Current Drug Targets (v.11, #11)

Among the four weapons of mass destruction and disruption (chemical, biological, radiological, and nuclear - CBRN), nuclear and radiological weapons are the most insidious. Serious debates are in progress about the danger- benefit equilibrium of even diagnostic and therapeutic radiological exposures. Although non-ionizing radiations like microwave are also harmful, the deleterious effects of ionizing radiation have been under the intense scrutiny by the scientific community, as well as the subject for deliberations of governments of the comity of nations. In the United States, considerable resources are being allocated by the departments of Defense (DoD), Homeland Security (DHS), and Health and Human Services (DHHS) in deciphering the mechanisms of radiation damage and develop drug(s) that can prevent these effects before, shortly after, and well after irradiation. The existing concepts of time, shielding, and distance are still the best practices of minimizing the deleterious effects of radiation exposure. Nevertheless, the development of a radiation countermeasure drug(s) is an important priority. Hence, the paucity of radiation countermeasures is a serious concern for physicians trying to protect the normal tissues surrounding a tumor during radiotherapy, for first responders who may have to enter a radiation field for rescue and recovery operations in a and#x201C;dirty bomband#x201D; attack by extremists, and for astronauts who may be involved in extended extra-vehicular activities during space explorations. Depending on the dose and dose rate of ionizing radiation exposure, normal tissue radiation injury can vary from short-term lethality to long-term carcinogenesis and fibrosis based on the severity of the radiation-induced multi organ dysfunction syndrome (Rad-MODS). The earliest visible manifestation of radiation in humans is nausea and vomiting. Hematological, gastrointestinal, central nervous system, and cardio-pulmonary sub-syndromes are manifested on exposure to progressively increasing doses of radiation. However, these sub-syndromes are not mutually exclusive and quite often overlap. Several studies are in progress to determine the various targets of radiation damage in these organ systems. These studies described in this volume include non-free radical scavenging mechanisms that modify radiation damage (Kim and McBride), nitric oxidetetrahydrobiopterin based novel molecular mechanisms (Berbee et al.), cellular mechanisms of injury and repair of bone marrow (Kulkarni et al.), organ specific normal tissue radiation injury of lungs (Williams et al., Monceau et al.), kidney (Cohen et al.), brain (Robbins et al.), and heart (Boerma and Hauer-Jensen). This special thematic edition of and#x201C;Targets for Potential Radioprotective Drugsand#x201D; is an attempt to collate the state of the art knowledge in this area written by established scientists in their own fields of expertise. In addition to collating the current state of the knowledge, the authors have succeeded in intertwining the findings of their own research with those of others. The readers of this issue will acquire insight into mechanisms of radiation damage to various organ systems to be applied to the development of novel mechanisms-based radiation countermeasure drug(s).

Modifying Radiation Damage by Kwanghee Kim, William H. McBride (1352-1365).
Radiation leaves a fairly characteristic footprint in biological materials, but this is rapidly all but obliterated by the canonical biological responses to the radiation damage. The innate immune recognition systems that sense and#x201C;dangerand#x201D; through direct radiation damage and through associated collateral damage set in motion a chain of events that, in a tissue compromised by radiation, often unwittingly result in oscillating waves of molecular and cellular responses as tissues attempt to heal. Understanding and#x201C;nature's whispersand#x201D; that inform on these processes will lead to novel forms of intervention targeted more precisely towards modifying them in an appropriate and timely fashion so as to improve the healing process and prevent or mitigate the development of acute and late effects of normal tissue radiation damage, whether it be accidental, as a result of a terrorist incident, or of therapeutic treatment of cancer. Here we attempt to discuss some of the non-free radical scavenging mechanisms that modify radiation responses and comment on where we see them within a conceptual framework of an evolving radiation-induced lesion.

Novel Strategies to Ameliorate Radiation Injury: A Possible Role for Tetrahydrobiopterin by Maaike Berbee, Qiang Fu, K. Sree Kumar, Martin Hauer-Jensen (1366-1374).
Novel pharmacological strategies are urgently needed to prevent or reduce radiation-induced tissue injury. Microvascular injury is a prominent feature of both early and delayed radiation injury. Radiation-induced endothelial dysfunction is believed to play a key role in the pathogenesis of post-irradiation tissue injury. Hence, strategies that could prevent or improve endothelial malfunction are expected to ameliorate the severity of radiation injury. This review focuses on the therapeutic potential of the nitric oxide synthase (NOS) cofactor 5,6,7,8-tetrahydrobiopterin (BH4) as an agent to reduce radiation toxicity. BH4 is an essential cofactor for all NOS enzymes and a critical determinant of NOS function. Inadequate availability of BH4 leads to uncoupling of the NOS enzyme. In an uncoupled state, NOS produces the highly oxidative radicals superoxide and peroxynitrite at the cost of NO. Under conditions of oxidative stress, such as after radiation exposure, BH4 availability might be reduced due to the rapid oxidation of BH4 to 7,8-dihydrobiopterin (7,8-BH2). As a result, free radical – induced BH4 insufficiency may increase the oxidative burden and hamper NOdependent endothelial function. Given the growing evidence that BH4 depletion and subsequent endothelial NOS uncoupling play a major role in the pathogenesis of endothelial dysfunction in various diseases, there is substantial reason to believe that improving post-irradiation BH4 availability, by either supplementation with it or modulation of its metabolism, might be a novel strategy to reduce radiation-induced endothelial dysfunction and subsequent tissue injury.

Hematological Targets of Radiation Damage by Shilpa Kulkarni, Sanchita P. Ghosh, Martin Hauer-Jensen, K. Sree Kumar (1375-1385).
Radiation-induced myelosuppression remains a rate-limiting factor of radiotherapy and chemotherapy. Therefore, hematological targets of radiation damage are of great significance for radiation oncology and normal tissue injury and protection. Protection of hematopoietic stem and progenitor cells is pivotal. In order to develop therapeutic targets, it is necessary to understand the mechanisms of stem cell renewal and differentiation. Recent advances in the molecular pathology of hematopoietic stem cells indicate a fine balance between various extrinsic and intrinsic signaling pathways in preserving the self-renewal and proliferative capacity of stem cells. Extrinsic signaling involves microenvironment niche factors such as neighboring stromal cells, osteoblasts, and adipocytes secreting cytokines, chemokines, and metalloproteinases; intrinsic regulation involves Wnt/hedgehog/Notch signaling, DNA damage-induced epigenetic alterations, telomere shortening, and early senescence. Various drugs including synthetic cytokine mimetics, cytokine stimulators, and DNA repair modulators are being tested as radioprotectants. Colony-stimulating factors are routinely used in clinics to treat neutropenia induced by chemotherapy and radiotherapy as well as to mobilize and expand progenitors used in autologous transplantation. However, toxicity has limited their use. The vitamin E isoform gamma tocotrienol, a potent free radical scavenger that has displayed promising anticarcinogenic properties, was recently shown to protect bone marrow (BM) from radiation injury and to stimulate hematopoiesis in a murine model. This chapter focuses on the potential targets of radiation damage in BM and speculates on the mechanisms of protection by and#947;- tocotrienol and how these mechanisms can contribute to radioprotection in general and to protection of BM during chemotherapy and radiotherapy in particular.

Treatment for Radiation-Induced Pulmonary Late Effects: Spoiled for Choice or Looking in the Wrong Direction? by Jacqueline P. Williams, Carl J. Johnston, Jacob N. Finkelstein (1386-1394).
Due to the radiosensitivity of the lung, toxic endpoints, in the form of radiation pneumonitis and pulmonary fibrosis, are relatively frequent outcomes following radiation treatment of thoracic neoplasms. Because of the potential lethal nature of these normal tissue reactions, they not only lead to quality-of-life issues in survivors, but also are deemed dose-limiting and thereby compromise treatment. The mitigation and treatment of lung normal tissue late effects has therefore been the goal of many investigations; however, the complexity of both the organ itself and its response to injury has resulted in little success. Nonetheless, current technology allows us to propose likely targets that are either currently being researched or should be considered in future studies.

Modulation of the Rho/ROCK Pathway in Heart and Lung after Thorax Irradiation Reveals Targets to Improve Normal Tissue Toxicity by Virginie Monceau, Nadia Pasinetti, Charlotte Schupp, Fred Pouzoulet, Paule Opolon, Marie-Catherine Vozenin (1395-1404).
The medical options available to prevent or treat radiation-induced injury are scarce and developing effective countermeasures is still an open research field. In addition, more than half of cancer patients are treated with radiation therapy, which displays a high antitumor efficacy but can cause, albeit rarely, disabling long-term toxicities including radiation fibrosis. Progress has been made in the definition of molecular pathways associated with normal tissue toxicity that suggest potentially effective therapeutic targets. Targeting the Rho/ROCK pathway seems a promising anti-fibrotic approach, at least in the gut; the current study was performed to assess whether this target was relevant to the prevention and/or treatment of injury to the main thoracic organs, namely heart and lungs. First, we showed activation of two important fibrogenic pathways (Smad and Rho/ROCK) in response to radiationexposure to adult cardiomyocytes; we extended these observations in vivo to the heart and lungs of mice, 15 and 30 weeks post-irradiation. We correlated this fibrogenic molecular imprint with alteration of heart physiology and long-term remodelling of pulmonary and cardiac histological structures. Lastly, cardiac and pulmonary radiation injury and bleomycin-induced pulmonary fibrosis were successfully modulated using Rho/ROCK inhibitors (statins and Y-27632) and this was associated with a normalization of fibrogenic markers. In conclusion, the present paper shows for the first time, activation of Rho/ROCK and Smad pathways in pulmonary and cardiac radiation-induced delayed injury. Our findings thereby reveal a safe and efficient therapeutic opportunity for the abrogation of late thoracic radiation injury, potentially usable either before or after radiation exposure; this approach is especially attractive in (i) the radiation oncology setting, as it does not interfere with prior anti-cancer treatment and in (ii) radioprotection, as applicable to the treatment of established radiation injury, for example in the case of radiation accidents or acts of terrorism.

Radiotherapy of thoracic and chest wall tumors, if all or part of the heart was included in the radiation field, can lead to radiation-induced heart disease (RIHD), a late and potentially severe side effect. RIHD presents clinically several years after irradiation and manifestations include accelerated atherosclerosis, pericardial and myocardial fibrosis, conduction abnormalities, and injury to cardiac valves. The pathogenesis of RIHD is largely unknown, and a treatment is not available. Hence, ongoing pre-clinical studies aim to elucidate molecular and cellular mechanisms of RIHD. Here, an overview of recent pre-clinical studies is given, and based on the results of these studies, potential targets for intervention in RIHD are discussed.

Renin-angiotensin System Blockers and Modulation of Radiation-Induced Brain Injury by M. E. Robbins, W. Zhao, M. A. Garcia-Espinosa, D. I. Diz (1413-1422).
Radiation-induced brain injury remains a major cause of morbidity in cancer patients with primary or metastatic brain tumors. Approximately 200,000 individuals/year are treated with fractionated partial or whole-brain irradiation, and > half will survive long enough (and#x2264;6 months) to develop radiation-induced brain injury, including cognitive impairment. Although short-term treatments have shown efficacy, no long-term treatments or preventive approaches are presently available for modulating radiation-induced brain injury. Based on previous preclinical studies clearly demonstrating that renin-angiotensin system (RAS) blockers can modulate radiation-induced late effects in the kidney and lung, we and others hypothesized that RAS blockade would similarly modulate radiation-induced brain injury. Indeed, studies in the last 5 years have shown that both angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin II type 1 receptor antagonists (AT1RAs) can prevent/ameliorate radiation-induced brain injury, including cognitive impairment, in the rat. The mechanistic basis for this RAS blocker-mediated effect remains the subject of ongoing investigations. Putative mechanisms include, i] blockade of Ang II/NADPH oxidase-mediated oxidative stress and neuroinflammation, and ii] a change in the balance of angiotensin (Ang) peptides from the pro-inflammatory and pro-oxidative Ang II to the anti-inflammatory and anti-oxidative Ang-(1-7). However, given that both ACEIs and AT1RAs are: 1] well-tolerated drugs routinely prescribed for hypertension, 2] exhibit some antitumor properties, and 3] can prevent/ameliorate radiationinduced brain injury, they appear to be ideal drugs for future clinical trials, offering the promise of improving the quality of life of brain tumor patients receiving brain irradiation.

Radiation nephropathy and other normal tissue radiation injuries can be successfully mitigated, and also treated, by antagonists of the renin-angiotensin system (RAS). This implies a mechanistic role for that system in radiation nephropathy, yet no evidence exists to date of activation of the RAS by irradiation. RAS antagonists, including angiotensin converting enzyme inhibitors and angiotensin receptor blockers, are the standard of care in the treatment of subjects with other chronic progressive kidney diseases, in which they exert benefit by reducing both glomerular and tubulo-interstitial injury. These drugs are likely to act in a similar way to mitigate radiation nephropathy.

Histone Deacetylase Inhibitors: New Promise in the Treatment of Immune and Inflammatory Diseases by Stephen J. Shuttleworth, Sarah G. Bailey, Paul A. Townsend (1430-1438).
The development of Histone Deacetylase (HDAC) inhibitors has, until recently, principally been driven by their potential as anti-cancer agents. However, there is emerging evidence that HDAC inhibitors could have utility in the treatment of chronic immune and inflammatory disorders, including rheumatoid arthritis, psoriasis, inflammatory bowel disease, multiple sclerosis, systemic lupus erythematosus, airway hyperresponsiveness and organ transplant rejection. Here we discuss the merits of various, structurally-distinct HDAC inhibitors as potential anti-inflammatory therapeutics and provide examples of the novel medicinal chemistry approaches being undertaken to realize HDAC as a druggable target in this clinical setting.

Methionine Aminopeptidases as Potential Targets for Treatment of Gastrointestinal Cancers and other Tumors by Jose L. Mauriz, Javier Martin-Renedo, Andres Garcia-Palomo, Maria J. Tunon, Javier Gonzalez-Gallego (1439-1457).
Methionine aminopeptidases (MetAP) are intracellular metalloproteins responsible for the removal of the initiator NH2-terminal methionine from newly synthesized proteins, thereby facilitating their intracellular translocation from the ribosome. Two types of MetAP enzymes, MetAP-1 (type-I) and MetAP-2 (type-II), which have a similar threedimensional structure despite a low homology in their sequences, have been described. Since the discovery that fumagillin, an irreversible MetAP-2 inhibitor, prevents angiogenesis, different studies have been carried out to analyze the role of MetAP proteins as potential targets in cancer treatment. Data obtained indicate that anticancer effect of MetAP-2 inhibitors may be a result of the combined effect of MetAP-2 inhibition in endothelial cells (anti-angiogenesis) and in tumor cells directly. Moreover, it has been recently described that MetAP-1 has a potential role in cell division, and MetAP-1-specific inhibition is able to induce apoptosis in both HeLa and HT-1080 cell lines. A new subtype of MetAP-1, called MetAP-1D, has been found overexpressed in samples from colon cancer patients, its inhibition resulting in a decreased cell growth. Although molecular mechanisms of action of these proteins are largely unknown, a significant progress has been made to understand their structure-function relationships and their physiological roles. Their potential as promising targets in cancer treatment and in the development of new antitumor agents is analyzed focusing on MetAP irreversible inhibitors. The present review summarizes recent research data on different molecules able to induce MetAP inhibition in gastrointestinal cancers and other tumors.

Effects of Cissampelos sympodialis Eichl. and its Alkaloid, Warifteine, in an Experimental Model of Respiratory Allergy to Blomia tropicalis by Ana Tereza Cerqueira-Lima, Neuza Maria Alcantara-Neves, Lain Carlos Pontes de Carvalho, Ryan Santos Costa, Jose Maria Barbosa-Filho, Marcia Piuvezam, Momtchilo Russo, Renato Barboza, Eduardo de Jesus Oliveira, Alexsandro Marinho, Camila Alexandrina Figueiredo (1458-1467).
Asthma is one of the most prevalent chronic diseases worldwide. Medicinal plants are historically used in its treatment. The plant Cissampelos sympodialis, known in Northeastern Brazil as and#x201C;Jarrinhaand#x201D;or and#x201C;Milonaand#x201D;, is used to treat some inflammatory conditions, including asthma. The objective of this study was to evaluate the potential of Cissampelos sympodialis EICHL. extract (CsE) and its isolated alkaloids, especially warifteine (Wa) on a Blomia tropicalis extract (BtE)-induced experimental model of allergy. The respiratory allergy was induced in AJ mice by the administration of BtE. Mice were orally treated with the 400 mg/kg of CsE or 8 mg/kg of total alkaloids fraction (TAF) or 4 mg/kg of Wa and the following parameters were analyzed: (a) total cell numbers in bronchoalveolar fluid (BAF); (b) differential cell numbers in BAF; (c) eosinophil peroxidase (EPO) activity in BAF; (d) IgE serum levels by passive cutaneous anaphylaxis; (e) IL-5, IL-13, IL-10, and IFN-and#947; levels in BAF; (f) histopathological alterations in the lung. The treatment of the animals with CsE, Wa or TAF led to a reduction in the numbers of total cells and eosinophils in BAF. The same reduction was observed in EPO levels in the BAF. The levels of IL-5 and IL-13 were also reduced in animals treated with Cissampelos sympodialis, while IL-10 levels were significantly increased in the BAF of CsE-treated animals. The treatment also decreased the density of inflammatory cells in the lung by histopathological examinations demonstrating the potential of this medicinal plant as new agent for asthma treatment.

Diabetic vascular complications are leading causes of acquired blindness, end-stage renal failure, a variety of neuropathies, and accelerated atherosclerosis, which may be involved in the disabilities and high mortality rates suffered by diabetic patients. Continuous hyperglycemia is involved in the pathogenesis of diabetic micro- and macrovascular complications via various metabolic pathways, and numerous hyperglycemia-induced metabolic and hemodynamic conditions exist, including increased generation of various types of advanced glycation end-products (AGEs). Recently, we demonstrated that glyceraldehyde-derived AGEs (Glycer-AGEs), the predominant components of toxic AGEs (TAGE), play an important role in the pathogenesis of angiopathy in diabetic patients. Moreover, a growing body of evidence suggests that the interaction of TAGE with the receptor for AGEs (RAGE) alters intracellular signaling, gene expression, and the release of pro-inflammatory molecules and elicits oxidative stress generation in numerous types of cells, all of which may contribute to the pathological changes observed in diabetic vascular complications. Therefore, the inhibition of TAGE formation, blockade of TAGE-RAGE interaction, and the suppression of RAGE expression or its downstream pathways are promising targets for therapeutic interventions against diabetic vascular complications. In this review, we discuss the pathophysiological role of the TAGE-RAGE-oxidative stress system and related therapeutic interventions for preventing the development and progression of diabetic vascular complications.