Antiviral effects of traditional herbal medicine Otsu-Ji-To against human coronavirus OC43, SARS-CoV-2, and its variants: in vitro and in vivo studies

Background and aim Otsu-Ji-To (OJT), also known as Eul-ja-tang in Korean, is a traditional herbal formula used to treat hemorrhoids. We investigated its antiviral effects against HCoV-OC43, SARS-CoV-2, and its variants. Experimental procedure Antiviral activity was determined using qRT-PCR, western blot analysis, immunofluorescent staining assay, plaque reduction assay, or CPE reduction assay. UHPLC-QTOF MS/MS and UHPLC-MRM analyses were used to identify and quantify the OJT’s chemical markers. To determine the mode of action, SARS-CoV-2 Mpro and PLpro activity assay kits and a cell-based RdRp activity assay system were used. SARS-CoV-2– or Omicron variant–infected K18-hACE2 transgenic mice and hamsters in vivo experiments were conducted. Results and conclusions We found that OJT inhibited HCoV-OC43 (IC 50 = 674.9 μg/mL), SARS-CoV-2 (IC 50 = 236.5 μg/mL), and its variants by reducing viral RNA and protein expression levels as well as attenuating cytopathic effects in vitro . We identified and quantified seven compounds from OJT’s herbal components. Baicalein, liquiritigenin, emodin, and rhein exhibited anti-SARS-CoV-2 activity by lowering viral protein expression levels. Furthermore, OJT inhibited the SARS-CoV-2 viral Mpro and PLpro in enzymatic activity assays as well as RNA-dependent RNA polymerase in a cell-based assay. In vivo , oral administration of OJT to wild-type SARS-CoV-2– or Omicron variant–infected K18-hACE2 transgenic mice and hamsters increased survival rates, reduced viral RNA and protein levels, or improved lung histology, suggesting OJT’s therapeutic potential for treating COVID-19. These results suggest that OJT could be a promising therapeutic agent for treating coronavirus infections.