Accepted Manuscript Antiviral activity of SA-2 against influenza A virus in vitro/vivo and its inhibition of RNA polymerase Jie Yu, Dechuan Wang, Jing Jin, Jun Xu, Mengwei Li, Hui Wang, Jie Dou, Changlin Zhou PII:

S0166-3542(16)30013-4

DOI:

10.1016/j.antiviral.2016.01.011

Reference:

AVR 3759

To appear in:

Antiviral Research

Received Date: 23 September 2015 Revised Date:

19 December 2015

Accepted Date: 19 January 2016

Please cite this article as: Yu, J., Wang, D., Jin, J., Xu, J., Li, M., Wang, H., Dou, J., Zhou, C., Antiviral activity of SA-2 against influenza A virus in vitro/vivo and its inhibition of RNA polymerase, Antiviral Research (2016), doi: 10.1016/j.antiviral.2016.01.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Antiviral activity of SA-2 against influenza A virus in vitro/vivo and its inhibition of RNA polymerase

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Jie Yu a,1, Dechuan Wang b,1, Jing Jin a, Jun Xu a, Mengwei Li a, Hui Wang a, Jie Dou a, Changlin Zhou a,*

State Key Laboratory of Natural Medicines, School of Life Science and Technology,

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China Pharmaceutical University, Nanjing, Jiangsu 210009, PR China; Department of Organic Chemistry, School of Science, China Pharmaceutical

University, Nanjing, Jiangsu 210009, PR China

Corresponding author:

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Tel: +86-25-8327-1323; Fax: +86-25-8327-1323; E-mail: [email protected] Address: School of Life Science and Technology, China Pharmaceutical University,

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24 Tong Jia Xiang, Nanjing 210009, PR China.

These authors contributed equally to this work.

ACCEPTED MANUSCRIPT Abstract

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A target-free and cell-based approach was applied to evaluate the anti-influenza

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properties of six newly synthesized benzoic acid derivatives. SA-2, the ethyl

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4-(2-hydroxymethyl-5-oxopyrrolidin-1-yl)-3-[3-(3-methylbenzoyl)-thioureido]

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benzoate (compound 2) was screened as a potential drug candidate. In a cytopathic

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effect

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oseltamivir-resistant mutant H1N1-H275Y influenza viruses in both virus-infected

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MDCK and A549 cells, with 50% effective concentrations (EC50) in MDCK cells of

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9.6, 19.2 and 19.8 µM respectively, and 50% cytotoxic concentration (CC50) of 444.5

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µM, showing competitive antiviral activity with oseltamivir in vitro. Orally

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administered SA-2 effectively protected mice infected with lethal doses of H1N1 or

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oseltamivir-resistant strain H1N1-H275Y, conferring 70% or 50% survival at a

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dosage of 100 mg/kg/d, reducing body weight loss, alleviating the influenza-induced

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acute lung injury, and reducing lung virus titer. Mechanistic studies showed that SA-2

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efficiently inhibited the activity of RNA polymerase and suppressed NP and M1

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levels during viral biosynthesis by interfering with gene transcription without having

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an obvious influence on virus entry and release. Based on these favourable findings,

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SA-2, a novel anti-influenza agent, with its potent anti-influenza activity in vitro and

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in vivo, could be a promising antiviral for the treatment of infection of influenza A

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viruses, including oseltamivir-resistant mutants.

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Keywords

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Benzoic acid derivatives; Influenza A virus; Antiviral activity; Oseltamivir-resistant;

SA-2

dose

dependently

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H1N1,

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RNA polymerase inhibition

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1. Introduction Influenza viruses are highly contagious and cause three to five million cases of

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severe respiratory illness and approximately 250 000 to 500 000 deaths annually

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(Fiore et al., 2008). Influenza A viruses include many subtypes; the H1N1 and H3N2

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subtypes infect humans, resulting in seasonal and pandemic infections. The highly

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pathogenic avian H5N1 and H7N9 subtypes have an estimated mortality rate of 60%

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and 30%, respectively. There is concern that these viruses could cause severe human

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influenza pandemics in case they would acquire human-to-human transmissibility (Li

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et al., 2014).

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Currently, other than vaccination, antiviral treatment is the most effective

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therapeutic method for the prevention and treatment of human influenza. M2 channel

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blockers (e.g., amantadine and rimantadine), inhibitors of viral RNA synthesis (e.g.,

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ribavirin and T-705) and neuraminidase (NA) inhibitors (e.g., oseltamivir) are three

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classes of anti-influenza virus drugs with different modes of action. However, clinical

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use of amantadine (or rimantadine) and ribavirin has met several limitations over

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recent decades because of the high frequency of emerging variants that are resistant

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for M2 channel blockers and the undesirable toxic side effects of ribavirin.

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The current standard-of-care antivirals for influenza cases are potent influenza virus

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NA inhibitors, such as zanamivir and oseltamivir (Schmidt, 2004; Moscona, 2005).

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While these agents possess high affinity and specificity for a variety of influenza

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viruses, they suffer from limitations in their efficacy due to adverse effects and drug

ACCEPTED MANUSCRIPT resistance (Thorlund et al., 2011). For oseltamivir, drug-resistant strains have

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increasingly emerged since 2007. There were many more reported cases of

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oseltamivir-resistant H1N1pdm09 infections with the H275Y NA mutation during

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2011 than during the first year of the pandemic (Leang et al., 2013). The appearance

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of drug-resistant influenza viruses caused by NA mutations limits the future utility of

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commonly used NA inhibitors, highlighting an urgent need for new classes of

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anti-influenza agents to combat potential human influenza pandemics.

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BANA-206 (Atigadda et al., 1999), a benzoic acid derivative, was reported to show

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sub-micromolar antiviral potency against influenza A virus. Acylthiourea derivatives

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(Sun et al., 2006; Burgeson et al., 2012) were reported for possessing broad-spectrum

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antiviral activity. Some compounds with enhanced anti-influenza activity have been

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successfully designed and synthesized by conjugation method, such as compound

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BTA938 (Tarbet, et al., 2014) and ZA-7-CA (Liu, et al., 2012). Here, in order to

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achieve potential influenza virus inhibitors with better antiviral activity, a series of

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novel benzoic acid derivatives (Fig. 1) were designed and synthesized by conjugation

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of benzoic acid with acylthiourea based on the combination principles as well as the

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principle of functional groups. In this study, six compounds were evaluated for

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antiviral activity against both H1N1 (including its oseltamivir-resistant H275Y strain)

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and H3N2 influenza A viruses in a cell culture-based assay. We further examined the

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efficacy of ethyl 4-(2-hydroxymethyl-5-oxopyrrolidin-1-yl)-3-[3-(3-methylbenzoyl)

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-thioureido] benzoate (C23H25N3O5S, compound 2, named SA-2) against the H1N1

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and H3N2 subtypes and the oseltamivir-resistant strain H1N1-H275Y in vitro and in

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ACCEPTED MANUSCRIPT vivo. Mechanistic studies were then conducted to investigate its antiviral target and

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mode of action. Our findings demonstrate the potential of SA-2 as an alternative

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antiviral agent against influenza A viruses, including oseltamivir-resistant mutants.

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2. Materials and methods

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2.1. Cells, viruses and plasmids

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Madin-Darby canine kidney cells (MDCK), human lung cancer cells (A549) and

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human embryonic kidney 293T cells were obtained from the American Type Culture

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Collection (ATCC, Manassas, VA). MDCK and 293T cell lines were cultured in

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DMEM, while the A549 cell line was cultured in RPMI-1640 medium, supplemented

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with 10% foetal bovine serum; all cell lines were cultured at 37°C in the presence of

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5% CO2.

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Influenza A viruses subtypes A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2) and

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an H1N1 oseltamivir-resistant mutant strain possessing a histidine-to-tyrosine

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substitution at position 275 (H275Y) of NA, were maintained at the Department of

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Microbiology, School of Life Science and Technology, China Pharmaceutical

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University. The NA of the A/FM/1/47-H275Y (H1N1-H275Y) oseltamivir-resistant

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virus was sequenced to confirm retention of the H275Y mutation. All these viruses

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were propagated in 10-day-old embryonated chicken eggs and stored at -80°C until

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use (Jung et al., 2010).

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Plasmids for expressing influenza virus (A/WSN/33) PB1, PB2, PA and NP, a

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polymerase expressing plasmid carrying an influenza virus-like RNA encoding firefly

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luciferase (vNS-Luc), and pCMV β-gal plasmids (Zhang et al., 2012) were kindly

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provided by Xin Ye, Institute of Microbiology, Chinese Academy of Sciences.

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2.2. Compounds Oseltamivir (OS) and oseltamivir carboxylate (OC) were purchased from

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MedChem Express (Shanghai, China). Ribavirin (RBV) was obtained from Sichuan

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Baili Pharmaceutical Co., Ltd. (Chengdu, China). Test compounds were synthesized

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at the School of Science, China Pharmaceutical University, according to published

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procedures (Wang et al., 2015). In brief, ethyl aminobenzoate, as the starting material,

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was reacted with diethyl bromomalonate. Subsequent reactions included cyclization,

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nitration and reduction. Then the product was treated with R-isothiocyanate to obtain

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the target compounds 1–6. The structures of the purified compounds were confirmed

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by 1H NMR (Bruker Avance 300) and the purity was determined to be

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95%. Each

compound was dissolved in dimethyl sulfoxide (DMSO) to a 50 mM concentration

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for use as stock solutions that were diluted to the required concentrations for in vitro

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studies.

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2.3. Determination of influenza A virus TCID50

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The TCID50 (50% tissue culture infectious dose) was determined by serial dilution

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of the influenza virus added to 100 µL of MDCK cells cultured at a density of 5 × 103

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cells/well in 96-well microplates. The infected cells were incubated at 37°C for 48 h

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and then 10 µL of Cell Counting Kit-8 (Dojindo, Japan) reagent (Takeuchi et al., 2003)

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was added to each well. After incubation for 2 h, absorbance at 450 nm was read on a

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plate reader. Influenza virus TCID50 was determined using the Reed-Müench method

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(Reed and Müench, 1938).

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2.4. Cytopathic effect (CPE) reduction assay MDCK cells and A549 cells (5 × 103 cells/well) were cultured in 96-well plates for

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24 h and infected with 100 TCID50 A/FM/1/47 (H1N1) virus, the oseltamivir-resistant

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strain H1N1-H275Y or A/Beijing/32/92 (H3N2) virus for 2 h. Then, the cells were

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incubated in the maintenance medium (DMEM / RPMI-1640 containing 0.5% foetal

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bovine serum) in the presence of 0.2% DMSO (a vehicle to dissolve test compounds)

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or different concentrations of chemicals. After a 48-h incubation, the cell viability was

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measured using Cell Counting Kit-8. The EC50 of test compounds were determined by

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fitting the curve of percent CPE versus the compound concentrations using GraphPad

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Prism 5.

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The inhibition rate of the test compounds was calculated using the following

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equation (Ding, et al., 2014): inhibition rate (%) = [(mean optical density of test –

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mean optical density of virus controls) / (mean optical density of cell controls – mean

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optical density of virus controls)] × 100.

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The CC50 of chemicals to MDCK or A549 cells were determined by procedures

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similar to those for EC50 determination but without virus infection. The selectivity

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index (SI) was calculated as the ratio of CC50 to EC50.

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2.5. Therapeutic efficacy study in mice

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Mouse-adapted A/FM/1/47 (H1N1) and its oseltamivir-resistant mutant strain

ACCEPTED MANUSCRIPT A/FM/1/47-H275Y were used as the challenge viruses. Six-week-old male ICR mice

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were purchased from the Laboratory Animal Center of Yangzhou University

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(Yangzhou, Jiangsu, China). The experiment was performed as previously described

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(Ding et al., 2014). Five mice per group were enrolled to determine the lung virus

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titers. On day 5 post-infection, mice were euthanized and lungs collected and

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homogenized. Lung homogenates were serially diluted and added to MDCK cells.

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Virus titer was measured by TCID50 method and expressed as TCID50/lung (Byrn et

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al., 2015). For oral gavage, SA-2 was first dissolved in a small quantity of DMSO,

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and then evenly mixed with 0.5% CMC-Na solution. The final concentration of

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DMSO was 5%. Animal experiments were conducted in accordance with protocols

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approved by the Animal Ethics and Experimentation Committee of China

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Pharmaceutical University.

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2.6. Inhibitory effects of SA-2 on different stages of viral replication

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An experiment focused on the effect of SA-2 on different stages of viral replication

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was carried out as previously described (Ding et al., 2014). In the first protocol

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(pre-infection), MDCK cells were pre-incubated with different concentrations of SA-2

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(10–80 µM) for 12 h before viral infection. In the second protocol (co-infection), the

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virus suspension and SA-2 were added to MDCK cells simultaneously in a mixture

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and incubated for 2 h. In both protocols, maintenance medium replaced the

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supernatant after viral adsorption, and the cultures were then incubated for 48 h. In the

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third protocol (post-infection), after adsorption of the influenza virus for 2 h,

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maintenance medium containing SA-2 was added to MDCK cells, and the cultures

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ACCEPTED MANUSCRIPT 152

were incubated for 48 h. Cell viability in each well was determined using Cell

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Counting Kit-8 and the inhibition rate was calculated.

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2.7. Hemagglutination assay Hemagglutination titrations of the A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2)

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strains were first determined using standard methods (Clarke and Casals, 1958), and

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the unit of hemagglutinin agglutination (HAU) for both strains were obtained. The

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hemagglutination assay was then performed in the same condition with 4 HAU/well

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of virus as previously described (Delogu et al., 2011). Negative controls contained no

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virus, and the sedimentation of red blood cells was observed in these controls.

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2.8. Neuraminidase (NA) inhibition assay

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Neuraminidase enzyme inhibition assays were carried out as previously described

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(Kim et al., 2013). Allantoic fluid containing the A/FM/1/47 (H1N1) or

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A/Beijing/32/92 (H3N2) influenza viruses was used. Chemicals were incubated with

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diluted allantoic fluid for 10 min at room temperature followed by the addition of 20

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µM fluorescent substrate 2’-(4-methylumbelliferyl)-α-D-N-acetylneuraminic acid

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(MUNANA; Sigma). Fluorescence was read after incubation with MUNANA for 60

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min using excitation and emission wavelengths of 360 and 450 nm, respectively.

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2.9. Western blot analysis

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In order to determine the viral protein expression level, MDCK cells in 6-well

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plates were infected with 1.5 × 103 TCID50 of the A/FM/1/47 (H1N1) influenza virus

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and treated with SA-2 (40 µM) or ribavirin (40 µM) at different exposure periods after

ACCEPTED MANUSCRIPT infection. In the interval of for instance 2–4 h post infection (hpi), compound was

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added at 2 h and then the medium was removed and replaced by fresh medium at 4h.

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Before fresh medium was added, cells were washed twice with PBS. Ten hours post

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infection, cells were lysed with 200 µL of Mammalian Protein Extraction Reagent

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(GENEray Biotechnology). The protein lysates were then applied to immunoblotting

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with anti-NP (Catalog No. GTX83054, GeneTex), anti-M1 (Catalog No. GTX125928,

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GeneTex), and anti-β-actin (Catalog No. SC-47778, Santa Cruz) antibodies,

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respectively. The blots were detected on a Bio-Imaging system and the band

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intensities of proteins were quantified using ImageJ and normalized to β-actin levels.

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2.10. RT-PCR and real-time qPCR analysis

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MDCK cells in 6-well plates were inoculated with 1.5 × 103 TCID50 of the

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A/FM/1/47 (H1N1) influenza virus for 2 h followed by the addition of 40 µM SA-2.

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Cells were collected 6 h post infection (Shih et al., 2010) and total RNA was extracted

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using TRIzol. Total RNA was reverse-transcribed using the PrimeScript

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Kit (Takara) to obtain cDNA. The RT-PCR experiment was performed as previously

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described (Chen et al., 2011) and the amplification conditions were: 95°C (5 min); 30

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cycles of 94°C (30 s), 54°C (30 s), and 72°C (20 s); and an extension at 72°C (10

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min). Each PCR product was analyzed by electrophoresis on a 2% (w/v) agarose gel

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and visualized using ethidium bromide staining.

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By using a 7300 qRT-PCR System (ABI) with a standard SYBR Green PCR

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protocol (Fan et al., 2014) according to the manufacturer’s instructions, the real-time

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qPCR amplification was performed with the following parameters: 95°C (10 min); 40

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cycles of 95°C (15 s) and 60°C (1 min); and 95°C (15 s), 60°C (1 min), 95°C (15 s).

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Each sample was run in triplicate alongside the endogenous control (GAPDH) to

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normalize reactions. The relative RNA levels of NP and M1 genes were calculated

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based on the 2

method (Livak and Schmittgen, 2001).

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The primers used to detect the viral mRNAs are described below.

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NP forward primer, 5'-CCCAGGATGTGCTCTCTGAT-3'

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NP reverse primer, 5'- TGAAAGGGTCTATTCCGACT-3'

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M1 forward primer, 5'-TTCTAACCGAGGTCGAAAC-3'

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M1 reverse primer, 5'-AAGCGTCTACGCTGCAGTCC-3'

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GAPDH forward primer, 5'-CACTCACGGCAAATTCAACGGCAC-3'

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GAPDH reverse primer, 5'-GACTCCACGACATACTCAGCAC-3'

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To measure the influence of SA-2 on influenza viral RNA polymerase activity,

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293T cells in a 24-well plate were transfected with plasmids for expressing influenza

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A virus (A/WSN/33 (H1N1)) PB1, PB2, PA, NP and vNS-Luc (150 ng of each) with

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pCMV β-gal (50 ng) as an internal control for 6 h followed by the addition of SA-2.

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After 24 h, the cell lysates were harvested and subjected to a luciferase assay (Zhang

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et al., 2012). The luciferase activity was normalized to β-gal activity.

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2.12. Statistical analysis

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All experiments were performed in triplicate. Data were analysed by one-way

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analysis of variance (ANOVA). The results were presented as the mean ± S.D.

ACCEPTED MANUSCRIPT Significant differences between control and treated groups were determined using an

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unpaired, two-tailed Student’s t test with significance set at P < 0.05. For survival

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studies, Kaplan-Meier survival curves were generated and compared by the Log-rank

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(Mantel-Cox) test using SPSS 17.0 software.

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3. Results

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3.1. Antiviral activity of SA-2 and its analogues against the A/FM/1/47 (H1N1) and

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A/Beijing/32/1 (H3N2) influenza viruses in vitro

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A cell culture-based screening system was established for influenza viruses,

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including A/FM/1/47 (H1N1), A/Beijing/32/92 (H3N2) and A/FM/1/47-H275Y, an

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oseltamivir-resistant strain of H1N1. Using a CPE reduction assay, CC50 and EC50

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values of compounds 1–6 were determined (Table 1). Oseltamivir carboxylate and

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ribavirin, two commonly used antiviral drugs, were used as positive controls to

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confirm the reliability of the assay.

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Compounds 2–5 protected cells from influenza virus infection with limited

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cytotoxicity, albeit with different potency. They were effective against both H1N1 and

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H3N2 viruses, as well as the oseltamivir-resistant strain, A/FM/1/47-H275Y. Notably,

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compound 2, ethyl 4-(2-hydroxymethyl-5-oxopyrrolidin-1-yl)-3-[3-(3-methylbenzoyl)

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-thioureido] benzoate (designated SA-2), had the highest potency of all six

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compounds, with EC50 values against H1N1 and H3N2 of 9.6 µM and 19.2 µM,

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respectively, which were comparable to those of oseltamivir carboxylate. Further,

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SA-2 (2) strongly inhibited the oseltamivir-resistant mutant strain H1N1-H275Y

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ACCEPTED MANUSCRIPT (EC50 = 19.8 µM) at a similar level to ribavirin, while the EC50 for oseltamivir

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reached as high as 1177.9 µM. These results indicate that SA-2 (2) effectively inhibits

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H1N1 and H3N2 influenza viruses in vitro, including the oseltamivir-resistant mutant.

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3.2. SA-2 inhibited the replication of A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2)

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influenza viruses in cell culture

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Based on the results of the CPE reduction assay (Table 1), the ability of compound

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2 (SA-2) to inhibit the replication of the virus in cell culture was explored using

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additional antiviral assays with MDCK and A549 cells. SA-2 exhibited a considerable

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protective effect when added to H1N1-infected MDCK cells at concentration of 80

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µM. The reduction of the CPE was confirmed by direct microscopic observation

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which detected far less CPE than in the DMSO control (Fig. 2A). The three strains

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(H1N1, H1N1-H275Y, and H3N2) were then tested with OC as a control. SA-2 (5–80

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µM) showed significant anti-influenza activity in a dose-dependent manner. The

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inhibition rates at the final concentration of 80 µM against H1N1 and H3N2 reached

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82.9% and 69.6%, respectively (Fig. 2B). For the oseltamivir-resistant (H1N1-H275Y)

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strain, the inhibition rate of SA-2 at 80 µM was 85.9%, while that of oseltamivir

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carboxylate (OC) was only 4.0%. In addition, SA-2 displayed clear anti-influenza

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activity in MDCK cells infected with A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2)

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viruses at different MOIs (1, 0.5, and 0.3) (Fig. 2C and D). An obvious inhibitory

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effect of SA-2 was observed at an MOI of 0.3 due to the lower infection dose. Similar

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anti-influenza effects were observed in influenza virus-infected A549 cells (Fig. 2E).

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ACCEPTED MANUSCRIPT All three virus strains were inhibited by SA-2 in a dose-dependent manner, and the

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rates of inhibition of the H1N1, H3N2 and H1N1-H275Y strains were 73.0%, 54.9%

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and 78.1%, respectively, when SA-2 was used at a concentration of 80 µM. The

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cytotoxicity of SA-2 in both cell lines was also examined. As shown in Fig. 2F, cell

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viability was maintained at 100% even at a 200 µM concentration. The CC50 values of

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SA-2 in MDCK and A549 cells were 444.5 µM and 580.3 µM, respectively.

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Taken together, these results indicate that SA-2 inhibits the replication of the

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A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2) influenza viruses, as well as the

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oseltamivir-resistant mutant strain, in a dose-dependent manner in both MDCK and

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A549 cell cultures, while also exhibiting limited cytotoxicity.

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3.3. Therapeutic efficacy of SA-2 against A/FM/1/47 (H1N1) influenza virus in vivo

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To evaluate the efficacy of SA-2 against influenza virus in vivo, a dose-response

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study in a virus-infected mouse model was performed. Fig. 3A illustrates the

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protocol for the experiments. On 'day 0', mice were infected intranasally with a

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lethal dose of A/FM/1/47 (H1N1) virus. Two hours post infection, SA-2, oseltamivir

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or saline (placebo) were administered twice daily for 5 days (days 0–4) by oral

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gavage. Each mouse was weighed daily for 14 days. On 'day 5', five mice per group

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were euthanized for lung pathological examinations. SA-2-treated mice were

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significantly protected from influenza-induced lethality, whereas 100% of the mice

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administered placebo died within 8 days (Fig. 3B). Treatment of mice with 50, 75 or

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100 mg/kg/d SA-2 increased survival to 30%, 40%, 70%, respectively, and there was

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a dose-dependent reduction in body weight loss (Fig. 3C). SA-2 treatment also led to a statistically significant reduction in influenza-induced

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lung pathology. On day 5 post-infection, signs of extensive lung damage, such as

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highly basophilic lining epithelium in the bronchioles, extensive cellular infiltrates

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with neutrophils, and cell necrosis, were observed in placebo group (Fig. 3D). In

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SA-2-treated mice, however, lung damage was relieved with increasing dosage.

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When SA-2 was administered at 100 mg/kg/d, approximately 60% of lung sections

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exhibited nearly normal lung architecture (similar to the ‘control’), while in the rest

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of the section, inflammatory infiltrates could be observed, although to a lesser extent

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than in the lungs of mice in the placebo group. Further, all the SA-2 treatment

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groups reduced lung virus titers on day 5 post-infection (Fig. 3E). The log virus

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titers calculated in SA-2 at 50, 75 and 100 mg/kg/day were 5.5 ± 0.3, 5.0 ± 0.2 and

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4.1 ± 0.5, respectively. SA-2 showed significant reduction at the dosage of 100

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mg/kg/day with log reduction of 1.8 compared to the placebo group (5.9 ± 0.3).

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These observations were supported by the measured lung index and histological

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scoring (Fig. 3F and G). SA-2 suppressed the influenza-induced increase in the lung

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index and histological score in a dose-dependent manner and had a significant effect

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at a dosage of 100 mg/kg/d (lung index: 1.2, histological score: 5.3) compared with

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the placebo group (lung index: 2.0, histological score: 9.0).

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These data indicate that SA-2 significantly enhances survival, reduces body

299

weight loss, inhibits lung consolidation and reduces lung virus titer in a

300

dose-dependent manner, which demonstrate the potent therapeutic efficacy of SA-2

ACCEPTED MANUSCRIPT in vivo against infection with the A/FM/1/47 (H1N1) influenza virus.

302

3.4. Effective protection of SA-2 on mice infected with oseltamivir-resistant

303

A/FM/1/47-H275Y virus

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The therapeutic efficacy of SA-2 was further explored in mice infected with the

305

oseltamivir-resistant virus A/FM/1/47-H275Y. All mice treated with placebo

306

succumbed to the disease by study day 7, and oseltamivir at a dose of 100 mg/kg/d

307

for 5 days was ineffective at protecting mice from mortality (Fig. 4A). SA-2

308

administered at 75 or 100 mg/kg/d provided 30% and 50% protection, respectively,

309

from death as well as a dose-dependent reduction in body weight loss compared with

310

the placebo (Fig. 4A and B).

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Similarly, the lungs of infected mice were harvested on day 5 post-infection and the

312

pathological changes were examined. As shown in Fig. 4C, extensive alveolar

313

damage and marked cellular infiltrates were observed in lungs of mice belonging to

314

placebo and oseltamivir groups. In contrast, all the SA-2 treatment groups (50, 75 or

315

100 mg/kg/d) showed less inflammation and fewer lesions in lung tissue. Dosing of

316

SA-2 at 100 mg/kg/d alleviated pneumonia symptoms, and the reductions in lung

317

index and histological score were significantly greater than that observed with

318

placebo or oseltamivir (Fig. 4D and E). These results suggest that SA-2 confers

319

effective protection to mice against the oseltamivir-resistant influenza virus

320

A/FM/1/47-H275Y.

321

3.5. SA-2 exhibited no obvious influence on virus entry and NA activity of the

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ACCEPTED MANUSCRIPT 322

A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2) influenza viruses To clarify the mechanism of activity against the influenza virus, the effect of SA-2

324

on virus entry was first investigated in a cell-based assay. The results consistently

325

indicated that the inhibition rates of both the A/FM/1/47 (H1N1) and

326

A/Beijing/32/92 (H3N2) viruses were much higher when SA-2 was added 2 h

327

post-infection (that is, after the virus entry step) than when SA-2 was added

328

'pre-infection' or 'co-infection' (Fig. 5A and B). For both virus strains, inhibition

329

rates were no greater than 30% when SA-2 was added to a final concentration of 80

330

µM before or during viral inoculation, which indicated that SA-2 had no influence

331

on viral adsorption and penetration. The results from the hemagglutination assay also

332

supported this inference. All tested concentrations of SA-2 were ineffective to

333

prevent the HA-mediated agglutination of red blood cells (Fig. 5C and D). To

334

examine whether SA-2 could affect the final step of virus progeny release from

335

infected cells, the NA inhibitory activity of SA-2 was assessed using the whole

336

influenza virion. The inhibition rates of SA-2 were less than 7.0% at all

337

concentrations tested. The results clearly showed that NA enzyme activity of H1N1

338

and H3N2 was hardly inhibited by SA-2, whereas it was suppressed by the influenza

339

NA inhibitor oseltamivir (Fig. 5E and F). All of these data indicate that SA-2 has no

340

effect on receptor binding and NA activity of the influenza virus.

341

3.6. SA-2 suppressed H1N1 influenza virus replication by inhibiting the activity of

342

RNA polymerase and interfering with NP and M1 expression during viral

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ACCEPTED MANUSCRIPT 343

biosynthesis As part of the effort to elucidate the mechanism of action, a time-of-addition study

345

was performed by exposing influenza-infected cells to SA-2 at different parts of the

346

virus replication cycle and measuring the expression of NP and M1 proteins 10 h

347

after infection. When SA-2 was added during the viral replication stage (2–4 or 4–6

348

h after infection), both NP and M1 protein levels were significantly reduced, while

349

no inhibitory activity was observed when SA-2 was added at the adsorption stage

350

(0–2 h) or at the release stage (6 or more hours after infection) (Fig. 6A and B). The

351

expression of NP and M1 genes were then examined at 6 h post-infection by

352

RT-PCR and real-time qPCR analysis. Compared with the DMSO control, SA-2

353

significantly reduced the mRNA levels of NP and M1 genes to 52.0% and 46.9%,

354

respectively (Fig. 6C and D). These results strongly suggest that SA-2 disrupts the

355

early

356

polymerase-mediated transcription and replication step.

intermediate

stage

of

viral

replication,

particularly

the

RNA

EP

to

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Subsequently, a luciferase experiment was conducted, and the results showed that

358

the activity of influenza virus RNA polymerase was significantly inhibited by SA-2

359

in a dose-dependent manner (Fig. 6E). SA-2 at concentrations of 40 and 80 µM

360

resulted in 68.1% and 75.0% reductions, respectively, of luciferase activity; SA-2,

361

showed a better inhibitory effect than did ribavirin (48.9% reduction at 40 µM). In

362

summary, SA-2 inhibits influenza virus replication by reducing viral polymerase

363

activity and interfering with RNA synthesis.

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ACCEPTED MANUSCRIPT 364

4. Discussion Outbreaks and epidemics of influenza are serious threats to human health. The

366

limited number of licensed drugs together with the continual emergence of viral

367

variants and drug-resistant mutants highlight the urgent need for antiviral agents with

368

novel mechanisms of action.

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365

Because the influenza virus produces a rapid and clear CPE followed by cell death

370

in vitro (Atkins et al., 2012), we initiated an antiviral evaluation of benzoic acid

371

derivatives using a cell-based approach. Compound 2 (SA-2) was found to be a

372

potent influenza inhibitor with limited cytotoxicity. In both virus-infected MDCK

373

and A549 cell models, SA-2 effectively inhibited the replication of the A/FM/1/47

374

(H1N1) and A/Beijing/32/92 (H3N2) influenza viruses, with EC50 values of 9.6 µM

375

and 19.2 µM, respectively, levels that are comparable to those for oseltamivir. In

376

addition, oral administration of SA-2 significantly protected mice infected with a

377

lethal dose of the A/FM/1/47 (H1N1) influenza virus, conferring 70% protection

378

from death at a dosage of 100 mg/kg/d, prolonging survival time, reducing body

379

weight loss, inhibiting lung consolidation, reducing lung virus titer, and decreasing

380

lung parameters. All these data demonstrate the potent anti-influenza activity of

381

SA-2 both in vitro and in vivo.

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Notably, permissive mutations enable influenza A (H1N1) viruses to acquire the

383

H275Y NA resistance mutation to oseltamivir without fitness loss, resulting in their

384

rapid global spread (Hurt, 2014; Tarbet et al., 2014). In the present study, a

385

drug-resistant mutant strain (A/FM/1/47-H275Y) selected by oseltamivir was used.

ACCEPTED MANUSCRIPT SA-2 displayed a sufficient antiviral effect to treat infection with H275Y-mutated

387

virus both in vitro and in vivo. In cell culture, the oseltamivir-resistant strain

388

A/FM/1/47-H275Y was susceptible to SA-2, with an EC50 value of 19.8 µM, while

389

that for oseltamivir was as high as 1177.9 µM. SA-2 also consistently showed

390

therapeutic

391

A/FM/1/47-H275Y in mice. Although oseltamivir at 100 mg/kg/d was ineffective at

392

protecting mice from mortality, 50% of virus-infected mice survived with alleviated

393

influenza-induced lung injury when treated with a 100 mg/kg/d dosage of SA-2.

394

These results suggest that SA-2 has effective inhibitory activity toward an

395

oseltamivir-resistant influenza virus. Moreover, considering the results we obtained

396

in mechanistic studies that indicated that SA-2 targeted RNA polymerase rather than

397

NA, this compound could potentially be used to treat infections caused by NA

398

inhibitor-resistant mutants of the influenza A virus.

against

the

oseltamivir-resistant

mutant

strain

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efficacy

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Viruses initiate their life cycle by attaching to host cell surface receptors, entering

400

cells, uncoating their viral nucleic acids, and replicating their genomes. After new

401

copies of viral proteins and genes are synthesized, these components assemble into

402

progeny virions, which exit the cell (Watanabe et al., 2010). The complete influenza

403

virus life cycle takes approximately 8 hours. The nucleoprotein (NP) and matrix

404

protein (M1) are major structural proteins that are essential for the integrity of a

405

virus (Wang et al., 2015). Their expression levels reflect the state of virus replication.

406

Consistent with results from hemagglutination and NA inhibition assays, results

407

from a time-of-addition assay suggested that the SA-2 antiviral effect may occur at

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ACCEPTED MANUSCRIPT an early to intermediate stage, particularly the RNA polymerase-mediated

409

transcription and replication step, but not at the steps of the virus entry or release.

410

Luciferase experiment, RT-PCR and real-time qPCR analysis confirmed the

411

inference that SA-2 suppressed influenza virus replication by inhibiting RNA

412

polymerase activity and interfering with gene transcription during viral biosynthesis.

413

SA-2 at concentrations of 40 and 80 µM resulted in 68.1% and 75.0% reductions,

414

respectively, of luciferase activity, which is a better inhibitory effect on RNA

415

polymerase than that of ribavirin (48.9% reduction at 40 µM). Meanwhile, the

416

mRNA levels of the NP and M1 genes were reduced to 52.0% and 46.9%,

417

respectively at 6 h post infection.

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408

Because the drug-resistance of influenza virus is frequently caused by amino acid

419

mutations in NA (e.g., R152K, H275Y, R292K) (Yen et al., 2014), inhibitors of viral

420

RNA synthesis in the influenza virus replication cycle have unique advantages

421

compared with NA inhibitors (Furuta, et al., 2009). Since SA-2 inhibits influenza

422

virus replication by reducing the viral polymerase activity and the action of point lies

423

in the phase of RNA synthesis, SA-2 should be considered a promising drug

424

candidate for the treatment of influenza, including influenza caused by existing and

425

potential drug-resistant mutants. However, based on the existing data we have

426

obtained, it is difficult to determine whether SA-2 acts directly on the viral

427

polymerase complex or the compound reduces polymerase activity indirectly by

428

interaction with host factors such as BUB3, CTNNB1 and CAMK2B, which are

429

essential for viral RNA transcription (Konig, et al., 2010; Watanabe, et al., 2015;

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ACCEPTED MANUSCRIPT Sasaki, et al., 2014). The presumed broad-spectrum antiviral activities, further

431

mechanism study, and more preclinical development of SA-2 will be the direction of

432

our future work.

433

Acknowledgments

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430

We sincerely thank Dr Xin Ye (Institute of Microbiology, Chinese Academy of

435

Sciences) for technical assistance with the luciferase assay. This work was supported

436

by the 111 Project (grant 111-2-07), the Priority Academic Program Development of

437

Jiangsu Higher Education Institutions (PAPD) and the Fundamental Research Funds

438

for the Central Universities (grant ZL2014SK0035).

439

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571

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Figure Legends

574

Fig. 1. Chemical structure of benzoic acid derivatives.

575

Fig. 2. Inhibitory effect of SA-2 on influenza A virus replication in MDCK and

576

A549 cells. (A) Microscopic images for A/FM/1/47 (H1N1) virus-infected and 80

577

µM SA-2-treated MDCK cells at 48 h post-infection. DMSO was used as a vehicle

578

control. (B) Rates of SA-2 (5–80 µM) inhibition of the A/FM/1/47 (H1N1),

579

A/Beijing/32/92

580

oseltamivir-resistant) influenza viruses in MDCK cells. Oseltamivir carboxylate (OC)

581

(10 µM) was used as a positive control. (C) Inhibitory rates of SA-2 in MDCK cells

AC C

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573

(H3N2)

and

A/FM/1/47-H275Y

(H1N1-H275Y;

ACCEPTED MANUSCRIPT infected with the A/FM1/1/47 (H1N1) and (D) A/Beijing/32/92 (H3N2) influenza

583

viruses at different MOIs (0.3, 0.5, and 1). (E) Rates of SA-2 (5–80 µM) inhibition

584

of the H1N1, H3N2 and H1N1-H275Y influenza viruses in A549 cells. Oseltamivir

585

carboxylate (OC) (10 µM) was used as a positive control. (F) Cellular toxicity of

586

SA-2 in MDCK and A549 cells. The results were presented as the mean ± S.D. of

587

three independent experiments, *P < 0.05; **P < 0.01; ***P < 0.001, compared with

588

oseltamivir carboxylate (OC)-treated group.

589

Fig. 3. Therapeutic efficacy of SA-2 against the A/FM/1/47 (H1N1) influenza virus

590

in mice. (A) Basic experimental protocol used to test SA-2 in mice infected with

591

influenza virus. ICR mice were intranasally infected with a mouse-adapted

592

A/FM/1/47 (H1N1) influenza virus (8 × LD50). Two hours later, ten mice per group

593

received placebo, SA-2 (50, 75, and 100 mg/kg/d), or oseltamivir (100 mg/kg/d)

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twice daily for 5 successive days (days 0 to 4) by oral gavage. (B) Survival and (C)

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body weight were monitored daily (n = 10). On day 5 post-infection, five mice per

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group were euthanized for lung pathological examinations. (D) H&E staining of

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sectioned lungs (n = 5). Some of the histopathological changes are indicated by

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arrows in pathological sections. (E) Lung virus titers, (F) Lung indices and (G)

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histological scores for mice infected with A/FM1/1/47 (H1N1) influenza virus (n =

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5). Data are represented as the mean ± S.D. *P < 0.05; **P < 0.01; ***P < 0.001,

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compared with the placebo group.

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Fig. 4. Protection of SA-2 in mice infected with oseltamivir-resistant strain

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A/FM/1/47-H275Y (H1N1-H275Y). Ten mice per group were intranasally infected

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Two hours post-infection, mice were administered placebo, SA-2 (50, 75, and 100

606

mg/kg/d), or oseltamivir (100 mg/kg/d) twice daily for 5 successive days by oral

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gavage. (A) Survival and (B) body weight were monitored daily (n = 10). On day 5

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post-infection, five mice per group were euthanized for lung pathological

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examinations. (C) H&E staining of sectioned lungs (n = 5). Some of the

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histopathological changes are indicated by arrows in pathological sections. (D) Lung

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indices and (E) histological scores for mice infected with the A/FM/1/47-H275Y

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influenza virus (n = 5). Data are presented as the mean ± S.D. *P < 0.05; **P < 0.01,

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compared with the placebo group.

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Fig. 5. SA-2 exhibited no obvious influence on virus entry and NA activity of the

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A/FM/1/47 (H1N1) and A/Beijing/32/92 (H3N2) influenza viruses. (A) Inhibition

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rates of SA-2 (10–80 µM) against H1N1 and (B) H3N2 influenza viruses with

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different infection protocols in MDCK cells. (C) Hemagglutination assays of various

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SA-2 concentrations (10, 40, and 80 µM) with 4 HAU/well of H1N1 and (D) H3N2

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viruses. DMSO was used as a vehicle control. (E) Neuraminidase (NA) inhibition

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assays of SA-2 (0.01–100 µM) against H1N1 and (F) H3N2 viruses. Oseltamivir

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carboxylate (OC) (1 nM) was used as a positive control.

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Fig. 6. SA-2 suppressed H1N1 influenza virus replication by inhibiting virus RNA

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polymerase and interfering with NP and M1 expression during viral biosynthesis. (A)

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MDCK cells were infected with 1.5 × 103 TCID50 of A/FM/1/47 (H1N1) influenza

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virus. SA-2 (40 µM) or ribavirin (40 µM) was added at the indicated periods. The

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and M1 antibodies. (B) The relative amounts of NP and M1 proteins were

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represented by band intensities normalized to β-actin. The relative intensity was

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calculated relative to the DMSO control. (C) MDCK cells were infected with 1.5 ×

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103 TCID50 of A/FM/1/47 (H1N1) influenza virus for 2 h to allow adsorption. Then,

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the media was replaced with fresh media with or without 40 µM SA-2. Cells were

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collected 6 h post-infection, and total RNA was extracted. Then, the mRNA

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expression levels of NP and M1 were determined using specific primers in a

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RT-PCR assay. (D) The relative RNA levels of NP and M1 genes were determined

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in a real-time qPCR experiment. Each sample was run in triplicate normalized to

636

GAPDH. The relative RNA levels of NP and M1 genes were calculated based on the

637

2

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with PB1, PB2, PA, NP (A/WSN/33 (H1N1)), vNS-Luc and pCMV β-gal plasmids

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for 6 h, then SA-2 (10, 40, and 80 µM) or ribavirin (40 µM) was added. After 24 h,

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the cell lysates were harvested for use in the luciferase assay, and their activities

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were normalized to β-gal activity. The relative luciferase activity was calculated

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relative to the DMSO control. The results are shown as the mean ± S.D. of three

643

independent experiments, *P < 0.05; **P < 0.01; ***P < 0.001, compared with the

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DMSO control.

method and relative to the DMSO control. (E) 293T cells were transfected

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Table 1

2

Structures, cytotoxicities and anti-influenza virus activities of newly synthesized

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benzoic acid derivatives.

Compound

CC50a (µM)

R

A/FM/1/47 (H1N1)

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EC50b (µM) for influenza virus (SIc) A/Beijing/32/92 (H3N2)

Wild-type

H275Y

>500

>160

>160

2

444.5

9.6 (46.3)

19.8 (22.4)

3

>500

72.2

4

398.2

96.2

5

>500

6

>500

Oseltamivir carboxylate

>500

Ribavirin

56.3

>160

19.2 (23.2)

SC

1

132.4

82.3

>160

85.3

128.6

141.4

142.2

>160

>160

7.6

1177.9

11.7

8.8

10.2

NDd

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73.2

4

Results were the mean values from three independent assays.

5

a

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by 50%.

7

b

8

virus-infected cells by 50%.

9

c

Selectivity index, CC50 / EC50.

10

d

Not determined.

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Concentration of chemicals required to reduce the viability of normal MDCK cells

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Concentration of chemicals required to improve the viability of influenza

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Novel benzoic acid derivatives were designed and synthesized with antiviral activity against influenza A viruses.




SA-2 (compound 2) significantly inhibited H1N1 and H3N2 in cell culture with




SA-2 showed potent therapeutic efficacy against the oseltamivir-resistant strain H1N1-H275Y both in vitro and in vivo.

SA-2 efficiently inhibited the activity of RNA polymerase during viral

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biosynthesis by interfering with gene transcription.

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EC50 values of 9.6 and 19.8 µM respectively.