Quinoline-Pyrimidine Hybrids as Multifunctional Drug Scaffolds: Structure-Activity Relationships, Target Validation, and Translational Challenges (A Review)

Molecular hybridization has become an important strategy in contemporary drug discovery for addressing challenges such as drug resistance, limited selectivity, and suboptimal pharmacokinetic performance. Among privileged heterocyclic scaffolds, quinoline and pyrimidine have attracted considerable interest owing to their presence in numerous bioactive molecules and approved therapeutics. The integration of these two pharmacophores combines the π-interaction capabilities of quinoline with the hydrogen-bonding potential of pyrimidine, generating hybrid frameworks with broad therapeutic applicability. This review provides a critical assessment of quinoline-pyrimidine hybrids reported across anticancer, antimalarial, antimicrobial, antiviral, and metabolic disease indications. Emphasis is placed on structure-activity relationships (SAR), molecular target engagement, and the level of experimental validation supporting proposed mechanisms of action. Mechanistically validated targets, including heme detoxification pathways, phosphodiesterase-5, P-glycoprotein, chitin synthase, and HIV-1 reverse transcriptase, are distinguished from targets supported primarily by computational studies. Comparative analysis identifies key structural determinants governing biological activity, including scaffold planarity, linker architecture, lipophilicity, and the hydrogen-bonding characteristics of the pyrimidine ring. Cross-therapeutic SAR trends are discussed to establish general design principles for future lead optimization. In addition, available data on drug-likeness, ADMET properties, and chemical space distribution are evaluated to assess scaffold developability and translational potential. Although no quinoline-pyrimidine hybrid has yet advanced to clinical application, growing mechanistic evidence, improved target validation, and advances in rational design support their continued development as versatile scaffolds for next-generation therapeutics.