Phosphodiesterase inhibitors: new opportunities for the treatment of asthma

cAMP-PDE signaling in COPD: Review of cellular, molecular and clinical features

Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of death among non-contagious diseases in the world. PDE inhibitors are among current medicines prescribed for COPD treatment of which, PDE-4 family is the predominant PDE isoform involved in hydrolyzing cyclic adenosine monophosphate (cAMP) that regulates the inflammatory responses in neutrophils, lymphocytes, macrophages and epithelial cells The aim of this study is to investigate the cellular and molecular mechanisms of cAMP-PDE signaling, as an important pathway in the treatment management of patients with COPD. In this review, a comprehensive literature review was performed about the effect of PDEs in COPD. Generally, PDEs are overexpressed in COPD patients, resulting in cAMP inactivation and decreased cAMP hydrolysis from AMP. At normal amounts, cAMP is one of the essential agents in regulating metabolism and suppressing inflammatory responses. Low amount of cAMP lead to activation of downstream inflammatory signaling pathways. PDE4 and PDE7 mRNA transcript levels were not altered in polymorphonuclear leukocytes and CD8 lymphocytes originating from the peripheral venous blood of stable COPD subjects compared to healthy controls. Therefore, cAMP-PDE signaling pathway is one of the most important signaling pathways involved in COPD. By examining the effects of different drugs in this signaling pathway critical steps can be taken in the treatment of this disease.

Cyclic nucleotide phosphodiesterase inhibition as a potential therapeutic target in renal ischemia reperfusion injury

AIMS

Ischemia/reperfusion (I/R) occurs in renal artery stenosis, partial nephrectomy and most commonly during kidney transplantation. It brings serious consequences such as DGF (Delayed Graft Function) or organ dysfunction leading to renal failure and ultimate death. There is no effective therapy to handle the consequences of Renal Ischemia/Reperfusion (I/R) injury. Cyclic nucleotides, cAMP and cGMP are the important second messengers that stimulate intracellular signal transduction for cell survival in response to growth factors and peptide hormones in normal tissues and in kidneys plays significant role that involves vascular tone regulation, inflammation and proliferation of parenchymal cells. Renal ischemia and subsequent reperfusion injury stimulate signal transduction pathways involved in oxidative stress, inflammation, alteration in renal blood flow leading to necrosis and apoptosis of renal cell.

MATERIALS AND METHODS

An extensive literature review of various search engines like PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was carried out. To understand the functioning of Phosphodiesterases (PDEs) and its pharmacological modulation in Renal Ischemia-Reperfusion Injury.

KEY FINDINGS

Current therapeutic options may not be enough to treat renal I/R injury in group of patients and therefore, the current review has discussed the general characteristics and physiology of PDEs and preclinical-studies defining the relationship between PDEs expression in renal injury due to I/R and its outcome on renal function.

SIGNIFICANCE

The role of PDE inhibitors in renal I/R injury and the clinical status of drugs for various renal diseases have been summarized in this review.

New Avenues for Phosphodiesterase Inhibitors in Asthma

Introduction

Phosphodiesterases (PDEs) are isoenzymes ubiquitously expressed in the lungs where they catalyse cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (GMP), which are fundamental second messengers in asthma, thereby regulating the intracellular concentrations of these cyclic nucleotides, their signaling pathways and, consequently, myriad biological responses. The superfamily of PDEs is composed of 11 families with a distinct substrate specificity, molecular structure and subcellular localization. Experimental studies indicate a possible role in asthma mainly for PDE3, PDE4, PDE5 and PDE7. Consequently, drugs that inhibit PDEs may offer novel therapeutic options for the treatment of this disease.

Areas Covered

In this article, we describe the progress made in recent years regarding the possibility of using PDE inhibitors in the treatment of asthma.

Expert Opinion

Many data indicate the potential benefits of PDE inhibitors as an add-on treatment especially in severe asthma due to their bronchodilator and/or anti-inflammatory activity, but no compound has yet reached the market as asthma treatment mainly because of their limited tolerability. Therefore, there is a growing interest in developing new PDE inhibitors with an improved safety profile. In particular, the research is focused on the development of drugs capable of interacting simultaneously with different PDEs, or to be administered by inhalation. CHF 6001 and RPL554 are the only molecules that currently are under clinical development but there are several new agents with interesting pharmacological profiles. It will be stimulating to assess the impact of such agents on individual treatable traits in specially designed studies.

Relaxation effects of Eriobotrya japonica toward tracheal smooth muscle via action mechanism on histamine-1 receptor and phosphodiesterase-5 enzyme

The Eriobotrya japonica leaves have the activity to relax the smooth muscle in the respiratory tract. However, the mechanism of action due to that activity has never been carried out. This study aims to determine the relaxation effects of E. japonica leaves extract in the isolated trachea of the guinea pigs through the inhibition of the histamine-1 (H-1) receptor and the phosphodiesterase-5 (PDE-5) enzyme. The determination of the relaxation effects was carried out by using histamine to contract smooth muscle within the tracheal tract, followed by adding cumulative concentrations of extract. Michaelis–Menten kinetics equation was used to determine the antagonist type of extract toward H-1 receptor. The understanding of mechanism of action of the extract toward PDE-5 enzyme was performed by incubating the smooth muscle using sildenafil. The percentage value of responses, originated from the relaxation effect of the extract toward the trachea was analyzed by using the t-independent test. The result showed that the extract was able to relax the smooth muscle, which was contracted by histamine, and there was a positive correlation between concentration and relaxation effect (P < 0.05; r = 0.973). The extract also antagonized the histamine as a noncompetitive antagonist. The incubation within the trachea with sildenafil demonstrated equal relaxation effect, produced by the extract. It can be concluded that E. japonica extract had relaxation effect within the isolated trachea as antagonist noncompetitive toward H-1 receptor and inhibitor of the PDE-5 enzyme.

Therapeutic opportunities in colon cancer: Focus on phosphodiesterase inhibitors

Despite novel technologies, colon cancer remains undiagnosed and 25% of patients are diagnosed with metastatic colon cancer. Resistant to chemotherapeutic agents is one of the major problems associated with treating colon cancer which creates the need to develop novel agents targeting towards newer targets. A phosphodiesterase is a group of isoenzyme, which, hydrolyze cyclic nucleotides and thereby lowers intracellular levels of cAMP and cGMP leading to tumorigenic effects. Many in vitro and in vivo studies have confirmed increased PDE expression in different types of cancers including colon cancer. cAMP-specific PDE inhibitors increase intracellular cAMP that leads to activation of effector molecules-cAMP-dependent protein kinase A, exchange protein activated by cAMP and cAMP gated ion channels. These molecules regulate cellular responses and exert its anticancer role through different mechanisms including apoptosis, inhibition of angiogenesis, upregulating tumor suppressor genes and suppressing oncogenes. On the other hand, cGMP specific PDE inhibitors exhibit anticancer effects through cGMP dependent protein kinase and cGMP dependent cation channels. Elevation in cGMP works through activation of caspases, suppression of Wnt/b-catenin pathway and TCF transcription leading to inhibition of CDK and survivin. These studies point out towards the fact that PDE inhibition is associated with anti-proliferative, anti-apoptotic and anti-angiogenic pathways involved in its anticancer effects in colon cancer. Thus, inhibition of PDE enzymes can be used as a novel approach to treat colon cancer. This review will focus on cAMP and cGMP signaling pathways leading to tumorigenesis and the use of PDE inhibitors in colon cancer.

Investigating the effect of sildenafil on improving lung function and quality of life in the patients with severe asthma

Introduction:

Phosphodiesterase inhibitors (PDEs) increase intracellular cyclic adenosine monophosphate, which results in a wide range of anti-inflammatory effects and pathologically leads to improve asthma disease. Because no human study has surveyed the effect of PDEs on pulmonary function, except some case reports and animal researches, we decided to perform a pilot study for evaluating the effect of sildenafil (PDE5) on pulmonary function in patients with severe asthma.

Methods:

This randomized controlled trials study was conducted on 20 patients with severe asthma in 2019 in Iran. For case group, was prescribed sildenafil (50 mg) daily and the control group received the placebo. In the beginning of the study and one month later, volume parameters, 6-minute walk distance (6MWD), and the quality-of-life questionnaire were measured and compared in the two groups.

Results:

Twenty patients were entered into this study. 8 patients (40%) were male and 12 (60%) were female. The results showed that mean forced vital capacity 1 in the sildenafil group turned from 1259 ± 170 to 1603 ± 527, while in the placebo group it changed from 1135 ± 125 to 1365 ± 251 (P-value = 0.215). There is no statistically significant difference between two groups. In addition, in comparison with placebo, sildenafil did not show any significant improvement in the volume parameters, the quality-of-life questionnaire scale, and 6MWD at the end of the study.

Conclusion:

According to present result can be concluded that sildenafil does not improve the severity of asthma and the quality of life in patients with severe asthma.

A comprehensive review on the potential therapeutic benefits of phosphodiesterase inhibitors on cardiovascular diseases

Phosphodiesterases are a group of enzymes that hydrolyze cyclic nucleotides, which assume a key role in directing intracellular levels of the second messengers' cAMP and cGMP, and consequently cell function. The disclosure of 11 isoenzyme families and our expanded knowledge of their functions at the cell and molecular level stimulate the improvement of isoenzyme selective inhibitors for the treatment of various diseases, particularly cardiovascular diseases. Hence, future and new mechanistic investigations and carefully designed clinical trials could help reap additional benefits of natural/synthetic PDE inhibitors for cardiovascular disease in patients. This review has concentrated on the potential therapeutic benefits of phosphodiesterase inhibitors on cardiovascular diseases.

Development of anticancer agents targeting the Hedgehog signaling

Hedgehog signaling is an evolutionarily conserved pathway which is essential in embryonic and postnatal development as well as adult organ homeostasis. Abnormal regulation of Hedgehog signaling is implicated in many diseases including cancer. Consequently, substantial efforts have made in the past to develop potential therapeutic agents that specifically target the Hedgehog signaling for cancer treatment. Here, we review the therapeutic agents for inhibition of the Hedgehog signaling and their clinical advances in cancer treatment.

PK/PD studies on non-selective PDE inhibitors in rats using cAMP as a marker of pharmacological response

In recent years, phosphodiesterase (PDE) inhibitors have been frequently tested for the treatment of experimental inflammatory and immune disorders. It is suggested that anti-inflammatory properties of PDE inhibitors are related to their ability to increase cAMP levels. The aim of this study was to verify the hypothesis that cAMP may be a useful marker of pharmacological response following administration of non-selective PDE inhibitors (pentoxifylline and (±)-lisofylline) to endotoxemic rats. Male Wistar rats were administered LPS (1 mg kg⁻¹, i.v.) simultaneously with either compound given at two doses (40 and 80 mg kg⁻¹, i.v.). Levels of cAMP and both compounds in animal plasma were measured by the validated HPLC methods. Pharmacokinetic-pharmacodynamic analysis was performed using basic and modified indirect response (IDR) models II in Phoenix WinNonlin. The results of this study indicate that, in contrast to pentoxifylline, (±)-lisofylline demonstrates a non-linear pharmacokinetics in rats with endotoxemia. In vitro study using human recombinant PDE4B and PDE7A revealed the occurrence of additive interaction between studied compounds. Moreover, (±)-lisofylline is a more potent inhibitor of PDEs compared to pentoxifylline, as evidenced by lower IC₅₀ values. Following administration of both compounds, levels of cAMP in rat plasma increased in a dose-dependent manner. The modified IDR model II better described cAMP levels over time profiles. The validity of the proposed marker was confirmed by measuring plasma TNF-α levels in the studied animals. In conclusion, cAMP may be used in future preclinical and clinical studies of some PDE inhibitors to evaluate the drug concentration–effect relationship.

Genetic Understanding of Stroke Treatment: Potential Role for Phosphodiesterase Inhibitors

Phosphodiesterase (PDE) gene family is a large family having at least 21 genes and multiple versions (isoforms) of the phosphodiesterase enzymes. These enzymes catalyze the inactivation of intracellular mediators of signal transduction such as cAMP and cGMP and therefore, play a pivotal role in various cellular functions. PDE inhibitors (PDEI) are drugs that block one or more of the five subtypes of the PDE family and thereby prevent inactivation of the intracellular cAMP and cGMP by the respective PDE-subtypes. The first clinical use of PDEI was reported almost three decades ago. Studies later found the ability of these compounds to increase the levels of ubiquitous secondary messenger molecules that can cause changes in vascular tone, cardiac function and other cellular events and thus these findings paved the way for their use in various medical emergencies. PDEs are found to be distributed in many tissues including brain. Therefore, new therapeutic agents in the form of PDEI are being explored in neurodegenerative diseases including stroke. Although studies have revealed their use in cerebral infarction prevention, their full-fledged application in times of neurological emergency or stroke in specific has been very limited so far. Nevertheless, recent investigations suggest PDE4 and PDE5 inhibitors to play a vital role in mitigating stroke symptoms by modulating signaling mechanisms in PDE pathway. Further, extensive research in terms of their pharmacological properties like dosing, drug specific activities, use of simultaneous medications, ancillary properties of these compounds and studies on adverse drug reactions needs to be carried out to set them as standard drugs of use in stroke.

Potential L-Type Voltage-Operated Calcium Channel Blocking Effect of Drotaverine on Functional Models

Drotaverine is considered an inhibitor of cyclic-3',5'-nucleotide-phophodiesterase (PDE) enzymes; however, published receptor binding data also support the potential L-type voltage- operated calcium channel (L-VOCC) blocking effect of drotaverine. Hence, in this work, we focus on the potential L-VOCC blocking effect of drotaverine by using L-VOCC-associated functional in vitro models. Accordingly, drotaverine and reference agents were tested on KCl-induced guinea pig tracheal contraction. Drotaverine, like the L-VOCC blockers nifedipine or diltiazem, inhibited the KCl-induced inward Ca²⁺- induced contraction in a concentration- dependent fashion. The PDE inhibitor theophylline had no effect on the KCl-evoked contractions, indicating its lack of inhibition on inward Ca²⁺ flow. Drotaverine was also tested on the L-VOCC-mediated resting Ca²⁺ refill model. In this model, the extracellular Ca²⁺ enters the cells to replenish the emptied intracellular Ca²⁺ stores. Drotaverine and L-VOCC blocker reference molecules inhibited Ca²⁺ replenishment of Ca²⁺-depleted preparations detected by agonist-induced contractions in post-Ca²⁺ replenishment Ca²⁺-free medium. Theophylline did not modify the Ca²⁺ store replenishment after contraction. It seems that drotaverine, but not theophylline, inhibits inward Ca²⁺ flux. The addition of CaCl₂ to Ca²⁺-free medium containing the agonist induced inward Ca²⁺ flow and subsequent contraction of Ca²⁺-depleted tracheal preparations. Drotaverine, similar to the L-VOCC blockers, inhibited inward Ca²⁺ flow and blunted the slope of CaCl₂-induced contraction in agonist containing Ca²⁺-free medium with Ca²⁺-depleted tracheal preparations. These results show that drotaverine behaves like L-VOCC blockers but, unlike PDE inhibitors using L-VOCC associated in vitro experimental models.

Trends in Modern Drug Discovery

Drugs discovered by the pharmaceutical industry over the past 100 years have dramatically changed the practice of medicine and impacted on many aspects of our culture. For many years, drug discovery was a target- and mechanism-agnostic approach that was based on ethnobotanical knowledge often fueled by serendipity. With the advent of modern molecular biology methods and based on knowledge of the human genome, drug discovery has now largely changed into a hypothesis-driven target-based approach, a development which was paralleled by significant environmental changes in the pharmaceutical industry. Laboratories became increasingly computerized and automated, and geographically dispersed research sites are now more and more clustered into large centers to capture technological and biological synergies. Today, academia, the regulatory agencies, and the pharmaceutical industry all contribute to drug discovery, and, in order to translate the basic science into new medical treatments for unmet medical needs, pharmaceutical companies have to have a critical mass of excellent scientists working in many therapeutic fields, disciplines, and technologies. The imperative for the pharmaceutical industry to discover breakthrough medicines is matched by the increasing numbers of first-in-class drugs approved in recent years and reflects the impact of modern drug discovery approaches, technologies, and genomics.

Interaction between phosphodiesterases in the regulation of the cardiac β-adrenergic pathway

In cardiac myocytes, the second messenger cAMP is synthesized within the β-adrenergic signaling pathway upon sympathetic activation. It activates Protein Kinase A (PKA) mediated phosphorylation of multiple target proteins that are functionally critical to cardiac contractility. The dynamics of cAMP are also controlled indirectly by cGMP-mediated regulation of phosphodiesterase isoenzymes (PDEs). The nature of the interactions between cGMP and the PDEs, as well as between PDE isoforms, and how these ultimately transduce the cGMP signal to regulate cAMP remains unclear. To better understand this, we have developed mechanistically detailed models of PDEs 1-4, the primary cAMP-hydrolyzing PDEs in cardiac myocytes, and integrated them into a model of the β-adrenergic signaling pathway. The PDE models are based on experimental studies performed on purified PDEs which have demonstrated that cAMP and cGMP bind competitively to the cyclic nucleotide (cN)-binding domains of PDEs 1, 2, and 3, while PDE4 regulation occurs via PKA-mediated phosphorylation. Individual PDE models reproduce experimentally measured cAMP hydrolysis rates with dose-dependent cGMP regulation. The fully integrated model replicates experimentally observed whole-cell cAMP activation-response relationships and temporal dynamics upon varying degrees of β-adrenergic stimulation in cardiac myocytes. Simulations reveal that as a result of network interactions, reduction in the level of one PDE is partially compensated for by increased activation of others. PDE2 and PDE4 exert the strongest compensatory roles among all PDEs. In addition, PDE2 competes with other PDEs to bind and hydrolyze cAMP and is a strong regulator of PDE interactions. Finally, an increasing level of cGMP gradually out-competes cAMP for the catalytic sites of PDEs 1, 2, and 3, suppresses their cAMP hydrolysis rates, and results in amplified cAMP signaling. These results provide insights into how PDEs transduce cGMP signals to regulate cAMP and how PDE interactions affect cardiac β-adrenergic response.

Emergency treatment of status asthmaticus with enoximone

This report describes the treatment of eight patients with status asthmaticus, six of whom were already maximally treated. They were consequently treated with enoximone, a selective phosphodiesterase III inhibitor, in their refractory phase. Bronchodilatation in these patients was immediate. No side-effects were observed. Enoximone appears to be a valuable addition to the treatment of status asthmaticus. I.V. administration bypasses inhalation incapability in severe asthma. It is likely to reduce or altogether prevent the need for resorting to secondary or tertiary high-tech therapy such as mechanical ventilation or anaesthetics, thus avoiding complications, as well as for transfers to specialized intensive care units. Not only do these aspects enable substantial cost savings, but they also may spare the patient a lot of anguish and a prolonged recovery.

Substituted oxadiazoles: a patent review (2010 - 2012)

INTRODUCTION

The oxadiazoles represent a class of five-membered heterocyclic compounds which are of considerable interest in different areas of medicinal chemistry and drug discovery. Oxadiazoles can exist in different regioisomeric forms and employ in various agents with a broad range of biological activities. This review covers the work reported on various biological activities of oxadiazole derivatives from 2010 to 2012.

AREAS COVERED

Oxadiazole derivatives attract great attention due to their different kinds of pharmaceutical activities including antiviral, antimicrobial, anticancer, anticonvulsant, antidiabetic and anti-inflammatory activity. This paper provides a general review of oxadiazole derivatives published in international journals and patented between 2010 and 2012.

EXPERT OPINION

Oxadiazoles have been used frequently in drug-like molecules as bioisosteres for ester and amide functionalities and displayed numerous prominent pharmacological effects. The broad pharmacological profile of oxadiazole derivatives has attracted the attention of many researchers to explore this scaffold to its multiple potential against several activities. Therefore, oxadiazole motif is likely to be present in other therapeutic molecules in the future.

The therapeutic profile of rolipram, PDE target and mechanism of action as a neuroprotectant following spinal cord injury

The extent of damage following spinal cord injury (SCI) can be reduced by various neuroprotective regimens that include maintaining levels of cyclic adenosine monophosphate (cyclic AMP), via administration of the phosphodiesterase 4 (PDE4) inhibitor Rolipram. The current study sought to determine the optimal neuroprotective dose, route and therapeutic window for Rolipram following contusive SCI in rat as well as its prominent PDE target and putative mechanism of protection. Rolipram or vehicle control (10% ethanol) was given subcutaneously (s.c.) daily for 2 wk post-injury (PI) after which the preservation of oligodendrocytes, neurons and central myelinated axons was stereologically assessed. Doses of 0.1 mg/kg to 1.0 mg/kg (given at 1 h PI) increased neuronal survival; 0.5 mg to 1.0 mg/kg protected oligodendrocytes and 1.0 mg/kg produced optimal preservation of central myelinated axons. Ethanol also demonstrated significant neuronal and oligo-protection; though the preservation provided was significantly less than Rolipram. Subsequent use of this optimal Rolipram dose, 1.0 mg/kg, via different routes (i.v., s.c. or oral, 1 h PI), demonstrated that i.v. administration produced the most significant and consistent cyto- and axo- protection, although all routes were effective. Examination of the therapeutic window for i.v. Rolipram (1.0 mg/kg), when initiated between 1 and 48 h after SCI, revealed maximal neuroprotection at 2 h post-SCI, although the protective efficacy of Rolipram could still be observed when administration was delayed for up to 48 h PI. Importantly, use of the optimal Rolipram regimen significantly improved locomotor function after SCI as measured by the BBB score. Lastly we show SCI-induced changes in PDE4A, B and D expression and phosphorylation as well as cytokine expression and immune cell infiltration. We demonstrate that Rolipram abrogates SCI-induced PDE4B1 and PDE4A5 production, PDE4A5 phosphorylation, MCP-1 expression and immune cell infiltration, while preventing post-injury reductions in IL-10. This work supports the use of Rolipram as an acute neuroprotectant following SCI and defines an optimal administration protocol and target for its therapeutic application.

Comparative monitoring of oral theophylline treatment in blood serum, saliva, and nasal mucus

BACKGROUND

Theophylline, used in the treatment for various pulmonary pathologies, is usually given orally with drug levels measured primarily in blood serum and occasionally in saliva. Although theophylline treatment is now not commonly used it has been effective to correct smell loss (hyposmia). This is important because 21 million people in the United States exhibit hyposmia and oral theophylline has corrected hyposmia in about 50% of these patients. This result suggests that oral theophylline may result in the drug not only appearing in the serum but also in nasal mucus, thereby playing a role in correcting hyposmia. No prior report of theophylline in nasal mucus has been made and no comparison of levels in nasal mucus, blood serum, or saliva has been previously reported.

PURPOSE

The aim was to determine, after oral theophylline treatment, if it is present in nasal mucus and, if present, to compare the levels with those in serum and saliva.

METHODS

Oral theophylline was given to 23 hyposmic patients at daily doses of 200, 300, 400, 600, and 800 mg for periods of 2-10 months. During each period, blood serum, saliva, and nasal mucus were collected and theophylline measured in each fluid.

RESULTS

Theophylline was found in nasal mucus and in saliva and blood serum at each drug dose in each patient to whom it was given. The mean level of theophylline in nasal mucus was 74% that of serum; mean level in saliva was 67% of serum; mean level in nasal mucus was 111% that in saliva.

CONCLUSIONS

Theophylline is present in nasal mucus after oral administration. Levels in nasal mucus reflect blood and saliva levels in a consistent manner and offer a simple convenient noninvasive method to monitor theophylline doses of the oral drug.

Phosphodiesterase inhibitors: history of pharmacology

The first pharmacological investigations of phosphodiesterase (PDE) inhibitors were developed with the clinical efficacies of drugs isolated from coffee, cacao and tea but only later their relevant ingredients were identified as xanthines that act as PDE. With its diuretic, inotropic and bronchodilating clinical efficacy, use of theophylline anticipated the clinical goals, which were later approached with the first-generation of weakly selective PDE inhibitors in the period from 1980 to 1990. Pharmacological and clinical research with these early compounds provided a vast pool of information regarding desired and adverse actions - although most of these new drugs had to be discontinued due to severe adverse effects. The pharmacological models for cardiac, vascular and respiratory indications were analysed for their PDE isoenzyme profiles, and when biochemical and molecular biological approaches expanded our knowledge of the PDE superfamily, the purified isoenzymes that were now available opened the door for more systematic studies of inhibitors and for generation of highly selective isoenzyme-specific drugs. The development of simple screening models and clinically relevant indication models reflecting the growing knowledge about pathomechanisms of disease are summarised here for today's successful application of highly selective PDE3, PDE4 and PDE5 inhibitors. The interplay of serendipitous discoveries, the establishment of intelligent pharmacological models and the knowledge gain by research results with new substances is reviewed. The broad efficacies of new substances in vitro, the enormous biodiversity of the PDE isoenzyme family and the sophisticated biochemical pharmacology enabled Viagra to be the first success story in the field of PDE inhibitor drug development, but probably more success stories will follow.

Bresol inhibits phosphodiesterase 4 gene expression and modulates the levels of select mediators of inflammation in human monocytic cells

Bresol-a poly-herbal formulation, has been reported to be effective against bronchial asthma and allergic rhinitis in children. In vivo studies have supported the anti-histaminic and anti-anaphylactic action of bresol. However, the mechanism of action of bresol in modulation of inflammation has not been studied at the cellular and molecular level. The present study was aimed to elucidate the mechanism(s) of action of bresol at the cellular and molecular levels, using human monocyte leukemia cells. The effects of bresol on phosphodiesterase 4B (PDE4B) gene expression were analyzed using human monocytic U937 leukemia cells. The ability of bresol to stimulate cAMP formation in these cells, as well as its effects on mediators of inflammation like tumor necrosis factor-α (TNFα), nitric oxide (NO), and cycloxygenase-2 (COX-2) in lipopolysaccharide (LPS)-stimulated U937 cells, were also studied. The results here indicated that bresol exhibited potential anti-inflammatory properties by inhibiting LPS-induced PDE4B gene expression in the cells. Bresol also dose dependently activated cAMP formation, and inhibited TNFα, NO, as well as COX-2 formation in the LPS-stimulated cells. Based upon the results, we concluded that the reported anti-inflammatory activity of bresol might be attributed to its abilities to inhibit PDE4B and thus elevate cAMP levels in human monocytes. The anti-inflammatory effects of bresol might also be a result of the capacity of bresol to modulate the formation of TNFα, NO, and COX-2 in monocytes.

Therapeutic potential of boron-containing compounds

Relative to carbon, hydrogen, nitrogen and oxygen, very little is currently known about boron in therapeutics. In addition, there are very few boron-containing natural products identified to date to serve as leads for medicinal chemists. Perceived risks of using boron and lack of synthetic methods to handle boron-containing compounds have caused the medicinal chemistry community to shy away from using the atom. However, physical, chemical and biological properties of boron offer medicinal chemists a rare opportunity to explore and pioneer new areas of drug discovery. Boron therapeutics are emerging that show different modes of inhibition against a variety of biological targets. With one boron-containing therapeutic agent on the market and several more in various stages of clinical trials, the occurrence of this class of compound is likely to grow over the next decade and boron could become widely accepted as a useful element in future drug discovery.

Update on roflumilast, a phosphodiesterase 4 inhibitor for the treatment of chronic obstructive pulmonary disease

Phosphodiesterase 4 (PDE4) is a member of the PDE enzyme superfamily that inactivates cyclic adenosine monophosphate and cyclic guanosine monophosphate, and is the main PDE isoenzyme occurring in cells involved in inflammatory airway disease such as chronic obstructive pulmonary disease (COPD). COPD is a preventable and treatable disease and is characterized by airflow obstruction that is not fully reversible. Chronic progressive symptoms, particularly dyspnoea, chronic bronchitis and impaired overall health are worse in those who have frequent, acute episodes of symptom exacerbation. Although several experimental PDE4 inhibitors are in clinical development, roflumilast, a highly selective PDE4 inhibitor, is the first in its class to be licensed, and has recently been approved in several countries for oral, once-daily treatment of severe COPD. Clinical trials have demonstrated that roflumilast improves lung function and reduces exacerbation frequency in COPD. Furthermore, its unique mode of action may offer the potential to target the inflammatory processes underlying COPD. Roflumilast is effective when used concomitantly with all forms of bronchodilator and even in patients treated with inhaled corticosteroids. Roflumilast thus represents an important addition to current therapeutic options for COPD patients with chronic bronchitis, including those who remain symptomatic despite treatment. This article reviews the current status of PDE4 inhibitors, focusing on the pharmacokinetics, efficacy and safety of roflumilast. In particular, it provides an overview of the effects of roflumilast on lung function and exacerbations, glucose homoeostasis and weight loss, and the concomitant use of long-acting beta(2)-adrenergic receptor agonists and short-acting muscarinic receptor antagonists.

Investigation of in vitro and in vivo anti-asthmatic properties of Siphonochilus aethiopicus

AIM OF THE STUDY

Asthma is a chronic inflammatory disease of the lungs, characterized by increased sensitivity to bronchoconstriction associated with infiltration of immune cells, mucus hypersecretion and structural remodelling of the airways. In South Africa, the indigenous plant Siphonochilus aethiopicus, is used by traditional health practitioners to treat colds, wheezing of the chest, coughs, influenza, sinus problems and mild asthma. In this study we aimed to investigate the potential anti-inflammatory and anti-allergic properties of S. aethiopicus in vitro and its efficacy in a mouse model of allergic asthma.

MATERIALS AND METHODS

The dried and powdered S. aethiopicus plant material was extracted separately with organic solvents (diethyl ether, ethanol) and water. Dried extracts as well as a purified furanoterpenoid compound present in the extracts were screened in vitro in a glucocorticoid and histamine H(1) receptor binding assay and a phosphodiesterase IV enzyme inhibition assay. Extracts were also evaluated for efficacy against ovalbumin (OVA)-induced allergic airway disease in mice.

RESULTS

Biological assaying of extracts of the plant and the isolated furanoterpenoid showed significant in vitro inhibition of glucocorticoid and histamine H(1) receptor binding and phosphodiesterase IV activity, supporting a possible anti-inflammatory, anti-allergic and bronchodilatory effect. Administration of S. aethiopicus extracts to OVA-sensitized and challenged mice significantly reduced lung inflammation and the percentage of eosinophils in bronchoalveolar lavage fluid but did not influence airway hyperreactivity.

CONCLUSION

This study provides evidence that S. aethiopicus has anti-inflammatory and anti-allergic properties in vitro and in vivo. These findings may support anecdotal accounts of its effectiveness against asthma, sinusitis, colds and flu.

GSK256066, an exceptionally high-affinity and selective inhibitor of phosphodiesterase 4 suitable for administration by inhalation: in vitro, kinetic, and in vivo characterization

Oral phosphodiesterase (PDE) 4 inhibitors such as roflumilast have established the potential of PDE4 inhibition for the treatment of respiratory diseases. However, PDE4 inhibitor efficacy is limited by mechanism-related side effects such as emesis and nausea. Delivering the inhibitor by the inhaled route may improve therapeutic index, and we describe 6-({3-[(dimethylamino)carbonyl]phenyl}sulfonyl)-8-methyl-4-{[3-methyloxy) phenyl]amino}-3-quinolinecarboxamide (GSK256066), an exceptionally high-affinity inhibitor of PDE4 designed for inhaled administration. GSK256066 is a slow and tight binding inhibitor of PDE4B (apparent IC(50) 3.2 pM; steady-state IC(50) <0.5 pM), which is more potent than any previously documented compound, for example, roflumilast (IC(50) 390 pM), tofimilast (IC(50) 1.6 nM), and cilomilast (IC(50) 74 nM). Consistent with this, GSK256066 inhibited tumor necrosis factor α production by lipopolysaccharide (LPS)-stimulated human peripheral blood monocytes with 0.01 nM IC(50) (compared with IC(50) values of 5, 22, and 389 nM for roflumilast, tofimilast, and cilomilast, respectively) and by LPS-stimulated whole blood with 126 pM IC(50). GSK256066 was highly selective for PDE4 (>380,000-fold versus PDE1, PDE2, PDE3, PDE5, and PDE6 and >2500-fold against PDE7), inhibited PDE4 isoforms A-D with equal affinity, and had a substantial high-affinity rolipram binding site ratio (>17). When administered intratracheally to rats, GSK256066 inhibited LPS-induced pulmonary neutrophilia with ED(50) values of 1.1 μg/kg (aqueous suspension) and 2.9 μg/kg (dry powder formulation) and was more potent than an aqueous suspension of the corticosteroid fluticasone propionate (ED(50) 9.3 μg/kg). Thus, GSK256066 has been demonstrated to have exceptional potency in vitro and in vivo and is being clinically investigated as a treatment for chronic obstructive pulmonary disease.

Development and validation of a transcreener assay for detection of AMP- and GMP-producing enzymes

Screening of AMP- and GMP-producing enzymes such as phosphodiesterases (PDEs), ligases, and synthetases would be simplified by the ability to directly detect unmodified nucleoside monophosphates. To address this need, we developed polyclonal and monoclonal antibodies that recognize AMP and GMP with nanomolar sensitivity and high selectivity vs. the corresponding triphosphate and 3',5'-cyclic monophosphate nucleotides that serve as substrates for many enzymes in these classes. One of these antibodies was used to develop a Transcreener AMP/GMP assay with a far red fluorescence polarization (FP) readout. This polyclonal antibody exhibited extremely high selectivity, with IC(50) ratios of 6,000 for ATP/AMP, 3,810 for cAMP/AMP, and 6,970 for cGMP/GMP. Standard curves mimicking enzymatic conversion of cAMP, cGMP, and ATP to the corresponding monophosphates yielded Z' values of >0.85 at 10% conversion. The assay reagents were shown to be stable for 24 h at room temperature, both before and after dispensing. The Transcreener AMP/GMP FP assay was used for enzymatic detection of cGMP- and cAMP-dependent PDEs 4A1A, 3A, and 9A2 and ATP-dependent ligases, acetyl CoA synthetase, and ubiquitin- activating enzyme (UBE1). Shifts of >100 mP were observed in the linear part of the progress curves for all enzymes tested, and the PDE isoforms exhibited the expected substrate and inhibitor selectivity. These studies demonstrate that direct immunodetection of AMP and GMP is a flexible, robust enzyme assay method for diverse AMP- and GMP-producing enzymes. Moreover, it eliminates many of the shortcomings of other methods including the need for fluorescently labeled substrates, the low signal:background inherent in substrate depletion assays, and the potential for interference with coupling enzymes.

Pyrazolopyridines as potent PDE4B inhibitors: 5-heterocycle SAR

Following the discovery of 4-(substituted amino)-1-alkyl-pyrazolo[3,4-b]pyridine-5-carboxamides as potent and selective phosphodiesterase 4B inhibitors, [Hamblin, J. N.; Angell, T.; Ballentine, S., et al. Bioorg. Med. Chem. Lett.2008, 18, 4237] the SAR of the 5-position was investigated further. A range of substituted heterocycles showed good potencies against PDE4. Optimisation using X-ray crystallography and computational modelling led to the discovery of 16, with sub-nM inhibition of LPS-induced TNF-α production from isolated human peripheral blood mononuclear cells.

Targeting cancer with phosphodiesterase inhibitors

IMPORTANCE OF THE FIELD

For many cancers, there has been a shift from management with traditional, nonspecific cytotoxic chemotherapies to treatment with molecule-specific targeted therapies that are used either alone or in combination with traditional chemotherapy and radiation therapy. Accumulating data suggest that multi-targeted agents may produce greater benefits than those observed with single-targeted therapies, may have acceptable tolerability profiles, and may be active against a broader range of tumour types. Thus, regulation of cyclic nucleotide signalling is properly regarded as a composite of multiple component pathways involved in diverse aspects of tumour cell function. The impairment of cAMP and/or cGMP generation by overexpression of PDE isoforms that has been described in various cancer pathologies, and the effects of PDE inhibitors in tumour models in vitro and in vivo, may offer promising insight into future cancer treatments because of the numerous advantages of PDE inhibitors.

AREAS COVERED IN THIS REVIEW

In this review, we focus on the expression and regulation of cyclic nucleotide phosphodiesterases (PDEs) in tumour progression and provide evidence that PDE inhibitors may be effective agents for treating cancer; the review covers literature from the past several years.

WHAT THE READER WILL GAIN

PDEs have been studied in a variety of tumours; data have suggested that the levels of PDE activity are elevated and, therefore, the ratio of cGMP to cAMP is affected. In addition, PDE inhibitors may be potential targets for tumour cell growth inhibition and induction of apoptosis. This review explores the prospects of targeting PDEs with therapeutic agents for cancer, as well as the shortcomings of this approach such as dose-limiting side effects, toxicity/efficacy ratio and selectivity towards tumour tissue. In addition, it includes opinions and suggestion for developing PDE inhibition for cancer treatment from initial concept to potential therapeutic application and final relevance in clinical use.

TAKE HOME MESSAGE

Impaired cAMP and/or cGMP generation upon overexpression of PDE isoforms has been described in various cancer pathologies. Inhibition of selective PDE isoforms, which raises the levels of intracellular cAMP and/or cGMP, induces apoptosis and cell cycle arrest in a broad spectrum of tumour cells and regulates the tumour microenvironment. Therefore, the development and clinical application of inhibitors specific for individual PDE isoenzymes may selectively restore normal intracellular signalling, providing antitumour therapy with reduced adverse effects.

The efficacy and safety of cilomilast in COPD

The aim of this review is to present the clinical data on the efficacy and safety of cilomilast in patients with chronic obstructive pulmonary disease (COPD). Over 6000 COPD patients received cilomilast during an extensive clinical development programme performed by GlaxoSmithKline (GSK).Five phase III randomized, double-blind, placebo-controlled, parallel-group pivotal studies were conducted in poorly reversible patients (<15% or <200 mL improvement over baseline in forced expiratory volume in 1 second (FEV(1)) after salbutamol). Patients were randomized to receive oral cilomilast 15 mg (n = 2088) or placebo (n = 1408) twice daily for 24 weeks. The co-primary efficacy variables were changes from baseline in trough (predose) FEV(1) and in total score of the St George's Respiratory Questionnaire (SGRQ).Additional studies were performed to investigate the anti-inflammatory actions of cilomilast by measuring inflammatory cells and mediators in biopsies and induced sputum; to assess the long-term effects of cilomilast; to assess the cardiac safety of cilomilast; and to assess the efficacy of cilomilast on hyperinflation. Results from one of the phase III and from one supportive study have been previously published.In the phase III pivotal studies, when averaged over 24 weeks, the mean change from baseline in FEV(1) in the cilomilast group showed improvement compared with placebo in all studies (range 24-44 mL treatment difference). When averaged over 24 weeks, there was a similar improvement in the mean total SGRQ score in both treatment groups with a decrease ranging from -1.8 to -4.2 units in the cilomilast group and 0.4 to -4.9 units in the placebo group. Only one study, however, showed both a statistically and clinically meaningful difference between the two treatment groups (treatment difference -4.1 units; p < 0.001). Although cilomilast was shown to reduce COPD exacerbations in some of these studies, there was no effect on the incidence of COPD exacerbations in a study specifically powered to detect a difference compared with placebo.No significant change was found in the primary endpoints of the anti-inflammatory studies, although some anti-inflammatory activity was detected, including a reduction in tissue CD8+ T lymphocytes and CD68+ macrophages in airway biopsies. In addition, studies did not demonstrate a consistent significant effect of cilomilast on hyperinflation.In all studies, adverse events associated with the gastrointestinal body system were reported more frequently in the cilomilast group than the placebo group and predominantly occurred within the first 2 weeks of initiating cilomilast therapy.During the cilomilast development programme a number of different endpoints were investigated to characterize the efficacy and safety of this second-generation phosphodiesterase 4 inhibitor. Safety assessments throughout the late-phase programme did not reveal any evidence of serious safety concerns with the use of cilomilast. Previous studies in phase II and early phase III had shown improvements in efficacy endpoints and some evidence of an anti-inflammatory mechanism of action. However, subsequent phase III studies failed to definitively confirm the earlier programme results, which led to termination of the development of cilomilast.

Combination therapy of lovastatin and rolipram provides neuroprotection and promotes neurorepair in inflammatory demyelination model of multiple sclerosis

Drug combination therapies for central nervous system (CNS) demyelinating diseases including multiple sclerosis (MS) are gaining momentum over monotherapy. Over the past decade, both in vitro and in vivo studies established that statins (HMG-CoA reductase inhibitors) and rolipram (phosphodiesterase-4 inhibitor; blocks the degradation of intracellular cyclic AMP) can prevent the progression of MS in affected individuals via different mechanisms of action. In this study, we evaluated the effectiveness of lovastatin (LOV) and rolipram (RLP) in combination therapy to promote neurorepair in an inflammatory CNS demyelination model of MS, experimental autoimmune encephalomyelitis (EAE). Combination treatment with suboptimal doses of these drugs in an established case of EAE (clinical disease score > or = 2.0) significantly attenuated the infiltration of inflammatory cells and protected myelin sheath and axonal integrity in the CNS. It was accompanied with elevated level of cyclic AMP and activation of its associated protein kinase A. Interestingly, combination treatment with these drugs impeded neurodegeneration and promoted neurorepair in established EAE animals (clinical disease score > or = 3.5) as verified by quantitative real-time polymerase chain reaction, immunohistochemistry and electron microscopic analyses. These effects of combination therapy were minimal and/or absent with either drug alone in these settings. Together, these data suggest that combination therapy with LOV and RLP has the potential to provide neuroprotection and promote neurorepair in MS, and may have uses in other related CNS demyelinating diseases.

Histopathology of vascular injury in Sprague-Dawley rats treated with phosphodiesterase IV inhibitor SCH 351591 or SCH 534385

Histopathological and immunohistochemical studies were conducted to characterize vascular injuries in rats treated with phosphodiesterase (PDE) IV inhibitors SCH 351591 or SCH 534385. Sprague-Dawley rats were administered PDE IV inhibitors by gavage at a range of doses and times. The two PDE IV inhibitors induced comparable levels of vascular injury, primarily in the mesentery and to a lesser extent in the pancreas, kidney, liver, small intestine, and stomach. Mesenteric vascular changes occurred as early as one hour, progressively developed over twenty-four to forty-eight hours, peaked at seventy-two hours, and gradually subsided from seven to nine days. The typical morphology of the vascular toxicity consisted of hemorrhage and necrosis of arterioles and arteries, microvascular injury, fibrin deposition, and perivascular inflammation of a variety of blood vessels. The incidence and severity of mesenteric vascular injury increased in a time- and dose-dependent manner in SCH 351591- or SCH 534385-treated rats. Mesenteric vascular injury was frequently associated with activation of mast cells (MC), endothelial cells (EC), and macrophages (MØ). Immunohistochemical studies showed increases in CD63 immunoreactivity of mesenteric MC and in nitrotyrosine immunoreactivity of mesenteric EC and MØ. The present study also provides a morphological and cellular basis for evaluating candidate biomarkers of drug-induced vascular injury.

Combined medication of lovastatin with rolipram suppresses severity of experimental autoimmune encephalomyelitis

Combinations of new medications or existing therapies are gaining momentum over monotherapy to treat central nervous system (CNS) demyelinating diseases including multiple sclerosis (MS). Recent studies established that statins (HMG-CoA reductase inhibitors) are effective in experimental autoimmune encephalomyelitis (EAE), an MS model and are promising candidates for future MS medication. Another drug, rolipram (phosphodiesterase-4 inhibitor) ameliorates the clinical severity of EAE via induction of various anti-inflammatory and neuroprotective activities. In this study, we tested whether combining the suboptimal doses of these drugs can suppress the severity of EAE. Prophylactic studies revealed that combined treatment with suboptimal doses of statins perform better than their individually administered optimal doses in EAE as evidenced by delayed clinical scores, reduced disease severity, and rapid recovery. Importantly, combination therapy suppressed the progression of disease in an established EAE case via attenuation of inflammation, axonal loss and demyelination. Combination treatment attenuated inflammatory T(H)1 and T(H)17 immune responses and induced T(H)2-biased immunity in the peripheral and CNS as revealed by serological, quantitative, and immunosorbant assay-based analyses. Moreover, the expansion of T regulatory (CD25(+)/Foxp3(+)) cells and self-immune tolerance was apparent in the CNS. These effects of combined drugs were reduced or minimal with either drug alone in this setting. In conclusion, our findings demonstrate that the combination of these drugs suppresses EAE severity and provides neuroprotection thereby suggesting that this pharmacological approach could be a better future therapeutic strategy to treat MS patients.

Can the anti-inflammatory potential of PDE4 inhibitors be realized: guarded optimism or wishful thinking?

PDE4 inhibitors have been in development as a novel anti-inflammatory therapy since the 1980s with asthma and chronic obstructive pulmonary disease (COPD) being primary indications. Despite initial optimism, none have yet reached the market. In most cases, the development of PDE4 inhibitors of various structural classes, including cilomilast, filaminast, lirimilast, piclamilast, tofimilast, AWD-12-281 (aka GSK 842470), CDP840, CI-1018, D-4418, IC485, L-826,141, SCH 351391 and V11294A has been discontinued due to lack of efficacy. A primary problem is the low therapeutic ratio of these compounds, which severely limits the dose that can be given. Indeed, for many of these compounds it is likely that the maximum tolerated dose is either sub-therapeutic or at the very bottom of the efficacy dose-response curve. Therefore, the challenge is to overcome this limitation. It is, therefore, encouraging that many 'new(er)' PDE4 inhibitors in development are reported to have an improved therapeutic window including tetomilast, oglemilast, apremilast, ONO 6126, IPL-512602 and IPL-455903 (aka HT-0712), although the basis for their superior tolerability has not been disclosed. In addition, other approaches are possible that may allow the anti-inflammatory activity of PDE inhibitors to be realized. Accordingly, this Commentary endorses the view of Spina (2008), published in the current issue of the British Journal of Pharmacology, that the therapeutic utility of PDE4 inhibitors to suppress inflammation still remains a viable concept.

Correlation between emetic effect of phosphodiesterase 4 inhibitors and their occupation of the high-affinity rolipram binding site in Suncus murinus brain

We employed an ex vivo [(3)H]rolipram binding experiment to elucidate the mechanism of emetic activity of phosphodiesterase 4 inhibitors. In Suncus murinus (an insectivore used for evaluation of emesis), emetic potential as well as ability to occupy the high-affinity rolipram binding site in brain membrane fraction in vivo were determined for phosphodiesterase 4 inhibitors. In vitro, [(3)H]rolipram bound to the membrane fraction of S. murinus brain with high affinity and its value was comparable to that for rat brain (K(d)=3.6 nM and 3.5 nM, respectively). The test compounds included denbufylline, rolipram, piclamilast, CDP840 and KF19514, each of which possessed similar affinities for the rolipram binding sites in both S. murinus and rat brain. In S. murinus, these compounds induced emesis via intraperitoneal administration. Their ED(50) values were as follows: denbufylline (1.4 mg/kg), rolipram (0.16 mg/kg), piclamilast (1.8 mg/kg), CDP840 (20 mg/kg), and KF19514 (0.030 mg/kg). In addition, these compounds occupied the high-affinity rolipram binding site in vivo as detected by dose-dependent reduction in capacity of ex vivo [(3)H]rolipram binding in brain membrane fractions. A clear correlation was observed between dose required to induce emesis and that to occupy the high-affinity rolipram binding site for individual phosphodiesterase 4 inhibitors. We conclude that the emetic effect of phosphodiesterase 4 inhibitors is caused at least in part via binding to the high-affinity rolipram binding site in brain in vivo.

Selective inhibitors for phosphodiesterase 3 and 4 in antigen-induced increase of cough reflex sensitivity in guinea pigs

Effects of the selective phosphodiesterase 3 (PDE3) inhibitor olprinone and the selective PDE4 inhibitor SB207499 were investigated on antigen-induced increase of cough reflex sensitivity and normal cough response to capsaicin in guinea pigs. Number of coughs elicited by inhalation of capsaicin (10(-8), 10(-6) and 10(-4)M) was counted 24h after an antigen challenge in actively sensitized guinea pigs, and then bronchoalveolar lavage (BAL) was performed. Olprinone (1 or 10mg/kg) or SB207499 (1 or 10mg/kg) was given twice intraperitoneally: One hour after the antigen challenge and 1h before the capsaicin provocation. SB207499, but not olprinone, significantly reduced both antigen-induced increase of cough response to inhaled capsaicin in sensitized animals and normal cough response in non-sensitized animals. SB207499, but not olprinone, also reduced the antigen-induced BAL eosinophilia. PDE4 inhibitors may be useful in treating cough associated with airway allergy via inhibiting both allergic airway inflammation and the common pathway of cough reflex.

Treating COPD with PDE 4 inhibitors

While the pathogenesis of chronic obstructive pulmonary disease (COPD) is incompletely understood, chronic inflammation is a major factor. In fact, the inflammatory response is abnormal, with CD8+ T-cells, CD68+ macrophages, and neutrophils predominating in the conducting airways, lung parenchyma, and pulmonary vasculature. Elevated levels of the second messenger cAMP can inhibit some inflammatory processes. Theophylline has long been used in treating asthma; it causes bronchodilation by inhibiting cyclic nucleotide phosphodiesterase (PDE), which inactivates cAMP. By inhibiting PDE, theophylline increases cAMP, inhibiting inflammation and relaxing airway smooth muscle. Rather than one PDE, there are now known to be more than 50, with differing activities, substrate preferences, and tissue distributions. Thus, the possibility exists of selectively inhibiting only the enzyme(s) in the tissue(s) of interest. PDE 4 is the primary cAMP-hydrolyzing enzyme in inflammatory and immune cells (macrophages, eosinophils, neutrophils). Inhibiting PDE 4 in these cells leads to increased cAMP levels, down-regulating the inflammatory response. Because PDE 4 is also expressed in airway smooth muscle and, in vitro, PDE 4 inhibitors relax lung smooth muscle, selective PDE 4 inhibitors are being developed for treating COPD. Clinical studies have been conducted with PDE 4 inhibitors; this review concerns those reported to date.

Phosphodiesterase inhibitors

Phosphodiesterases are a diverse family of enzymes that hydrolyse cyclic nucleotides and thus play a key role in regulating intracellular levels of the second messengers cAMP and cGMP, and hence cell function. Theophylline and papaverine have historically been used therapeutically and are known to be weak inhibitors of PDE, but to what extent this contributed toward their clinical efficacy was poorly defined. However, the discovery of 11 isoenzyme families and our increased understanding of their function at the cell and molecular level provides an impetus for the development of isoenzyme selective inhibitors for the treatment of various diseases. This review focuses on the development of PDE3 inhibitors for congestive heart failure, PDE4 inhibitors for inflammatory airways disease and most successfully, PDE5 inhibitors for erectile dysfunction.

Efficacy and safety of roflumilast in the treatment of asthma

BACKGROUND

The central role of chronic inflammation of the airways in asthma pathogenesis is supported by the efficacy of corticosteroids in controlling clinical symptoms. However, the search continues for potentially safer anti-inflammatory alternatives. Roflumilast is an oral, once-daily phosphodiesterase type 4 inhibitor with anti-inflammatory activity in preclinical models of asthma and chronic obstructive pulmonary disease.

OBJECTIVE

To investigate the dose-ranging efficacy and safety of roflumilast in patients with mild-to-moderate asthma.

METHODS

Patients (N = 693) were randomized in a double-blind, parallel-group, phase 2/3 study. After a 1- to 3-week placebo run-in period, patients (mean forced expiratory volume in 1 second [FEV1], 73% of predicted) were randomized to receive 100, 250, or 500 microg of roflumilast once daily for 12 weeks. The primary end point was change from baseline in FEV1; secondary end points included change from baseline in morning and evening peak expiratory flow.

RESULTS

Roflumilast use significantly increased FEV1 (P < .001 vs baseline). Improvements from baseline in FEV1 at the last visit were 260, 320, and 400 mL for the 100-, 250-, and 500-microg dose groups, respectively. Roflumilast, 500 microg, was superior to roflumilast, 100 microg, by 140 mL in improving FEV1 (P = .002). There were also significant improvements from baseline in morning and evening peak expiratory flow in all the dose groups (P < or = .006). Roflumilast was well tolerated at all doses tested. Most adverse events were mild to moderate in intensity and transient.

CONCLUSION

These results support the emerging role of roflumilast, 500 microg/d, in the treatment of asthma.