Chicago BioSolutions, Inc.

Influenza A virus (IAV) is a zoonotic pathogen responsible for seasonal and pandemic flu. The extensive genetic and antigenic diversity within and between IAV phylogenetic groups presents major challenges for developing universal vaccines and broad-spectrum antiviral therapies. Current interventions provide limited protection due to the virus’s high mutation rate and capacity for immune evasion. Recent advancements in viral hemagglutinin (HA)-targeting small-molecule entry inhibitors offer a promising avenue to overcome these limitations. Here, we present structural and functional analyses of two group 2 HA-specific small-molecule inhibitors recently identified by our team. Cryogenic electron microscopy (cryo-EM) structures revealed that these inhibitors bind a conserved pocket within the HA stalk, likely interfering with the conformational rearrangements necessary for membrane fusion and viral entry. Structure-guided mutagenesis confirmed the critical roles of key interacting residues and uncovered distinct resistance profiles between the two compounds, as well as in comparison to Arbidol, a previously reported HA inhibitor. Notably, our structural analysis highlights intrinsic barriers to achieving cross-group inhibition with current small-molecule designs. To address this, we propose an alternative strategy for broadening antiviral coverage. Together, these findings provide mechanistic insights into IAV entry inhibition and a foundation for the rational design of next-generation anti-influenza therapeutics.

We discovered a series of imidazo[1,2-a]pyrimidines as potent, group 2 selective inhibitors of influenza A viruses (IAVs) that target the hemagglutinin-mediated viral entry process. Preliminary hit-to-lead optimization efforts afforded promising IAV inhibitors with improved activity against infectious H7N7 and H3N2 viruses. We now report a more comprehensive cycle of structure-activity relationship studies and optimization of metabolic stability, and overall druglike properties of this series of imidazo[1,2-a]pyrimidines, which allowed in the identification of two lead compounds that show promise as preclinical candidates. Compounds 10 and 12 inhibited pseudotyped H7N1 with EC₅₀ values of 0.09 and 0.47 μM, respectively. They were among the most potent compounds in the viral replication assay when tested against infectious H3N2 IAV, and they also demonstrated remarkable activity against avian influenza viruses; these data designated these imidazo[1,2-a]pyrimidines as potent broad-spectrum group 2 IAV inhibitors. Compounds 10 and 12 exhibit dissimilar but desirable drug metabolism and pharmacokinetics (DMPK) profiles, and therefore they offer different options for specific and effective patient treatment.