Baloxavir (S-033447) is a first-in-class cap-dependent endonuclease inhibitor of the influenza virus polymerase PA subunit.

Baloxavir (BXA) inhibits viral RNA transcription via selective inhibition of cap-dependent endonuclease (CEN) activity in enzymatic assays, and inhibits viral replication in infected cells without cytotoxicity in cytopathic effect assays, and it also inhibits cap-dependent endonuclease (CEN) and CEN/RdRp activities with IC50 values of 2.5 nM and 1.6 nM, respectively, while low potency (IC50 >40 nM) is observed against RdRp activity[2]. The median EC50 values at baseline for Baloxavir (BXA) are 17.96 nM for A/H1N1pdm, 4.48 nM for A/H3N2, and 18.67 nM for type B virus[1]. Baloxavir shows high potency against influenza A and B viruses with mean EC90 of 0.46 - 0.98 nM and 2.2-3.4 nM, respectively[2] and it also shows broad potency against various subtypes of influenza A viruses (H1N2, H5N1, H5N2, H5N6, H7N9 and H9N2). Additionally, serial passages of the viruses in the presence of Baloxavir result in isolation of PA/I38T variants with reduced BXA susceptibility[2]. Baloxavir (BXA) has a high inhibitory potency against CEN activity of the tested viral ribonucleoprotein complexes (vRNPs) from influenza A and B viruses with mean IC50 values of 1.4-3.1 nM and 4.5-8.9 nM, respectively, indicating that Baloxavir has broad spectrum activities.