Expanding the landscape of covalent drug discovery: irreversible targeting of non-cysteine residues.

Abstract

Covalent inhibitors have re-emerged as a powerful class of therapeutics due to their prolonged target engagement, ability to decouple pharmacokinetic availability from pharmacodynamic outcome, potential for high selectivity and ability to modulate traditionally 'undruggable' targets. Historically, cysteine residues have dominated covalent drug discovery owing to their unique nucleophilicity and relative scarcity in the proteome. However, the desire to broaden the chemical scope of covalent therapeutics has driven a surge of interest in irreversibly targeting other amino acid residues, such as lysine, serine, tyrosine, threonine and histidine. This review explores the outstanding questions and challenges in developing covalent inhibitors beyond cysteine, highlighting current warhead chemistries, strategies for achieving selectivity, proteomic mapping advances, assessment of the intrinsic reactivity of the electrophiles targeting non-cysteine residues and opportunities for expanding the druggable proteome. We also discuss future directions and the pharmacological implications of non-cysteine covalent modifications in therapeutic contexts.