Favipiravir, an anti-influenza drug against life-threatening RNA virus infections

Abstract

Favipiravir has been developed as an anti-influenza drug and licensed as an anti-influenza drug in Japan. Additionally, favipiravir is being stockpiled for 2 million people as a countermeasure for novel influenza strains. This drug functions as a chain terminator at the site of incorporation of the viral RNA and reduces the viral load. Favipiravir cures all mice in a lethal influenza infection model, while oseltamivir fails to cure the animals. Thus, favipiravir contributes to curing animals with lethal infection. In addition to influenza, favipiravir has a broad spectrum of anti-RNA virus activities in vitro and efficacies in animal models with lethal RNA viruses and has been used for treatment of human infection with life-threatening Ebola virus, Lassa virus, rabies, and severe fever with thrombocytopenia syndrome. The best feature of favipiravir as an antiviral agent is the apparent lack of generation of favipiravir-resistant viruses. Favipiravir alone maintains its therapeutic efficacy from the first to the last patient in an influenza pandemic or an epidemic lethal RNA virus infection. Favipiravir is expected to be an important therapeutic agent for severe influenza, the next pandemic influenza strain, and other severe RNA virus infections for which standard treatments are not available.

Keywords: Antiviral agent; Chain termination; Ebola; Favipiravir; Influenza; Resistant virus.

Fig. 1

Chemical structures of favipiravir and its related compounds. A. Favipiravir (6-fluoro-3-hydroxy-2-pyrazinecarboxamide). T-705 is the code number of favipiravir. B. Guanosine, inosine, 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), ribavirin, and ribosyl favipiravir. Guanosine, inosine, and AICAR are biosynthesized in the body, and ribavirin and ribosyl favipiravir are synthesized nucleoside analogues. They have similar chemical structures, but favipiravir and the other two compounds differ because they contain pyrazine, triazole, and imidazole that are marked by red circles.

Fig. 2

Cascade of fever induction by influenza and antipyretic action of cinnamyl compounds and NSAIDs. Influenza infection induces interferon (IFN) production that subsequently induces interleukin (IL)-1 production to act on the hypothalamus. Next, cyclooxygenase is expressed to induce prostaglandin (PG)E₂ production and generate fever. Cinnamyl compounds from medicinal herbs, anti-IFN antibody, anti-IL-1 antibody, and nonsteroidal anti-inflammatory drugs (NSAIDs) (aspirin) act at each step of this process. Cinnamyl compounds and NSAIDs inhibit the induction of IFN and cyclooxygenase activity, respectively, in the fever cascade to work as antipyretics in influenza. Regarding pneumonia, IL-12 and IFN-γ production are induced in the bronchoalveolar fluid (BALF) on the second and third day of infection, respectively, and an increase in IL-12 levels reduces the viral load in the BALF and the area of pneumonia throughout the lungs.

Fig. 3

Comparison of the efficacy of favipiravir and oseltamivir in lethal influenza virus infection. Mice were infected with 3 × 10⁴ plaque forming units of influenza A/PR/8/34 virus and were orally administered favipiravir and oseltamivir at doses of 200 and 400 mg/kg/day for 5 days beginning at 1 h post-infection (n = 14). The results presented in this figure were obtained from a representative experiment. *P < .01 compared to 0.5% methylcellulose solution-treated controls and oseltamivir-treated groups (log-rank test). The authors obtained permission from the Antiviral Chemistry and Chemotherapy to reuse this figure (Takahashi et al., 2003).

Fig. 4

Replication cycle of influenza and sites of action of anti-influenza drugs. Influenza virus hemagglutinin (HA) binds to its receptor, sialic acid on the cell surface and is taken up into endosomes by endocytosis. Endosomes are gradually acidified to produce late endosomes, viral matrix-2 (M2) ion channels acidify viral particles, and the structure of trimeric HA molecules changes and shows membrane fusion activity. The endosomal membrane and envelope are then fused, and the genomic RNA and RdRp complex in the virus particle are released into the cytoplasm (uncoating) and transported to the nucleus through the cytoplasm. The transcription (replication) of genomic RNA by the RdRp complex occurs in the nucleus, and the genomic RNA is abundantly produced. Genomic RNA lacks the Cap structure required for mRNA function, and the Cap portion of the host mRNA is removed by a Cap-dependent endonuclease. Then, the Cap portion is coupled to genomic RNA, and viral mRNA synthesis is complete (Cap-snatching). Viral proteins are synthesized from the mRNA, and viral proteins and genomic RNA are transported to the cell surface and bud from the membrane to form viral particles. Since the budding virus is bound to the sialic acid on the infected cell surface via the HA protein of virus particles, it is unable to leave the infected cell and infect new cells. For this reason, the sialic acid-HA bond is cleaved by neuraminidase (NA) on the surface of the virus particle, and the virus particle is released from the surface of the infected cell and proceeds to the next round of infection. Amantadine blocks uncoating by inhibiting acidification mediated by the M2 protein in the virus particle in late endosome, and thus the infection is unable to be completed. Favipiravir inhibits viral RNA synthesis by terminating chain elongation at its incorporated site, and no new RNA is generated in the cell. Baloxavir marboxil prevents Cap-snatching, and the viral mRNA is not produced, resulting in a failure to produce viral proteins and infectious viruses. Genomic RNA is synthesized and remains in the cell. NA inhibitors block the cleavage of the sialic acid-HA bond in the virus on the surface of infected cells and prevent the spread of the viral infection. NA inhibitors allow genomic RNA synthesis. Favipiravir inhibits viral RNA synthesis, while baloxavir and NAIs allow viral RNA synthesis. Although viral spread is inhibited by baloxavir and NAIs, viral RNA is synthesized, and the pool of genomic RNA serves as a rich source of resistant viruses.

Fig. 5

Chain termination and lethal mutagenesis. A. Chain termination Favipiravir is converted to favipiravir-RTP and incorporated into the elongating RNA strand. Then, chain elongation stops at the site of favipiravir incorporation, and elongation does not proceed because favipiravir functions as a chain terminator. This RNA-favipiravir(−RdRp) complex will not be repaired by the proofreading enzyme and would be disposed of as unnecessary RNA, resulting in the extinction of the viral genome (Rocha-Pereira et al., 2012; Vanderlinden et al., 2016). B. Lethal mutagenesis Ribavirin is incorporated into the elongating RNA strand, and viral RdRp continues to elongate until completion. Next, the RNA strand with multiple ribavirin incorporation sites further incorporates ribavirin or serves as mRNA for viral protein synthesis. The incorporation of ribavirin in the viral RNA causes the mismatch of base pairs (transition mutation), and translation of this RNA causes mutations in the amino acid sequence of the protein, resulting in impaired function. Viral proteins with impaired function fail to replicate or produce infectious viral particles, and viral infection ceases. Thus, lethal mutagenesis terminates viral infection through a different mechanism than chain termination.