Nitric Oxide Production‐Inhibitory Activity of Limonoids from Azadirachta indica and Melia azedarach

Seventy‐three limonoids isolated from three Meliaceae plants, Azadirachta indica, A. indica var. siamensis, and Melia azedarach, or semi‐synthesized from the Meliaceae limonoids, were evaluated for their inhibitory activity against nitric oxide (NO) production in mouse macrophage RAW 264.7 cells induced by lipopolysaccharide (LPS), as a primary screening test for anti‐inflammatory agents. Of the compounds tested, 21 compounds exhibited inhibitory activity (IC50 4.6 – 58.6 μm) without any significant toxicity (IC50 > 100 μm) which were more potent than l‐NMMA (NO‐production inhibitory activity, IC50 65.6 μm; cytotoxicity, IC50 > 100 μm), and among which, nine compounds, i.e., 17‐hydroxy‐15‐methoxynimbocinol (6), ohchinin (20), 1‐cis‐cinnamoyl‐1‐decinnamoylohchinin (24), salannin (27), methyl nimbidate (32), isosalannin (55), nimbolinin D (58), mesendanin E (69), and 7‐deacetylgedunin (73) exhibited potent inhibitory activity (IC50 4.6 – 29.3 μm). In particular, compounds 6 (IC50 7.3 μm), an azadirone‐type limonoid, and 73 (IC50 4.6 μm), a gedunin‐type limonoid, exhibited remarkable activity. Western blot analysis revealed that 27 and 73 reduced the expression levels of the inducible NO synthase and cyclooxygenase‐2 proteins in a concentration‐dependent manner. These findings suggest that limonoids of A. indica, A. indica var. siamensis, and M. azedarach, and their semi‐synthetic derivatives may be effective against inflammation.