Correlation of enhanced 6-mercaptopurine cytotoxicity with increased phosphoribosylpyrophosphate levels in Chinese hamster ovary cells treated with 3-aminobenzamide.

3-Aminobenzamide (3AB) has been used widely to inhibit the nuclear enzyme poly(ADP-ribose) polymerase (EC 2.4.2.30) and study the involvement of poly(ADP-ribose) synthesis in DNA repair and other cellular functions. 3AB (3 mM) potentiates the cytotoxicity of 6-mercaptopurine (MP) and azathioprine in CHO-K1 cells with dose enhancement factors at 10% survival of 30-fold. In synchronized cells, 3AB is required during G1 and early S phase to obtain potentiation of MP cytotoxicity. There is a small but significant depletion of cellular NAD in MP-treated cells. As demonstrated by flow cytometric analysis, 20-40 microM MP causes an accumulation of cells in early S phase of the cell cycle. 3AB (3 mM) has no effect on cell cycle distribution; however, in the presence of MP, a similar accumulation is seen by 2-5 microM MP. 3AB and MP per se have no effect on phosphoribosylpyrophosphate levels, but coincubation causes a 30-fold increase in phosphoribosylpyrophosphate levels, reaching a maximum by 1.5 microM MP and declining to basal levels by 10 microM MP. There was a good correlation between the 3AB dose-dependent increase in cell killing and rise in phosphoribosylpyrophosphate levels.