|Systematic (IUPAC) name|
|Pregnancy cat.||B3 (AU) C (US)|
|Legal status||Prescription Only (S4) (AU) ℞-only (CA) POM (UK) ℞-only (US)|
|Metabolism||Minimal (mostly to acetyl metabolites)|
|Mol. mass||151.249 g/mol|
|(what is this?)|
Amantadine (trade name Symmetrel, by Endo Pharmaceuticals) is a drug that has U.S. Food and Drug Administration approval for use both as an antiviral and an antiparkinsonian drug. It is the organic compound 1-adamantylamine or 1-aminoadamantane, meaning it consists of an adamantane backbone that has an amino group substituted at one of the four methyne positions. Rimantadine is a closely related derivative of adamantane with similar biological properties.
Apart from medical uses, this compound is useful as a building block, allowing the insertion of an adamantyl group.
According to the U.S. Centers for Disease Control and Prevention, 100% of seasonal H3N2 and 2009 pandemic flu samples tested have shown resistance to adamantanes, and amantadine is no longer recommended for treatment of influenza in the United States. Additionally, its effectiveness as an antiparkinsonian drug is undetermined, with a 2003 Cochrane Review concluding that there was insufficient evidence in support or against its efficacy and safety.
Amantadine was approved by the U.S. Food and Drug Administration in October 1966 as a prophylactic agent against Asian influenza and eventually received approval for the treatment of Influenzavirus A in adults. In 1969, the drug was also discovered by accident to help reduce symptoms of Parkinson's disease, drug-induced extrapyramidal syndromes, and akathisia.
Amantadine is a weak antagonist of the NMDA type glutamate receptor, increases dopamine release, and blocks dopamine reuptake. This makes it a weak therapy for Parkinson's disease. Although, as an antiparkinsonian it can be used as monotherapy; or together with L-DOPA to treat L-DOPA-related motor fluctuations (i.e., shortening of L-DOPA duration of clinical effect, probably related to progressive neuronal loss) and L-DOPA-related dyskinesias (choreiform movements associated with long-term L-DOPA use, probably related to chronic pulsatile stimulation of dopamine receptors).
Despite a 2003 Cochrane review of the scientific literature concluding that there is inadequate evidence to support the use of amantadine for Parkinson's, the drug continues to be prescribed for this indication.
Amantadine is no longer recommended for treatment of influenza A infection.
For the 2008/2009 flu season, the United States' Centers for Disease Control and Prevention (CDC) found that 100% of seasonal H3N2 and 2009 pandemic flu samples tested have shown resistance to adamantanes. The CDC issued an alert to doctors to prescribe the neuraminidase inhibitors oseltamivir and zanamivir instead of amantadine and rimantadine for treatment of current circulating flu.
Amantadine is frequently used to treat the chronic fatigue often experienced by patients with multiple sclerosis. Additionally, there have been anecdotal reports and a small number of pilot studies that show low-dose amantadine as a potential treatment for ADHD. Limited data has shown that amantadine may help to relieve SSRI-induced sexual dysfunction.
Some open-label uncontrolled studies have found it to possess value as an adjunct to antidepressant therapy.
Amantadine has been associated with several central nervous system (CNS) side effects, likely due to amantadine's dopaminergic and adrenergic activity, and to a lesser extent, its activity as an anticholinergic. CNS side effects include nervousness, anxiety, agitation, insomnia, difficulty in concentrating, and exacerbations of pre-existing seizure disorders and psychiatric symptoms in patients with schizophrenia or Parkinson's disease. The usefulness of amantadine as an anti-parkinsonian drug is somewhat limited by the need to screen patients for a history of seizures and psychiatric symptoms.
Physical and chemical properties
Amantadine may be prepared by reacting adamantane with bromine or nitric acid to give the bromide or nitroester at position one. Reaction of either compound with acetonitrile affords the acetamide, which is hydrolyzed to give 1-adamantylamine:
Dosage and mechanism of action
A starting dose is often 100 mg once daily. Many influenza A strains (and virtually all H1N1 "swine flu" strains) are resistant to amantadine, so a failure at this dose is likely. For its anti-Parkinsonian effects, a starting dose of 300 mg once daily is normal, but can be increased to a limit of about 400 mg.
The mechanisms for amantadine's antiviral and antiparkinsonian effects appear to be unrelated.
- The mechanism of Amantadine's antiviral activity involves interference with a viral protein, M2 (an ion channel), which is required for the viral particle to become "uncoated" once taken inside a cell by endocytosis. Influenza B does not possess M2 channels, and thus the drug is ineffective towards all Influenza B strains.
- The mechanism of its antiparkinsonian effect is poorly understood. The drug has many effects in the brain, including release of dopamine and norepinephrine from nerve endings. It appears to be a weak NMDA receptor antagonist as well as an anticholinergic, specifically a nicotinic alpha-7 antagonist like the similar pharmaceutical memantine.
Amantadine appears to act through several pharmacological mechanisms, but no dominant mechanism of action has been identified. It is a dopaminergic, noradrenergic and serotonergic substance, blocks monoamine oxidase A and NMDA receptors, and seems to raise beta-endorphin/beta-lipotropin levels.
Research in brain injury
In a 2012 study, 184 patients with severe traumatic brain injury were treated with amantadine or placebo for four weeks. In this study, the drug accelerated functional brain recovery during treatment. However, the placebo group had improved just as much as the amantadine group at six weeks — two weeks after the drug administration ended.
In 2005, Chinese poultry farmers were reported to have used amantadine to protect birds against avian influenza. In Western countries and according to international livestock regulations, amantadine is approved only for use in humans. Chickens in China have received an estimated 2.6 billion doses of amantadine. Avian flu (H5N1) strains in China and southeast Asia are now resistant to amantadine, although strains circulating elsewhere still seem to be sensitive. If amantadine-resistant strains of the virus spread, the drugs of choice in an avian flu outbreak will probably be restricted to the scarcer and costlier oseltamivir and zanamivir, which work by a different mechanism and are less likely to trigger resistance.
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