Covalent drugs reimagined: from cysteine-targeted inhibitors to proteome-informed precision therapeutics

Covalent molecular design has undergone a profound transformation from a historically empirical and risk-associated strategy into a quantitatively guided and proteome-informed therapeutic approach. Modern targeted covalent inhibitors (TCIs) integrate high-affinity non-covalent recognition with finely tuned electrophilic warheads, where potency and selectivity are governed by both reversible binding (Kᵢ) and covalent reaction kinetics (k_inact). The composite parameter k_inact/Kᵢ provides a unifying framework for optimizing covalent efficiency within biologically relevant constraints. This review presents a comprehensive medicinal chemistry perspective that integrates structure-activity relationships (SAR), structure-property relationships (SPR), and kinetic behavior across diverse warhead classes, including acrylamides, sulfur(VI) fluorides, nitriles, boronates, and reversible covalent electrophiles. Particular emphasis is placed on how electronic, steric, and environmental factors govern context-dependent reactivity and proteome-wide selectivity. Expanding beyond cysteine, we discuss emerging strategies for targeting lysine, tyrosine, histidine, and methionine within a microenvironment-driven framework of ligandability. We further highlight the central role of chemoproteomics-especially activity-based protein profiling (ABPP)-as a quantitative engine that enables direct measurement of target engagement, residue mapping, and iterative optimization through engagement-driven SAR. The integration of ligand-first and electrophile-first approaches is presented as a unified discovery strategy that expands access to cryptic and previously undruggable sites. Finally, we examine the extension of covalent chemistry into targeted protein degradation and event-driven pharmacology, alongside current limitations in predictive modeling and kinetic translation. Collectively, covalent drug discovery is reframed as a precision discipline grounded in mechanistic insight, quantitative design, and proteome-scale validation.