Favipiravir (T-705), a broad spectrum inhibitor of viral RNA polymerase

Abstract

Favipiravir (T-705; 6-fluoro-3-hydroxy-2-pyrazinecarboxamide) is an anti-viral agent that selectively and potently inhibits the RNA-dependent RNA polymerase (RdRp) of RNA viruses. Favipiravir was discovered through screening chemical library for anti-viral activity against the influenza virus by Toyama Chemical Co., Ltd. Favipiravir undergoes an intracellular phosphoribosylation to be an active form, favipiravir-RTP (favipiravir ribofuranosyl-5'-triphosphate), which is recognized as a substrate by RdRp, and inhibits the RNA polymerase activity. Since the catalytic domain of RdRp is conserved among various types of RNA viruses, this mechanism of action underpins a broader spectrum of anti-viral activities of favipiravir. Favipiravir is effective against a wide range of types and subtypes of influenza viruses, including strains resistant to existing anti-influenza drugs. Of note is that favipiravir shows anti-viral activities against other RNA viruses such as arenaviruses, bunyaviruses and filoviruses, all of which are known to cause fatal hemorrhagic fever. These unique anti-viral profiles will make favipiravir a potentially promising drug for specifically untreatable RNA viral infections.

Keywords: RNA viruses; RNA-dependent RNA polymerase; broad spectrum anti-viral agent; favipiravir; influenza virus; viral hemorrhagic fever.

Figure 1.

Chemical structure of favipiravir (T-705).

Figure 2.

Effects of favipiravir-RTP, favipiravir and favipiravir-RMP on RNA-dependent RNA polymerase (RdRp) activity of Influenza virus (Based on Furuta et al., 2005⁸⁾). RdRp activity was assessed by incorporation of ³²P-GTP into a nascent RNA strand. Results are means ± standard deviations (n = 3). *, P < 0.01; results significantly different from those for the controls by the Tukey test.

Figure 3.

Incorporation of favipiravir-RTP and GTP into a nascent RNA strand, and inhibition of influenza virus RdRp (Based on Sangawa et al., 2013¹⁵⁾). (A) Incorporation of favipiravir-RTP and GTP at the position of G11+2. The ³²P-labeled pGEM-7zf (+) DNA run-off transcript with a 5′Cap1 structure (Cap1-pGEM-mRNA), crude influenza virus RdRp containing a viral genome, and nucleotides including favipiravir-RTP were incubated. Reaction products were then electrophoresed. Lane 1–5: Cap1-pGEM-mRNA and crude enzyme solution + 50 µmol/L CTP; Lane 2, 3: Conditions of lane 1 + 100 and 1,000 µmol/L GTP; Lane 4, 5: Conditions of lane 2 + 100 and 1,000 µmol/L favipiravir-RTP. (B) Inhibition of influenza virus RdRp by favipiravir-RTP. The ³²P-labeled pGEM-7zf (+) DNA run-off transcript with a 5′Cap1 structure (Cap1-pGEM-mRNA), crude influenza virus RdRp containing a viral genome, and nucleotides including favipiravir-RTP were incubated. Reaction products were then electrophoresed. Lane 1: Cap1-pGEM-mRNA; Lane 2–6: Cap1-pGEM-mRNA + crude enzyme solution; Lane 3–6: Conditions of lane 2 + 50 µmol/L CTP, 100 µmol/L ATP, 50 µmol/L GTP; Lanes 4–6: Conditions of lane 3 + 10, 100, and 1,000 µmol/L favipiravir-RTP. * Elongated RNA was detected when GTP, ATP, and CTP were added to the reaction mixture.

Figure 4.

Schematic representation of the activation mechanism of favipiravir (Based on Furuta et al., 2013⁹⁾). Favipiravir is incorporated into cells, and converted to favipiravir ribofuranosyl phosphates by host cell enzymes. The triphosphate form, favipiravir-RTP, inhibits the influenza viral RNA polymerase activity.

Figure 5.

Influenza virus susceptibility testing to favipiravir in plaque reduction assay in MDCK cells (Based on Sleeman et al., 2010²²⁾). Influenza viruses were included seasonable strains A(H1N1), A(H1N1)pdm09, A(H3N2), and B; highly pathogenic avian influenza virus (H5N1) isolated from human. These strains contain ones resistant to oseltamivir or zanamivir, and several ones resistant to both NA inhibitors. Changes to NA were detected by surveillance criteria (Sheu et al., 2008⁵⁹⁾). EC₅₀, 50% effective concentration. ○; NA inhibitor sensitive, ●; Oseltamivir resistant, ▲; Oseltamivir and Zamanivir resistant.

Figure 6.

Therapeutic effects of favipiravir in the mice infection model (Based on Furuta et al., 2013⁹⁾). Ten female Balb/c mice were infected with 100% lethal dose of influenza virus either of A/Victoria/3/75 (H3N2), A/Osaka/5/70 (H3N2) or A/Duck/MN/1525/81 (H5N1) and monitored for survival to day 21. Favipiravir was orally administered 1 hr post-infection twice (Osaka, Duck) or four times (Victoria) a day for 5 days. **, p < 0.001 compared to control group (Yates-corrected Chi-square test). ++, p < 0.001 compared to control group (Kaplan-Meier method, Log-rank test.

Figure 7.

Treatment of lethal Lassa virus infection in guinea pigs with favipiravir (Based on Safronetz D et al. 2015³²⁾). (A) Treatment began 48 hours after challenge. Groups of nine guinea pigs were challenged with a lethal dose of guinea pig adapted–Lassa virus (GPA-LASV) and treated subcutaneously once daily for two weeks with favipiravir (150 or 300 mg/kg/day), ribavirin (50 mg/kg/day) or vehicle placebo. (B) Groups of six guinea pigs were challenged with a lethal dose of GPA-LASV. Beginning on days 5, 7, and 9 post-challenge, favipiravir treatment (300 mg/kg/day, once daily subcutaneously for 14 consecutive days) was initiated in the respective group of animals. Each study followed survival for up to 42 days post-infection. *p < 0.05, **p < 0.01, ***p < 0.001 compared to placebo; a; p < 0.05 and b; p < 0.001 compared to ribavirin.