Paxlovid is the first approved oral treatment for coronavirus disease 2019 and includes nirmatrelvir, a protease inhibitor targeting the main protease (Mpro) of SARS-CoV-2, as one of the key components. While some specific mutations emerged in Mpro were revealed to significantly reduce viral susceptibility to nirmatrelvir in vitro, there is no report regarding resistance to nirmatrelvir in patients and animal models for SARS-CoV-2 infection yet. We recently developed xenograft tumors derived from Calu-3 cells in immunodeficient mice and demonstrated extended replication of SARS-CoV-2 in the tumors. In this study, we investigated the effect of nirmatrelvir administration on SARS-CoV-2 replication. Treatment with nirmatrelvir after virus infection significantly reduced the replication of the parental SARS-CoV-2 and SARS-CoV-2 Omicron at 5 days post-infection (dpi). However, the virus titers were completely recovered at the time points of 15 and 30 dpi. The virus genomes in the tumors at 30 dpi were analyzed to investigate whether nirmatrelvir-resistant mutant viruses had emerged during the extended replication of SARS-CoV-2. Various mutations in several genes including ORF1ab, ORF3a, ORF7a, ORF7b, ORF8, and N occurred in the SARS-CoV-2 genome; however, no mutations were induced in the Mpro sequence by a single round of nirmatrelvir treatment, and none were observed even after two rounds of treatment. The parental SARS-CoV-2 and its sublineage isolates showed similar IC50 values of nirmatrelvir in Vero E6 cells. Therefore, it is probable that inducing viral resistance to nirmatrelvir in vivo is challenging differently from in vitro passage.
Keywords: COVID-19; Drug resistance; Long-term infection model; Nirmatrelvir; SARS-CoV-2.