Measuring potential doubling times of murine tumors using flow cytometry

Flow cytometric analysis of bromodeoxyuridine (BrdUrd) labeled cells has led to the description of various methods for determining population potential doubling times (Tpot) from samples obtained at a single time point. There have been several reports of results from human tumor samples using these various methods of analysis, but little documentation of the validity of these approaches in experimental tumor models. Recently, we reported results using in vitro cell cultures and determined the most reliable method for these. In this paper we report the results of this methodology when applied to two in vivo murine tumor models (MCaK and FSA). The analytic approach tested is summarized as follows: The relative movement (RMlu(t], a measure of the mean DNA fluorescence of BrdUrd-labeled cells that have not undergone division at the time of sampling, and a quantity v, a function of the fractions of BrdUrd-labeled divided and undivided cells, were measured for a series of time points following labeling. From each value of RMlu(t) an estimate of the length of S-phase (TS) was computed and from each v and TS a value for Tpot was found. These results were compared to the values of TS and Tpot obtained by fitting all the values simultaneously. Tumor MCaK values of TS and Tpot of 9.6 and 28.0 h were obtained from fitting all of the data. Tumor FSA gave values of 16.8 and 42.3 h for TS and Tpot, respectively. The results of this analysis show that single time point measurements can give reliable estimates of TS and Tpot.