High-level, unstable adriamycin resistance in a Chinese hamster mutant cell line with double minute chromosomes.

An Adriamycin-resistant Chinese hamster V79 line was isolated previously in this laboratory. There was about a 5-fold increase in Adriamycin resistance in this mutant as determined from survival curve measurements. Using this low-level Adriamycin-resistant mutant, a cell line with a high level of resistance was isolated after a multistep selection process culminating in continuous growth of the cells in medium containing 5.0 micrograms Adriamycin/ml. These cells are about 3000 times more resistant towards the cytotoxic effects of Adriamycin than is the parental V79 line. This high-level resistance phenotype is unstable and lost upon culture in the absence of drug. The highly resistant cells also showed increased cross-resistance to actinomycin D, Colcemid, and vincristine compared to the low-level resistant cells. Cytogenetic studies showed that these mutant cells contained increased numbers of double minute chromosomes and that the number of double minutes decreased proportionately with the reduction of Adriamycin resistance in cultures changed to drug-free medium. Adriamycin uptake assays demonstrated that there was a further decrease in net uptake relative to the low-level resistant mutant.