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Amantadine is a synthetic tricyclic amine that has been used in medicine for over five decades. Initially approved as an antiviral agent for influenza A, it later gained prominence in the treatment of Parkinson’s disease and drug-induced extrapyramidal symptoms. Its unique pharmacological profile—combining antiviral, dopaminergic, and NMDA receptor antagonistic properties—makes it a versatile though now less commonly used drug. This report provides a concise yet thorough review of amantadine, covering its chemistry, mechanisms of action, therapeutic applications, adverse effects, and current place in clinical practice.

Chemistry and Pharmacology

Amantadine (1-adamantanamine) has a rigid, cage-like adamantane structure that confers high lipophilicity, allowing it to cross the blood-brain barrier readily. It is well absorbed orally, with peak plasma concentrations reached in 1–4 hours, and has a long half-life of 10–31 hours, permitting once- or twice-daily dosing. The drug is primarily excreted unchanged in urine, necessitating dose adjustment in renal impairment.

Mechanisms of Action

Amantadine exerts its effects through multiple pathways:

1. Antiviral Mechanism: Against influenza A, it inhibits the viral M2 ion channel protein, which is essential for uncoating the virus after entry into host cells. This action prevents viral replication. However, widespread resistance has limited its use.
   
2. Dopaminergic Effects: It increases dopamine release from presynaptic terminals and inhibits dopamine reuptake, indirectly boosting dopaminergic transmission. This explains its efficacy in Parkinson’s disease.
   
3. NMDA Receptor Antagonism: Amantadine acts as a non-competitive antagonist at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors, reducing excitatory neurotransmission. This mechanism is thought to underlie its benefit in levodopa-induced dyskinesias and possibly neuroprotection.
   
4. Anticholinergic Properties: Weak anticholinergic effects may also contribute to its antiparkinsonian action, though this is not its primary mechanism.
   
   

Influenza A

Amantadine was historically used for prophylaxis and treatment of influenza A in adults and children. When started within 48 hours of symptom onset, it reduces illness duration by about 1–2 days. However, due to the rapid emergence of resistant strains (e.g., the H3N2 and pandemic H1N1 strains became nearly universally resistant), the CDC no longer recommends amantadine for influenza. It has been largely replaced by neuraminidase inhibitors like oseltamivir and zanamivir.

Parkinson’s Disease

Amantadine is used as monotherapy for early, mild Parkinson’s disease or as an adjunct to levodopa/carbidopa. It can improve bradykinesia, rigidity, and tremor. More notably, it is one of the few drugs effective in reducing levodopa-induced dyskinesias—a common complication of long-term levodopa therapy. Its effect on dyskinesias is attributed to NMDA receptor antagonism. The drug may also provide mild symptomatic benefit for gait and freezing episodes.

Drug-Induced Extrapyramidal Symptoms

Amantadine is used to treat extrapyramidal side effects (e.g., dystonia, parkinsonism, akathisia) caused by antipsychotic medications. It is often considered a second-line option after anticholinergics like benztropine, but its different mechanism makes it useful when anticholinergics are contraindicated or ineffective.

Other Off-Label Uses

Amantadine has been explored for fatigue in multiple sclerosis, traumatic brain injury, stroke recovery, and as an adjunct for depression. The evidence for these indications is mixed, and use is not standard.

Adverse Effects

The most common side effects are mild and include dizziness, (https://farmaciaacaba.it) insomnia, nervousness, nausea, and dry mouth. More serious but less common effects include livedo reticularis (a purplish skin discoloration, especially on limbs, due to vasomotor changes), peripheral edema, and orthostatic hypotension. Neuropsychiatric effects—such as confusion, hallucinations, anxiety, and agitation—are more frequent in elderly patients or those with renal impairment. Rarely, amantadine can cause seizures or neuroleptic malignant syndrome when withdrawn abruptly.

Contraindications and Precautions

Amantadine is contraindicated in patients with hypersensitivity, uncontrolled epilepsy, or severe renal failure. Caution is needed in patients with renal impairment (dose adjustment based on creatinine clearance), hepatic disease, psychiatric disorders, and those at risk of falls due to dizziness or orthostatic hypotension. It should be used during pregnancy only if clearly needed; it passes into breast milk, so nursing is not recommended.

Drug Interactions

Concomitant use with anticholinergics can intensify anticholinergic side effects. CNS stimulants may increase the risk of agitation. Thiazide diuretics can reduce renal excretion of amantadine, raising its plasma level and toxicity risk. Alcohol should be avoided.

Current Clinical Status

Amantadine remains a second-line or third-line agent for influenza A due to resistance. In Parkinson’s disease, it is still used, particularly for dyskinesias, but newer extended-release formulations (e.g., ADS-5102, approved in 2017 for dyskinesias) have revived interest. The drug’s low cost and unique mechanism make it a valuable option when other therapies fail. Its role in drug-induced extrapyramidal symptoms is established but not first-line.

Conclusion

Amantadine is a multifaceted drug with a rich history in antiviral and neurologic therapy. While its antiviral utility has waned, its benefits in Parkinson’s disease and drug-induced movement disorders persist. Understanding its mechanisms, proper dosing, and adverse effect profile allows clinicians to use it effectively, especially in patients with levodopa-induced dyskinesias or antipsychotic-induced parkinsonism. Ongoing research into its NMDA receptor effects may reveal further applications in neuroprotection and recovery after brain injury. Despite newer alternatives, amantadine remains a relevant tool in the pharmacological armamentarium.