Targeting protein–protein interactions with reversible covalent modalities: Non‐cysteine chemistries

Protein-protein interactions (PPIs) are central to diverse cellular functions, and represent a rapidly expanding class of therapeutic targets. Advancements in covalent drug design have enabled small-molecule drugs to overcome challenges associated with engaging these targets, such as limited durations of action and difficult-to-drug (expansive, featureless and solvent-exposed) binding sites. Among them, reversible covalent modulators have emerged as a promising approach, by offering a balance of selectivity, sustained engagement, and reduced toxicity. These molecules form transient conjugates with nucleophilic residues at PPI interfaces (and beyond). Cysteine is one such extensively targeted nucleophile in covalent drug discovery and, therefore, a popular amino acid commonly leveraged for reversible covalent drugs. However, because of limitations in its availability and mutational susceptibility, alternative chemistries involving other residues are highly sought. In this review, we discuss emerging reversible electrophile-nucleophile chemistries, with a particular focus on serine and lysine, each of which have the potential to unlock new opportunities for PPI modulations. We also explore specific PPI case studies supporting the repertoire of reversible covalent chemistries beyond cysteine-based strategies.