1. Introduction
Influenza viruses belong to the
Orthomyxoviridae
family. Among these, influenza A, B, and C, are known to infect human hosts and
cause acute respiratory infections [
1].
Influenza A is prone to antigenic variation and is capable of interspecies
transmission. Moreover, this variant is often the cause of major flu pandemics [
2,
3,
4]. Influenza viruses have the glycoproteins
hemagglutinin (HA) and neuraminidase (NA) on their surface, as well as Matrix-2
(M2) proton channels (
Figure 1). The
presence of HA and NA gives influenza viruses their ability to adapt to and
evade host immune responses, which necessitates the invention of new
preventative vaccines each flu season.
The prominent viral coat structures are emphasized,
including the two glycoproteins hemagglutinin and neuraminidase. The M2 proton
channel is also displayed. Near the bottom of the figure is the surface of the
respiratory tract. Made with BioRender.com.
A viral life cycle is composed of five (5) stages:
viral entry, viral uncoating, viral replication, assembly and budding, and
viral release from the host cell [
5]. HA is a
sialic acid receptor-binding molecule that mediates the entry of the influenza
virus into the target cell and is, therefore, the main target for a host body’s
neutralizing enzymes [
6]. NA enzymes are then
responsible for cleaving the glycosidic linkages of viral neuraminic acids,
which allows the release of these new influenza particles to spread throughout
the infected organism [
7]. These unique
surface proteins, as well as each viral life stage, provide influenza
antivirals with different targets for therapeutic action.
There are various influenza antiviral drugs
currently in the developmental pipeline. However, very few have been approved
for use by human patients. Currently, for the treatment, prevention, and
management of post-influenza complications, there are a handful of drug classes
to choose from. This review explores the uses, mechanisms, emerging resistance,
and current efficacy data of the most widely prescribed antivirals including
umifenovir, the three most widely used NA inhibitors (oseltamivir, zanamivir, and
peramivir), the M2 inhibitors, and the cap-dependent endonuclease inhibitor
baloxavir marboxil. Other influenza antiviral drugs exist but are not as widely
prescribed. These include laninamivir and favipiravir, which were approved for
influenza treatment in Japan in 2010 [
8] and
2014 [
9], respectively. These, and others like
them, have limited efficacy data, and clinical studies lack information about
the potential for viral resistance, which currently prevents their widespread
use. As such, they, and others like them, are not discussed in this research.
2. Umifenovir
Umifenovir (Arbidol) is
a broad-spectrum antiviral that acts against viral HA, specifically [
10]. Developed in the 1970s by the collaborative
efforts of the Chemical–Pharmaceutical Scientific Research Institute of Russia,
the Scientific Research Institute of Medical Radiology in Obninsk, and the
Leningrad-Pasteur Scientific Research Institute for Epidemiology and
Microbiology, Umifenovir is currently approved only in Russia and China for the
treatment of influenza A and B, prophylaxis, and post-influenza complications [
11,
12,
13], though it does exhibit anti-influenza C
activity as well [
10]. Umifenovir
is a controversial drug; due to a lack of reproducible lab results [
14] and limited toxicity data outside of Russia, it
has yet to gain global use and remains unapproved for influenza treatment in
many countries. Information on umifenovir is difficult to find in the West,
largely due to the language barrier, as key information including early
clinical trial designs and results is often available only in Russian [
13]. There are, however, many Russian reports
describing umifenovir’s anti-influenza activity against various strains, such
as influenza A (H5N1) and the 2009 A (H1N1) variant [
15,
16,
17].
Umifenovir is considered an inhibitor of various
enveloped and non-enveloped RNA viruses based on its insertion into membrane
lipids, leading to the inhibition of membrane fusion between virus particles
and plasma membranes, as well as interfering with the fusion between virus
particles and the membranes of endosomes (
Figure 2) [
10,
14,
18]. In influenza strains,
umifenovir interacts with HA, causing an increase in HA stability and
preventing its transition into the fusing state [
19,
20,
21].
Umifenovir may also be immunomodulatory, which would allow it to interfere with
induction and macrophage activation [
11].
Umifenovir shows antioxidant activity, which presumably counteracts virus
activity [
22]. As this drug is not well known
outside of Russia and China, this section examines the recent and notable
in
vitro,
in vivo, and clinical studies about umifenovir’s efficacy as
influenza treatment.
The current understanding of umifenovir’s method of
action is based on its insertion into membrane lipids, leading to the
inhibition of membrane fusion between virus particles and plasma membranes, as
well as interfering with the fusion between virus particles and the membranes
of endosomes. Made with BioRender.com.
Russian
in vitro studies are plentiful and
report IC50s for umifenovir in the 2.5–16 μM range [
13,
15,
16,
23,
24,
25].
One of the best sources of information on this drug currently is the I.I.
Mechnikov Research Institute of Vaccines and Sera, Russian Academy of Medical
Sciences, Moscow, Russia, and its affiliates. Most notably, these labs have
performed tests
in vivo [
25,
26],
in
vitro [
17,
21,
23,
27,
28,
29], and clinical
trials [
30,
31] gauging the effectiveness of
umifenovir against influenza strains, as well as other types of viruses. A
recent
in vitro study showed, using an MDCK cell-based enzyme-linked immunosorbent
assay, that influenza A and B viruses from the 2012-2014 flu seasons were
inhibited by umifenovir. Moreover, no markers of resistance were found in
viruses isolated from umifenovir-treated patients [
25].
Another
in vitro study examined nasal swabs from 57 umifenovir-treated
patients, with influenza A(H1N1), A(H3N2), and influenza B strains and found no
sign of resistance [
26]. An
in vivo
study also showed that umifenovir was effective against influenza A(H3N2) in
orally treated mice at the daily doses of 15 mg/kg or 20 mg/kg [
30]. Another notable
in vivo study explored
the effectiveness of umifenovir in post-influenza complications, specifically
Staphylococcus
aureus pneumonia, following the infection of the California 2009 A(H1N1)
strain in mice. This study showed that oral 40 or 60 mg/kg/day doses increased
the survival rate in mice from 0% to 90%. Furthermore, after dissection, the
lungs of the treated mice displayed less severe histopathologic lesions as
compared to the control group [
26].
Two clinical studies also examined patients with
either influenza or acute respiratory tract infection. The first clinical trial
enrolled 215 patients aged 18-74 years and split them into placebo (
n=106)
and treatment (
n=109) groups. The treatment group received umifenovir
200 mg four times a day for 5 days [
31]. The
second clinical trial enrolled 359 patients aged 18-65 years and split them
into treatment (
n=181) and placebo (
n=178) groups. The treatment
group received 800 mg/day for 5 days [
30]. In
both trials, both the influenza and acute respiratory tract infection patients
were grouped. The patients in the umifenovir treatment group in both trials
recovered faster and displayed fewer complications. Still, it is difficult to
parse out what the results mean for umifenovir’s efficacy
against influenza alone [
30]. These studies
reported no adverse effects attributed to umifenovir.
Umifenovir efficacy testing has been performed in
labs in other countries as well, though such studies remain scarce. Studies out
of China reported the efficacy of umifenovir against influenza A variants. An
in
vivo study from Wuhan University showed that 24 hours before virus
exposure, at doses of 50 or 100 mg/kg/day for 6 days, umifenovir significantly
reduced the rate of infection and mortality in mice infected with an influenza
A strain [
18]. An
in vitro study also
conducted at Wuhan University showed that umifenovir was effective against two
influenza A(H1N1) strains, responsible for both seasonal and pandemic
influenza, in MDCK cells via an MTT assay [
32].
Afterward, an
in vivo study on mice found that umifenovir
treatment at oral doses of 90-180 mg/kg/day reduced viral lung titers and
lesions. Additionally, the secretion of lung and macrophage cytokines was
downregulated [
32]. A more recent
in vitro study
from the First Affiliated Hospital of Guangzhou Medical University, Guangzhou,
China showed that umifenovir inhibited other local influenza A(H1N1) variants,
including A(H3N2) and A(H9N2), with IC50s ranging from 4.4 to 12.1μM [
33]. The
in vitro experiment performed
shortly after on mice and ferrets showed that the survival rates of
influenza-infected mice, given 25 mg/ml and 45 mg/ml umifenovir, were 40% and
50%, respectively. Moreover, these mice displayed reduced viral lung titers.
The ferret data also showed a decrease in fever symptoms duration in umifenovir
treatment groups as compared to controls [
33].
A clinical trial conducted by the Department of Respiratory Diseases, in Beijing,
China tested the efficacy of umifenovir on influenza on 125 influenza-infected
patients. Of these patients, 59 were in the treatment group and 66 were in the
placebo group. This clinical study reported that at a dose of 200 mg,
administered orally 3 times per day for 5 days, the treatment group saw a
significant reduction in symptoms and a median duration of illness of around 72
hours, compared to the placebo group’s 96 hours. Adverse effects were not
attributed to umifenovir [
34].
At the Department of Biotechnology and
Environmental Biology, RMIT University, Bundoora, Victoria, Australia, both
in
vivo and
in vitro testing revealed that umifenovir neither reduced
lung viral titers nor caused a significant reduction of lung consolidation in
mice after oral and intraperitoneal administration and intranasal challenge
with a local influenza A(H3N2) strain. In cells, the therapeutic indices for
influenza A and B were in the range of 1.9-8.5 and umifenovir was more
effective against influenza A(H3N2) than rimantadine or amantadine [
14]. Overall, the available studies indicate that
umifenovir is an effective and broad-spectrum antiviral that works against
several human pathogenic respiratory viruses, although its actual effectiveness
remains in question until lab results are reproducible, globally.
3. Neuraminidase Inhibitors
NA inhibitors target the viral enzyme neuraminidase
to inhibit viral release and are effective against influenza A and B [
35]. NA inhibitors, as their name suggests, are a
class of drugs that inhibit the actions of NA enzymes [
35]. NA cleaves the terminal sialic acid from the
carbohydrate residue on the surface of host cells, which influenza virus
envelopes. This promotes the release of the virus from the infected cells
which, in turn, allows the virus to spread [
35].
NA inhibitors block the active site of this enzyme, which reduces viral
shedding [
5,
35]. In this way, replication can
be blocked by NA inhibitors, which prevent virions from being released from the
surface of the infected host cells (
Figure 3)
[
7].
These antivirals prevent neuraminidase from acting
on terminal sialic acid from the carbohydrate residue on the surface of the
host cells, thereby inhibiting viral release and further replication. Made with
BioRender.com.
As of the time of writing, of the four antivirals
approved for the treatment of influenza in the United States, three, including
oseltamivir (Tamiflu), zanamivir (Relenza Diskhaler), and peramivir (Rapivab),
are NA inhibitors [
36]. The recommended
oseltamivir dosage for the treatment of acute influenza infection in adults,
beginning within 2 days of symptom onset, is 75 mg taken orally twice daily for
5 days [
37]. For prophylaxis, oseltamivir can
be taken once daily for up to 42 days [
38,
39].
Oseltamivir is taken as a prodrug(oseltamivir phosphate) and converted by
hepatic esterases into its active metabolite oseltamivir carboxylate, which has
high bioavailability [
38]. The recommended
zanamivir dosage for the treatment of acute influenza in adults, beginning
within 2 days after symptom onset, is 10 mg via oral inhalation twice daily for
5 days [
40]. For prophylaxis, zanamivir can be
taken once daily for up to 28 days [
40]. Up to
15% of the dose is absorbed in the lungs [
7,
40].
The recommended dosage of peramivir for the treatment of acute influenza in
adults, beginning within 2 days after symptom onset, is a single dose of 600 mg
taken intravenously [
41]. Peramivir displays a
low binding affinity to human plasma (<30%) [
41].
However, in healthy adult volunteers, the peak concentration of peramivir in
both pharyngeal and bronchial epithelial lining fluid samples was greater than
the IC50 value for influenza [
42].
Whether NA inhibitors are genuinely effective
treatments for influenza A and B has been questioned in the past due to the
sloppy clinical trials involving the drugs [
43].
One large meta-analysis found that many of the clinical trials contained bias,
and several (possibly) had an active substance as their placebo [
43]. Several studies concluded that NA inhibitors
shorten the duration of influenza symptoms, although not in all patients [
43,
44,
45,
46,
47,
48,
49]. While using NA inhibitors for prophylaxis
is effective, the use of oseltamivir increases the chance of adverse effects,
such as nausea, vomiting, psychiatric effects, and renal events in adults,
along with vomiting in children [
43].
Zanamivir produces fewer adverse effects than the other two drugs in this
class, possibly due to its lower bioavailability and inhalation route, while
peramivir produces the most adverse effects, possibly due to its intravenous
route of administration [
43]. The balance
between their potential adverse effects and their potential benefits should be
carefully weighed before drug administration.
Resistance to NA inhibitors is drug-specific;
however, given the similar structure shared by the drugs in this class,
resistance to one can affect the activity of others. Amino acid substitutions
in either the NA catalytic site or the HA receptor binding site of influenza
viruses can cause resistance to NA inhibitors to arise [
50]. The H275Y amino acid substitution of the
neuraminidase gene found in various influenza A viruses provides resistance
towards oseltamivir and peramivir. Similarly, E119E/V (found in influenza
A(H3N2) and A(H7N9)) causes resistance to oseltamivir and R292K causes
resistance to all three NA inhibitors, though lower resistance rates are
observed for zanamivir [
50,
51,
52]. While
resistance to NA inhibitors can crop up in circulating strains, it is generally
seen as rare [
53,
54,
55], especially for zanamivir
[
56]. Regardless of its rarity, close
monitoring for global NA inhibitor susceptibility is still required [
50].
4. M2 Inhibitors (Adamantanes)
Adamantanes are a class
of anti-influenza antivirals used specifically for treating type A influenza
infections, although mass viral resistance has limited their recent use. There
are only two members of this class, namely amantadine hydrochloride (Symmetrel)
and rimantadine hydrochloride (flumadine), or simply amantadine and
rimantadine, both of which are symmetric tricyclic amines [
57]. Adamantanes are also called M2 inhibitors or
M2 ion-channel inhibitors based on their mechanism of action [
58]. M2 ion-channel inhibitors target the stage of
viral uncoating. M2 proteins are responsible for forming the proton channels
that lower the pH of the viral interior right before the dissociation of the matrix
protein, which eventually leads to the uncoating of the viral genome during
replication [
5,
59]. By inhibiting these ion
channels, amantadine and rimantadine specifically inhibit the replication of
influenza A strains (
Figure 4) [
60].
These antivirals target the stage of viral
uncoating and prevent it from happening altogether. This stops the virus from
proceeding to the replication stage. Made with BioRender.com.
Amantadine and rimantadine are given in similar
dosages administered orally, that is, 100 mg tablets and a syrup formulation of
50 mg/5ml [
60]. The dosage for adults, for the
treatment and prevention of influenza A, is 100 mg every 12 hours. Both drugs
achieve peak levels within the body at around 3-5 hours after dosing.
Amantadine is excreted unchanged by the kidneys but rimantadine undergoes
extensive hepatic metabolism before renal excretion [
61,
62].
Common side effects of adamantanes are minor central nervous system complaints,
such as anxiety, difficulty concentrating, insomnia, dizziness, and headaches,
as well as gastrointestinal upset. Rarer but well- documented side effects
include antimuscarinic effects, orthostatic hypotension, and congestive heart
failure. Drug–drug interactions can occur within a large number of drug
classes, including antihistamines and anticholinergic drugs, which further
limits their usage [
60,
63,
64].
Rimantadine is the structural analog of amantadine
and is seen as the superior drug due to its larger volume of distribution,
higher concentration in respiratory secretion, and more extensive metabolism
that results in fewer central nervous system side effects [
60,
65]. However, rimantadine shares its
specificity, mechanism of action, and potential for resistance with amantadine [
66]. Cross-resistance to both drugs occurs when a
single amino acid is substituted in the transmembrane portion of the M2
protein. Resistance has been noted to emerge as early as 2–4 days after the
start of the therapy in up to 30% of the patients infected with strains that
showed susceptibility to either drug [
60].
Many studies have demonstrated influenza resistance to this drug class [
67,
68,
69,
70,
71,
72,
73,
74,
75]. Due to the widespread resistance to M2
inhibitors exhibited by influenza A strains, these drugs are not currently
recommended for the prevention or treatment of influenza in the United States [
60,
72,
73].
5. CAP-Dependent Endonuclease Inhibitors
The cap-dependent endonuclease, found within the
RNA polymerase subunit of influenza viruses, plays a crucial role in
facilitating the cap-snatching process during the creation of viral mRNA. This
process is essential for the replication of the virus [
76]. Baloxavir marboxil (xofluza), or
baloxavir, was approved for the treatment of uncomplicated influenza first in
Japan and then in the United States in 2018, followed shortly thereafter by
several other countries [
77,
78], making it the
sole approved member of the antiviral class known as cap-dependent endonuclease
inhibitors [
5]. Baloxavir is a prodrug
metabolized via hydrolysis into its active metabolite, baloxavir acid [
79]. Baloxavir acid targets the replication stage
of the viral life cycle and selectively inhibits the endonuclease activity of
the polymerase acidic protein, one of the subunits of RNA polymerase [
80]. The targeted endonuclease is a virus-specific
enzyme required for viral gene transcription [
81]
which provides baloxavir its specificity. Through the inhibition of
cap-dependent endonuclease, baloxavir can inhibit viral replication for both
influenza A and B viruses [
5,
79] (
Figure 5).
These antivirals target the replication stage of
the viral life cycle and selectively inhibit the endonuclease activity of the
polymerase acidic protein, one of the subunits of RNA polymerase. Through
inhibition of cap-dependent endonuclease, these antivirals inhibit influenza
viral replication. Made with BioRender.com.
Baloxavir is metabolized in the liver mainly by the
enzyme UGT1A3, with minor contributions by CYP3A4. To date, no serious
drug-drug interactions have been documented, even with co-administered CYP3A
and UGT inhibitors, such as probenecid [
5,
82].
Co-administration with medicines containing polyvalent cations, such as
antacids, lowers the bioavailability of baloxavir. Baloxavir is mainly excreted
in the feces, with minor excretions in the urine. Moreover, in patients with
renal and hepatic impairments, baloxavir showed no altered pharmacokinetic
properties [
5,
82]. Baloxavir is suggested for
use in patients 12 years of age and older who have been symptomatic for a
maximum of 48 hours and only for acute uncomplicated influenza [
5,
82]. In this regard, it is an inferior
alternative to other antivirals that are also generally suggested for
prophylaxis as well as influenza treatment. Baloxavir, however, is the
preferred choice in patients where the use of NA inhibitors is contraindicated.
Since it has a half-life of about 79 hours, baloxavir is given in a single-dose
regimen [
5,
82]. In this regard, it is a
superior treatment to other multi-dose regimens, as patient compliance is an
issue with multiple-dose treatment plans.
While baloxavir can treat viruses resistant to NA
inhibitors, the main problem in using baloxavir alone is the speed by which
influenza viruses develop resistance towards it. Both influenza A and B can
develop resistance, though A more so than B [
5].
In an
in vitro study, it was found that viruses substituted at I38 in
the polymerase acidic protein, which resulted in reduced susceptibility to
baloxavir [
83]. Indeed, one clinical study
that used this drug to treat influenza A(H3N2) reported that even after a
single dose, a small subset of influenza patients developed resistance to it,
with an overall rate of 19.5% resistance [
77],
while another clinical study showed resistance appearing between 8%-10% [
84]. Interestingly, previous results reported a
resistance rate of only 2.2%, however, the patients treated previously had
contracted the 2009 A(H1N1) variant, the strain responsible for the 2009
pandemic [
77].
In recent years, baloxavir resistance was only
observed at the rates of 0.5% and 0.1% during the 2018-2019 and 2019-2020 flu
seasons, respectively [
85]. These results
imply that baloxavir resistance varies across influenza strains and the drug
remains a valid choice for treatment [
86].
Additionally, when co-administered with oseltamivir, synergistic properties
were shown between the two drugs. Moreover, resistance and drug-drug
interactions were avoided [
87,
88,
89].
Additionally, a recent study showed a lack of drug-drug interactions between
baloxavir and NA inhibitors, though it failed to report improved clinical
outcomes when compared to treatment plans consisting of a single antiviral [
90]. These results suggest that if widespread viral
resistance to baloxavir, NA inhibitors, or both occur in the future,
co-administering baloxavir with an NA inhibitor may be the most effective
treatment regimen to bypass resistance.