SN-38 is an active metabolite of the topoisomerase I inhibitor irinotecan. The mechanism behind its antitumor effect in colorectal cancer is not fully understood. In this study, we examined the response of colon cancer cell lines with different metastatic potential to SN-38. The parental human colon cancer cell line KM12C and its two highly metastatic derivatives KM12SM and KM12L4a were cultivated in 5% CO2 at 37°C for 24 h and then exposed to SN-38 (2.5 µg/ml) at 37°C for 4, 24 and 48 h, respectively. The cell cycle was measured by flow cytometry, apoptotic activity was determined by flow cytometry and immunocytochemistry and the expression of topoisomerase I, Bax and survivin proteins were examined by Western blot. The exposure of the cells to SN-38 induced S-phase and G2 arrest (P<0.0001) and the KM12L4a cells had the highest response in a time-dependent manner (P<0.0001). The rates of apoptosis in the KM12SM (P=0.001) and KM12L4a cell lines (P=0.01) were increased time-dependently, though there was no such change in the KM12C cells. The expression of topoisomerase I protein was decreased in each cell line tested and the expression of Bax protein was increased, especially in KM12L4a. In conclusion, the effect of SN-38 on the colon cancer cell lines was mediated via conducting S-phase and G2 arrest and apoptosis. This effect was found in the cell lines with higher metastatic potentials, indicating that SN-38 can be used to treat advanced colon cancers.

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