Sara Cherry of the Perelman School of Medicine spoke about the arduous process of developing antiviral drugs to treat COVID-19. “Most of the compounds that work in cells ultimately fail in animal studies for lots of reasons,” she said. Of the compounds that remain, just a few make it to the animal testing stage.
https://www.scientificamerican.com/article/there-are-few-good-covid-antivirals-but-that-could-be-changing/ Scientific American