Exploration of 1,2,3-triazolo fused triterpenoids as inhibitors of human coronavirus 229E targeting the viral nsp15 protein.

The coronavirus disease-19 (COVID-19) pandemic has raised major interest in innovative drug concepts to suppress human coronavirus (HCoV) infections. We previously reported on a class of 1,2,3-triazolo fused betulonic acid derivatives causing strong inhibition of HCoV-229E replication via the viral nsp15 protein, which is proposedly related to compound binding at an intermonomer interface in hexameric nsp15. In the present study, we further explored the structure-activity relationship (SAR), by varying the substituent at the 1,2,3-triazolo ring as well as the triterpenoid skeleton. The 1,2,3-triazolo fused triterpenoids were synthesized by a multicomponent triazolization reaction, which has been developed in-house. Several analogs possessing a betulin, oleanolic acid, or ursolic acid core displayed favorable activity and selectivity (EC50 values for HCoV-229E: 1.6-3.5 µM), but neither of them proved as effective as the lead compound containing betulonic acid. The 18ß-glycyrrhetinic acid-containing analogs had low selectivity. The antiviral findings were rationalized by in silico docking in the available structure of the HCoV-229E nsp15 protein. The new SAR insights will aid the further development of these 1,2,3-triazolo fused triterpenoid compounds as a unique type of coronavirus inhibitors.