Repair of sublethal damage in two human tumor cell lines grown as multicellular spheroids

Applicability of the linear-quadratic model to single and fractionated radiotherapy schedules: an experimental study

The aim of this study was to examine the applicability of the linear-quadratic (LQ) model to single and fractionated irradiation in EMT6 cells. First, the α/β ratio of the cells was determined from single-dose experiments, and a biologically effective dose (BED) for 20 Gy in 10 fractions (fr) was calculated. Fractional doses yielding the same BED were calculated for 1-, 2-, 3-, 4-, 5-, 7-, 15- and 20-fraction irradiation using LQ formalism, and then irradiation with these schedules was actually given. Cell survival was determined by a standard colony assay. Differences in cell survival between pairs of groups were compared by t-test. The α/β ratio of the cells was 3.18 Gy, and 20 Gy in 10 fr corresponded to a BED3.18 of 32.6 Gy. The effects of 7-, 15- and 20-fraction irradiation with a BED3.18 of 32.6 Gy were similar to those of the 10-fraction irradiation, while the effects of 1- to 5-fraction irradiation were lower. In this cell line, the LQ model was considered applicable to 7- to 20-fraction irradiation or doses per fraction of 2.57 Gy or smaller. The LQ model might be applicable in the dose range below the α/β ratio.

Compatibility of the repairable-conditionally repairable, multi-target and linear-quadratic models in converting hypofractionated radiation doses to single doses

We investigated the applicability of the repairable-conditionally repairable (RCR) model and the multi-target (MT) model to dose conversion in high-dose-per-fraction radiotherapy in comparison with the linear-quadratic (LQ) model. Cell survival data of V79 and EMT6 single cells receiving single doses of 2-12 Gy or 2 or 3 fractions of 4 or 5 Gy each, and that of V79 spheroids receiving single doses of 5-26 Gy or 2-5 fractions of 5-12 Gy, were analyzed. Single and fractionated doses to actually reduce cell survival to the same level were determined by a colony assay. Single doses used in the experiments and surviving fractions at the doses were substituted into equations of the RCR, MT and LQ models in the calculation software Mathematica, and each parameter coefficient was computed. Thereafter, using the coefficients and the three models, equivalent single doses for the hypofractionated doses were calculated. They were then compared with actually-determined equivalent single doses for the hypofractionated doses. The equivalent single doses calculated using the RCR, MT and LQ models tended to be lower than the actually determined equivalent single doses. The LQ model seemed to fit relatively well at doses of 5 Gy or less. At 6 Gy or higher doses, the RCR and MT models seemed to be more reliable than the LQ model. In hypofractionated stereotactic radiotherapy, the LQ model should not be used, and conversion models incorporating the concept of the RCR or MT models, such as the generalized linear-quadratic models, appear to be more suitable.

Dose dependence of the growth rate of multicellular tumour spheroids after irradiation

The present study investigated differences in the growth rate of multicellular tumour spheroids of the MCF-7 line of human breast cancer before and after their irradiation. Growth of the spheroids was analysed according to a model based on a Gompertz function. In this model, normalization to a common initial volume is achieved in a way that enables meaningful comparisons to be made between the results obtained for each spheroid. For irradiated spheroids the model includes an additional term to take account of sterilized cells. We found that the growth rate observed before irradiation is not fully recovered by irradiated spheroids and that growth recovery reduces with higher irradiation doses. Surviving fractions obtained at doses below 3 Gy are comparable with those found in clonogenic assays on spheroids of the same cellular line. At larger doses, discrepancies between the different studies are considerable.

Spheroids in radiobiology and photodynamic therapy

Spheroids are tridimensional aggregates of tumor cells coming from one or several cell clones. This model, which mimics the micro-tumors structure and some of their properties, shows oxygen, pH and nutrient gradients inducing a necrotic area in the center of the spheroid. Analysis of spheroids, cultured under static or stirred conditions, can be performed on whole spheroids or dissociated spheroids. The spheroids sensitivity to ionizing radiation and photodynamic therapy can be altered by oxygen status, damage repair, intercellular commmunications and apoptosis induction, as in experimental tumor models. In radiobiology, the similarity of radiation response between spheroids and tumor xenograft bearing mice makes the spheroids to be a good alternative model to in vivo irradiation studies. In photodynamic therapy, spheroids lead to a better understanding of the own tumor response without interactions with vascular system. Finally, despite the quality of spheroid model, only the use of new technology for analysis of spheroid populations will help to increase their experimental use, particularly in preclinical oncology.

Heterogeneity in the fractionation sensitivities of human tumor cell lines: studies in a three-dimensional model system

PURPOSE

Current concepts to optimize the therapeutic gain of radiotherapy by hyperfractionation assume that human tumors are less sensitive to fractionation than late reacting normal tissues. The aim of this study was to investigate the extent of the intercell line heterogeneity of fractionation sensitivity of a wide variety of human tumor cell lines in a three-dimensional model system under fully oxic conditions using schedules with one to eight fractions. Biological characteristics of the tumors that correlate with fractionation sensitivity should be identified.

METHODS AND MATERIALS

A total of 21 cell lines from human tumors maintained as multicellular spheroids consisting of 1000-1500 cells were given fractionated irradiation within a total treatment time of maximally 50 h. Complete dose-spheroid control curves were determined for each fractionation scheme. The spheroid control data were adequately described by the linear quadratic model assuming Poisson statistics. In addition, the induction of a G2 block by a fractionated test dose of seven 3 Gy fractions given at 6-h intervals was determined in spheroid cells using flow cytometry of propidium bromide stained cell nuclei.

RESULTS

The fractionation sensitivities of human tumor cells in multicellular spheroids could be characterized by alpha/beta values, ranging from 2.8-37 Gy in dependence on the cell line. The log normally distributed alpha/beta values were positively correlated with the percentage increase in G2/M phase after the fractionated test dose compared to the controls (r = 0.72, p < 0.01), and were associated with the degree of tumor differentiation (p = 0.01, ANOVA F-test). No significant correlation between the log (alpha/beta) values and the surviving fractions at 2 Gy (SF2) or the total doses with 2 Gy per fraction necessary to control 50% of the spheroids (SCD50) was observed. Despite the intercell line variability of the alpha/beta values, the SCD50 values of the different cell lines, given with one and eight fractions or one fraction and 2 Gy per fraction, were closely associated (Spearman rank correlation coefficients: r = 0.89 or r = 0.90, p < 0.0001).

CONCLUSION

Human tumor cell lines showed a marked heterogeneity in the fractionation sensitivity when irradiated as multicellular spheroids and assayed in situ using the spheroid control end point. Therefore, the therapeutic gain of altered fractionation also depends on those biological characteristics of each individual tumor that affects its fractionation sensitivity. Parameters that correlate with fractionation sensitivity of the tumor lines in the spheroid system were identified as grade of tumor differentiation and percentage increase in G2/M cells at the end of an eight-fraction schedule.

Radiosensitivity, repair capacity, and stem cell fraction in human soft tissue tumors: an in vitro study using multicellular spheroids and the colony assay

Radiation doses necessary to control 50% of spheroids (SCD50) were determined for five human soft tissue tumor lines after single dose and fractionated irradiation. Spheroids with 1000-1500 cells were used throughout. A similar number of cells per spheroid resulted in different sized spheroids for the respective cell lines. The parameters alpha, beta, and the number of regenerating cellular units per spheroid (SRU) were estimated from the spheroid control data using a direct fit according to the linear quadratic model assuming Poisson statistics. The number of spheroid regenerating cellular units was also determined from the growth delay at doses required for 10% spheroid control. In addition, alpha, beta, and the fraction of clonogenic cells of the five cell lines were obtained from a soft agar colony forming assay. The most precise parameter for radiation sensitivity was the SCD50, with a coefficient of variation smaller than 5%. SCD50 values ranged from 5.9 to 11.0 Gy for the five soft tissue tumor lines. Two of the five cell lines showed significantly higher alpha values and lower calculated survival fractions after 2 Gy (SF2) in the soft agar clonogenic assay than in the spheroid control assay. This points to a resistance-enhancing effect in the spheroid system. Whereas the fractions of SRU from the number of cells per spheroid, estimated from the spheroid control and growth delay assays, agreed well, no significant correlation existed between the fraction of SRU and the fraction of clonogenic cells in the soft agar colony forming assay. The alpha/beta ratios as a descriptive measure of the fractionation sensitivity of the tumor cell spheroids in the spheroid control assay corresponded well with those derived from the dose-cell survival data using a soft agar colony forming assay. Two of the five cell lines showed high fractionation sensitivities with alpha/beta values smaller than 5 Gy while those of the remaining three ranged from 7.8 to 10.8 Gy. Spheroids are structurally more similar to in vivo tumors than monolayer cultures. From the observed lack of correlation in the radiosensitivity parameters alpha and SF2 as well as in the fraction of SRU or clonogenic cells obtained from the spheroid control assay or the colony forming assay, one would expect even greater differences between results from colony forming assays and the radiosensitivity of in vivo tumors, at least for human soft tissue sarcomas.

Effectiveness in inhibition of recovery of cell survival by cisplatin and carboplatin: influence of treatment sequence

Clinical protocols have been designed to combine platinum-based drugs and radiation in the treatment of cancer. The rationale for this approach has been developed from preclinical studies demonstrating that platinum compounds can potentiate the cytotoxic effects of radiation toward cells. In the present study multicellular spheroids derived from squamous cell carcinoma cell line HN-1 have been used to study the effects of both cisplatin and carboplatin when administered prior to, concurrently, and after irradiation treatment. To study the influence of platinum compounds on sublethal damage repair, single and split doses of radiation were applied. Growth delay and proportion cured spheroids served as endpoints. Both cisplatin and carboplatin had no potentiating effect when administered 24 hr prior to irradiation. When administered 3 hr after completion of irradiation procedures, growth delay after single and split doses were enhanced to the same extent. The drug enhancement ratio for cisplatin was larger (1.5) than for carboplatin (1.2). Both single and split doses were enhanced by the same factor, which was interpreted as no effect on sublethal damage repair. When platinum compounds were present in the target cells at the time of irradiation, especially the split dose radiation response was strongly enhanced: the drug enhancement ratio was 3.9 for cisplatin and 3.2 for carboplatin. Recovery from sublethal damage was totally repressed. This study shows that platinum compounds can potentiate radiation and that for maximum effect the sequence of the two treatment modalities is of utmost importance. Moreover, these results may in part explain the heterogeneous outcomes of trials combining platinum compounds and radiation.

Radiosensitivity of different human tumor cells lines grown as multicellular spheroids determined from growth curves and survival data

Five human tumor cell lines were grown as multicellular tumor spheroids (MTS) to determine whether multicellular tumor spheroids derived from different types of tumors would show tumor-type dependent differences in response to single-dose irradiation, and whether these differences paralleled clinical behavior. Multicellular tumor spheroids of two neuroblastoma, one lung adenocarcinoma, one melanoma, and a squamous cell carcinoma of the oral tongue, were studied in terms of growth delay, calculated cell survival, and spheroid control dose50 (SCD50). Growth delay and cell survival analysis for the tumor cell lines showed sensitivities that correlated well with clinical behavior of the tumor types of origin. Similar to other studies on melanoma multicellular tumor spheroids our spheroid control dose50 results for the melanoma cell line deviated from the general pattern of sensitivity. This might be due to the location of surviving cells, which prohibits proliferation of surviving cells and hence growth of melanoma multicellular tumor spheroids. This study demonstrates that radiosensitivity of human tumor cell lines can be evaluated in terms of growth delay, calculated cell survival, and spheroid control dose50 when grown as multicellular tumor spheroids. The sensitivity established from these evaluations parallels clinical behavior, thus offering a unique tool for the in vitro analysis of human tumor radiosensitivity.