“…Indeed Cys145 of M pro reacts reversibly with the nitrile group of the clinically-used drugs 1 and 2. 14,15,42 Many investigational M pro inhibitors, however, employ highly reactive electrophiles for covalent reaction with Cys145, including e.g., aldehydes, a-ketoamides, and Michael acceptors, 19,43,44 which may potentially compromise safety, as reported for some clinically-used small-molecules bearing reactive electrophiles; 26,45 The use of electrophilic groups with relatively low intrinsic reactivity is thus desirable. The observation that the glactam of both 1 and 2 is stable in cells 14,15 likely reects its reduced reactivity compared to more reactive electrophiles, indicating that covalently reacting g-lactams may have potential for development of safe COVID-19 therapeutics.…”