Measuring the complexity of cell cycle arrest and killing of drugs: Kinetics of phase‐specific effects induced by Taxol

Background: Paclitaxel (Taxol) is known to act mainly in mitosis, interfering with microtubule dynamics, but effects on the other cells cycle phases have been reported also. However, a comparative picture of perturbation and killing in the G 1 , S and G 2 M phases after drug treatment is lacking. The approach developed by our group tackles the problem of the complexity of cell cycle effects with the aid of a computer program simulating cell cycle progression and new quantities measuring cell‐cycle arrest and death. Methods: The program generates data that were compared with those given by absolute cell counts and by different flow cytometry techniques, enabling us to follow the fate of G 1 and G 2 M blocked cells either re‐entering the cycle or dying, distinguishing cytostatic and cytotoxic effects. Apoptosis was analyzed in order to refine the description of cytotoxic effects. Results: We estimated the number of blocked and dead cells after short‐term Taxol treatments in a range of concentrations and post‐drug incubation times. G 2 M block was immediately active at low concentrations but was reversible, becoming irreversible only at the highest concentrations. G 1 block became active later, allowing cell cycle progression of cells initially in G 1 , but was still active 48 h post‐treatment, at intermediate concentrations. S‐phase delay was detected after 24 h. The death rate was much higher within G 1 than G 2 M blocked cells. Conclusions: Our analysis unraveled the complexity of cell cycle effects of the drug, and revealed the activity of G 1 checkpoint, hidden by a prompter but less cytotoxic G 2 M block. Cytometry 37:113–124, 1999. © 1999 Wiley‐Liss, Inc.