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Background. The COVID-19 pandemic, caused by the SARS-CoV-2 coronavirus, originated in Wuhan, China, has claimed millions of lives around the world. In this regard, the search for effective drugs, including the repurposing of existing ones, has become an urgent task. A promising treatment strategy appears to be drug disruption of viral reproduction. RNA-dependent RNA polymerase (RdRp) is the central subunit of the RNA synthesis process for all positive-strand RNA viruses and is therefore an attractive target for antiviral inhibitors.The aim of this work is an experimental study of the antiviral activity of various drugs based on 6-fluoro-3-hydroxy-2-pyrazinecarboxamide (Favipiravir) in vitro and in vivo against the SARS-CoV-2 coronavirus (COVID-19).Material and methods. The experiments were carried out on a permanent culture of African green monkey kidney cells — Vero Cl008. The effectiveness of the drugs was assessed by suppressing the reproduction of the virus in vitro. Biological activity was assessed by titration of the virus-containing suspension in Vero Cl008 cell culture by the formation of negative colonies. Syrian golden hamsters orally infected with the SARS-CoV-2 virus, variant B, were used in the study. The effectiveness of the drug was assessed by the coefficient of therapeutic action.Results. The results of the study revealed that the compounds FP-1 and Avifavir in the concentration range of 100–400 µg/ml almost completely suppress the reproduction of the SARS-CoV-2 virus; the CTI index for the drug FP-1 was 4, for Avifavir it was 2. The ED₅₀ value for FP-1 was 26 µg/ml, for Avifavir it was 36 µg/ml. Preparations T-705 and Coronavir revealed antiviral activity only at extremely high concentrations. The CTI was 1. During the study on Syrian golden hamsters orally infected with the SARS-CoV-2 virus, variant B, at a dose of 5×105 PFU, it was shown that the use of Avifavir and FP-1 has a high protective efficacy, while Coronavir and T-705 cause a moderate suppression of virus reproduction in the target organ. According to the complex of clinical-virological, biochemical, and hematological indicators, the disease severity index (DSI) and the therapeutic index (TI) were calculated. For the drug Avifavir, the DSI was 0.269; the TI was 71.3% with a probability of 99.9%.Conclusion. Of the studied compounds, Avifavir and FP-1 showed the highest antiviral activity.

In May 2020 the Russian Ministry of Health granted fast-track marketing authorization to RNA polymerase inhibitor AVIFAVIR (favipiravir) for the treatment of COVID-19 patients. In the pilot stage of Phase II/III clinical trial, AVIFAVIR enabled SARS-CoV-2 viral clearance in 62.5% of patients within 4 days, and was safe and well-tolerated. Clinical Trials Registration. NCT04434248. [ABSTRACT FROM AUTHOR]