|Trade names||Revia, Vivitrol|
|By mouth, intramuscular|
|Biological half-life||4 h (naltrexone),
13 h (6β-naltrexol)
|Chemical and physical data|
|Molar mass||341.401 g/mol|
|3D model (Jmol)|
|Melting point||169 °C (336 °F)|
|(what is this?)|
The closely related medication, methylnaltrexone, is used to treat opioid-induced constipation but does not treat addiction as it does not cross the blood brain barrier. Nalmefene is similar to naltrexone and is used for the same purposes as naltrexone. Naltrexone should not be confused with naloxone, which is used in emergency cases of opioid overdose.
Naltrexone is marketed under the trade name Revia and Vivitrol. In the United States as of 2016, naltrexone tablets cost about $0.91 per day. The extended-release injections cost about $1248.50 USD per month (41.62 USD per day). A combination of naltrexone with bupropion is used to treat obesity in the United States.
The main use of naltrexone is for the treatment of alcoholism. Naltrexone has been shown to decrease the amount and frequency of drinking. It does not appear to change the percentage of people drinking. Its overall benefit has been described as "modest".
The Sinclair method is a method of using opiate antagonists such as naltrexone to treat alcoholism. The patient takes the medication about an hour (and only then) before drinking to avoid side effects that arise from chronic use. The opioid blocks the positive reinforcement effects of alcohol and allows the person to stop or reduce drinking.
Naltrexone helps patients overcome opioid addiction by blocking the effects of opioid drugs. It has little effect on opioid cravings. Naltrexone has in general been better studied for alcoholism than in for opioids. It is more frequently used for alcoholism, despite its original approval by the FDA in 1984 for opioid addiction.
A 2011 review of studies suggested that naltrexone, when taken by mouth, was not superior to placebo or to no medication, nor was it superior to benzodiazepine or buprenorphine. Because of the poor quality of the reviewed studies, the authors found insufficient evidence to support naltrexone therapy when taken by mouth for an opioid use disorder. While some patients do well with the oral formulation, it must be taken daily, and a patient whose cravings become overwhelming can obtain opioid intoxication simply by skipping a dose. Due to this issue, the usefulness of oral naltrexone in opioid use disorders is limited by the low retention in treatment. Oral naltrexone remains an ideal treatment only for a small part of the opioid-addicted population, usually those with a stable social situation and motivation. With additional contingency management support, naltrexone is effective in a broader population.
Extended-release depot injections of naltrexone, administered once per month, have proven somewhat effective in treating opioid abuse, an approach that avoids the compliance issue that arises with oral formulations.
The most common side effects reported with naltrexone are non-specific gastrointestinal complaints such as diarrhea and abdominal cramping. These adverse effects are analogous to the symptoms of opioid withdrawal, as the mu receptor blockade will increase GI motility.
Naltrexone has been reported to cause liver damage (when given at doses higher than recommended). It carries an FDA boxed warning for this rare side effect. Due to these reports, some physicians may check liver function tests prior to starting naltrexone, and periodically thereafter. Concerns for liver toxicity initially arose from a study of non-addicted obese patients receiving 300 mg of naltrexone. Subsequent studies have suggested limited toxicity in other patient populations.
Naltrexone should not be started until several (typically 7-10) days of abstinence from opioids has been achieved. This is due to the risk of acute opioid withdrawal if naltrexone is taken, as naltrexone will displace most opioids from their receptors. The time of abstinence may be shorter than 7 days, depending on the half-life of the specific opioid taken. Some physicians use a naloxone challenge to determine whether an individual has any opioids remaining. The challenge involves giving a test dose of naloxone and monitoring for opioid withdrawal. If withdrawal occurs, naltrexone should not be started.
It is important that a person does not attempt to use opioids while using naltrexone. Although naltrexone blocks the opioid receptor, it is possible to override this blockade with high doses of opioids. This is quite dangerous, and may lead to opioid overdose, respiratory depression, and death. Similarly, a person will not show a normal response to opioid pain medications when taking naltrexone. In a supervised medical setting pain relief is possible, but may require higher than usual doses, and the individual should be closely monitored for respiratory depression. All individuals taking naltrexone are encouraged to keep a card or a note in their wallet in case of an injury or another medical emergency. This is to let medical personnel know that special procedures are required if opiate-based painkillers are to be used.
There has been some controversy regarding the use of opioid receptor antagonists such as naltrexone in the long-term management of opioid dependence, due to the effect of these agents in sensitizing opioid receptors. That is, after therapy, the opioid receptors continue to have increased sensitivity for a period during which the patient is at increased risk of opioid overdose. This effect reinforces the necessity of monitoring therapy, and provision of patient support measures by medical practitioners.
Naltrexone should not be used by persons with acute hepatitis or liver failure, or those with recent opioid use (typically 7–10 days).
A naltrexone treatment study by Anton et al., released by the National Institutes of Health in February 2008 and published in the Archives of General Psychiatry, has shown that alcoholics having a certain variant of the opioid receptor gene (G polymorphism of SNP Rs1799971 in the gene OPRM1), known as Asp40, demonstrated strong response to naltrexone and were far more likely to experience success at cutting back or discontinuing their alcohol intake altogether, while for those lacking the gene variant, naltrexone appeared to be no different from placebo. The G allele of OPRM1 is most common in individuals of Asian descent, with 60% to 70% of people of Chinese, Japanese, and Indian ancestry having at least one copy, as opposed to 30% of Europeans and few Africans.
Because of the characteristics of the patient group in the US, the first study was done on white patients and the next without regard for ethnicity. Anton et al. found that patients of African descent did not have much success with naltrexone in treatment for alcohol dependence because of lacking the relevant gene.
As white patients with the gene had a five times greater rate of success in reducing drinking when given naltrexone than did patients without the gene, when used in a protocol of Medical Management (MM), Anton et al. concluded,
"Because almost 25% of the treatment-seeking population carries the Asp40 allele, genetic testing of individuals before naltrexone treatment might be worth the cost and effort, especially if structured behavioral treatment were not being considered." This would enable treatment to be targeted by genetics to patients for whom it would be most effective. They noted, "Naltrexone is relatively easy to administer and free of serious adverse effects and, as we observed in the Asp40 carriers we studied, it appears to be highly effective."
Studies have found naltrexone to be more efficacious among certain white subjects, because of the genetic basis, than among black subjects, who generally do not carry the relevant gene variant. A 2009 study of naltrexone as an alcohol dependence treatment among African Americans failed to find any statistically significant differences between naltrexone and a placebo. Studies have suggested that carriers of the G allele may experience higher levels of craving and stronger "high" upon alcohol consumption, compared to carriers of the dominant allele, and naltrexone somewhat blunts these responses, leading to a reduction in alcohol use in some studies.
Mechanism of actionEdit
Naltrexone and its active metabolite 6β-naltrexol are antagonists at the μ-opioid receptor (MOR), the κ-opioid receptor (KOR) to a lesser extent, and to a far lesser and possibly insignificant extent, at the δ-opioid receptor (DOR). The Ki affinity values of naltrexone at the MOR, KOR, and DOR have been reported as 0.0825 nM, 0.509 nM, and 8.02 nM, respectively, demonstrating a KOR/MOR binding ratio of 6.17 and a DOR/MOR binding ratio of 97.2.
The blockade of opioid receptors is the basis behind naltrexone's action in the management of opioid dependence—it reversibly blocks or attenuates the effects of opioids. Its mechanism of action in alcohol dependence is not fully understood, but as an opioid receptor antagonist is likely to be due to the modulation of the dopaminergic mesolimbic pathway (one of the primary centers for risk-reward analysis in the brain, and a tertiary "pleasure center") which is hypothesized to be a major center of the reward associated with addiction that all major drugs of abuse are believed to activate. Mechanism of action may be antagonism to endogenous opiates such as tetrahydropapaveroline, whose production is augmented in the presence of alcohol.
Structure and pharmacokineticsEdit
Naltrexone can be described as a substituted oxymorphone – here the tertiary amine methyl-substituent is replaced with methylcyclopropane. Naltrexone is the N-cyclopropylmethyl derivative of oxymorphone.
Naltrexone is metabolized mainly to 6β-naltrexol by the liver enzyme dihydrodiol dehydrogenase. Other metabolites include 2-hydroxy-3-methoxy-6β-naltrexol and 2-hydroxy-3-methoxy-naltrexone. These are then further metabolized by conjugation with glucuronide.
The plasma half-life of naltrexone and its metabolite 6β-naltrexol are about 4 hours and 13 hours, respectively.
Additionally, naltrexone implants that are surgically implanted are available, although they are authorized for export only (i.e. not for use within Australia). Naltrexone implants have been studied in Australia with encouraging results.
The FDA authorized use of injectable naltrexone (Vivitrol) for opioid addiction using a single study. This study was run out of Russia, a country where opioid agonists such as methadone and buprenorphine are not available. Therefore, this single trial of naltrexone was performed not by comparing it to the best available, evidence-based treatment (methadone or buprenorphine) but by comparing it with a placebo. In addition, the study failed to follow up on participants to document post-treatment overdose - a key measure for opioid substitution therapies. Furthermore, Vivitrol costs about $1,100 a month, compared to $11 a month for generic naltrexone. These factors led to criticism of the study's design and ethics, and, by extension, of the FDA's approval of injectable naltrexone for opioid addiction based on this study.
Naltrexone is sometimes used in the treatment of dissociative symptoms such as depersonalization and derealization. Some studies suggest it might help. Other small, preliminary studies have also shown benefit. It is thought that blockade of the KOR by naltrexone and naloxone is responsible for their effectiveness in ameliorating depersonalization and derealization. Since these drugs are less efficacious in blocking the KOR relative to the MOR, higher dose than typically used seem to be necessary.
"Low-dose naltrexone" (LDN) describes the "off-label" use of naltrexone at low doses for diseases not related to chemical dependency or intoxication, such as multiple sclerosis. More research needs to be done before it can be recommended for clinical use.
Although there are scientific studies showing its efficacy in some conditions such as fibromyalgia, other, more dramatic claims for its use in conditions like cancer and HIV have less scientific support. This treatment has received significant attention on the Internet, especially through websites run by organizations promoting its use.
A study done by The Chicago Stop Smoking Research Project at the University of Chicago found that naltrexone did not help to improve a subject's chance of attaining smoking cessation when compared to a placebo.
Some studies suggest that self-injurious behaviors present in persons with developmental disabilities (including autism) can sometimes be remedied with naltrexone. In these cases, it is believed that the self-injury is being done to release beta-endorphin, which binds to the same receptors as heroin and morphine. If the "rush" generated by self-injury is removed, the behavior may stop.
There are indications that naltrexone might be beneficial in the treatment of impulse control disorders such as kleptomania, compulsive gambling, or trichotillomania (compulsive hair pulling), but there is conflicting evidence of its effectiveness for gambling. A 2008 case study reported successful use of naltrexone in suppressing and treating an internet pornography addiction.
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