Potency, selectivity and cell cycle dependence of catechols in human tumour cells in vitro

Enhancement of the potency and melanoma-selectivity of redox agents was sought by two different approaches. In screening a series of catechols, derivatives of moderate half-life (dopa, dopamine, noradrenaline, 3,4-dihydroxybenzylamine, 3,4-dihydroxyphenylacetic acid; t1/2 12-33 hr) had significant toxicity (D37 20-30 microM) and selectivity for melanoma cells compared with HeLa. Less stable catechols (5-hydroxy- and 6-hydroxydopamine; t1/2 4 and 5 hr respectively) were toxic but lacked selectivity whereas more stable derivatives (4-hydroxyanisole, 2,3-dihydroxybenzoic acid; t1/2 greater than 72 hr) were less potent (D37 greater than 100 microM) and had poor selectivity. Gossypol, a complex catechol derivative, exhibited significant toxicity (D37 7.7 microM) but little selectivity. Enzymes capable of reacting with components of the culture medium and known to continuously generate hydrogen peroxide (glucose-6-oxidase) or superoxide ion (xanthine oxidase) exhibited a similar degree of selectivity as dopa, indicating that active oxygen species are more important mediators of catechol toxicity than quinones. Rhodamine 123, a cationic dye preferentially taken up by some tumour cells, was accumulated equally by melanoma and HeLa yet had a similar selectivity to that of dopa. In the second approach, the potency of dopa was found to be greatly enhanced during early S phase. This phenomenon, found with cells synchronised both by mitotic shake off and by 24 hr accumulation in G1S in the presence of 5 mM hydroxyurea, occurred during a period in which the proportion of cells in S phase cells was low. These results indicate that human cells are extremely sensitive to extracellular active oxygen species during a relatively short period in early S phase, and selective killing of asynchronous melanoma cells therefore requires agents capable of sustaining a redox effect for at least one cell cycle.