New Hybrid Benzothiazole Derivatives from Gallic and Syringic Acid as a Potential Multifunctional Skin Disease

Multifunctional drugs represent an emerging strategy for treating complex skin disorders and melanoma. A series of benzothiazole-based hybrids incorporating gallic and syringic acid moieties was synthesized and evaluated as multifunctional agents for skin-related applications. Six hydrazone (GAHYDR1–3) and acyl-hydrazone (GACIN1–3) derivatives were obtained and fully characterized. Hydroxylated compounds showed the strongest antioxidant activity, with GAHYDR1 and GACIN1 displaying low DPPH IC50 values and high FRAP reducing power. UV–Vis studies revealed strong UVA–UVB absorption, with molar extinction coefficients comparable to or exceeding those of PBSA. Photoprotective evaluation showed SPF values up to 10.09 (GACIN2) and broad-spectrum behavior for selected derivatives. Antioxidant activity remained substantially stable over 3 months in solution. Antiproliferative assays against Colo38, A375, and HaCaT cell lines indicated generally low cytotoxicity toward non-tumor cells. Notably, GAHYDR3 exhibited selective activity against A375 melanoma cells (IC50 = 8.75 µM; SI = 8.12). Overall, phenolic substitution emerged as a key determinant of biological activity, highlighting hydroxylated benzothiazole hybrids as promising antioxidant and photoprotective agents, with GAHYDR3 representing a potential lead for anti-melanoma development.