Rhenium(V) Complexes as Cysteine-Targeting Coordinate Covalent Warheads

[Image: see text] Interest in covalent enzyme inhibitors as therapeutic agents has seen a recent resurgence. Covalent enzyme inhibitors typically possess an organic functional group that reacts with a key feature of the target enzyme, often a nucleophilic cysteine residue. Herein, the application of small, modular Re(V) complexes as inorganic cysteine-targeting warheads is described. These metal complexes were found to react with cysteine residues rapidly and selectively. To demonstrate the utility of these Re(V) complexes, their reactivity with SARS-CoV-2-associated cysteine proteases is presented, including the SARS-CoV-2 main protease and papain-like protease and human enzymes cathepsin B and L. As all of these proteins are cysteine proteases, these enzymes were found to be inhibited by the Re(V) complexes through the formation of adducts. These findings suggest that these Re(V) complexes could be used as a new class of warheads for targeting surface accessible cysteine residues in disease-relevant target proteins.