Structure-Activity Relationships Defining the Cytotoxicity of Catechol Analogues against Human Malignant Melanoma¹

The cytotoxic activities of three new synthetic catechol analogues, β-[(p-hydroxyphenyl)amino]alanine (Compound 1), N^b-(p-hydroxyphenyl)ornithine (Compound 2), and N^b-(m-hydroxyphenyl)ornithine (Compound 3), were determined against 10 human melanoma and 5 nonmelanoma cell lines. Activities of l-DOPA and 3,4-dihydroxybenzylamine were also measured. Dose-response curves were obtained and concentrations in µg/ml required to give 90% inhibition of colony formation (IC₉₀) were calculated. Using a cutoff IC₉₀ of <2.5 as a definition of activity, Compound 2 was active in 6 of 10 melanoma and 0 of 5 nonmelanoma cell lines while both Compound 1 and l-DOPA methyl ester were active in 1 of 10 melanomas and 0 of 5 nonmelanomas. Compound 3 was inactive in all cell lines and all IC₉₀ values exceeded 100. 3,4-Dihydroxybenzylamine was active in 3 of 8 melanomas and 1 of 5 nonmelanomas. Regression analysis of IC₉₀ values for Compound 2 and tyrosinase levels in the 15 cell lines yielded a correlation coefficient of 0.93 (P < 0.001). By contrast, a similar analysis for 3,4-dihydroxybenzylamine gave a correlation coefficient of 0.17 (P > 0.05). Spectrophotometric data indicated that Compounds 1 and 2 were oxidized by tyrosinase to quinones. Cytotoxic activity was blocked by the tyrosinase inhibitor phenylthiocarbamide. Since the rates of activation of Compounds 1 and 2 were identical, the higher activity of Compound 2 was probably due to its higher lipophilicity and greater intracellular accumulation. Compounds 1 and 2 exhibited greater potency and selectivity against malignant melanoma than did the natural product l-DOPA methyl ester.