| Summary: | (<i>Z</i>)-5-Benzylidene-2-phenylthiazol-4(5<i>H</i>)-one ((<i>Z</i>)-BPT) derivatives were designed by combining the structural characteristics of two tyrosinase inhibitors. The double-bond geometry of trisubstituted alkenes, (<i>Z</i>)-BPTs <b>1</b>–<b>14</b>, was determined based on the <sup>3</sup><i>J</i><sub>C,Hβ</sub> coupling constant of <sup>1</sup>H-coupled <sup>13</sup>C NMR spectra. Three (<i>Z</i>)-BPT derivatives (<b>1</b>–<b>3</b>) showed stronger tyrosinase inhibitory activities than kojic acid; in particular, <b>2</b> was to be 189-fold more potent than kojic acid. Kinetic analysis using mushroom tyrosinase indicated that <b>1</b> and <b>2</b> were competitive inhibitors, whereas <b>3</b> was a mixed-type inhibitor. The in silico results revealed that <b>1</b>–<b>3</b> could strongly bind to the active sites of mushroom and human tyrosinases, supporting the kinetic results. Derivatives <b>1</b> and <b>2</b> decreased the intracellular melanin contents in a concentration-dependent manner in B16F10 cells, and their anti-melanogenic efficacy exceeded that of kojic acid. The anti-tyrosinase activity of <b>1</b> and <b>2</b> in B16F10 cells was similar to their anti-melanogenic effects, suggesting that their anti-melanogenic effects were primarily owing to their anti-tyrosinase activity. Western blotting of B16F10 cells revealed that the derivatives <b>1</b> and <b>2</b> inhibited tyrosinase expression, which partially contributes to their anti-melanogenic ability. Several derivatives, including <b>2</b> and <b>3</b>, exhibited potent antioxidant activities against ABTS cation radicals, DPPH radicals, ROS, and peroxynitrite. These results suggest that (<i>Z</i>)-BPT derivatives <b>1</b> and <b>2</b> have promising potential as novel anti-melanogenic agents.
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