Pentoxifylline does not spare acute radiation reactions in rat lung and skin

Targeting Tumor Necrosis Factor Alpha to Mitigate Lung Injury Induced by Mustard Vesicants and Radiation

Pulmonary injury induced by mustard vesicants and radiation is characterized by DNA damage, oxidative stress, and inflammation. This is associated with increases in levels of inflammatory mediators, including tumor necrosis factor (TNF)α in the lung and upregulation of its receptor TNFR1. Dysregulated production of TNFα and TNFα signaling has been implicated in lung injury, oxidative and nitrosative stress, apoptosis, and necrosis, which contribute to tissue damage, chronic inflammation, airway hyperresponsiveness, and tissue remodeling. These findings suggest that targeting production of TNFα or TNFα activity may represent an efficacious approach to mitigating lung toxicity induced by both mustards and radiation. This review summarizes current knowledge on the role of TNFα in pathologies associated with exposure to mustard vesicants and radiation, with a focus on the therapeutic potential of TNFα-targeting agents in reducing acute injury and chronic disease pathogenesis.

Pathological observation of the effects of exposure to radioactive microparticles on experimental animals

Internal radiation exposure from neutron-induced radioisotopes that were environmentally activated following an atomic bombing or nuclear accident should be considered for a complete picture of the pathologic effects on survivors. Inhaled hot particles expose neighboring tissues to very high doses of particle beams, which can cause local tissue damage. Experimentally, a few μm of 55MnO2 powder was irradiated with neutrons at a nuclear reactor in order to generate 56MnO2 that emits β-rays. Rats were irradiated via inhalation. Pathological changes in various rat tissues were examined. In addition, the 56Mn β energy spectrum around the particles was calculated to determine the local dose rate and the cumulative dose. This review focuses on our latest pathological findings in lungs with internal radiation injury and discusses the pathological changes of early event damage caused by localized, very high-dose internal radiation exposure, including apoptosis, elastin stigma, emphysema, hemorrhage and severe inflammation. The pathological findings of lung tissue due to internal radiation exposure of 0.1 Gy were severe, with no pathological changes observed due to external exposure to γ radiation at a dose of 2.0 Gy. Therefore, it is suggested that new pathological analysis methods for internal exposure due to radioactive microparticles are required.

Biological impacts on the lungs in rats internally exposed to radioactive 56MnO2 particle

To understand the radiation effects of the atomic bombing of Hiroshima and Nagasaki among the survivors, radiation from neutron-induced radioisotopes in soil should be considered in addition to the initial radiation directly received from the bombs. ⁵⁶Mn, which emits both β particles and γ-rays, is one of the dominant radioisotopes created in soil by neutrons from the bomb. Thus we investigated the biological effects of internal exposure to ⁵⁶MnO₂ particle in the lung of male Wistar rats comparing to the effects of external ⁶⁰Co-γ irradiation. Absorbed doses of internal irradiation of lungs were between 25 and 65 mGy in ⁵⁶MnO₂-exposed animals, while the whole body doses were between 41 and 100 mGy. Animals were examined on days 3 and 61 after the exposure. There were no remarkable pathological changes related to ⁵⁶MnO₂ particle exposure. However, mRNA and protein expressions of aquaporin 5 increased significantly in the lung tissue on day 3 postexposure in ⁵⁶MnO₂ groups (by 1.6 and 2.9 times, respectively, in the highest dose group). Smad7 mRNA expression was also significantly elevated by 30% in the highest dose group of ⁵⁶MnO₂. Our data demonstrated that internal exposure to ⁵⁶MnO₂ induced significant biological responses including gene expression changes in the lungs, while external ⁶⁰Co-γ irradiation of 2 Gy did not show any changes.

Early radiation-induced oral pain signaling responses are reduced with pentoxifylline treatment

Radiation-induced acute oral mucositis is associated with inflammation and pain. In other realms of pain research, nociceptors are known to be activated by inflammatory cytokines; for example, tumor necrosis factor alpha (TNF-α) can activate transient receptor potential ion channels on sensory neurons. But there is an unclear relationship between inflammatory cytokines and molecular mediators of pain in radiation-induced mucositis (RIM) and radiation-associated pain (RAP). In this prospective, analytical, experimental pilot study, a common drug (pentoxifylline [PTX]) was used with the goal of inhibiting TNF-α signaling in mice that underwent lingual irradiation to induce severe acute oral RIM/RAP. Body weight and glossitis scores were recorded daily. Eye wiping behaviors were assayed as a surrogate measure of oral discomfort (which is possible due to cross-sensitization of the mandibular and ophthalmic branches of the trigeminal nerve). Quantitative real-time reverse transcription polymerase chain reaction was performed on irradiated tongue tissue to measure changes in expression of TNF-α, its receptor, nuclear factor kappa-light-chain-enhancer of activated B cells, transient receptor potential vanilloid type 1 (TRPV1), and transient receptor potential vanilloid type 4 (TRPV4). Responsiveness of afferent sensory trigeminal neurons to TNF-α, a TRPV1 agonist (capsaicin), and a partial TRPV4 agonist (histamine) was measured via calcium imaging. Although PTX treatment did not reduce glossitis severity or mitigate weight loss in mice with RIM/RAP, it did inhibit the upregulation of TNF-α's receptor that normally accompanies RIM, and it also reduced neuronal responsiveness to each of the aforementioned chemical stimuli. These results provide provisional evidence that inhibition of TNF-α signaling with PTX treatment may serve as a useful tool for reducing pain in head and neck cancer patients.

Impact of Local High Doses of Radiation by Neutron Activated Mn Dioxide Powder in Rat Lungs: Protracted Pathologic Damage Initiated by Internal Exposure

Internal radiation exposure from neutron-induced radioisotopes environmentally activated following atomic bombing or nuclear accidents should be considered for a complete picture of pathologic effects on survivors. Inhaled hot particles expose neighboring tissues to locally ultra-high doses of β-rays and can cause pathologic damage. ⁵⁵MnO₂ powder was activated by a nuclear reactor to make ⁵⁶MnO₂ which emits β-rays. Internal exposures were compared with external γ-rays. Male Wistar rats were administered activated powder by inhalation. Lung samples were observed by histological staining at six hours, three days, 14 days, two months, six months and eight months after the exposure. Synchrotron radiation—X-ray fluorescence—X-ray absorption near-edge structure (SR–XRF–XANES) was utilized for the chemical analysis of the activated ⁵⁶Mn embedded in lung tissues. ⁵⁶Mn beta energy spectrum around the particles was calculated to assess the local dose rate and accumulated dose. Hot particles located in the bronchiole and in damaged alveolar tissue were identified as accumulations of Mn and iron. Histological changes showed evidence of emphysema, hemorrhage and severe inflammation from six hours through eight months. Apoptosis was observed in the bronchiole epithelium. Our study shows early event damage from the locally ultra-high internal dose leads to pathogenesis. The trigger of emphysema and hemorrhage was likely early event damage to blood vessels integral to alveolar walls.

Apoptosis and Bcl-2 Expression in Irradiated Lungs and the Effect of Pentoxifylline

We measured number of bcl-2, apoptotic, neutrophil, and surfactant apoprotein D (SP-D) positive cells in irradiated rat lungs during different time points after the sublethal whole-thorax irradiation of rats. We also investigated the influence of pentoxifylline (PTX) therapy on these markers. Wistar rats were given 15 Gy thoracic irradiation and PTX (35 mg/kg) twice a week. Animals were examined histologically and imunohistochemically at intervals from 1-12 weeks. In non-treated rats compared with treated rats, bcl-2 expression was significantly inhibited from 4 weeks after irradiation. A higher apoptosis presence in non-treated rats from 4 weeks was found and apoptosis development in PTX-treated animals was delayed and started 8 weeks after irradiation. Similar differences were measured during neutrophil granulocytes examination. Neutrophil penetration in non-treated rats was found 5 weeks after irradiation in contrast to the RP onset of PTX-treated animals 8 weeks after irradiation. The number of SP-D positive cells in non-treated rats observed until 5 weeks after irradiation was higher than in the control group. PTX-treated animals expressed higher number of SP-D positive cells during the whole experiment than the control group. We suggest that apoptosis is linked to neutrophil granulocyte actions during the RP onset and that PTX-therapy causes diminished inflammation development.

Current Status of Targeted Radioprotection and Radiation Injury Mitigation and Treatment Agents: A Critical Review of the Literature

As more cancer patients survive their disease, concerns about radiation therapy-induced side effects have increased. The concept of radioprotection and radiation injury mitigation and treatment offers the possibility to enhance the therapeutic ratio of radiation therapy by limiting radiation therapy-induced normal tissue injury without compromising its antitumor effect. Advances in the understanding of the underlying mechanisms of radiation toxicity have stimulated radiation oncologists to target these pathways across different organ systems. These generalized radiation injury mechanisms include production of free radicals such as superoxides, activation of inflammatory pathways, and vascular endothelial dysfunction leading to tissue hypoxia. There is a significant body of literature evaluating the effectiveness of various treatments in preventing, mitigating, or treating radiation-induced normal tissue injury. Whereas some reviews have focused on a specific disease site or agent, this critical review focuses on a mechanistic classification of activity and assesses multiple agents across different disease sites. The classification of agents used herein further offers a useful framework to organize the multitude of treatments that have been studied. Many commonly available treatments have demonstrated benefit in prevention, mitigation, and/or treatment of radiation toxicity and warrant further investigation. These drug-based approaches to radioprotection and radiation injury mitigation and treatment represent an important method of making radiation therapy safer.

Internal exposure to neutron-activated 56Mn dioxide powder in Wistar rats-Part 2: pathological effects

To fully understand the radiation effects of the atomic bombing of Hiroshima and Nagasaki among the survivors, radiation from neutron-induced radioisotopes in soil and other materials should be considered in addition to the initial radiation directly received from the bombs. This might be important for evaluating the radiation risks to the people who moved to these cities soon after the detonations and probably inhaled activated radioactive "dust." Manganese-56 is known to be one of the dominant radioisotopes produced in soil by neutrons. Due to its short physical half-life, 56Mn emits residual radiation during the first hours after explosion. Hence, the biological effects of internal exposure of Wistar rats to 56Mn were investigated in the present study. MnO2 powder was activated by a neutron beam to produce radioactive 56Mn. Rats were divided into four groups: those exposed to 56Mn, to non-radioactive Mn, to 60Co γ rays (2 Gy, whole body), and those not exposed to any additional radiation (control). On days 3, 14, and 60 after exposure, the animals were killed and major organs were dissected and subjected to histopathological analysis. As described in more detail by an accompanying publication, the highest internal radiation dose was observed in the digestive system of the rats, followed by the lungs. It was found that the number of mitotic cells increased in the small intestine on day 3 after 56Mn and 60Co exposure, and this change persisted only in 56Mn-exposed animals. Lung tissue was severely damaged only by exposure to 56Mn, despite a rather low radiation dose (less than 0.1 Gy). These data suggest that internal exposure to 56Mn has a significant biological impact on the lungs and small intestine.

The use of antioxidants in radiotherapy-induced skin toxicity

Radiation-induced skin damage is one of the most common complications of radiotherapy. In order to combat these side effects, patients often turn to alternative therapies, which often include antioxidants. Antioxidants such as those in the polyphenol chemical class, xanthine derivatives, tocepherol, sucralfate, and ascorbate have been studied for their use in either preventing or treating radiotherapy-induced skin damage. Apart from their known role as free radical scavengers, some of these antioxidants appear to alter cytokine release affecting cutaneous and systemic changes. We review the role of antioxidants in treating and preventing radiation-induced skin damage as well as the possible complications of using such therapy.

Pentoxifylline enhances the radioprotective properties of γ-tocotrienol: differential effects on the hematopoietic, gastrointestinal and vascular systems

The vitamin E analog γ-tocotrienol (GT3) is a potent radioprotector and mitigator. This study was performed to (a) determine whether the efficacy of GT3 can be enhanced by the addition of the phosphodiesterase inhibitor pentoxifylline (PTX) and (b) to obtain information about the mechanism of action. Mice were injected subcutaneously with vehicle, GT3 [400 mg/kg 24 h before total-body irradiation (TBI)], PTX (200 mg/kg 30 min before TBI), or GT3+PTX before being exposed to 8.5-13 Gy TBI. Overall lethality, survival time and intestinal, hematopoietic and vascular injury were assessed. Cytokine levels in the bone marrow microenvironment were measured, and the requirement for endothelial nitric oxide synthase (eNOS) was studied in eNOS-deficient mice. GT3+PTX significantly improved survival compared to GT3 alone and provided full protection against lethality even after exposure to 12.5 Gy. GT3+PTX improved bone marrow CFUs, spleen colony counts and platelet recovery compared to GT3 alone. GT3 and GT3+PTX increased bone marrow plasma G-CSF levels as well as the availability of IL-1α, IL-6 and IL-9 in the early postirradiation phase. GT3 and GT3+PTX were equally effective in ameliorating intestinal injury and vascular peroxynitrite production. Survival studies in eNOS-deficient mice and appropriate controls revealed that eNOS was not required for protection against lethality after TBI. Combined treatment with GT3 and PTX increased postirradiation survival over that with GT3 alone by a mechanism that may depend on induction of hematopoietic stimuli. GT3+PTX did not reduce GI toxicity or vascular oxidative stress compared to GT3 alone. The radioprotective effect of either drug alone or both drugs in combination does not require the presence of eNOS.

Amelioration of the pathological changes induced by radiotherapy in normal tissues

Damage to normal tissues remains the most important limiting factor in the treatment of cancer by radiotherapy. In order to deliver a radiation dose sufficient to eradicate a localised tumour, the normal tissues need to be protected. A number of pharmacological agents have been used experimentally, and some clinically, to alleviate radiation damage to normal tissues but at present there is no effective clinical treatment to protect normal tissues against radiation injury. This paper reviews the efficacy of pharmacological substances used after radiation exposure. The limited evidence available suggests that radiation insult, like many other tissue injuries, is amenable to pharmacological intervention. However, care must be taken in the administration of these substances for the management of different aspects of radiation damage because there appears to be a tissue-specific response to different pharmacological agents. Also, one must be aware of the limitations of results obtained from animal models, which do not necessarily correlate to benefits in the clinic; the conflicting results reported with some modifiers of radiation damage; and the toxicity of these substances and radiation doses used in published studies. Conflicting results may arise from differences in the pathophysiologic processes involved in the development of radiation lesions in different tissues, and in the markers used to assess the efficacy of treatment agents.

DNA damage in cultured skin microvascular endothelial cells exposed to gamma rays and treated by the combination pentoxifylline and α-tocopherol

PURPOSE

This in vitro study aims at evaluating the effect of the combination of pentoxifylline (PTX) and trolox (Tx), the water-soluble analogue of alpha-tocopherol, on the oxidative state and DNA damage in dermal microvascular endothelial cells exposed to doses up to 10 Gy of ionizing radiation.

MATERIALS AND METHODS

Confluent primary cultures of dermal endothelial cells were gamma irradiated at 3 and 10 Gy, and 0.5 mM of both drugs, PTX and Tx, was added either before (15 min) or after (30 min or 24 h) irradiation. Reactive oxygen species (ROS), measured by the dichlorodihydrofluorescein diacetate assay, and DNA damage, assessed by the comet and micronucleus assays, were measured at different times after exposure (0 - 21 days).

RESULTS

The PTX/Tx treatment decreased the early and delayed peak of ROS production by a factor of 2.8 in 10 Gy-irradiated cells immediately after irradiation and the basal level by a factor of 2 in non-irradiated control cells. Moreover, the level of DNA strand breaks, as measured by the comet assay, was shown to be reduced by half immediately after irradiation when the PTX/Tx treatment was added 15 min before irradiation. However, unexpectedly, it was decreased to a similar extent when the drugs were added 30 min after radiation exposure. This reduction was accompanied by a 2.2- and 3.6-fold higher yield in the micronuclei (MN) frequency observed on days 10 and 14 post-irradiation, respectively.

CONCLUSION

These results suggest that oxidative stress and DNA damage induced in dermal microvascular endothelial cells by radiation can be modulated by early PTX/Tx treatment. These drugs acted not only as radical scavengers, but they were also responsible for the increased MN frequency in 10 Gy-irradiated cells. Thus, these drugs may cause a possible interference with DNA repair processes.

Significance of endothelial dysfunction in the pathogenesis of early and delayed radiation enteropathy

This review summarizes the current state of knowledge regarding the role of endothelial dysfunction in the pathogenesis of early and delayed intestinal radiation toxicity and discusses various endothelial-oriented interventions aimed at reducing the risk of radiation enteropathy. Studies published in the biomedical literature during the past four decades and cited in PubMed, as well as clinical and laboratory data from our own research program are reviewed. The risk of injury to normal tissues limits the cancer cure rates that can be achieved with radiation therapy. During treatment of abdominal and pelvic tumors, the intestine is frequently a major dose-limiting factor. Microvascular injury is a prominent feature of both early (inflammatory), as well as delayed (fibroproliferative) radiation injuries in the intestine and in many other normal tissues. Evidence from our and other laboratories suggests that endothelial dysfunction, notably a deficiency of endothelial thrombomodulin, plays a key role in the pathogenesis of these radiation responses. Deficient levels of thrombomodulin cause loss of vascular thromboresistance, excessive activation of cellular thrombin receptors by thrombin, and insufficient activation of protein C, a plasma protein with anticoagulant, anti-inflammatory, and cytoprotective properties. These changes are presumed to be critically involved in many aspects of early intestinal radiation toxicity and may sustain the fibroproliferative processes that lead to delayed intestinal dysfunction, fibrosis, and clinical complications. In conclusion, injury of vascular endothelium is important in the pathogenesis of the intestinal radiation response. Endothelial-oriented interventions are appealing strategies to prevent or treat normal tissue toxicity associated with radiation treatment of cancer.

Association between plasma angiotensin-converting enzyme level and radiation pneumonitis

Angiotensin-converting enzyme (ACE) plays an important role in pulmonary fibrosis and may be involved in the development of radiation-induced lung damage. The objective of this study was to evaluate the predictive value of plasma ACE in radiation pneumonitis (RP). Patients with stage I-III lung cancer were treated with radiotherapy with or without chemotherapy. ACE levels were measured using enzyme-linked immunosorbent assay before radiotherapy (pre-RT) and when a median dose of 45 Gy (Range: 40-48 Gy) was reached (during-RT). The primary end point was > or = grade 2 RP. Statistic significances were evaluated with independent T-test and chi-square. Thirty-nine patients were enrolled in this study, among which 33.3% experienced > or = grade 2 RP. ACE levels, either pre-RT or during-RT, were significantly lower in the RP group than in the non-RP group (P=0.02 and 0.03, respectively). Nine out of the 19 patients (47.4%) with pre-RT ACE levels < or = 462 ng/mL experienced RP, versus 3 of 19 (15.8%) patients with ACE levels > 462 ng/mL (P=0.04). This study suggested that plasma ACE as a predictive factor for radiation pneumonitis deserves further study.

The role of pentoxifylline as a modifier of radiation therapy

Pentoxifylline (Ptx), a hemorrheologic methylxanthine derivative, is of interest in radiation oncology for several reasons. First, improvement of tumor perfusion might result in better oxygenation and thus radiosensitivity. In addition, the drug also influences cytokine-mediated inflammation. The role of cytokines in the progression of radiation reactions in both tumor and normal tissues therefore provides further opportunities to combine Ptx with ionising radiation in order to improve the therapeutic ratio. This review summarizes preclinical and clinical data in both tumor and normal tissues. Regarding radiosensitization of tumors, a large body of evidence suggests that Ptx improves tumor oxygenation and sensitizes p53 mutant tumors. However, these findings have not translated into positive clinical studies to date. None of three published clinical trials attempting to enhance the effectiveness of radiotherapy with Ptx had a satisfactory design. There is also little evidence to prove that Ptx reduces acute side effects of radiotherapy. The only possible exception is a small randomized trial of lung radiotherapy. Regarding established late sequelae, numerous non-randomized clinical trials described healing of soft tissue necrosis and improvement of trismus and fibrosis after several weeks of Ptx or Ptx plus vitamin E. However, is not unequivocally clear that the combination with vitamin E indeed is superior. The literature data suggest that radiation necrosis can be treated more effectively than fibrosis and that certain improvements might be functional and transient, with less influence on the chronic structural damage induced by ionising radiation. The ultimate individual outcome might depend, for example, on the stage of fibrosis progression, the size of the lesion and comorbid conditions such as diabetes and arteriosclerosis. Some of these factors will influence the actual amount of drug available in the targeted region. It is therefore necessary to evaluate Ptx in larger clinical trials with less baseline variation and to improve the recording of long-term results.

Modulation of DNA Damage by Pentoxifylline and α-Tocopherol in Skin Fibroblasts Exposed to Gamma Rays

Previous in vivo studies showed the combination pentoxifylline (PTX) and alpha-tocopherol was highly efficient in reducing late radiation-induced skin damage. The present work aimed at investigating the molecular and cellular mechanisms involved in the effects of this combination. Primary cultures of confluent dermal fibroblasts were gamma-irradiated in the presence of PTX and trolox (Tx), the water-soluble analogue of alpha-tocopherol. Drugs were added either before or after radiation exposure and were maintained over time. Their antioxidant capacity and their effect on radiation-induced ROS production was assessed together with cell viability and clonogenicity. DNA damage formation was assessed by the alkaline comet assay and by the micronucleus (MN) test. Cell cycle distribution was also determined. The combination of PTX/ Tx was shown to reduce both immediate and late ROS formation observed in cells after irradiation. Surprisingly, decrease in DNA strand breaks measured by the comet assay was observed any time drugs were added. In addition, the micronucleus test revealed that for cells irradiated with 10 Gy, a late significant increase in MN formation occurred. The combination of PTX/Tx was shown to be antioxidant and to decrease radiation-induced ROS production. The observed effects on DNA damage at any time the drugs were added suggest that PTX/Tx could interfere with the DNA repair process.

Pharmacological intervention to prevent or ameliorate chronic radiation injuries

Until the 1990s, chronic radiation-induced normal-tissue injury was viewed as being due solely to the delayed mitotic death of parenchymal or vascular cells; these injuries were held to be inevitable, progressive, and untreatable. It is now clear that parenchymal and vascular cells are active participants in the response to radiation injury rather than passive observers dying as they attempt to divide. This offers fundamentally new approaches to radiation injury because it allows for the possibility of pharmacological interventions directed at modulating steps in the cascade of events leading to expression of injury. Such interventions would be relevant to both cancer patients and victims of radiation accidents. Prophylaxis and treatment of chronic radiation injuries have been experimentally shown in multiple organ systems (eg, lung, kidney, soft tissue) and with fundamentally different pharmacological agents (eg, corticosteroids, angiotensin-converting enzyme inhibitors, pentoxifylline, superoxide dismutase). For the most part, this has been achieved using clinically relevant radiation and drug schedules and with agents that have already been approved for human use. Unfortunately, assessment of the utility of these agents for clinical use has been minimal, and there are no established mechanisms for any of the experimental or clinical successes. Clinical development of pharmacological approaches to modification of chronic radiation injuries could lead to significant improvement in survival and quality of life for radiotherapy patients and for victims of radiation accidents or nuclear terrorism.

Endothelial cell loss in irradiated optic nerves

OBJECTIVE

Radiation optic neuropathy usually occurs months to years after exposure of the anterior visual pathways to ionizing radiation. It is characterized by high signal on gadolinium-enhanced T1-weighted magnetic resonance imaging. Radiation-induced endothelial cell damage resulting in blood-nerve barrier breakdown is hypothesized to produce this pattern, but histologic evidence of this in the optic nerve is lacking. We attempted to evaluate the effect of radiation on endothelial cells in the optic nerve.

DESIGN

Case-controlled histologic study.

METHODS

We studied the optic nerves of 16 enucleated eyes from patients with uveal melanoma treated with proton beam irradiation, 6 from normal eyes and 5 from eyes with unirradiated uveal melanomas. Binding of Ulex europaeus agglutinin I (UEA-I) lectin was used to identify endothelial cells in single paraffin sections. Transverse and longitudinal sections of vessels were counted in masked fashion.

RESULTS

There were 49.4+/-6.9 transversely sectioned endothelial cells per millimeter of nerve in 6 optic nerves exposed to 0 to 1000 cGyE ("low-dose") compared with 17.3+/-5.3 in 10 nerves exposed to 5500 to 7000 cGyE ("high-dose") (P = 0.002). Longitudinally sectioned vessels stained with UEA-I were separately identified, with 11.5+/-2.1 in the low-dose group and 5.6+/-1.6 in the high-dose group (P = 0.044). The thickness and staining of the endothelial cell layer appeared greater in the high-dose group. Endothelial cell counts did not correlate with age, gender, acuity, or interval after irradiation.

CONCLUSIONS

Increased radiation dosage to the optic nerve correlates with smaller numbers of endothelial cells.

Striking regression of subcutaneous fibrosis induced by high doses of gamma rays using a combination of pentoxifylline and alpha-tocopherol: an experimental study

PURPOSE

To establish a successful treatment of subcutaneous fibrosis developing after high doses of gamma rays, suitable for use in clinical practice.

METHODS AND MATERIALS

We used an animal model of acute localized gamma irradiation simulating accidental overexposure in humans. Three groups of 5 Large White pigs were irradiated using a collimated 192Ir source to deliver a single dose of 160 Gy onto the skin surface (100%) of the outer side of the thigh. A well-defined block of necrosis developed within a few weeks which had healed after 26 weeks to leave a block of subcutaneous fibrosis involving skin and skeletal muscle. One experimental group of 5 pigs was dosed orally for 26 weeks starting 26 weeks after irradiation with 1600 mg/120 kg body weight of pentoxifylline (PTX) included in the reconstituted food during its fabrication, and another group of 5 was dosed orally for the same period with a daily dose of 1600 mg/120 kg body weight of PTX combined with 2000 IU/120 kg body weight of alpha-tocopherol. Five irradiated control pigs were given normal food only. Animals were assessed for changes in the density of the palpated fibrotic block and in the dimensions of the projected cutaneous surface. Depth of scar tissue was determined by ultrasound. Physical and sonographic findings were confirmed by autopsy 26 weeks after treatment started. The density, length, width, and depth of the block of fibrotic scar tissue, and the areas and volume of its projected cutaneous surface, were compared before treatment, 6 and 13 weeks thereafter, and at 26 weeks.

RESULTS

The experimental animals exhibited no change in behavior and no abnormal clinical or anatomic signs. No modifications were observed in the block of fibrotic scar tissue of pigs dosed with PTX alone. However, significant softening and shrinking of this block were noted in the pigs dosed with PTX + alpha-tocopherol 13 weeks after treatment started and at autopsy, when mean regression was approximately 30% for length, approximately 50% for width and depth, and approximately 70% for area and volume. Histologic examination showed completely normal muscle and subcutaneous tissue surrounding the residual scar tissue. The 50% decrease in the linear dimensions of the scar tissue, were comparable to the results obtained in our previous clinical studies, and were highly significant compared to the clinical and autopsy results for the controls. Histologic examination of the residual scar tissue revealed tissue which was more homogenous and less cellular and inflammatory than in control and PTX-dosed pigs. The tissular and cellular immunolocalization of tumor necrosis factor alpha (TNFalpha) was similar in the residual fibrotic tissues of all three groups of pigs, whereas the immunostaining of transforming growth factor beta-1(TGFbeta-1) diminished much more in the residual fibrotic scar tissue of the PTX + alpha-tocopherol-dosed pigs than in the two other groups.

CONCLUSIONS

The present results showed a striking regression of the subcutaneous fibrotic scar tissue that develops as a consequence of high doses of gamma rays.

Effect of high-dose pentoxifylline on acute radiation-induced lung toxicity in a rat lung perfusion model

PURPOSE

The purpose of this study was to study the effect of high-dose oral pentoxifylline on radiation-induced acute lung injury as assessed with a rat lung perfusion model.

METHODS AND MATERIALS

Adult male Sprague-Dawley rats were used throughout this study. A preliminary experiment determined that treatment with 2 g/liter pentoxifylline in drinking water resulted in an average consumption of 1.38 g/m2/day, which is comparable to the maximum tolerated dosage in humans. Seventy-two rats were irradiated to the left hemithorax with single fraction doses ranging from 10 through 18 Gy. Half were treated with 2 g/liter pentoxifylline in drinking water from 1 week before radiation through 8 weeks after radiation. Lung vascular perfusion scanning was performed at 3, 4, 5, 6, and 8 weeks after radiation using 99mTc-macroaggregated albumin. The lung perfusion ratio was defined as the number of counts due to radioactivity within the irradiated left lung region of interest divided by the number of counts within the region of the nonirradiated right lung. This lung perfusion ratio has been shown to decrease with radiation-induced lung injury.

RESULTS

Although radiation led to a decreased lung perfusion ratio in all groups, those receiving pentoxifylline maintained higher ratios than irradiated controls from 3-5 weeks, especially for those receiving 15 or 18 Gy. However, from 6 through 8 weeks the irradiated controls exhibited partial recovery of lung perfusion ratio, whereas the pentoxifylline groups did not. By 8 weeks after 15 and 18 Gy, lung perfusion ratios were significantly higher for the irradiated controls than for pentoxifylline-treated rats-a reversal of the pattern observed at 3-5 weeks.

CONCLUSIONS

The protection by pentoxifylline against radiation-induced acute lung injury was transient and limited to the first 5 weeks after radiation. Subsequent recovery from lung injury was inhibited by this drug at later times within the acute phase.

Effect of pentoxifylline on radiation-induced lung and skin toxicity in rats

PURPOSE

There is currently substantial clinical interest in pentoxifylline as an inhibitor of radiation-related normal tissue injury. To further assess this drug's potential toxicity-sparing effects, pentoxifylline was studied in rats using a radiation-induced lung injury model.

METHODS AND MATERIALS

Adult male rats were exposed to either sham irradiation or a single fraction of 21 Gy delivered to the left hemithorax. Four study groups were defined: those that received neither radiation nor pentoxifylline, those that received pentoxifylline (500 mg/L in drinking water) but no irradiation, those that underwent irradiation without pentoxifylline, and those that received both pentoxifylline and radiation. Lung injury was measured by changes in relative left:right lung perfusion ratios derived from quantitative gamma camera imaging of 99mTechnetium-macroaggregated albumin uptake in the pulmonary circulation. Serial scans were done over a 40-week period following radiation. Skin toxicity was also assessed. After 40 weeks, the animals were killed, and lung tissue was assayed for angiotensin converting enzyme activity as a marker for endothelial cell damage.

RESULTS

Both groups of radiated (with or without pentoxifylline) rats showed equivalent acute sharp decreases in left:right lung perfusion ratios compared to the nonirradiated groups, reaching a mean nadir value of 0.29 at week 4. Irradiated lung perfusion in subsequent weeks in the radiation-only group showed minimal recovery, with a plateau mean ratio of 0.37 (0.36-0.39). However, there was apparent later recovery of lung perfusion in the radiation with pentoxifylline group from weeks 14 through 40, to a mean ratio of 0.47 (0.43-0.52) (p < 0.01 compared to the radiation-only group). Angiotensin converting enzyme activity correlated closely with lung perfusion data. No effect of pentoxifylline on acute or late skin toxicity was detected.

CONCLUSIONS

This study suggests that pentoxifylline does not have any measurable effect on acute lung injury following hemithoracic irradiation in rats, but does result in sparing of later lung toxicity.

Caffeine consumption is associated with decreased severe late toxicity after radiation to the pelvis

PURPOSE

Recent studies have suggested that pentoxifylline, a methylxanthine, can prevent or ameliorate late radiation injury in animals and humans. Caffeine is a commonly consumed methylxanthine that provides a model for evaluating the impact of this category of drugs on radiation injury. A retrospective study was undertaken to determine if there is an association between caffeine consumption and a lower incidence of late radiation toxicity.

METHODS AND MATERIALS

From 1984 through 1990, 82 patients with cervical cancer and 53 patients with endometrial cancer were treated with primary or adjuvant radiation therapy at the University of Washington. Patients were interviewed regarding ingestion of caffeine-containing beverages, and average daily caffeine consumption during the time of radiotherapy was estimated. The evaluable patients (42 cervical, 31 endometrial) were stratified by quantity of caffeine consumption for correlation with the incidence of radiation toxicity.

RESULTS

Acute radiation toxicity was not associated with caffeine consumption for cervical or endometrial cancer. There was a nonstatistically significant trend toward a decrease in overall late radiation toxicity with increased caffeine intake for cervical cancer patients. Subgroup analysis revealed this trend to be attributable to a decreased incidence of severe late radiation injury in cervical cancer patients who consumed higher levels of caffeine at the time of their radiotherapy (p = 0.02). This relationship was not observable for late toxicity in the endometrial cancer patients due to the low incidence of severe late injury following radiation for that disease.

CONCLUSIONS

This investigation is supportive of previous studies showing a radioprotective effect for pentoxifylline, and suggests that the mechanisms of radioprotection may be common to methylxanthines as a drug class.

On radiation damage to normal tissues and its treatment. II. Anti-inflammatory drugs

In addition to transiently inhibiting cell cycle progression and sterilizing those cells capable of proliferation, irradiation disturbs the homeostasis effected by endogenous mediators of intercellular communication (humoral component of tissue response to radiation). Changes in the mediator levels may modulate radiation effects either by assisting a return to normality (e.g., through a rise in H-type cell lineage-specific growth factors) or by aggravating the damage. The latter mode is illustrated with reports on changes in eicosanoid levels after irradiation and on results of empirical treatment of radiation injuries with anti-inflammatory drugs. Prodromal, acute and chronic effects of radiation are accompanied by excessive production of eicosanoids (prostaglandins, prostacyclin, thromboxanes and leukotrienes). These endogenous mediators of inflammatory reactions may be responsible for the vasodilatation, vasoconstriction, increased microvascular permeability, thrombosis and chemotaxis observed after radiation exposure. Glucocorticoids inhibit eicosanoid synthesis primarily by interfering with phospholipase A2 whilst non-steroidal anti-inflammatory drugs prevent prostaglandin/thromboxane synthesis by inhibiting cyclooxygenase. When administered after irradiation on empirical grounds, drugs belonging to both groups tend to attenuate a range of prodromal, acute and chronic effects of radiation in man and animals. Taken together, these two sets of observations are highly suggestive of a contribution of humoral factors to the adverse responses of normal tissues and organs to radiation. A full account of radiation damage should therefore consist of complementary descriptions of cellular and humoral events. Further studies on anti-inflammatory drug treatment of radiation damage to normal organs are justified and desirable.