How the Virus Enters the Cell
To understand how this group of drugs works, it is necessary to examine the natural process of influenza virus entry into a healthy cell in detail. It all starts when the virus binds to specific glycoprotein receptors on the cell membrane. Following this, it enters the cell via receptor-mediated endocytosis, becoming enclosed in a specialized vesicle called an endosome.
Next, the cellular proton pump is activated. It actively pumps hydrogen ions ($H^+$) from the cytoplasm directly into the endosome, creating a pronounced acidic environment inside it. This acidification leads to two crucial consequences. First, it alters the spatial conformation of the viral hemagglutinin protein, triggering the fusion of the endosomal membrane with the viral envelope. Second, it opens M2 proton channels — specialized pH-dependent proteins located in the viral envelope.
Through these open channels, hydrogen ions flood into the viral particle (causing virion acidification). It is this acidic environment inside the virion that causes the dissociation, or physical separation, of ribonucleoproteins from the matrix protein. This process is known as viral "uncoating." Ultimately, the viral genome is successfully released into the cell cytoplasm, where its replication begins.
Mechanism of Action of Adamantanes
Adamantane derivatives, which include Amantadinum and Rimantadinum, act at the earliest stage of viral replication. Their primary pharmacological target is the selective blockade of the influenza virus M2 proton channels.
The pharmacodynamic effect is that the drugs physically prevent protons from entering the virion. Consequently, acidification of its internal environment is prevented. As a result, the critically important process of ribonucleoprotein dissociation from the matrix is disrupted. The viral genome remains securely trapped inside the envelope, and its release into the cytoplasm is completely blocked.
Because these drugs interfere with an early stage of the viral lifecycle, they are most effective for prophylaxis or at the very onset of the disease. Additionally, scientific data indicate that adamantane derivatives may also influence late stages of viral infection. This effect is believed to be related to alterations in hemagglutinin conformation.
Comparison of Agents: Amantadine and Rimantadine
Both drugs are administered 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 infectious disease practice nowadays. Its primary niche in modern medicine is the treatment of Parkinson's disease. Its mechanism of action in parkinsonism involves promoting dopamine release while simultaneously blocking glutamate receptors in the neostriatum.
- Rimantadine (Remantadin, Orvirem). Also active solely against influenza A virus. It has no activity against influenza B because this pathogen genetically lacks the M2 protein. Rimantadine is actively used for prophylaxis during seasonal epidemics (its preventive efficacy is estimated at 70–90%), as well as for treatment at the early stages of the disease, as it significantly alleviates symptoms and accelerates recovery.
Rimantadine crosses the blood-brain barrier (BBB) and placenta well. 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 Challenges
The safety profile of adamantane derivatives has several distinct features. The most pronounced adverse reactions occur within the Central Nervous System (CNS). Patients frequently complain of insomnia, ataxia (impaired coordination of movements), and marked impairment in concentration. Other neurological disturbances may also occur. Comparative tolerability shows that adverse effects occur significantly less frequently with rimantadine than with amantadine therapy.
Furthermore, M2 channel blockers cause undesirable gastrointestinal side effects, including loss of appetite, nausea, and abdominal pain.
However, the major drawback of this drug class is drug resistance. Viral resistance to adamantane derivatives develops extremely rapidly and very frequently. Statistically, resistance emerges in 30% of patients within the first 5 days of antiviral therapy, which severely limits their use in modern clinical practice.