Collateral sensitivity to methotrexate in cells resistant to adriamycin.

Chinese hamster ovary (CHO) cells, maintained and treated in log-phase growth, are extremely sensitive to Adniamycin but are very resistant to methotrexate-induced cell killing. The survival response to Adniamycin is biphasic, with the sensitive population showing a D0 of 0.08 pg/mI for 1 hr, while essen tially no cell killing is produced by methotrexate treatments of up to 500 @.tg/mI for either 1 or 13 hr. Cells surviving Adniamycin treatment (5 @sg/ml for 1 hr) and isolated in colony form were more resistant (D0 = 0.2 pg/mI) to subsequent treatments with Adniamycin than were the parental population, although the survival response was still biphasic. These Adniamycin-resist ant cells (CHO-R/ADR), however, had become sensitive to methotrexate, with a nearly 50% cell killing achieved by treat ment with 5 @tg/ml for 1 hr. Uptake studies indicate that the acquired Adniamycin resistance is not due to a decrease in Adniamycin uptake, whereas the increase in methotrexate sen sitivity is, in part, due to an increased uptake and a lower efflux of methotrexate. Levels of dihydrofolate neductase activity in the Adniamycin-sensitive methotrexate-resistant CHO cells are substantially higher (34%) than levels in Adriamycin-resistant (CHO-R/ADR) cultures. Titration of CHO cells with methotrex ate doses of up to 500 @tg/ml(for only 1 hn) reduced the dihydrofolate reductase activity to the level found in CHO-R/ ADR cells treated with doses up to 5 pg/mI for 1 hr. Survival studies confirmed that Adniamycin resistance could be con ferred to the CHO cells in a dose-dependent fashion by pre treating normal cultures with methotrexate. These and other data suggest that the collateral methotrexate sensitivity in the CHO-R/ADR cells is on the basis of both decreased levels of dihydrofolate reductase and increased net uptake of metho trexate. Dihydrofolate reductase levels seem to be a measure of Adniamycin sensitivity. These results may have important implications in the use of these two drugs, either alone or in combination, in cancer chemotherapy.