the tail-flick test. Combined with favorable calculated physiochemical properties compared to varenicline, our findings suggest that 6b should be considered for development as a potential pharmacotherapy for treating nicotine addiction and other CNS disorders.

Synthesis and nicotinic acetylcholine receptor in vitro and in vivo pharmacological properties of 2'-fluoro-3'-(substituted phenyl)deschloroepibatidine analogues of 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine

Herein, we report the synthesis and nicotinic acetylcholine receptor (nAChR) in vitro and in vivo pharmacological properties of 2'-fluoro-3'-(substituted phenyl)deschloroepibatidines 5b-g, analogues of 3'-(4-nitrophenyl) compound 5a. All compounds had high affinity for α4β2-nAChR and low affinity for α7-nAChR. Initial electrophysiological studies showed that all analogues were antagonists at α4β2-, α3β4-, and α7-nAChRs. The 4-carbamoylphenyl analogue 5g was highly selective for α4β2-nAChR over α3β4- and α7-nAChRs. All the analogues were antagonists of nicotine-induced antinociception in the tail-flick test. Molecular modeling docking studies using the agonist-bound form of the X-ray crystal structure of the acetylcholine binding protein suggested several different binding modes for epibatidine, varenicline, and 5a-g. In particular, a unique binding mode for 5g was suggested by these docking simulations. The high binding affinity, in vitro efficacy, and selectivity of 5g for α4β2-nAChR combined with its nAChR functional antagonist properties suggest that 5g will be a valuable pharmacological tool for studying the nAChR and may have potential as a pharmacotherapy for addiction and other central nervous system disorders.

The antinociceptive effects of nicotinic partial agonists varenicline and sazetidine-A in murine acute and tonic pain models

Nicotinic agonists display a wide-range profile of antinociceptive activity in acute, tonic, and chronic pain models. However, their effectiveness is limited by their unacceptable side effects. We investigated the antinociceptive effects of two new α4β2* nicotinic partial agonists, varenicline and sazetidine-A, in acute thermal and tonic pain mouse models. Both drugs failed to induce significant effects in the tail-flick and hot-plate tests after subcutaneous administration. However, they blocked nicotine's effects in these tests at very low doses. In contrast to acute pain tests, varenicline and sazetidine-A dose-dependently induced an analgesic effect in the mouse formalin test after systemic administration. Their antinociceptive effects were mediated, however, by different nicotinic acetylcholine receptor (nAChR) subtypes. Sazetidine-A effects were mediated by β2* nAChR subtypes, whereas varenicline actions were attributed to α3β4 nAChRs. Moreover, low inactive doses of varenicline blocked nicotine's actions in phase II of the formalin test. Overall, our results suggest that the antagonistic actions of varenicline at low doses are mediated by β2*-nAChRs and at higher doses as an agonist by α3β4*-nAChRs. In contrast, both actions of sazetidine-A are mediated by β2*-nAChR subtypes. These results suggest that nicotinic partial agonists possess analgesic effects in a rodent tonic pain model and may provide a potential treatment for the treatment of chronic pain disorders.

Varenicline blocks β2*-nAChR-mediated response and activates β4*-nAChR-mediated responses in mice in vivo

INTRODUCTION

The smoking cessation aid, varenicline, has higher affinity for the alpha4beta2-subtype of the nicotinic acetylcholine receptor (α4β2*-nAChR) than for other subtypes of nAChRs by in vitro assays. The mechanism of action of acute varenicline was studied in vivo to determine (a) subtype activation associated with physiological effects and (b) dose relationship as an antagonist of nicotine.

METHODS

Acute doses of saline, nicotine, and varenicline were given to mice, and locomotor depression and hypothermia were measured. Subunit null mutant mice as well as selective antagonists were used to study mode of action of varenicline as an agonist. Varenicline as an antagonist of nicotine was also investigated.

RESULTS

Varenicline evokes locomotor depression and hypothermia at higher doses than necessary for nicotine. Null mutation of the α7- or β2-nAChR subunit did not decrease the effectiveness of varenicline; however, null mutation of the β4 subunit significantly decreased the magnitude of the varenicline effect. Effects of the highest dose studied were blocked by mecamylamine (general nAChR antagonist) and partially antagonized by hexamethonium (largely peripheral nAChR antagonist). No significant block was seen with ondansetron antagonist of 5-hydroxytryptamine 3 receptor. Using a dose of nicotine selective for β2*-nAChR subtype effects with these tests, dose-dependent antagonism by varenicline was seen. Effective inhibitory doses were determined and appear to be in a range consistent with binding affinity or desensitization of β2*-nAChRs.

CONCLUSIONS

Varenicline acts as a functional antagonist of β2*-nAChRs, blocking certain effects of nicotine. At higher doses, varenicline is an agonist of β4*-nAChRs producing physiological changes in mice.

Synthesis of 6-(het) ary Xylocydine analogues and evaluating their inhibitory activities of CDK1 and CDK2 in vitro

A series of purine nucleoside analogues bearing an aryl and hetaryl group in position 6 were prepared and their biological activities were assessed by in vitro CDK1/Cyclin B1 and CDK2/Cyclin A2 kinase assay. From the synthesized chemicals, three Xylocydine derivatives 3h, 3i, and 3j exhibited specific inhibitory activities on CDK2/Cyclin A2 with IC(50) values of 4.6, 4.8, and 55 μM, respectively. Those three compounds all induced G1/S phase arrest in Human epithelial carcinoma cell line (HeLa), and the results suggested they may inhibit CDK2 activity in vitro. Furthermore, molecular modeling study, their docking into Cyclin Dependant Kinase 2 (CDK2) active site showed high docking scores. Taken together, these data suggest that, those three compounds are good inhibitors of CDK2 for studying this kinase signal transduction pathway in cell system.

Varenicline blocks nicotine intake in rats with extended access to nicotine self-administration

RATIONALE

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine. Varenicline, a partial α4β2 nicotinic acetylcholine receptor (nAChR) agonist, is effective in reducing nicotine craving and relapse in smokers, suggesting that α4β2 nAChRs may play a key role in nicotine dependence. In rats, the effect of varenicline on nicotine intake has only been studied with limited access to the drug, a model of the positive reinforcing effect of nicotine. Varenicline has not been tested on the increase in motivation to take nicotine in nicotine-dependent rats.

OBJECTIVES

The present study evaluated the effects of varenicline on nicotine intake in rats with extended access to nicotine self-administration (23 h/day), a condition leading to the development of nicotine dependence. We hypothesized that varenicline's effects on nicotine self-administration would be greater in rats with extended than limited access to the drug and after forced abstinence rather than during baseline self-administration.

RESULTS

Varenicline dose-dependently decreased nicotine self-administration in rats with limited (1 h/day) and extended (23 h/day) access. Despite an increased sensitivity to the motivational effects of abstinence on nicotine intake compared with limited-access rats, varenicline was equally effective in decreasing nicotine intake in dependent rats with extended access to nicotine.

CONCLUSION

These results suggest that α4β2 nAChRs are critical in mediating the positive reinforcing effects of nicotine but may not be a key element underlying the negative reinforcement process responsible for the increased nicotine intake after abstinence in dependent subjects.

The effects of nicotine, varenicline, and cytisine on schedule-controlled responding in mice: differences in α4β2 nicotinic receptor activation

Nicotine, varenicline, and cytisine are pharmacotherapies for tobacco dependence; the extent to which their in vivo effects vary as a function of differences in nicotinic acetylcholine receptor agonism is not clear. Male C57BL/6J mice responding under a fixed ratio 30 schedule of food delivery were used to establish the potency and time course of nicotine, varenicline, and cytisine; antagonism was examined with the non-competitive, non-selective antagonist mecamylamine and the competitive, α4β2 nicotinic receptor antagonist dihydro-β-erythroidine (DHβE). Intraperitoneal nicotine, varenicline, and cytisine dose-dependently decreased responding; nicotine was more potent (ED(50) value=0.83 mg/kg) than varenicline (ED(50) value=2.51 mg/kg) and cytisine (ED(50) value=2.97 mg/kg). The agonists had a similar time course including a rapid onset (5 min or less) and relatively short duration of action (30 min). Mecamylamine dose-dependently attenuated the rate-decreasing effects of a fixed dose of nicotine (1.78 mg/kg), varenicline (5.6 mg/kg), and cytisine (5.6 mg/kg). Mecamylamine (1mg/kg) produced parallel rightward shifts in the dose-response curves for nicotine (3.3-fold), varenicline (3.1-fold), and cytisine (2.3-fold). In contrast, DHβE (3.2mg/kg) produced 2-fold antagonism of nicotine and did not antagonize varenicline or cytisine. The data strongly suggest that nicotinic acetylcholine receptors mediate the effects of the agonists to decrease operant responding in mice. However, α4β2 receptor agonism appears to contribute partially to the rate-decreasing effects of nicotine but not to the rate-decreasing effects of varenicline and cytisine. Differential activation of α4β2 receptors in vivo might contribute to differences in the effectiveness of these smoking cessation aids.

Varenicline and mecamylamine attenuate locomotor sensitization and cross-sensitization induced by nicotine and morphine in mice

The present study focused on the evaluation of behavioural sensitization and cross-sensitization induced by nicotine and morphine in mice. First, we revealed that after 9days of nicotine administration (0.175mg/kg, free base), every other day and following its 7-day withdrawal, challenge doses of nicotine (0.175mg/kg) and morphine (5mg/kg) induced locomotor sensitization in mice. When we examined the influence of varenicline, a partial alpha4beta2 nicotinic receptor agonist (0.5, 1 and 2mg/kg) and mecamylamine (0.5, 1 and 2mg/kg), a non-selective nicotinic receptor antagonist, we found that both agents attenuated the acquisition and expression of nicotine sensitization as well as locomotor cross-sensitization between nicotine and morphine. Our results indicate similar cholinergic mechanisms involved in the locomotor stimulant effects of nicotine and morphine in mice, and as such these data may suggest that nicotinic neurotransmission could be a potential target for developing pharmacotherapeutic strategies to treat and prevent nicotine and/or opioid addiction.

Preclinical Pharmacology, Efficacy and Safety of Varenicline in Smoking Cessation and Clinical Utility in High Risk Patients

Smoking is still the most prominent cause of preventable premature death in the United States and an increasing cause of morbidity and mortality throughout the world. Although the current treatments such as nicotine replacement therapy (NRT) and bupropion are effective, long-term abstinence rates are low. Mechanism studies suggest that the pleasurable effects of smoking are mediated predominantly by nicotine, which activates the brain reward system by activation of brain α4β2 nicotinic acetylcholine receptors (nAChRs). Varenicline is a novel α4β2 nAChR partial agonist and has been found to be even more effective than NRT or bupropion in attenuating smoking satisfaction and in relieving craving and withdrawal symptoms after abstinence. Thus, varenicline has been recently approved to be a first-line medication for smoking cessation in the United States and European countries. Varenicline is generally well tolerated in healthy adult smokers, with the most commonly reported adverse effects being nausea, insomnia, and headache. However, growing postmarketing data has linked varenicline to an increase in neuropsychiatric symptoms such as seizures, suicidal attempts, depression, and psychosis as well as serious injuries potentially relating to unconsciousness, dizziness, visual disturbances, or movement disorders. Therefore, new safety warnings are issued to certain high risk populations, such as patients with mental illness and operators of commercial vehicles and heavy machinery. In particular, pilots, air traffic controllers, truck and bus drivers have been banned from taking varenicline.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 3'-(substituted phenyl)epibatidine analogues. Nicotinic partial agonists

In 1992, John Daly et al. reported the isolation and structure determination of epibatidine. Epibatidine's unique structure and its potent nicotinic agonist activity have had a tremendous impact on nicotine receptor research. This research has led to a better understanding of the nicotinic acetylcholine receptor (nAChR) pharmacophore and to epibatidine analogues with potential as pharmacotherapies for treating various CNS disorders. In this study, we report the synthesis, receptor binding ([(3)H]epibatidine and [(125)I]iodoMLA), and in vivo pharmacological properties (mouse tail flick, hot plate, hypothermia, and spontaneous activity) of a series of 3'-(substituted phenyl)epibatidine analogues (5a-m). Results from these studies have added to the understanding of the nAChR pharmacophore and led to nicotinic partial agonists that may have potential for smoking cessation. All the analogues had affinities for the alpha4beta2 nAChR similar to epibatidine (1). 3'-(3-Dimethylaminophenyl)epibatidine (5m) has a nicotinic partial agonist pharmacological profile similar to the smoking cessation drug varenicline. Other analogues are partial agonists with varying degrees of nicotinic functional agonist and antagonist activity. 3'-(3-Aminophenyl)epibatidine (5j) is a more potent functional agonist and antagonist in all tests than varenicline. 3'-(3-Fluorophenyl)epibatidine and 3'-(3-chlorophenyl)epibatidine (5c and 5e) are more potent than varenicline when tested as agonists in four pharmacological tests and antagonists when evaluated against nicotine in the analgesia hot-plate test.

Effect of varenicline on the acute and repeated locomotor responses to nicotine in rats

The objective of this study was to evaluate the efficacy of varenicline, a novel partial agonist at alpha 4 beta 2 and full agonist at alpha 7 nicotinic acetylcholine receptor (nAChR) subtypes, in blocking the locomotor effects of acute or repeated treatments with nicotine (0.4 mg/kg, s.c.) in rats. Varenicline (0.3-3 mg/kg, s.c.) by itself enhanced the basal locomotor activity in naive rats while it had an inhibitory effect on acute nicotine-induced hyperlocomotion. Varenicline (0.3-3 mg/kg) did not change the nicotine-evoked conditioned locomotion, but when administered to nicotine-sensitized rats (0.1 and 1 mg/kg), reduced the expression of nicotine sensitization. In another set of experiments, varenicline (1 mg/kg) administered during the second withdrawal period (days 11-14) to nicotine-treated rats, attenuated the reestablishment of the expression of nicotine sensitization. Our pharmacological analyses further support the hypothesis that varenicline might be a useful treatment for smoking cessation considering its actions on the locomotor and reinforcing effects of nicotine without inhibition of conditioned locomotion.

EPIBATIDINE ANALOGS SYNTHESIZED FOR CHARACTERIZATION OF NICOTINIC PHARMACOPHORES-A REVIEW

In 1992 Daly and co-workers reported the isolation of a new natural product, epibatidine. Future studies showed that epibatidine was an nAChR ligand with analgesic potency 200-400 times greater than that of morphine. However, its potential as a new drug was limited by its toxic side effects, probably resulting from its activity at a number of nAChR subtypes. Epibatidine's unique structure and potent activity made it an ideal lead structure for the development of nAChR ligands with reduced side effects and better nAChR subtype selectivity. This review presents the synthetic methods we have used to synthesize a number of epibatidine agonists, antagonists, and mixed agonists/antagonists to better characterize the α4β2 nAChR pharmacophore and hopefully provide compounds that have potential for treating nicotine addiction.