ABSTRACT
BACKGROUND: Earlier in vitro studies show that irradiation with an ultra-low dose-rate of 15 mGy/h delivered with [3H]-valine leads to loss of clonogenicity in hypoxic T-47D cells. Here, the aim was to determine if [3H]-valine could be used to deliver low dose-rate irradiation in a colorectal cancer model. METHODS: Clonogenicity was measured in cultured cancer cell line HT29 irradiated with 15 mGy/h combined with intermittent hypoxia. Mice with HT29 xenografts were irradiated by repeated injections of [3H]-valine intravenously. The activity in the tumor tissue was measured by scintillation counting and tumor growth, hypoxic fraction and tritium distribution within tumors were assessed by pimonidazole staining and autoradiography. RESULTS: Ultra-low dose-rate irradiation decreased clonogenicity in hypoxic colorectal cancer cells. In vivo, the tumor growth, hypoxic fraction and weight of the mice were similar between the treated and untreated group. Autoradiography showed no [3H]-valine uptake in hypoxic tumor regions in contrast to aerobic tissue. CONCLUSION: Continuous low-dose-rate irradiation was well tolerated by aerobic tissue. This indicates a potential use of low dose-rate irradiation to target hypoxic tumor cells in combination with high dose-rate irradiation to eradicate the well oxygenated tumor regions. However, [3H]-valine is not the appropriate method to deliver ultra-low dose-rate irradiation in vivo.
Subject(s)
Colorectal Neoplasms/pathology , Colorectal Neoplasms/radiotherapy , Tritium/therapeutic use , Tumor Hypoxia/radiation effects , Valine/therapeutic use , Animals , Cell Line, Tumor , Cell Survival/radiation effects , HT29 Cells , Humans , Mice , Mice, Inbred BALB C , Mice, Nude , Radiotherapy Dosage , Valine/chemistry , Xenograft Model Antitumor AssaysABSTRACT
PURPOSE: To investigate in detail the earlier observed combined effect of low dose-rate ß-irradiation delivered at a dose-rate of 15 mGy/h and continued intermittent hypoxia that leads to extensive cell death after approximately 3-6 weeks. MATERIAL AND METHODS: Continuous low dose-rate ß-irradiation at a dose rate of 15, 1.5 or 0.6 mGy/h was given by incorporation of [(3)H]-labelled valine into cellular protein. The cells were cultivated in an atmosphere with 4% O2 using an INVIVO2 hypoxia glove box. Clonogenic capacity, cell-cycle distribution and cellular respiration were monitored throughout the experiments. RESULTS: After 3-6 weeks most cells died in response to the combined treatment, giving a surviving fraction of only 1-2%. However, on continued cultivation a few cells survived and restarted proliferation as the cellular oxygen supply increased with the reduced cell number. Irradiating the T-47D cells grown in an atmosphere with 4% O2 at dose-rates 10 and 25 times lower than 15 mGy/h did not have a pronounced effect on the clonogenic capacity with surviving fractions of 60-80%. CONCLUSIONS: Treatment of T-47D cells with low dose-rate ß-irradiation leads to a specific effect on intermittent hypoxic cells, inactivating more than 98% of the cells in the population. Given improved oxygen conditions, the few surviving cells can restart their proliferation.
