Studies on the radiobiology of a rat sarcoma treated in situ and assayed in vitro

Drug and radiation resistance in spheroids: cell contact and kinetics

Cells from multicellular spheroids are often more resistant than monolayers to drugs and radiation. While explanations for resistance can be based on differences in cell cycle distribution, inability of the drug to penetrate the spheroid, or the presence of hypoxic cells, these mechanisms do not adequately explain resistance to all agents. Small spheroids (containing about 25-50 cells) exposed to ionizing radiation, hyperthermia, photodynamic therapy, or topoisomerase II inhibitors, are more resistant to killing than monolayers; the close three-dimensional contact in spheroids has been implicated in this resistance. Proposed mechanisms for the 'contact effect' include gap junctional 'reciprocity', cell shape mediated changes in (repair-related) gene expression, and alterations in chromatin packaging which influence DNA repair. The consequences of the contact effect are especially important for multifraction exposures. Another form of resistance can be demonstrated during repetitive treatments; 'regrowth resistance' reflects the capacity of spheroid cells to proliferate more efficiently to compensate for cell killing.

Sensitising effect of misonidazole on NHIK 1922 cells irradiated in vitro or as solid tumour in athymic nude mice

The sensitising effect of the hypoxic cell sensitiser misonidazole (MIS) on NHIK 1922 cells irradiated either as single cells in vitro or as tumours in vivo in air-breathing and in dead mice was investigated with subsequent determination of in vitro colony-forming ability. In absence of MIS no significant difference was found between the dose-response curves after irradiation of tumours in dead mice and of cells in suspension under hypoxic conditions. After irradiation of tumours in air-breathing mice the same enhancement ratio of 1.4 was found whether MIS was administered in doses of 250 or 1000 micrograms/g body weight. In dead mice the enhancement ratio was 1.3 and 2.6 after 250 and 1000 micrograms/g, respectively. The sensitising effect of MIS for cells irradiated in suspension under extremely hypoxic conditions corresponded to that found in dead mice when a comparable concentration of MIS was used.

Brain-tumor therapy. Quantitative analysis using a model system

A recently developed colony-formation assay has been used to evaluate in vivo 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) therapy of a transplantable rat brain-tumor model. A comparison of the in vitro colony-forming capacity of treated and untreated tumor cells permits calculation of the fraction of clonogenic tumor cells surviving in vivo therapy; The plateau that we previously observed o the BCNU dose-response curve is not the result of repair of potentially lethal damage, since no change in the 0.1% of surviving clonogenic tumor cells occurs during the first 2 to 4 days after treatment. Although reanalysis of the dose-response curve indicates that sublethal damage exists, its repair is probably minimal. The most likely explanation for the observed limitation of the BCNU effect is the drug's failure to reach all clonogenic cells. A dose of BCNU that kills more than 99.9% of clonogenic tumor cells within 30 minutes of treatment results in only a 60% decrease in tumor weight by Day 14. This disparity is explained by retarded removal of dead cells, and, along with a previously determined 90% cell-kill threshold necessary to appreciate increased animal survival, demonstrates the inherent limitations of measurements of tumor size (including brain scans and clinical patient evaluations) in evaluating the efficacy of brain-tumor therapy. Following at LD10 dose of BCNU the surviving clonogenic tumor cells increase in number after latency period of 2 to 4 days; during regrowth the cell doubling time is 40 hours. Marked variability in tumor response and regrowth was noted. The determination of information regarding disturbed tumor cell kinetics and tumor heterogeneityis essential for the proper planning of combination chemotherapy and multimodality regimens.

Development of an in vitro colony formation assay for the evaluation of in vivo chemotherapy of a rat brain tumor

An in vitro colony formation assay for the evaluation of in vivo brain tumor therapy has been developed. When plated, disaggregated cells derived from solid tumors proliferated to form relatively homogeneous colonies after a latency period of 2 to 6 days. Increasing concentrations of fetal calf serum enhanced colony-forming efficiency (CFE) with a plateau between 7 and 16%. Supplementation with either irradiated feeder cells (10(3) to 10(5) cells per dish), or medium conditioned by 1 to 3 days of in vitro incubation with the same cell line, doubled the CFE. The density of tumor cells (untreated or previously treated with chemotherapeutic agents) did not affect the CFE when a minimum of 10(4) total cells (tumor plus feeder) were plated. Therefore, in this system the optimal experimental conditions for evaluating chemotherapy and radiotherapy require incubation of disaggregated tumor cells for 12 days in medium containing 10% of fetal calf serum and enough feeder cells to provide a minimum of 10(4) cells per dish. The CFE for untreated tumors was 18 +/- 10% (+/-S.D.), demonstrating that there is significant biological variation. The assay appeared sensitive, with reproducible results, when applied to individual chemically treated tumors. An estimate of the percentage of clonogenic cells affected by in vivo chemotherapy may be obtained by comparing the CFE of cells from treated and untreated tumors. This assay can measure up to a 5 log(10) cell kill, and it should prove to be valuable in developing more effective regimens for the treatment of solid tumors in animals and man.

Cellular radiosensitivity of a rat brain tumor

The response of a model brain tumor system in the rat to single doses of x irradiation has been studied. Solid tumors were exposed in situ, removed, dissociated into single cells, and grown in tissue culture. The fraciton of surviving clonogenic cells wasdetermined as a function of x-ray dose level for tumors irradiated in anesthetized, air-breathing rats, and for tumors irradiated in rats killed 5 minutes prior to irradiation bynitrogen gas asphyxiation. The parameters of the survival curve for tumors irradiatedin air-breathing rats were: DO= 295 rads; DQ= 350 rads; and n= 3.0. Similarly, thesurvival parameters for tumors irradiated in nitrogen gas asphyxiated rats were: DO=625 rads; DQ=675 rads; and n= 3.0. The survival curve of these tumor cells when assayed in in vitro tissue culture only has parameters of: DO= 225 rads; DQ= 350 rads; andn= 4.7. Data form the in vivo to in vivo tumor cell survival curves suggest that there is no significant hypoxic fraction in this brain tumor.

A comparison of the effects of radiation on tumour growth delay and cell survival. The effect of radiation quality

Measurements have been made of the effects of 250 kV X-rays and cyclotron produced neutrons on the delay in growth of a rat fibrosarcoma (RIB5C) and on tumour cell survival assayed in vitro after irradiation in vivo. For doses above 300 rads of neutrons the RBE for cell survival was greaer than that for tumour growth delay. This may be due to X rays causing a greater delay in cell proliferation than neutrons for a given level of cell survival. This would be the opposite effect to that found by BBarendsen and Broerse (1969), irradiating a rat rhabdomyosarcoma. Another possibility is that meassurements of cellular radio-sensitivity which involve removal of cells from their normal environment may lead to incorrect estimates of cell survival in situ. A plot of the RBE for growth delay against the reciprocal of the neutron dose indicated that the dose level at which the RBE became dependent on the fraction of hypoxic tumour cells was larger than that for cell survival, indicating a smaller "effective" hypoxic fraction than that estimated from the cell-survival curves.