KEYWORDS:
Influenza virus, antiviral, baloxavir, susceptibility, replicative
capacity
INTRODUCTION
Influenza is an acute infectious disease caused by the influenza virus
and the worldwide epidemics each year result in approximately 3-5
million seriously ill cases and approximately 290,000-650,000
deaths.1 Antiviral treatment is recommended for the
management of influenza infections, particularly in high risk
individuals such as elderly and immunocompromised persons. Neuraminidase
inhibitors (NAIs: oseltamivir, zanamivir, peramivir) are widely used as
the current treatment for influenza,2 while
adamantanes, M2 ion channel inhibitors, are no longer used due to
widespread resistance in circulating influenza
viruses.3 However, A(H1N1) viruses developed
oseltamivir resistance in the 2007-09 influenza
seasons,4,5,6 emphasizing the need for antivirals with
a novel mechanism of action.
Baloxavir marboxil (BXM) became available for the treatment of
uncomplicated influenza in otherwise healthy and high-risk patients in a
number of countries, following its approval in Japan and the United
States in 2018.7,8. Baloxavir acid (BXA), the active
form of BXM, selectively and potently blocks a catalytic center of
cap-dependent endonuclease (CEN) located in the polymerase acid (PA)
protein of the influenza polymerase complex, which consists of PA,
polymerase basic 1 (PB1) and PB2 subunits.9,10 The CEN
is highly conserved across all types of influenza
viruses11 and plays an essential role in the
transcription, protein synthesis, and viral genome
replication,12 and therefore BXA displays
broad-spectrum activity against influenza A, B, C, and D
viruses.13,14 In clinical trials, single-dose BXM
treatment was superior to placebo in relieving influenza symptoms and
additionally, superior to both Oseltamivir and placebo in reducing the
viral load.7,8 However, amino acid (AA) substitutions
at position I38 (T/M/F) in the PA subunit have been identified as the
most common treatment-emergent substitutions associated with reduced
susceptibility to BXA.10,15 Influenza surveillance
studies conducted in Japan during the 2018-19 influenza season confirmed
treatment-emergence of PA/I38T and PA/I38M variants in A(H3N2)-infected
subjects.16,17 A(H1N1) and A(H3N2) viruses harboring
PA/I38T substitution were detected in some few subjects without prior
BXM-treatment, suggesting the possibility of human-to-human transmission
of the variant viruses.17,18 In addition to the I38
substitutions, E23K/G, A37T and E199G substitutions were identified in
the PA subunit that affect BXA susceptibility by less than
10-fold.10,19 20 Therefore,
consecutive monitoring of variant viruses with reduced BXA
susceptibility is required to identify new potential genetic markers for
the purpose of influenza surveillance.
It has been well demonstrated that mutations in NA conferring resistance
to NAIs can negatively impact the viral replicative capacity, but
additional HA mutations can also compensate these fitness
cost.21 K229R in the PB1 subunit of influenza A
viruses confers resistance to the viral RNA polymerase inhibitor
favipiravir, and the fitness cost caused by this mutation can be
compensated by a P653L substitution in PA that restores the fitness
while maintaining favipiravir resistance.22 Therefore,
AA substitutions located at distal position from drug-binding sites may
impact drug sensitivity or compensate impaired fitness.
Here, we report phenotypic analyses of AA substitutions in PA, PB1, and
PB2 subunits which were detected in clinical trials and influenza
surveillance. This additional information on BXA susceptibility and
replicative capacity of viruses with these substitutions will further
support influenza surveillance.