Characterization of a human small-cell lung cancer cell line resistant to a new water-soluble camptothecin derivative, DX-8951f

DX-8951f, a water-soluble and non-pro-drug analogue of camptothecin, exhibi ts a strong inhibitory action on DNA topoisomerase I(Topo I) and iii vitro cytotoxicity against various human cancer cell lines, In order to elucidate the mechanisms of its cytotoxicity, we established a DX-8951f-resistant ce ll line, SBC-3/DXCL1, from human small cell lung cancer cells (SBC-3) by st epwise 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-1 1, SN-38 and camptothecin, but not to non-Topo I targeting agents such as c isplatin, adriamycin, etoposide, and vincristine. The mechanisms of resista nce of SBC-3/DXCL1 cells to DX-8951f were examined, Intracellular accumulat ion 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/DXCL 1 cells was the same as that of the parent cells, the Topo I of SBC-3/DXCL1 cells aas resistant to the inhibitory effects of DX8951f and SN-38, Immuno blotting 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/D XCL1 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, DX8951f 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 DX8951f in SBC-3/DXC L1 cells, Therefore, SBC-3/DXCL1 cells may have a unique mechanism of resis tance to Topo I-directed antitumor drugs.