Extraction, Phytochemical Profiling, and Computational Evaluation of Corymbia citriodora Essential Oil as a COX-2 Inhibitor: Steam vs. Microwave-Assisted Distillation, GC–MS/MS, Docking, DFT, and MD Simulations

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Background/Objectives: Chronic inflammation sustained by cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) underlies many human diseases, while the cardiovascular liabilities of coxibs have renewed interest in plant-derived alternatives. This study characterised the volatile composition of Corymbia citriodora essential oil under two distillation regimes and identified constituents able to inhibit human COX-2 and iNOS. Methods: Oils obtained by steam distillation (SD) and microwave-assisted steam distillation (MSD) were profiled by GC–MS/MS. All identified constituents were screened with SASA Vina against human COX-2 (PDB: 5KIR, 3LN1), ovine COX-1 (4COX) and human iNOS (3E7G), with native-ligand re-docking validating each protocol (RMSD 0.39–0.84 Å). The lead compound underwent density functional theory (B3LYP/3-21G/CPCM), 100 ns all-atom molecular dynamics, MM/GBSA and MM/PBSA decomposition, and pkCSM ADMET prediction. Results: Sixty-three constituents were identified (99.85% SD; 99.97% MSD). MSD enriched oxygenated monoterpenes (93.23% versus 88.27%), citronellal rose from 38.24% to 48.41%. (Z,Z,Z)-1,5,9,9-Tetramethyl-1,4,7-cycloundecatriene ranked highest at COX-2 (−8.0 to −8.2 kcal/mol) and also bound COX-1 (−8.8 kcal/mol) and iNOS (−6.8 kcal/mol). DFT indicated high kinetic stability (ΔE = 6.12 eV; η = 3.06 eV) and purely non-covalent hydrophobic binding. The COX-2 complex remained stable over 100 ns (Cα RMSD 0.17 ± 0.02 nm), with MM/GBSA and MM/PBSA binding energies of −23.98 and −21.95 kcal/mol and Val523 as the principal hotspot. ADMET prediction returned 96.61% human intestinal absorption and no mutagenicity, hepatotoxicity or hERG I blockade. Conclusions: MSD offers a faster route to a citronellal-enriched oil, and C. citriodora hydrocarbon sesquiterpenes emerge as chemically stable, multi-target COX-2/iNOS scaffolds. Comparable binding at COX-1 indicates that isoform selectivity remains to be established; in vitro enzymatic and cell-based validation is therefore required.

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