Comparative study of the radiosensitizing and cell cycle effects of vinflunine and vinorelbine, in vitro.

BACKGROUND: Vinca alkaloids are an important class of anticancer agents and semisynthetic vinca alkaloids are developed to improve the therapeutic index of this class of drugs. In the present study, a direct comparison was made between vinflunine and vinorelbine regarding their radiosensitizing and cell cycle effects. METHODS: Four human tumour cell lines were tested under identical experimental conditions, using equitoxic concentrations of vinflunine and vinorelbine. RESULTS: Vinflunine and vinorelbine induced a comparable radiosensitizing effect (p-value never below 0.01) when cells were incubated for 24 h immediately prior to radiation. Regarding the cell cycle effects, a statistically significant concentration-dependent G2/M block was seen after 24 h incubation with vinorelbine in all tested cell lines. Similar results, with small cell line-related differences, were observed with vinflunine. CONCLUSION: The radiosensitizing effects of both semisynthetic vinca alkaloids were comparable (not statistically different) and nearly always cell line-specific and concentration-dependent. The cell cycle effects could be related to the observed radiosensitizing effects. Considering the more favourable toxicity profile of vinflunine, this agent might be more promising than vinorelbine for chemoradiation studies in the clinic.

The relation between deoxycytidine kinase activity and the radiosensitising effect of gemcitabine in eight different human tumour cell lines.

BACKGROUND: Gemcitabine (dFdC) is an active antitumour agent with radiosensitising properties, shown both in preclinical and clinical studies. In the present study, the relation between deoxycytidine kinase (dCK) activity and the radiosensitising effect of gemcitabine was investigated in eight different human tumour cell lines. METHODS: Tumour cells were treated with dFdC (0-100 nM) for 24 h prior to radiotherapy (RT) (gamma-Co60, 0-6 Gy, room temperature). Cell survival was determined 7, 8, or 9 days after RT by the sulforhodamine B test. dCK activity of the cells was determined by an enzyme activity assay. RESULTS: A clear concentration-dependent radiosensitising effect of dFdC was observed in all cell lines. The degree of radiosensitisation was also cell line dependent and seemed to correlate with the sensitivity of the cell line to the cytotoxic effect of dFdC. The dCK activity of our cell lines varied considerably and differed up to three fold from 5 to 15 pmol/h/mg protein between the tested cell lines. In this range dCK activity was only weakly related to radiosensitisation (correlation coefficient 0.62, p = 0.11). CONCLUSION: Gemcitabine needs to be metabolised to the active nucleotide in order to radiosensitise the cells. Since dFdCTP accumulation and incorporation into DNA are concentration dependent, the degree of radiosensitisation seems to be related to the extent of dFdCTP incorporated into DNA required to inhibit DNA repair. The activity of dCK does not seem to be the most important factor, but is clearly a major factor. Other partners of the intracellular metabolism of gemcitabine in relation to the cell cycle effects and DNA repair could be more responsible for the radiosensitising effect than dCK activity.

Cell cycle effects of vinflunine, the most recent promising Vinca alkaloid, and its interaction with radiation, in vitro.

PURPOSE: Vinflunine (VFL) is a novel third generation Vinca alkaloid with superior antitumour activity in preclinical models and an anticipated more favourable toxicity profile compared to the other Vinca alkaloids. METHOD: We investigate the radiosensitising properties of VFL and its cell cycle effects in four human tumour cell lines (ECV304, MCF-7, H292, and CAL-27). The sulforhodamine B test was used to determine cell survival, and cell cycle analysis was performed by flow cytometry. Radiosensitisation (RS) was represented by dose enhancement factors (DEFs). RESULTS: Twenty-four hours treatment with VFL before radiation caused dose-dependent RS in all cell lines. This was most pronounced in ECV304 cells with RS already at VFL concentrations that reduced cell survival by 10% (IC10). DEFs ranged from 1.57 to 2.29 in the different cell lines. A concentration-dependent G2/M block was observed (starting at 4 h of incubation). After maximal G2/M blockade cells started cycling again, mainly by mitosis, while a small portion of cells started a polyploid cell cycle. Also drug removal immediately caused recycling of cells and induction of a polyploid cell population. The polyploid cell population was most impressively noticeable after prolonged incubation with VFL (48 h), in particular in CAL-27 and ECV304. This was never observed in a tested normal fibroblast cell line (Fi 360). The fate of these cells is of particular interest, but yet uncertain. CONCLUSION: VFL has radiosensitising potential. The exact role of the cell cycle effects of VFL in its radiosensitising mechanism is still not fully elucidated and requires further study.

The radiosensitising effect of difluorodeoxyuridine, a metabolite of gemcitabine, in vitro.

PURPOSE: Gemcitabine is an active antitumour agent with radiosensitising properties. Gemcitabine is rapidly metabolised, intracellularly as well as extracellularly, by deoxycytidine deaminase to difluorodeoxyuridine (dFdU), a compound with little antitumour activity. However, plasma concentrations are maintained for a prolonged period (>24 h) at levels known to cause growth inhibition. This is the first study that investigates the radiosensitising potential of dFdU in vitro. METHODS: ECV304 and H292, human cancer cells, were treated with different concentrations dFdU (0-100 microM) during 24 h before radiation treatment (RT). The schedule dependency of the radiosensitising effect was studied by varying the interval between dFdU and radiation treatment. In addition, the cell cycle effect of dFdU was investigated with flow cytometry, and the induction of apoptosis under radiosensitising conditions was determined by Annexin V staining and caspase 3 cleavage. RESULTS: dFdU caused a clear concentration-dependent radiosensitising effect in both ECV304 and H292 cells. Dose enhancement factor (DEF) increased with an increasing concentration of dFdU: DEFs were 1.10, 1.60 and 2.17 after treatment with 10, 25 and 50 microM dFdU, respectively, in ECV304 cells and 1.08, 1.31 and 1.60 after treatment with 25, 50 and 100 microM, respectively, in H292 cells. DEFs decreased with an increasing interval of 0-24 h between dFdU treatment and radiation. Under radiosensitising conditions, the combination dFdU and radiation resulted in an increased induction of apoptosis. In addition, the cell cycle effect of dFdU, an arrest at the early S phase, is comparable with the cell cycle effect of gemcitabine. CONCLUSIONS: dFdU, the main metabolite of gemcitabine, causes a concentration- and schedule- dependent radiosensitising effect in vitro. Since the metabolite is present in plasma for a long period (>24 h) after treatment with gemcitabine, it might be partly responsible for the interaction between radiotherapy and gemcitabine. This observation might have important consequences for the optimal schedules of the combination gemcitabine and radiation therapy.

Cell cycle effect of gemcitabine and its role in the radiosensitizing mechanism in vitro.

PURPOSE: The mechanism of radiosensitization by gemcitabine is still unclear. It has been hypothesized that the accumulation of cells in early S phase may play a role in enhancing radiosensitivity. METHODS AND MATERIALS: The schedule dependency of the radiosensitizing effect was studied in ECV304, human bladder cancer cells, and H292, human lung cancer cells, by varying the incubation time and time interval between gemcitabine and radiation treatment. To determine the role of cell cycle perturbations in the radiosensitization, the influence of gemcitabine on the cell cycle at the moment of radiation was investigated by flow cytometry. RESULTS: The radiosensitizing effect increased with a longer incubation period: Dose enhancement factors varied from 1.30 to 2.82 in ECV304 and from 1.04 to 1.78 in H292 after treatment during 8-32 h, respectively. Radiosensitization decreased with an increasing interval: Dose enhancement factors varied from 2.26 to 1.49 in ECV304 and from 1.45 to 1.11 in H292 after an interval 0-24 h, respectively. Cells were blocked in the early S phase of the cell cycle by gemcitabine. The highest percentage S-phase cells was observed after treatment with the schedules that resulted in the highest radiosensitizing effect. CONCLUSIONS: We observed a clear schedule-dependent radiosensitization by gemcitabine. Our findings demonstrated a correlation between gemcitabine-induced early S-phase block and the radiosensitizing effect.

Comparison of the sulforhodamine B assay and the clonogenic assay for in vitro chemoradiation studies.

PURPOSE: Since there is a growing interest in preclinical research on interactions between radiation and cytotoxic agents, this study focused on the development of an alternative to the very laborious clonogenic assay (CA). METHODS: The colorimetric sulforhodamine B (SRB) assay was compared to the clonogenic assay for radiosensitivity testing in two lung cancer cell lines (A549, H292), one colon cancer cell line (HT-29) and one breast cancer cell line (MCF-7). In addition, the combination of the radiosensitizing agent gemcitabine and radiation was investigated with both assays. RESULTS: The dose-response curves obtained with the SRB assay and the CA were very similar up to 6 Gy. The radiosensitivity parameters (SF(2), alpha, beta, MID and ID(50)) obtained from the SRB assay and the CA were not significantly different between H292, A549 and MCF-7 cells. The radiation dose-response curves for A549 and H292 cells pretreated with 4 n M gemcitabine for 24 h clearly showed a radiosensitizing effect with both assays. The dose-enhancement factors obtained with the SRB assay and the CA were 1.80 and 1.76, respectively, for A549 cells, and 1.52 and 1.41 for H292 cells. CONCLUSIONS: The SRB assay was shown to be as useful as the more traditional CA for research on chemotherapy/radiotherapy interactions in cell lines with moderate radiosensitivity. This assay will be used for more extensive in vitro research on radiosensitizing compounds in these cell lines.