Evidence suggests that the anticancer agents etoposide (VP16-213) and teniposide (VM26) produce DNA breaks and cytotoxicity by interaction with type II topoisomerase. Therefore, levels of type II topoisomerase may influence sensitivity to VP16-213 and VM26. We have characterized four lung carcinoma-derived cell lines for natural sensitivity or resistance to VP16-213 and VM26. Included in this study were two small cell lung carcinoma lines (SW900 and SW1271), an adenocarcinoma line (A549), and a large cell carcinoma (H157). SW1271 was the most sensitive line with a median inhibitory concentration for cell proliferation of 0.5 µm for VM26 and 2.7 µm for VP16-213, and SW900 was the most resistant with median inhibitory concentration values of 2.0 and 16 µm, respectively. A549 and H157 cells were intermediate in sensitivity to these drugs. Alkaline elution techniques were used to study in vivo formation and repair of single and double strand DNA breaks. Single strand DNA breaks were observed in SW1271 cells exposed to as little as 10 nm VM26 or 100 nm VP16-213 for 1 h, whereas SW900 cells required exposure to 10-fold higher concentrations of VM26 or VP16-213 to produce similar results. Single strand DNA breaks predominated only in SW1271 and A549 cells and then, only at low drug concentrations, whereas the ratios between single and double strand DNA breaks decreased at higher drug concentrations. Plots of cytotoxicity versus single and double strand DNA breakage revealed that cytotoxicity produced by both drugs was more closely related to double strand DNA break formation in all four cell lines. DNA breaks appeared rapidly upon addition of drug, reaching plateaus in DNA breaks within 30 min, and repair of both single and double strand DNA breaks occurred rapidly with time to repair one-half of the DNA breaks of 20 to 60 min in all four cell lines upon removal of drug, arguing against repair as a mechanism for drug resistance. DNA breakage was also observed in nuclei isolated from SW900 and SW1271 cells in similar magnitude to that observed in the respective cells. Results indicate that DNA breakage plateaus may reflect a steady-state equilibrium established between the drug and its nuclear target, possibly type II topoisomerase, and suggest that natural resistance to VP16-213 and VM26 may be due to different enzyme levels in sensitive and naturally resistant cells.


Preliminary results of this study were communicated at the 23rd Annual Meeting of the American Association for Cancer Research (1).

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