Further investigations of the effects of the hypoxic-cell radiosensitizer, Ro-07-0582, on local control of a mouse tumour

Enhanced response to radiotherapy in tumours deficient in the function of hypoxia-inducible factor-1

BACKGROUND AND PURPOSE

To test the hypothesis that deficiency in expression of the transcription factor, HIF-1, renders tumours more radioresponsive than HIF-1 proficient tumours.

PATIENTS AND METHODS

Tumours comprising mouse hepatoma cells lacking HIF-1beta (and thereby HIF-1 function) were grown in nude mice and radiation-induced growth delay compared with that seen for wild-type tumours and tumours derived from HIF-1beta negative cells where HIF-1 function had been restored.

RESULTS

The xenografts that lack HIF-1 activity take longer to establish their growth and are more radioresponsive than both parental xenografts and those with restored HIF-1 function. Pre-treatment of the HIF-1 deficient xenografts with the hypoxic radiosensitizer misonidazole, had little effect on radioresponse. In contrast this treatment radiosensitized the parental xenografts. In spite of this, no difference in oxygenation status was found between the tumour types as measured by Eppendorf O(2)-electrodes and by binding of the hypoxic cell marker NITP. Admixing wild type and HIF-1 deficient cells in the same tumour at ratios of 1 in 10 and 1 in 100 restores the growth of the mixed tumours to that of a 100% HIF-1 proficient cell population. However, when comparing the effects of radiation on the mixed tumours, radioresponsiveness is maintained in those tumours containing the high proportion of HIF-1 deficient cells.

CONCLUSIONS

The differences in radioresponse do not correlate with tumour oxygenation, suggesting that the hypoxic cells within the HIF-1 deficient tumours do not contribute to the outcome of radiotherapy. Thus, hypoxia impacts on tumour radioresponsiveness not simply because of the physio-chemical mechanism of oxygen with radiation-induced radicals causing damage 'fixation', but also because hypoxia/HIF-1 promotes expression of genes that allow tumour cells to survive under these adverse conditions. Further, the results from the cell mixing experiments uncouple the growth promoting effects of HIF-1 and the underlying mechanism by which HIF-1 may increase radiation resistance in solid tumours.

Radioimmunotherapy for pancreatic carcinoma using (131)I-labeled monoclonal antibody Nd2 in xenografted nude mice

We investigated the biodistribution, radiolocalization, and radioimmunotherapeutic potential of (131)I-labeled Nd2 in athymic nude mice bearing human pancreatic carcinoma xenografts. (131)I-Nd2 was accumulated at high levels in the tumor, in contrast to blood, liver, spleen, and other normal organs. The tumor was clearly delineated in scintigraphs. The volumes of tumors of mice injected with 7.4 MBq of (131)I-Nd2 were 80% less than those of tumors before injection of radiolabeled Nd2. Fibrous or vacuolar degeneration was seen in histological sections of tumors of 7-week-treated mice. The growth of tumors in mice treated with misonidazole, a hypoxic cell radiosensitizer, and then injected twice with 3.7 MBq of (131)I-Nd2 was suppressed over 7 weeks. Neither leucocytopenia nor thrombocytopenia was severe after injection of radiolabeled Nd2. Thus (131)I-labeled Nd2 may have clinical application in the radioimmunotherapy of pancreatic cancer.

Tumor hypoxia: its impact on cancer therapy

The presence of radiation resistant cells in solid human tumors is believed to be a major reason why radiotherapy fails to eradicate some such neoplasms. The presence of unperfused regions containing hypoxic cells may also contribute to resistance to some chemotherapeutic agents. This paper reviews the evidence that radiation resistant hypoxic cells exist in solid tumors, the assumptions and results of the methods used to detect hypoxic cells, and the causes and nature of tumor hypoxia. Evidence that radiation resistant hypoxic cells exist in the vast majority of transplanted rodent tumors and xenografted human tumors is direct and convincing, but problems with the current methodology make quantitative statements about the magnitude of the hypoxic fractions problematic. Evidence that radiation resistant hypoxic cells exist in human tumors is considerably more indirect than the evidence for their existence in transplanted tumors, but it is convincing. However, evidence that hypoxic cells are a significant cause of local failure after optimal clinical radiotherapy or chemotherapy regimens is limited and less definitive. The nature and causes of tumor hypoxia are not definitively known. In particular, it is not certain whether hypoxia is a chronic or a transient state, whether hypoxic cells are proliferating or quiescent, or whether hypoxic cells have the same repair capacity as aerobic cells. A number of new methods for assessing hypoxia are reviewed. While there are still problems with all of the new techniques, some of them have the potential of allowing the assessment of hypoxia in individual human tumors.

Radiation therapy of rat brain tumor using misonidazole as hypoxic cell sensitizer

Rat brain tumor was used as a model to evaluate radiation therapy with and without misonidazole. BD-IX rats were implanted intracerebrally with an ethylnitrosourea-induced glioma. Three series of experiments were performed, with radiation given 14 days after inoculation of the glioma clone. In each series, the following radiation doses were given: 500 rads once, 1,000 rads once; and 1,000 rads twice, every time with or without two different doses of misonidazole. Radiation therapy significantly prolonged survival when compared to the longevity of the control group. The dose of 1,000 rads given twice was highly effective and the life-span of tumor-bearing rats increased from 72% to 121%. Misonidazole plus irradiation negated the prolongation of survival, achieved with radiation therapy alone.

Radiation sensitization and chemopotentiation: RSU 1069, a compound more efficient than misonidazole in vitro and in vivo

Electron affinity as measured by the one-electron reduction potential, E17, is the major factor influencing radiosensitizing efficiency in vitro. RSU 1069 has an electron affinity (E17 = -398 mV) similar to misonidazole; however, the ability of this compound to sensitize hypoxic cells is considerably greater than that of misonidazole, e.g. 0.2 mM RSU 1069 gives an enhancement ratio of 2.2 compared to 1.5 for the same concentration of misonidazole. Radiosensitization studies with the MT tumour in vivo also showed RSU 1069 to be a more efficient sensitizer than misonidazole. An administered dose of only 0.08 mg g-1 RSU 1069 yielded an enhancement of 1.8 to 1.9 using tumour cell survival and tumour cure as end-points. The ability of RSU 1069 to potentiate the cytotoxic action of melphalan towards the MT tumour was also examined. RSU 1069 (0.08 mg g-1) given to mice 1 h before melphalan resulted in an enhancement of 3.0. In contrast, previous studies had shown with a series of nitroimidazoles including misonidazole that Ro 03-8799 was the most effective potentiating agent, but this only gave an enhancement of 2.3 at a 10-fold higher dose than RSU 1069. RSU 1069 is a compound of substantial promise both as a radiosensitizer and chemopotentiating agent and warrants further investigation.

Tumor-to-tumor variability in the hypoxic fractions of experimental rodent tumors

Paired determinations of the radiation responses of normally-aerated and artificially hypoxic rodent tumors, performed to measure the hypoxic fractions of the tumors, were obtained from our own laboratories and from the literature. The data were reanalyzed to assess whether the variabilities in the radiation responses of the normally-aerated and artificially hypoxic tumors were similar. If there were large differences in the hypoxic fractions of individual tumors within the experiments, the variability in the data from aerobic tumors would be expected to be greater than the variability in the data from artificially hypoxic tumors (which should all be brought to uniform hypoxia and therefore uniform radioresistance). The analyses revealed the variability to be as great or greater for hypoxic tumors as for normally-aerated tumors. This finding suggests that factors other than tumor-to-tumor differences in oxygenation produce most of the variability in the radiation responses of individual tumors from an experimental tumor line.

Hypoxic fractions of solid tumors: experimental techniques, methods of analysis, and a survey of existing data

Hypoxic fractions are measured by indirect techniques, which compare the response of tumors to large single doses of radiation given under normal aeration and artificial hypoxia. This paper reviews hypoxic fraction measurements and measurement techniques, giving particular attention to the biological, technical, and statistical aspects of the assays; the implicit assumptions underlying the analyses; and the dependence of the determinations on the assay conditions and the tumor and host characteristics. The three major hypoxic fraction assay techniques (paired survival curve, clamped tumor control, and clamped growth delay) share common biological assumptions. They require that the survival curves of naturally and artificially hypoxic cells have the same slope and intercept. They assume that the majority of the cells are either fully oxic or fully hypoxic. They assume that the methods used to induce artificial hypoxia leave no oxygenated regions and that tumor cells rendered artificially hypoxic are no less viable than cells in normally-aerated tumors. The universal validity of these assumptions is questionable. Each technique uses additional special assumptions and each may measure a different population of hypoxic cells. This paper reviews 92 hypoxic fraction determinations in 42 tumor systems. Radiobiologically hypoxic cells appear to be present in the majority of macroscopic solid rodent tumors. The hypoxic fraction was found to increase as the tumor size increased from microscopic to macroscopic; the dependence of hypoxic fraction on tumor size at macroscopic sizes was less clear. The site of tumor implantation, the use of anesthesia, and certain host characteristics may influence the hypoxic fraction. The hypoxic fraction generally did not depend on the tumor growth rate, transplantation history, or histology. These findings indicate that hypoxic cells are a common feature of solid tumors in rodents and provide no evidence that hypoxic cells should not be present in human tumors.

Effect of misonidazole on the radiosensitivity and repair of potentially lethal damage of L5178Y ascites tumor cells

The radiosensitizing effect of low concentrations of misonidazole was investigated by using L5178Y cells growing as an ascites tumor in DBA-2 mice. The cells were irradiated in vivo with graded doses of X-rays in the presence or absence of 0.1-0.5 mg/g body weight of misonidazole. Then cell survival was assayed in vitro by plating cells in soft agar medium. By analyzing the X-ray survival curves with or without misonidazole, the dose-modifying effects were determined. The results indicated that the slope and shoulder of the survival curves were greatly modified by the treatment with misonidazole. The dose-modifying factor in terms of the D0 ratio between the drug-treated and untreated control cells was increased as the drug concentration was increased. Further, it was revealed that the isoeffect dose ratios, estimated by the linear quadratic equation of Chadwick and Leenhouts, were higher at a low-radiation dose range. This is due to the suppression of the shoulder region of survival curves for the drug-treated cells. The inhibition of the X-ray-induced repair of potentially lethal damage was apparent with 0.1 mg/g body weight of misonidazole. The inhibition became more effective as the drug concentration increased.

Misonidazole enhancement of radiation-induced growth delay in rat rhabdomyosarcoma tumours exposed to accelerated carbon and neon ions

The response of a rat rhabdomyosarcoma tumour was assessed by measurements of radiation-induced growth delay resulting from administration of the hypoxic cell sensitizer misonidazole in combination with single and fractionated doses of X-rays and charged-particle radiation. Enhancement ratios of 1.8--2.1 were obtained following single doses of misonidazole (500 mg/kg i.p.) and 225 kV X-rays. Single doses of misonidazole with either carbon-ion or neon-ion radiation in the 4 cm extended-peak ionization region led to enhancement ratios of 1.2--1.3. When combined misonidazole (300 mg/kg i.p.) and X-ray treatments were given in four daily fractions, the enhancement ratios decreased to 1.2--1.5. However, a four-fraction schedule using either carbon-ion or neon-ion radiation in combination with misonidazole gave enhancement ratios of 1.1--1.3, which are similar to the values obtained for single-dose schedules with the sensitizer and charged-particle radiation.