RNHC inhibits SARS-CoV-2 in vitro but is mutagenic in mammalian cells

ABSTRACT

Background: We previously showed that β-D-N4-hydroxycytidine (rNHC) and its orally bioavailable prodrug, molnupiravir, acts as a broad-spectrum antiviral against coronaviruses in vitro and in vivo through lethal mutagenesis. Molnupiravir is currently in clinical trials for the treatment of SARS-CoV-2 infection. However, there are concerns that rNHC could be metabolized to dNHC and cause mutations in host cells. We examined the in vitro antiviral and mammalian cell mutagenic activity of three different nucleoside/base analogs, rNHC, favipiravir, and ribavirin, on SARS-CoV-2. We further examined the in vitro genotoxicity of a panel of antiviral nucleotide/nucleoside analogs, including rNHC, using a modified HPRT gene mutation assay. Methods: A549-hACE2 cells were infected with SARS-CoV-2 in the presence of nucleoside analogs. After 48 hours, the supernatants were collected and viral RNA was extracted. We constructed multiplexed-Primer ID libraries from viral RNA and sequenced them using MiSeq. HPRT knockout assays were performed using CHO-K1 cells treated with a panel of nucleotide/nucleoside analogs for 32 days. After 6-thioguanine selection, resistant cell colonies were counted as a measure of HPRT knockout mutations in host cells, and HPRT mRNA was sequenced from selected colonies. Results: rNHC showed dose-dependent antiviral and mutagenic effects against SAR-CoV-2 in vitro. In the 10 μM group, we found 7-fold and 14-fold increases in the overall substitution rate and the C to U mutation rate, respectively. The HPRT assay showed an rNHC dose-dependent increase in the number of resistant colonies with HPRT gene mutations. Other analogs showed no significant increase in the number of 6-thioG resistant colonies except for a slight increase with favipiravir (Fig 1a). Most colonies had missense substitutions or frame-shift deletions within HPRT mRNA, with most being distinct. Conclusion: rNHC showed a dose-dependent inhibition and mutagenic effect of SAR-CoV-2 in vitro. However, rNHC would be expected to be metabolized into the deoxynucleotide pool (by host RNR), resulting in DNA mutation of dividing mammalian cells. We demonstrated such mutagenic potential in a simple mammalian cell detection scheme. Molnupiravir has considerable potential as an orally bioavailable direct acting antiviral against SARS-CoV2 early in infection, especially in high risk patients. However, clinical use should be carefully considered in light of its potential mutagenic effects on the host.