Background: An unusual feature of influenza viral messenger RNA (mRNA) synthesis is its dependence upon host cell mRNAs as a source of capped RNA primers. A crucial activity of the influenza polymerase is to steal these primers by binding and cleaving the caps from host mRNAs. The recent structural analysis of the cap-binding fragment of the influenza virus PB2 protein has highlighted the importance of the mesoionic properties of the N7-methylguanine (N7mG) component of the mRNA cap in this interaction. Methods: A series of mesoionic heterocycles with 5,6-fused ring systems analogous to the N7mG component of mRNA cap structures were synthesized and examined for the ability to inhibit the cap-binding activity of the influenza virus RNA polymerase complex using a bead-based in vitro cap-binding assay. Results: None of the compounds tested were able to significantly inhibit binding and subsequent endonucleolytic cleavage of a synthetic radiolabelled capped mRNA substrate by recombinant influenza virus polymerase in vitro. Conclusions: Compounds analogous to the mesoionic N7mG component of mRNA cap structures comprise a large class of potential inhibitors of the influenza virus polymerase. Although this preliminary assessment of a small group of related analogues was unsuccessful, further screening of this class of compounds is warranted.
