ROLE OF HETEROCYCLIC SCAFFOLDS IN MODERN DRUG DESIGN: STRUCTURE–ACTIVITY RELATIONSHIPS, THERAPEUTIC APPLICATIONS AND EMERGING COMPUTATIONAL APPROACHES

Heterocyclic scaffolds are central structural elements in medicinal chemistry because incorporationofnitrogen,oxygen orsulfurintoaringcan altermolecularshape, electronicdistribution,ionisation,hydrogen-bondingcapacity,lipophilicityand metabolicbehaviour.Thesefeaturesallowmedicinalchemiststotunetarget recognitionanddrug-likepropertieswhileretainingacompactandsynthetically modifiable framework. Nitrogen-containing heterocycles are particularly prominent in approvedsmall-moleculemedicines;aninfluentialanalysisofUSFDA-approved pharmaceuticalsreportedthat59% ofuniquesmall-moleculedrugsinitsdataset contained at least one nitrogen heterocycle [1]. A later analysis of 321 unique smallmolecule drugs approved during 2013–2023 reported that 82% contained at least one nitrogen heterocycle[12].These valuesarisefrom differentapprovalperiodsand datasetsandshouldnotbeinterpretedasauniversalordirectlycomparable prevalenceestimate.Thepresentnarrativereviewdiscussesthecontributionof heterocyclic scaffolds to modern drug design, with emphasis on scaffold selection, structure–activityrelationships(SAR),representativetherapeuticapplications, physicochemicaloptimisationandcomputationalapproaches.Five-andsixmemberedmonocyclicrings,fusedsystemsandsaturatedheterocyclesare considered,includingimidazole,triazole,pyridine,pyrimidine,indole,quinoline, piperidineandmorpholinemotifs.Selectedmarketeddrugsillustratehow heterocycles can act as recognition elements, tune basicity and solubility, or serve as bioisostericreplacements.Thereview alsoexaminestheintegrationofmolecular docking,ligand-basedmodelling,virtualscreeningandmachine-learningmethods intoheterocycle-focuseddiscovery.Importantly,aheterocyclicringdoesnot independentlyguaranteepotency,selectivityorfavourablepharmacokinetics;its value depends on substitution pattern, three-dimensional context, target biology and whole-moleculeproperties.Currentchallengesincludepromiscuousbinding, metabolic liabilities, solubility limitations, synthetic accessibility, selectivity and overreliance on computational scores. A rational workflow combining medicinal-chemistry design,experimentaltestinganditerativeoptimisationisthereforeessentialfor translating heterocyclic hits into safe and effective drug candidates.