text: Neuraminidase inhibitors are useful for combating influenza infection: zanamivir, administered by inhalation; oseltamivir, administered orally; peramivir administered parenterally, that is through intravenous or intramuscular injection; and laninamivir which is in phase III clinical trials.
  There are two major proteins on the surface of influenza virus particles. One is the lectin haemagglutinin protein with three relatively shallow sialic acid-binding sites and the other is enzyme sialidase with the active site in a pocket. Because of the relative deep active site in which low-molecular-weight inhibitors can make multiple favorable interactions and approachable methods of designing transition-state analogues in the hydrolysis of sialosides, the sialidase becomes more attractive anti-influenza drug target than the haemagglutinin. After the X-ray crystal structures of several influenza virus sialidases were available, the structure-based inhibitor design was applied to discover potent inhibitors of this enzyme.
  The unsaturated sialic acid (N-acetylneuraminic acid [Neu5ac]) derivative 2-deoxy-2, 3-didehydro--N-acetylneuraminic acid (Neu5Ac2en), a sialosyl cation transition-state (Figure 2) analogue, is believed the most potent inhibitor core template. Structurally modified Neu5Ac2en derivatives may give more effective inhibitors.
  Many Neu5Ac2en-based compounds have been synthesized and tested for their influenza virus sialidase inhibitory potential. For example:
  The 4-substituted Neu5Ac2en derivatives (Figure 3), 4-amino-Neu5Ac2en (Compound 1), which showed two orders of magnitude better inhibition of influenza virus sialidase than Neu5Ac2en5 and 4-guanidino-Neu5Ac2en (Compound 2), known as Zanamivir, which is now marketed for treatment of influenza virus as a drug, have been designed by von Itzstein and coworkers. A series of amide-linked C9 modified Neu5Ac2en have been reported by Megesh and colleagues as NEU1 inhibitors.