Concurrent agonist-antagonist administration for the analysis and treatment of drug dependence

[Combinations of pharmacological treatments in smoking cessation. A systematic review]

INTRODUCTION

The effectiveness of the three validated smoking cessation medications, nicotine replacement therapy, varenicline and bupropion, may be insufficient, in hard-core smokers.

OBJECTIVES

This systematic review investigates the efficacy of combinations of different medications in smoking abstinence and their tolerability.

RESULTS

Three randomized controlled trials (RCTs) compared the combined medications with varenicline and nicotine patches vs. varenicline; two found an increase in abstinence rates with the combined medications. In one study, the beneficial effect was only observed in heavy smokers. The four RCTs comparing the combined medications with varenicline and bupropion (vs. varenicline) demonstrated an increase in abstinence rates with the combined medications, most often in heavy smokers who are very dependent on tobacco. The results of the three RCTs comparing the combined medications with bupropion and nicotine replacement therapy vs. varenicline were discordant. Three studies included other molecules (mecamylamine, selegiline, sertraline, buspirone). Combined medications were well tolerated.

CONCLUSION

Combination treatments can achieve higher smoking abstinence rates than monotherapies, especially in smokers who have failed to quit (Hard-core smokers). Treatment with a combination of varenicline and nicotine replacement therapy is a therapeutic option in smoking cessation.

NAChR α4β2 Subtype and their Relation with Nicotine Addiction, Cognition, Depression and Hyperactivity Disorder

BACKGROUND

Neuronal α4β2 nAChRs are receptors involved in the role of neurotransmitters regulation and release, and this ionic channel participates in biological process of memory, learning and attention. This work aims to review the structure and functioning of the α4β2 nAChR emphasizing its role in the treatment of associated diseases like nicotine addiction and underlying pathologies such as cognition, depression and attention-deficit hyperactivity disorder.

METHODS

The authors realized extensive bibliographic research using the descriptors "Nicotine Receptor α4β2" and "cognition", "depression", "attention-deficit hyperactivity disorder", besides cross-references of the selected articles and after analysis of references in the specific literature.

RESULTS

As results, it was that found 179 relevant articles presenting the main molecules with affinity to nAChR α4β2 related to the cited diseases. The α4β2 nAChR subtype is a remarkable therapeutic target since this is the most abundant receptor in the central nervous system.

CONCLUSION

In summary, this review presents perspectives on the pharmacology and therapeutic targeting of α4β2 nAChRs for the treatment of cognition and diseases like nicotine dependence, depression and attention-deficit hyperactivity disorder.

The contribution of agonist and antagonist activities of α4β2* nAChR ligands to smoking cessation efficacy: a quantitative analysis of literature data

RATIONALE AND OBJECTIVE

Two mechanisms underlie smoking cessation efficacies of α4β2* nicotinic acetylcholine receptor (nAChR) agonists: a "nicotine-like" agonist activity reduces craving by substituting for nicotine during a quit attempt, and a "nicotine-blocking" antagonist activity attenuates reinforcement by competing with inhaled nicotine during a relapse. To evaluate the contribution of each mechanism to clinical efficacy, we estimated the degree of agonist and antagonist activities of nicotine replacement therapy (NRT), varenicline, cytisine, and the discontinued nAChR agonists dianicline, ABT-418, ABT-089, CP-601927, and CP-601932, relative to the functional effects of nicotine from smoking.

METHODS

Functional activities that occur in vivo with clinical doses were predicted from literature data on binding and functional potencies at the target α4β2 nAChR, as well as at α6β2* nAChRs, and from estimates of free drug exposures in human brain. Agonist activity is comprised of nAChR activation and desensitization, which were expressed as percentages of desensitization and activation by nicotine from smoking. Antagonist activity was expressed as the reduction in nAChR occupancy by nicotine during smoking in the presence of an agonist.

RESULTS

Comparisons with odds ratios at end of treatment suggest that extensive α4β2 and α6β2* nAChR desensitization combined with α6β2* nAChR activation at similar levels as nicotine from smoking is associated with clinical efficacy (NRT, varenicline, cytisine, ABT-418). Effective competition with inhaled nicotine for α4β2 and α6β2* nAChRs further improves clinical efficacy (varenicline). Other discontinued nAChR agonists have lower agonist and antagonist activities at α4β2 nAChRs and are inactive or less efficacious than NRT (dianicline, ABT-089, CP-601927, CP-601932).

CONCLUSION

Three pharmacological effects appear to be key factors underlying smoking cessation efficacy: the degree of activation of α6β2* nAChRs, desensitization of α4β2 and α6β2* nAChRs (agonist activity), and the reduction of nicotine occupancy at α4β2 and α6β2* nAChRs (antagonist activity). No single activity is dominant, and the level of smoking cessation efficacy depends on the profile of these activities achieved at clinical doses. While adequate agonist activity alone seems sufficient for a clinical effect (e.g., NRT, cytisine), clinical efficacy is improved with substantial competitive antagonism of α4β2 nAChRs, i.e., if the drug has a dual agonist-antagonist mechanism of action (e.g., varenicline).

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

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.

The nicotinic acetylcholine receptor partial agonist varenicline and the treatment of drug dependence: a review

Drug dependence is a chronic brain disease characterized by recurrent episodes of relapse, even when the person is motivated to quit. Relapse is a major problem and new pharmacotherapies are needed to prevent relapse episodes. The nicotinic acetylcholine receptor (nAChR) plays an important role in nicotine dependence, alcohol consumption and cue-induced cocaine craving. Stimulation of the nAChR has been found to alter and modulate cell firing in brain areas important for the maintenance of drug dependence. Varenicline, an alpha4beta2 nAChR partial agonist and an alpha7 nAChR full agonist registered for the treatment of nicotine dependence, significantly reduces nicotine craving and prevents relapse. In addition, varenicline reduces alcohol consumption in rats. Based on a review of the available literature, we hypothesize a potential role for varenicline in the prevention of relapse in patients recovering from drug dependence other than nicotine dependence.

Disrupting nicotine reinforcement: from cigarette to brain

Cigarette smoking is a tenacious addiction that is maintained to a significant extent by the reinforcing effects of nicotine. An emerging theme in smoking cessation treatment is the development of methods for interfering with these reinforcing effects. By attenuating nicotine reinforcement, treatments may enhance a smoker's chances of successfully remaining abstinent. Several treatment approaches will be described, including the use of denicotinized cigarettes, nicotine vaccines, nicotinic receptor agonists and antagonists, and modulators of brain reinforcement processes. These techniques highlight the numerous sites along the path between the cigarette and the brain that can be targeted for intervention. In addition to unimodal therapies, treatment combinations will be discussed that might more effectively block cigarette reward and thereby further enhance smoking abstinence.

Varenicline: progress in smoking cessation treatment

Varenicline is the first pharmacological aid for smoking cessation treatment to be approved in almost a decade. Varenicline is an alpha4beta2 nicotinic acetylcholine receptor partial agonist. It may assist those who wish to quit smoking by producing partial activation at nicotinic receptors to ease craving and withdrawal symptoms; and by simultaneously blocking the effects of nicotine from cigarettes smoked, thereby reducing the satisfaction of continued smoking. The effectiveness of varenicline as an aid for smoking cessation has been demonstrated in several clinical trials versus placebo and sustained release bupropion. In addition, varenicline is well tolerated and has an acceptable safety profile. As an aid for smoking cessation, varenicline offers progress in the treatment of tobacco dependence.

Rationale, pharmacology and clinical efficacy of partial agonists of alpha4beta2 nACh receptors for smoking cessation

Most smokers repeatedly fail in their attempts to stop smoking because of the addictive nature of the nicotine in tobacco products. Nicotine dependence is probably mediated through the activation of multiple subtypes of neuronal nicotinic acetylcholine receptor (nAChR), among which the mesolimbic alpha(4)beta(2) subtype has a pivotal role. Here, we discuss the rationale for and the design of alpha(4)beta(2) nAChR partial agonists as novel treatments for tobacco addiction. Such agents are expected to exhibit a dual action by sufficiently stimulating alpha(4)beta(2)-nAChR-mediated dopamine release to reduce craving when quitting and by inhibiting nicotine reinforcement when smoking. Potent and selective alpha(4)beta(2) nAChR partial agonists that exhibit dual agonist and antagonist activity in preclinical models can be identified. The validity of this approach is demonstrated by the clinical efficacy of the alpha(4)beta(2) nAChR partial agonist varenicline, which has significantly better quit rates than do other treatments and offers a new option for smoking cessation pharmacotherapy.

A randomized, controlled trial to assess the efficacy and safety of a transdermal delivery system of nicotine/mecamylamine in cigarette smokers

AIMS

To determine the efficacy and safety of nicotine transdermal therapy co-administered with the nicotine antagonist, mecamylamine, compared to a nicotine transdermal patch alone (21 mg nicotine + 6 mg mecamylamine, 21 mg nicotine + 3 mg mecamylamine, and 21 mg nicotine + 0 mg mecamylamine).

DESIGN

Multi-center (n = 4), double-blind, randomized, parallel group, repeat-dose study.

SETTING

Clinical laboratory.

PARTICIPANTS

A total of 540 subjects were enrolled into the study-135 from each of four sites; 180 patients in each of three treatment arms.

INTERVENTION

Treatment was administered for the first 6 weeks of the 8-week study. Patients were instructed to continue smoking for the first 2 weeks of treatment.

MEASUREMENTS

The primary efficacy parameter was 4-week continuous abstinence after the quit date, confirmed with an expired carbon monoxide of < 10 parts per million.

FINDINGS

Analysis of the 4-week continuous abstinence for the intent-to-treat population showed overall rates of 29% (nicotine + 6 mg mecamylamine), 29% (nicotine + 3 mg mecamylamine) and 23% (nicotine only) using the slip definition which allows smoking in the first 2 weeks after the quit date. Statistical analyses revealed no significant treatment differences. Analyses using the strict definition (no smoking after the quit date) yielded similar non-significant group differences (29%, 27%, 26%).

CONCLUSION

If adding mecamylamine to nicotine replacement therapy (NRT) improves the chances of success at stopping smoking, the results of this study suggest that the effect is very small.

Pharmacological profile of the alpha4beta2 nicotinic acetylcholine receptor partial agonist varenicline, an effective smoking cessation aid

The preclinical pharmacology of the alpha4beta2 nicotinic acetylcholine receptor (nAChR) partial agonist varenicline, a novel smoking cessation agent is described. Varenicline binds with subnanomolar affinity only to alpha4beta2 nAChRs and in vitro functional patch clamp studies in HEK cells expressing nAChRs show that varenicline is a partial agonist with 45% of nicotine's maximal efficacy at alpha4beta2 nAChRs. In neurochemical models varenicline has significantly lower (40-60%) efficacy than nicotine in stimulating [(3)H]-dopamine release from rat brain slices in vitro and in increasing dopamine release from rat nucleus accumbens in vivo, while it is more potent than nicotine. In addition, when combined with nicotine, varenicline effectively attenuates the nicotine-induced dopamine release to the level of the effect of varenicline alone, consistent with partial agonism. Finally, varenicline reduces nicotine self-administration in rats and supports lower self-administration break points than nicotine. These data suggest that varenicline can reproduce to some extent the subjective effects of smoking by partially activating alpha4beta2 nAChRs, while preventing full activation of these receptors by nicotine. Based on these findings, varenicline was advanced into clinical development and recently shown to be an effective and safe aid for smoking cessation treatment.

Nicotine and nonnicotine factors in cigarette addiction

RATIONALE

A great deal of research supports the role of nicotine in cigarette addiction. However, the effectiveness of nicotine replacement therapy (NRT) as a smoking cessation treatment has fallen short of initial hopes. A key reason may be that NRT does not address nonnicotine components of smoking reinforcement. These include constituents that provide reinforcing sensory stimulation, components that minimize excessive irritation from inhaled nicotine and other pharmacologically active compounds in cigarette smoke.

OBJECTIVE

Studies using various paradigms to dissociate nicotine from other components of smoking are summarized.

RESULTS

Nonnicotine components provide many rewarding effects, often surpassing the direct effects of nicotine. Substitutes for the sensory effects of smoking may be effective in relieving craving for cigarettes and in facilitating smoking cessation. Moreover, techniques for devaluing smoking-related cues may decrease craving and enhance subsequent abstinence. Promising approaches for devaluing smoke cues include extinction-based treatments employing denicotinized cigarettes and the use of nicotinic agonist and/or antagonist treatment during the weeks leading up to a quit attempt. Recent studies suggest that incorporating these approaches into a treatment program may significantly increase smoking abstinence rates. Preliminary findings also suggest that replacement of the effects of monoamine oxidase inhibitors contained in cigarette smoke may enhance quit rates.

CONCLUSIONS

While current NRT methods have been the mainstay of smoking cessation treatment and will likely continue to serve a useful role, the next stage of progress will likely entail the development of tools designed with recognition of the importance of nonnicotine components of cigarette smoking.

Nicotine addiction and treatment

The persistence of cigarette smoking despite widespread awareness of adverse health effects results from an underlying addiction to nicotine. Unaided attempts to quit smoking are generally unsuccessful. This article discusses nicotine addition and therapeutic techniques that have been or are being developed to relieve smoking withdrawal symptoms and promote abstinence from smoking. These techniques include nicotine chewing gum, skin patches, nasal sprays, and inhalers, as well as pharmacotherapies such as mecamylamine and clonidine, serotonergic treatments such as buspirone, and antidepressants such as buproprion. A nondrug approach using cigarette substitutes that mimic the airway sensations produced by cigarette smoke is also discussed.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.