Establishment and Characterization of Resistant Cells to Etoposide (VP16) from a Mouse Breast Cancer Cell Line, FM3A

We established 4 cell lines resistant to VP16 from a mouse breast cancer cell line, FM3A. The IC(50) values of all 4 resistant strains were approximately 2 μg/ml as measured by colony formation in soft agar; about 40 times higher than that of parent cell (0.05 μg/ml). These cells showed a cross‐resistance to VM26, a compound related to VP16, but not to a variety of other antitumor drugs including adriamycin, mitomycin C, cis‐platinum, 5‐fluorouracil, bleomycin, vincristine, 4‐hydroperoxycyclophosphamide, methotrexate and cytosine arabinoside. Topoisomerase II, the putative target of VP16, was partially purified from cells, and was assayed using knotted P4 phage DNA as a substrate. However, no significant difference was observed between enzymes from resistant cells and from the parent cells in either activity per cell or sensitivity to VP16. On the other hand, the resistance of these cell lines to VP16 was greatly reduced by adding a calcium antagonist, verapamil, to the soft agar at a concentration as low as 5μM, at which the viability of cells was hardly affected. A similar verapamil‐induced reduction in the resistance of the cells to VM26 was also observed. These results suggest that the acquired resistance may be largely due to an altered membrane permeability to drugs, which may be overcome by verapamil, rather than to an altered topoisomerase II.