Serendipitous Discovery of a Potent Influenza Virus A Neuraminidase Inhibitor

We have previously reported a potent neuraminidase inhibitor that comprises a carbocyclic analogue of zanamivir in which the hydrophilic glycerol side chain is replaced by the hydrophobic 3-pentyloxy group of oseltamivir. This hybrid inhibitor showed excellent inhibitory properties in the neuraminidase inhibition assay (Ki =0.46 nM; Ki (zanamivir) =0.16 nM) and in the viral replication inhibition assay in cell culture at 10(-8) M. As part of this lead optimization, we now report a novel spirolactam that shows comparable inhibitory activity in the cell culture assay to that of our lead compound at 10(-7) M. The compound was discovered serendipitously during the attempted synthesis of the isothiourea derivative of the original candidate. The X-ray crystal structure of the spirolactam in complex with the N8 subtype neuraminidase offers insight into the mode of inhibition.