A Topoisomerase II Inhibitor, NK109, Induces DNA Single‐ and Double‐strand Breaks and Apoptosis

2,3‐(Methylenedioxy)‐5‐methyl‐7‐hydroxy‐8‐methoxybenzo[c]phenanthridinium hydrogensulfate di‐hydrate, called NK109, is a benzo[c]phenanthridine derivative, which inhibits DNA topoisomerase II activity by stabilizing the DNA‐enzyme‐drug complex, and shows strong growth‐inhibitory effects on several human cancer cells. In the present study, NK109 treatment induced DNA fragmentation and a rise in the level of cytoplasmic nucleosomes, which are markers of apoptosis, in human small‐cell lung carcinoma SBC‐3 cells. These effects were inhibited by zinc ions and enhanced by cycloheximide or actinomycin D. Dose‐dependent single‐ and double‐strand DNA breaks were observed, using alkaline and neutral elution assays, in SBC‐3 cells treated with more than 0.2 μM NK109 for 4 h. Treatment with NK109 caused more DNA single‐ and double‐strand breaks than treatment with an equimolar amount of VP‐16. These results suggest that NK109 induces DNA strand breaks and apoptosis. In addition, it appears that this process does not require protein or RNA synthesis, but involves a specific endonuclease which is inhibited by zinc ions.