Adamantane derivatives

How the virus enters the cell

To understand how drugs in this group work, it is necessary to examine in detail the natural process of influenza virus entry into a healthy cell. It all starts with the virus binding to specific glycoprotein receptors on the cell membrane. Afterwards, it enters the cell via receptor-mediated endocytosis, ending up enclosed in a special vesicle called an endosome.

Next, the cellular proton pump comes into play. It begins actively pumping hydrogen ions ($H^+$) from the cytoplasm directly into the endosome, creating a markedly acidic environment inside it. This acidification leads to two crucial consequences. First, the spatial conformation of the viral protein hemagglutinin changes, which triggers the fusion of the endosomal membrane with the viral envelope itself. Second, M2 proton channels open — specific pH-dependent proteins located in the viral envelope.

Through these open channels, hydrogen ions massively flood into the viral particle (virion acidification occurs). It is the acidic environment inside the virion that causes dissociation, meaning the physical separation of ribonucleoproteins from the matrix protein. This process is called viral "uncoating." As a result, the viral genome is successfully released into the cell's cytoplasm, where its replication begins.

Mechanism of action of adamantanes

Adamantane derivatives, which include Amantadinum and Rimantadinum, act at the very early stage of viral replication. Their main pharmacological target is the selective blockade of the M2 proton channels of the influenza virus.

The pharmacodynamic effect is that the drugs physically prevent protons from entering the virion. This prevents the acidification of its internal environment. As a result, the critically important process of ribonucleoprotein dissociation from the matrix is disrupted. The viral genome remains securely locked inside the envelope, and its release into the cytoplasm is completely blocked.

Since the drugs interfere with the early stage of the viral life cycle, they are most effective for prevention or at the very beginning of the disease. Additionally, scientific data indicate that adamantane derivatives may also affect later stages of viral infection. This effect is believed to be associated with alterations in hemagglutinin conformation.

Drug comparison: Amantadine and Rimantadine

Both drugs are taken orally after meals and exhibit high bioavailability exceeding 90%. However, their clinical applications differ significantly today.

• Amantadine (Midantan). This is one of the oldest antiviral drugs. It is effective exclusively against influenza A virus. Interestingly, it is rarely used in infectology nowadays. Its primary niche in modern medicine is the treatment of Parkinson's disease. The mechanism of action in parkinsonism is that the drug promotes dopamine release while simultaneously blocking glutamate receptors in the neostriatum.
• Rimantadine (Remantadine, Orvirem). It is also active solely against influenza A virus. It has no effect whatsoever on influenza B virus because this pathogen genetically lacks the M2 protein. Rimantadine is actively used for prophylaxis during seasonal epidemics (its prophylactic efficacy is estimated at 70–90%), as well as for treatment at the early stage of the disease, as it noticeably alleviates symptoms and accelerates recovery.

Rimantadine readily crosses the blood-brain barrier (BBB) and placenta. The main differences between rimantadine and amantadine lie in their pharmacokinetics: rimantadine is characterized by intensive metabolism (about 75% of the drug is degraded) and a significantly longer half-life ranging from 24 to 36 hours.

Side effects and clinical issues

The safety profile of adamantane derivatives has several specific features. The most pronounced adverse reactions occur in the Central Nervous System (CNS). Patients often complain of insomnia, ataxia (impaired coordination of movements), as well as marked impairment of concentration. Other neurological disorders may also occur. At the same time, comparative tolerability shows that side effects during rimantadine therapy occur significantly less frequently than with amantadine.

In addition, M2 channel blockers cause adverse gastrointestinal reactions. These include loss of appetite, nausea, and abdominal pain.

However, the main drawback of this drug group is the problem of resistance. Viral resistance to adamantane derivatives develops extremely rapidly and very frequently. According to statistics, resistance emerges in 30% of patients within the first 5 days of antiviral therapy, which severely limits their use in modern clinical practice.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed mechanism of receptor-mediated viral endocytosis
• The role of hemagglutinin in viral and endosomal membrane fusion
• Mechanism of action of amantadine in Parkinson's disease (effects on dopamine and glutamate receptors in the neostriatum)
• Pharmacokinetic features of rimantadine: crossing the blood-brain barrier and placenta
• Mechanisms of rapid resistance development to M2 channel blockers

Other topics in the "Antivirals" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.