Anti-allergic principles of Rhinacanthus nasutus leaves

Three naphthoquinone derivatives, rhinacanthin-C (1), -D (2) and -N (3) were isolated from the extract of Rhinacanthus nasutus Kurz leaves and were tested for anti-allergic effect. The result indicated that all three compounds possessed very potent anti-allergic activity against antigen-induced β-hexosaminidase release as a marker of degranulation in RBL-2H3 cells with IC50 values of 6.9, 8.9 and 6.4 μM, respectively. In addition, the effects of rhinacanthin-C, -D and -N on antigen-induced release of TNF-α and IL-4 were also examined. It was found that rhinacanthin-C showed the most potent on antigen-induced TNF-α release with an IC50 value of 0.7 μM, followed by rhinacanthin-D (IC50=3.8 μM) and rhinacanthin-N (IC50=10.3 μM), whereas those for IL-4 were rhinacanthin-D (IC50=5.4 μM), rhinacanthin-C (IC50=7.0 μM) and rhinacanthin-N (IC50=12.0 μM), respectively. The mechanisms in the late phase reaction of rhinacanthin-C, -D and -N were found to inhibit TNF-α and IL-4 gene expression in antigen-induced TNF-α and IL-4 releases on from RBL-2H3 cells as dose-dependent manners.The structure-activity trends of rhinacanthin-C,-D and-N on the inhibition of TNF-α release are as follow; substitution with octadienoic acid (rhinacanthin-C) conferred much higher activity than that of benzodioxo carboxylic acid ester (rhinacanthin-D) as well as naphthalene carboxylic acid ester (rhinacanthin-N). For the inhibition of IL-4 release, the substitution with octadienoic acid (rhinacanthin-C) and benzodioxo carboxylic acid ester (rhinacanthin-D) possessed the effect two-fold higher than that of naphthalene carboxylic acid ester (rhinacanthin-N).As regards active constituents for anti-allergic activity of R. nasutus, rhinacanthin-C, -D and -N are responsible for anti-allergic effect of this plant on both the early phase and late phase reactions. The finding may support the traditional use of R. nasutus leaves for treatment of allergy and allergy-related diseases.