Attempts to study drug resistance in vitro have focused on continuous exposure of cell lines to cytotoxic agents that results in marked resistance. In an effort to examine drug resistance in an environment that is more typical of a clinical setting, monolayered human breast cancer cells (MCF-7) were exposed weekly for one hour to doxorubicin at a concentration that is achieved in vivo (0.1 microg/ml). After 20 weeks, the treated cell line (C20) was found to be resistant to doxorubicin by a factor of 2.0-2.5 at the 10 and 1% cell survival level compared to parent MCF-7 cells. Additionally, cross resistance to other chemotherapeutic agents including etoposide, vincristine, cisplatin, and mitomycin C was observed. Similar to other models of in vitro drug resistance, no modification of radiosensitivity was observed in C20 cells. The p170 glycoprotein was not overexpressed on C20 cells as assessed by the anti-p170 glycoprotein monoclonal antibody C219 staining nor was mRNA for the mdr-1 or MRP gene overexpressed. In addition, the mdr-blocking agent verapamil had no effect on the level of resistance encountered in tissue culture. C20 cells did not differ from the parent cell line with respect to cell cycle distribution, doubling time, GSH, GSH peroxidase, GSH reductase or GSH transferase levels. After a one-hour exposure to doxorubicin, lower intracellular doxorubicin levels were found in C20 cells compared to the parent line which provides at least a partial explanation for resistance, although the mechanism for this is unclear. Although the magnitude of resistance observed in the C20 cell line is low compared to other in vitro models of drug resistance, the modest level of drug resistance is probably sufficient to explain drug resistance seen clinically. The model of non-mdr mediated drug resistance presented may be a more relevant model for the evaluation of drug resistance in vitro.

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