<p>The mechanism by which amantadine exerts its antiviral activity is not clearly understood. It appears to mainly prevent the release of infectious&nbsp;viral&nbsp;nucleic acid&nbsp;into the host cell by interfering with the function of the transmembrane domain&nbsp;of the viral M2 protein. In certain cases, amantadine is also known to prevent virus assembly during virus replication. It does not appear to interfere with the immunogenicity of inactivated influenza A virus vaccine.<br />
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Amantadine inhibits the replication of influenza A virus isolates from each of the subtypes, i.e., H1N1, H2N2 and H3N2. It has very little or no activity against influenza&nbsp;B virus&nbsp;isolates. A quantitative relationship between the&nbsp;in vitrosusceptibility of influenza A virus to amantadine and the clinical response to therapy has not been established in man.&nbsp;Sensitivity&nbsp;test results, expressed as the concentration of amantadine required to inhibit by 50% the growth of virus (ED50) in tissue culture vary greatly (from 0.1 &micro;g/mL to 25.0 &micro;g/mL) depending upon the assay protocol used, size of virus inoculum, isolates of influenza A virus strains tested, and the cell type used. Host cells in tissue culture readily tolerated amantadine up to a concentration of 100 &micro;g/mL.</p>