Radiation protectors: the unexpected benefits

Formation of two centre three electron bond by hydroxyl radical induced reaction of thiocoumarin: evidence from experimental and theoretical studies

Radiation chemical studies of thioesculetin (1), a thioketone derivative of coumarin, were performed by both pulse radiolysis technique and DFT calculations. Hydroxyl (^•OH) radical reaction with 1 resulted transients absorbing at 320, 360 and 500 nm. To identify the nature of the transients, the reaction was studied with specific one-electron oxidant (N₃^•) radical, where 360 nm band was absent. The transient absorption at 500 nm was concentration-dependent. The overall impression for ^•OH radical reaction was that the transient absorbing at 320, 360 and 500 nm was due to sulphur centred monomer radical, hydroxysulfuranyl and dimer radical of 1 respectively. The equilibrium constant between the monomer to dimer radical was 3.75 × 10⁴ M^-1. From the transients' redox nature, it was observed that 57 and 24% of ^•OH radical yielded to oxidising and reducing products respectively. Further, the product analysis by HPLC suggested that the dimer radical disproportionate to esculetin and thioesculetin. DFT energy calculation for all the possible transients revealed that dimer radical has the lowest energy. The HOMO of 1 and its monomer radical suggested that the electron density was localised on the sulphur atom. The bond length between the two sulphur atoms in dimer radical was 2.88 Å which was less than the van der Waals distance. Bond order between the two sulphur atoms was 0.55, suggesting that the bond was two centre three electron (2c-3e). From TD-DFT calculation, the electronic transition of dimer radical was at 479 nm which was in close agreement with the experimental value. The nature of the electronic transition was σ → σ* from a 2c - 3e bond.

Neuroprotective Role of Selected Antioxidant Agents in Preventing Cisplatin-Induced Damage of Human Neurons In Vitro

Chemotherapy-induced peripheral neuropathy (CIPN) is a side effect of platinum-based chemotherapy and decreases the quality of life of cancer patients. We compared neuroprotective properties of several agents using an in vitro model of terminally differentiated human cells NT2-N derived from cell line NT2/D1. Sodium azide and an active metabolite of amifostine (WR1065) increase cell viability in simultaneous treatment with cisplatin. In addition, WR1065 protects the non-dividing neurons by decreasing cisplatin caused oxidative stress and apoptosis. Accumulation of Pt in cisplatin-treated cells was heterogeneous, but the frequency and concentration of Pt in cells were lowered in the presence of WR1065 as shown by X-ray fluorescence microscopy (XFM). Transition metals accumulation accompanied Pt increase in cells; this effect was equally diminished in the presence of WR1065. To analyze possible chemical modulation of Pt-DNA bonds, we examined the platinum L_III near edge spectrum by X-ray absorption spectroscopy. The spectrum found in cisplatin-DNA samples is altered differently by the addition of either WR1065 or sodium azide. Importantly, a similar change in Pt edge spectra was noted in cells treated with cisplatin and WR1065. Therefore, amifostine should be reconsidered as a candidate for treatments that reduce or prevent CIPN.

Radioprotective effects of gallic acid in mice

Radioprotecting ability of the natural polyphenol, gallic acid (3,4,5-trihydroxybenzoic acid, GA), was investigated in Swiss albino mice. Oral administration of GA (100 mg/kg body weight), one hour prior to whole body gamma radiation exposure (2–8 Gy; 6 animals/group), reduced the radiation-induced cellular DNA damage in mouse peripheral blood leukocytes, bone marrow cells, and spleenocytes as revealed by comet assay. The GA administration also prevented the radiation-induced decrease in the levels of the antioxidant enzyme, glutathione peroxidise (GPx), and nonprotein thiol glutathione (GSH) and inhibited the peroxidation of membrane lipids in these animals. Exposure of mice to whole body gamma radiation also caused the formation of micronuclei in blood reticulocytes and chromosomal aberrations in bone marrow cells, and the administration of GA resulted in the inhibition of micronucleus formation and chromosomal aberrations. In irradiated animals, administration of GA elicited an enhancement in the rate of DNA repair process and a significant increase in endogenous spleen colony formation. The administration of GA also prevented the radiation-induced weight loss and mortality in animals (10 animals/group) exposed to lethal dose (10 Gy) of gamma radiation. (For every experiment unirradiated animals without GA administration were taken as normal control; specific dose (Gy) irradiated animals without GA administration serve as radiation control; and unirradiated GA treated animals were taken as drug alone control).

Chelation of lysosomal iron protects against ionizing radiation

Ionizing radiation causes DNA damage and consequent apoptosis, mainly due to the production of hydroxyl radicals (HO•) that follows radiolytic splitting of water. However, superoxide (O2•−) and H2O2 also form and induce oxidative stress with resulting LMP (lysosomal membrane permeabilization) arising from iron-catalysed oxidative events. The latter will contribute significantly to radiation-induced cell death and its degree largely depends on the quantities of lysosomal redox-active iron present as a consequence of autophagy and endocytosis of iron-rich compounds. Therefore radiation sensitivity might be depressed by lysosome-targeted iron chelators. In the present study, we have shown that cells in culture are significantly protected from ionizing radiation damage if initially exposed to the lipophilic iron chelator SIH (salicylaldehyde isonicotinoyl hydrazone), and that this effect is based on SIH-dependent lysosomal stabilization against oxidative stress. According to its dose-response-modifying effect, SIH is a most powerful radioprotector and a promising candidate for clinical application, mainly to reduce the radiation sensitivity of normal tissue. We propose, as an example, that inhalation of SIH before each irradiation session by patients undergoing treatment for lung malignancies would protect normally aerated lung tissue against life-threatening pulmonary fibrosis, whereas the sensitivity of malignant lung tumours, which usually are non-aerated, will not be affected by inhaled SIH.

Radioprotective Effects of Honeybee Venom (Apis mellifera) Against 915-MHz Microwave Radiation–Induced DNA Damage in Wistar Rat Lymphocytes: In Vitro Study

The aim of this study is to investigate the radioprotective effect of bee venom against DNA damage induced by 915-MHz microwave radiation (specific absorption rate of 0.6 W/kg) in Wistar rats. Whole blood lymphocytes of Wistar rats are treated with 1 μg/mL bee venom 4 hours prior to and immediately before irradiation. Standard and formamidopyrimidine-DNA glycosylase (Fpg)–modified comet assays are used to assess basal and oxidative DNA damage produced by reactive oxygen species. Bee venom shows a decrease in DNA damage compared with irradiated samples. Parameters of Fpg-modified comet assay are statistically different from controls, making this assay more sensitive and suggesting that oxidative stress is a possible mechanism of DNA damage induction. Bee venom is demonstrated to have a radioprotective effect against basal and oxidative DNA damage. Furthermore, bee venom is not genotoxic and does not produce oxidative damage in the low concentrations used in this study.

A pilot study of addition of amifostine to melphalan, carboplatin, etoposide, and cyclophosphamide with autologous hematopoietic stem cell transplantation in pediatric solid tumors-A pediatric blood and marrow transplant consortium study

Limited information is available regarding the use of amifostine in pediatric hematopoietic stem cell transplant (HSCT) patients. Melphalan, carboplatin, etoposide +/- cyclophosphamide is a commonly used preparatory regimen in pediatric solid tumor HSCT. Therefore, we decided to determine the feasibility of the addition of amifostine (750 mg/m b.i.d. x 4 d) to melphalan (200 mg/m), carboplatin (1200 mg/m), and etoposide (800 mg/m) (level 1) and escalating doses of cyclophosphamide (3000 mg/m and 3800 mg/m, levels 2 and 3, respectively) followed by autologous HSCT. Thirty-two patients with a variety of pediatric solid tumors were studied. Seventeen patients were accrued at level 1, 9 at level 2, and 6 at level 3. Major toxicities during the administration of the preparatory regimen were hypocalcemia, emesis, and hypotension. Hypocalcemia required aggressive calcium supplementation during the conditioning phase. No dose limiting toxicities were encountered at level 3. Amifostine at 750 mg/m b.i.d. for 4 days can be administered with a double alkylator regimen consisting of melphalan (200 mg/m), cyclophosphamide (up to 3800 mg/m), carboplatin (1200 mg/m), and etoposide (800 mg/m) with manageable toxicities.

Relationship between phosphorylated histone H2AX formation and cell survival in human microvascular endothelial cells (HMEC) as a function of ionizing radiation exposure in the presence or absence of thiol-containing drugs

Human microvascular endothelial cells (HMEC) were exposed to ionizing radiation at doses ranging from 0 to 16 Gy in either the presence or absence of the active thiol forms of amifostine (WR1065), phosphonol (WR255591), N-acetyl-l-cysteine (NAC), captopril or mesna. Each of these clinically relevant thiols, administered to HMEC at a dose of 4 mM for 30 min prior to irradiation, is known to exhibit antioxidant properties. The purpose of this investigation was to determine the relationship(s), if any, between the frequency of radiation-induced histone H2AX phosphorylation at serine 139 (gamma-H2AX) in cells and subsequent survival, as assessed by colony-forming ability, in exposed cell populations as a function of the presence or absence of each of the five thiol compounds during irradiation. gamma-H2AX formation in irradiated cells, as a function of relative DNA content, was quantified by bivariant flow cytometry analysis with FITC-conjugated gamma-H2AX antibody and nuclear DAPI staining. gamma-H2AX formation in cells was measured as the relative fold increase as a function of the treatment conditions. The frequency of gamma-H2AX-positive cells increased with increasing dose of radiation followed by a dose- and time-dependent decay. The most robust response for gamma-H2AX formation occurred 1 h after irradiation with their relative frequencies decreasing as a function of time 4 and 24 h later. To assess the effects of the various thiols on gamma-H2AX formation, all measurements were made 1 h after irradiation. WR1065 was not only effective in protecting HMEC against gamma-H2AX formation across the entire dose range of radiation exposures used, but it was also significantly more cytoprotective than either its prodrug (WR2721) or disulfide (WR33278) analogue. WR1065 had no significant effect on gamma-H2AX formation when administered immediately or up to 30 min after radiation exposure. An inhibitory effect against gamma-H2AX formation induced by 8 Gy of radiation was expressed by each of the thiols tested. NAC, captopril and mesna were equally effective in reducing the frequency of gamma-H2AX formation, with both WR1065 and WR255591 exhibiting a slightly more robust protective effect. Each of the five thiols was effective in reducing the frequency of gamma-H2AX-positive cells across all phases of the cell cycle. In contrast to the relative ability of each of these thiols to inhibit gamma-H2AX formation after irradiation, NAC, captopril and mesna afforded no protection to HMEC as determined using a colony-forming survival assay. Only WR1065 and WR255591 were effective in reducing the frequencies of radiation-induced gamma-H2AX-positive cells as well as protecting against cell death. These results suggest that the use of gamma-H2AX as a biomarker for screening the efficacy of novel antioxidant radioprotective compounds is highly problematic since their formation and disappearance may be linked to processes beyond simply the formation and repair of radiation-induced DSBs.

Radioprotectants: Adding Quality of Life to Survivorship?

OBJECTIVES

To explore the use of radioprotectants in oncology and their affect on quality of life (QOL).

DATA SOURCES

Textbooks, manuals, and journals in radiation oncology and cancer nursing.

CONCLUSION

Many studies have shown that the use of amifostine has decreased the intensity and severity of treatment-related side effects as well as improving QOL. However, more research is needed in the development of newer agents and refining older agents.

IMPLICATIONS FOR NURSING PRACTICE

With the advent of combined modality treatments, overall response and survival benefit has increased as well as enhanced toxicities affecting one's QOL. Agents such as radioprotectants and advances in supportive care have assisted in minimizing treatment-related side effects. Strides toward providing quality, compassionate care remain the goal of oncology nurses, while enhancing the QOL for cancer patients.

Preventing radiation induced xerostomia

Radiation induced xerostomia is a frequent consequence of radiotherapy (RT) for head-neck cancer (HNC) patients, when parotid glands are included in the radiation fields. Although early appearing xerostomia may be alleviated with the use of pilocarpine, persistent chronic xerostomia affects more than 70% of HNC patients treated with post-operative or radical radiotherapy and significantly impairs the quality of life potentially cured patients. The present manuscript reviews and discusses the current technological (conformal and intensity modulated RT) and pharmacological (amifostine) developments aiming to prevent the severity and reduce incidence of both acute and late radiation xerostomia in patients with HNC.

Prospective randomised trial of amifostine cytoprotection in myeloma patients undergoing high-dose melphalan conditioned autologous stem cell transplantation

In this prospective multicentre trial, 90 patients undergoing autologous stem cell transplantation (ASCT) were randomised to receive (n=43) or not receive (n=47) amifostine 910 mg/m(2) prior to melphalan 200 mg/m(2). Patients were monitored for regimen-related toxicity, engraftment, supportive care, response and survival. Both groups underwent ASCT at a median of 8 months from diagnosis and were matched for disease characteristics, prior therapy and pre-ASCT disease responsiveness. Amifostine infusional side-effects were frequent, occurring in 65% of patients, but of mild severity. Amifostine use was associated with a reduction in the median grade of oral mucositis (1 vs 2, P=0.01) and the frequency of severe (WHO grades 3 or 4) mucositis (12 vs 33%, P=0.02), but no reduction in the requirement for parenteral nutrition or analgesic use. Conversion to complete remission post-ASCT occurred in 30 and 14% of the amifostine and control groups, respectively (P=0.09). With a median follow-up of 35 months, there was no statistically significant difference between the median progression-free or overall survival times for the two groups. We conclude that amifostine can be safely administered prior to high-dose melphalan and significantly reduces the frequency and severity of therapy-induced oral mucositis.

Radioprotective effect of amifostine on cells from cancer prone patients and healthy individuals studied by the G2 and PCC assays

PURPOSE

To investigate whether amifostine is effective at reducing the yield of chromatid breaks when present during G(2)-phase irradiation of human normal cells and cells from cancer prone patients, as well as to study the mechanisms underlying the radioprotective effect of amifostine.

MATERIALS AND METHODS

G(2) chromosomal radiosensitivity in the presence or absence of amifostine was studied in healthy donors, cancer patients, ataxia-telangietasia (A-T) patients and five human lymphoblastoid cell lines with genes predisposing to cancer. The yield of chromatid breaks following gamma-irradiation in G(2) phase was obtained at the subsequent metaphase using the G(2) assay. For scoring chromatid damage directly in G(2) or G(0) phase, premature chromosome condensation was used.

RESULTS

When amifostine was present during irradiation, the mean yield of radiation-induced chromatid breaks as visualized by the G(2) assay was significantly reduced in healthy donors (t-test, p=0.001), in cells from cancer patients (p=0.001) and in cell lines from patients with genes predisposing to cancer (p=0.01) except ATM(-/-) (0.1<p<0.2). However, when chromatid breaks were scored directly in G(2) or G(0) phase using premature chromosome condensation, the presence of amifostine did not affect the yields obtained.

CONCLUSION

Amifostine reduces the mean yield of chromatid breaks in normal cells and in cells from cancer prone patients when present during G(2) irradiation. Although the precise mechanisms of radioprotection caused by amifostine remain unclear, the results obtained using premature chromosome condensation reveal that amifostine does not act on cells only as a free radical scavenger and as a repair enhancer of DNA damage.

Amifostine induces anaerobic metabolism and hypoxia-inducible factor 1α

PURPOSE

The cytoprotective mechanism of amifostine (WR-2721) implies free radical scavenging and DNA repair activities. We investigated additional cytoprotective pathways involving intracellular hypoxia and the activation of the hypoxia-inducible factor (HIF) pathway, a key transcription factor regulating glycolysis, angiogenesis and apoptosis, which is also linked with radioresistance.

MATERIALS AND METHODS

The glucose and oxygen levels in the peripheral blood of patients receiving 1000 mg amifostine were determined at various time-points in order to investigate the metabolic changes induced by amifostine. MDA468 breast tumor cell lines were incubated with a high amifostine concentration (10 m M) to overcome the natural resistance of cancer cells to influx of the non-hydrolyzed WR-2721, and the HIF1 alpha protein levels were determined by Western blot analysis. In vivo experiments with Wistar rats were performed in order to assess immunohistochemically changes in the intracellular accumulation of HIF1 alpha induced by amifostine (200 mg/kg).

RESULTS

By 30 min following amifostine administration, the hemoglobin oxygen saturation and pO(2) levels had increased in the peripheral blood while glucose levels had reduced, providing evidence that normal tissue metabolism switches to glycolytic pathways. Incubation of cell lines with amifostine resulted in HIF1 alpha induction. In Wistar rats administration of amifostine resulted in increased HIF1 alpha accumulation in normal tissues.

CONCLUSIONS

Since it is doubtful whether dephosphorylation of amifostine to the active metabolite WR-1065 occurs within tumoral tissues (an acidic environment that lacks vascular alkaline phosphatase activity), intracellular hypoxia and upregulation of HIF1 alpha represents an additional, normal tissue-specific, amifostine cytoprotective pathway.

Amifostine in clinical oncology: current use and future applications

Amifostine (Ethyol), an inorganic thiophosphate, is a selective broad-spectrum cytoprotector of normal tissues that provides cytoprotection against ionizing radiation and chemotherapeutic agents, thus preserving the efficacy of radiotherapy and chemotherapy. This review summarizes the preclinical data and clinical experience with amifostine, and provides insight into future clinical directions. Amifostine, an inactive pro-drug, is transformed to an active thiol after dephosphorylation by alkaline phosphatase found in the normal endothelium. The absence of alkaline phosphatase in the tumoral endothelium and stromal components, and the hypovascularity and acidity of the tumor environment, may explain its cytoprotective selectivity. The cytoprotective mechanism of amifostine is complicated, involving free radical scavenging, DNA protection and repair acceleration, and induction of cellular hypoxia. Intravenous administration of amifostine 740-900 mg/m(2) before chemotherapy and 250-350 mg/m(2) before each radiotherapy fraction are widely used regimens. The US Food and Drug Administration has approved the use of amifostine as a cytoprotector for cisplatin chemotherapy and for radiation-induced xerostomia. Ongoing trials are being conducted to determine the efficacy of amifostine in reducing radiation-induced mucositis and other toxicities. Novel schedules and routes of administration are under investigation, and may further simplify the use of amifostine and considerably broaden its applications.

Treatment with cisplatin and etoposide in patients with neuroendocrine tumors

BACKGROUND

Patients with malignant endocrine pancreatic tumors (EPTs) are responsive to combinations of chemotherapy with streptozotocin and 5-fluorouracil/doxorubicin, whereas patients with malignant carcinoids are not. For both categories of patients, alpha-interferon and/or somatostatin analogs can produce long-lasting responses. Cisplatin in combination with etoposide has been suggested to be effective in patients with malignant neuroendocrine carcinomas. The authors used this therapy as second-line or third-line treatment in patients with poorly differentiated and/or rapidly progressing disease.

METHODS

Thirty-six patients with histopathologically verified malignant neuroendocrine tumors were included: Eighteen tumors were of foregut origin, of which 5 were atypical, and 15 tumors were EPTs, of which 4 were poorly differentiated endocrine carcinomas. Three tumors were of midgut origin. The median patient age was 47.5 years. The median duration of disease from the time of diagnosis was 12 months. All patients had metastatic disease. Thirty of 36 patients had received previous treatment. Etoposide was given at a dose of 100 mg/m(2) per day for 3 days, and cisplatin was given at a dose of 45 mg/m(2) on Days 2 and 3 as a continuous intravenous infusion that was repeated every 4 weeks.

RESULTS

Ten of 18 patients with foregut carcinoids (56%) responded radiologically and/or biochemically, with a median duration of 9 months; and 7 of 14 patients with EPTs (50%) responded radiologically and/or biochemically, with a median duration of 9 months. No difference in response was seen between patients with atypical or typical foregut carcinoids or between patients with well differentiated or poorly differentiated endocrine pancreatic carcinoma. Nineteen of 36 patients (53%) experienced World Health Organization (WHO) Grade 1-2 nephrotoxicity, and 23 patients (64%) suffered from WHO Grade 3-4 neutropenia.

CONCLUSIONS

The combination of cisplatin and etoposide can produce significant responses in patients with heavily pretreated and poorly differentiated/rapidly progressing neuroendocrine tumors. The toxicity is considerable, and nephrotoxicity is the dose limiting factor.

Sparing of radiation-induced damage to the physis: fractionation alone compared to amifostine pretreatment

PURPOSE

The purpose of this study was to determine the relative benefits of sparing longitudinal bone growth by fractionation alone compared to pretreatment with amifostine, a chemical that provides differential radioprotection of normal tissues.

METHODS AND MATERIALS

Twenty-four weanling 4-week-old male Sprague-Dawley rats were randomized into 2 overall treatment groups: fractionation alone (n = 12) and amifostine pretreatment (n = 12). The distal femur and proximal tibia in the right leg of each animal were exposed to a therapeutic X-irradiation dose (17.5 Gy total in 3 or 5 fractions) with the contralateral left leg as control. In 12 of the animals, amifostine (100 mg/kg) was administered intraperitoneally 20 min before radiation exposure. Six weeks later, growth was calculated based upon measurement of the bone lengths.

RESULTS

Fractionated radiation resulted in a mean percent overall limb growth loss of 21. 1 +/- 7.0%. The addition of amifostine brought the mean percent overall limb growth loss to 16.3% +/- 4.6%, which showed a strong trend toward significance compared to fractionation alone (p = 0. 061). The addition of radioprotection with amifostine to 5 fractions irradiation significantly reduced the femoral and overall percentage growth arrest and limb length discrepancy compared to 5 fractions alone.

CONCLUSIONS

These results support further investigation of amifostine and other radioprotectants in combination with fractionation for use in growing children requiring radiotherapy to the extremity for malignant tumors.

Cytoprotection by WR-1065, the active form of amifostine, is independent of p53 status in human malignant glioma cell lines

PURPOSE

This study tests the hypothesis that p53 status, i.e. wild type versus mutant form, is a determinant in radiation protection of human glioma cells by WR-1065, the active thiol form of amifostine (WR-2721).

MATERIALS AND METHODS

The cytoprotective effectiveness of WR-1065 when present during irradiation was investigated using four well-characterized human glioma cell lines. The p53 positive lines were U87 and D54, and the mutant p53 lines were U251 (mutant at codon 273; CGT/CAT; Arg/His) and A172 (mutant at codon 242; TGC/TTC; Cys/Phe). Treatment conditions included exposure of cells to a range of doses (0-10Gy) alone or in combination with 4mM of WR-1065 added 30min prior to irradiation. Resultant survival curves were obtained using a clonogenic assay and protection factors, the ratio of terminal slopes +/- WR-1065, were determined for each glioma cell line.

RESULTS

The Do values of wild-type U87 and D54 were 1.62 and 1.89Gy while those of p53 mutants U251 and A172 were 1.64 and 1.68 Gy, respectively. Protection factors were determined to be 2.4 and 1.9 for U87 and D54, and 2.6 and 2.8 for U251 and A172, respectively.

CONCLUSIONS

The p53 status of the four human glioma cell lines tested was not a predictor for either their relative sensitivity to ionizing radiation or ability to be protected by WR-1065. It is concluded that cytoprotection exhibited by cells exposed to WR-1065 during irradiation is independent of their p53 status.

A randomized study of very accelerated radiotherapy with and without amifostine in head and neck squamous cell carcinoma

PURPOSE

The aim of this study was to assess whether amifostine could minimize acute mucositis induced by a very accelerated irradiation regimen in patients with advanced head and neck squamous cell carcinoma (HNSCC).

METHODS AND MATERIALS

Between May 1996 and February 1998, 26 patients with an inoperable nonmetastatic Stage IV HNSCC were entered in this study. The treatment consisted of very accelerated radiotherapy given 64 Gy in 3.5 weeks. The patients were randomized to receive or not 150 mg/m(2), amifostine (Ethyol, U.S. Bioscience) 15-30 min prior to each radiation session.

RESULTS

Of the 13 patients who received amifostine, definitive interruption of amifostine occurred in 5 cases (38%), due to tolerance problems (vomiting, liver enzyme elevation, generalized erythema). The distribution of Grade 4 mucositis (WHO) was 1 case versus 8 cases, with and without amifostine, respectively. The mean duration of "at least Grade 3" mucositis (WHO) was 25.1 days versus 49.2 days with and without amifostine (p = 0.03). In the amifostine group, 11/13 of the patients required a feeding tube (nasogastric tube or medical gastrostomy), because of acute mucositis, whereas in the control group a feeding tube was necessary in all cases. The mean duration of the use of this feeding tube was 1 month versus 2.5 months with and without amifostine respectively (p < 0.01). Local-regional control was not different between both arms with a median follow-up of 15 months.

CONCLUSION

Despite the limited number of patients, this pilot randomized study suggests that amifostine was able to markedly reduce the severity and duration of mucositis induced by very accelerated radiotherapy. However, the tolerance of this twice daily amifostine schedule was relatively poor.

Sparing radiation-induced damage to the physis by radioprotectant drugs: laboratory analysis in a rat model

The radioprotectant compound amifostine (S-2[3-aminopropylamino]-ethylphosphorothioic acid), administered prior to radiotherapy, has been demonstrated to provide differential protection of normal cells from the damaging effects of ionizing radiation. The aim of this pilot was to determine if amifostine could preserve the integrity of, or minimize the damage to, the physis during exposure to radiation in an animal model. Thirty weanling Sprague-Dawley rats were randomized into five groups of six animals each. Groups 1 and 2 received a single exposure to radiation consisting of 12.5 and 17.5 Gy, respectively. Groups 3 and 4 received similar exposures of 12.5 and 17.5 Gy, respectively, but with prior administration of amifostine at 100 mg/kg. Group 5 (control) received neither radiation nor amifostine. At 6 weeks, femoral and tibial lengths were measured in treated and untreated hindlimbs and compared with the baseline lengths to calculate growth. Concordant with previous reports in the literature, the radiation doses of 12.5 and 17.5 Gy reduced net femoral growth in length by a mean of 23% (range = 12-33%, SD = 7.41) and 59% (range = 54-64%, SD = 4.45), respectively, in the irradiated limb. Amifostine reduced anticipated growth loss normally resulting from a single 12.5-Gy radiation dose by 48.9% in the femur, 13.1% in the tibia, and 27.6% overall in the total limb (p < or = 0.05). Similarly, anticipated growth loss from a single 17.5-Gy radiation dose was reduced by 30.8% in the femur, 20.3% in the tibia, and 25.7% overall in the total limb (p < or = 0.05). Amifostine administered prior to clinically relevant radiation exposures significantly reduced the amount of anticipated growth arrest in our animal model.

Mammalian cell polyamine homeostasis is altered by the radioprotector WR1065

Mammalian cells become more susceptible to radiation-induced death and mutagenesis when restricted in their production of the natural polyamines putrescine, spermidine and spermine. The effects of polyamine deprivation are reversed by N-(2-mercaptoethyl)-1, 3-diaminopropane (WR1065), a simple aminothiol that has been extensively studied for its radioprotectant properties. Because this compound and its oxidized derivative WR33278 bear some resemblance to the polyamines, it was hypothesized that radioprotection by WR1065 or its metabolites is derived, at least in part, from their ability to supplement the natural polyamines. To evaluate the ability of these aminothiol compounds to emulate polyamine function in intact cells, rat liver hepatoma (HTC) cells were treated with radioprotective doses of WR1065; the ability of this compound to affect various aspects of normal polyamine metabolism was monitored. Although cellular WR1065 was maintained at levels exceeding those of the polyamines, this aminothiol did not have any polyamine-like effect on the initial polyamine biosynthetic enzyme, ornithine decarboxylase, or on polyamine degradative reactions. On the contrary, treatment with relatively low levels of WR1065 resulted in an unexpected increase in putrescine and spermidine synthesis. WR1065 treatment enhanced the stability, and consequently the activity, of ornithine decarboxylase. This stabilization seems to result from a WR1065-induced delay in the synthesis of antizyme, a critical regulatory protein required in the feedback modulation of polyamine synthesis and transport. The increase in cellular spermidine induced by WR1065 might explain its antimutagenic properties, but is probably not a factor in protection against cell killing by radiation. This is the first evidence that compounds can be designed to control polyamine levels by targeting the activity of the regulatory protein antizyme.

[Chemoprotectors. Mechanisms of action and clinical applications]

Toxicity of chemotherapy remains an important point in the care of patients with malignancies. Since a few years, new compounds without any intrinsic anti tumoral activity have been developed to decrease the toxicity or to enhance the activity of anti-cancer drugs. From those used against the toxic activity of anti-cancer drugs, two classes could be isolated: the chemoprotectors that interact through a specific of chemotherapy in normal cells, and the chemocorrectors that enhance the spontaneous recovery after exposition to cytotoxic drugs. The most widely used chemoprotector remains the 2-mercaptoethanesulfonate (mesna) which protects against the bladder toxicity of ifosfamide and cyclophosphamide. However, two new drugs, amifostine and dextrazoxane have been recently or will be approved in France against the toxicity of cisplatin and anthracyclines, respectively. Mechanism of action and clinical applications of these new drugs in cancer chemotherapy are reviewed.

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.

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.

Influence of a radioprotector WR-638 on the lymphoid compartment of the irradiated rat thymus: a flow cytometric analysis

The T cell composition of the thymus of X-ray irradiated (3.5 Gy) Wistar rat protected with WR-638 was analyzed by flow cytometry using monoclonal antibodies directed to the Thy 1.1, CD43, CD2, CD5, CD4, CD8 and class I and II MHC antigens. It was shown that this dose of X-rays caused cyclic changes in thymic cellularity manifested as: primary involution (until day 2), primary regeneration (from days 2 to 14), secondary involution (from days 14 to 21) and secondary regeneration (from days 21 to 30). WR-638 reduced the magnitude of thymocyte depletion in the primary involutive phase of the irradiated thymi, primarily as a result of protection of Thy 1.1high+ CD2low+ CD5high+ CD4+ CD8+ class I antigen high+ subpopulations of thymocytes. In the early regenerative phase, WR-638 accelerated the regeneration of CD4-CD8- and CD4-CD8+ thymocyte subsets, followed by subsequent increase of CD4+CD8+ and CD4+CD8- thymocyte subsets. Secondary involutive and regenerative phases in protected animals were characterized by higher absolute cell number of almost all thymocyte subpopulations in comparison with those in irradiated, non-protected animals.

Amifostine-mediated protection of normal bone marrow from cytotoxic chemotherapy

Amifostine (US Bioscience, West Conshohocken, PA; Ethyol, WR-2721), a phosphorylated thiol developed by the United States Army as a protective agent for military personnel in the event of nuclear warfare, has shown protection of normal tissues from the cytotoxic effects of therapeutic radiation and chemotherapy with preservation of cytotoxic effects on the tumor. The basis of this selective protection derives from the relatively rapid uptake and anabolism of Amifostine into normal tissues and minimal, slower uptake into tumor tissue. Preclinical investigations have demonstrated protection of bone marrow stem cells from the toxic effects of radiation and chemotherapy. Several controlled clinical trials demonstrated this hematoprotective effect. In patients given 1.5 g/m2 cyclophosphamide and month later given Amifostine (740 mg/m2) followed by the same dose of cyclophosphamide, the median nadir neutrophil count was significantly increased and duration of neutropenia was significantly reduced by pretreatment with Amifostine. In women with stage III/IV ovarian cancer treated with 1 g/m2 cyclophosphamide and 100 mg/m2 cisplatin +/- Amifostine 910 mg/m2, treatment with Amifostine before cyclophosphamide and cisplatin resulted in a significant decrease in both the incidence and duration of hospital stays for neutropenic fever compared to cyclophosphamide and cisplatin alone. There were equivalent rates of response and duration of survival in both groups. Other studies have shown Amifostine protects bone marrow purged in vitro with 4-hydroperoxycyclophosphamide before autologous bone marrow transplantation. This preservation of marrow stem cells resulted in a statistically significant decrease in time to marrow engraftment, need for platelet transfusions and antibiotics, and duration of hospital stay. Amifostine-mediated protection of normal bone marrow illustrated in preclinical experiments is also evident in clinical trials. Amifostine preserves trilineage stem cells (red blood cells, platelets, and white blood cells) in contrast to the lineage-specific effects of the colony-stimulating factors. Theoretically, Amifostine and the colony-stimulating factors should provide complementary benefits to bone marrow recovery and function after cytotoxic therapies. These observations offer the promise of using high doses of chemotherapy to exploit antitumor, dose-response relationships in clinical trials.

Effects of WR-1065 and WR-151326 on survival and neoplastic transformation in C3H/10T1/2 cells exposed to TRIGA or JANUS fission neutrons

We demonstrated the ability of aminothiols WR-1065 and WR-151326, each at concentration 1 mM, to protect C3H/10T1/2 cells against the transforming effects of fission neutrons under two distinct sets of experimental conditions. Experiments with WR-1065 were performed with stationary cultures of C3H/10T1/2 cells, and a TRIGA reactor-generated fission neutron field at the Armed Forces Radiobiology Research Institute (USA). Experiments with WR-151326 were performed with proliferating cultures of C3H/10T1/2 cells and a JANUS reactor-generated fission neutron field at the Argonne National Laboratory (USA). Radioprotectors were present before, during, and after irradiation for total-periods of 35 min (WR-151326; 10 min pre-incubation) or 1 h (WR-1065; 30 min pre-incubation). Bioavailability of WR-1065 and WR-151326 in extracellular medium under experimental conditions simulating those of the transformation experiments was studied by measuring oxidation rates in the presence of attached C3H/10T1/2 cells in plateau and exponential phase of growth for periods of up to 5 h. Estimated half-lives for autoxidation of WR-1065 or WR-151326 were approximately 8 min or 1 h regardless of the proliferative status of cells. In the absence of WR-compounds, dose-response data for transformation induction by neutrons from TRIGA and JANUS reactors were fitted to a common curve with a linear coefficient of about 7 x 10(-4)/Gy. WR-151326 and WR-1065 were found to provide significant radioprotection by factors of 1.79 +/- 0.08 and 3.23 +/- 0.19, respectively, against fission neutron-induced neoplastic transformation. No significant protection against neutron-induced cell lethality was observed.

Comparison of behavioral and radioprotective effects of WR-2721 and WR-3689

The behavioral effects of the radioprotectant agents ethiofos, S-2-(3-aminopropylamino)ethylphosphorothioic acid (WR-2721) and S-2-(3-methylaminopropyl)aminoethylphosphorothioic acid (WR-3689) were evaluated in rats trained to respond under a multiple fixed-interval 120-s, fixed-ratio 50-response (mult FI FR) schedule of milk reinforcement. Each compound produced dose-dependent reductions in responding under both schedules over the same dose range (100-180 mg/kg, IP); ED50s indicated that WR-3689 was slightly more potent than WR-2721. On several performance measures, WR-3689 produced greater decrements during a second dose-effect determination, whereas WR-2721 had more pronounced effects during the initial one. In a second series of studies, low (56 mg/kg) and high (180 mg/kg) doses of both drugs were tested for radioprotective effects in rats responding under an FR-50 schedule of milk reinforcement and exposed to a nonlethal (5 gray, Gy) or lethal (10 Gy) dose of ionizing radiation (60Co gamma rays). Neither dose of radiation altered FR response rates on the day of exposure (day 1). Five Gy of gamma radiation produced a 25-40% reduction in response rates on days 2-5 (24-72 h) after exposure. Neither dose of WR-2721 or WR-3689 provided significant protection against these performance decrements. All groups exposed to 10 Gy experienced a progressive decline in FR responding on days 2-5 after exposure. Performance of groups that received pretreatment with the 180-mg/kg dose of either drug or the 56-mg/kg dose of WR-3689 was maintained at significantly higher levels than saline-treated controls on days 4-5 after exposure to 10 Gy; however, even at these higher levels of performance response rates remained below 50% of preirradiation control levels. Subsequently, 56 and 180 mg/kg WR-3689 and 180 mg/kg WR-2721 were found to provide protection against the lethal consequences of the 10-Gy exposure. Thus, neither WR-2721 nor WR-3689 afforded any significant short-term protection against radiation-induced performance decrements when these drugs were administered at either behaviorally ineffective or behaviorally disruptive doses. Rather, the beneficial effects of these drugs paralleled their ability to antagonize radiation-induced lethality.

Postirradiation treatment with granulocyte colony-stimulating factor and preirradiation WR-2721 administration synergize to enhance hemopoietic reconstitution and increase survival

These studies tested whether WR-2721 could be used to protect hemopoietic stem cells, which after irradiation could be stimulated by granulocyte colony-stimulating factor (G-CSF) to proliferate and reconstitute the hemopoietic system. Female C3H/HeN mice were administered WR-2721 (4 mg/mouse, i.p.) 30 min before 60Co irradiation and G-CSF (2.5 micrograms/mouse/day, s.c.) from days 1-16 after irradiation. In survival studies, saline, G-CSF, WR-2721, and WR-2721 + G-CSF treatments resulted in LD50/30 values of 7.85 Gy, 8.30 Gy, 11.30 Gy, and 12.85 Gy, respectively. At these LD50/30 values, the dose reduction factor (DRF) of 1.64 obtained in combination-treated mice was more than additive between the DRF's of G-CSF-treated mice (1.06) and WR-2721-treated mice (1.44). Bone marrow and splenic multipotent hemopoietic stem cell (CFU-s) and granulocyte-macrophage progenitor cell (GM-CFC) recoveries were also accelerated most in mice treated with WR-2721 + G-CSF. In addition, mice treated with WR-2721 + G-CSF exhibited the most accelerated peripheral blood white cell, platelet, and red cell recoveries. These studies (a) demonstrate that therapeutically administered G-CSF accelerates hemopoietic reconstitution from WR-2721-protected stem and progenitor cells, increasing the survival-enhancing effects of WR-2721 and (b) suggest that classic radioprotectants and recombinant hemopoietic growth factors can be used in combination to reduce risks associated with myelosuppression induced by radiation or radiomimetic drugs.

New protective agents for bone marrow in cancer therapy

WR-2721 is a unique bone marrow protective agent which also protects other normal tissues against the toxic effects of radiation therapy and chemotherapy, especially with cisplatin and alkylating agents. In addition to protection against acute toxicities, WR-2721 has also been shown to protect against mutagenesis, cell transformation, and carcinogenesis which might be termed subacute and late toxicities. The protective mechanisms are not fully elucidated but oxygen free radical scavenging certainly plays a role and effects of enzymatic DNA repair mechanisms have also been reported. In the field of reduction of toxicity from anticancer treatment modalities, the properties of CSFs and other substances have been contrasted with WR-2721 the former being stimulators of bone marrow (and the immune system) rather than true protectors. Nonetheless, they also have a clinical role and combinations of a protector and stimulator may prove useful in the future. WR-2721 offers the prospect of increasing the therapeutic index of current cancer treatment and may allow the administration of higher than currently would be considered maximal tolerated doses (MTD). This may, in turn, lead to improved efficacy and the full potential of this agent is now being explored in a variety of randomized, controlled clinical trials.