Evidences of
oseltamivir resistant
influenza patients raised the need of novel
neuraminidase inhibitors. In this study, five
oseltamivir analogs PMC-31-PMC-36, synthesised according to the outcomes of a rational design analysis aimed to investigate the effects of substitution at the 5-amino and 4-amido groups of
oseltamivir on its
antiviral activity, were screened for their inhibition against
neuraminidase N1 and N3. The
enzymes used as models were from the
avian influenza A H7N1 and H7N3 viruses. The
neuraminidase inhibition assay was carried out by using recombinant species obtained from a baculovirus expression system and the
fluorogenic substrate MUNANA. The assay was validated by using
oseltamivir carboxylate as a reference inhibitor. Among the tested compounds, PMC-36 showed the highest inhibition on N1 with an IC(50) of 14.6±3.0nM (
oseltamivir 25±4nM), while PMC-35 showed a significant inhibitory effect on N3 with an IC(50) of 0.1±0.03nM (
oseltamivir 0.2±0.02nM). The analysis of the inhibitory properties of this panel of compounds allowed a preliminary assessment of a structure-activity relationship for the modification of the 4-amido and 5-amino groups of
oseltamivir carboxylate. The substitution of the acetamido group in the
oseltamivir structure with a 2-butenylamido moiety reduced the observed activity, while the introduction of a propenylamido group was well tolerated. Substitution of the free 5-amino group of
oseltamivir carboxylate with an
azide, decreased the activity against both N1 and N3. When these structural changes were both introduced, a dramatic reduction of activity was observed for both N1 and N3. The alkylation of the free 5-amino group in
oseltamivir carboxylate introducing an isopropyl group seemed to increase the inhibitory effect for both N1 and N3 neuraminidases, displaying a more pronounced effect against N1.