Induction of concentration-dependent blockade in the G2 phase of the cell cycle by cancer chemotherapeutic agents.

The mitotic cell selection procedure for cell cycle anal ysis was utilized with Chinese hamster ovary fibroblasts to determine the transition points in G,, i.e., the age in G, at which cells become refractory to drug-induced progres sion blockade, for several cancer Chemotherapeutic agents and antimetabolites over a 1000-fold concentration range. The G transition points for five anticancer drugs (actinomycin D, Adriamycin, lucanthone, mitomycin C, and bleomycin) varied linearly as a function of the logarithm of the drug concentration between the S-G_,boundary at low concentrations and prometaphase (45 min prior to the end of karyokinesis) at high concentrations. Very low concentrations produced an incomplete blockade with some cells continuing to progress through the cycle. Above a certain concentration the transition point did not decrease further but attained a minimum value at 45 min prior to the end of karyokinesis, implying that once a cell has entered prometaphase it is completely refractory to drug action and proceeds through mitosis without pertur bation. In contrast, the age at which cells are refractory to treatment with three antimetabolites did not show a drug concentration dependence. Over wide ranges of concen tration, hydroxyurea, cycloheximide and puromycin re sulted in transition points of 116 (the S-G boundary), 56, and 58, min respectively, prior to the end of karyokinesis. We interpret these results to indicate a physical mech anism, distinct from pleotrophic action, by which the anticancer antibiotics block the progression of G cells.