Mathematical modeling to distinguish cell cycle arrest and cell killing in chemotherapeutic concentration response curves

Concentration response experiments are utilized widely to characterize the response of tumor cell lines to chemotherapeutic drugs, but the assay methods are non-standardized and their analysis based on phenomenological equations. To provide a framework for better interpretation of these experiments, we have developed a mathematical model in which progression through the tumor cell cycle is inhibited by drug treatment via either cell cycle arrest or entrance into cell death pathways. By fitting concentration response data, preferably over a dynamic range, the contributions of these mechanisms can be delineated. The model was shown to fit well experimental data for three glioma cell lines treated with either carmustine or etoposide. In each cell line, the major mechanism of tumor cell inhibition was cell death for carmustine in contrast to cell cycle arrest for etoposide. The model also provides a possible interpretation for the acquired in vitro resistance of U87 cells to carmustine as an accelerated desensitization to cell killing effects. This approach will aid in understanding better the action of chemotherapeutic agents on tumor cells.