Structural modification of (‒)-xanthorrhizol isolated from Curcuma aromatica rhizomes to analogs with potent nitric oxide inhibitory activities.

(‒)-Xanthorrhizol (1), a natural sesquiterpenoid isolated from Curcuma aromatica rhizomes, exhibits notable anti-inflammatory activity by inhibiting nitric oxide (NO) production. To enhance its NO inhibitory activity, a series of 22 analogs were synthesized through chemical modifications, including alkylation, esterification, hydrogenation, oxidation, and oximination. The NO inhibitory activities of these analogs were evaluated in LPS-stimulated RAW264.7 macrophage cells using the Griess method, while cytotoxicity was assessed with the MTT assay. Among the tested compounds, the oxime analog 23 demonstrated the strongest NO inhibitory activity (IC50 = 9.4 µM, SI = 13.2), significantly outperforming the reference drug aminoguanidine (IC50 = 29.5 µM, SI > 5.4). Structure-activity relationship (SAR) analysis revealed that electron-donating substituents and increased molecular rigidity generally decreased NO inhibition potency. However, some modifications also increased cytotoxicity against RAW264.7 cells. In addition, the oximation of xanthorrhizol enhanced NO inhibitory activity approximately 5-fold compared with the parent compound, while maintaining similarly low toxicity. These SAR findings underscore the critical influence of the oxime substructure in modulating NO inhibitory activity. Notably, the oxime analog 23 emerges as a promising lead candidate for the development of anti-inflammatory agents targeting NO-mediated pathways.