Selective incorporation and specific cytocidal effect as the cellular basis for the antimelanoma action of sulphur containing tyrosine analogs

Tyrosine analogs are good candidates for developing melanoma chemotherapy b ecause melanogenesis is inherently toxic and uniquely expressed in melanocy tic cells, Sulphur containing substrate (tyrosine) analogs, N-acetyl-4-S-cy steaminylphenol (NAcCAP) and N-propionyl-4-S-cysteaminylphenol (NPrCAP), ha ve been shown to have potent antimelanoma activity in mice bearing melanoma , Both NAcCAP and NPrCAP show selective cytotoxicity towards melanoma cell lines. But the mechanism leading to selectivity is not clear as these drugs are also toxic to other cell lines to a lesser extent. Here we show that t hese drugs have both cytostatic and cytocidal effects, which could account for this. Cytostatic effect is suggested by DNA flow cytometry. The drug ca uses cell cycle changes in four human cell lines (normal skin fibroblasts, HeLa cells, and melanoma cell lines, C32 and SK-MEL-23) in a dose-dependent manner blocking cells in S phase with concomitant decrease in the number o f cells in G(1) phase. There is also a gradual decrease in cells in G(2) M phases. The dose-concentration curves give IC50 values in the range of 50 -400 mu M and the melanotic melanoma cell Line SK-MEL-23 has the lowest IC5 0 value consistent with our hypothesis that these drugs are selective towar ds melanoma cells. The concentration-dependent accumulation of cells in S p hase suggest a cytostatic effect as a consequence of inhibition of DNA synt hesis in agreement with [H-3]thymidine incorporation assay. There is a high ly specific uptake of [C-14]NAcCAP and irreversible damage to DNA synthesis machinery in SK-MEL-23 cells, indicating a melanotic-specific cytocidal ef fect as well. Trypan blue exclusion study and competitive inhibition assay indicated that visible cytocidal effect occurs slowly and oxidative stress resulting from tyrosinase mediated oxidation of the drug appears to be the underlying mechanism. The primary antimelanoma effect of cysteaminylphenols derives from a selective cytostatic effect, but is followed by a specific cytocidal action rendering the drugs useful for targeted melanoma chemother apy.