Melanogenesis-inhibitory activities of limonoids and tricyclic diterpenoids from Azadirachta indica.

The chemical constituents of the roots and bark of Azadirachta indica were investigated, leading to the isolation of six tricyclic diterpenoids and four limonoids including a new compound, azadirachtin J (4). The structures were elucidated on the basis of NMR spectroscopic techniques, mass spectrometry as well as comparison with the literature. Furthermore, melanogenesis-inhibitory activities of the isolated compounds were evaluated. As a result, compounds 1-3 and 10 exhibited superior inhibitory activities against melanogenesis with no, or almost no, toxicity to the cells (86.5-105.1% cell viability). Western blot analysis showed that compounds 1 and 3 exhibited melanogenesis inhibitory activities in α-MSH-stimulated B16 melanoma cells due to, at least in part, inhibition of the expression of MITF, followed by a decrease in the expression of tyrosinase, TRP-1, and TRP-2. Compounds 1 and 3 exhibited tyrosinase inhibitory activities (IC50 values of 44.86 μM and 69.85 μM respectively). Docking results confirm that the active inhibitors strongly interact with tyrosinase residues.