Current Pharmaceutical Design - Volume 12, Issue 27, 2006

The most fascinating aspect in the process of drug development is the astonishing modification of perspective from which a newly discovered pharmaceutical agent can be viewed. Sometimes it may only be a matter of definition as to whether a pharmaceutical action is labeled as effect or side effect. Primary effects become side effects, and side effects may soon become the new focus of research. One of the most intriguing stories in this context has been told by phospodiesterase 5 (PDE 5)- inhibitors. PDE-5 is one of eleven members of the mammalian phosphodiesterase family that hydrolyzes cyclic guanosine monophosfate (cGMP) and cyclic adenosine monophosphate. PDE-5 degrades cGMP in smooth muscle cells, reducing the muscular tone. In addition, PDE-5 is involved in other physiological processes, such as neurogenesis and apoptosis. PDE-5 inhibitors were initially developed as new therapeutic principles for the treatment of cardiovascular disease. When sildenafil was investigated in clinical studies on coronary artery disease, it turned out to be of limited anti-anginal potential. However, a substantial number of volunteers, participating in the initial studies, reported a notable side effect: sildenafil appeared to enhance penile erections. Soon further research focused on this particular side effect, and erectile dysfunction (ED) became the first indication for which sildenafil was approved shortly thereafter in 1998. Sildenafil, vardenafil and tadalafil, are now approved for the treatment of ED. However, for their action on the cardiovascular system, new therapeutic applications have been proposed for PDE-5 inhibitors in the treatment of some specific cardiovascular disease, such as pulmonary arterial hypertension. This issue of Current Pharmaceutical Design focuses on the most recent knowledges regarding PDE-5 inhibitors, from molecular and biochemical aspects of PDE-5 to pharmacological properties and therapeutic applications of PDE-5 inhibitors. Lin et al. [1] carefully analyse the tissue expression, distribution and regulation of PDE-5. In their analysis the authors intend not only to inspire the development of the “next generation” PDE-5-specific inhibitors but also the design of drugs targeting other PDEs. Supuran et al. [2] describe PDE-5 inhibitors differentiation based on receptorial selectivity, chemical structure, pharmacokinetic and efficacy profile. Aversa et al. [3] report a systematic review on the use of PDE-5 inhibitors in ED, with a careful clinical differentiation among PDE-5 inhibitors according to the most recent and selected literature in the field. About 70% of ED population report the presence of one or more comorbidities (i.e., hypertension, diabetes, cardiovascular disease, dyslipidemia) which may impair endothelial function. The recent discovery that chronic not on-demand administration of PDE- 5 inhibitors may improve erectile and endothelial response in men previously unresponding to on-demand regimes, opens a new scenario in the treatment of men with ED and comorbidities. Reffelman and Kloner [4] discuss in their review about basic mechanisms, pharmacological and patho-physiological aspects of PDE-5 inhibitors, including theoretical concepts on adverse cardiovascular effects. Moreover, they analyse the effects of PDE-5 inhibitors in the cardiovascular system, as measured in various human and animal studies, with respect to both descriptive studies and statistical evaluations. They speculate also on novel therapeutic applications of these pharmaceutical agents in various cardiovascular diseases. In June 2005 sildenafil was approved in the United States for treatment of pulmonary arterial hypertension.Furthermore, recent basic science reports focus on potentially direct cardioprotective, preconditioning-like and anti-apoptotic effects, which might open another interesting field of research close to the initial roots in cardiovascular pathophysiology. It's my hope that the issue be helpful for the scientific and clinical community working in this area. I would like to thank all the authors for their important contributions. References [1] Lin CS, Lin G, Xin ZC, Lue TF. Expression, Distribution and Regulation of Phosphodiesterase 5. Curr Pharm Des 2006; 12(27): 3439-3457. [2] Supuran C, Mastrolorenzo A, Barbaro G, Scozzafava A. PDE5 Inhibitors - Drug design and differentiation based on selectivity, pharmacokinetic and efficacy profiles. Curr Pharm Des 2006; 12(27): 3459-3465. [3] Aversa A, Bruzziches R, Pili M, Spera G. Phosphodiesterase 5 inhibitors in the treatment of erectile dysfunction. Curr Pharm Des 2006; 12(27): 3467-3484. [4] Reffelmann T, Kloner RA. Cardiovascular effects of phosphodiesterase 5 inhibitors. Curr Pharm Des 2006; 12(27): 3485-3494.

Phosphodiesterase 5 (PDE5) is one of eleven members of the mammalian phosphodiesterase family that hydrolyzes cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP). Best known as the target of the impotence drug sildenafil, PDE5 degrades cGMP in smooth muscle cells so as to maintain the contracted state of contractile organs such as the penis, blood vessels, uterus, and intestines. In addition, it regulates numerous other physiological processes such as neurogenesis and apoptosis. Like all other PDEs, PDE5 is dimeric; each subunit is approximately 100 kd in size and has two allosteric cGMP-binding sites and a catalytic domain. Protein kinase G (PKG)- mediated phosphorylation and allosteric cGMP binding upregulate PDE5 activity, while PP1 phosphatase-mediated dephosphorylation downregulates. Sildenafil and other selective inhibitors inhibit PDE5 by binding to the catalytic site. From two promoters a single PDE5A gene at human chromosome 4q26 encodes three alternatively spliced isoforms (PDE5A1-3) that differ in the N-terminus. The PDE5A promoter is located upstream of the three isoform-specific first exons (in the order of A1-A3-A2) and consists of a 139-bp core, a 308-bp upstream enhancer, and a 156-bp downstream enhancer. The weaker 182-bp PDE5A2 promoter is located between the A3- and A2-specific exons and contains an indispensable Sp1-binding sequence. Both promoters are responsive to cGMP or cAMP stimulation, and several studies have demonstrated regulation of PDE5 expression possibly through these promoters. Virtually all tissues and cell types express PDE5, with heart and cardiomyocytes being contentious. PDE5A1 and PDE5A2 are ubiquitous, but PDE5A3 is specific to smooth muscle.

The discovery that inhibition of phosphodiesterase-5 (PDE5) reduces the degradation of cGMP, allowing erectile function to occur by relaxation of penile smooth muscle, represents a revolutionary approach or the treatment of erectile dysfunction (ED). Three PDE5 inhibitors (sildenafil, tadalafil, and vardenafil) are clinically available at this time, and extensive drug design efforts are registered for finding agents with a better activity, enhanced selectivity and reduced side effects. Many classes of such compounds have been reported, belonging to diverse chemical entities. The drug design has been very much facilitated after the report of the X-ray crystal structure of the PDE5 catalytic domain in complex with the three clinically used derivatives. PDE5 inhibitor therapy, has been found to be effective in special clinical populations, such as those with prostate cancer, diabetes, and cardiovascular disease. The duration of action of sildenafil and vardenafil is of about 4 hours, whereas that of tadalafil is of about 36 hours, and the overall safety of the treatments is good. There is a risk of hypotension if nitrates are given concurrently with the PDE5 inhibitors. Common side-effects include headache, facial flushing, nasal congestion, dyspepsia and transient visual impairment. There are pharmacological interactions between these drugs and other medications metabolized by the cytochrome P450 (P3A4 isoform), such as the azole antifungals, erythromycin and the HIV protease inhibitors.

Erectile dysfunction (ED) has multifactor pathogenesis, with neurological, vascular, endocrinological and psychogenic components described. However, about 50-85% of ED population report the presence of one or more comorbidities i.e. hypertension, diabetes, cardiovascular disease, dyslipidemia which all impair endothelial function and, erection is a basically vascular event that necessitates an intact endothelium to occur. Hence, ED may be mostly considered as the clinical manifestation of a disease affecting penile circulation as a part of a generalized vascular disorder due to atherosclerosis. Orally active drugs, i.e. phosphodiesterase type-5 inhibitors (PDE5-i), are a group of on-demand drugs licensed for ED treatment and appear to offer advantages over past therapies in terms of ease of administration and cost, and they are now widely advocated as first-line therapy. The recent discovery that chronic not on-demand administration of these drugs may improve erectile and endothelial response in men previously unresponding to on-demand regimes, opens a new scenario in the treatment of men with ED and comorbidities. Finally, the recent approval of PDE5-i sildenafil for the treatment of pulmonary arterial hypertension represents the new challenge for these class of drugs. Aim of this article will be to provide an update on the pathophysiology of ED and how to use of different available PDE5-i in approaching sexual dysfunctional men, pointing out on their characteristic of efficacy and safety and different indications in special subpopulations.

Phosphodiesterase 5 inhibitors, such as sildenafil, vardenafil and tadalafil, are now approved for the treatment of erectile dysfunction. They inhibit the cGMP-specific isoform 5 of phosphodiesterase, resulting in cGMP accumulation, which, for example in smooth muscle cells, reduces muscular tone. In the cardiovascular system, they slightly reduce arterial systemic blood pressure. This moderate effect was also shown in combination with many antihypertensive drugs. But the important contraindication is the concomitant use of PDE 5 inhibitors with any drug serving as a nitric oxide donor, as this combination can lead to significant arterial hypotension. Caution is needed in patients on alpha-blocking agents. In general, this class of drugs was not shown to exhibit direct deleterious effects on the myocardium or promote arrhythmias. Furthermore, statistical evaluations did not demonstrate an increased risk for patients taking PDE 5 inhibitors in comparison with an adequate control population. Many patients suffering from erectile dysfunction may be characterized by multiple cardiovascular risk factors or even ischemic heart disease, suggesting an increased baseline risk. While in many forms of erectile dysfunction, these agents seem to be very effective, it becomes clear that endothelial dysfunction is an attractive target of PDE 5 inhibitors and may also be the underlying cause in many types of erectile dysfunction. In addition, these agents seem to be very effective in lowering pulmonary arterial pressure, which might provide the opportunity to treat primary and some forms of secondary pulmonary hypertension, perhaps in combination with inhaled nitric oxide or other pulmonary arterial vasodilators. Sildenafil was approved for treatment of primary arterial hypertension in the U.S. in June 2005. Recently, direct cardioprotective effects were described in animal research, resembling preconditioning-like effects, which may, under certain conditions, also be applicable in clinical research.

In this issue of Current Pharmaceutical Design, new insights and strategies in treating inflammation-related diseases is reviewed by experts. A large number of studies have demonstrated that inflammation plays a critical role in the pathogenesis of various central and peripheral diseases such as age-related neurological disorders, atherosclerosis, sepsis and rheumatoid arthritis. It is well known that the inflammatory reaction is very complex and multifactorial process resulting from the generation of various pro-inflammatory mediators. The issue consists of six reviews. The first review provides recent information on the signal transduction pathways of inflammatory genes expression. Dr. Chen [1] describes the current understanding of the NF-κB activation pathway, a key factor of inflammatory signal transduction, and the possible signal pathways of intercellular adhesion molecule-1 (ICAM-1) and cyclooxygenase-2 (COX-2). Furthermore, some potential anti-inflammatory natural products such as flavonids modulating ICAM-1 and COX-2 expression are also discussed. Dr. Yossi et al. [2] reviews the pathological role of inflammation in neurodegenerative diseases including Alzheimer's and Parkinson's diseases. In addition, authors also discuss the possible reasons accounting for the equivocation in clinical trials of using anti-inflammatory drugs (AIDs). Accordingly, administration of a AIDs with better blood brain barrier (BBB) transport and more selectivity for COX-2 at correct timing may be more effective in treating neurodegenerative diseases. Dr. Wang et al., [3] highlights the association of oxidative stress and neurodegenerative diseases and provide recent evidence confirming the beneficial effect of antioxidants in treating neurodegeneration and suppressing glia-mediated inflammation. Furthermore, the neuroprotective effects of different kinds of antioxidants are discussed and compared, which may be useful for future clinical therapy. Sepsis, a systemic inflammatory response syndrome, is very complicated and heterogeneous accompanied by a high mortality. However, an ideal therapy to improve the survival of patients with sepsis has not yet established. Dr. Wu [4] reviews recent therapies for septic shock including traditional therapies and potential treatments targeting the individual element of the inflammatory cascade or transcription factors based on several animal and clinical studies. Nitric oxide (NO), synthesized by the enzyme nitric oxide synthase (NOS), plays an important regulatory/modulatory role in physiological and pathological conditions. However, high amount of NO produced by inducible NOS (iNOS) may cause tissue damage and organ dysfunction, which is often observed in sepsis and septic shock. Thus, attenuation of excess formation of NO by NO scavengers may be a beneficial strategy in treating septic shock. Dr. Harbrecht [5] describes some characteristics of ideal NO scavengers including specific for binding NO and optimal pharmacodynamic profiles to maximize therapeutic efficacy and minimize side effects. In addition, Dr. Harbrecht also introduces and evaluates the properties of various NO scavengers and their therapeutic effect in animal studies of septic shock. Rheumatoid arthritis (RA) is a progressive debilitating inflammatory disease with an incidence of about 2-3%. The last review by Dr. Cuzzocrea [6] will discuss the role of NO and oxygen-derived free radicals especially superoxide and hydroxyl radical and peroxynitrite in RA and further evaluates the pharmacological effect of superoxide dismutase mimetic (SODm) in arthritis. Current evidence suggest that reducing free radical generation by antioxidants or SOD may be a potential therapy in arthritis. Finally, I would like to thank all authors for their contribution. References [1] Chen CC. Signal transduction pathways of inflammatory genes expression and therapeutic implications. Curr Pharm Design 2006; 12(27): 3497-3508. [2] Yossi GS, Melamed E, Offen D. Anti-inflammatory drugs in the treatment of neurodegenerative diseases: Current state. Curr Pharm Design 2006; 12(27): 3509-3519. [3] Wang JY, Wen LL, Huang YN, Chen YT, Gu MC. Dual effects of antioxidants in neurodegeneration: Direct neuroprotection against oxidative stress and indirect protection via suppression of glia-mediated inflammation. Curr Pharm Design 2006; 12(27): 3521-3533. [4] Wu CC. Possible therapies of septic shock: based on animal studies and clinical trials. Curr Pharm Design 2006; 12(27): 3535-3541. [5] Harbrecht BG. Therapeutic use of nitric oxide scavengers in shock and sepsis. Curr Pharm Design 2006; 12(27): 3543- 3549. [6] Cuzzocrea S. Role of nitric oxide and reactive oxygen species in arthritis. Curr Pharm Design 2006; 12(27): 3551-3570.

Intercellular adhesion molecule-1 (ICAM-1), an inducible cell adhesion glycoprotein of the immunoglobulin supergene family and cyclooxygenase-2 (COX-2), an inducible prostaglandin G/H synthase, are overexpressed by proinflammatory mediators in a wide variety of cell types. These stimuli increase ICAM-1 or COX-2 expression primarily through activation of ICAM-1 or COX-2 gene transcription. The architecture of the ICAM-1 or COX-2 promoter is complex, containing a large number of binding site for inducible transcription factors, the most important of which is NF-κB. NF-κB acts in concert with other transcription factors or transcriptional coactivators which facilitate the assembly of distinct stereospecific transcription complexes on the ICAM-1 or COX-2 promoter. These transcription complexes presumably mediate the induction of ICAM-1 or COX-2 expression in different cell types and in response to different stimuli. In this review, I summarize the current understanding of ICAM-1 and COX-2 gene regulation with a particular emphasis on the transcription factors or coactivators, and signal transduction pathways critical for their expression. A PKC-dependent c-Src pathway activating NF-κB or GAS to enhance ICAM-1 or COX-2 gene expression is discussed. Furthermore, natural products and novel agents targeting on the transcription factor with potential anti-inflammation and anti-tumor activity are also discussed.

Increasing evidence indicates that inflammation is involved in the pathogenesis of many neurological, particularly neurodegenerative diseases. Even if inflammation is not a primary causative process, its presence may contribute to the continued loss of CNS neurons. Therefore, it seems reasonable to propose that use of anti-inflammatory drugs might diminish the cumulative effects of inflammation in the brain. Indeed, some epidemiological studies performed to date, especially in Alzheimer's disease, suggests that sustained use of anti-inflammatory drugs (AIDs) may prevent or slow down the progression of neurodegenerative diseases. However, small number of clinical trials carried out so far using AIDs, were minimal and equivocal in their outcome. Potential reasons for these mixed results include timing of AIDs administration, nonselective inhibition of cyclooxygenase (COX), inappropriate use of particular antiinflammatory drugs for a given disease or disease progression/ severity, sub-optimal dose in target site, or limited penetration to the brain through the blood-brain barrier (BBB). Therefore, design of AIDs for the treatment of neurodegenerative diseases based upon better BBB penetration, and with minimal adverse events, would be appropriate. In addition, relevant genetic differences among patients should be considered planning new AIDs, for improved efficacy. Furthermore, due to the possible co-involvement of oxidative stress and excitotoxicity in the pathogenesis of these diseases, combination therapy with antioxidants or glutamate antagonists or a multi-potent drug might be much more effective in successfully treating neurodegenerative diseases.

Oxidative stress, in which production of highly reactive oxygen species (ROS) and reactive nitrogen species (RNS) overwhelms antioxidant defenses, is a feature of many neurological diseases and neurodegeneration. ROS and RNS generated extracellularly and intracellularly by various processes initiate and promote neurodegeneration in CNS. ROS and RNS can directly oxidize and damage macromolecules such as DNA, proteins, and lipids, culminating in neurodegeneration in the CNS. Neurons are most susceptible to direct oxidative injury by ROS and RNS. ROS and RNS can also indirectly contribute to tissue damage by activating a number of cellular pathways resulting in the expression of stress-sensitive genes and proteins to cause oxidative injury. Moreover, oxidative stress also activates mechanisms that result in a glia-mediated inflammation that also causes secondary neuronal damage. Associated with neuronal injuries caused by many CNS insults is an activation of glial cells (particularly astrocytes and microglia) at the sites of injury. Activated glial cells are thus histopathological hallmarks of neurodegenerative diseases. Even though direct contact of activated glia with neurons per se may not necessarily be toxic, the immune mediators (e.g. nitric oxide and reactive oxygen species, pro-inflammatory cytokines and chemokines) released by activated glial cells are currently considered to be candidate neurotoxins. Therefore, study of the protective role of antioxidant compounds on inhibition of the inflammatory response and correcting the fundamental oxidant/antioxidant imbalance in patients suffering from neurodegenerative diseases are important vistas for further research. The purpose of this review is to summarize the current evidence in support of this critical role played by oxidative stress of neuronal and glial origin in neurodegenerative diseases. The mechanistic basis of the neuroprotective activity of antioxidants does not only rely on the general free radical trapping or antioxidant activity per se in neurons, but also the suppression of genes induced by pro-inflammatory cytokines and other mediators released by glial cells. We propose that combinations of agents which act at sequential steps in the neurodegenerative process can produce additive neuroprotective effects. A cocktail of multiple antioxidants with anti-inflammatory agents may be more beneficial in the prevention of neurodegenerative disease. A clearer appreciation of the potential therapeutic utility of antioxidants would emerge only when the complexity of their effects on mechanisms that interact to determine the extent of oxidative damage in vivo are more fully defined and understood.

The intention of this review is to give a brief overview of the continuously expanding field of sepsis therapy based on recent studies with animal models and clinical trials. Over the past few years, it has become apparent that the mechanisms controlling this disease are more complex than was previously thought, with factors such as free radicals, nuclear factors, and enzyme co-factors all contributing in the control of the pathogenesis of sepsis as well and improvements in the morbidity. Recent advances at the molecular biology level have facilitated the development of a whole new field of research. In addition, a number of groups have also shown that free radicals can modulate the expression of several genes. Probably an effect that is due to an interaction between free radicals and transcription factors. Further elucidation of the signals that influence the production and actions of free radicals will, without doubt, further our understanding of the numerous pathophysiological processes involved in sepsis. For these reasons, there is considerable interest in alternative treatment modalities which focus on recent animal studies. These recent experimental approaches to the therapy of sepsis are discussed in light of each step in the complex inflammatory cascade involved and compared to traditional approaches to the prevention and therapy of sepsis and septic shock.

Nitric oxide (NO) is a reactive radical produced by the enzyme nitric oxide synthase (NOS) and it plays an important role in a large number of biological pathways. NO can be produced under normal physiologic conditions and contribute to homeostasis but, when produced in excess, it may lead to tissue injury and organ dysfunction. The regulation of NOS activity and expression is becoming increasingly understood. NOS enzyme inhibitors as tools to decrease excessive NO synthesis have received the most attention and have been subjected to the greatest experimental study. Compounds that scavenge excessive NO have been developed and have shown promise in a number of experimental models but have received considerably less attention as potential therapeutic agents. In this article, the use of NO scavengers in two conditions in which excessive NO appears to be pathophysiologically significant, shock and sepsis, is reviewed. The relevant biology of NO in these disease states is presented and the therapeutic potential of NO scavengers in clinical care is explored.

A vast amount of circumstantial evidence implicates oxygen-derived free radicals, especially reactive oxygen species and nitric oxide as mediators of inflammation and/or tissue destruction in inflammatory and arthritic disorders. The aim of the current article is to overview the recent developments in this field, as it relates to the roles of nitric oxide (NO) and reactive oxygen species in the pathogenesis of this condition. The first part of the review focuses on the biochemical impact of NO and reactive oxygen species. The second part of the review deals with the novel findings related to the recently identified regulatory roles of the inducible isoform of nitric oxide synthase (iNOS) in the expression of pro-inflammatory mediators in inflammation. Reactive oxygen species can initiate a wide range of toxic oxidative reactions. These include initiation of lipid peroxidation, direct inhibition of mitochondrial respiratory chain enzymes, inactivation of glyceraldehyde-3phosphate dehydrogenase, inhibition of membrane sodium/potassium ATP-ase activity, inactivation of membrane sodium channels, and other oxidative modifications of proteins. All these toxicities are likely to play a role in the pathophysiology of inflammation. Reactive oxygen species are all potential reactants capable of initiating DNA single strand breakage, with subsequent activation of the nuclear enzyme poly (ADP ribose) synthetase (PARS), leading to eventual severe energy depletion of the cells, and necrotic-type cell death. Recently it has been demonstrated that iNOS inhibitor prevents the activation of poly (ADP ribose) synthetase, and prevents the organ injury associated with inflammation. Although the severity and duration of inflammation may dictate the timing and extent of NOS expression, it is now evident that the up-regulation of NOS can take place during sustained inflammation. Thus, induced nitric oxide, in addition to being a "final common mediator" of inflammation, is essential for the up-regulation of the inflammatory response. Furthermore, a picture of a pathway is evolving that contributes to tissue damage both directly via the formation of reactive oxygen species, with them associated toxicities, and indirectly through the amplification of the inflammatory response.