Current Drug Targets (v.17, #16)

Meet Our Editorial Board Member by Stefania Vetrano (1827-1827).

A Central Role for ATP Signalling in Glial Interactions in the CNS by Andrea Rivera, Ilaria Vanzulli, Arthur M. Butt (1829-1833).
The purine ATP has a prominent regulatory role in CNS function and pathology due to its actions on glial cells - microglia, astrocytes and oligodendrocytes. ATP serves as an apparently ubiquitous 'gliotransmitter' that is released by astrocytes and other cells to activate purine receptors on neighbouring cells. In pathology, the release of ATP mediates both tissue damage and repair by its direct effects on glial cell integrity and survival. The actions of ATP on glia are mediated via a wide range of receptors, broadly divided into ionotropic P2X and metabotropic (G-protein coupled receptors (GPCR)) P2Y receptors, of which there are multiple subtypes (P2X1-P2X7 and P2Y1-P2Y14). ATP-mediated interactions between astrocytes and microglia are at the centre of immune responses in the CNS, with prominent roles for the P2X4, P2X7, P2Y1, P2Y6 and P2Y12 receptor subtypes. In oligodendrocytes, P2X7 and P2Y1 receptor subtypes have a bipartite function in respectively mediating oligodendrocyte destruction and protection. Purine receptors mediate glial pathology, with prominent roles in ischemia, neuroinflammation, Multiple Scelerosis, neuropathic pain and traumatic injury. Notably, glial ATP signalling may be altered with ageing and is implicated in impaired myelination and immunity in Alzheimer's disease. Hence, glial purine receptors provide potential therapeutic targets in multiple neuropathologies, but the 'Jeckyll and Hyde' nature of purine signalling underscores the importance of further research and a comprehensive understanding of the roles of the different purine receptors in mediating tissue damage and repair.

The Emerging Role of the Cannabinoid Receptor Family in Peripheral and Neuro-immune Interactions by Orla Haugh, June Penman, Andrew J. Irving, Veronica A. Campbell (1834-1840).
The classical endogenous cannabinoid (CB) system is composed of the endocannabinoid signalling molecules, 2-arachidonoyl glycerol (2-AG) and anandamide (AEA) and their G-protein coupled receptors (GPCR), CB1 and CB2 which together constitutes the endocannabinoid system (ECS). However, putative, novel lipid-sensing CB receptors have recently been identified, including the orphan GPR55 and GPR18 receptors that are regulated by cannabinoid-like molecules and interact with CB system. CB receptors and associated orphan GPCRs are expressed at high levels in the immune and/or central nervous systems (CNS) and regulate a number of neurophysiological processes, including key events involved in neuroinflammation. As such, these receptors have been identified as emerging therapeutic targets for a number of brain disorders in which neuroinflammation is a key feature, including multiple sclerosis (MS) and Alzheimer's disease (AD). This review will consider the role of the wider cannabinoid receptor superfamily in mediating immune function with a focus on the immune processes that contribute to neuroinflammatory conditions.

Sphingosine-1-phosphate receptor (S1PR) modulators have entered clinical practice as immune-modulators for the treatment of multiple sclerosis (MS). Pharmacologic modulation of S1PR expression on lymphocytes inhibits these cells capacity to respond to the S1P gradient within regional lymph nodes (LNs) (and thymus) that promotes their exit into peripheral circulation. The resultant peripheral blood restricted lymphopenia is considered to underlie the capacity of S1PR modulators to reduce new inflammatory lesion formation in MS in the absence of global immune suppression. These modulators also regulate entry of selective lymphocyte populations and dendritic cells (DCs) into LNs and modulate sphingosine-1-phosphate (S1P) cell signaling networks that govern the generation of specific cell subsets within LNs. S1PR modulators that access the CNS can also have functional effects within this compartment since S1PRs are expressed by cells comprising the blood brain barrier (BBB) and by those within the parenchyma, including neurons, astrocytes, oligodendrocytes and microglia. Absence of S1P1 receptor (S1PR1) on astrocytes reduces disease severity in experimental autoimmune encephalomyelitis (EAE). Even under conditions that inhibit cell responses to the natural ligand, S1PR modulators can continue to induce active signaling responses; such responses may be relevant for promoting neuroprotection and augmenting tissue repair within the CNS.

The Role of the Oxysterol/EBI2 Pathway in the Immune and Central Nervous Systems by Aleksandra Rutkowska, Kumlesh K. Dev, Andreas W. Sailer (1851-1860).
Oxysterols are pleiotropic messengers interacting with multiple receptor systems. One of the cognate receptors for oxysterols is EBI2, a G protein-coupled receptor highly expressed in the cells of the immune system. Here we discuss the receptor's role in the adapted immunity and inflammation as well as the receptor's expression and function in the CNS with the focus on astrocytes. We also discuss expression and signalling of oxysterol-producing enzymes such as CH25H and CYP7B1 in the CNS and the immune system. These steps will help to elucidate a possible role for this pathway in the physiology of the central and peripheral nervous system and its possible link to human disease.

Protease-Activated Receptor 2: Are Common Functions in Glial and Immune Cells Linked to Inflammation-Related CNS Disorders? by Trevor J. Bushell, Margaret R. Cunningham, Kathryn A. McIntosh, Serge Moudio, Robin Plevin (1861-1870).
Protease-activated receptors (PARs) are a novel family of G-protein coupled receptors (GPCRs) whose activation requires the cleavage of the N-terminus by a serine protease. However, recent evidence reveals that alternative routes of activation also occur, that PARs signal via multiple pathways and that pathway activation is activator- dependent. Given our increased understanding of PAR function both under physiological and pathophysiological conditions, one aspect that has remained constant is the link between PAR2 and inflammation. PAR2 is expressed in immune cells of both the innate and adaptive immune system and has been shown to play a role in several peripheral inflammatory conditions. PAR2 is similarly expressed on astrocytes and microglia within the CNS and its activation is either protective or detrimental to CNS function depending on the conditions or disease state investigated. With a clear similarity between the function of PAR2 on both immune cells and CNS glial cells, here we have reviewed their roles in both these systems. We suggest that the recent development of novel PAR2 modulators, including those that show biased signalling, will further increase our understanding of PAR2 function and the development of potential therapeutics for CNS disorders in which inflammation is proposed to play a role.

Astrocytes: Adhesion Molecules and Immunomodulation by Shane Liddelow, Daniel Hoyer (1871-1881).
Protection of neurons, as well as maintenance of their general homeostasis and trophic support is performed by glial cells. Astrocytes, the most abundant glial cell, increase in size and number evolutionarily such that invertebrates contain fewer small astrocytes, while humans have large multi-branched astrocytes that constitute up to 60% of central nervous system (CNS) cells. Astrocytes provide neurotrophic support, induce synaptogenesis and are integral for maintenance and pruning of synapses in the adult. Following injury and in disease, their ability to respond to, and initiate initial responses to injury/disease is increasingly apparent - with alterations in function that disrupt the internal milieu of the CNS, which ultimately may lead to deficits in neuronal health and functioning. Additionally, it seems certain astrocytes serve as liaison between the CNS and the peripheral immune system. In this role, astrocytes function much like macrophages, capable of phagocytosis and production of antigens to recruit additional immune cells. Aside from up-regulation of traditional immune modulators after injury, astrocytes express many adhesion molecules such as VCAM1, NCAM1 and ICAM1, which represent potential drug targets in a number of CNS inflammatory diseases. These molecules may facilitate astrocyte-lymphocyte interactions ultimately aiding recruitment of immune cells into the CNS. Due to their immense numbers and widespread presence in the brain, combined with their capacity for rapid proliferation, astrocytes are well-positioned as gate-keepers of CNS immune responses. Beyond roles in immune and inflammatory activities, astrocytes also affect a complex array of neural functions and behaviours like mood, sleep, and pain, as well as altering the CNS capacity for recovery following injury/illness. Further, their complex effects on synaptic function suggest that astrocytes are key players in neuropsychiatric and neurodegenerative diseases.

IL-17A and Multiple Sclerosis: Signaling Pathways, Producing Cells and Target Cells in the Central Nervous System by Frank Kolbinger, Christine Huppertz, Anis Mir, Franco Di Padova (1882-1893).
Multiple sclerosis (MS) is an immune mediated demyelinating disease of the central nervous system (CNS). The importance of immune cells to MS pathology is supported by clinical data linking the depletion of T and B cells, or the prevention of their migration into the brain with significant reduction in relapses and development of new lesions. In vitro studies, preclinical animal models and encouraging data with the anti-IL-17A antibody secukinumab in a small proof of concept study in man, indicate that IL-17A, a key interleukin associated with many inflammatory and autoimmune diseases, may be involved in MS. Not only cells involved in adaptive immune responses such as Th17 cells and cytotoxic T cells, or innate immune responses such as mucosa-associated invariant T (MAIT) cells and ??T cells, but also CNS resident cells such as astrocytes and oligodendrocytes might contribute to the local production of IL-17A. IL-17A synergizes with other proinflammatory cytokines, by inducing the release of additional cytokines, mediators of tissue damage and chemokines, that recruit new inflammatory cells. IL-17A adversely affects the functions of microglia, astrocytes, oligodendrocytes, neurons, neural precursor cells and endothelial cells. Blockade of IL-17A might be beneficial to MS patients not only by inhibiting inflammation and tissue destruction, but also by enhancing repair processes.

Recent advances in the self-shielded cyclotrons, improved targets, videomonitored hot cells design, and automated PET radiopharmaceutical (RPs) synthesis modules, utilizing computer-controlled graphic user interphase (GUI) has revolutionized PET molecular imaging technology for basic biomedical research and theranostics to accomplish the ultimate goal of evidence-based personalized medicine. Particularly, [18F]HX4: (3-[18F]fluoro-2-(4-((2-nitro-1Himidazol-1-yl)methyl)-1H-1,2,3,-triazol-1- yl)-propan-1-ol), 18F-FAZA: 1-(5-[18F]Fluoro-5-deoxy-?-D-arabinofuranosyl)-2- nitroimidazole, and 18F-FMSIO: 18F-Ffluoromisonidazole to assess tumor hypoxia, [18F]FB-VAD-FMK: [18F]4-fluorobenzylcarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone to determine in vivo apoptosis, 64Cu-PTSM: 64Cu-Pyrualdehyde Bis-NMethylthiosemicarbazone for brain and myocardial perfusion imaging, and 68Ga-DOTATOC: 68Ga- DOTAD-Phy1-Tyr3-octreotide and 68Ga-DOTANOC: 68Ga-(1,4,7,10-tetraazacyclododecane- N,N',N'',N'''-tetraacetic acid)-1-NaI3-octreotide for neuroendocrine and neural crest tumors have demonstrated great promise in personalized theranostics. Furthermore, multimodality imaging with 124IPET/ CT and 18FDG-PET/CT rationalizes 131I treatment in thyroid cancer patients to prevent cost and morbid toxicity. In addition to 18F-labeled PET-RPs used in clinical practice, novel discoveries of chemical reactions including transition metal-mediated cross-coupling of carbon-carbon, carbonheterocarbon, and click chemistry at ambient temperature with significantly reduced synthesis times, labeled even with short-lived radionuclides such as 11C, has facilitated development of novel PET-RPs. These innovative approaches to synthesize PET-RPs and efficient image acquisition capabilities have improved the resolution of multimodality imaging and significantly reduced the radiation exposure to patients as well as healthcare professionals. Future developments in novel PET-RPs, utilizing automated microfluidic synthesis modules and multifunctional nanoparticles, will improve biomarker discovery, internal dosimetry, pharmacokinetics, immunotherapy, and stem cell tracking in regenerative medicine. This review provides recent developments in the synthesis of clinically-significant cyclotron and generator- based PET-RPs with potential applications in cardiovascular diseases, neurodegenerative diseases, and cancer to accomplish the ultimate goal of evidence-based personalized theranostics.

Targeting ADAM17 Sheddase Activity in Cancer by Armando Rossello, Elisa Nuti, Silvano Ferrini, Marina Fabbi (1908-1927).
A disintegrin and metalloprotease (ADAM)17 is a sheddase, capable of releasing the ectodomains of membrane proteins such as growth factors (e.g. Epidermal Growth Factor Receptor ligands), cytokines and their receptors, adhesion and signaling molecules. These activities regulate several physiological and pathological processes including inflammation, tumor growth and metastatic progression. In this review, we will summarize ADAM17 biology and focus on its role in cancer and the possible usage of ADAM17 inhibitors in cancer therapy. Recent achievements in this area include the development of small molecule metalloprotease inhibitors with enhanced specificity for ADAM17, monoclonal antibodies, and synthetic short RNA molecules for gene silencing. These approaches successfully inhibited cancer cell growth and invasiveness or sensitized them to cytotoxic drugs, ionizing radiations or targeted therapies, in preclinical studies. These findings suggest the repositioning of ADAM17 inhibitors, which have yet proven unsuccessful as anti-inflammatory agents, for the development of new anti-cancer therapies, particularly in EGFR ligand-dependent cancers. Future studies should address ADAM17 inhibitors as short-term treatments in combination with different anti-cancer therapies.

Topoisomerase I (Topo I) is a nuclear enzyme engaged in adjustment of DNA topological structure during cell cycle by cleaving and reannealing one of the two strands of the DNA double helix. Inhibition of this enzyme results in DNA strand breaks, ultimately leads to apoptosis and cell death; additionally it is in raised level in solid tumors contrasted with healthy tissues. Consequently, Topo I has a great potential as a target for the treatment of tumors. Although significant anti-tumor activity of first Topo I inhibitor, camptothecin (CPT), was observed on colon, lung, ovarian, breast, liver, pancreas and stomach cancers, CPT and its clinical derivatives (topotecan and irinotecan) have several restrictions. In addition to their low water solubility and cell resistance to CPTs, lactone ring opening causes a reduction in cytotoxic activity and severe side effects in physiological conditions (pH: 7.4, 37°C). Although numerous efficient nano drug delivery systems were developed for CPT and its derivatives to compensate the handicaps of these compounds, none of them has been approved so far. On the other hand, organic non-CPT compounds have been searched; indolocarbazoles, indenoisoquinolines and dibenzonaphthyridines have been applied to clinical development. Especially, indenoisoquinolines and dibenzonaphthyridines have favorable characteristics compared to CPTs: They are chemically stable; they have the ability to overcome cell resistance; they stabilize enzyme-DNA cleavage complexes more persistently. In addition to the approaches based on organic compounds, recently some metal complexes (e.g., platinum, gold, copper, cobalt, zinc, vanadium, ruthenium) have also been reported as inhibitors of Topo I. This review will discuss the whole aspects of strategies targeting Topo I in cancer chemotherapy from past to the recent progresses.

Phospholipase A2 Isoforms as Novel Targets for Prevention and Treatment of Inflammatory and Oncologic Diseases by Nagendra Sastry Yarla, Anupam Bishayee, Lakshmipathi Vadlakonda, Ramakrishna Chintala, Govinda Rao Duddukuri, Pallu Reddanna, Kaladhar S.V.G.K. Dowluru (1940-1962).
Phospholipase A2s (PLA2s) are group of enzymes, which cleave phospholipids specifically at sn-2 position to liberate free fatty acid, mostly arachidonic acid (AA) and lysophospholipids (LPLs). Inhibition of PLA2 prevents the liberation of AA and LPLs. Hence, researchers have been considering PLA2s could be a better therapeutic target than the downstream enzymes cyclooxygense and lipoxygenase. Several isoforms of PLA2s exist; they are mainly divided into secretory PLA2s (sPLA2), cytosolic PLA2s (cPLA2), and calcium independent PLA2s (iPLA2), platelet activating factor- acyl hydrolase (PAF-AH), lysosomal PLA2 (LPLA2), adipose-specific PLA2 (Ad- PLA). Each isoform of PLA2s is different in its chemical structure and physiological functions. sPLA2s (Groups IIA, V and X) are well characterized as proinflammatory mediating enzymes but their role in cancer is controversial. Groups IVA, IVB and IVC cPLA2s are present in humans but only Group IVA cPLA2 plays key role in pathophysiology of various cancers and inflammation. The role of iPLA2 in inflammation and cancer is limited. Lipoprotein associated PLA2 (Group VIIA PLA2), a PAF-AH isoform, has key role in atherosclerosis. Several isoform specific PLA2 inhibitors have been developed and some of the PLA2s inhibitors are currently under clinical trials for various inflammatory and oncologic diseases. This review focuses on the recent experimental evidences to support the notion that PLA2s are causally implicated in the pathobiology of cancer and inflammatory related disorders and discuss the potential utility of isoform specific PLA2 inhibitors as preventive and/or therapeutic agents.

Aspirin Intolerance: Experimental Models for Bed-to-Bench by Masamichi Yamashita (1963-1970).
Aspirin is the oldest non-steroidal anti-inflammatory drug (NSAID), and it sometimes causes asthma-like symptoms known as aspirin-exacerbated respiratory disease (AERD), which can be serious. Unwanted effects of aspirin (aspirin intolerance) are also observed in patients with food-dependent exercise-induced anaphylaxis, a type I allergy disease, and aspirin-induced urticaria (AIU). However the target and the mechanism of the aspirin intolerance are still unknown. There is no animal or cellular model of AERD, because its pathophysiological mechanism is still unknown, but it is thought that inhibition of cyclooxygenase by causative agents leads to an increase of free arachidonic acid, which is metabolized into cysteinyl leukotrienes (cysLTs) that provoke airway smooth muscle constriction and asthma symptoms. As the bed-tobench approach, to confirm the clinical discussion in experimental cellular models, we have tried to develop a cellular model of AERD using activated RBL-2H3 cells, a rat mast cell like cell line. Indomethacin (another NSAID and also causes AERD), enhances in vitro cysLTs production by RBL-2H3 cells, while there is no induction of cysLTs production in the absence of inflammatory activation. Since this suggests that all inflammatory cells with activation of prostaglandin and cysLT metabolism should respond to NSAIDs, and then I have concluded that aspirin intolerance should be separated from subsequent bronchoconstriction. Evidence about the cellular mechanisms of NSAIDs may be employed for development of in vitro AERD models as the approach from bench-to-bed.