As part of our continuous search for novel
tyrosinase inhibitors, we designed 5,6-dihydroimindazo[2,1-b]thiazol-3(2H)-one (DHIT) derivatives based on the structure of MHY773; a potent
tyrosinase inhibitor with a
2-iminothiazolidin-4-one template. Of the 11 DHIT derivatives synthesized using a Knoevenagel condensation, three DHIT derivatives 1a (IC50 = 36.14 ± 3.90 μM), 1b (IC50 = 0.88 ± 0.91 μM), and 1f (IC50 = 17.10 ± 1.01 μM) inhibited mushroom
tyrosinase more than
kojic acid (IC50 = 84.41 ± 2.87 μM). Notably, compound 1b inhibited mushroom
tyrosinase around 100- and 3.3-fold more potently than
kojic acid and MHY773, respectively. Lineweaver-Burk plots demonstrated that compounds 1b and 1f competitively inhibited mushroom
tyrosinase, and in silico docking results supported our kinetic results and indicated that these two compounds bind more strongly to the active site of
tyrosinase than
kojic acid. Docking simulation results using a human
tyrosinase homology model confirmed the abilities of 1b and 1f to strongly inhibit human
tyrosinase. B16F10 murine
melanoma cells were used to investigate whether these two compounds display
tyrosinase inhibitory activities and anti-melanogenesis effects in cells. Both compounds were found to significantly and dose-dependently inhibit cellular
tyrosinase activity and intracellular and extracellular
melanin production more potently than
kojic acid. The similarities observed between the cellular
tyrosinase and melanogenesis inhibitory effects of 1b and 1f suggest their observed anti-melanogenic effects were due to
tyrosinase inhibition. These results indicate that compounds 1b and 1f, which possess the DHIT template, are promising candidates as anti-browning agents and therapeutic agents for
hyperpigmentation disorders.