Baloxavir marboxil susceptibility of influenza viruses from the Asia-Pacific, 2012–2018.

Abstract Baloxavir Marboxil (BXM) is an influenza polymerase inhibitor antiviral that binds to the endonuclease region in the PA subunit of influenza A and B viruses. To establish the baseline susceptibility of viruses circulating prior to licensure of BXM and to monitor for susceptibility post-BXM use, a cell culture-based focus reduction assay was developed to determine the susceptibility of 286 circulating seasonal influenza viruses, A(H1N1)pdm09, A(H3N2), B (Yamagata/Victoria) lineage viruses, including neuraminidase inhibitor (NAI) resistant viruses, to Baloxavir Acid (BXA), the active metabolic form of BXM. BXA was effective against all influenza subtypes tested with mean EC 50 values (minimum-maximum) of 0.7 ± 0.5 nM (0.1–2.1 nM), 1.2 ± 0.6 nM (0.1–2.4), 7.2 ± 3.5 nM (0.7–14.8), and 5.8 ± 4.5 nM (1.8–15.5) obtained for A(H1N1)pdm09, A(H3N2), B(Victoria lineage), and B(Yamagata lineage) influenza viruses, respectively. Using reverse genetics, amino acid substitutions known to alter BXA susceptibility were introduced into the PA protein resulting in EC 50 fold change increases that ranged from 2 to 65. Our study demonstrates that currently circulating viruses are susceptible to BXA and that the newly developed focus reduction assay is well suited to susceptibility monitoring in reference laboratories. Highlights • A focus reduction assay was developed to determine baloxavir susceptibility in seasonal influenza viruses. • 286 viruses of all seasonal influenza subtypes and lineages tested were susceptible to baloxavir. • The focus reduction assay was amenable to detecting viruses with reduced susceptibility to baloxavir. • PA N protein substitutions at position 38 derived by reverse genetics resulted in up to a 65 fold reduction in baloxavir EC 50. [ABSTRACT FROM AUTHOR]