Melanogenesis provides a unique target for the development of antitumour agents specific for
malignant melanoma. Among the anti-
melanoma compounds 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 its efficacy as an anti-
melanoma agent, we synthesized the R- and S-enantiomers (99% enantiomer excess) of alpha-methyl-
4-S-cysteaminylphenol (alpha-Me-4-S-CAP) and alpha-ethyl-
4-S-cysteaminylphenol (alpha-Et-4-S-CAP) by coupling 4-hydroxythiophenol with the oxazolines obtained from the (R)- and (S)-enantiomers of 2-amino-1-propanol and
2-amino-1-butanol, respectively. The enantiomers of
alpha-Me-4-S-CAP and
alpha-Et-4-S-CAP were found to be better substrates for
tyrosinase than the natural substrate,
L-tyrosine. In vitro experiments showed that all four enantiomers were highly cytotoxic to pigmented B16-F1
melanoma cells, the effect being 70-fold and 160-fold greater than that on non-pigmented B16-G4F
melanoma cells and 3T3 fibroblasts, respectively. The cytotoxic effect against B16-F1 cells was completely inhibited by
phenylthiourea, a
tyrosinase inhibitor, or by
N-acetyl-L-cysteine, which increases the intracellular
reduced glutathione (GSH) level. 4-S-CAP and the enantiomers were taken up into B16-F1 cells at comparable rates, but showed varying rates of GSH depletion that were inversely correlated to the cytotoxicity. These results suggest that the use of enantiomers would increase the efficacy of
tyrosinase-dependent cytotoxic
phenols.