Antiallergic Properties of Biflavonoids Isolated from the Flowers of <em>Mesua ferrea</em> Linn.

The methanolic extract from the flowers of Mesua ferrea Linn. (Calophyllaceae) showed significant hyaluronidase inhibitory activity. Following a bioassay-guided separation of the extract, two biflavonoids, viz., mesuaferrone-A (1) and mesuaferrone-B (2), were isolated, along with ten flavonoids (3–12), two xanthones (13 and 14), three triterpenes (15–17), a phenylpropanoid (18), and five aromatics (19–24). Among the isolates, 1 and 2 (IC50 = 51.1 µM and 54.7 µM, respectively) exhibited hyaluronidase inhibitory activity equivalent to that of the commercially available antiallergic agents disodium cromoglycate (64.8 μM) and ketotifen fumarate (76.5 μM). These biflavonoids (1 and 2) are 8-8″ linked dimers that are composed of naringenin (1a) or apigenin (3), with their corresponding monomers lacking inhibitory activity (IC50 > 300 μM). In addition, 1 and 2 (IC50 = 49.4 µM and 49.2 µM, respectively) inhibited the release of β-hexosaminidase, which is a marker of antigen-IgE-mediated degranulation, in rat basophilic leukemia (RBL-2H3) cells. These inhibitory activities were more potent than those of the antiallergic agents tranilast and ketotifen fumarate (IC50 = 282 μM and 158 μM, respectively), as well as one of the corresponding monomers (1a; IC50 > 100 μM). Nonetheless, these effects were weaker than those of the other monomer (3; IC50 = 6.1 μM)