In our previous study, we documented the anti-melanogenic efficacy of calebin-A (CBA), which is a curcuminoid analog. The effects of its newly synthesized analogs, i.e., bisdemethoxy calebin (BD), demethoxycalebin-1 (DA1), demethoxycalebin-2 (DA2), and tetrahydrocalebin-A (THCBA), on melanogenesis have not been examined yet. Herein, we evaluated these four CBA analogs to determine their impacts on the enzymatic activity of mushroom tyrosinase. Additionally, we examined their effects on melanogenesis and the tyrosinase activity in B16F10 mouse and MNT-1 human melanoma cells. The antioxidant activity of the analogs was also assessed. Our results revealed that BD was ineffective, while DA1 and DA2 showed similar antioxidant activities, with THCBA exhibiting the greatest antioxidant activity. Next, the diphenolase activity assay results revealed that DA1 showed the most excellent inhibitory efficacy, DA2 and BD showed similar inhibition profiles, and THCBA was ineffective. In addition, the results of the monophenolase activity showed a similar pattern, except that THCBA suppressed the activity. The four analogs were evaluated for any cytotoxicity over a 48 h duration in B16F10 and HaCaT keratinocytes, where DA1 exerted cytotoxicity at the concentration of 50 µM. Based on this, the analogs were evaluated over a 10–50 µM concentration range, while DA1 was evaluated over 10–35 µM. BD showed the greatest efficacy at multiple concentrations in significantly diminishing melanogenesis in hormone-stimulated B16F10 cells, while DA1 and DA2 suppressed melanin at 35 and 50 µM, respectively, and THCBA stimulated melanogenesis at 50 µM. In addition, BD and DA1 suppressed tyrosinase activity in B16F10 cells, with no effect in the case of DA2 and THCBA analogs. However, in MNT-1 cells, only DA1 showed efficacy in suppressing melanin production while the other three analogs were ineffective. Interestingly, BD and DA1 suppressed MNT-1 cell tyrosinase activity. Collectively, our results indicate that of the four analogs, DA1 merits further investigation as a potential compound for hyperpigmentation skin disorders. Additional research is necessary to delineate the molecular mechanisms underlying the melanogenesis-inhibitory effect of CBA analogs and examine their efficacy in diminishing melanogenesis in normal human melanocytes.
