Discovery and structural optimization of 3-O-ß-Chacotriosyl betulonic acid saponins as potent fusion inhibitors of Omicron virus infections.

The recent global Omicron epidemics underscore the great need for the development of small molecule therapeutics with appropriate mechanisms. The trimeric spike protein (S) of SARS-CoV-2 plays a pivotal role in mediating viral entry into host cells. We continued our efforts to develop small-molecule SARS-CoV-2 entry inhibitors. In this work, two sets of BA derivatives were designed and synthesized based on the hit BA-1 that was identified as a novel SARS-CoV-2 entry inhibitor. Compound BA-4, the most potent one, showed broad inhibitory activities against pOmicron and other pseudotyped variants with EC50 values ranging 2.73 to 5.19 µM. Moreover, pSARS-CoV-2 assay, SPR analysis, Co-IP assay and the cell-cell fusion assay coupled with docking and mutagenesis studies revealed that BA-4 could stabilize S in the pre-fusion step to interfere with the membrane fusion, thereby displaying promising inhibition against Omicron entry.