Pyrazinone Derivatives in Medicinal Chemistry: SAR Insights and Emerging Therapeutic Applications

Pyrazinones remain a comparatively underexplored class of nitrogen heterocycles, despite their recurrence in bioactive natural products and their emergence as a privileged scaffold engaging viral polymerases, kinases, proteases, and G-protein-coupled receptors. Unlike prior reviews, typically confined to a single therapeutic class or to synthetic methodology alone, this review is the first to integrate the natural occurrence, synthetic access, structure-activity relationships, and clinical translatability of pyrazinone derivatives within a single framework, and in doing so identifies structural determinants that recur across nine otherwise unrelated therapeutic areas rather than being specific to any one target. Three such determinants emerge consistently: halogenation at C-3/C-5 improves potency and metabolic stability across kinase, protease, and antifungal series alike; hydrogen-bond donor/acceptor substituents are well tolerated at N-1/C-6 but sharply disruptive when placed adjacent to the lactam at N-3/C-2, reflecting a conserved role for this region in target recognition; and lipophilic extensions at N-1/C-6 raise cellular potency at the recurring cost of selectivity. Inherent limitations are critically examined, including lactam-lactim tautomeric equilibria, challenges in regioselective functionalization, and the trade-off between aqueous solubility and membrane permeability, alongside ADMET considerations such as metabolic stability and off-target toxicity. A central finding of this synthesis is that, across almost every therapeutic class surveyed, reported activity data remain confined to in vitro measurements, without matching pharmacokinetic or translatability data, indicating that the principal bottleneck to clinical progression is evaluative rather than a shortage of potent chemical matter. Clinically relevant examples, including the antiviral drug favipiravir and the clinical-stage candidates RWJ-671818 and BI 730357, nonetheless demonstrate that this bottleneck is surmountable and illustrate the scaffold's translational potential. Recent advances, emerging resistance mechanisms, and future directions, including multi-target-directed ligand design, structure-based optimization, and AI-assisted drug design, are also discussed. By consolidating this fragmented literature into a single cross-scaffold structure-activity framework and by pinpointing where pharmacokinetic and toxicological characterization, rather than potency optimization, most limits clinical translation, this review provides a practical, decisionoriented resource for researchers designing the next generation of pyrazinone-based therapeutics.