Rapid analysis of drug effects on the cell cycle

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Abstract
Using a flow cytometric technique to analyse DNA content and chromatin structure simultaneously, the following parameters of cell cycle progression were estimated in control and drug‐treated L1210 cell cultures: (a) the kinetics of cell exit from the G1 phase; (b) the probability of cell exit from the indeterminate portion of the G1 phase, measured as the half‐time of cell residence in that state; (c) the duration of the deterministic portion of G1 phase; (d) the rates of cell transit through selected “windows” in S phase; (e) the rate of cell entrance into mitosis; (f) the mean duration of the cell cycle (Tc). These parameters are obtained in a single stathmokinetic experiment from measurements of individual samples withdrawn at 30 min‐1 hr intervals from Vinblasatine‐treated cultures. In the same experiment mitotic indices are obtained with high statistical accuracy, and may be used to determine the terminal point of drug action. In addition to cell cycle analysis the method makes it possible to detect drug‐induced changes in nuclear chromatin that are manifested by varying sensitivity of DNA in situ to denaturation by acid. Such changes were found to be associated with defective chromatin condensation, altered histone modifications or intercalation of the drugs into DNA. Using this technique the effects of sodium n‐butyrate and two new antitumor drugs on L1210 cells were investigated.