Diastereomeric Resolution Yields Highly Potent Inhibitor of SARS-CoV-2 Main Protease.

SARS-CoV-2 is the causative agent behind the COVID-19 pandemic. The main protease (M pro , 3CL pro ) of SARS-CoV-2 is a key enzyme that processes polyproteins translated from the viral RNA. M pro is therefore an attractive target for the design of inhibitors that block viral replication. We report the diastereomeric resolution of the previously designed SARS-CoV-2 M pro α-ketoamide inhibitor 13b . The pure ( S,S,S )-diastereomer, 13b-K , displays an IC 50 of 120 nM against the M pro and EC 50 values of 0.8-3.4 μM for antiviral activity in different cell types. Crystal structures have been elucidated for the M pro complexes with each of the major diastereomers, the active ( S,S,S )-13b ( 13b-K ), and the nearly inactive ( R,S,S )-13b ( 13b-H ); results for the latter reveal a novel binding mode. Pharmacokinetic studies show good levels of 13b-K after inhalative as well as after peroral administration. The active inhibitor ( 13b-K ) is a promising candidate for further development as an antiviral treatment for COVID-19.