Cytotoxic effect of substitution at 2-, 6-, and 2,6-positions in ascorbic acid on malignant cell line

In recent years L-ascorbic acid (AA) and its isomers have raised considerable interest as anticancer agents, although the mechanism has remained largely unknown. AA isomers are nearly identical in their physical and chemical properties but differ widely in their biological properties. AA, a lactone sugar, has a number of reactive positions, especially at 2- and 6-. Although there are a number of reports on the cytotoxic effect of AA and its isomers on malignant and nonmalignant cell lines, no work has been reported on the comparative effects of substitutions at these active sites. This study, then, investigates the comparative cytotoxicity of such substitutes on the malignant leukemia P388 cell line in culture. We tested a series of 2-, 6- and 2,6- disubstituted AA-derivatives, comprising the following: i) substitution at 2-position: -PO4, -SO4, O-Me, O-octadecyl; ii) substitution at 6-position: -PO4, -SO4, -palmitate, -stearate; and iii) substitution at 2,6-position: -dipalmitate. About 50,000 P388 cells/ml were incubated with and without AA derivatives in a final concentration of 1000, 500, 100, 10 and 1 microg/ml in triplicate and counted after 72 hrs. All 2-substituted and the 2,6-substituted AA derivatives tested were nontoxic and ineffective in preventing cell growth. In contrast, all 6-substituted AA derivatives were very toxic at all levels, even at the lowest concentration. These results suggest that substitution at 2-, 6- and 2,6-positions in AA have a different effect on toxicity. The 2-, and 2,6-substituted AA derivatives are stable compounds, resistant to hydrolysis which render them inactive. The cytotoxicity of the 6-substituted derivatives may be explained by one of the following mechanisms, yet to be explored: i) the hydrolysis rate may differ; or ii) the chemical structure itself may affect toxicity. Further studies are in progress to understand the mechanism.