Inflammation & Allergy-Drug Targets (Discontinued) - Volume 5, Issue 2, 2006

The focus of this special issue of Inflammation & Allergy - Drug Targets is on anti-inflammation. The inflammatory response protects the body against infection and injury, but can itself become deregulated with deleterious consequences to the host. Several endogenous biochemical pathways activated during defense reactions can counter-regulate inflammation. It is increasingly recognized that chronic inflammation develops as a consequence of the failure of these systems to attenuate generation of inflammatory mediators. Bioactive eicosanoids represent the main mediators of inflammation. Synthesis of the eicosanoids (Reviewed in this issue by Dr. Mario Romano) originates when arachidonic acid is converted to the prostaglandins through the prostaglandin synthase pathways. Arachidonic acid can also be oxygenated by lipoxygenases to form HPETEs. 5-lipoxygenase catalyzes the production of leukotriene (LT) A4, which is in turn hydrolyzed to produce LTB4; alternatively, the addition of a glutathione moiety in the presence of glutathione S-transferase produces LTC4 and LTD4. In contrast to the LTs and the prostaglandins, the lipoxins (LXs), an acronym for lipoxygenase interaction products, are endogenously produced eicosanoids with anti-inflammatory actions. LX synthesis begins with the oxygenation of arachidonic acid by 15-lipoxygenase to form 15-HETE. Within leukocytes, 15-HETE is converted to LXA4 by 5-lipoxygenase and epoxide hydrase. LXs were first identified by Charles Serhan and colleagues in 1984 from purified fractions of leukocyte suspensions that were coincubated with the ionophore A-23187 and 15-hydroperoxyeicosatetraenoic acid. During the last 20 years, significant efforts have been directed toward identifying the physiological actions of LXs in the inflammatory response and to design metabolically stable analogs that could have been used for therapeutic purposes (Reviewed in this issue by Dr. John Parkinson). In contrast to the proinflammatory eicosanoids, LXs are proposed to act as endogenous "braking signals" in inflammation. Aspirin not only inhibits eicosanoids generation but can also trigger the formation of the 15-epi-LXs that are usually referred to as the aspirin-triggered LXs (ATLs). Thus, cyclo-oxygenase-2 acetylation by aspirin modifies its activity to generate 15Rhydroxyeicosatetraenoic acid, which can be oxygenated to produce 15(R)-epi-LXA4. Similarly to LXA4, ATL exerts potent anti-inflammatory actions. Synthetic ligands of LXA4 and ATLs have facilitated the characterization of distinct receptors for LXA4 that mediate the anti-inflammatory signals. These studies provide evidence that ATLa can directly or indirectly modulate T cell effector function in the setting of Th1- and Th2-dependent inflammation and adaptive immunity. In addition, a surprising interaction of LXs and glucocorticoids has been demonstrated in the context of the broad anti-inflammatory activity of these agents that represent the most potent endogenous anti-inflammatory mediators in mammals (Reviewed by Dr. Mauro Perretti). Nitric oxide (NO) is also regarded as an anti-inflammatory mediator. NO, synthesized from L-arginine by a family of constitutive and inducible NO synthases, is a small, diffusible, highly reactive molecule that serves a variety of functions in the cardiovascular system and accounts for most of the endothelium-dependent vasodilation. NO is considered a double sword mediator, since its excessive generation in the context of inflammatory states leads to vasodilation a key hallmark of inflammation (Reviewed by Dr. Giuseppe Cirino). In addition to controlling vascular tone and vasodilation however, NO also regulates adhesive interactions at the endothelium surface. Thus, exposure of endothelial cells to NO inhibits E-selectin, intercellular adhesion molecule and vascular cell adhesion molecule-1 expression, limiting the release of secretable cytokines IL-6 and IL-8 and prevents nuclear translocation of nuclear factor (NF)- B, suggesting that similar to LXA4 and ATLs, NO might act as a braking signal in regulating vascular inflammation. Development of anti-inflammatory agents that limit eicosanoid production and release NO seems the logical exploitation of our increasing understanding of the cross-talk between pro- and anti-inflammatory mediators. In the recent years, several of these agents, called the COX-inhibitor NO-donating agents (CINODs) have been reported (Reviewed by Dr. Stefano Fiorucci). Interaction of NO-releasing aspirin with the LXs system, and the ability of ATLs to trigger NO formation (Reviewed by Dr. John Wallace), highlight the tight interaction between these two functionally distinct families of mediators. This issue is a journey through these mediators and describes how exploitation of anti-inflammation can be used to treat human diseases. A special thanks goes to all the authors who have accepted the invitation to spend some of their time in writing these reviews, and to Samina Khan, the Senior Manager Publications at Bentham Science Publishers Ltd.

Lipoxins (LX) and their 15-epimers, aspirin triggered lipoxins (ATL) are emerging as major promoters of resolution of the inflammatory reaction. These eicosanoids, that carry a tetraene chromophore, derive from sequential lipoxygenase (LO) metabolism of arachidonic acid. Three principal routes of LX/ATL biosynthesis have been uncovered. One involves cooperation between 15- and 5-LO, one other requires interactions between 12-and 5-LO and a third is characterized by 5-LO transformation of intermediary products generated by aspirin-acetylated cyclooxygenase (COX)-2. Thus, in a large majority of cases the biosynthesis of these eicosanois requires transcellular metabolic exchange during cell-cell interactions. LX and ATL are rapidly metabolized and inactivated by monocyte 15-hydroxyprostaglandin dehydrogenase (PGDH). A number of stable analogs that resist inactivation and retain biological activity has been synthesized. Accumulating evidence suggests that these analogs may have a potential therapeutic impact in a variety of diseases characterized by neutrophil-mediated persistent inflammation, such as reperfusion injury, gastro-intestinal and renal inflammatory disorders, periodontitis. Clinical evaluation of LXA4 and 15-epi-LXA4 formation and their pharmacological regulation may be now achieved using recently developed ELISA assays, that allow large-scale measurements in human biological fluids.

Lipoxin A4 (LXA4) and lipoxin B4 (LXB4) were first identified in 1984 by Serhan and colleagues as 5- and 15- lipoxygenase interaction products of activated leukocytes. Endogenous transcellular biosynthesis of LXA4 and LXB4 occurs via interaction of leukocytes with epithelium, endothelium or platelets. Acetylation of cyclooxygenase-2 (COX-2) by aspirin can trigger 15-epi-LXA4 (ATL) biosynthesis. Elucidating the pharmacological actions of lipoxins and ATL was facilitated by total synthesis of LXA4 in 1988 by Nicolaou and colleagues. In 1994, Fiore and colleagues used [3H]-LXA4 to identify the cDNA for a human G-protein-coupled, high affinity LXA4 and ATL receptor (ALX-R/FPRL-1), providing the first hints for the molecular basis of lipoxin actions. The recognition that lipoxins and ATL undergo rapid, prostaglandin dehydrogenase (PGDH)-mediated metabolic inactivation led do the design and synthesis of first-generation PGDH-resistant LXA4, LXB4 and ATL analogs in 1995-1998 by Serhan, Petasis and colleagues. These relatively stable pharmacological agents, together with myeloid-specific ALX-R-expressing transgenic mice, have provided powerful tools to explore lipoxin functions in vivo. Here we briefly review the substantial body of evidence supporting the lipoxin→ ALX-R pathway as a novel and potent mechanism for preventing/resolving acute inflammation. Emphasis will also be placed on recent findings that lipoxins play new roles in "immunomodulation" via regulation of macrophage, dendritic cell, and T-lymphocyte effector functions in the setting of polarized T-helper cell responses (Th1 and Th2). These studies suggest roles for lipoxins as novel regulators of allergy and adaptive immunity and that lipoxins may have therapeutic potential in chronic immune disorders.

This review will emphasize the concept of anti-inflammation and propose that better understanding of the resolution phase of the inflammatory response organized by the host against inflammatory insults can lead to the identification of novel targets for drug development. Under the umbrella of anti-inflammation, we discuss here recent discoveries in the biology of annexin 1 and glucocorticoids. The fact that annexin 1 and another anti-inflammatory mediator, lipoxin A4, converge onto a specific membrane receptor (termed ALX) might help understanding the resolution phase of inflammation, and strengthen the use of this target for innovative drug development. In addition, glucocorticoids (GC), historically linked to annexin 1, are widely use in the clinical control of several pathologies with an inflammatory etiology, though their use is often burdened by several side effects. The development of new GC with more specific or improved mechanisms, e.g. the nitro-steroids, would go along this wave and, potentially, could be of large applicability. The two mediators/targets here illustrated are to be taken as examples of the clues that the study of anti-inflammation might give to the pharmaceutical industry for innovative drug discovery.

There are several pre-clinical studies on the involvement of NO in inflammation. From this large amount of information it is clear that virtually every cell and many immunological parameters are modulated by NO. Thus, the final outcome is that NO cannot be rigidly classified as an anti-inflammatory or pro-inflammatory molecule. This peculiar aspect of the pathophysiology of NO has hampered the development of new drugs based on the concepts developed. Recent therapeutic approach are targeted to increase endogenous NO by activating the gene and some promising early data are available. At the present stage one of the most promising approach in the inflammation field is represented by a new class of NO-releasing compounds namely NO-NSAIDs that have recently enrolled in phase 2 clinical studies.

Non-steroidal anti-inflammatory drugs (NSAIDs) and cyclo-oxygenase (COX)-2 selective inhibitors (COXIBs) are widely used drugs. However, their use is hampered by gastrointestinal, cardiovascular and renal side effects. Nitric oxide (NO)-releasing NSAIDs, NO-NSAID, are a new class of anti-inflammatory and analgesic drugs generated by adding a nitroxybutyl or a nitrosothiol moiety to the parent NSAID via a short-chain ester linkage. While efficacy of nitrosothiol-NSAIDs still awaits investigation, nitroxybutyl-NO-NSAIDs have been extensively studied in humans. The combination of balanced inhibition of the two main COX isoforms with release of NO confers to NO-NSAIDs reduced gastrointestinal and cardiorenal toxicity. It is suggested that the NO, which is released as the compounds are broken down, may counteract the consequences of the NSAID-induced decrease in gastric mucosal prostaglandins. Recent clinical trials with NO-NSAIDs have provided data consistent with pre-clinical observations.

The use of nonsteroidal anti-inflammatory drugs is associated with an incidence of severe gastrointestinal adverse events of 2-4%, the most common of which is bleeding. These events are largely attributable to the ability of these drugs to suppress prostaglandin and thromboxane synthesis. Prostaglandins perform a number of important functions in the gastrointestinal tract, particularly with respect to resistance of the mucosa to injury. Nitric oxide also appears to be a key mediator of gastrointestinal mucosal defence, and this has been exploited in the development of a novel class of anti-inflammatory drugs, called "NO-NSAIDs", which exhibit little if any gastrointestinal toxicity. NONSAIDs are more effective than traditional NSAIDs in reducing pain and inflammation. Another class of inflammatory mediators that contribute to gastrointestinal mucosal defence are the lipoxins. These products of arachidonic acid metabolism can increase the resistance of the stomach to the damaging effects of aspirin. Indeed, aspirin can trigger the formation of a lipoxin by the stomach which acts to diminish the damaging effects of this drug. Lipoxins and nitric oxide are important mediators of mucosal defence in the stomach (and elsewhere in the gastrointestinal tract) and represent attractive therapeutic targets for reducing the incidence of gastric ulceration.