Cisplatin down-regulates topoisomerase I activity in lung cancer cell lines

Many clinical studies have reported that irinotecan has reproducible antitumor activity against lung cancer. Both cisplatin and SN-38 are key drugs in the treatment of lung cancer, and their combination is one of the most promising regimens available. Using lung cancer cell lines, ABC-1 and SBC-3, we examined the cytotoxic effect of the schedule, as well as the effect of cisplatin on topoisomerase I activity. Cytotoxicity was determined by MTT assay. ABC-1 or SBC-3 cells were incubated with or without various concentrations of both drugs in 96-well microplates for 72 or 96 hours in a humidified 5% CO ₂ atmosphere at 37°C. Synergism was evaluated by median-effect plot analysis and a combination index isobologram method by Chou and Talalay. After ABC-1 or SBC-3 cells had been exposed to 10 μM cisplatin for one hour, topoisomerase I activities were determined by supercoiled-DNA relaxation assay. Synergism was observed in ABC-1 and SBC-3 cells when cisplatin was given first, followed by SN-38 (7-ethyl-10-hydroxycamptothecin) and cisplatin. Topoisomerase I activity decreased at 1-2 hours after exposure to cisplatin and recovered gradually after 4-5 hours of cisplatin exposure in both ABC-1 and SBC-3 cells. Accordingly, pretreatment with cisplatin will have an impact on the sensitivity to SN-38.