Remdesivir impurity 9-d4 is deuterium labeled Remdesivir. Remdesivir (GS-5734), a nucleoside analogue with effective antiviral activity, has EC50s of 74 nM for SARS-CoV and MERS-CoV in HAE cells, and 30 nM for murine hepatitis virus in delayed brain tumor cells. Remdesivir is highly effective in the control of SARS-CoV-2 (COVID-19) infection in vitro[1][2].