Comparison of in vivo anti-melanoma effect of enantiomeric alpha-methyl- and alpha-ethyl-4-S-cysteaminylphenol.

Melanogenesis appears to be a unique target to develop anti-tumour agents specific for malignant melanoma. Among the anti-melanoma compounds that we have examined, 4-S-cysteaminylphenol (4-S-CAP), a phenolic amine, was found to have the most promising anti-melanoma effects. To further improve the efficacy as anti-melanoma agents, we have recently synthesized enantiomers of alpha-Me-4-S-CAP and alpha-Et-4-S-CAP. The enantiomers were found to be good substrates for tyrosinase. In vitro experiments showed that the enantiomers were highly cytotoxic to B16-F1 melanoma cells, and the cytotoxic effect was proved to be tyrosinase-dependent. In the present study, in vivo cytotoxicity experiments showed that i.p. administration of R-alpha-Me-4-S-CAP and S-alpha-Et-4-S-CAP (and 4-S-CAP) strongly inhibited the subcutaneous growth of B16 melanoma in mice, while the corresponding enantiomers were much less effective. Similarly, i.p. treatment with R-alpha-Me-4-S-CAP or S-alpha-Et-4-S-CAP, but not with 4-S-CAP, caused strong depigmentation of follicular melanocytes in C57BL black mice. Among 4-S-CAP and the enantiomers, only R-alpha-Et-4-S-CAP caused a moderate decrease in blood pressure in spontaneously hypertensive rats. These results confirm that the use of enantiomers increases the efficacy of tyrosinase-dependent cytotoxic phenolic amines.