Quantitation of the radiotherapeutic importance of naturally-hypoxic normal tissues from collated experiments with rodents using single doses

How quickly does FLASH need to be delivered? A theoretical study of radiolytic oxygen depletion kinetics in tissues

Purpose. Radiation delivered over ultra-short timescales ('FLASH' radiotherapy) leads to a reduction in normal tissue toxicities for a range of tissues in the preclinical setting. Experiments have shown this reduction occurs for total delivery times less than a 'critical' time that varies by two orders of magnitude between brain (∼0.3 s) and skin (⪆10 s), and three orders of magnitude across different bowel experiments, from ∼0.01 to ⪆(1-10) s. Understanding the factors responsible for this broad variation may be important for translation of FLASH into the clinic and understanding the mechanisms behind FLASH.Methods.Assuming radiolytic oxygen depletion (ROD) to be the primary driver of FLASH effects, oxygen diffusion, consumption, and ROD were evaluated numerically for simulated tissues with pseudorandom vasculatures for a range of radiation delivery times, capillary densities, and oxygen consumption rates (OCR's). The resulting time-dependent oxygen partial pressure distribution histograms were used to estimate cell survival in these tissues using the linear quadratic model, modified to incorporate oxygen-enhancement ratio effects.Results. Independent of the capillary density, there was a substantial increase in predicted cell survival when the total delivery time was less than the capillary oxygen tension (mmHg) divided by the OCR (expressed in units of mmHg/s), setting the critical delivery time for FLASH in simulated tissues. Using literature OCR values for different normal tissues, the predicted range of critical delivery times agreed well with experimental values for skin and brain and, modifying our model to allow for fluctuating perfusion, bowel.Conclusions. The broad three-orders-of-magnitude variation in critical irradiation delivery times observed inin vivopreclinical experiments can be accounted for by the ROD hypothesis and differences in the OCR amongst simulated normal tissues. Characterization of these may help guide future experiments and open the door to optimized tissue-specific clinical protocols.

Enabling ultra-high dose rate electron beams at a clinical linear accelerator for isocentric treatments

BACKGROUND AND PURPOSE

Radiotherapy delivery with ultra-high dose rates (UHDR) has consistently produced normal tissue sparing while maintaining efficacy for tumour control in preclinical studies, known as the FLASH effect. Modified clinical electron linacs have been used for pre-clinical studies at reduced source-surface distance (SSD) and novel intra-operative devices are becoming available. In this context, we modified a clinical linac to deliver 16 MeV UHDR electron beams with an isocentric setup.

MATERIALS AND METHODS

The first Varian TrueBeam (SN 1001) was clinically operative between 2009-2022, it was then decommissioned and converted into a research platform. The 18 MeV electron beam was converted into the experimental 16 MeV UHDR. Modifications were performed by Varian and included a software patch, thinner scattering foil and beam tuning. The dose rate, beam characteristics and reproducibility were measured with electron applicators at SSD = 100 cm.

RESULTS

The dose per pulse at isocenter was up to 1.28 Gy/pulse, corresponding to average and instantaneous dose rates up to 256 Gy/s and 3⋅105 Gy/s, respectively. Beam characteristics were equivalent between 16 MeV UHDR and conventional for field sizes up to 10x10cm2 and an overall beam reproducibility within ± 2.5% was measured.

CONCLUSIONS

We report on the first technical conversion of a Varian TrueBeam to produce 16 MeV UHDR electron beams. This research platform will allow isocenter experiments and deliveries with conventional setups up to field sizes of 10x10 cm2 within a hospital environment, reducing the gap between preclinical and clinical electron FLASH investigations.

A phenomenological model of proton FLASH oxygen depletion effects depending on tissue vasculature and oxygen supply

Introduction

Radiation-induced oxygen depletion in tissue is assumed as a contributor to the FLASH sparing effects. In this study, we simulated the heterogeneous oxygen depletion in the tissue surrounding the vessels and calculated the proton FLASH effective-dose-modifying factor (FEDMF), which could be used for biology-based treatment planning.

Methods

The dose and dose-weighted linear energy transfer (LET) of a small animal proton irradiator was simulated with Monte Carlo simulation. We deployed a parabolic partial differential equation to account for the generalized radiation oxygen depletion, tissue oxygen diffusion, and metabolic processes to investigate oxygen distribution in 1D, 2D, and 3D solution space. Dose and dose rates, particle LET, vasculature spacing, and blood oxygen supplies were considered. Using a similar framework for the hypoxic reduction factor (HRF) developed previously, the FEDMF was derived as the ratio of the cumulative normoxic-equivalent dose (CNED) between CONV and UHDR deliveries.

Results

Dynamic equilibrium between oxygen diffusion and tissue metabolism can result in tissue hypoxia. The hypoxic region displayed enhanced radio-resistance and resulted in lower CNED under UHDR deliveries. In 1D solution, comparing 15 Gy proton dose delivered at CONV 0.5 and UHDR 125 Gy/s, 61.5% of the tissue exhibited ≥20% FEDMF at 175 μm vasculature spacing and 18.9 μM boundary condition. This percentage reduced to 34.5% and 0% for 8 and 2 Gy deliveries, respectively. Similar trends were observed in the 3D solution space. The FLASH versus CONV differential effect remained at larger vasculature spacings. A higher FLASH dose rate showed an increased region with ≥20% FEDMF. A higher LET near the proton Bragg peak region did not appear to alter the FLASH effect.

Conclusion

We developed 1D, 2D, and 3D oxygen depletion simulation process to obtain the dynamic HRF and derive the proton FEDMF related to the dose delivery parameters and the local tissue vasculature information. The phenomenological model can be used to simulate or predict FLASH effects based on tissue vasculature and oxygen concentration data obtained from other experiments.

Determining the parameter space for effective oxygen depletion for FLASH radiation therapy

There has been a recent revival of interest in the FLASH effect, after experiments have shown normal tissue sparing capabilities of ultra-high-dose-rate radiation with no compromise on tumour growth restraint. A model has been developed to investigate the relative importance of a number of fundamental parameters considered to be involved in the oxygen depletion paradigm of induced radioresistance. An example eight-dimensional parameter space demonstrates the conditions under which radiation may induce sufficient depletion of oxygen for a diffusion-limited hypoxic cellular response. Initial results support experimental evidence that FLASH sparing is only achieved for dose rates on the order of tens of Gy/s or higher, for a sufficiently high dose, and only for tissue that is slightly hypoxic at the time of radiation. We show that the FLASH effect is the result of a number of biological, radiochemical and delivery parameters. Also, the threshold dose for a FLASH effect occurring would be more prominent when the parameterisation was optimised to produce the maximum effect. The model provides a framework for further FLASH-related investigation and experimental design. An understanding of the mechanistic interactions producing an optimised FLASH effect is essential for its translation into clinical practice.

Heavy charged particle beam therapy and related new radiotherapy technologies: The clinical potential, physics and technical developments required to deliver benefit for patients with cancer

In the UK, one in two people will develop cancer during their lifetimes and radiotherapy (RT) plays a key role in effective treatment. High energy proton beam therapy commenced in the UK National Health Service in 2018. Heavier charged particles have potential advantages over protons by delivering more dose in the Bragg peak, with a sharper penumbra, lower oxygen dependence and increased biological effectiveness. However, they also require more costly equipment including larger gantries to deliver the treatment. There are significant uncertainties in the modelling of relative biological effectiveness and the effects of the fragmentation tail which can deliver dose beyond the Bragg peak. These effects need to be carefully considered especially in relation to long-term outcomes.In 2019, a group of clinicians, clinical scientists, engineers, physical and life scientists from academia and industry, together with funding agency stakeholders, met to consider how the UK should address new technologies for RT, especially the use of heavier charged particles such as helium and carbon and new modes of delivery such as FLASH and spatially fractionated radiotherapy (SFRT).There was unanimous agreement that the UK should develop a facility for heavier charged particle therapy, perhaps constituting a new National Ion Research Centre to enable research using protons and heavier charged particles. Discussion followed on the scale and features, including which ions should be included, from protons through helium, boron, and lithium to carbon, and even oxygen. The consensus view was that any facility intended to treat patients must be located in a hospital setting while providing dedicated research space for physics, preclinical biology and clinical research with beam lines designed for both in vitro and in vivo research. The facility should to be able to investigate and deliver both ultra-high dose rate FLASH RT and SFRT (GRID, minibeams etc.). Discussion included a number of accelerator design options and whether gantries were required. Other potential collaborations might be exploited, including with space agencies, electronics and global communications industries and the nuclear industry.In preparation for clinical delivery, there may be opportunities to send patients overseas (for 12C or 4He ion therapy) using the model of the National Health Service (NHS) Proton Overseas Programme and to look at potential national clinical trials which include heavier ions, FLASH or SFRT. This could be accomplished under the auspices of NCRI CTRad (National Cancer Research Institute, Clinical and Translational Radiotherapy Research Working Group).The initiative should be a community approach, involving all interested parties with a vision that combines discovery science, a translational research capability and a clinical treatment facility. Barriers to the project and ways to overcome them were discussed. Finally, a set of different scenarios of features with different costs and timelines was constructed, with consideration given to the funding environment (prer-Covid-19) and need for cross-funder collaboration.

Saving normal tissues - a goal for the ages

Almost since the earliest utilization of ionizing radiation, many within the radiation community have worked toward either preventing (i.e. protecting) normal tissues from unwanted radiation injury or rescuing them from the downstream consequences of exposure. However, despite over a century of such investigations, only incremental gains have been made toward this goal and, with certainty, no outright panacea having been found. In celebration of the 60th anniversary of the International Journal of Radiation Biology and to chronicle the efforts that have been made to date, we undertook a non-rigorous survey of the articles published by normal tissue researchers in this area, using those that have appeared in the aforementioned journal as a road map. Three 'snapshots' of publications on normal tissue countermeasures were taken: the earliest (1959-1963) and most recent (2013-2018) 5-year of issues, as well as a 5-year intermediate span (1987-1991). Limiting the survey solely to articles appearing within International Journal of Radiation Biology likely reduced the number of translational studies interrogated given the basic science tenor of this particular publication. In addition, by taking 'snapshots' rather than considering the entire breadth of the journal's history in this field, important papers that were published during the interim periods were omitted, for which we apologize. Nonetheless, since the journal's inception, we observed that, during the chosen periods, the majority of studies undertaken in the field of normal tissue countermeasures, whether investigating radiation protectants, mitigators or treatments, have focused on agents that interfere with the physical, chemical and/or biological effects known to occur during the acute period following whole body/high single dose exposures. This relatively narrow approach to the reduction of normal tissue effects, especially those that can take months, if not years, to develop, seems to contradict our growing understanding of the progressive complexities of the microenvironmental disruption that follows the initial radiation injury. Given the analytical tools now at our disposal and the enormous benefits that may be reaped in terms of improving patient outcomes, as well as the potential for offering countermeasures to those affected by accidental or mass casualty exposures, it appears time to broaden our approaches to developing normal tissue countermeasures. We have no doubt that the contributors and readership of the International Journal of Radiation Biology will continue to contribute to this effort for the foreseeable future.

Hyperthermia: The Optimal Treatment to Overcome Radiation Resistant Hypoxia

Regions of low oxygenation (hypoxia) are a characteristic feature of solid tumors, and cells existing in these regions are a major factor influencing radiation resistance as well as playing a significant role in malignant progression. Consequently, numerous pre-clinical and clinical attempts have been made to try and overcome this hypoxia. These approaches involve improving oxygen availability, radio-sensitizing or killing the hypoxic cells, or utilizing high LET (linear energy transfer) radiation leading to a lower OER (oxygen enhancement ratio). Interestingly, hyperthermia (heat treatments of 39⁻45 °C) induces many of these effects. Specifically, it increases blood flow thereby improving tissue oxygenation, radio-sensitizes via DNA repair inhibition, and can kill cells either directly or indirectly by causing vascular damage. Combining hyperthermia with low LET radiation can even result in anti-tumor effects equivalent to those seen with high LET. The various mechanisms depend on the time and sequence between radiation and hyperthermia, the heating temperature, and the time of heating. We will discuss the role these factors play in influencing the interaction between hyperthermia and radiation, and summarize the randomized clinical trials showing a benefit of such a combination as well as suggest the potential future clinical application of this combination.

Therapeutic potential of using the vascular disrupting agent OXi4503 to enhance mild temperature thermoradiation

PURPOSE

The response of tissues to radiation with mild temperature hyperthermia is dependent on the interval between the two modalities. This study investigated the effect that the vascular disrupting agent OXi4503 had on this time-interval interaction.

METHODS

The normal right rear foot of female CDF1 mice or foot-implanted C3H mammary carcinomas were locally irradiated (230 kV X-rays) and heated (41.5 °C for 60 min) by foot immersion in a water bath. OXi4503 (50 mg/kg) was injected intraperitoneally 1.5 h before irradiating. Irradiation was performed either in the middle of the heating period (simultaneous treatment) or at 1 or 4 h prior to starting the heating (sequential treatments). Response was the percentage of mice showing local tumour control at 90 days or skin moist desquamation between days 11-23. From the radiation dose response curves the dose producing tumour control (TCD(50)) or moist desquamation (MDD50) in 50% of mice was calculated.

RESULTS

The TCD(50) and MDD50 values for radiation alone were 54 Gy and 29 Gy, respectively. Simultaneously heating the tissues enhanced radiation response, the respective TCD(50) and MDD50 values being significantly (chi-square test, p < 0.05) reduced to 33 Gy and 14 Gy. A smaller enhancement was obtained with a sequential treatment in both tissues. OXi4503 enhanced the radiation response of tumour and skin. Combined with radiation and heat, the only effect was in tumours where OXi4503 prevented the decrease in sensitisation seen with the sequential treatment.

CONCLUSION

Combining OXi4503 with a sequential radiation and heat treatment resulted in a 1.4-fold therapeutic gain.

ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs--threshold doses for tissue reactions in a radiation protection context

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It was instigated following a recommendation in Publication 103 (ICRP, 2007), and it provides updated estimates of 'practical' threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the haematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40-50years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. A variety of biological response modifiers have been used to help reduce late reactions in many tissues. These include antioxidants, radical scavengers, inhibitors of apoptosis, anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, growth factors, and cytokines. In many cases, these give dose modification factors of 1.1-1.2, and in a few cases 1.5-2, indicating the potential for increasing threshold doses in known exposure cases. In contrast, there are agents that enhance radiation responses, notably other cytotoxic agents such as antimetabolites, alkylating agents, anti-angiogenic drugs, and antibiotics, as well as genetic and comorbidity factors. Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events. For these two tissues, a threshold dose of 0.5Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgement further.

Radiobiological rationale and patient selection for high-LET radiation in cancer therapy

The rationale for introducing ion beams in cancer therapy is the high level of physical selectivity that can be achieved with ions, equal or even better than with proton beams or modern photon techniques, as well as the potential advantage of high-LET radiations for some tumour types and sites. The radiobiological arguments for high-LET radiation in cancer therapy are reviewed: reduction of OER in the case of hypoxic and poorly-reoxygenating tumours, and the lesser importance of repair phenomena which are a problem in controlling repair-proficient photon-resistant tumours. Fast neutrons were the first type of high-LET radiation used clinically, and were often applied under suboptimal technical conditions. Nevertheless, useful clinical information was derived from the neutron experience. A greater benefit from neutrons than from conventional radiotherapy was found for several tumour sites. The present discussion is limited to the results for salivary gland tumours and prostatic adenocarcinoma. Based on the fast neutron experience, radiobiological arguments, and the added benefit of excellent physical selectivity of ion beams, the potential clinical indications for high-LET ions are discussed: hypoxic, slowly growing and well-differentiated photon-resistant tumours. One of the main remaining issues is the selection of individual patients for high- or low-LET radiation. Since the physical selectivity of ions now matches that obtained with other techniques, the selection of patients will be based only on the radiobiological characteristics of the tumour.

Preclinical studies on how to deal with patient intolerance to nicotinamide and carbogen

BACKGROUND AND PURPOSE

Accelerated radiation carbogen nicotinamide (ARCON) therapy is generally well tolerated, but some patients experience intolerance to nicotinamide and carbogen (95% O(2)+5% CO(2)). This study investigated the effect of reducing both the nicotinamide dose and carbogen CO(2) content on radiation response.

MATERIALS AND METHODS

A C3H mouse mammary carcinoma grown in the right rear foot of female CDF1 was used and treated when at 200 mm(3). Nicotinamide was intraperitoneally injected 20 min prior to irradiation. Carbogen (CO(2) content of either 2 or 5%, balance O(2)) breathing was started 5 min before, and continued during, additional treatments. Radiation was given locally to tissues of restrained non-anaesthetised mice either as a single or fractionated (10 fractions in 12 days) schedule. The endpoints were local tumour control at 90 days, development of moist desquamation 11-23 days after treatment of normal foot skin, or tumour oxygenation measured with the Eppendorf electrode.

RESULTS

The TCD50 values in this tumour following single or fractionated radiation treatment were 52 and 71Gy, respectively. Carbogen (5% CO(2) content) breathing with every radiation treatment in the fractionated schedule significantly (Chi-squared test; P<0.05) enhanced radiation response (ER 1.25). Significant enhancements were also seen with nicotinamide given either as 10x120 mg/kg (ER 1.25), 6x120 mg/kg (ER 1.11) or 10x90 mg/kg (ER 1.18), although the 6x120 schedule was significantly less effective than 10x120. Combining nicotinamide with carbogen resulted in ERs of 1.39-1.44, and these were independent of the nicotinamide treatments. There was also no significant difference in the enhancement of tumour radiation response or improved tumour oxygenation status if the CO(2) content of the gas breathing was varied from 0% (i.e. 100% O(2)) to 2 or 5% (balance O(2)), although a CO(2) content of 2% did give a smaller enhancement of radiation-induced normal skin damage than 5%.

CONCLUSIONS

Both the nicotinamide dose, but not the frequency, and carbogen CO(2) content may be reduced in patients experience intolerance without any significant loss of sensitisation.

The role of oxygen in cutaneous photodynamic therapy

Photodynamic therapy (PDT) is based on the dye-sensitized photooxidation of biological matter in the target tissue, and utilizes light activated drugs for the treatment of a wide variety of malignancies. Skin is a target organ for PDT, because of the increasing incidence of skin cancers and the easy accessibility to photosensitizing drugs and light. Skin oxygen tension changes dramatically during and after PDT and seems to be an important treatment parameter. Experimental approaches to modulate oxygen tension (e.g., hyperbaric oxygenation, hyperthermia, or perfluorocarbons) have been studied mainly in animals, and some of these techniques may have the potential to be applied in humans to improve the efficacy and safety of PDT. The main purpose of this review is to provide the reader with current information on cutaneous oxygen physiology and oximetry, the role of oxygen and singlet oxygen (1O2) in PDT, and approaches to modulate skin oxygen tension. The literature indicates that it may be possible to utilize transcutaneous oxygen measurements as a valuable measure of the clinical effectiveness of PDT and as an in situ predictor of the energy required to elicit a biological response. Consequently the effectiveness of PDT can be manipulated by modulating skin oxygen tension.

Nicotinamide as a radiosensitizer in tumours and normal tissues: the importance of drug dose and timing

BACKGROUND AND PURPOSE

Nicotinamide is a radiation sensitizer currently undergoing clinical testing. This was an experimental study to determine the importance of drug dose and time interval between drug administration and irradiation for radiosensitization.

MATERIALS AND METHODS

Nicotinamide (50-500 mg/kg) was injected intraperitoneally into CDFI or C3H mice and drug plasma pharmacokinetics were determined by HPLC. Radiosensitization was measured in tumours and normal tissues after local irradiation. The tumours were a C3H mammary carcinoma, the KHT sarcoma and the SCCVII carcinoma. Tumour response was assessed using either growth delay (C3H) or clonogenic survival (KHT/SCCVII). Normal tissue toxicities evaluated included early responding skin (development of moist desquamation of the foot) and late responding bladder (reservoir function estimated by cystometry) and lung (ventilation rate measured by plethysmography).

RESULTS

All nicotinamide peak plasma concentrations were seen within 30 min after injection. Irradiating tumours at peak times resulted in enhancement ratios (ERs) of 1.27 (C3H), 1.75 (KHT) and 1.45 (SCCVII) with high nicotinamide doses and 1.27 (C3H), 1.28 (KHT) and 1.36 (SCCVII) after giving clinically relevant doses (100-200 mg/kg). Lower ERs were observed when the time interval between drug injection and irradiation was increased beyond the peak time. Irradiating normal tissues at peak times after injecting 100-200 mg/kg nicotinamide gave ERs of 1.20 (skin), 0.90 (bladder) and 1.02 (lung).

CONCLUSIONS

Clinically achievable doses of nicotinamide will enhance tumour radiation damage while having minimal effects in normal tissues, but for the best tumour effect radiation should be given at the time of peak plasma drug concentrations.

Radiosensitisation in normal tissues with oxygen, carbogen or nicotinamide: therapeutic gain comparisons for fractionated x-ray schedules

METHODS

Radiosensitisation with oxygen, carbogen or nicotinamide alone and oxygen or carbogen combined with nicotinamide was compared in early and late responding normal tissues in rodents. X-ray treatments were delivered as single doses or fractionated schedules of 2 fractions in 1 day, 2, 12 and 36 fractions in an overall time of 12 days and 10 fractions in 5 or 12 days. Acute skin reactions, survival of intestinal crypts, breathing rate, reduction in the packed red-cell volume and clearance of 51Cr-EDTA were used as assays of epidermal, gut, lung and renal damage.

RESULTS

Relative to air-breathing mice, carbogen or oxygen produced a small, and not always significant, increase in sensitivity (enhancement ratios < or = 1.15) in gut, lung and kidneys; however, in skin a dose enhancement of 1.2-1.3 was observed. The effect of nicotinamide in air, carbogen or oxygen was studied only in lung and gut. The drug produced variable but generally significant increases in radiosensitisation ( < or = 1.26) in all three gases. Relative to treatments in air, enhancement ratios for nicotinamide alone were usually slightly higher than those observed when either carbogen or oxygen were administered without the drug. With all three modifiers (i.e. oxygen, carbogen, nicotinamide alone or for the drug-gas combinations) there was no significant change in the enhancement ratios observed as the number of radiation dose fractions was varied.

CONCLUSIONS

Comparisons with fractionated X-ray studies done previously in rodent tumours indicate that a therapeutic benefit, relative to lung, gut and renal damage, would be observed with oxygen or carbogen alone but not with nicotinamide alone. The greatest gain would be achieved with the combination of carbogen and nicotinamide, with which a benefit was observed even relative to epidermal damage. These results indicate that some decrease in normal tissue tolerance could be observed when using these modifiers in clinical radiotherapy and, although small, the appropriate dose reductions should be considered; caution should be exercised especially when carbogen and nicotinamide are used in conjunction with the more radical accelerated schedules.

Radiosensitization of mouse skin by oxygen and depletion of glutathione

PURPOSE

To determine the oxygen enhancement ratio (OER) and shape of the oxygen sensitization curve of mouse foot skin, the extent to which glutathione (GSH) depletion radiosensitized skin, and the dependence of such sensitization on the ambient oxygen tension.

METHODS AND MATERIALS

The feet of WHT mice were irradiated with single doses of 240 kVp x-rays while mice were exposed to carbogen or gases with oxygen/nitrogen mixtures containing 8-100% O2. The anoxic response was obtained by occluding the blood supply to the leg of anesthetized mice with a tourniquet, surrounding the foot with nitrogen, and allowing the mice to breathe 10% O2. Further experiments were performed to assess the efficacy of this method to obtain an anoxic response. Radiosensitivity of skin was assessed using the acute skin-reaction assay. Glutathione levels were modified using two schedules of DL-buthionine sulphoximine (BSO) and diethylmaleate (DEM), which were considered to produce extensive and intermediate levels of GSH depletion in the skin of the foot during irradiation.

RESULTS

Carbogen caused the greatest radiosensitization of skin, with a reproducible enhancement of 2.2 relative to the anoxic response. The OER of 2.2 is lower than other reports for mouse skin. This may indicate that the extremes of oxygenation were not produced, although there was no direct evidence for this. When skin radiosensitivity was plotted against the logarithm of the oxygen tension in the ambient gas, a sigmoid curve with a K value of 17-21% O2 in the ambient gas was obtained. Depletion of GSH caused minimal radiosensitization when skin was irradiated under anoxic or well-oxygenated conditions. Radiosensitization by GSH depletion was maximal at intermediate oxygen tensions of 10-21% O2 in the ambient gas. Increasing the extent of GSH depletion led to increasing radiosensitization, with sensitization enhancement ratios of 1.2 and 1.1, respectively, for extensive and intermediate levels of GSH depletion. In mice exposed to 100% O2, a significant component of skin radiosensitivity was due to diffusion of oxygen directly through the skin. Pentobarbitone anesthesia radiosensitized skin in mice exposed to 100% O2 by a factor of 1.2, but did not further sensitize skin in mice exposed to carbogen.

CONCLUSIONS

Glutathione levels and the local oxygen tension at the time of irradiation were important determinants of mouse foot skin radiosensitivity. The extent to which GSH levels altered the radiosensitivity of skin was critically dependent on the local oxygen tension. These results have significant implications for potential clinical application of GSH depletion.

Nicotinamide and other benzamide analogs as agents for overcoming hypoxic cell radiation resistance in tumours. A review

Oxygen deficient hypoxic cells, which are resistant to sparsely ionising radiation, have now been identified in most animal and some human solid tumours and will influence the response of those tumours to radiation treatment. This hypoxia can be either chronic, arising from an oxygen diffusion limitation, or acute, resulting from transient stoppages in microregional blood flow. Although clinical attempts to overcome hypoxia have met with some success, the results have been far from satisfactory, and efforts are still being made to find better methods. Extensive experimental studies, especially in the last decade, have shown that nicotinamide and structurally related analogs can effectively sensitise murine tumours to both single and fractionated radiation treatments and that they do so in preference to the effects seen in mouse normal tissues. The earliest studies suggested that this enhancement of radiation damage was the result of an inhibition of the repair mechanisms, as was well documented in vitro. However, recent studies in mouse tumours have shown that the primary mode of action actually involves a reduction in tumour hypoxia. More specifically, these drugs prevent transient cessations in blood flow, thus inhibiting the development of acute hypoxia. This novel discovery led to the suggestion that the potential role of these agents as radiosensitizers would be when combined with treatments that overcame chronic hypoxia. The first attempt to demonstrate this combined nicotinamide with hyperthermia and found that the enhancement of radiation damage by both agents together was greater than that seen with each agent alone. Similar results were later seen for nicotinamide combined with a perfluorochemical emulsion, carbogen breathing, and pentoxifylline, and in all these studies the effects in tumours were always greater than those seen in appropriate normal tissues. Of all the analogs, it is nicotinamide itself which has been the most extensively studied as a radiosensitizer in vivo and the one that shows the greatest effect in animal tumours. It is also an agent that has been well established clinically for the treatment of a variety of disorders, with daily doses of up to 6 g being considered reasonably safe and associated with a low incidence of side effects. This human dose is equivalent to 100-200 mg/kg in mice and such doses will maximally sensitize murine tumours to radiation. These findings have now resulted in phase I/II clinical trials of nicotinamide, in combination with carbogen breathing, as a potential radiosensitizing treatment.

Hypoxic fractions measured in murine tumors and normal tissues using the comet assay

PURPOSE

To apply the alkaline comet assay to the detection of radiobiologically hypoxic cells in solid tumors and normal tissues of mice, and to examine the influence of strand break repair on the oxygen enhancement ratio measured using the alkaline comet assay.

METHODS AND MATERIALS

In previous studies, we found that hypoxic fraction in squamous cell carcinomas growing in C3H mice could be reliably and easily measured using the alkaline comet assay. The comet assay applies fluorescence microscopy and image analysis to examine patterns of migration of deoxyribonucleic acid from individual cells embedded in agarose and exposed to an electric field. This method has sufficient resolution to detect subpopulations of hypoxic cells which show about 3 x fewer strand breaks than aerobic cells after irradiation.

RESULTS

Fast rejoining kinetics in vitro are comparable to those measured in vivo, and rejoining of strand breaks in hypoxic tumor cells occurs at a similar rate as rejoining in aerobic cells. Little residual damage was detectable using the comet assay in tumors 4-24 h following 15 Gy, allowing repeat measurements to be performed. Bone marrow and testis, but not liver, spleen, or jejunum contained a small fraction of hypoxic cells when mice breathed 10% oxygen during irradiation.

CONCLUSION

The comet assay confirms that some normal tissues may border on hypoxia. Rejoining of strand breaks occurs rapidly in both oxic and hypoxic cells so that the oxygen enhancement ratio remains relatively constant with time after irradiation. Interestingly, a smaller oxygen enhancement ratio was observed in tumors than was expected, probably as a result of the presence of acutely hypoxic cells.

Distribution of the hypoxia marker CCI-103F in canine tumors

PURPOSE

The purpose of this study was to identify the prevalence and distribution of hypoxic tumor cells in spontaneous canine tumors, and to relate these parameters to various tumor and patient characteristics, such as tumor volume, tumor type, or tumor location.

METHODS AND MATERIALS

Hypoxic tumor cells were labeled in vivo in 32 primary malignant canine tumors by bioreductive binding of the nitroimidazole hypoxia marker CCI-103F. CCI-103F was given at 40 mg/kg i.v. Tumors were completely excised, and CCI-103F adducts were detected in histologic sections (mean, 138 sections-per-tumor) by peroxidase-antiperoxidase immunostaining. Area fraction (area labeled/total area examined) of labeled regions was measured via computer assisted image analysis. In tumors with a volume < 100 cm3, each cubic centimeter of tumor was examined; in larger tumors 100 randomly selected 1 cm3 samples were examined.

RESULTS

There were 13 soft-tissue sarcomas, 11 mast-cell tumors, five carcinomas, two lymphosarcomas, and one melanoma. Tumors varied from < .001 to > 2000 cm3. Labeled cells were present in 31 of 32 canine tumors examined, and varied between 0 and 35%. Mean (+/- SD) % label was 12.2% +/- 16.7%; 13 of the 32 dogs had % labeled area < 5.0%. The area fraction was not related to tumor site, tumor type, tumor volume, presence and degree of necrosis or tumor grade. Dog characteristics such as sex, age, and body size did not affect the degree of labeling of tumors. CCI-103F adducts were randomly distributed grossly, and at the microscopic level were not found near blood vessels or regions containing mitoses. Labeling was seen in a variety of normal tissues; not all binding in normal tissues could be attributed to hypoxia.

CONCLUSION

CCI-103F labeling of hypoxic regions in tumors provides a nonradioactive method of detecting nitroimidazole adducts at the cellular level, and allows concurrent histologic examination. The pattern of labeling is consistent with detection of hypoxic tumor cells arising from oxygen diffusion limitations. This method may have clinical applicability in the detection of tumor hypoxia.

Pharmacokinetics of varying doses of nicotinamide and tumour radiosensitisation with carbogen and nicotinamide: clinical considerations

Plasma concentrations, after administration of varying doses of nicotinamide, were measured in CBA male mice using a newly-developed high performance liquid chromatography assay. In all dose groups, peak levels were observed within the first 15 min after an i.p. administration of 0.1, 0.2, 0.3 or 0.5 mg g-1 of nicotinamide. There was a clear dose-dependent increase in plasma concentration with increasing dose, with almost a five-fold lower concentration (1.0 vs 4.9 mumol ml-1) achieved with a dose of 0.1 mg g-1 compared with 0.5 mg g-1, respectively. The half-life of nicotinamide increased from 1.4 h to 2.2 h over the dose range (P < 0.01). Comparisons with previous pharmacokinetic data in humans show that clinically-relevant oral doses of 6 and 9 g in humans give plasma levels slightly higher than those achieved at 1 h with doses of 0.1 to 0.2 mg g-1 in mice. Tumour radiosensitisation with carbogen alone, and with carbogen combined with varying doses of nicotinamide (0.05 to 0.5 mg g-1), was investigated using a 10-fraction in 5 days X-ray schedule. Relative to air-breathing mice, a statistically significant increase in sensitisation was observed with both a local tumour control and with an in vivo/in vitro excision assay (P < or = 0.007). With the local control assay, a trend was observed towards lower enhancement ratios (ERs) with decreasing nicotinamide dose (from 1.85 to 1.55); carbogen alone was almost as effective as when combined with 0.1 mg g-1 of nicotinamide. With the excision assay, ERs for carbogen combined with nicotinamide increased with decreased levels of cell survival. At a surviving fraction of 0.02, enhancement ratios of 1.39-1.48 were obtained for carbogen plus 0.1 to 0.3 mg g-1 of nicotinamide. These were lower than those seen with the two higher doses of 0.4 to 0.5 mg g-1 (ERs = 1.63-1.69).

Effects of hyperbaric oxygen and a perfluorooctylbromide emulsion on the radiation responses of tumors and normal tissues in rodents

Perfluorochemical emulsions are being examined in many laboratory and clinical studies as possible adjuncts to radiotherapy and chemotherapy. The studies reported here examine the clinical potential of hyperbaric oxygen (HBO) in combination with a highly concentrated perfluorochemical emulsion (Oxygent) containing 100% w/v perfluorooctylbromide (PFOB). HBO alone produced only a small improvement in the radiation response of BA1112 tumors in WAG/rij rats, while regimens combining HBO with Oxygent produced much greater radiation sensitization. A sham emulsion, formulated without the O2-carrying PFOB, did not alter the radiation response of the tumors in comparison with that seen with HBO alone. Neither HBO nor Oxygent plus HBO altered the radiosensitivity of bone marrow progenitor cells in BALB/c mice. HBO alone augmented skin reactions in BALB/c mice, but addition of Oxygent did not alter the skin reactions in comparison to those seen with HBO alone. Regimens combining Oxygent with HBO selectively increased the radiation sensitivity of tumors relative to normal tissues, thereby enhancing the therapeutic ratio. These results support the potential usefulness of perfluorochemical emulsions and HBO in clinical radiation therapy.

A therapeutic benefit from combining normobaric carbogen or oxygen with nicotinamide in fractionated X-ray treatments

The ability of normobaric oxygen and carbogen (95% O2 + 5% CO2) combined with nicotinamide to enhance the radiosensitivity of two rodent adenocarcinomas and of mouse skin and kidneys, using a 10 fraction radiation schedule, was compared with the effect of radiation in air with and without the drug. Tumour response was assayed using local control and regrowth delay, and compared with acute skin reactions, decreased renal 51Cr-EDTA clearance and reduction in haematocrit. Nicotinamide increased the radiation sensitivity of CaNT tumours under all three different oxygen concentrations tested (21, 95 and 100% oxygen). The effect was statistically significant for oxygen and carbogen but not for air; the combination of nicotinamide with carbogen gave the greatest increase in tumour radiosensitivity. Relative to treatments in air without the drug, the enhancement ratios (ER) at the TCD50 level were 1.17, 1.65 and 1.83 for CaNT tumours irradiated in air, oxygen or carbogen and injected with nicotinamide 1 h before each fraction. The ER in CaRH tumours irradiated in carbogen plus the drug was 1.83, which was greater, but statistically not significantly different, to that seen with carbogen alone (ER = 1.68). In skin, relative to air without the drug, the increase in radiosensitivity by nicotinamide was greater in oxygen and carbogen than in air (1.29, 1.36 and 1.08, respectively). The ERs for both assays of renal damage were similar and lower than those in skin: less than or equal to 1.07, less than or equal to 1.13 and less than or equal to 1.16 for irradiations done in air, oxygen and carbogen plus nicotinamide, relative to air alone. A comparison of these results in the tumours and normal tissues showed that a significant therapeutic benefit was obtained with normobaric oxygen and carbogen combined with nicotinamide. This benefit is greater than observed with other radiosensitizers tested so far. Toxic side effects of the treatment are unlikely in a clinical situation, since prolonged administration of nicotinamide is well tolerated in man. The combination of normobaric carbogen with nicotinamide could be an effective method of enhancing tumour radiosensitivity in clinical radiotherapy where hypoxia limits the outcome of treatment.

Immunohistochemical detection of a hypoxia marker in spontaneous canine tumours

An immunoperoxidase technique has been used to detect the in vivo binding of a 2-nitroimidazole hypoxia marker in histochemical sections of a variety of excised canine tumours. The binding occurred 10-12 cell diameters away from tumour blood vessels, consistent with the expected location of hypoxic cells in tissues in which oxygen concentration gradients are established by diffusion. Hypoxic fractions ranging from 4 to 13% have been estimated on the basis of morphometric analysis of multiple tumour sections. The binding of the marker was restricted to the cytoplasm of the cells. The marker appeared in regions adjacent to necrosis but also in regions free of necrosis. As in earlier autoradiography studies, binding was occasionally observed in cells adjacent to tumour blood vessels. Generally, binding to normal tissues was not observed. However, binding to smooth muscle cells surrounding arterioles in some sections of normal tissue and tumour tissue was observed.

Tissue distribution of 14C- and 3H-labelled misonidazole in the tumor-bearing mouse

The retention of labelled misonidazole (MISO) was measured in a range of normal tissues in the mouse 24 hr after the intravenous injection of [14C]MISO (ring labelled) and [3H]-MISO (side-arm labelled). For [14C]MISO the 24 hr tissue retention, in order of the highest to the lowest levels (excluding pathways of excretion), was esophageal epithelium, liver, foot pad, eyelid, lung, subcutaneous lung tumor (A110), esophageal wall, uterus, eye ball, blood, salivary gland, spleen, voluntary muscle, pancreas, inguinal fat. It was assumed that the 14C represented MISO metabolite(s) bound to macromolecules. An approximately similar pattern was observed for [3H]MISO, but a higher percentage of the injected activity per gram of tissue was retained, probably due to the presence of tritiated water in the tissues. It has generally been assumed that significant levels of MISO binding are restricted to hypoxic tissues, for example tumors, but the present results show that significant levels of binding can also occur in apparently normoxic tissues. The explanation is put forward that this binding may be due to local high levels of nitroreductase capacity.

Manipulation of the radiosensitivity of pig epidermis by changing the concentration of oxygen and halothane in the anaesthetic gas mixture

A gas mixture of halothane, oxygen and nitrous oxide has been used to anesthetize pigs for irradiation. The effects of various concentrations of halothane and oxygen on the radiosensitivity of the epidermis were examined after irradiation with single doses of beta-rays from strontium-90 plaques. The incidence of moist desquamation was used as an endpoint, and experiments were compared on the basis of the dose associated with a 50 per cent incidence of moist desquamation (ED50 +/- SE). For pigs inspiring an anaesthetic gas mixture of 2 per cent halothane, approximately 70 per cent oxygen and approximately 30 per cent nitrous oxide the ED50 for moist desquamation was 27.32 +/- 0.52 Gy. A similar ED50 value of 27.39 +/- 1.20 Gy was obtained when 4 per cent halothane was used in place of 2 per cent. When the pigs were breathing air (approximately 21 per cent oxygen) in place of oxygen and nitrous oxide the ED50 values were increased significantly to 31.25 +/- 0.94 Gy and 33.72 +/- 1.08 Gy for 2, and 4 per cent halothane, respectively. This change in the radiosensitivity of the epidermis was represented by dose modification factors of approximately 1.13 and approximately 1.23 for 2 and 4 per cent halothane, respectively. Irradiation with a high oxygen concentration in the inspired gas mixture did not result in any significant variation of the dose required to produce moist desquamation in 50 per cent of the fields irradiated for dorsal, lateral and ventral positioned skin fields on the flank. However, pigs breathing air and halothane during irradiation showed marked differences in the radiosensitivity of the various sites on the flank, with ED50 values for moist desquamation of approximately 37 Gy and 26-30 Gy for dorsal and ventral positioned fields, respectively. This marked difference in radiosensitivity suggests variations in the physiological compensation over the flank when pigs are breathing oxygen at low concentrations under anaesthesia.

Autoradiographic study of tritium-labeled misonidazole in the mouse

The localization of tritiated misonidazole metabolites in a number of normal tissues in the mouse is reported from autoradiography. The labeled misonidazole was injected at 750 or 75 mg/kg body weight (Rel. Sp. Act. 74 and 740 MBq/mg respectively). The grain count ratio, parenchyma:stroma, for selected tissues was: liver (centrilobular zone) 13; meibomian gland (acini) 68, (duct) 116; esophagus (keratinized layer) 61; enamel organ 17. It is concluded that there are a number of tissues which will accumulate MISO metabolites although they may not all be hypoxic.

Microscopic distribution of misonidazole in mouse tissues

Mice were injected with tritiated misonidazole (750 mg/kg-1), killed after 24h and the excised tissues prepared for autoradiography (ARG) to identify sites of accumulation. The previously reported high grain count associated with bound misonidazole metabolite(s) was observed in the liver. The ratio of grain count in the emulsion above the centrilobular hepatocytes to the count over connective tissue (stroma) was 12. A higher count ratio for 'target' cells to stroma was observed in the following cells/tissues: meibomian gland (ducts 110, acini 65), oesophagus (keratinised layer 60), incisor (enamel organ 17), nasal septum (subepithelial glands 13). For some of these tissues the explanation might appear to lie with localised hypoxia, but for others which were probably normoxic there is as yet no obvious reason for these findings.

Protective effect of hypoxia in the ram testis during single and split-dose X-irradiation

Spermatogonial stem-cell survival in the ram was studied after single (6 Gy) and split-dose (2 x 3 Gy, interval 21-24 h) X-irradiation both under normal and hypoxic conditions. Hypoxia was induced by inflation of an occluder implanted around the testicular artery. The occluders were inflated about 10 min before irradiation and deflated immediately after. Stem-cell survival was measured at 5 or 7 weeks after irradiation by determination of the Repopulation Index (RI) in histological testis sections. The RI-values after fractionated irradiation were only half those after single dose irradiation. Hypoxia had a protective effect on the stem-cell survival. After split-dose irradiation under hypoxic conditions two times more stem cells survived than under normal oxic conditions; the RI-values increased from 34% (oxic) to 68% (hypoxic). This effect of hypoxia was also found after single dose irradiation where the RI-values increased from 68% (oxic) to 84% (hypoxic). The development of the epithelium in repopulated tubules was also studied. Under hypoxia, a significantly higher fraction of tubules with complete epithelium was found after single (38 vs. 4%) as well as after split-dose irradiation (12 vs. 0%).

Variations in the spectrum of lesions produced in the DNA of cells from mouse tissues after exposure to gamma-rays in air-breathing or in artificially anoxic animals

Gamma-ray-induced DNA-protein crosslinks (dpc) are preferentially induced in cultured cells irradiated at very low oxygen tensions (Meyn et al. 1987). Since some cells within mouse tumors may be radiobiologically hypoxic, dpc may also be induced in such cells after irradiation in vivo. To examine this possibility, mice bearing either an FSa or NFSa fibrosarcoma in their hind legs were whole-body irradiated either while breathing atmospheric oxygen or 15 min after cervical dislocation, which induces uniform anoxia. DNA single-strand breaks (ssb) and dpc were then assayed both in tumors and normal tissues by alkaline elution. The level of dpc was inferred from the observed increase in ssb yield after digestion of the cell lysates with proteinase K. In addition, cell suspensions were irradiated in vitro, on ice, exposed to atmospheric oxygen tensions. Few dpc were detected in the DNA from tumor cells irradiated in vitro; however, in cells from both FSa and NFSa tumors irradiated in situ there was a significant level of protein-concealed ssb, and thus of dpc. These data are most likely the result of the relative hypoxia of a proportion of cells from both the FSa and NFSa tumor in the air-breathing animals. Induction of dpc was further enhanced in the DNA from tumor cells irradiated under anoxic conditions. A significant level of dpc was also observed in jejunal and spleen cells irradiated in vivo; however, since a significant level of protein-concealed breaks was also observed in cells irradiated in vitro, oxygenation appears not to be the only parameter capable of modifying the proportion of protein-concealed ssb, and the effects of proteinase K on the DNA elution rate for normal mouse tissues may be complex.

Is misonidazole binding to mouse tissues a measure of cellular pO2?

Misonidazole (MISO), a hypoxic cell radiosensitizer, forms covalently-linked adducts to cellular molecules as a result of bioreductive metabolism, a process which is strongly dependent upon oxygen concentration. MISO binding to liver tissue taken from air-breathing mice was three to five times greater than binding to other normal tissues. The relative binding of [14C]MISO to various mouse tissue cubes in vitro was measured by autoradiography as a function of defined oxygen concentrations, and standard curves (binding rate vs oxygen concentration) were generated. The oxygen concentration for half-maximum binding as well as the maximum and minimum binding rates (grains per 100 micron 2) observed for liver tissue were not significantly different from those measured for brain or heart tissue. These results, along with previously published data on MISO binding to isolated hepatocytes in vitro, suggest that the elevated binding to liver in vivo may result, in part, from the organ existing at a significantly lower pO2 than other normal tissues. They also suggest that this drug adduct procedure could be developed as a sensitive method for the quantitative measurement of tissue pO2 at the cellular level.

DNA damage in normal and neoplastic mouse tissues after treatment with misonidazole in vivo

Alkaline elution has been used to examine the integrity of DNA isolated from various tissues from mice treated with misonidazole (MISO). High doses (1-3 mg/g) of MISO caused extensive DNA strand breakage in cells isolated from two fibrosarcoma tumors that were known to contain hypoxic cells, and also in cells from certain normal tissues (liver and kidney in particular). The incidence of strand breaks gives further support to the suggestion that MISO can be metabolically nitroreduced beyond the singly reduced nitro radical-anion in some normal tissues as well as in hypoxic tumor cells, generating DNA-reactive species. Nitroreductases must therefore be able to compete successfully with molecular oxygen for the MISO nitro radical-anion in such tissues.

Tumor radiosensitization with concomitant bone marrow radioprotection: a study in mice using diethyldithiocarbamate (DDC) under oxygenated and hypoxic conditions

We have established, both in vitro and in vivo, that Diethyldithiocarbamate (DDC) protects mammalian cells from radiation. The in vivo protection, when non-toxic concentrations of DDC are present one-half hour before irradiation, is reflected by a dose modification factor (DMF) of 1.9 based on LD50/30 and 1.5 using survival of CFUs as an endpoint. Further experiments in vivo have extended our knowledge to the differential radioprotective effects of DDC on the bone marrow of animals breathing room air compared to a 5.5% oxygen in nitrogen mixture. The DMF (LD50/30) for DDC in air breathing animals, previously established as 1.9, can be contrasted with a DMF, obtained in the present study, of 1.2 for animals irradiated in the hypoxic state. Moreover the DMF (CFUs survival) previously established at 1.5 for air breathing animals, can be compared to a value of 1.3, obtained in the present study, for mice irradiated under hypoxic conditions. Modification of the dose response by DDC, for both bone marrow and tumor, was also examined in animals bearing a RIF sarcoma. Although protection of the bone marrow was confirmed (DMF = 2.1), the striking finding was that the tumor cells were sensitized, in both air breathing and nitrogen breathing animals, by the addition of DDC one-half hour before the radiation exposure. Moreover, the tumor radiosensitization, a factor greater than 2, in air breathing animals, appeared to be independent of dose (D0 = 200 rad, with or without DDC). The tumor radiosensitization was even more marked in the nitrogen breathing mice, in which a factor of 10 difference in survival was noted, together with a tendency towards greater sensitization at radiation doses in the clinical range. The results, demonstrating bone marrow radioprotection by DDC (aerobic greater than hypoxic) with concomitant tumor radiosensitization (hypoxic greater than aerobic) strongly suggest a large therapeutic gain factor (TGF) which could be further explored in a clinical setting.

The effect of breathing 100% oxygen on lung response to radiation in mice

The response of the lung after single doses of radiation was measured in mice breathing air, or 100% oxygen, or in air-breathing mice given the hypoxic cell sensitizer misonidazole 30 min before irradiation. There was a clear enhancement of only the pneumonitis response in the mice breathing oxygen when breathing rate or lethality was used to assess injury. Less enhancement of the late fibrotic reaction was observed in these animals. No enhancement of either phase of lung response was observed in the misonidazole-treated mice. Dose reduction factors (DRF) were estimated from these data and used to calculate oxygen concentration in the lung, giving values ranging between 187 and 250 micron oxygen.

The influence of X ray dose levels on normal tissue radioprotection by WR-2721

A variation in the degree of radioprotection by WR-2721 with X ray dose level is detectable in several normal tissue studies. A similar effect in tumors has been attributed to differential protection of oxic and hypoxic cells. For normal tissues it was previously postulated that it resulted from greater protection of 1 hit damage at low doses, with less protection of multihit damage. However, more extensive analysis of the normal tissue data, including both single dose and fractionated results show that it is not a universal effect in all normal tissues. It now seems more likely that varying PF values result from differential protection of cells at different oxygen tensions, even though the heterogeneity of oxygenation may not be detectable in the response to X rays alone.

Oxygen-dependent protection of radiation lung damage in mice by WR 2721

The modification of early and late radiation damage to the mouse lung by oxygen and WR 2721 has been studied by measurement of breathing rate, lethality, pleural fluid and hydroxyproline content. Protection by hypoxia and sensitization by hyperoxia of early radiation pneumonitis were demonstrated. There was a tendency for the protective effect of WR 2721 to decrease as the breathed oxygen concentration was raised above normal levels. WR 2721 protection of the late damage was higher (PF = 1.6-1.65) than was seen for early pneumonitis (PF = 1.3-1.35) when either breathing rate or lethality were used. Protection factors (PF) gained from measurements of pleural fluid at a year after treatment were similar to those for other endpoints of late damage (PF = 1.7). In contrast, the measurement of fibrosis through determination of lung hydroxyproline at 1 year gave a somewhat lower protection factor for WR 2721. In the same experiments the degree of epilation on the dorsal thorax was scored at 6 weeks. One hundred per cent oxygen gave enhancement (dose enhancement factor (DEF) = 1.2), 9 per cent oxygen reduced damage (DEF less than 0.7) and WR 2721 gave PF values in excess of 1.4 at all oxygen concentrations used. This showed that the radiation response of hair follicles was more sensitive to WR 2721 or to changes of oxygen than the lung. The results presented indicate a competitive interaction between WR 2721 and oxygen for the same injury site causing a shift in the oxygen K curve to higher oxygen concentrations. The validity of applying functional or survival measurements to assess the extent of pulmonary fibrosis is discussed.

The oxygen dependence of protection by aminothiols: implications for normal tissues and solid tumors

This paper reviews the role of oxygen in the protection of both normal tissues and tumors in vivo by WR-2721. Although the presence of hypoxic cells in tumors is well accepted, data are presented that suggest there is a range of oxygen concentrations and thus OERs in normal tissues in vivo, too, and that some may, in fact, be radiobiologically hypoxic. Thus, the range of protection factors in normal tissues (which appears unrelated to tissue drug levels), as well as tumors can perhaps be explained by the range of OERs in these same tissues. The question of whether protection decreases with radiation dose per fraction is related to the distribution of oxygen in both normal tissues and tumors, i.e., whether oxygen is homogeneously or heterogeneously distributed among the cells. It is hypothesized that sulphydryl compounds may equalize the OER differential between tumors and normal tissues and thus remove the natural advantage (radioresistance) of the tumor when treated with radiation. Thus, no loss of therapeutic benefit would occur when sulphydryl compounds were given with radiation if the normal tissue were better oxygenated than the tumor.

Perfluorochemical emulsions can increase tumor radiosensitivity

An oxygen-carrying perfluorochemical emulsion enhanced the effectiveness of radiation therapy in two transplantable solid tumors in mice. The perfluorochemical emulsion had no effect on tumor growth after x-irradiation, but delayed tumor growth significantly when administered to oxygen-breathing mice before or during irradiation.

Modification of the radiation response of the mouse kidney by misonidazole and WR-2721

The radiation response of the mouse kidney has been assayed after a range of X ray doses given with or without the nitroimidazole misonidazole or the aminothiol WR-2721. Sensitization and protection of the kidney were investigated by comparing the X ray dose needed to achieve a particular level of injury in the presence or absence of the drug. Two functional assays and kidney weight at sacrifice were used to obtain dose response curves. Urine output and 51Chromium EDTA excretion were used as functional assays at 25 and 49 weeks after irradiation. They demonstrated no radio-sensitization by misonidazole with 1, 2 or 5 fractions of X rays. Significant radioprotection was seen when 400 mg kg 1 WR-2721 was given before single X ray doses (PF = 1.34). Similar radioprotection was observed when renal weight at 1 year after irradiation was used as the third assay of damage. These results confirm that the kidney responds as a well-oxygenated normal tissue with only a small protection being afforded against radiation injury by WR-2721.

Radioprotection by WR-2721 in vitro at low oxygen tensions: implications for its mechanisms of action

Radioprotection of spheroids of Chinese hamster V79 cells by WR-2721 was found to be a function of spheroid size, with the greatest dose-modifying effect by the protector observed for spheroids almost large enough to contain radioresistant "anoxic" cells. The nature of the response suggested that most of the protective effect was due to the presence of an increased hypoxic fraction in the drug-treated spheroids. Similarly, when single-cell suspensions were irradiated at various oxygen tensions, one component of radioprotection by WR-2721 was found to be highly dependent upon the available oxygen. Two mechanisms of radioprotection of V79 cells by WR-2721 were thus demonstrated: a modest, oxygen-independent effect, presumably due to hydrogen donation, and an oxygen-depleting effect, which is of maximal significance for cells or tissues which would otherwise be partially sensitized by low levels of oxygen.

Skin sensitization by misonidazole: a demonstration of uniform mild hypoxia

Skin reactions on irradiated mouse feet were used to measure the radiosensitization of normal tissues by misonidazole (MISO). Fractionation schedules of 1, 2, 5 and 10 daily doses of X-rays were combined with either 100 mg/kg or 670 mg/kg MISO. When unanaesthetized mice were irradiated in air, significant sensitization was observed with both the high and low drug doses, in all fractionation schedules. There was no decrease in sensitization with fractionation, even using fractions as small as 5 Gy. This indicates that many of the cells in mouse skin may be marginally hypoxic, and that sensitization at low doses is possible. Irradiation in O2 without MISO rendered the skin more sensitive to X-rays than in air. MISO given 30 min before single doses of radiation further sensitized the skin, but for 10 fractions in O2 no MISO sensitization was detected. There was little evidence for cytotoxic killing in skin by MISO. Repair of radiation damage was slightly reduced when MISO was present, during or after irradiation.

Interaction of radiosensitizers and WR-2721. I. Modification of skin radioprotection

We have studied the radiomodifying action in mouse skin of WR-2721 and misonidazole (MISO) when used alone or in combination. The radioprotection with WR-2721 was drug-dose dependent and highly influenced by the O2 concentration at the time of irradiation. Significant sensitizaton was observed with MISO, especially in air-breathing mice. The combination of WR-2721 and MISO produced a radiation response intermediate between the resistant and sensitive responses to either drug alone. The precise degree of sensitivity was dependent on the relative doses of protector and sensitizer. We have also studied the interaction of both drugs in terms of drug-induced lethality, which showed a clear toxic interaction. The WR-2721 LD50 was reduced by a factor of 1.4 with only 200 mg/kg of MISO. We conclude that the combination of WR-2721 and MISO shows an interaction in terms of drug toxicity and radiation response, such that the radioprotection of skin is reduced or even abolished with low doses of MISO.

Radioprotection of mouse skin by WR-2721: the critical influence of oxygen tension

The epidermal clone assay has been used to study the radioprotective effect of WR-2721 on mouse skin under different conditions of oxygenation and under anoxia. The skin has shown a progressive decrease in sensitivity as the inspired gas was changed from 100% oxygen towards 0% oxygen. Compared with mice breathing 100% oxygen, those breathing air are partially protected. The inspired oxygen concentration to given half the full oxygen effect is 10--12%. The radioprotection observed with 400 mg/kg WR-2721 is markedly dependent on the ambient oxygen concentration. The protection factor is 1.1 or less in mice breathing 5%, 1% of 0% oxygen. Protection is maximal (1.95) in air and in 50% oxygen and diminishes to 1.6 at higher oxygen tensions.