The radioprotector WR-2721 reduces neutron-induced mutations at the hypoxanthine-guanine phosphoribosyl transferase locus in mouse splenocytes when administered prior to or following irradiation

Mitigating the risk of radiation-induced cancers: limitations and paradigms in drug development

The United States radiation medical countermeasures (MCM) programme for radiological and nuclear incidents has been focusing on developing mitigators for the acute radiation syndrome (ARS) and delayed effects of acute radiation exposure (DEARE), and biodosimetry technologies to provide radiation dose assessments for guiding treatment. Because a nuclear accident or terrorist incident could potentially expose a large number of people to low to moderate doses of ionising radiation, and thus increase their excess lifetime cancer risk, there is an interest in developing mitigators for this purpose. This article discusses the current status, issues, and challenges regarding development of mitigators against radiation-induced cancers. The challenges of developing mitigators for ARS include: the long latency between exposure and cancer manifestation, limitations of animal models, potential side effects of the mitigator itself, potential need for long-term use, the complexity of human trials to demonstrate effectiveness, and statistical power constraints for measuring health risks (and reduction of health risks after mitigation) following relatively low radiation doses (<0.75 Gy). Nevertheless, progress in the understanding of the molecular mechanisms resulting in radiation injury, along with parallel progress in dose assessment technologies, make this an opportune, if not critical, time to invest in research strategies that result in the development of agents to lower the risk of radiation-induced cancers for populations that survive a significant radiation exposure incident.

Radioprotectants to reduce the risk of radiation-induced carcinogenesis

PURPOSE

Development of radioprotective agents has focused primarily on cytoprotection from relatively high doses of therapeutic radiation and nuclear disasters. Epidemiological studies and radiobiological models report the potential for stochastic effects from relatively low-dose radiation exposure. Diagnostic studies like computed tomography (CT) expose the patient to a small but significant amount of radiation, which has been reported to increase the risk for carcinogenesis. Young patients expected to undergo multiple CT studies may benefit from a protective agent given prior to CT. This review includes published data of agents that have been shown to protect against radiation-induced carcinogenesis. A discussion follows regarding the data that describes the extent of radiation exposure during CT, as well as technical modifications, which also reduce radiation exposure.

RESULTS/CONCLUSIONS

Most experiments have used in vivo animal models or in vitro cell lines. Ethical barriers prevent large-scale human studies, although, there are two prospective human studies from the Chernobyl nuclear accident. Collectively, all of these studies provide evidence of statistically significant reductions in radiation-induced carcinogenesis. Protection is achieved by several mechanisms, which include free radical scavenging, caloric restriction, non-steroidal anti-inflammatory agents, humoral factors, and an oxidative agent. Enhanced efficacy is achieved when targeting multiple mechanisms. The data presented provides the scientific foundation for future development of a radioprotectant that may reduce the risk of carcinogenesis from low-dose exposure when certain at-risk populations undergo diagnostic studies like CT.

Amifostine protection against induced DNA damage in gamma-irradiated Escherichia coli cells depend on recN DNA repair gene product activity

Amifostine is the most effective radioprotector known and the only one accepted for clinical use in cancer radiotherapy. In this work, the antigenotoxic effect of amifostine against gamma-rays was studied in Escherichia coli cells deficient in DNA damage repair activities. Assays of irradiated cells treated with amifostine showed that the drug reduced the genotoxicity induced by radiation in E. coli wild-type genotypes and in uvr, recF, recB, recB-recC-recF mutant strains, but not in recN defective cells. Thus, the mechanism of DNA protection by amifostine against gamma-radiation-induced genotoxicity appears to involve participation of the RecN protein that facilitates repair of DNA double-strand breaks. The results are discussed in relation to amifostine's chemopreventive potential.

Studies of thioguanine-resistant lymphocytes induced by in vivo irradiation of mice

The frequency of Hprt-deficient lymphocytes in mice after in vivo gamma irradiation, has been found to vary as a function of time elapsed after exposure and irradiation dose. The frequency of mutant lymphocytes in spleen was determined using an in vitro, clonogenic assay for thioguanine-resistant T-lymphocytes. Mice were exposed to single doses of 0-400 cGy from cesium-137 or to eight daily doses of 50 cGy. The time to maximum-induced mutant frequency was 3 weeks. The dose response was strikingly curvilinear at 3-5 weeks after irradiation, but less precisely defined for 10-53 weeks after exposure, being fit by either linear or quadratic dependence. Three weeks after eight daily 50 cGy exposures, mutant frequency was elevated above controls and mice exposed to 50 cGy (which were not distinct from the nonirradiated controls), but only 17% in that of mice given a single 400 cGy fraction. This fractionation effect and the curvilinearity of the early dose-response curve suggested that saturation of repair increased the yield of mutations at higher acute doses. The decline of spleen mutant frequency in mice observed between 5 and 10 weeks after irradiation may reflect selection against some mutants. The marked variation of mutant frequency, as a function of time after irradiation and of dose rate, emphasize the need to evaluate these variables carefully and consistently in future studies.

Free radical scavenger edaravone suppresses x-ray-induced apoptosis through p53 inhibition in MOLT-4 cells

Edaravone, a clinical drug used widely for the treatment of acute cerebral infarction, is reported to scavenge free radicals. In the present study, we investigated the radioprotective effect of edaravone on X-ray-induced apoptosis in MOLT-4 cells. Apoptosis was determined by the dye exclusion test, Annexin V binding assay, cleavage of caspase, and DNA fragmentation. We found that edaravone significantly suppressed the X-ray-induced apoptosis. The amount of intracellular ROS production was determined by the chloromethyl-2',7'-dichlorodihydro-fluorescein diacetate system. We found that the intracellular ROS production by X-irradiation was completely suppressed by the addition of edaravone. The accumulation and phosphorylation of p53 and the expression of p21(WAF1), a target protein of p53, which were induced by X-irradiation, were also suppressed by adding edaravone. We conclude that the free radical scavenger edaravone suppresses X-ray-induced apoptosis in MOLT-4 cells by inhibiting p53.

Zebrafish as a model system to screen radiation modifiers

Zebrafish (Danio rerio) is a bona fide vertebrate model system for understanding human diseases. It allows the transparent visualization of the effects of ionizing radiation and the convenient testing of potential radioprotectors with morpholino-modified oligonucleotides (MO) knockdown. Furthermore, various reverse and forward genetic methods are feasible to decipher novel genetic modifiers of radioprotection. Examined in the review are the radioprotective effects of the proposed radiomodifiers Nanoparticle DF-1 (C-Sixty, Inc., Houston, TX) and Amifostine (WR-2721, Ethyol), the DNA repair proteins Ku80 and ATM, as well as the transplanted hematopoietic stem cells in irradiated zebrafish. The presence of any of these sufficiently rescued the radiation-induced damages in zebrafish, while its absence resulted in mutagenic phenotypes as well as an elevation of time- and dose-dependent radiation-induced apoptosis. Radiosensitizers Flavopiridol and AG1478, both of which block progression into the radioresistant S phase of the cell cycle, have also been examined in zebrafish. Zebrafish has indeed become a favorite model system to test for radiation modifiers that can potentially be used for radiotherapeutic purposes in humans.

Effects of thiols on topoisomerase-II alpha activity and cell cycle progression

Thiol containing compounds exhibiting antioxidant properties are currently being evaluated for use in cytoprotection and chemoprevention. Many of these have also been found to be effective in inhibiting cell cycle progression and cellular proliferation. N-Acetyl-L-cysteine (L-NAC), along with its nonmetabolically active stereoisomer N-acetyl-D-cysteine (D-NAC), together with captopril and dithiothreitol (DTT) were investigated to assess their effects on cell cycle progression as determined by flow cytometry. Topoisomerase-IIa (topo-II alpha) activity, an enzyme involved in DNA synthesis, was also monitored as a function of drug dose using a kinetoplast DNA (kDNA) decatenation assay. Chinese hamster ovary (CHO) AA8 cells were exposed to each thiol at concentrations ranging from 4 microM to 4 mM for a period of 3 h. Following the removal of the thiols, cell cultures were followed for an additional 5 h to assess changes in cell cycle progression. L-NAC, which also serves as a precursor for glutathione (GSH) synthesis, effectively inhibited topo-IIa activity by at least 50% at all concentrations tested. Associated with this reduction in enzyme activity was a sixfold increase in the relative number of cells accumulating in G2phase. D-NAC, which is unable to participate in GSH synthesis, was only half as effective as L-NAC at each concentration tested in inhibiting topo-IIa activity as well as perturbing cell progression through G2. In comparison, captopril, an inhibitor of angiotensin converting enzyme (ACE), had little effect on the progression of cells into G2 phase. In contrast to the repressive effects of L-NAC and D-NAC, it enhanced topo-IIa activity over control values by approximately 20%. DTT, a well characterized thiol known to be capable of reducing disulphides in proteins, was observed to be relatively ineffective in either perturbing cell cycle progression or affecting topo-IIa activity. This suggests an involvement of a mechanism(s) in addition to thiol mediated affects on reduction/oxidation processes. The inhibitory effects of L-NAC and D-NAC on topo-IIa activity, in contrast to the other two thiols, may be due in part to the presence of amine groups which could allow for their participation in polyamine related processes. The difference in the magnitude of the effect exhibited by L-NAC, as compared to D-NAC, on the repression topo-IIa activity also suggests a role for GSH in this process. Inhibition of cellular progression and proliferation by thiols can therefore be mediated by diverse mechanisms which include both cycle-phase specific (i.e. L-NAC and D-NAC) and non cell cycle specific (i.e. captopril) processes.

Reduced apoptosis and increased deletion mutations at Aprt locus in vivo in mice exposed to repeated ionizing radiation

Exposure to ionizing radiation (IR) is a risk factor for carcinogenesis because it is a mutagen. However, a single 4-Gy whole body X-ray exposure only induced a modest increase of mutations at the Aprt reporter gene locus in mouse T cells. Intriguingly, when the same dose of IR was given in a fractionated protocol (1 Gy x 4 at weekly intervals), there was a strong induction of Aprt mutations in T cells. Many of these were mutations that arose via interstitial deletions inclusive of Aprt or by intragenic deletions. We hypothesized that the weekly fractionated X-ray exposures select for somatic cells with reduced p53 expression and/or reduced apoptosis, which, in turn, may have facilitated the accumulation of interstitial deletions, as in p53-deficient mice. We indeed found that splenocytes of mice with three previous exposures (1 Gy x 4 in total) were more resistant to X-ray-induced apoptosis than those of mice exposed to X-rays for the first time (1 Gy total). Thus, repeated X-ray radiation selects for reduced apoptosis in vivo. However, this reduced apoptosis is p53-independent, because p53 induction and the up-regulation of genes downstream of p53, such as Bax and p21, were similar between the 1-Gy and 1 Gy x 4 groups. Reduced apoptosis probably allows the generation of more mutations, particularly deletion mutations. Because both reduced apoptosis and increased somatic mutation are risk factors for carcinogenesis, they may contribute to the paradigm in which different radiation exposure schemes are varied in their efficiency in inducing lymphomagenesis.

Usefulness of the SOS Chromotest in the study of medicinal plants as radioprotectors

PURPOSE

The aim of this work is to investigate the usefulness of a modified protocol of the SOS Chromotest to detect antigenotoxicity activities against gamma-rays of plant extracts with proven antioxidant activity, and to elucidate the antigenotoxic mechanisms involved in radioprotection using this system.

MATERIALS AND METHODS

The methodology developed was assayed with amifostine, the most studied radioprotector, and with Phyllanthus orbicularis HBK, Cymbopogon citratus (DC) Stapf and Pinus caribaea Morelet extracts, using pre- and post-treatment procedures.

RESULTS

The P. caribaea and C. citratus extracts were antigenotoxic against gamma-rays when the cells were pre-treated with both extracts, suggesting a possible antigenotoxic action through a free radical scavenging mechanisms. Amifostine and the P. orbicularis extract were also antigenotoxic under pre- and post-treatment conditions, indicating that several antimutagenic components of this plant extract may also operate by some intracellular mechanism, unlike its antioxidant activity.

CONCLUSIONS

The results have demonstrated the usefulness of the modified SOS Chromotest assay in the screening of phytochemical radioprotectors as well as in the study of their antimutagenic mechanisms.

A potential synergistic anticancer effect of paclitaxel and amifostine on endometrial cancer

Although paclitaxel is one of the most effective chemotherapeutic agents, its usefulness is still limited in advanced and recurrent endometrial cancer. Amifostine protection of normal tissues against the side effects of chemotherapeutic agents has been clinically proven in cancer patients; however, its application in endometrial cancer has not been fully evaluated. We have investigated the use of paclitaxel and amifostine in controlling the growth of poorly differentiated endometrial cancer cells, Hec50co, in vitro and in vivo. Our studies show that amifostine had direct anticancer effects on endometrial cancer cells in vitro by arresting the cell cycle at the G1 phase and inducing apoptosis. Amifostine also inhibited s.c. tumor growth in athymic mice. Paclitaxel IC50 value was reduced from 14 to 2 nmol/L with pretreatment of a single dose of 178 micromol/L of amifostine for 72 hours. Amifostine also synergized with paclitaxel in the arrest of the cell cycle at the G2-M phase and in the induction of apoptosis. This two-drug regimen inhibited s.c. tumor growth as well as improved mouse survival significantly more than paclitaxel alone. Amifostine also significantly improved paclitaxel-induced cytotoxic effects on peripheral blood profiles. Our studies show that amifostine has direct anticancer effects on endometrial cancer. Our data have also shown a potential anticancer synergy between amifostine and paclitaxel in vitro and in vivo, whereas amifostine maintained a protective role in peripheral blood profiles. The dual specificity of amifostine action should be further investigated.

Protection against ionizing radiation by antioxidant nutrients and phytochemicals

The potential of antioxidants to reduce the cellular damage induced by ionizing radiation has been studied in animal models for more than 50 years. The application of antioxidant radioprotectors to various human exposure situations has not been extensive although it is generally accepted that endogenous antioxidants, such as cellular non-protein thiols and antioxidant enzymes, provide some degree of protection. This review focuses on the radioprotective efficacy of naturally occurring antioxidants, specifically antioxidant nutrients and phytochemicals, and how they might influence various endpoints of radiation damage. Results from animal experiments indicate that antioxidant nutrients, such as vitamin E and selenium compounds, are protective against lethality and other radiation effects but to a lesser degree than most synthetic protectors. Some antioxidant nutrients and phytochemicals have the advantage of low toxicity although they are generally protective when administered at pharmacological doses. Naturally occurring antioxidants also may provide an extended window of protection against low-dose, low-dose-rate irradiation, including therapeutic potential when administered after irradiation. A number of phytochemicals, including caffeine, genistein, and melatonin, have multiple physiological effects, as well as antioxidant activity, which result in radioprotection in vivo. Many antioxidant nutrients and phytochemicals have antimutagenic properties, and their modulation of long-term radiation effects, such as cancer, needs further examination. In addition, further studies are required to determine the potential value of specific antioxidant nutrients and phytochemicals during radiotherapy for cancer.

Chemical radioprotection: a critical review of amifostine as a cytoprotector in radiotherapy

The use of chemical radioprotectors represents an obvious strategy to improve the therapeutic index in radiotherapy. Amofostine (WR-2721) has recently been approved for use in head and neck cancer to protect against radiation-induced xerostomia. Currently, the question has arisen whether amifostine could be used for radioprotection in broader terms. Amifostine may have the potential to enable intensified treatment by ameliorating mucosal reactions that are often a limiting factor in accelerated fractionation or concomitant chemoradiation. However, it has as yet not been clarified whether sufficient amifostine to reduce mucositis can be administered before each radiation fraction without causing unacceptable toxicity. Also, the optimal dosage and schedule of amifostine in chemoradiation combinations have not yet been established. The major concern related to radioprotectiors is the potential hazard of collateral tumor protection. A number of clinical studies have concluded that amifostine does not reduce antitumor efficacy. However, not even the largest study conducted, with over 300 patients, has sufficient statistical power to detect a clinically significant reduction in tumor control rate. To put this issue ultimately to a rest, a clinical trial with a sufficient accrual to definitely rule out a tumor protective effect of amifostine needs to be conducted. Substances reducing radiation-induced toxicity by modulating the biological response to radiation injury may represent an alternative concept in radioprotection. However, such agents are still at a developmental stage.

Inhibition of spontaneous metastases formation by amifostine

Amifostine was investigated for its ability to inhibit spontaneous metastases formation using the well-characterized murine sarcoma, Sa-NH. Amifostine was administered intraperitoneally at a dose of 50 mg/kg every other day for 6 days to C3Hf/Kam mice until tumors reached an average size of 8-8.5 mm in diameter. Amifostine was again administered immediately after surgical removal of the tumor-bearing limbs by amputation, and then once more 2 days later. Twenty-one days later, animals were evaluated for the presence of spontaneously developed pulmonary metastases. Nontumor-bearing control animals were sham treated using the same dosing and surgery schedules. Treatment with amifostine appeared to slightly delay tumor growth, that is, 13 vs. 12 days for tumors to reach an average diameter of 8 mm. Amifostine reduced both the incidence of pulmonary metastases formed in experimental animals from 77% to 57% (p < 0.05), and their average number per animal from 12.8 +/- 5.4 (SEM) to 2.9 +/- 1.1 (SEM). The effect of amifostine exposure on serum levels of the angiogenesis inhibitor angiostatin was also determined using Western blot analysis. Consistent with the antimetastatic effect, exposure of animals to 50 mg/kg of amifostine resulted in a 4-fold enhanced serum level of angiostatin above control levels. This phenomenon occurred in tumor-bearing and nontumor-bearing animals. The effects of amifostine on matrix metalloproteinase (MMP) enzymatic activity was also determined using gelatin zymography. Conditioned growth medium collected from Sa-NH cells grown to confluency was exposed to various concentrations of SH, i.e., 2-[(aminopropyl)amino]ethane-thiol (WR-1065), the active thiol form of amifostine, for either 30 min or 18 hr. WR-1065, as a function of increasing dose and time, inhibited the enzymatic activities of MMP-2 and MMP-9. At a concentration and time of exposure likely to be achieved in vivo, that is, 40 microM and 30 min, MMP-2 and MMP-9 activities were reduced to between 30% and 40% of control values. Consistent with these affects, WR-1065 was also found to be effective in inhibiting the ability of Sa-NH cells to migrate through Matrigel membranes. After an 18-hr exposure under in vitro conditions, WR-1065 at concentrations of 4, 40 and 400 microM, and 4 mM, inhibited Sa-NH migration to 11%, 44%, 81% and 97% of control values, respectively. The abilities of amifostine and its active thiol WR-1065 to stimulate angiostatin production in mice, and to inhibit the MMP enzymatic activities and invasion ability of Sa-NH cells under in vitro conditions, are consistent with the observed antimetastatic effects exhibited against Sa-NH tumors growing in vivo.

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.

Radioprotective thiolamines WR-1065 and WR-33278 selectively denature nonhistone nuclear proteins

Differential scanning calorimetry was used to study the interactions of nuclei isolated from Chinese hamster V79 cells with the radioprotector WR-1065, other thiol compounds, and polyamines. Differential scanning calorimetry monitors denaturation of macromolecules and resolves the major nuclear components (e.g. constrained and relaxed DNA, nucleosome core, and nuclear matrix) of intact nuclei on the basis of thermal stability. WR-1065 treatment (0.5-10 mM) of isolated nuclei led to the irreversible denaturation of nuclear proteins, a fraction of which are nuclear matrix proteins. Denaturation of 50% of the total nonhistone nuclear protein content of isolated nuclei occurred after exposure to 4.7 mM WR-1065 for 20 min at 23 degrees C. In addition, a 22% increase in the insoluble protein content of nuclei isolated from V79 cells that had been treated with 4 mM WR-1065 for 30 min at 37 degrees C was observed, indicating that WR-1065-induced protein denaturation occurs not only in isolated nuclei but also in the nuclei of intact cells. From the extent of the increase in insoluble protein in the nucleus, protein denaturation by WR-1065 is expected to contribute to drug toxicity at concentrations greater than approximately 4 mM. WR-33278, the disulfide form of WR-1065, was approximately twice as effective as the free thiol at denaturing nuclear proteins. The proposed mechanism for nucleoprotein denaturation is through direct interactions with protein cysteine groups with the formation of destabilizing protein-WR-1065 disulfides. In comparison to its effect on nuclear proteins in isolated nuclei, WR-1065 had only a very small effect on non-nuclear proteins of whole cells, isolated nuclear matrix, or the thiol-rich Ca(2+)ATPase of sarcoplasmic reticulum, indicating that WR-1065 can effectively denature protein only inside an intact nucleus, probably due to the increased concentration of the positively charged drug in the vicinity of DNA.

Inhibitory effects of WR-2721 and cysteamine on tumor initiation in mammary glands of pregnant rats by radiation

We evaluated the effect of WR-2721 [S-2-(3-aminopropylamino)-ethylphosphorothioic acid] and cysteamine (2-mercaptoethylamine) on the development of radiation-induced mammary tumors in rats. Pregnant rats were treated with WR-2721 or cysteamine 30 min prior to whole-body irradiation with gamma rays from a (60)Co source at a dose of 1.5 or 2.6 Gy. Additional pregnant rats were given saline and then exposed to gamma rays at a dose of 0, 1.5 or 2.6 Gy as a control. All rats were implanted with pellets of diethylstilbestrol, a tumor promoter, 1 month after termination of nursing and were observed for 1 year to detect palpable mammary tumors. No mammary tumors developed in the saline-injected nonirradiated rats. However, when rats were irradiated with 1.5 or 2. 6 Gy after saline treatment, the incidence of mammary tumors was high (71.4 and 92.3%, respectively). Administration of WR-2721 or cysteamine prior to irradiation with 1.5 Gy significantly decreased the tumor incidence (23.8 and 20.8%, respectively). Tumor prevention by either agent was less effective at the higher dose. The appearance of the first mammary tumor occurred later in rats treated with WR-2721 or cysteamine than in the control rats. An increasing rate of adenocarcinoma in the control group was observed with increasing dose from 1.5 Gy up to 2.6 Gy. However, the development of adenocarcinoma did not increase after pretreatment with WR-2721 or cysteamine in rats irradiated with 2.6 Gy. Many of the mammary tumors that developed in the control rats were of the ER(+)PgR(+) type. Administration of WR-2721 produced no tumors of the ER(+)PgR(+) type. Cysteamine treatment increased the development of ER-negative tumors. The serum concentration of progesterone was significantly higher in rats treated with WR-2721 or cysteamine than in the control rats. On the other hand, the estradiol-17beta concentration was reduced by treatment with WR-2721, but not significantly compared to the control. WR-2721 and cysteamine had no effect on the prolactin concentration of the irradiated rats. The results suggest that administration of WR-2721 or cysteamine prior to the irradiation has a potent preventive effect on theinitiation phase during mammary tumorigenesis.

The sulfhydryl containing compounds WR-2721 and glutathione as radio- and chemoprotective agents. A review, indications for use and prospects

Radio- and chemotherapy for the treatment of malignancies are often associated with significant toxicity. One approach to reduce the toxicity is the concomitant treatment with chemoprotective agents. This article reviews two sulfhydryl compounds, namely the agent WR-2721 (amifostine), a compound recently registered for use in human in many countries, and the natural occurring compound glutathione (GSH). GSH is not registered as a chemoprotective agent. WR-2721 is an aminothiol prodrug and has to be converted to the active compound WR-1065 by membrane-bound alkaline phosphatase. WR-1065 and GSH both act as naturally occurring thiols. No protective effect on the tumour has been found when these compounds are administered intravenously. There is even in vitro evidence for an increased anti-tumour effect with mafosfamide after pretreatment with WR-2721, and in vivo after treatment with carboplatin and paclitaxel. Randomized clinical studies have shown that WR-2721 and GSH decrease cisplatin-induced nephrotoxicity and that WR-2721 reduces radiation radiotherapy-induced toxicity. Side-effects associated with WR-2721 are nausea, vomiting and hypotension, GSH has no side-effects. An exact role of WR-2721 and GSH as chemoprotectors is not yet completely clear. Future studies should examine the protective effect of these drugs on mucositis, cardiac toxicity, neuro- and ototoxicity, the development of secondary neoplasms and their effect on quality of life.

Radiation-induced long-lived radicals which cause mutation and transformation

Using electronic spin resonance (ESR), we found a new type of radical with a long life-time in cells (T1/2>20 h) and which may play a more important role in the induction of mutation and transformation than either the active, short-lived, H, or OH radicals. When cells were treated with dimethyl sulfoxide (DMSO) and l-ascorbic acid (AsA) just before irradiation, the short-lived radicals were well-scavenged. On the other hand, if cells were treated with the scavengers 20 min after irradiation, then AsA scavenged the long-lived radicals, but DMSO did not. AsA treatment 20 min after the start of irradiation drastically reduced both the frequencies of mutation at the hypoxanthine guanine phosphoribosyl transferase (HGPRT) locus in human cells and morphological transformations in mouse m5S cells, but DMSO treatment did not. In addition, AsA treatment 20 h after irradiation also reduced the mutation frequency in human cells. These results suggested that mutations and morphological transformation are probably caused by the presence of long-lived radicals in the cells, rather than by short lived radicals, and that AsA reacts efficiently with long-lived radicals, resulting in a decrease of the mutations and transformations induced.

Reducing the toxicity of anticancer therapy: new strategies

Cytoprotective agents offer opportunities to reduce the treatment-related toxicity of anticancer therapy and perhaps to increase the dose and dose intensity of radiation and chemotherapy. One such agent is amifostine, an organic thiophosphate. Amifostine selectively protects normal tissues and provides broad-spectrum protection for a variety of organs while remaining minimally toxic. Clinical studies have demonstrated that amifostine protects against myelotoxicity, nephrotoxicity, neurotoxicity, mucositis and esophagitis in patients treated with alkylating and platinum agents, paclitaxel and radiation therapy. In addition, preclinical studies suggest the possibility of protection against anthracycline-induced cardiotoxicity and radiation- and chemotherapy-induced mutagenicity. Preclinical and clinical studies have not demonstrated any diminution of antitumor efficacy. Amifostine is well tolerated in doses of 740 or 910 mg/m2. The most common side effects requiring treatment are transient hypotension, which responds to intravenous fluids, and nausea and vomiting, effectively treated with 5-HT3 antagonists and dexamethasone.

Genomic instability and tumorigenic induction in immortalized human bronchial epithelial cells by heavy ions

Carcinogenesis is postulated to be a progressive multistage process characterized by an increase in genomic instability and clonal selection with each mutational event endowing a selective growth advantage. Genomic instability as manifested by the amplification of specific gene fragments is common among tumor and transformed cells. In the present study, immortalized human bronchial (BEP2D) cells were irradiated with graded doses of either 1GeV/nucleon 56Fe ions or 150 keV/micrometer alpha particles. Transformed cells developed through a series of successive steps before becoming tumorigenic in nude mice. Tumorigenic cells showed neither ras mutations nor deletion in the p16 tumor suppressor gene. In contrast, they harbored mutations in the p53 gene and over-expressed cyclin D1. Genomic instability among transformed cells at various stage of the carcinogenic process was examined based on frequencies of PALA resistance. Incidence of genomic instability was highest among established tumor cell lines relative to transformed, non-tumorigenic and control cell lines. Treatment of BEP2D cells with a 4 mM dose of the aminothiol WR-1065 significantly reduced their neoplastic transforming response to 56Fe particles. This model provides an opportunity to study the cellular and molecular mechanisms involved in malignant transformation of human epithelial cells by heavy ions.

The inhibition of radiation-induced mutagenesis by the combined effects of selenium and the aminothiol WR-1065

In order to evaluate the anti-mutagenic effects of the potential chemoprotective compounds selenium and (S)-2-(3-aminopropylamino)ethylphosphorothioic acid (WR-1065), CHO AA8 cells were exposed to both compounds either individually or in combination prior to irradiation. Mutation frequency following exposure to 8 Gy was evaluated by quantitation of the mutations detected at the hprt locus of these cells. Protection against radiation-induced mutation was observed for both 30 nM sodium selenite or 4 mM WR-1065. In addition, the protection against mutation induction provided by the combination of these agents appeared additive. In contrast, sodium selenite did not provide protection against radiation toxicity when provided either alone or in conjunction with WR-1065. In order to evaluate the possible mechanisms of the anti-mutagenic effects observed in these cells, glutathione peroxidase (GPx) activity was evaluated following exposure to the chemopreventative compounds. The addition of sodium selenite to the culture media resulted in a 5-fold increase in GPx activity, which was unaltered by the presence of the WR-1065. Northern analysis of RNA derived from these cells indicated that selenium supplementation resulted in a marginal increase in the mRNA for the cytosolic GPx (GSHPx-1) which was insufficient to account for the stimulation of GPx activity observed in cellular extracts. These results suggest that selenium and WR-1065 offer protection via independent mechanisms and that GPx stimulation remains a possible mechanism of the anti-mutagenic effect of selenium.

The preclinical basis for broad-spectrum selective cytoprotection of normal tissues from cytotoxic therapies by amifostine (Ethyol)

Administered prior to cytotoxic chemotherapy or radiation, the aminothiol amifostine provides broad-spectrum cytoprotection of various normal tissues without attenuating antitumour response. The basis for the selectivity of action resides in the anabolism of amifostine at the normal tissue site by membrane-bound alkaline phosphatase. Dephosphorylation to the free thiol, WR-1065, is followed by rapid uptake into normal tissues by a carrier mediated, facilitated diffusion process; in contrast, uptake into tumour tissue is slow to negligible. Preclinical studies have shown that pretreatment with amifostine provides protection of normal tissues from the cytotoxic effects of alkylating agents, organoplatinums, anthracyclines, taxanes and radiation. Normal tissues protected include bone marrow, kidney, neural tissues, the heart, intestinal crypt cells and pulmonary tissues. Additionally, the mutagenic and carcinogenic effects of these modalities are also attenuated. With respect to bone marrow, preclinical studies have shown significant protection of progenitor cells that give rise to the red and white cells and platelets. Comparative in vitro and in vivo studies using murine and human tumour xenografts show no decrease of antitumour effects of these same therapies despite the protection of normal organs. The unique preclinical profile of amifostine serves as a model for the clinical development programme for this important new broad-spectrum cytoprotective agent.

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.

Dietary supplements of antioxidants reduce hprt mutant frequency in splenocytes of aging mice

The level of spontaneous and gamma-radiation-induced mutations in the hypoxanthine-guanine phosphoribosyl-transferase (hprt) locus as well as the decrease in frequency of these mutations in mice of various age pretreated with dietary supplements of an antioxidant mixture (vitamins C, E, beta-carotene, rutin, selenium, zinc) were studied in splenocytes of young (8-14-week-old) and aged (102-110-week-old) male C57BL/6 mice. The frequency of spontaneous mutations in splenocytes of 102-110-week-old mice was higher by 68-88% than that in mice aged 8-14 weeks. On gamma-irradiation (0.5-5.0 Gy) of mice, the frequency of radiation-induced mutations (Vf assay) in aged mice was 2.3 to 3.6 times (depending on dose) higher than in young ones. Daily supplements of an antioxidant mixture to the diet of mice prior to irradiation showed an antimutagenic effect. The values of mutant frequency reduction factor (MFRF) for 14-110-week-old mice fed with dietary antioxidants during 6 weeks prior to gamma-irradiation with doses of 2.0 and 5.0 Gy were 5.4 and 3.7, respectively. The frequency of radiation-induced mutations prevented or not prevented by antioxidants was much higher in aged mice than in young ones.

Mutant frequency at the H-2K class 1 and HPRT genes in T lymphocytes from the X-ray-exposed mouse

The frequency of H-2Kk and HPRT-deficient T cells was measured in the H-2Kb, kDd,k genotype mouse 8-10 weeks after X-ray exposure at doses up to 6 Gy to compare the mutant frequency (MF) of an autosomal gene with that of an X-chromosomal gene. H-2K mutants were enriched by magnetic cell separation (MACS) using the H-2Kk-specific monoclonal antibody H100.5/28 and were isolated by limiting dilution cloning. Finally, the mutant phenotype was verified by flow cytometric analysis in a representative number of clones. The frequency of HPRT-deficient T cells rises from 2.5 x 10(-6) at 0 Gy to a maximum of 1.13 x 10(-4) at 4 Gy, and decreases to 2.9 x 10(-5) at 6 Gy. The H-2K- MF in the non-irradiated mouse was 8.4 x 10(-7). It increases with dose to a maximum of 8.1 x 10(-6) at 4 Gy and declines to 3.3 x 10(-6) at 6 Gy. The H-2K- MF measured depends on the monoclonal antibody used for the isolation of mutants. In a pilot study with another H-2Kk-specific monoclonal antibody (11.4.1), the spontaneous MF was four times higher than in experiments with the H100.5/28 monoclonal antibody. The expression of other class 1 antigens was investigated in H-2K- clones. The H-2Dd antigen had also disappeared in six of 41 clones from irradiated animals. This gene is situated at a distance of 1500 kb from the K-locus. The H-2Kb antigen was present in every investigated clone. In the discussion a model is presented that explains the shape of the dose-response curve of MF by selection against mutants in vivo systems under homeostasis. The results of the present investigation indicate that observed X-ray mutagenicity depends on many factors and that several genes have to be explored before reliable risk estimates are possible.

Protection of normal tissues from the cytotoxic effects of chemotherapy and radiation by amifostine (WR-2721): preclinical aspects

Amifostine is a radioprotective agent that prevents radiation- and chemotherapy-induced cellular injury through free-radical scavenging, hydrogen donation, and inhibition of DNA damage. Amifostine is metabolised and accumulated to a much greater extent in normal cells than in tumour cells. As a result, it exerts a protective effect from toxicity on normal tissues induced by chemo- or radiotherapy without reducing the antitumour effects of cancer therapy. Extensive preclinical studies have shown that amifostine protects against radiation damage and against the myelotoxic, nephrotoxic and neurotoxic effects of chemotherapeutic agents such as alkylating agents and platinum compounds. In some cases, the antitumour effects of these agents have been potentiated by amifostine. Amifostine has also been shown to protect against radiation- and chemotherapy-induced mutagenesis and, as a result, carcinogenesis. Use of amifostine allows for safer and more effective administration of radio- and anticancer therapy.

Dose and dose-rate dependence of the frequency of HPRT deficient T lymphocytes in the spleen of the 137Cs gamma-irradiated mouse

The frequency of hypoxanthine phosphoribosyl transferase (HPRT) deficient splenic T lymphocytes was measured in the 137Cs gamma-irradiated mouse by the T cell cloning method. Doses from 0.3 to 6 Gy were applied at the dose-rates 0.5 Gy/min, 1 Gy/day and 1 Gy/week. Mutants were determined 8-10 and 30-40 weeks after the end of exposure. Radiation-induced mutant frequency (MFi) was calculated by subtracting the age corrected spontaneous mutant frequency (MFsp) from total mutant frequency (MF) found in irradiated animals. Data were fitted to linear and linear-quadratic dose-response models. MFi depended markedly on dose, dose-rate and time after exposure. When mutants were determined 8-10 weeks after acute irradiation (0.5 Gy/min) the dose-effect curve fitted the linear-quadratic equation MFi = 6.9 x 10(-6) Gy + 1.2 x 10(-6) Gy2, whereas in low dose-rate experiments (1 Gy/day, 1 Gy/week) the dose-effect curves were linear. The slope of the linear regression was about 3 x 10(-6). When low dose-rate-irradiated animals were killed 30-40 weeks after irradiation, MFi was about one-third of that observed after 8 weeks. The dose dose-rate effectiveness factor (DDREF) for radiation mutagenicity was calculated in animals that had been exposed 8-10 weeks previously. For doses < 2 Gy the reduction in effectiveness was about 1.5 when the irradiation dose-rate was < or = 1 Gy/day. For higher doses DDREF was 3-5.

Normal and reverse dose-rate effect for the induction of mutants in somatic cells by ionizing radiation

In radiobiology the reduction of the dose-rate in general diminishes the degree of the biological effect per unit dose. This phenomenon is characterized by the dose-rate effectiveness factor (DREF). DREF is the factor by which a risk per unit dose obtained from data at high dose and high dose-rate overestimates the risk at low doses and/or low dose-rates. In general, DREF is in the range of 2 to 10. In the first part of this review, a short survey of the modern microdosimetric approach for a better understanding of radiation load on the cellular level and the significance of dose-rate is given. Experiments on the influence of dose-rate on the mutagenicity of ionizing radiation in cultured cells are reviewed. In contrast to other biological effects, in most experiments the reduction of the dose-rate had no or even a reverse dose-rate effect (DREF < or = 1). In the second part results on the influence of dose-rates on the induction of HGPRT-deficient T-lymphocytes in mice irradiated in vivo are given. Mutagenicity decreases with dose-rate and DREF values between 3-10 were measured. Possible reasons for the discrepancy between in vitro and in vivo experiments are discussed.