Design of next-generation covalent inhibitors: Targeting residues beyond cysteine

Covalent protein inhibitors have found considerable utility in medicinal chemistry and chemical biology. Chemical probes and drugs have been rationally designed and fine-tuned to engage cysteine residues predominantly using electrophilic acrylamide warheads that deliver inhibitors with improved pharmacological efficacy, duration and selectivity over reversible binders. However, cysteine is rarely present in protein binding sites and so there is a need to advance inhibitors that engage alternative amino acid residues. This review describes recent progress in the development of covalent small molecules that target residues beyond cysteine. Numerous electrophiles have been incorporated into inhibitors that selectively label various reactive amino acid side chains including lysine, tyrosine, histidine, serine, threonine, glutamate, aspartate and methionine. Several synthetic strategies have facilitated these advances, particularly in the area of sulfonyl-exchange chemistry, though other emerging approaches are presented. We hope the review will inspire future research into next-generation covalent protein inhibitors that positively effect therapeutic discovery and drive fundamental biological insights through novel chemical probe development.