3CLpro-1 is an antiviral drug related to rupintrivir which acts as a 3CL protease inhibitor and was originally developed for the treatment of human enterovirus 71. It is one of the most potent of a large series of compounds developed as inhibitors of the viral enzyme 3CL protease, with an in vitro IC<sub>50</sub> of 200 nM. It also shows activity against coronavirus diseases such as SARS and MERS, and is under investigation as a potential treatment agent for the viral disease COVID-19.