LAUSR

Molnupiravir (MOV) is used to treat COVID‐19. In cells, MOV is converted to the ribonucleoside analog N4‐hydroxycytidine (NHC) and incorporated into the SARS‐CoV‐2 RNA genome during its replication, resulting in RNA mutations. The widespread accumulation of such mutations inhibits SARS‐CoV‐2 propagation. Although safety assessments by many regulatory agencies across the world have concluded that the genotoxic risks associated with the clinical use of MOV are low, concerns remain that it could induce DNA mutations in patients, particularly because numerous in vitro studies have shown that NHC is a DNA mutagen. In this study, we used HiFi sequencing, a technique that can detect ultralow‐frequency substitution mutations in whole genomes, to evaluate the mutagenic effects of MOV in E. coli and of MOV and NHC in mouse lymphoma L5178Y cells and human lymphoblastoid TK6 cells. In all models, exposure to these compounds increased genome‐wide mutation frequencies in a dose‐dependent manner, and these increases were mainly composed of A:T → G:C transitions. The NHC exposure concentrations used for mammalian cells were comparable to those observed in the plasma of humans who received clinical doses of MOV.