The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has resulted in millions of deaths and threatens public health and safety. Despite the rapid global spread of COVID-19 vaccines, effective oral antiviral drugs are urgently needed. Here, we describe the discovery of S-217622, the first oral noncovalent, nonpeptidic SARS-CoV-2 3CL protease inhibitor clinical candidate. S-217622 was discovered via virtual screening followed by biological screening of an in-house compound library, and optimization of the hit compound using a structure-based drug design strategy. S-217622 exhibited antiviral activity in vitro against current outbreaking SARS-CoV-2 variants and showed favorable pharmacokinetic profiles in vivo for once-daily oral dosing. Furthermore, S-217622 dose-dependently inhibited intrapulmonary replication of SARS-CoV-2 in mice, indicating that this novel noncovalent inhibitor could be a potential oral agent for treating COVID-19.

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has resulted in millions of deaths and threatens public health and safety. Despite the rapid global spread of COVID-19 vaccines, effective oral antiviral drugs are urgently needed. Here, we describe the discovery of S-217622, the first oral non-covalent, non-peptidic SARS-CoV-2 3CL protease inhibitor clinical candidate. S-217622 was discovered via virtual screening followed by biological screening of an in-house compound library, and optimization of the hit compound using a structure-based drug-design strategy. S-217622 exhibited antiviral activity in vitro against current outbreaking SARS-CoV-2 variants and showed favorable pharmacokinetic profiles in vivo for once-daily oral dosing. Furthermore, S-217622 dose-dependently inhibited intrapulmonary replication of SARS-CoV-2 in mice, indicating that this novel non-covalent inhibitor could be a potential oral agent for treating COVID-19.

The 2′-5′-oligoadenylate synthetase (OAS)/RNase L pathway is an innate immune system that protects hosts against pathogenic viruses and bacteria through cleavage of exogenous single-stranded RNA; however, this system's selective targeting mechanism remains unclear. Here, we identified an mRNA quality control factor Dom34 as a novel restriction factor for a positive-sense single-stranded RNA virus. Downregulation of Dom34 and RNase L increases viral replication, as well as half-life of the viral RNA. Dom34 directly binds RNase L to form a surveillance complex to recognize and eliminate the exogenous RNA in a manner dependent on translation. Interestingly, the feature detected by the surveillance complex is not the specific sequence of the viral RNA but the ‘exogenous nature’ of the RNA. We propose the following model for the selective targeting of exogenous RNA; OAS3 activated by the exogenous RNA releases 2′-5′-oligoadenylates (2–5A), which in turn converts latent RNase L to an active dimer. This accelerates formation of the Dom34-RNase L surveillance complex, and its selective localization to the ribosome on the exogenous RNA, thereby promoting degradation of the RNA. Our findings reveal that the selective targeting of exogenous RNA in antiviral defense occurs via a mechanism similar to that in the degradation of aberrant transcripts in RNA quality control.