Characterization of a Human Small‐cell Lung Cancer Cell Line Resistant to a New Water‐soluble Camptothecin Derivative, DX‐8951f

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Abstract
DX‐8951f, a water‐soluble and non‐pro‐drug analogue of camptothecin, exhibits a strong inhibitory action on DNA topoisomerase I (Topo I) and in vitro cytotoxicity against various human cancer cell lines. In order to elucidate the mechanisms of its cytotoxicity, we established a DX‐8951f‐resistant cell line, SBC‐3/DXCL1, from human small cell lung cancer cells (SBC‐3) by stepwise exposure to DX‐8951f. SBC‐3/DXCL1 cells were approximately 400 times more resistant to DX‐8951f than parent cells. The SBC‐3/DXCL1 cells showed a high degree of cross‐resistance to other Topo I inhibitors such as CPT‐11, SN‐38 and camptothecin, but not to non‐Topo I targeting agents such as cisplatin, adriamycin, etoposide, and vincristine. The mechanisms of resistance of SBC‐3/DXCL1 cells to DX‐8951f were examined. Intracellular accumulation of DX‐8951f by SBC‐3 and SBC‐3/DXCL1 cells did not differ significantly. Although the Topo I activity of nuclear extracts obtained from SBC‐3/DXCL1 cells was the same as that of the parent cells, the Topo I of SBC‐3/DXCL1 cells was resistant to the inhibitory effects of DX8951f and SN‐38. Immunoblotting using anti‐Topo I antibody demonstrated similar protein levels of Topo I in SBC‐3 and SBC‐3/DXCL1 cells. The active Topo I protein of SBC‐3/DXCL1 was eluted by a high concentration of NaCl (0.4 N) compared with that of SBC‐3 (0.3 N). DX‐8951f stabilized the DNA‐Topo I cleavable complex from SBC‐3 cells, as measured by Topo I‐mediated cleavage assay. In SBC‐3/DXCL1 cells, DX‐8951f also stabilized the DNA‐Topo I complex, but with a 10‐fold lower efficiency. These results suggest that a qualitative change in Topo I contributes, at least partially, to the resistance to DX‐8951f in SBC‐3/DXCL1 cells. Therefore, SBC‐3/DXCL1 cells may have a unique mechanism of resistance to Topo I‐directed antitumor drugs.