Abnormal melanogenesis results in excessive production of
melanin, leading to
pigmentation disorders. As a key and rate-limiting
enzyme for melanogenesis,
tyrosinase has been considered an important target for developing therapeutic agents of pigment disorders. Despite having an (E)-β-phenyl-α,β-unsaturated carbonyl scaffold, which plays an important role in the potent inhibition of
tyrosinase activity, cinnamic
acids have not attracted attention as potential
tyrosinase inhibitors, due to their low
tyrosinase inhibitory activity and relatively high hydrophilicity. Given that cinnamic
acids' structure intrinsically features this (E)-scaffold and following our experience that minute changes in the chemical structure can powerfully affect
tyrosinase activity, twenty less hydrophilic
cinnamamide derivatives were designed as potential
tyrosinase inhibitors and synthesised using a Horner-Wadsworth-Emmons reaction. Four of these cinnmamides (4, 9, 14, and 19) exhibited much stronger mushroom
tyrosinase inhibition (over 90% inhibition) at 25 µM compared to
kojic acid (20.57% inhibition); crucially, all four have a 2,4-dihydroxy group on the β-phenyl ring of the scaffold. A docking simulation using
tyrosinase indicated that the four cinnamamides exceeded the binding affinity of
kojic acid, and bound more strongly to the active site of
tyrosinase. Based on the strength of their
tyrosinase inhibition, these four cinnamamides were further evaluated in B16F10
melanoma cells. All four cinnamamides, without cytotoxicity, exhibited higher
tyrosinase inhibitory activity (67.33 - 79.67% inhibition) at 25 μM than
kojic acid (38.11% inhibition), with the following increasing inhibitory order:
morpholino (9) = cyclopentylamino (14) < cyclohexylamino (19) < N-methylpiperazino (4) cinnamamides. Analysis of
tyrosinase activity and
melanin content in B16F10 cells showed that the four cinnamamides dose-dependently inhibited both cellular
tyrosinase activity and
melanin content and that their inhibitory activity at 25 μM was much better than that of
kojic acid. The results of
melanin content analysis well matched those of the cellular
tyrosinase activity analysis, indicating that
tyrosinase inhibition by the four cinnamamides is a major factor in the reduction of
melanin production. These results imply that these four cinnamamides with a 2,4-dihydroxyphenyl group can act as excellent anti-melanogenic agents in the treatment of
pigmentation disorders.