The cytotoxicity of cysteinylcatechols and related compounds to human melanoma cells in vitro.

L-3,4-Dihydroxyphenylalanine (L-dopa) and its structural analogs are known to be potently cytotoxic to melanoma cells. We examined the effects of cysteinylcatechols and related compounds, which were newly synthesized as cysteinyl derivatives of L-dopa, on the growth of human melanoma cells in vitro, and their actions were compared with those of L-dopa. 4-S- and 3-S-Cysteinylcatechols showed significantly more potent cytotoxicity to melanoma cells than did L-dopa, and 2-S-cysteinylhydroquinone was next to the catechols in potency. The mechanism of action may involve interaction with the melanocyte-specific enzyme, tyrosinase, for which the cysteinylcatechols could become a better substrate than L-dopa itself. 4-S-Cysteaminylphenol was almost comparable to L-dopa in cytotoxicity, suggesting that this phenol might be oxidized to the corresponding catechol by tyrosinase within the melanoma cells.