Exploration of Compounds with 2-Phenylbenzo[d]oxazole Scaffold as Potential Skin-Lightening Agents through Inhibition of Melanin Biosynthesis and Tyrosinase Activity

Inspired by the potent tyrosinase inhibitory activity of phenolic compounds with a 2-phenylbenzo[<i>d</i>]thiazole scaffold, we explored phenolic compounds <b>1</b>-<b>15</b> with 2-phenylbenzo[<i>d</i>]oxazole, which is isosterically related to 2-phenylbenzo[<i>d</i>]thiazole, as novel tyrosinase inhibitors. Among these, compounds <b>3</b>, <b>8</b>, and <b>13</b>, featuring a resorcinol structure, exhibited significantly stronger mushroom tyrosinase inhibition than kojic acid, with compound <b>3</b> showing a nanomolar IC₅₀ value of 0.51 μM. These results suggest that resorcinol plays an important role in tyrosinase inhibition. Kinetic studies using Lineweaver-Burk plots demonstrated the inhibition mechanisms of compounds <b>3</b>, <b>8</b>, and <b>13</b>, while docking simulation results indicated that the resorcinol structure contributed to tyrosinase binding through hydrophobic and hydrogen bonding interactions. Additionally, these compounds effectively inhibited tyrosinase activity and melanin production in B16F10 cells and inhibited B16F10 tyrosinase activity in situ in a concentration-dependent manner. As these compounds showed no cytotoxicity to epidermal cells, melanocytes, or keratinocytes, they are appropriate for skin applications. Compounds <b>8</b> and <b>13</b> demonstrated substantially higher depigmentation effects on zebrafish larvae than kojic acid, even at 800- and 400-times lower concentrations than kojic acid, respectively. These findings suggest that 2-phenylbenzo[<i>d</i>]oxazole is a promising candidate for tyrosinase inhibition.