Spirostaphylotrichin X from a Marine-Derived Fungus as an Anti-influenza Agent Targeting RNA Polymerase PB2

A new spirocyclic γ-lactam, named spirostaphylotrichin X (<b>1</b>), and three related known spirostaphylotrichins (<b>2</b>–<b>4</b>) were isolated from the marine-derived fungus <i>Cochliobolus lunatus</i> SCSIO41401. Their structures were determined by spectroscopic analyses. Spirostaphylotrichin X (<b>1</b>) displayed obvious inhibitory activities against multiple influenza virus strains, with IC<sub>50</sub> values from 1.2 to 5.5 μM. Investigation of the mechanism showed that <b>1</b> inhibited viral polymerase activity and interfered with the production of progeny viral RNA. Homogeneous time-resolved fluorescence, surface plasmon resonance assays, and a molecular docking study revealed that <b>1</b> could inhibit polymerase PB2 protein activity by binding to the highly conserved region of the cap-binding domain of PB2. These results suggest that <b>1</b> inhibits the replication of influenza A virus by interfering with the activity of PB2 protein and that <b>1</b> represents a new type of potential lead compound for the development of anti-influenza therapeutics.