Current Pharmaceutical Design - Volume 12, Issue 8, 2006

In response to hormonal stimulation, phospholipases are activated to release arachidonic acid from membrane phospholipids. Free arachidonate can then metabolized nonenzymatically, contributing to oxidative stress, or through the actions of different types of oxygenase: cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 monooxygenases. The arachidonic acid metabolites produced, or eicosanoids, are a large series of lipidderived mediators capable of producing a multitude of physiologic effects in the local environment. They play important roles in a variety of signaling pathways both in physiologic and pathophysiologic conditions. For many years, arachidonic acid metabolism has become one of the most active area of fundamental and applied research. Researchers and pharmaceutical companies early focused their attention on new agents able to interfere with metabolic steps in the degradation of arachidonic acid or eicosanoid receptors. The aim of this hot topic is to highlight the latest developments in the pharmaceutical design of some specific arachidonic acid pathway metabolites or enzymes. Thus, over a decade ago, Professor Jason D. Morrow reported that a series of prostaglandin (PG)1-like compounds were produced by the free radical-catalyzed peroxidation of arachidonic acid, independent of the cyclooxygenase enzyme. Since then, this researcher and others have accumulated a large body of evidence indicating that quantification of these unique products of lipid peroxidation, now termed isoprostanes, provides a reliable marker of oxidant injury both in vitro and in vivo. In the first review, the mechanisms involved in isoprostanes formation, their biological activities at a cellular level and the future research related to the development of pharmacological approaches to modulate their formation and action in vivo will be discussed [1]. Thromboxane A2 (TXA2) and prostacyclin (PGI2) are two key metabolites produced by the cyclooxygenase pathway via thromboxane synthase and prostacyclin synthase, respectively. TXA2 has been implicated in various pathophysiological conditions due to its potent activating effects on platelet aggregation and smooth muscle contraction. In the second review of this hot topic, Dr. Jean-Michel Dogné, co-guest editor, aims to describe the physiological properties of TXA2, thromboxane synthase and thromboxane receptors [2]. Two sections are dedicated to a presentation of each class of TXA2 modulators with the advantages and disadvantages they offer and to recent studies performed with the most interesting TXA2 modulators in major pathologies such as myocardial infarction and thrombosis, atherosclerosis, diabetes, pulmonary embolism, septic shock, preeclampsia, and asthma. PGI2 is the main arachidonic acid metabolite in vascular walls and has opposing biological properties to TXA2. Indeed, PGI2 represents the most potent endogenous inhibitor of platelet aggregation and is also a strong antihypertensive agent through its vasodilatory effects on vascular beds. Understanding the molecular mechanisms of PGI2 biosynthesis and signaling is crucial to the development of therapeutic approaches to regulate PGI2 functions. Thus, Dr. Ke-He Ruan, co-guest editor provides information regarding the most current advances in the findings of the molecular mechanisms for PGI2 biosynthesis and for PGI2 signaling through its cell membrane receptors and nuclear peroxisome proliferator-activated receptors [3]. Prostaglandin E2 is the most common prostanoid with a variety of bioactivities and has been implicated in various pathologies. Prostaglandin E synthase (PGES), which converts cyclooxygenase (COX)-derived prostaglandin H2 to PGE2, occurs in multiple forms with distinct enzymatic properties, modes of expression, cellular and subcellular localizations and intracellular functions. In their review, Makoto Murakami and Dr. Ichiro Kudo highlight the latest understanding of the expression, regulation and functions of these three PGES enzymes, in particular mPGES-1 of which recent gene targeting studies have revealed that this enzyme represents a novel target for anti-inflammatory and anti-cancer drugs [4]. Prostaglandins, PGI2, TXA2 and lipoxins are rapidly metabolized by initial oxidation of their 15(S)-hydroxyl group catalyzed by NAD+-linked 15-hydroxyprostaglandin dehydrogenase (15-PGDH). The 15-keto products of this enzyme exhibit greatly reduced biological activities. Therefore, this enzyme has been considered the key enzyme responsible for the inactivation of these biologically active eiocosanoids. Moreover, studies on the regulation of enzyme expression and activity by physiological and pharmacological agents have begun to uncover its significant roles in cancer, inflammation and reproduction. In the fifth review, Dr. Hsin-Hsiung Tai provides insight into structural characterization, transcriptional regulation, biological functions and catalytic mechanism of this enzyme...... [5].

The isoprostanes (IsoPs) are a series of novel prostaglandin-like compounds formed in vivo in humans from the free radical-catalyzed peroxidation of arachidonate independent of the cyclooxygenase. While quantification of these compounds is a highly accurate measure of oxidant stress in vivo in humans, IsoPs also possess potent biological activity and likely mediate certain aspects of oxidative injury. The purpose of this review is to summarize selected aspects of our knowledge regarding the bioactivity of the IsoPs. I will first briefly highlight mechanisms involved in IsoP formation. Subsequently, I will discuss the biological activities of certain IsoPs that are formed in abundance in vivo and focus on two compounds, 15-F2t-IsoP and 15-E2t-IsoP, that have been studied in the greatest detail. This review will then examine, at a molecular level, mechanisms by which IsoPs exert their bioactivity. It has been shown that they are ligands for various eicosanoid receptors, in particular, the thromboxane receptor. In addition, I will discuss the controversial evidence that a unique IsoP receptor(s) exists. Finally, I will offer avenues for future research related to the development of pharmacological approaches to modulate IsoP formation and action in vivo and thus decrease the pathophysiological sequelae of oxidative injury.

Arachidonic acid (AA) metabolites are key mediators involved in the pathogenesis of numerous cardiovascular, pulmonary, inflammatory, and thromboembolic diseases. One of these bioactive metabolites of particular importance is thromboxane A2 (TXA2). It is produced by the action of thromboxane synthase on the prostaglandin endoperoxide H2 (PGH2) which results from the enzymatic transformation of AA by the cyclooxygenases. It is a potent inducer of platelet aggregation, vasoconstriction and bronchoconstriction, and has been involved in a series of major pathophysiological conditions. Therefore, TXA2 receptor antagonists, thromboxane synthase inhibitors and drugs combining both properties have been developed by different laboratories since the early 1980s. Several compounds have been launched on the market and others are under clinical evaluation. In the first part of this review, we will describe the physiological properties of TXA2, thromboxane synthase and thromboxane receptors. The second part is dedicated to a description of each class of thromboxane modulators with the advantages and disadvantages they offer. In the third part, we aim to describe recent studies performed with the most interesting thromboxane modulators in major pathologies: myocardial infarction and thrombosis, atherosclerosis, diabetes, pulmonary embolism, septic shock, preeclampsia, and asthma. Each pathology will be systematically reviewed. Finally, in the last part we will highlight the latest perspectives in drug design of thromboxane modulators and in their future therapeutic applications such as cancer, metastasis and angiogenesis.

Prostacyclin (PGI2) is one of the major vascular protectors against thrombosis and vasoconstriction, caused by thromboxane A2. Understanding the molecular mechanisms of PGI2 biosynthesis and signaling is crucial to the development of therapeutic approaches to regulate PGI2 functions. This review provides information regarding the most current advances in the findings of the molecular mechanisms for PGI2 biosynthesis in the endoplasmic reticulum (ER) membrane through the coordination between PGI2 synthase and its upstream enzymes, cyclooxygenase-1 (COX-1) or -2 (COX-2), and for PGI2 signaling through its cell membrane receptors and nuclear peroxisome proliferator-activated receptors. The substrate presentation from the COXs to PGI2 synthase and its cell membrane receptor/G protein coupling sites, as characterized by our group, are discussed in detail. The association between the regulation of the biosynthesis and signaling of PGI2 with the pathophysiological processes of PGI2-related diseases is also discussed. The molecular knowledge of PGI2 biosynthesis and signaling will help to design the next generation of drugs, specifically targeting the regulation of PGI2 functions, which will undoubtedly provide advances in cardiovascular protection and the treatment of PGI2-related diseases.

Prostaglandin E synthase (PGES), which converts cyclooxygenase (COX)-derived prostaglandin (PG) H2 to PGE2, occurs in multiple forms with distinct enzymatic properties, modes of expression, cellular and subcellular localizations and intracellular functions. Two of them are membrane-bound enzymes and have been designated as mPGES-1 and mPGES-2. mPGES-1 is a perinuclear protein belonging to the MAPEG (for membrane-associated proteins involved in eicosanoid and GSH metabolism) family. This enzyme is markedly induced by proinflammatory stimuli, is down-regulated by anti-inflammatory glucocorticoids, and is functionally coupled with cyclooxygenase (COX)-2 in marked preference to COX-1. mPGES-2 is synthesized as a Golgi membrane-associated protein, and the proteolytic removal of the N-terminal hydrophobic domain leads to the formation of a mature cytosolic enzyme. This enzyme is rather constitutively expressed in various cells and tissues and is functionally coupled with both COX-1 and COX-2. Cytosolic PGES (cPGES) is constitutively expressed in a wide variety of cells and is functionally linked to COX-1 to promote immediate PGE2 production. This review highlights the latest understanding of the expression, regulation and functions of these three PGES enzymes. In particular, recent gene targeting studies of mPGES-1 have revealed that this enzyme represents a novel target for anti-inflammatory and anti-cancer drugs.

NAD+-linked 15-hydroxyprostaglandin dehydrogenase (15-PGDH) catalyzes the oxidation of 15(S)-hydroxyl group of prostaglandins and lipoxins resulting in the formation of 15-keto metabolites which exhibit greatly reduced biological activities. Therefore, this enzyme has been considered the key enzyme responsible for the inactivation of prostaglandins and lipoxins. Both the cDNA and the genomic DNA of the 15-PGDH gene have been cloned. Structural characterization, transcriptional regulation and biological functions of this enzyme have been investigated. Molecular modeling corroborated with site-directed mutagenesis has identified key residues and domains involved in coenzyme and substrate binding. Catalytic mechanism has been proposed. Studies on the regulation of enzyme expression and activity by physiological and pharmacological agents have begun to uncover its significant roles in cancer, inflammation and reproduction. Apparently, 15-PGDH works with cyclooxygenase-2 to control the cellular levels of prostaglandins. Their reciprocal regulation within the same cells appears to determine the fate of the cells. Because of its ability to inactivate both prostaglandins and lipoxins of two opposite biological activities, the roles of 15-PGDH in cancer and inflammation are particularly intriguing and challenging. Future investigations in these areas are warranted.

We have chemically synthesized several stable analogs of the naturally occurring hepoxilins, 12-LO products derived from arachidonic acid, which we found to have promising actions in a variety of test systems of disease. The analogs, PBTs, afford chemical and biological stability to the hepoxilin molecule. This article reviews some of our latest observations with the PBTs in the areas of inflammation (inhibition of the bleomycin-evoked lung fibrosis in mice in vivo), platelet aggregation (antagonism of the thromboxane receptor in human platelets in vitro) and thrombosis (inhibitors in vivo), and cancer (apoptosis of the human leukemia cell line, K562 in vitro and in vivo). The demonstration that the PBTs are active in vivo suggests that they can serve as a platform for their further development as novel therapeutics in disease.

Since the discovery of COX-2, a second subtype of cyclooxygenase, selective inhibitors or "coxibs" were developed with the idea that this isoform was inducible at the site of inflammation whereas COX-1 was expressed constitutively in several tissues including gastric epithelium. This new class of non steroidal anti-inflammatory agents was though to be safer for ulcerations of the gastroinstestinal mucosa observed with non selective COX-2 inhibitors. Nevertheless, at the end of September 2004, Merck & Co announced the voluntary withdrawal of rofecoxib (Vioxx) worldwide because of an increased risk of cardiovascular events. This decision raised serious concerns about safety of selective COX-2 inhibitors which are actively marketed today, and the ones currently under development. The mechanism of this cardiovascular toxicity could lie in the inhibition of COX-2 itself, and thus be a class effect. On the other hand, these cardiovascular side effects could be limited on rofecoxib and be dependent on its chemical and/or pharmacological own properties. This hypothesis is undermined by the unexpected findings of one colon cancer study which has shown that celecoxib might also increase the chance of heart attack and stroke in some patients. In this review, we compared the different coxibs marketed to date on the basis of their clinical, pharmacological and chemical properties with the aim of providing some clues in the understanding of their potential or revealed "cardiovascular effects".

The evolution of research on drug protein binding is discussed with the unbound concentration (Cu) and the unbound fraction (fu) as protagonists. Particular attention is paid to the mechanisms via which alterations in binding affect the pharmacokinetics (PK) and the effect, or independently the pharmacodynamics (PD). Apart from albumin, the important α-acid glycoprotein (AGP), as well as specific drug classes and applications in the clinic and development (routine monitoring, cancer and HIV therapy, allometry) are addressed. The flaws with the classical method of indirectly calculating the Cu or the unbound PK/PD parameters, based on the fu in vitro, are related to the intrinsic complexity and variability in the outcomes. Increased focus is urged on directly estimating the unbound PK/PD and also on using population statistical methods.

Innovations in biotechnology have made possible the development of several new systemic therapies for psoriasis - the "biologicals", a new group of compounds including monoclonal antibodies, fusion proteins and recombinant proteins. These novel biotechnological advances potentially offer designer drugs, which interfere with specific targets in the pathophysiological network of psoriasis and are thus much safer. The therapeutic strategy of biologicals is based on the knowledge derived from pathogenetic studies, which have focused on targeting disease relevant T-cell- or mediatorsystems. Important targets include inactivation of soluble mediators such as tumor-necrosis-factor-α, the blockade of receptors for cytokines, adhesion molecules and the interference with T-cell activation by antigen-presenting cells. In addition, recombinant cytokines are able to modulate the immunological balance of this chronic inflammatory skin disease. Currently, up to forty agents are under investigation for the treatment of psoriasis. Four of these agents, alefacept, efalizumab, etanercept and infliximab have already impacted on routine clinical practice. Current developments in the treatment of psoriasis with biologicals are reviewed.

Beta-endorphin were detected in the endocrine pancreas and seem able to influence insulin and glucagon release. Hence, endogenous opioids could have a role in glucoregulation and in the pathogenesis of obesity beyond the previously detected effects on appetite. Metabolic abnormalities, such as hyperinsulinemia, insulin-resistance and obesity, are common features of polycystic ovary syndrome (PCOS), and seem to have a pathogenetic role in this disorder. A link between opioids and PCOS-related hyperinsulinism is suggested by the finding of altered central opioid tone and elevated β-endorphins levels, directly correlated with body weight, in these patients. Furthermore, naloxone and naltrexone significantly reduce the insulin response to glucose load only in hyperinsulinemic PCOS patients. This effect is obtained chiefly through an improvement of insulin clearance. Naltrexone is also able to ameliorate the abnormal gonadotrophins secretion and to improve the ovarian responsiveness in obese PCOS women undergoing ovulation induction with exogenous GnRH. Such effects are believed to be obtained through an amelioration of hyperinsulinemia. Gonadal steroids modulate the opioid system both centrally and in peripheral districts. Nevertheless, the decline of ovarian function does not abolish the opioidergic control of glucoregulation. Post-menopausal period is characterised by a high prevalence of hyperinsulinemia and insulin-resistance. In particular, an association between hyperinsulinemia and increased opioid activity was found in postmenopausal women showing a central body fat distribution. Both naloxone and naltrexone ameliorate the metabolic imbalance also when it appears in the climacteric period, and mainly by increasing insulin clearance. The benefits of naltrexone may represent in the future a useful tool for the treatment of women with hyperinsulinism in the clinical practice.