Biological properties and response to x-rays of first-generation transplants of spontaneous mammary carcinomas in C3H mice

Variations of the hypoxic fraction in the SCC VII tumors after single dose and during fractionated radiation therapy: assessment without anesthesia or physical restraint of mice

Variations of the hypoxic fraction (HF) after single dose (13 Gy or 4 Gy) and during fractionated (5 fractions of 4 Gy, 1 or 2 fractions per day) radiation therapy were studied in SCC VII tumors implanted subcutaneously in the hind legs of C3H/He/Jms mice using the paired survival curve method. Whole-body irradiation was delivered to tumor-bearing mice without anesthesia or physical restraint, because both are known to increase the HF artificially. The HF decreased after a single 13 Gy dose in a biphasic fashion: extremely rapidly within 1 hr and comparatively slowly during the following 12-72 hr. On the other hand, nearly no fall of HF was observed in 24 hr following a single 4 Gy dose. Also, reoxygenation was found to occur more rapidly in the interfraction period as the number of fractions of 4 Gy increased irrespective of differences of interfraction time. However, the HF just before each radiation fraction was significantly higher than the pretreatment level for both fractionated regimens. Thus, the reoxygenation patterns observed after single low and high doses of irradiation were different from each other, and reoxygenation in each interfraction period did not always proceed in a similar manner to that after single low dose irradiation. Reoxygenation was facilitated as fractionated radiation therapy proceeded, but it was not sufficient for the HF to remain at a level comparable to that before irradiation.

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.

The effect of recovery from potentially lethal damage on the determination of repair and repopulation in a murine tumour

Repair and repopulation following X irradiation of clamped-off murine anaplastic MT tumours was investigated using the established method of (Dn-D1)/(n-1). Repair was complete in 4 h, similar in extent to that reported in other tumours, and within the range of that reported for normal tissues. Subsequent repopulation commenced after 4 days and was equivalent to 1.8 Gy/day recovered dose, corresponding to a clonogenic cell number doubling time of 1.8 days. However, estimates of repair and repopulation may have been in error because the chronically hypoxic cells in this tumour alone have the ability to recover from potentially lethal damage (PLD) and so are more radioresistant than cells rendered acutely hypoxic by clamping. Because of this, even clamping off tumours at irradiation does not render all cell populations equally radioresistant, and so reoxygenation between fractions could result in an underestimate of repair and repopulation. Further, the differing sensitivity between acutely and chronically hypoxic cells renders the apparent OER a function of dose (i.e., oxygen not truly dose-modifying to chronically hypoxic cells). Consequently it is incorrect to assume a constant OER in order to compare repair in tumours irradiated under hypoxic conditions with that in normal tissues irradiated under aerobic conditions. It will be argued here that in the case of the present tumour neither reoxygenation nor the choice of OER will have qualitatively altered the conclusion reached from the conventional method.

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.

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.

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

The potential benefit of a perfect radiosensitizer has been assessed by computing the sensitization ratios that would be observed in a mixed population of oxic and hypoxic cells if different reoxygenation rates existed. The sensitizer has been assumed to be as effective as oxygen, completely non-toxic and freely diffusible to all hypoxic cells within the tumour. The calculations have been made for several different clinical fractionation regimes, namely 30, 20, 9 or 6 fractions, all with the same ret dose (NSD = 1700 rets). These calculations have allowed us to deduce how large the observed sensitization would be for differing rates of reoxygenation and for the different fractionation schemes. The size of the extrapolation number is seen to be an important parameter in these calculations. They have allowed us to indicate how much reoxygenation would be needed to abolish the benefit from (and hence the need for) a perfect radiosensitizer.

In vivo assessment of basic 2-nitroimidazole radiosensitizers

The radiosensitizing efficiencies of 4 structural analogues of misonidazole (MISO) have been compared with that of the parent compound. Three of these were charged basic compounds, previously shown in vitro to be 10 times more efficient. Enhancement ratios were measured from pairs of tumour growth-delay curves for the mouse fibrosarcoma SA Fab. Two routes of administration and ranges of drug dose and intervals between injection and irradiation were tested. Drug concentrations in blood, brain and tumor were measured using high-performance liquid chromatography. The peak concentration in tumours coincided with the peak in radiosensitization: 20 min after i.v. injection and 40 min after i.p. injection. The concentration in tumours was similar for either route. Comparison of radiosensitizing efficiency on the basic of equal administered dose showed no difference between the 5 compounds, but after equimolar doses the charged compounds achieved lower tumour concentrations. Comparison of sensitizing efficiency on the basis of tumour concentration showed that they were 3 times more potent than MISO, as predicted from their higher electron-affinity. The resultant improvement in radiosensitization at low, clinically relevant, concentrations is so slight that any therapeutic benefit would depend on reduced drug toxicity in man.

Differences in vascular pattern between the spontaneous and the transplanted C3H mouse mammary carcinoma

The transplanted C3H tumor differs from the spontaneous in its multi-focal origin and in its increasing loss of differentiation. Loss of differentiation is principally important in that even the partial branching into lobes, which occurs in the spontaneous tumour, is lost and that the parenchyma no longer consists of single-layered and multi-layered tubules, as in the spontaneous tumour, but of multi-layered only. Absence of branching removes the interlobular pressures, which in spontaneous tumours lead to infarction of the efferent vessels and sinus formation. Consequently; sinuses are absent from the transplanted tumour and the circulation is thereby improved. In contrast, since the vascular pattern of the spontaneous tumour is preserved and capillaries do not penetrate into the multi-layered tubules, although these have developed into solid cylinders, the diffusion pathway of oxygen is extended, resulting in necrosis at their cores. Accumulating necrotic fluid is at first removed by lymphatics, but subsequently invades the efferent vascular system. These changes are reflected in the pattern of tumour growth and in the radiation responses.

Recovery from sublethal damage by acutely hypoxic tumour cells in vivo and in vitro

The ability of acutely hypoxic tumour cells to recover from sublethal damage following irradiation in vivo and in vitro has been measured using a single tumour system. The methods of assay were tumour growth delay, local tumour control and tumour cell survival in vitro following treatment in vivo or in vitro. Tumours in vivo or cells in vitro rendered acutely hypoxic during irradiation were irradiated with either single doses or two doses 24 hours apart. Cells left in situ had a greater capacity for recovery than those treated either in vivo or in vitro and then assayed in vitro. It is suggested that tumours may not show a systematically reduced capacity for recovery relative to normal tissues, unless chronically hypoxic tumour cells have a reduced capacity for recovery and determine the response. However, the results imply that deductions as to the ability of tumour cells to recover from sublethal damage (whether chronically hypoxic or not), which rely on in vitro assays, may underestimate the extent of recovery.

The effect of recovery from potentially lethal damage on the determination of reoxygenation in a murine tumour

The pattern of reoxygenation in the murine anaplastic MT tumour was investigated using the established method of determining the hypoxic fraction, at intervals after a priming X-ray dose, from test doses given either to unclamped or clamped-off tumours. Little reoxygenation was apparent whilst the tumour was increasing in size for 12--72 hours after a single dose of 20.3 Gy, but extensive reoxygenation was evident whilst the tumour was shrinking at nine days after a dose of 50 Gy. However, the degree of reoxygenation may have been underestimated, especially after the smaller priming dose. This is because only the chronically hypoxic cells in this tumour have the ability to recover from potentially lethal damage (PLD) and so are more radioresistant than cells rendered acutely hypoxic by clamping. Because of this, even when tumours are clamped off during irradiation, the resulting survival curve is biphasic and the apparent effect of the clamp becomes a function of the X-ray dose used. The larger the dose, the smaller the observed effect of the clamp, so the greater the apparent hypoxic fraction and hence the smaller the apparent degree of reoxygenation.

The effect of vascular occlusion on the thermal sensitization of a mouse tumour

The effect of occluding the blood supply to a mouse tumour (with a metal clamp) has been studied for both irradiation and heating. Local heat was applied by immersion in a water bath for one hour at 42.8 degrees C or for 15 minutes at 44.8 degrees C. Occlusion of the blood supply during heating has a profound cytotoxic effect on the tumour, even in the absence of irradiation. Most tumours treated with 42.8 degrees C for one hour under clamped conditions were locally controlled whether they were irradiated or not. Tumours heated with their blood supply unobstructed showed a lesser sensitivity to heat, seen as an increased sensitivity to X rays with a thermal enhancement ratio of 1.8--2.6. With the shorter period of more intense heat (44.8 degrees C for 15 min), the effect of increasing the clamping time before heating was studied. The proportion of tumours locally controlled increased from 33% if the clamp was applied immediately before heating to 83% if the clamp was present for 60 minutes before heating commenced. No cures were observed for heat applied immediately before clamping, or immediately after release of the clamp. Accumulation of metabolic products or pH changes are implicated as the factors which alter the thermal sensitivity of these tumour cells.

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

The tumour used, designated MT1, is a more radiosensitive form of the anaplastic MT tumour previously described. No explanation for the increased radiosensitivity was found, but it was shown not to be due to infection or to a change in immunological status, growth rate or histology. The sensitivity has remained constant throughout the present work. No cytotoxicity in the tumour was observed when 1 mg/g body weight of Ro-07-0582 was injected immediately after a single dose of X-rays; indeed a small protective effect was seen. A radiosensitization enhancement of 1-5 was achieved with a relatively low drug dose of Ro-07-0582 in a 5F/4d fractionated regime. The interval between the injection of a low dose of Ro-07-0582 and the start of irradiation was found to be critical, the optimum interval being 45-60 min. The subsequent incidence of distant metastases was not increased by the use of Ro-07-0582 at the time of "primary" tumour irradiation.

Hypoxic cell radiosensitizers and local control by X-ray of a transplanted tumour in mice

Tumour experiments including local control after X-irradiation have been performed, using a new technique that eliminates the need for anaesthetics in restraining the animals. This system has been used to investigate the degree of sensitization that can be achieved with ICRF 159 and 4 strongly electron-affinic radiosensitizers, nifurpipone dihydrochloride, metronidazole, Ro-11-3696 and Ro-07-0582. No significant enhancement of the radiation effect was observed with ICRF 159. Significant sensitization was achieved by all 4 nitro-heterocyclic compoinds, Ro-07-0582 being the most effective, metronidazole and Ro-11-3696 the next, and nifurpipone dihydrochloride the least effective. For Ro-07-0582 and metronidazole, several concentrations were investigated, and the interval between injection with Ro-07-0582 and irradiation was varied: an interval of 30 min gave more sensitization than an interval of 90 min. The results from the local control experiments using Ro-07-0582 have been compared with those obtained from regrowth delay experiments. The radiosensitization obtained by the Ro-07-0582 increased with the X-ray dose above 25 gray. Both metronidazole and Ro-07-0582 gave significant enhancement effect at serum concentrations which can be achieved in man.

Tumour regression as a guide to prognosis: a study with experimental animals

An analysis has been made of the relationship between regression during therapy and the probability of local control in a group of mice treated with single doses and fractionated doses of X rays. The tumours were first generation transplants from spontaneous mammary carcinomas in C3H mice and were irradiated with single doses 3F/4 days, 9F/10 days, 9F/18 days or 15F/18 days. The size of the smallest radiation dose approached those encountered in clinical radiotherapy. A significant correlation was observed between the shrinkage during the treatment period and the local control at 150 days, for three of the four fractionated schedules. A weaker correlation was observed for shrinkage within a week after single doses, and for shrinkage during treatment with nine fractions in 18 days. It is postulated that the rate of shrinkage is an inherent characteristic of each individual tumour and does not reflect the number of cells killed. However, the shrinkage in some tumours during the course of therapy may result in more extensive reoxygenation which makes these tumours more sensitive to subsequent doses of fractionated course. The degree of shrinkage within the period of fractionated irradiation was found to be a useful prognostic guide to ultimate local control in individual tumours.

Radiosensitizing and cytocidal effects on hypoxic cells of RO-07-0582, and repair of x-ray injury, in an experimental mouse tumour

The delay in regrowth to 10 mm diameter of a transplanted carcinoma in mice was used to estimate the effect of the hypoxic-cell radiosensitizer Ro-07-0582. When 1 mg/g body wt. was given before a single dose of X-rays, a dose-enhancement ratio of 2-0 was found. When the drug was given immediately after irradiation, a large cytotoxic effect was observed, equivalent to an enhancement ratio of 1-3. These results were confirmed by determining the X-ray doses required for the local control of 50% of the tumours at 80 days after irradiation. The capacity of the tumour for repair of sublethal X-ray injury within 24 h was similar to that for several normal tissues.