The potential benefit from a perfect radiosensitizer and its dependence on reoxygenation

Tumor hypoxia in cancer therapy

Human solid tumors are invariably less well-oxygenated than the normal tissues from which they arose. This so-called tumor hypoxia leads to resistance to radiotherapy and anticancer chemotherapy as well as predisposing for increased tumor metastases. In this chapter, we examine the resistance of tumors to radiotherapy produced by hypoxia and, in particular, address the question of whether this resistance is the result of the physicochemical free radical mechanism that produces resistance to radiation killing of cells in vitro. We conclude that a major part of the resistance, though perhaps not all, is the result of the physicochemical free radical mechanism of the oxygen effect in sensitizing cells to ionizing radiation. However, in modeling studies used to evaluate the effect of fractionated irradiation on tumor response, it is essential to consider the fact that the tumor cells are at a wide range of oxygen concentrations, not just at the extremes of oxygenated and hypoxic. Prolonged hypoxia of the tumor tissue also leads to necrosis, and necrotic regions are also characteristic of solid tumors. These two characteristics--hypoxia and necrosis--represent clear differences between tumors and normal tissues and are potentially exploitable in cancer treatment. We discuss strategies for exploiting these differences. One such strategy is to use drugs that are toxic only under hypoxic conditions. The second strategy is to take advantage of the selective induction under hypoxia of the hypoxia-inducible factor (HIF)-1. Gene therapy strategies based on this strategy are in development. Finally, tumor hypoxia can be exploited using live obligate anaerobes that have been genetically engineered to express enzymes that can activate nontoxic prodrugs into toxic chemotherapeutic agents.

Optimization of tumour control probability in hypoxic tumours by radiation dose redistribution: a modelling study

Tumour hypoxia is a known cause of clinical resistance to radiation therapy. The purpose of this work was to model the effects on tumour control probability (TCP) of selectively boosting the dose to hypoxic regions in a tumour, while keeping the mean tumour dose constant. A tumour model with a continuous oxygen distribution, incorporating pO(2) histograms published for head and neck patients, was developed. Temporal and spatial variations in the oxygen distribution, non-uniform cell density and cell proliferation during treatment were included in the tumour modelling. Non-uniform dose prescriptions were made based on a segmentation of the tumours into four compartments. The main findings were: (1) Dose redistribution considerably improved TCP for all tumours. (2) The effect on TCP depended on the degree of reoxygenation during treatment, with a maximum relative increase in TCP for tumours with poor or no reoxygenation. (3) Acute hypoxia reduced TCP moderately, while underdosing chronic hypoxic cells gave large reductions in TCP. (4) Restricted dose redistribution still gave a substantial increase in TCP as compared to uniform dose boosts. In conclusion, redistributing dose according to tumour oxygenation status might increase TCP when the tumour response to radiotherapy is limited by chronic hypoxia. This could potentially improve treatment outcome in a subpopulation of patients who respond poorly to conventional radiotherapy.

Endogenous markers of two separate hypoxia response pathways (hypoxia inducible factor 2 alpha and carbonic anhydrase 9) are associated with radiotherapy failure in head and neck cancer patients recruited in the CHART randomized trial

PURPOSE

Randomized controlled trials have generally shown a benefit from accelerated radiotherapy in head and neck squamous cell carcinoma (HNSCC). However, the large randomized United Kingdom trial CHART (Continuous Hyperfractionated Accelerated Radiotherapy) failed to show a benefit of strongly accelerated over standard radiotherapy (RT) in 918 patients with HNSCC. In this study, we investigated the impact of tumor hypoxia on the outcome of HNSCC patients in the CHART trial. There are two distinct hypoxia inducible factors (HIFs) that control different gene response pathways and we assessed them both with endogenous markers of hypoxia, hypoxia inducible factor HIF-2 alpha (HIF-2) and carbonic anhydrase CA9, an indicator of HIF-1 alpha (HIF-1) function.

METHODS

Tissue from pre-RT biopsies performed in 198 of 918 patients recruited was analyzed for the immunohistochemical expression of HIF-2 and CA9.

RESULTS

A significant association of high HIF2 and of high CA9 reactivity with poor locoregional control (P < .0001 and P = .0002, respectively) and poor survival (P = .0004 and 0.002, respectively) was noted. In multivariate analysis, HIF-2 and CA9 maintained their independent prognostic significance. Coexpression of both pathways had an additive effect, supporting their independent role. The uni-directional hypothesis, that a benefit from randomization to CHART should be seen in the nonhypoxic tumors, was supported by the data (one-tailed P = .04).

CONCLUSION

Expression of endogenous markers of hypoxia for the HIF-1 and HIF-2 pathway is strongly associated with radiotherapy failure. Using immunohistochemical methods it is possible to identify subgroups of HNSCC patients who are highly curable with radiotherapy, or who are excellent candidates for clinical trials on hypoxia-targeting drugs in two distinct pathways.

Inducible repair and the two forms of tumour hypoxia--time for a paradigm shift

Clinical experience shows that there is a therapeutic window between 60 and 70 Gy where many tumours are eradicated, but the function of the adjacent normal tissues is preserved. This implies much more cell kill in the tumour than is acceptable in the normal tissue. An SF2 of 0.5 or lower is needed to account for the eradication of all tumour cells, while an SF2 of 0.8 or higher is needed to explain why these doses are tolerated by normal tissues. No such systematic difference is known between the intrinsic sensitivity of well-oxygenated normal and tumour cells. The presence of radioresistant hypoxic cells in tumours makes it even more difficult to understand the clinical success. However, there is experimental evidence that starved cells lose their repair competence as a result of the depletion of cellular energy charge. MRS studies have shown that low ATP levels are a characteristic feature of solid tumours in rodents and man. In this paper we incorporate the concept of repair incompetence in starving, chronically hypoxic cells. The increased sensitivity of such cells has been derived from an analysis of mammalian cell lines showing inducible repair. It is proportional to the SF2 and highest in resistant cells. The distinction between acutely hypoxic radioresistant cells and chronically hypoxic radiosensitive cells provides the key to the realistic modelling of successful radiotherapy. It also opens new conceptual approaches to radiotherapy. We conclude that it is essential to distinguish between these two kinds of hypoxic cells in predictive assays and models.

Vasculature and microenvironmental gradients: the missing links in novel approaches to cancer therapy?

This paper illustrates how the concept of the malignant cell per se as the prime and only target in cancer therapy may be erroneous. The micro-vasculature evoked to satisfy nutritional requirements of solid tumors, and the inadequacy of this nutrition for all tumor cells, provide novel targeting concepts. The vascular architecture and the microenvironmental gradients (VAMP) will differ from one tumor to another and may determine whether current therapies succeed or fail. Many agents have a different toxicity or mode of action at the pathophysiological oxygen tensions that prevail in solid tumors. This warrants more attention. The hypoxic cell or the immature proliferating endothelial cell may provide tumor specificity that is more general than, and greater than, that conferred by the process of malignant transformation. The poor vasculature of solid tumors is often regarded as a problem by the oncologist. It limits the access of cytotoxic drugs, monoclonal antibodies, cytokines, etc. It also leads to hypoxic radioresistance because of diffusion limited chronic hypoxia and perfusion limited intermittent hypoxia, resulting from transient vessel closure. However, it can also be seen as a potential target, since prolonged vessel occlusion can lead to an avalanche of cell death. Strategies to prevent further expansion of the vascular network (anti-angiogenesis) should stabilize tumors and prevent further growth. Vascular targeting, aiming to damage the microvascular function and cause occlusion, can lead to extensive cell death. The target may relate to the excessive proliferation of endothelial cells in tumors or to abnormal functional aspects, such as altered cell shape (influencing permeability) adhesiveness to leukocytes or steps in the coagulation cascade. These microvascular features and microenvironmental gradients, and the phenotypic consequences of them, have been relatively neglected. The altered milieu and inadequate neovasculature is a common feature of all types of solid tumor, whereas the genetic changes that can give rise to a malignancy are very variable, from tumor site to site and even within a site from individual to individual. It seems, therefore, that therapies that could be of widespread general applicability might more easily be found from the micro-environmental or anti-vascular approaches than from gene therapy targeted at specific oncogenes. This approach will require cross fertilisation between scientists from quite disparate backgrounds, whose paths seldom cross, and who may not read, or even scan, each other's literature. If the endothelium or the low oxygen tension in subsets of tumor cells are the key to successful cancer treatment in mice, there are considerable implications for screening methods in vitro and for predictive and prognostic tests made on homogenized tumor samples.

Pilot study of local hyperthermia, radiation therapy, etanidazole, and cisplatin for advanced superficial tumours

Five patients (six hyperthermia sites) with advanced superficial tumours were treated with combined etanidazole, cisplatin, local hyperthermia, and radiation therapy as part of a Phase I pilot study. Treatment was given once weekly and consisted of etanidazole 3 gm/m2 IV bolus, cisplatin 50 mg/m2 IV bolus, hyperthermia for 60 min with a target temperature of 43 degrees C, and radiation therapy 500 cGy/fraction (median total dose 3000 cGy) for a total of six weeks. Blood levels of etanidazole were taken during treatment at week 1 and week 4. Etanidazole drug exposure was calculated using the trapezoidal rule and expressed as the area under the curve (AUC) of plasma concentration x time. Five of six treatment sites had received prior irradiation. Prior chemotherapy had been given in three patients and tamoxifen therapy given in the other two patients. The median follow-up time is 34 months; 3/5 patients have died of disease. The most significant toxicity was grade I or II nausea and vomiting associated with 19/32 treatments (59%) and a second degree burn in 2/6 fields. None of the five patients experienced peripheral neuropathy, skin ulceration, or needed surgical repair. In addition, there was mild renal toxicity; pharmacokinetic analysis showed a 28-75% increase in the week 1 to week 4 AUC in three patients, all of whom had a decrease in creatinine clearance over the same time of 15-47%. This pilot study suggests this combined modality therapy can be delivered without major complications and that renal function, determined by creatinine clearance, affects clearance of etanidazole and alters the AUC. Therefore, monitoring renal function is important in patients receiving etanidazole in addition to other nephrotoxic agents such as cisplatin. The impact of etanidazole on the therapeutic index of hyperthermia, radiation therapy and cisplatin may be worth of study, especially since a positive interaction between these modalities is found in laboratory models.

Enhancement of tumor radiosensitivity and reduced hypoxia-dependent binding of a 2-nitroimidazole with normobaric oxygen and carbogen: a therapeutic comparison with skin and kidneys

To evaluate the therapeutic potential of normobaric oxygen and carbogen as hypoxic-cell sensitizers, both radiosensitization in a mouse mammary carcinoma, mouse skin and kidneys, and the reduction in the proportion of hypoxic tumor cells were quantified in mice breathing air, oxygen, or carbogen. Local tumor control, acute skin reactions, reduced renal clearance, and hematocrit were used as assays. X rays as 10 fractions in 5 days were given to skin and tumors and 10F/12 days to kidneys. In the tumor study, the pre-irradiation breathing time was varied from 2 to 20 min. Hypoxic cells, before and during a 10F/5 day schedule, were quantified using a 2-nitroimidazole with a theophylline side chain. Bioreductively reduced metabolites of this probe were localized in hypoxic cells that were then stained using an immunofluorescent technique and analyzed by flow cytometry. The fraction of cells with high fluorescence intensity was 19% in air, 9% in oxygen, and 3% in carbogen-breathing mice. For all three gases, hypoxia-dependent binding was similar in non-irradiated tumors and those treated with four or nine fractions. Both gases significantly enhanced tumor radiosensitivity (ER = 1.3 to 1.6) and carbogen was slightly more effective than oxygen. With carbogen, maximum sensitization was observed with a 5 min pre-irradiation breathing interval. With oxygen, pre-irradiation breathing times of 2-20 min gave similar sensitization. In skin an enhancement ratio of 1.2 was observed, whereas enhancement ratios for both renal endpoints were significantly lower (1.0 to 1.07). Relative to both tissues, there was therefore a substantial therapeutic gain by irradiating CaNT tumors under both gases, especially with carbogen.

Biological response in vitro to pulsed high dose rate electrons from a clinical accelerator

Several studies of biological response to ionizing radiation of high absorbed dose rates have been performed, often with conflicting results. The aim of this study was to establish whether a difference between irradiation at high dose rates and at more conventional dose rates could be verified. Pulsed 50 MeV electrons from a clinical accelerator were used both for the high dose rate experiments (mean dose rate: 3.8 x 10(2) Gy/s) and the reference experiments (mean dose rate: 9.6 x 10(-2) Gy/s). In this study V-79 cells were irradiated in vitro. The experiments were carried out under both oxic and anoxic conditions. No significant difference in relative biological effectiveness (RBE) or oxygen enhancement ratio (OER) was observed at the different dose rates investigated.

Effect of normobaric oxygen on tumor radiosensitivity: fractionated studies

The sensitizing ability of 100% normobaric oxygen was investigated in a mouse mammary carcinoma (CaNT) using a variety of fractionated regimens. Both regrowth delay and local control were used as assays of tumor response. With both assays, there was a similar and significant increase in radiosensitivity for all fractionated schedules. Enhancement ratios ranged from 1.24 to 1.45, the highest increase being observed with a 30 fraction schedule given in an overall time of 6 weeks. Thus, in CaNT tumors normobaric oxygen is a far more efficient radiosensitizer in fractionated treatments than the oxygen-mimetic compound misonidazole; an oxygen effect being observed at doses per fraction as low as 1.8 Gy. These results suggest strongly that normobaric gases could play an important role in the clinical management of tumors where hypoxia may limit the outcome of radiotherapy.

Final report of the phase I trial of the hypoxic cell radiosensitizer SR 2508 (etanidazole) Radiation Therapy Oncology Group 83-03

In a Phase I trial SR 2508 was administered by rapid intravenous infusion to 102 patients receiving radiation therapy. The dose-limiting toxicity was peripheral sensory neuropathy (PN) which was related to the cumulative dose administered. The highest single daily dose, 3.7 g/m2, was tolerated without toxicity. The lowest cumulative toxic dose was 21.6 g/m2, and the highest non-toxic dose was 40.8 g/m2. Grade 1 neuropathies were mild and self-limited; grade 2 neuropathies were long-lasting and debilitating. In a retrospective analysis, the risk of developing neurotoxicity was related to the cumulative drug exposure calculated by the area-under-the-curve (AUC) of plasma concentration versus time. There was an increased incidence of neuropathy in patients with a cumulative AUC of greater than or equal to 36 mM-hr. At a total dose of 34 g/m2 over 6 weeks, the incidence of Grade 1 neuropathy was approximately 30%; no grade 2 neuropathy occurred at this dose and schedule. Additional toxicities observed included nausea and vomiting (6%), skin rash (4%), and transient arthralgias (3%). One patient had transient abnormalities in liver function tests of unknown etiology. (In a more recent Phase II trial neutropenia has been observed which may be related to SR2508). Approximately three times more SR 2508 is tolerable compared to misonidazole, and it appears that severe neuropathy can be avoided by monitoring individual patient pharmacokinetic parameters. Evaluation of the efficacy of this hypoxic cell sensitizer is in progress.

Hypoxia and reoxygenation in human melanoma xenografts

Tumor hypoxia and reoxygenation pattern following single dose (10.0 Gy) and fractionated (7 fractions of 2.0 Gy, 1 fraction per day) irradiation were studied in five human melanoma xenograft lines using the paired survival curve method. The hypoxic fractions differed significantly among the melanoma lines; they were found to be 6 +/- 3% (E.E.), 22 +/- 8% (E.F.), 31 +/- 11% (G.E.), 45 +/- 17% (M.F.), and 15 +/- 5% (V.N.). There were no clear correlations between hypoxic fraction and tumor volume-doubling time or vascular density, suggesting that intrinsic cellular characteristics, for example, rate of oxygen consumption and ability to retain clonogenicity under hypoxic stress, also may play an important role for the magnitude of the hypoxic fractions in the melanomas. Reoxygenation was rapid and extensive in all melanoma lines; 12-24 hr after the single dose irradiation or the last fraction of the fractionated irradiation, the hypoxic fractions were similar to those in untreated tumors and stayed at that level up to at least 10 days after irradiation. The hypoxic fractions 1-10 days after irradiation tended to be higher after fractionated than after single dose irradiation, but the differences were not statistically significant. There was a positive correlation between the hypoxic fractions in untreated tumors and the hypoxic fractions after irradiation and reoxygenation, suggesting that it may be possible to predict radiation resistance caused by hypoxia from the hypoxic fractions in tumors before start of radiation therapy. However, hypoxia is probably not a major cause of failure in the radiation therapy of malignant melanoma.

Misonidazole in fractionated radiotherapy of a murine mammary carcinoma: comparison of tumor and normal tissue response

The potential therapeutic benefit of misonidazole was tested in radiotherapy with 1, 2, 5, and 10 equal fractions, using as endpoints local tumor control (TCD50) of murine mammary carcinoma MDAH-MCa-4 and leg contracture at the TCD50, measured 120 days after irradiation. In controls and misonidazole-treated mice, the TCD50 increased with the number of fractions, from 66.7 to 114.6 Gy in controls, and from 43.3 to 75.7 Gy with misonidazole. At doses of greater than or equal to 0.1 mg/g body weight, misonidazole reduced the TCD50 in all fractionation schedules; however, because of toxicity, 1.0 and 0.6 mg/g could be given with only 1 or 2 fractions. Leg contracture at the TCD50 was greatest (14.5 mm) in control mice treated with a single dose of radiation, and was least (7.2 to 7.4 mm) in those treated with a single dose of radiation preceded by 1.0 or 0.6 mg misonidazole/g body weight. With 0.1 mg misonidazole/g, the leg contracture at the TCD50 was less (9.8 to 12.2 mm with the various schedules) than in controls (12.0 to 14.5 mm) for 1, 5, or 10 fractions. Therefore, a therapeutic gain could be obtained by using misonidazole with 1, 2, 5, or 10 fractions, but the greatest gain occurred with 1 fraction, with high doses of misonidazole, that is, 0.6 to 1.0 mg/g.

Polarography of Walker Tumor Submitted to Radiotherapy

A Polarographic study of oxigen was done in 57 rats inoculated with Walker 256 tumor and platinum electrode implanted in muscle and in tumor. The goal of the research was the study of oxygen in tumor before and after irradiation. Tumor growth caused a decrease in tumoral oxygen. Oxygen was always lower in the tumor than in the muscle. Radiotherapy with 2000 rad (but not with 1000 rad) increased oxygen in the tumor.

Sensitizers and radiation dose fractionation: results and interpretations

Misonidazole is generally regarded as having been a clinical failure as a radiation sensitizer. It is hoped that the newer sensitizers SR-2508 and Ro 03-8799 will give better results because single dose studies with animal tumors have indicated that these two drugs give higher enhancement ratios than misonidazole at clinically tolerated doses. Other factors may also have influenced the clinical efficacy of misonidazole, however, particularly reoxygenation during the course of the fractionated treatments. In this paper reoxygenation in animal tumors and experimental studies in which fractionated radiation doses have been combined with sensitizers are reviewed. It is concluded that, even for dose fractions of 2 Gy, reoxygenation may not completely eliminate the influence of hypoxic cells on tumor response, when large total doses are given. Problems associated with tumor heterogeneity are also discussed to highlight the desirability of selecting the most suitable patients for clinical studies. Poorly reoxygenating tumors, rapidly growing tumors and tumors in patients in whom oxygen delivery to tissue is compromised are those whose control is most likely to be improved by combining radiation sensitizers with conventional treatment. However effective sensitizers should also allow fractionation schedules to be modified, to achieve a therapeutic gain, by taking advantage of differences in repair or repopulation between the tumor and critical normal tissue, without having to consider possible detrimental effects on reoxygenation.

Response of human tumour xenografts to fractionated X-irradiation

The response of two human tumour xenografts to single dose and fractionated X-rays has been tested using regrowth delay as the assay. The tumours were line transplanted cells from a moderately well-differentiated squamous carcinoma of the tonsillar fossa (XJ) and an undifferentiated carcinoma of the floor of the mouth (XR). Comparison of the dose response curves for single doses in air, clamped, or after misonidazole administration, led to estimates of the hypoxic fraction (approximately 15%) and the sensitizer enhancement ratio (less than or equal to 1.6). When 5 daily fractions were used, the effect of misonidazole (miso) was lost and reoxygenation appeared to be effective in both tumours. Comparison of single doses and 5 fractions in clamped tumours, and in those sensitized by miso, allowed the sparing effect of fractionation to be estimated. When analysed by the linear quadratic model the alpha/beta ratios were found to be in the range of 6.4-9.2 Gy and 6.8-16.0 Gy for the two tumours. These values are in good agreement with murine tumours (assayed in vivo or in vitro), with human tumour cells assayed in vitro, and with analyses of fractionated clinical data for skin cancer.

Eighth annual Juan del Regato lecture. Chemical modifiers of radiosensitivity--theory and reality: a review

In this review the poor clinical gains from hyperbaric oxygen (HBO) and misonidazole (MISO) are discussed critically. The biggest factor reducing clinical gains is almost certainly reoxygenation. Other possible reasons include vasoconstrictive self-limitation of HBO and neurotoxicity of MISO, so that the radiosensitization of any hypoxic cells in human tumors was not adequate. Nevertheless, there have been some positive clinical results, so that hypoxic cells can sometimes be a problem in some tumors, especially those of the head and neck, even after multiple fraction radiotherapy. While hypoxic cell radioresistance is obviously only one form of radioresistance it is a large factor of resistance when hypoxic cells are present. Current developments are briefly reviewed: the 'new' clinical sensitizers Ro-03-8799 and SR-2508 which should be 3 to 10 times more efficient than MISO if viable hypoxic cells are present; and methods of measuring which human tumors might have significant numbers of hypoxic viable cells.

Hypoxic cell radiosensitizers: expectations and progress in drug development

When misonidazole (MISO) was introduced into clinical trials there were great expectations that the cure rate of many tumors would be dramatically increased. The lack of efficacy of MISO discouraged further studies with hypoxic cell sensitizers. In recent years superior sensitizers SR 2508 and RO-03-8799 have been introduced into the clinic. SR 2508 is less neurotoxic than MISO, allowing more than three times the total amount of drug to be administered. Furthermore, based on the analysis of a patient's plasma pharmacokinetic profile, neurotoxicity may be largely avoidable. RO-03-8799 is superior in that it produces a higher sensitizer enhancement ratio than MISO for the same administered dose. Unlike with MISO and SR 2508, the dose of RO-03-8799 that can be administered is limited by acute toxicity with no cumulative toxicity having yet been encountered. The lack of overlapping toxicities of RO-03-8799 and SR 2508 may permit their simultaneous use with radiation thereby further increasing the utility of this class of compounds. Study design has improved and the expected clinical benefit from sensitizers has been clarified. Sensitizers, like particle radiation therapy and hyperthermia will, if successful, effect the rate of local tumor control, but cannot improve the cure rate of patients with preexisting metastatic disease. Taking into account the need to optimize reoxygenation, the various reasons for tumor radioresistance other than hypoxia, and the lower oxygen and sensitizer enhancement ratios at 200 cGy per fraction, it is likely that sensitizers will provide some clinical benefit for patients with selected tumor types. Future trials with sensitizers may not only provide clinical benefit but may help answer the question as to the role of hypoxia in clinical radiotherapy.

Dose-response relationships for human tumors: implications for clinical trials of dose modifying agents

Clinical benefit from dose modifying agents depends upon the effectiveness of the agents and the steepness of dose response curves for the local control of human tumors by radiotherapy. We have analyzed the two prospective trials and the many retrospective analyses of clinical data from the literature to determine what dose increment is needed to increase local control from 40 to 60%. This increment ranges from 3 to greater than 35%. Thus a dose modifying factor of at least 1.03 (to greater than 1.35) will be necessary for clinical detection of the benefit of a new modality, even if 135 patients are included in each arm of a trial. Two dose levels in the new treatment arm would ensure that therapeutic advantage could be assessed, and would also generate prospective dose response information.

Initial report of the phase I trial of the hypoxic cell radiosensitizer SR-2508

From March 15, through August 31, 1983, 37 patients have been entered on the RTOG Phase I trial of SR-2508. The drug was given intravenously three times weekly for three weeks. The starting total dose was 11.7 g/m2 and the highest total dose given was 32 g/m2. The lower lipophilicity of SR-2508 has produced the expected decrease in terminal half-life (5.4 hrs) of drug excretion and increase in total drug excreted unchanged in the urine (71%) compared to misonidazole or desmethylmisonidazole. The maximum single dose (3.7 g/m2) administered was well tolerated. With multiple doses peripheral neuropathy is the dose-limiting toxicity. The lowest cumulative dose producing toxicity was 21.6 g/m2, the highest non-toxic dose was 29.7 g/m2. The use of an individual patient's drug exposure as measured by the area under the curve of drug concentration vs time may be an excellent predictor of toxicity. This may eventually permit individualization of dose and prevention of serious toxicity. A single dose of 2 g/m2 will produce a tumor concentration of drug (approx. 100 micrograms/ml) that will yield a sensitizer enhancement ratio of 1.5 to 1.7. Using a starting dose of 2 g/m2 three times weekly, patients are now being studied on a five week drug schedule to further evaluate predictability of drug toxicity in preparation for clinical trials of drug efficacy.

Site dependent response of tumours to combined heat and radiation

In previous experiments, large differences in thermal sensitisation were observed for tumours grown on the tails of the chest of mice. The present work reports the results of experiments to compare the response of tumours in four different sites to the radiosensitising effects of both heat and misonidazole. Factors influencing tumour response, e.g., tumour growth rate, blood flow, temperature uniformity, temperature increase during heating and drug availability, were also studied. Tumour response and most of the parameters measured varied according to the site of tumour implantation. Growth rate, blood flow and natural tumour temperature are all likely to be important. However, there appears to be no simple relationship by which tumour response could be predicted, although heat dose, the product of temperature elevation above the natural level and treatment time, may be the most relevant parameter. Clearly the choice of implant site does influence response to treatment. Tumours grown on the extremities may be poor models for human tumours, because of their low natural temperatures.