What is/are Varenicline?
Varenicline (trade name Chantix in the USA and Champix in Canada, Europe and other countries, marketed by Pfizer, usually in the form of varenicline tartrate), is a prescription medication used to treat smoking addiction. Varenicline is a nicotinic receptor partial agonist - it stimulates nicotine receptors more weakly than nicotine itself does. In this respect it is similar to cytisine and different from the nicotinic antagonist, bupropion, and nicotine replacement therapies (NRTs) like nicotine patches and nicotine gum. As a partial agonist it both reduces cravings for and decreases the pleasurable effects of cigarettes and other tobacco products. Through these mechanisms it can assist some patients to quit smoking.
Varenicline is indicated for smoking cessation. It is more effective than NRTs and nicotine agonists. In a 2006 randomized controlled trial sponsored by Pfizer, after one year the rate of continuous abstinence was 10% for placebo, 15% for bupropion and 23% for varenicline.In a 2009 meta-analysis of 101 studies funded by Pfizer, varenicline was found to be more effective than bupropion (odds ratio 1.40) and NRTs (odds ratio 1.56).
A Cochrane systematic review concluded that varenicline improved the likelihood of successfully quitting smoking by two- to three-fold relative to pharmacologically unassisted attempts. Varenicline was more efficacious than bupropion in this regard but not statistically superior to nicotine replacement therapy. The FDA has approved its use for twelve weeks. If smoking cessation has been achieved it may be continued for another twelve weeks. Varenicline has not been tested in those under 18 years old or pregnant women and therefore is not recommended for use by these groups.
Nausea occurs commonly in people taking varenicline. Other less common side effects include headache, difficulty sleeping, and abnormal dreams. Rare side effects reported by people taking varenicline compared to placebo include change in taste, vomiting, abdominal pain, flatulence, and constipation. In a recent meta-analysis paper by Leung et al, it has been estimated that for every 5 subjects taking varenicline at maintenance doses (1 mg twice daily), there will be an event of nausea, and for every 24 and 35 treated subjects, there will be an event of constipation and flatulence respectively. Gastrointestinal side-effects are important factors compromising the compliance of varenicline.
Depression and suicide
In November 2007, the FDA announced it had received post-marketing reports that patients using varenicline for smoking cessation had experienced several serious side-effects, including suicidal ideation and occasional suicidal behavior, erratic behavior, and drowsiness. On February 1, 2008 the FDA issued an alert to further clarify its findings, noting that "it appears increasingly likely that there is an association between Chantix and serious neuropsychiatric symptoms." It is unknown whether the psychiatric symptoms are related to the drug or to nicotine withdrawal symptoms, although not all patients had stopped smoking. The FDA also recommended that health care professionals and patients watch for behavioral and mood changes. In May 2008, Pfizer updated the safety information associated with varenicline, noting that "some patients have reported changes in behavior, agitation, depressed mood, suicidal thoughts or actions."
As of July 1, 2009, the US Food and Drug Administration requires Chantix (varenicline) to carry a black box warning, the agency's strongest safety warning, due to public reports of side effects including depression, suicidal thoughts, and suicidal actions.
On June 16, 2011, the FDA issued a safety announcement that Chantix may be associated with "a small, increased risk of certain cardiovascular adverse events in patients who have cardiovascular disease."
On July 4, 2011, four scientists published a review of double-blind studies in the Canadian Medical Association Journal. They found that varenicline has increased risk of serious adverse cardiovascular events compared with placebo. Expert commentary in the same issue of CMAJ raised doubts about the methodology of the review, concerns which were echoed by the European Medicines Agency. Of specific concern were "the low number of events seen, the types of events counted, the higher drop-out rate in people receiving placebo, the lack of information on the timing of events, and the exclusion of studies in which no-one had an event." In contrast, a meta analysis published in BMJ in 2012 included all double-blind RCTs of varenicline vs. placebo (including 8 trials with no events among 1596 participants), focused on events occurring during drug exposure or within 30 days after discontinuation, and analyzed findings using 4 summary estimates. The BMJ meta-analysis found no significant increase in cardiovascular serious adverse events associated with varenicline use for any of the measures, and found negligible variation in the evidence over 22 independent trials with 9,232 subjects.
BMJ published an article later that year that reported on a national cohort study in Denmark with 35,852 smokers using either bupropion or varenicline. This study found no significant differences in the rates of cardiovascular events by drug.
Despite the discussion in academic circles, to date, no direct experimental evidence suggests varenicline's action at the α3β4 and/or α7 dopamine receptors results in cardiovascular effects.
Mechanism of action
Varenicline is a partial agonist of the α4β2 subtype of the nicotinic acetylcholine receptor. In addition it acts on α3β4 and weakly on α3β2 and α6-containing receptors. A full agonism was displayed on α7-receptors.
Acting as a partial agonist varenicline binds to, and partially stimulates, the α4β2 receptor without producing a full effect like nicotine. Thus varenicline does not greatly increase the downstream release of dopamine. Due to its competitive binding on these receptors, varenicline blocks the ability of nicotine to bind and stimulate the mesolimbic dopamine system, akin to the action of buprenorphine in the treatment of opioid addiction.
Varenicline also acts as an agonist at 5-HT3 receptors, which may contribute to mood altering effects of varenicline.
This article uses material from the Wikipedia article Varenicline, which is released under the Creative Commons Attribution-Share-Alike License 3.0.