The part of the influenza polymerase PA subunit featuring endonuclease activity is a target for anti-influenza therapies, including the FDA-approved drug Xofluza. A general feature of endonuclease inhibitors is their ability to chelate Mg2+ or Mn2+ ions located in the enzyme's catalytic site. Previously, we screened a panel of flavonoids for PA inhibition and found luteolin and its C-glucoside orientin to be potent inhibitors. Through structural analysis, we identified the presence of a 3',4'-dihydroxyphenyl moiety as a crucial feature for sub-micromolar inhibitory activity. Here, we report results from a subsequent investigation exploring structural changes at the C-7 and C-8 positions of luteolin. Experimental IC50 values were determined by AlphaScreen technology. The most potent inhibitors were C-8 derivatives with inhibitory potencies comparable to that of luteolin. Bio-isosteric replacement of the C-7 hydroxyl moiety of luteolin led to a series of compounds with one-order-of-magnitude-lower inhibitory potencies. Using X-ray crystallography, we solved structures of the wild-type PA-N-terminal domain and its I38T mutant in complex with orientin at 1.9 Å and 2.2 Å resolution, respectively.

1st Faculty of Medicine Charles University Kateřinská 1660 121 08 Prague Czech Republic

Department of Biochemistry Faculty of Science Charles University Hlavova 8 128 00 Prague Czech Republic

Department of Organic Chemistry Faculty of Science Charles University Hlavova 8 128 00 Prague Czech Republic

Institute of Molecular Genetics of the Czech Academy of Sciences Vídeňská 1083 140 00 Prague Czech Republic

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Gilead Sciences and IOCB Research Center Flemingovo n 2 166 10 Prague Czech Republic

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