Calculating potential doubling time using monoclonal antibodies specific for two halogenated thymidine analogues

Estimating cell death in G2M using bivariate BrdUrd/DNA flow cytometry

BACKGROUND

In an accompanying paper (Asmuth et al.) it was found necessary to include cell death explicitly to estimate parameters of cell proliferation. The use of bivariate flow cytometry to estimate the phase durations and the doubling times of cells labeled with thymidine analogues is well established. However, these methods of analysis do not consider the possibility of cell death. This report demonstrates that estimating cell death in G(2)/M is possible.

METHODS

Mathematical models for the experimental quantities, the fraction of labeled undivided cells, the fraction of labeled divided cells, and the relative movement were developed. These models include the possibility that, of the cells with G(2)/M DNA content, only a certain fraction will divide, with the remainder dying after some time T(R). Simulation studies were conducted to test the possibility of using simple methods to estimate phase durations and cell death rates.

RESULTS

Cell death alters the estimates of phase transit times in a rather complex manner that depends on the lifetime of the doomed cells. However, it is still possible to obtain estimates of the phase durations of cells in S and G(2)/M and the death rates of cells in G(2)/M.

CONCLUSIONS

The methods presented herein provide a new way to characterize cell populations that includes cell death rates and common measurements of cell proliferation.

A stochastic model for dual label experiments: an analysis of the heterogeneity of S phase duration

We develop a statistical approach for the study of S phase duration in experiments using sequential pulses of two thymidine analogues. Cell entrance into S phase is assumed to follow a possibly nonhomogeneous Poisson process, and the duration in S phase independently follows an unspecified distribution, thus allowing the possibility of variable S phase duration times for cells. Several conclusions regarding experimental design considerations are reached. The availability of three labelled cell subgroups comprising cells receiving exactly one of the two thymidine analogues or receiving both thymidine analogues at least doubles the efficiency of the mean S phase duration estimate compared to conventional estimates based on two labelled groups. Increasing the duration between the two thymidine analogue pulses can also dramatically increase the efficiency. The modelling technique was applied to fourteen uveal melanomas from patients who received in vivo injections of two thymidine analogues, bromodeoxyuridine (BrdUrd) and iododeoxyuridine (IdUrd). Counts of labelled cells were consistent with steady-state time homogeneous entry into S phase and nonhomogeneous spatial entry into S phase.

Three parameter flow cytometric analysis for simultaneous detection of cytokeratin, proliferation associated antigens and DNA content

The analysis of the cell cycle distributions by univariate flow cytometric DNA measurement has been widely applied in the clinic to determine kinetic parameters of human malignancies. A common problem with measurements of cell cycle phase distributions in tumor biopsy material is the presence of non-malignant diploid cells. Furthermore, such a static measurement might not be accurate enough to describe the dynamic process of cell proliferation. For this purpose alternative methods have been developed to include BrdUrd incorporation or the presence of intrinsic proliferation associated markers such as PCNA or Ki67-Ag into the analysis. However, the presence of nonmalignant diploid cells will influence also these bivariate analyses, especially in case of DNA-diploidy of the tumor cells. Here we present a three parameter flow cytometric assay based on the simultaneous detection of cytokeratin, DNA and a proliferation associated marker, such as BrdUrd, PCNA or Ki67-Ag. Based on the presence of cytokeratin, epithelial cells can be selected for a detailed cell cycle analysis. This method can be applied to frozen tissue, which makes this assay useful for multicentre clinical studies.

Specific staining of iododeoxyuridine and bromodeoxyuridine in tumors double labelled in vivo: a cell kinetic analysis

The simultaneous and specific staining of iododeoxyuridine (IdUrd) and chlorodeoxyuridine (CldUrd) allows for more accurate estimates of potential doubling time (Tpot). Because CldUrd is not approved for human use, the procedure was adapted for the staining of IdUrd and bromodeoxy-uridine (BrdUrd). The fluorescein isothiocyanate-conjugated B44 antibody (B44-FITC) stained both IdUrd and BrdUrd in tumor nuclei labelled singly with one or the other pyrimidine analogue. However, when MCaK tumors in exponential growth in vivo were pulse labelled with both IdUrd and BrdUrd, the staining of BrdUrd was not seen, and the labelling pattern reflected specificity to IdUrd. These observations were confirmed using tumors pulse labelled with IdUrd and/or BrdUrd at 6 h and/or 0.3 h prior to tumor removal in all possible combinations. Simultaneous specific staining of BrdUrd by Br3 and of IdUrd by B44-FITC was documented by quantification of labelling indices (LIs) from double-labelled tumors. The specificity of B44-FITC for IdUrd in double-labelled tumors was due to a greater affinity of this antibody for IdUrd than for BrdUrd. This technique allowed for two independent estimates of LI and Tpot when tumors were double labelled for 3.0 and 5.5 h. Both IdUrd and BrdUrd are approved for clinical use, and this double-labelling technique should prove to be valuable for measuring the cell kinetics of solid tumors in vivo.

Simultaneous cytokinetic measurement of aneuploid tumors and associated diploid cells following continuous labelling with chlorodeoxyuridine

Cells from a murine tumor, MCa-K, were continuously labelled with the thymidine analogue chlorodeoxyuridine (CldUrd) and analyzed by bivariate flow cytometry in order to measure the growth fraction (GF) and potential doubling time (Tpot) of both the DNA-aneuploid tumor cells and the associated DNA-diploid cells. MCa-K has a DNA index of 1.7, rendering two, partially overlapping, populations observable with labelled and unlabelled cells in each population. The data from these tumors may be divided into three regions of differing DNA content, with one region containing a pure DNA-diploid population, a second region with both cell types, and a third region including only DNA-aneuploid cells. Equations are presented to characterize the fractions of labelled cells in each region as a function of labelling time and cell type, thereby permitting estimation of the proliferative properties of the populations. These equations include the possibility that DNA-aneuploid cells cease cycling both in G1 and in S phase to account for the observed numbers of unlabelled cells with S phase contents. The estimated value of Tpot of the DNA-diploid cells is 126.0 h with a GF of 42%, while that of the DNA-aneuploid cells is 36.9 h with a GF of 69%. It is also estimated that between 2% and 6% of all DNA-aneuploid cells starting DNA synthesis cease cycling, leading to 25% of the cells having an S-phase DNA content being noncycling.

Double labelling to obtain S phase subpopulations: application to determine cell kinetics of diploid cells in an aneuploid tumour

We studied the cell kinetics of the murine mammary carcinoma MCa-K using iododeoxyuridine (IdUrd) and chlorodeoxyuridine (CldUrd) given at different times as independently detectable labels of S phase cells. The presence of IdUrd and CldUrd, and the amount of DNA were measured by three-colour flow cytometry making it possible to define three subpopulations within S phase and to measure the progression through the cell cycle during the time following labelling. In DNA histograms of these subpopulations, the diploid and aneuploid cells (which had a DNA index of 1.7) are essentially completely separated. From appropriate combinations of cells labelled with IdUrd only, CldUrd only, or both, it was possible to construct separate DNA distributions for the labelled diploid and aneuploid cells at the times of administration of each label. The kinetics of the diploid and aneuploid cells could be calculated for individual tumours from these two time points without having to make corrections for the presence of the second population. The diploid and aneuploid populations had indistinguishable S and G2 + M phase durations, T(S) and T(G2 + M), of about 9 and 2 h; however, the potential doubling time values for the aneuploid and diploid populations were 30.2 and 101.2 h respectively.