Exploring the Size Limit of Templates for Inhibitors of the M2 Ion Channel of Influenza A Virus

Amantadine inhibits the M2 proton channel of influenza A virus) yet its clinical use has been limited by the rapid emergence of amantadine-resistant virus strains. We have synthesized and characterized a series of polycyclic compounds designed as ring-contracted or ring-expanded analogues of amantadine. Inhibition of the wild-type (wt) M2 channel and the A/M2-S31N and A/M2-V27A mutant ion channels were measured in Xenopus oocytes using two-electrode voltage clamp (TEV) assays. Several bisnoradamantane and noradamantane derivatives :inhibited the wt ion channel. The compounds bind to a primary site delineated by Val27, Ala30, and Ser31, though ring expansion restricts the positioning in the binding site. Only the :smallest analogue 8 was found to inhibit the S31N mutant ion channel. The structure activity relationship obtained by TEV assay was confirmed by plaque reduction assays with A/H3N2 influenza virus carrying wt M2 protein.