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 G₁, S and G₂M 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 G₁ and G₂M 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. G₂M block was immediately active at low concentrations but was reversible, becoming irreversible only at the highest concentrations. G₁ block became active later, allowing cell cycle progression of cells initially in G₁, 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 G₁ than G₂M blocked cells. Conclusions: Our analysis unraveled the complexity of cell cycle effects of the drug, and revealed the activity of G₁ checkpoint, hidden by a prompter but less cytotoxic G₂M block.