In addition to the field of antibacterial drugs, targeting novel sites in the design of antiviral drugs is also a prevalent strategy employed against drug resistance. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is capable of inducing respiratory infections and, in severe cases, fatality. The emergence of multiple mutated variants such as Omicron and Delta during the coronavirus disease 2019 pandemic has prompted scientists to explore novel treatments to combat drug resistance in these mutants. Presently, the Food and Drug Administration has approved three antiviral drugs encompassing an RNA-dependent RNA polymerase inhibitor and a main protease (M
pro) inhibitor; however, resistant strains against both classes of drugs have been identified. In order to discover new mechanisms for antiviral therapy, papain-like protease (PL
pro) has been investigated as a potential target. Building upon the reported non-covalent PL
pro inhibitor XR8–24 and covalent PL
pro inhibitor Cp7, the design of a covalent PL
pro inhibitor, named Jun11313, has been accomplished. The co-crystal structure revealed that the thiophene moiety of Jun11313 occupied the same hydrophobic pocket as ubiquitin Val70 (Val70
Ub). Based on this observation, the structure of Jun11313 has been optimized, resulting in Jun12682 that targets both Val70
Ub and the BL2 groove (
Fig. 1B), exhibited inhibitory activity against Omicron, Delta variants as well as three nirmatrelvir-resistant virus strains [
2].