Interpreting cell cycle effects of drugs: the case of melphalan

Multiple effects usually occur in the cell cycle, during and after the exposure to a drug, while treated cells flowing through the cycle encounter G(1), S and G(2)M checkpoints. We developed a simulation tool connecting the microscopic level of the cellular response in G(1), S and G(2)M with the experimental data of growth inhibition and flow cytometry. We found that multiple-often not intuitive-combinations of cytostatic and cytotoxic effects can be in keeping with the observations. This multiplicity of interpretation can be strongly reduced by considering together data with different methods, ideally reaching a reconstruction of the underlying cell cycle perturbations. Here, we propose an experimental plan including a time course of DNA flow cytometry and absolute cell count measurements with several drug concentrations and a limited number of flow cytometric DNA-Bromodeoxyuridine and TUNEL analyses, coupled with computer simulation. We showed its use in the attempt to define the complete time course of the effects of melphalan on three cancer cell lines. After drug treatment, each subset of cells experienced blocks and lethality in all phases of the cell cycle, but the dynamics was different, the differences being strongly dose-dependent. Our approach allows a better appreciation of the complexity of the cell cycle phenomena associated with drug treatment. It is expected that such level of understanding of the time- and dose-dependence of the cytostatic and cytotoxic effects of a drug might support rational therapeutic design.