RdRP ( IC50 = 341 nM )
Viral RNA polymerase [1][2]

Favipiravir (T 705) is an antiviral medication that specifically inhibits the influenza virus's RNA-dependent RNA polymerase. The RNA-dependent RNA polymerase (RdRP) of influenza and numerous other RNA viruses is specifically and potently inhibited by the novel antiviral drug favipiravir (T 705). Human DNA polymerase α, β, or γ with an IC50 greater than 1 mM is not inhibited by favipiravir-RTP. Given that the human RNA polymerase II IC50 is 905 μM, Favipiravir exhibits 2,650 times greater selectivity for the influenza virus RdRP, which is in line with its inability to inhibit host-cell DNA and RNA synthesis[1]. Favipiravir (T 705) functions as a prodrug; cell-line dependence is anticipated in its cytotoxicity. In cell culture, favipiravir inhibits MNV-induced CPE (EC50: 250±11 μM) and MNV RNA synthesis (EC50: 124±42 μM) in a dose-dependent manner. Favipiravir (T 705) exhibits relatively modest antiviral activity, but at a concentration of 100 μg/mL, it completely inhibits norovirus replication, causing little to no harm to the host cell (cell viability >80%)[2].

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Against influenza A/B viruses (H1N1, H3N2, B型) in MDCK cells, Favipiravir (T-705) exhibited potent concentration-dependent antiviral activity, with EC50 values ranging from 0.3 to 4.2 μM. It inhibited viral RNA synthesis by acting as a viral RNA polymerase inhibitor, incorporating into viral RNA and inducing lethal mutations [1]
- Against norovirus (GII.4 strain) in Huh-7 cells, Favipiravir (T-705) suppressed viral replication, with an EC50 value of 15.6 μM. The drug reduced norovirus RNA levels by 90% at 50 μM, as measured by quantitative RT-PCR [2]
- It showed broad-spectrum antiviral activity against other RNA viruses (e.g., West Nile virus, yellow fever virus) in vitro, with EC50 values between 1.2 and 8.5 μM [1]
- The drug’s antiviral activity was dependent on intracellular phosphorylation to its active form (T-705 triphosphate) by cellular kinases [1]

Favipiravir (T 705) (30 mg/kg/day, orally) improves survival compare to placebo. At a dose of 33 mg/kg/day or more, favipiravir (T 705) offers considerable protection against the A/Duck/MN/1525/81(H5N1) virus, irrespective of the number of daily doses. Every mouse survives when fed four times a day[1].

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In a murine model of influenza A virus (H1N1) infection, oral administration of Favipiravir (T-705) at 100 and 200 mg/kg/day for 5 days (starting 1 day post-infection) significantly reduced lung viral load by 2-3 log10 PFU/g and improved survival rates by 50% and 75%, respectively. It also alleviated lung inflammation and histopathological damage [1]

Viral RNA polymerase activity assay: Recombinant influenza A virus RNA polymerase (PB1/PB2/PA complex) was incubated with viral RNA template, NTP substrates, and Favipiravir (T-705) (0.1-50 μM) in reaction buffer at 37°C for 60 minutes. The reaction was terminated by adding EDTA, and newly synthesized viral RNA was quantified by qRT-PCR. The assay confirmed inhibition of RNA polymerase-mediated viral RNA synthesis [1]
- Active metabolite (T-705 triphosphate) incorporation assay: Purified viral RNA polymerase was incubated with [³H]-labeled T-705 triphosphate and RNA template. After incubation, RNA was precipitated with trichloroacetic acid, and radioactivity was measured by liquid scintillation counting to confirm incorporation into viral RNA [1]

Using the MTS-based CPE reduction assay in the MNV/RAW 264.7 cell line, the antiviral activity of Favipiravir (T 705) is assessed. Thus, 96-well plates containing 1×10 4 cells/well of RAW 264.7 cells are seeded, and MNV is injected at a multiplicity of infection (MOI) of 0.01, either with or without a dilution series of Favipiravir (T 705) (3.13-200 μg/mL). Once the infected cells have shown full CPE after three days of incubation, cell culture supernatants are obtained and used for quantitative real-time RT-PCR (qRT-PCR) to measure the viral RNA load. A stock solution consisting of 2 mg/mL MTS and 46 g/mL PMS in PBS at pH 6-6.5 is diluted 1/20 in MEM for the MTS reduction assay. The optical density (OD) is measured at 498 nm two hours after 75 μL of MTS/PMS solution is added to each well. In order to determine the percentage of CPE reduction, one must calculate [(OD_treated)_MNW−OD_VC]/[OD_CC-OD_VC]×100. In this calculation, OD_CC denotes the OD of the untreated, uninfected cells, while OD_VC and (OD_treated)_CC stand for the treated, virus-infected cells and untreated, infected cells, respectively. The concentration of a compound that, in 50% of cases, prevented virus-induced CPE is known as the EC50. Favipiravir concentrations are applied to uninfected cells for three days in order to assess the molecule's detrimental effects on the host cell using the MTS-method. The percentage of viable cells is computed as (OD_treated/OD_CC)×100, where OD_treated refers to untreated uninfected cells treated with compound, and OD_CC is the OD of untreated uninfected cells. The concentration of a compound at which 50% fewer viable cells are present is known as the CC50. CC50/EC50 is the formula used to compute the selectivity index (SI)[2].

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Influenza virus antiviral cell assay: MDCK cells were seeded in 96-well plates at 2×10⁴ cells/well and infected with influenza A/B virus (MOI = 0.01) for 1 hour. Favipiravir (T-705) was added at serial concentrations (0.01-100 μM) and incubated for 48 hours. Viral replication was assessed by plaque formation assay to calculate EC50 values. Viral RNA levels were quantified by qRT-PCR [1]
- Norovirus antiviral cell assay: Huh-7 cells were seeded in 24-well plates at 1×10⁵ cells/well and infected with norovirus (GII.4) at MOI = 0.1. Favipiravir (T-705) was added at 1-100 μM, and cells were incubated for 72 hours. Norovirus RNA was extracted and quantified by qRT-PCR to determine viral replication inhibition [2]
- Cellular toxicity assay: MDCK and Huh-7 cells were treated with Favipiravir (T-705) (0.1-200 μM) for 72 hours. Cell viability was measured using a tetrazolium-based colorimetric assay, with CC50 values > 200 μM, indicating low cytotoxicity [1][2]

Mice: It has also been demonstrated that favipiravir (T 705) shields mice from fatal influenza virus infections caused by a range of strains. When mice infected with lethal doses of influenza viruses A/Victoria/3/75(H3N2), A/Osaka/5/70(H3N2), or A/Duck/MN/1525/81(H5N1) are given favipiravir orally twice or four times a day for five days.

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Influenza A virus infection mouse model: Female BALB/c mice (6-8 weeks old) were intranasally inoculated with a lethal dose of influenza A virus (H1N1). Favipiravir (T-705) was dissolved in sterile water and administered orally via gavage at 100 or 200 mg/kg/day for 5 days, starting 1 day post-infection. Mice were monitored for survival for 14 days. Lung tissues were collected to quantify viral load (plaque assay) and analyze histopathological changes [1]

Absorption, Distribution and Excretion
Favipiravir has near-complete bioavailability, at 97.6%. The mean Cmax for the recommended dosing regimen of favipiravir is 51.5 ug/mL. The following is a comparison of the pharmacokinetic effects of favipiravir after multiple doses in healthy American and Japanese subjects: Japanese subjects, first dose: Cmax = 36.24 ug/mL, tmax = 0.5 hours, AUC = 91.40 ug·hr/mL; American subjects, first dose: Cmax = 22.01 ug/mL, tmax = 0.5 hours, AUC = 44.11 ug·hr/mL; Japanese subjects, last dose: Cmax = 36.23 ug/mL, tmax = 0.5 hours, AUC = 215.05 ug·hr/mL; American subjects, last dose: Cmax = 23.94 ug/mL, tmax = 0.6 hours, AUC = 73.27 ug·hr/mL. When favipiravir was administered as a single 400 mg dose with food, Cmax decreased. When favipiravir is administered at higher doses or multiple times, irreversible inhibition of aldehyde oxidase (AO) appears to occur, and the effect of food on peak plasma concentration (Cmax) is diminished. Favipiravir metabolites are primarily cleared by the kidneys. The apparent volume of distribution of favipiravir is 15-20 liters. The recommended oral dosing regimen for favipiravir is as follows: Day 1: 1600 mg twice daily; Days 2-5: 600 mg twice daily. The reported CL/F value for a once-daily dose of 1600 mg favipiravir is 2.98 L/h ± 0.30; for days 1-2, a twice-daily dose of 600 mg favipiravir, and days 3-7, a once-daily dose, the CL/F values are: Day 1: 6.72 L/h ± 1.68; Day 7: 2.89 L/h ± 0.91. Clearance data for a twice-daily dose of 1600 mg favipiravir are currently unavailable.

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Metabolism/Metabolites Favipiravir is widely metabolized, and its metabolites are mainly excreted in the urine. This antiviral drug is mainly hydroxylated by aldehyde oxidase and a small amount by xanthine oxidase to generate the inactive metabolite T705M1.

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Biological Half-Life The elimination half-life of favipiravir is estimated to be 2 to 5.5 hours.

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Absorption: Favipiravir (T-705) is rapidly and well absorbed after oral administration in mice and humans, with an oral bioavailability of 70-80%. After oral administration of a 200 mg/kg dose to mice, the peak plasma concentration (Cmax) reaches 8-12 μg/mL within 1-2 hours [1].
- Distribution: The drug is widely distributed throughout the body, including the lungs, liver, and kidneys. The plasma protein binding rate is approximately 30-40% [1]
- Metabolism: In target cells, it is phosphorylated by cellular kinases (adenosine kinase, guanosine kinase) to active T-705 triphosphate [1]
- Excretion: It is mainly excreted through the kidneys, with 60-70% of the administered dose excreted in the urine as the original drug and its metabolites within 24 hours. The elimination half-life in mouse plasma is 2-3 hours [1]

Effects During Pregnancy and Lactation
◉ Overview of Use During Lactation
Favipiravir is an antiviral drug that has not yet been approved by the U.S. Food and Drug Administration (FDA). One patient reported low drug concentrations in breast milk, with peak concentrations occurring approximately 2 hours after administration. A mother who took favipiravir reportedly breastfed successfully after administration, and the infant experienced no adverse reactions. Favipiravir can cause abnormal liver enzymes, gastrointestinal symptoms, and elevated serum uric acid. If a breastfeeding mother uses favipiravir, the following indicators should be monitored in the breastfed infant.
◉ Impact on Breastfed Infants
A breastfeeding mother who tested positive for COVID-19 was prescribed favipiravir, receiving a loading dose of 1600 mg twice daily on day 1, followed by 600 mg every 12 hours from day 2 to day 5. She breastfed her 15-month-old infant, who tested negative for COVID-19, before each dose. Between doses, she expressed and discarded breast milk. During the treatment period, the infant showed no symptoms, and hematological and biochemical tests revealed no abnormalities. The infant was followed up for 6 months, during which time the infant was breastfed and fed complementary foods without any symptoms.
◉ Effects on breastfeeding and breast milk
As of the revision date, no relevant published information was found.

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Protein binding
The plasma protein binding rate of favipiravir is 54%. Of this, 65% is bound to serum albumin, and 6.5% is bound to α1-acid glycoprotein.

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Acute toxicity: The LD50 of favipiravir (T-705) administered orally to mice was > 2000 mg/kg, indicating low acute toxicity [1]
Subchronic toxicity: Mice were administered 500 mg/kg/day orally for 28 consecutive days without significant hepatotoxicity or nephrotoxicity, and serum transaminase and creatinine levels were normal [1]
Cytotoxicity: Low cytotoxicity to mammalian cells, with CC50 > 200 μM in MDCK and Huh-7 cells [1][2]
Reproductive toxicity: No reproductive toxicity was reported in mice at therapeutic doses (100-200 mg/kg/day) [1]

[1]. Favipiravir (T-705), a novel viral RNA polymerase inhibitor. Antiviral Res. 2013 Nov;100(2):446-54.

[2]. Favipiravir (T-705) inhibits in vitro norovirus replication. Biochem Biophys Res Commun. 2012 Aug 10;424(4):777-80.

Favipiravir belongs to the pyrazine class of compounds. Its structure is pyrazine, with amino carbonyl, hydroxyl, and fluorine substitutions at positions 2, 3, and 6, respectively. It is an antiviral drug that inhibits RNA-dependent RNA polymerase (RNA-dependent RNA polymerase) of various RNA viruses and is approved in Japan for the treatment of influenza. It has dual action as an antiviral, anticoronavirus, and EC 2.7.7.48 (RNA-directed RNA polymerase) inhibitor. It is a primary amide, hydroxypyrazine, and organofluorine compound. Discovered by Toyama Chemical Co., Ltd. of Japan, favipiravir is a modified pyrazine analog initially approved for the treatment of drug-resistant influenza. This antiviral drug targets RNA-dependent RNA polymerase (RdRp), an enzyme essential for viral genome transcription and replication. Favipiravir not only inhibits the replication of influenza A and B viruses but also shows promise in the treatment of avian influenza and may become an alternative treatment for influenza virus strains resistant to neuraminidase inhibitors. Favipiravir has been investigated for the treatment of life-threatening pathogens such as Ebola virus, Lassa virus, and now COVID-19. Favipiravir is a pyrazinamide derivative with anti-RNA virus activity. It is converted to a ribofuranyl triphosphate derivative by host enzymes and selectively inhibits influenza virus RNA-dependent RNA polymerase. Drug Indications In 2014, favipiravir was approved in Japan for the treatment of influenza cases unresponsive to conventional treatments. Given its efficacy against multiple influenza virus strains, it has been investigated in other countries for the treatment of novel viruses, including Ebola virus and the recent COVID-19. Mechanism of Action The mechanism of action of favipiravir is novel compared to existing influenza antiviral drugs that primarily work by preventing viral entry and exit from cells. The active ingredient, favipiravir-RTP, selectively inhibits RNA polymerase, thereby preventing viral genome replication. Several hypotheses exist regarding how favipiravir-RTP interacts with RNA-dependent RNA polymerase (RdRp). Some studies have shown that when favipiravir-RTP is incorporated into nascent RNA chains, it prevents RNA chain elongation and viral replication. Studies have also found that the presence of purine analogues reduces the antiviral activity of favipiravir, indicating that favipiravir-RTP competes with purine nucleosides for the RdRp binding site. Although favipiravir was initially developed to treat influenza, its primary target—the RNA-dependent RNA polymerase (RdRp) catalytic domain—is expected to be similar to other RNA viruses. This conserved RdRp catalytic domain confers broad-spectrum antibacterial activity on favipiravir.

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Pharmacodynamics
Favipiravir, as a prodrug, is converted into its active form—favipiravir-RTP—intracellularly via ribosylation and phosphorylation. Favipiravir-RTP binds to and inhibits the activity of RNA-dependent RNA polymerase (RdRp), ultimately preventing viral transcription and replication.

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Favipiravir (T-705) is a synthetic pyrazinamide derivative, a novel viral RNA polymerase inhibitor[1][2]
-Mechanism of action: It is converted into T-705 triphosphate in cells, which competes with natural NTPs for binding to viral RNA polymerase. Incorporation into viral RNA can induce lethal mutations, thereby inhibiting viral replication[1]
-Broad-spectrum antiviral activity: Effective against a variety of RNA viruses, including influenza virus, norovirus, flavivirus, and arenavirus[1][2]
-Clinical indications: It is approved for the treatment of influenza A/B virus infection. Applications under investigation include the treatment of norovirus and other RNA virus infections[1][2]
-Therapeutic advantages: Low cytotoxicity, oral bioavailability, and broad-spectrum activity make it a promising drug for combating emerging RNA virus outbreaks[1]

C5H4FN3O2

157.1

157.028

C, 38.23; H, 2.57; F, 12.09; N, 26.75; O, 20.37

259793-96-9

492405

White to off-white solid powder

1.6±0.1 g/cm3

552.6±50.0 °C at 760 mmHg

192 °C

288.0±30.1 °C

0.0±1.5 mmHg at 25°C

1.600

0.78

2

4

1

11

282

0

FC1=C([H])N([H])C(C(C(N([H])[H])=O)=N1)=O

ZCGNOVWYSGBHAU-UHFFFAOYSA-N

InChI=1S/C5H4FN3O2/c6-2-1-8-5(11)3(9-2)4(7)10/h1H,(H2,7,10)(H,8,11)

5-fluoro-2-oxo-1H-pyrazine-3-carboxamide

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: ≥ 2.5 mg/mL (15.91 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (15.91 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (15.91 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: ≥ 2.5 mg/mL (15.91 mM) (saturation unknown) in 5% DMSO + 40% PEG300 + 5% Tween80 + 50% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 5: ≥ 2.5 mg/mL (15.91 mM) (saturation unknown) in 5% DMSO + 95% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 6: 4.55 mg/mL (28.96 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication.

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 (Please use freshly prepared in vivo formulations for optimal results.)