A review of the safety of favipiravir – a potential treatment in the COVID-19 pandemic?

Background: Repurposing broad-spectrum antivirals is an immediate treatment opportunity for 2019 coronavirus disease (COVID-19). Favipiravir is an antiviral previously indicated for influenza and Ebola, which has shown some promise in early trials for treatment of COVID-19. We aim to review existing favipiravir safety evidence, which is vital to informing the potential future use of favipiravir in COVID-19. Methods: A search was conducted across EMBASE and MEDLINE databases, supplemented by relevant grey-literature and ClinicalTrials.gov. All studies assessing the use of favipiravir in humans by 27 March 2020 were considered for inclusion. Further analysis of available safety data from phase 2 and 3 studies was undertaken. Data extracted were adverse events (AEs) grade 1–4, serious AEs and discontinuation for AEs. Specific AEs of interest highlighted in early-phase studies, including gastrointestinal AEs and hyperuricaemia, were also examined. Results: Twenty-nine studies were identified as potential sources of evidence of the clinical safety of favipiravir. Six were phase 2 and 3 studies reporting relevant safety data for statistical comparison, representing a total of 4299 participants, an estimated 175 person-years-of-follow-up (PYFU). Comparator drugs were oseltamivir, umifenovir, lopinavir/ritonavir or placebo. Study follow-up was between 5 and 21 days. The proportions of grade 1–4 AEs on favipiravir was 28.2% vs 28.4% (P = n.s.) in the comparison arms. The proportion of discontinuations due to AEs on favipiravir was 1.1% vs 1.2% (P = n.s.) in the comparison arms. For serious AEs the proportion was 0.4% in both arms (P = n.s.). There were significantly fewer gastrointestinal AEs occurring on favipiravir vs comparators [8.7% vs 11.5%; P = 0.003]. Favipiravir showed significantly more uric acid elevations than comparators [5.8% vs 1.3%; P<0.0001]. Conclusions: Favipiravir demonstrates a favourable safety profile regarding total and serious AEs. However, safety concerns remain: hyperuricaemia, teratogenicity and QTc prolongation have not yet been adequately studied. Favipiravir may be safe and tolerable in short-term use, but more evidence is needed to assess the longer-term effects of treatment. Given the limitations of the evidence and unresolved safety concerns, caution is warranted in the widespread use of favipiravir against pandemic COVID-19.