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

All for one, though not one for all: team players in normal tissue radiobiology

PURPOSE

As part of the special issue on 'Women in Science', this review offers a perspective on past and ongoing work in the field of normal (non-cancer) tissue radiation biology, highlighting the work of many of the leading contributors to this field of research. We discuss some of the hypotheses that have guided investigations, with a focus on some of the critical organs considered dose-limiting with respect to radiation therapy, and speculate on where the field needs to go in the future.

CONCLUSIONS

The scope of work that makes up normal tissue radiation biology has and continues to play a pivotal role in the radiation sciences, ensuring the most effective application of radiation in imaging and therapy, as well as contributing to radiation protection efforts. However, despite the proven historical value of preclinical findings, recent decades have seen clinical practice move ahead with altered fractionation scheduling based on empirical observations, with little to no (or even negative) supporting scientific data. Given our current appreciation of the complexity of normal tissue radiation responses and their temporal variability, with tissue- and/or organ-specific mechanisms that include intra-, inter- and extracellular messaging, as well as contributions from systemic compartments, such as the immune system, the need to maintain a positive therapeutic ratio has never been more urgent. Importantly, mitigation and treatment strategies, whether for the clinic, emergency use following accidental or deliberate releases, or reducing occupational risk, will likely require multi-targeted approaches that involve both local and systemic intervention. From our personal perspective as five 'Women in Science', we would like to acknowledge and applaud the role that many female scientists have played in this field. We stand on the shoulders of those who have gone before, some of whom are fellow contributors to this special issue.

In vivo Study to Evaluate the Protective Effects of Amifostine on Radiation-Induced Damage of Testis Tissue

OBJECTIVE

To investigate the early protective effects of amifostine against radiation-induced damage on rat testis tissue.

METHODS

Eighty adult male Wistar rats were randomized to 4 groups: Saline solution was given to group A for control, 200 mg/kg amifostine (WR-2721) to group B, a single fraction of 6 Gy local irradiation to testes in group C and 200 mg/kg amifostine 15-30 min before 6 Gy testicular irradiation to group D. Animals were sacrificed 3 weeks after treatment and their testes were removed for macroscopic, microscopic and ultrastructural histopathological examination.

RESULTS

The weights, widths and lengths of testes in the last 3 groups had decreased significantly when compared with the control group, but the decrease in widths after irradiation was found to be significantly less only in the amifostine plus radiation group. There was a significant reduction of testis weights in relation to the individual body weights in the irradiated testes compared with the other groups (p < 0.005), while there was no significant change of testis weight/total body weight ratio in amifostine plus irradiation group. Spermatogonium A and primary spermatocyte counts were also less in the treatment groups, and primary spermatocyte numbers were significantly higher in amifostine plus radiation group when compared with radiation alone group (p < 0.005). Pretreatment with amifostine reduced the decrease of primary spermatocyte counts by a factor of 1.28. Electron microscopic analysis did not show any cytotoxic effect of amifostine alone, and furthermore, ultrastructural findings were normal with the addition of amifostine prior to irradiation, though there was damage in the radiation exposure group.

CONCLUSION

Amifostine when given alone by itself appears to cause adverse alterations in testis tissue; however, it has a radioprotective effect on spermiogenetic cells when used prior to radiation.

Models for Evaluating Agents Intended for the Prophylaxis, Mitigation and Treatment of Radiation Injuries Report of an NCI Workshop, December 3–4, 2003

To develop approaches to prophylaxis/protection, mitigation and treatment of radiation injuries, appropriate models are needed that integrate the complex events that occur in the radiation-exposed organism. While the spectrum of agents in clinical use or preclinical development is limited, new research findings promise improvements in survival after whole-body irradiation and reductions in the risk of adverse effects of radiotherapy. Approaches include agents that act on the initial radiochemical events, agents that prevent or reduce progression of radiation damage, and agents that facilitate recovery from radiation injuries. While the mechanisms of action for most of the agents with known efficacy are yet to be fully determined, many seem to be operating at the tissue, organ or whole animal level as well as the cellular level. Thus research on prophylaxis/protection, mitigation and treatment of radiation injuries will require studies in whole animal models. Discovery, development and delivery of effective radiation modulators will also require collaboration among researchers in diverse fields such as radiation biology, inflammation, physiology, toxicology, immunology, tissue injury, drug development and radiation oncology. Additional investment in training more scientists in radiation biology and in the research portfolio addressing radiological and nuclear terrorism would benefit the general population in case of a radiological terrorism event or a large-scale accidental event as well as benefit patients treated with radiation.

Preclinical modeling of improved amifostine (Ethyol) use in radiation therapy

Amifostine (Ethyol) has been evaluated clinically as a radioprotective agent for the prevention of xerostomia and mucositis for patients receiving radiotherapy (RT). Currently, amifostine is approved for the prevention of xerostomia in head and neck cancer patients receiving RT when administered intravenously (IV) before RT. For the clinician, there would be several advantages to administering the drug subcutaneously and to being able to show its protective effects on mucositis. The authors have developed a rat RT model to examine the protective effects of amifostine after IV and subcutaneous (SC) administration in a mucositis model. Rats (5 per group) were given 200 mg/kg (human dose equivalent of approximately 1,300 mg/m(2)) of amifostine either IV or SC, and their head and neck regions were exposed to 15.3 Gy of gamma radiation 0.5, 2, 4, and 8 hours after amifostine administration. For 10 days after treatment, the oral cavities of the rats were examined for signs of mucositis. Mucosal erythema and mucosal edema were scored according to 0 through 5 and 0 through 2 scales, respectively, with the scores added to indicate overall mucositis. The average mucositis score for the untreated animals was 3.5. Rats were protected from mucositis up to 4 hours when given amifostine either IV or SC. Rats that received amifostine SC, but not IV, were protected from mucositis 8 hours after administration. Preliminary pharmacokinetic data have revealed slightly higher active metabolite (WR-1065) levels in the parotid gland and small intestine in the rats given amifostine SC compared with IV and equivalent levels in the plasma and kidney. The data showed that SC administration of amifostine gave radioprotection comparable to IV administration up to 4 hours before RT and may be more effective than IV administration at longer pretreatment intervals.

Amifostine: mechanisms of action underlying cytoprotection and chemoprevention

Amifostine is an important drug in the new field of cytoprotection. It was developed by the Antiradiation Drug Development Program of the US Army Medical Research and Development Command as a radioprotective compound and was the first drug from that Program to be approved for clinical use in the protection of dose limiting normal tissues in patients against the damaging effects of radiation and chemotherapy. Its unique polyamine-like structure and attached sulfhydryl group give it the potential to participate in a range of cellular processes that make it an exciting candidate for use in both cytoprotection and chemoprevention. Amifostine protects against the DNA damaging effects of ionizing radiation and chemotherapy drug associated reactive species. It possesses anti-mutagenic and anti-carcinogenic properties. At the molecular level, it has been demonstrated to affect redox sensitive transcription factors, gene expression, chromatin stability, and enzymatic activity. At the cellular level it has important effects on growth and cell cycle progression. This review focuses on relating its unique chemical design to mechanisms of action that underlie its broad usefulness as both a cytoprotective and chemopreventive agent for use in cancer therapy.

Successful brief captopril treatment in experimental radiation nephropathy

Experimental renal irradiation is followed by a well-defined sequence of events leading to kidney failure. Inhibitors of angiotensin-converting enzyme can prevent the structural and functional changes that occur after renal irradiation, which suggests that the renin-angiotensin system plays a key role in their evolution. We therefore evaluated captopril, used for short intervals, in a total body irradiation model of radiation nephropathy. Irradiated 7- to 8-week-old rats that were treated with captopril from 3.5 to 9.5 weeks after irradiation had better kidney function and survival than irradiated animals treated at earlier or later intervals. At 26 weeks after irradiation, kidney function of these animals was similar to that of irradiated animals treated continuously with captopril, but their subsequent survival was less. Animals irradiated at 7 to 8 weeks of age and treated with captopril from 6 to 9 weeks after irradiation had better function and survival than animals treated at earlier or later intervals. Irradiated 15-week-old animals had significant functional and survival benefit from continuous captopril treatment but no protection from a 6-week interval of therapy. We conclude that radiation nephropathy may be significantly attenuated by the use of captopril from 3.5 to 9.5 weeks after irradiation in young animals. Although older animals did not appear to benefit from a short course of captopril, these data suggest that the renin-angiotensin system is important in the sequential expression of renal radiation injury, particularly between 3.5 and 9.5 weeks after irradiation.

Amifostine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential as a radioprotector and cytotoxic chemoprotector

Amifostine (WR-2721) was originally developed as a radioprotective agent. In animals, it protects normal tissues from the damaging effects of irradiation and, as shown in more recent studies, of several cytotoxic agents. Protection of tumours is generally reduced compared with that of normal tissues in animals, suggesting that amifostine may increase the therapeutic window of cytotoxic therapies. Clinical data concerning amifostine suggest that cytotoxic chemotherapy-induced haematological toxicity and cisplatin-induced neurotoxicity, nephrotoxicity and ototoxicity are decreased upon administration of amifostine prior to cytotoxic drugs. Similarly, amifostine reduces damage to normal tissues caused by radiotherapy. Available data show that this protection is achieved without adversely affecting tumour response or patient survival. In 1 large trial, the reduction in cyclophosphamide- and cisplatin-related toxicities manifested as a decrease in the incidence and severity of neutropenia-related fever and sepsis and in the number of patients with ovarian cancer who discontinue therapy before completion of treatment, thus improving the tolerability of this antineoplastic regimen. In addition, the incidences of cisplatin-induced nephro- and neurotoxicity were reduced. Increased doses of cytotoxic therapy have also been administered when amifostine was given prior to therapy, which may increase tumour response. The predominant adverse effect associated with amifostine are hypotension, nausea and vomiting, somnolence and sneezing. Thus, amifostine is likely to be a useful adjuvant to the treatment of patients with malignancy, particularly those receiving cyclophosphamide plus cisplatin. discontinued therapy before completion of treatment, thus improving the tolerability of this antineoplastic regimen. In addition, the incidences of cisplatin-induced.

Amelioration of radiation nephropathy by acetylsalicylic acid

This investigation was carried out to assess the amelioration by two antithrombotic drugs of radiation nephropathy in mice. Mouse kidneys were given split-dose irradiation to total doses between 17 and 22 Gy. A first group of animals was given acetylsalicylic acid (ASA) in drinking water, a second received daltroban, a thromboxane A2/prostaglandin H2 receptor antagonist, and a third received normal tap water, serving as a control. Both antithrombotic drugs were started 1 week prior to the irradiation and were given throughout the whole follow-up period. Renal function was assessed every 4 weeks from 18 weeks after the start of irradiation onwards by measuring the [51Cr] EDTA retention and haematocrit. The dose of ASA (600 mg/kg/day) caused an inhibition of thromboxane A2 and prostacyclin biosynthesis to 19 +/- 10 (mean +/- SEM) and 85 +/- 22%, respectively, as assessed by the excretion of their urinary metabolites. A significant sparing effect on the renal function after irradiation was observed in the ASA-treated animals. Using the latency time to reach 4% residual plasma activity of [51Cr] EDTA, a dose-modifying factor of 1.19 was calculated. No effect was seen with daltroban (10 mg/kg/day). Histopathological analysis of the kidneys at 12 months after irradiation demonstrated a substantially lower level of damage in the ASA-treated mice compared with daltroban-treated and radiation-only animals. These data indicate that long-term treatment with ASA is effective in reducing renal functional impairment after irradiation.

Treatment of radiation nephropathy with ACE inhibitors

PURPOSE

A previous study showed that radiation nephritis could be treated with captopril, an angiotensin-converting-enzyme inhibitor. These studies were designed to determine whether other angiotensin-converting-enzyme inhibitors would be effective, whether captopril would inhibit the development of the histopathologic lesions typical of radiation nephritis, and whether captopril could be used to treat the nephropathy observed in bone marrow transplant recipients conditioned with total body irradiation.

METHODS AND MATERIALS

In radiation nephritis studies, rats were given 17-27 Gy bilateral renal irradiation in 5 fractions. Six months after irradiation animals were stratified by blood urea nitrogen and assigned to no treatment, or treatment with captopril (500 mg/l) or enalapril (50 mg/l) in the drinking water. A subset of animals was sacrificed for histopathology after 3 months; the remaining animals continued on drugs for 7 months. In the bone marrow transplant nephropathy study, rats received 14-17 Gy total body irradiation in 6 fractions over 3 days followed by syngeneic bone marrow transplant. Six months after irradiation, animals were stratified by blood urea nitrogen and assigned to no treatment, or treatment with captopril (500 mg/l). Animals remained on drugs for 6 months. In all studies animals were followed with periodic renal function tests.

RESULTS

In the radiation nephritis study, survival and renal function were significantly enhanced by both captopril and enalapril, but there were no significant differences between the drugs. The histopathologic severity of the lesions of radiation nephritis correlated with the degree of renal dysfunction; and in irradiated animals with equal initial azotemia, captopril-treated rats developed less severe renal lesions. Finally, captopril also prolonged survival and preserved renal function in this rat bone marrow transplant nephropathy model.

CONCLUSION

Angiotensin-converting-enzyme inhibitors are an effective treatment for both radiation nephritis and bone marrow transplant nephropathy.

Radiation and chemotherapy sensitizers and protectors

Radiosensitizers and radioprotectors are part of the chemical modifier approach to cancer therapy whereby the state of the tumor cells and/or normal tissues are modified such that a therapeutic gain is achieved using conventional radiation or chemotherapy. Radiosensitization can be achieved by the use of oxygen-mimetic compounds, agents that alter DNA sensitivity to irradiation, maneuvers that alter DNA repair processes, and manipulation of tissue oxygenation. Standard chemotherapeutic agents such as cisplatin can be utilized in a manner that optimizes the radiosensitization properties. Protection and sensitization can occur by altering the thiol status of the cell. The chemical modifiers field is both developing novel approaches to cancer treatment and increasing the understanding of basic cancer biology.

Radioprotection of normal tissues of the mouse by hypoxic breathing

Hypoxic breathing during irradiation has been advocated as a therapeutic modality, to increase the efficacy of radiotherapy. In this form of treatment, the total and daily X-ray dose is increased by a factor of 1.25, on the assumption that all normal tissues in the beam will be protected to a similar extent by breathing gas containing a reduced oxygen concentration (usually 10%). To test this concept, we have determined the effect of varying the inspired oxygen tension on the radiosensitivity of 3 normal tissues in the mouse (kidney, jejunum and skin), and have compared these results with data from the literature for mouse lung. Reduction of the inspired oxygen tension from 21% (air) to 7-8% led to much greater radioprotection of skin (protection factor 1.37) than of lung (1.09). Protection factors for jejunum and kidney were 1.16 and 1.36 respectively. The results show that the extent of radioprotection afforded by hypoxic breathing is tissue dependent, and that great care must be taken clinically in choosing the increased radiation dose to be used in conjunction with hypoxic breathing.

Role of glutathione peroxidase in the radiation response of mouse kidney

Glutathione peroxidase (GSH-Px) has been implicated in mediating the radioprotective effects of glutathione (GSH). This hypothesis was tested in vivo by determining the effect of GSH-Px depletion on the radiation response of murine kidneys. Renal GSH-Px levels were depleted to 17% of control values by feeding animals a selenium deficient diet for 6 weeks; this had no significant effect on renal levels of GSH or GSH-S-transferase (GST). However, we also tested the effect of direct depletion of GSH to 3-4% of control values, using a combination of DL-buthionine sulphoximine (BSO) and diethyl maleate (DEM). Kidneys with normal or depleted levels of GSH-Px and/or GSH were irradiated with 240kVp X rays (2 fractions, 7 days apart to minimize intestinal injury). Mice breathed 7% oxygen during irradiation. Renal damage was assessed at 20, 25, and 32 weeks after the first fraction of X rays, in terms of reduced hematocrit and renal clearance of 51Cr-EDTA. Depletion of GSH-Px levels to 17% of control did not alter renal radiosensitivity, but depletion of GSH to 3-4% of control values radiosensitized the kidney by a factor of 1.4. Depletion of both GSH and GSH-Px together did not radiosensitize the kidney any more than was achieved by GSH depletion alone.

Comparative biodistribution and radioprotection studies with three radioprotective drugs in mouse tumors

The organ level biodistribution and tumor radioprotective properties of three drugs have been compared: WR-2721 (NSC 296961), WR-3689 (NSC 327729), and WR-77913 (NSC 318809). The three drugs have similar distribution patterns in normal mouse tissues. At 30 minutes after intraperitoneal injection, highest levels of 35S from radiolabeled protector are found in kidney and submandibular salivary gland, with lowest levels in brain and moderately low values in tumor and skin. Three of four tumors examined take up less WR-3689 than the other two protectors. For the three protectors, the dose modifying factors for the RIF-1 tumor irradiated in vivo and assayed in vitro are 1.5-1.7, but do not vary as predicted by differential uptake of drug into this neoplasm. In RIF-1, WR-3689 is taken up most avidly, but the three drugs tend to be equally protective.

Protection of acute and late radiation damage of the gastrointestinal tract by WR-2721

WR-2721 was investigated for its protective effect against acute and late damage produced by irradiation of the esophagus, small intestine, and colon of mice. The microcolony assay was used to measure the acute response of the small intestine and the colon, and an LD50 assay (within the 28- to 42-day time range) was used to measure acute esophageal damage. A dose of WR-2721 at 400 mg/kg, injected 30 min prior to irradiation, resulted in a protection factor (PF) of 1.6 against radiation damage in these three regions of the gastrointestinal tract. Lethality and histology scores were applied to determine late radiation damage to the rectum, at times ranging from 3 to 15 months after irradiation. Deaths occurred after doses of 20 Gy and above throughout the postirradiation period. WR-2721 increased the survival of mice; the PF calculated from the LD50 values was 1.5. PFs of animal survival did not vary during the observation period. Histological studies showed evidence of ulceration, fibrosis, and vascular changes as late radiation damage. WR-2721 protected against radiation-induced histological damage with a PF of 1.3. There was no qualitative difference between the types of histological damage observed in the group undergoing only irradiation and the group treated with WR-2721. Biochemical measurements of fibrosis by hydroxyproline determination of collagen 16 months after irradiation showed an increase in collagen per milligram wet weight of rectal tissue in all irradiated groups, but no increase in the amount of collagen per 5 mm segment of the rectum. Thus it appears that the apparent fibrosis is a result of atrophy rather than collagen accumulation. We conclude that WR-2721 is indeed effective at protection against late damage from large single doses of radiation to the rectum as measured histologically and also improves the long-term survival of the mice, although the target cells for this damage are not known.

Protection of mouse lung by WR-2721 after fractionated doses of irradiation

The radioprotective effect of WR-2721 on mouse lung has been studied after single doses, 4 or 7 equal fractions of X rays. Using breathing rate and lethality to measure lung injury up to 1 year after radiation, significant protection against both pneumonitis at 7 months and fibrosis at 12 months was observed using 300 mg/kg of WR-2721. The degree of radioprotection was similar for pneumonitis and fibrosis and was not less after doses per fraction of 4.0 Gy. These data indicate that protection of mouse lung by WR-2721 will not be less in a multifractionated schedule of radiation, at least for doses per fraction greater than 4.0 Gy.

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.

Radiation induced renal damage in mice: influence of overall treatment time

The influence of overall treatment time on the radiation response of the mouse kidney was studied by varying the time over which 2 or 5 fractions of X-rays were administered. Two functional assays (urine output and 51Cr-EDTA excretion), and renal weight at sacrifice were used to obtain dose-response curves and estimate isoeffective doses. Split dose experiments showed Elkind recovery of about 5 Gy in 24 h. With a 7-day interval between fractions a transitory increase in isoeffect dose was observed in the first experiment. In the second, more extensive, experiment the recovered dose did not increase significantly even if the interval between two fractions was prolonged up to 25 days. Therefore, if slow repair occurred it was not worth more than 1 Gy because this was the limit of resolution of the assays used. As overall time was prolonged to 60 days an additional 1-2 Gy were recovered: it is difficult to explain this delayed sparing effect on the basis of a compensatory proliferative response, because the labelling indices of the likely target cells in the kidney are so low. Whatever the mechanisms involved, an increase in overall time had only a slight effect on isoeffect dose in these experiments and values for the "T" exponent were low (0.0-0.09). Recovery from sublethal injury between fractions has a much larger effect.