Effects of Antineoplastic Drugs on Plateau-Phase Cultures of Mammalian Cells. II. Bleomycin and Hydroxyurea2

Abstract

The effects of antineoplastic agents bleomycin (BLM) and hydroxyurea (HOU) were studied on plateau-phase cultures of Chinese hamster cells grown in MEM supplemented with 15% fetal calf serum. In these cultures, clonigenic cells belong essentially to the fraction of the population which is not synthesizing DNA at the time of trypsinization and reseeding. The dose-survival and time-survival curves for both log- and plateau-phase cells treated with BLM were characterized by an initial upward concavity. Plateau-phase cells were slightly more resistant to BLM than log-phase cells. The time-survival curve for plateau-phase cells treated with HOU exhibited an initial very broad shoulder followed by an exponential region, in contrast to the same curve for log-phase cells in which an initial exponential drop was followed by a survival plateau. These results suggest 2 distinct mechanisms of lethal action for HOU in the 2 phases of growth. In studies on the effects of the drugs on labeled thymidine-pulse incorporation at various times after treatment, HOU inhibited incorporation in the fraction of the population synthesizing in the plateau phase but BLM increased incorporation which peaked in a parasynchronous way about 12 hours after treatment. Thus, BLM-treated cells appeared to be stimulated in a way similar to that of plateau-phase cells recalled into cycle by serum addition. The value of plateau-phase systems for testing antineoplastic agents is discussed.