Possible role of cyclic AMP phosphodiesterases in the actions of ibudilast on eosinophil thromboxane generation and airways smooth muscle tone

Emerging Potential of the Phosphodiesterase (PDE) Inhibitor Ibudilast for Neurodegenerative Diseases: An Update on Preclinical and Clinical Evidence

Neurodegenerative diseases constitute a broad range of central nervous system disorders, characterized by neuronal degeneration. Alzheimer's disease, Parkinson's disease, amyolotrophic lateral sclerosis (ALS), and progressive forms of multiple sclerosis (MS) are some of the most frequent neurodegenerative diseases. Despite their diversity, these diseases share some common pathophysiological mechanisms: the abnormal aggregation of disease-related misfolded proteins, autophagosome-lysosome pathway dysregulation, impaired ubiquitin-proteasome system, oxidative damage, mitochondrial dysfunction and excessive neuroinflammation. There is still no effective drug that could halt the progression of neurodegenerative diseases, and the current treatments are mainly symptomatic. In this regard, the development of novel multi-target pharmaceutical approaches presents an attractive therapeutic strategy. Ibudilast, an anti-inflammatory drug firstly developed as an asthma treatment, is a cyclic nucleotide phosphodiesterases (PDEs) inhibitor, which mainly acts by increasing the amount of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), while downregulating the pro-inflammatory factors, such as tumor necrosis factor-α (TNF-α), macrophage migration inhibitory factor (MIF) and Toll-like receptor 4 (TLR-4). The preclinical evidence shows that ibudilast may act neuroprotectively in neurodegenerative diseases, by suppressing neuroinflammation, inhibiting apoptosis, regulating the mitochondrial function and by affecting the ubiquitin-proteasome and autophagosome-lysosome pathways, as well as by attenuating oxidative stress. The clinical trials in ALS and progressive MS also show some promising results. Herein, we aim to provide an update on the emerging preclinical and clinical evidence on the therapeutic potential of ibudilast in these disorders, discuss the potential challenges and suggest the future directions.

Ibudilast Suppresses MUC5AC Mucus Production through Inhibition of ERK1/2 Phosphorylation

Mucus hypersecretion is a hallmark of respiratory diseases, and excess airway mucus can worsen these conditions. Therefore, it is important to control the production of airway mucus in the treatment of respiratory diseases. The phosphodiesterase inhibitor ibudilast has been reported to be effective in treating sputum and postnasal drip in patients with chronic airway inflammation. On the basis of the hypothesis that ibudilast could inhibit mucus production in the airway, in the present study, we examined the effects of ibudilast on the production of MUC5AC, a major protein component of mucus. In in vitro studies using NCI-H292 cells, ibudilast suppressed MUC5AC production induced by various stimuli. In addition, ibudilast inhibited extracellular signal-regulated kinase (ERK)1/2 phosphorylation and MUC5AC gene transcription. Furthermore, it attenuated MUC5AC production and Muc5ac mRNA expression in lipopolysaccharide-treated mice in vivo. Collectively, these findings demonstrate that ibudilast has an inhibitory effect on mucus production, which could at least partly be attributed to the inhibition of ERK1/2 phosphorylation and the repression of MUC5AC gene transcription.

A double-blind, randomized, placebo-controlled pilot trial to determine the efficacy and safety of ibudilast, a potential glial attenuator, in chronic migraine

Background

Chronic migraine (CM) is problematic, and there are few effective treatments. Recently, it has been hypothesized that glial activation may be a contributor to migraine; therefore, this study investigated whether the potential glial inhibitor, ibudilast, could attenuate CM.

Methods

The study was of double-blind, randomized, placebo-controlled, two-period crossover design. Participants were randomized to receive either ibudilast (40 mg twice daily) or placebo treatment for 8 weeks. Subsequently, the participants underwent a 4-week washout period followed by a second 8-week treatment block with the alternative treatment. CM participants completed a headache diary 4 weeks before randomization throughout both treatment periods and 4 weeks after treatment. Questionnaires assessing quality of life and cutaneous allodynia were collected on eight occasions throughout the study.

Results

A total of 33 participants were randomized, and 14 participants completed the study. Ibudilast was generally well tolerated with mild, transient adverse events, principally nausea. Eight weeks of ibudilast treatment did not reduce the frequency of moderate to severe headache or of secondary outcome measures such as headache index, intake of symptomatic medications, quality of life or change in cutaneous allodynia.

Conclusion

Using the current regimen, ibudilast does not improve migraine with CM participants.

Reduction of alcohol drinking of alcohol-preferring (P) and high-alcohol drinking (HAD1) rats by targeting phosphodiesterase-4 (PDE4)

RATIONALE

Phosphodiesterase-4 (PDE4) and neuroimmune signaling have been posited to regulate alcohol drinking.

OBJECTIVES

This study evaluated the involvement of PDE4 and Il22ra2 on ethanol (EtOH) intake by alcohol-preferring (P) and high-alcohol-drinking (HAD1) rats.

METHODS

Exp 1 determined the dose-response effects of PDE4 inhibitors, rolipram, and Ro 20-1724, on 2 h/day free-choice EtOH intake by adult P and HAD1 rats. Exps 2-3 examined the effects of repeated administration with the PDE4 inhibitors on EtOH or sucrose intake and locomotor behavior. Exp 4 determined Pde4-associated gene expression differences in subregions of the extended amygdala, between high- and low-alcohol-consuming rat lines. Exp 5 evaluated the effects of infusing short hairpin RNA to knock down Il22ra2 in the nucleus accumbens (NAc) shell on a 24-h free-choice EtOH drinking by P rats.

RESULTS

Administration of rolipram or Ro 20-1724 reduced EtOH intake by P rats; Ro 20-1724 reduced EtOH intake by HAD1 rats. Repeated rolipram or Ro 20-1724 exposure reduced EtOH intake by P and HAD1 rats. PDE4 inhibition induced motor impairment during the first hour of EtOH intake by P rats. Higher gene expression levels for PDE4A were found in the NAc shell of P vs NP rats. ShRNAs targeting Il22ra2 in the NAc shell significantly reduced chronic EtOH intake.

CONCLUSIONS

PDE4 and neuroinflammatory/immune signaling pathways could represent molecular targets for the treatment of alcohol use disorders in genetically predisposed subjects. This study underscores the importance of testing compounds over multiple days and rat lines when determining efficacy to disrupt excessive alcohol intake.

Carboxyamidotriazole: a novel inhibitor of both cAMP-phosphodiesterases and cGMP-phosphodiesterases

Carboxyamidotriazole (CAI) is a non-cytotoxic anti-tumor drug, which also shows considerable anti-inflammatory effects in a variety of animal models of inflammation. The exact target and mechanism of CAI were not clearly understood yet. In the present study, we demonstrate that CAI is a non-selective phosphodiesterase (PDE) inhibitor, which provides comprehensive inhibitions of both adenosine 3',5'-cyclic monophosphate specific PDE (cAMP-PDE) and guanosine 3',5'-cyclic monophosphate specific PDE (cGMP-PDE) isolated from rat brain, mouse pulmonary tissue, primary mouse peritoneal macrophages, RAW264.7 cells, Lewis lung carcinoma (LLC) cells and lymphocytic leukemia cells (L1210) with moderate potencies (IC50≈0.5-30μM). The comprehensive elimination of PDE activities in living LLC cells by CAI results in accumulation of intracellular cAMP and cGMP, which can be visualized by fluorescence resonance energy transfer (FRET)-based cyclic nucleotide sensors. The stimulation by 30μM CAI yielded ~1.5-fold greater cGMP responses compared with 10μM sildenafil citrate, whereas the influence of 30μM CAI on cAMP levels was similar as that of 100μM 3-isobutyl-1-methylxanthine (IBMX). The non-selective inhibitory effect of CAI on cAMP-PDE and cGMP-PDE increases the likelihood for CAI to affect the balance between the levels of intracellular cyclic nucleotides cAMP and cGMP, then a variety of cellular signaling pathways that regulate cell functions and even related disease processes. When examining the widely proven anti-tumor and anti-inflammatory activities of CAI, it is important to affirm its comprehensive inhibitory effect on PDEs, which makes it superior to some selective PDE inhibitors in a way.

Pretreatment with antiasthmatic drug ibudilast ameliorates Aβ 1-42-induced memory impairment and neurotoxicity in mice

Amyloid-β peptide (Aβ) is thought to be associated with the progressive neuronal death observed in Alzheimer's disease (AD). However, effective neuroprotective approaches against Aβ neurotoxicity are unavailable. Here, we investigated possible preventive effects of ibudilast, as a pharmacologic phosphodiesterase inhibitor, currently used for treatment of inflammatory diseases such as asthma, on Aβ 1-42-induced neuroinflammatory, apoptotic responses and memory impairment. We found that pretreatment with ibudilast (4 or 12 mg/kg, i.p.) significantly ameliorated impaired spatial learning and memory in intracerebroventricularly (ICV) Aβ 1-42-injected mice, as evidenced by decrease in escape latency during acquisition trials and increase in exploratory activities in the probe trial in Morris water maze (MWM) task, and by increase in the number of correct choices and decrease in latency to enter the shock-free compartment in Y-maze test. Further study showed that ibudilast prevented generation of pro-inflammatory cytokines such as NF-κB p65 and TNF-α as well as pro-apoptotic molecule caspase-3 activation and anti-apoptotic protein Bcl-2 downregulation in both hippocampus and cortex of ICV Aβ 1-42-injected mice. Taken together, our findings suggest that ibudilast has preventive effects on Aβ-induced cognitive impairment via inhibiting neuroinflammatory and apoptotic responses.

Harnessing the clinical efficacy of phosphodiesterase 4 inhibitors in inflammatory lung diseases: dual-selective phosphodiesterase inhibitors and novel combination therapies

Phosphodiesterase (PDE) 4 inhibitors have been in development as a novel anti-inflammatory therapy for more than 20 years, with asthma and chronic obstructive pulmonary disease (COPD) being primary indications. Despite initial optimism, only one selective PDE4 inhibitor, roflumilast (Daxas (®)), has been approved for use in humans and available in Canada and the European Union in 2011 for the treatment of a specific population of patients with severe COPD. In many other cases, the development of PDE4 inhibitors of various structural classes has been discontinued due to lack of efficacy and/or dose-limiting adverse events. Indeed, for many of these compounds, it is likely that the maximum tolerated dose is either subtherapeutic or at the very bottom of the efficacy dose-response curve. Thus, a significant ongoing challenge that faces the pharmaceutical industry is to synthesize compounds with therapeutic ratios that are superior to roflumilast. Several strategies are being considered, but clinically effective compounds with an optimal pharmacophore have not, thus far, been reported. In this chapter, alternative means of harnessing the clinical efficacy of PDE4 inhibitors are described. These concepts are based on the assumption that additive or synergistic anti-inflammatory effects can be produced with inhibitors that target either two or more PDE families or with a PDE4 inhibitor in combination with other anti-inflammatory drugs such as a glucocorticoid.

Ibudilast: a review of its pharmacology, efficacy and safety in respiratory and neurological disease

Ibudilast is a relatively nonselective phosphodiesterase inhibitor which has been marketed for almost 20 years in Japan for treating asthma. More recently it has been found to have anti-inflammatory activity in both the peripheral immune system and in the CNS via glial cell attenuation. This CNS-directed anti-inflammatory activity is of potential use in the treatment of multiple sclerosis, neuropathic pain, and in the improved efficacy and safety of opioids by decreasing opioid tolerance, withdrawal and reinforcement. Its suitable pharmacokinetics and generally good tolerability make it a promising potential treatment for these conditions.

Evidence for a role for cyclic AMP in modulating the action of 5-HT and an excitatory neuropeptide, FLP17A, in the pharyngeal muscle of Caenorhabditis elegans

The feeding activity of the nematode Caenorhabditis elegans is regulated by an anatomically well-defined network of 20 enteric neurones that employs small molecule and neuropeptidergic signalling. Two of the most potent excitatory agents are 5-HT and the neuropeptide FLP17A. Here we have examined the role of cAMP in modulating their excitatory actions by pharmacological manipulation of the level of cAMP. Application of the membrane permeable cAMP analogue, dibutyryl-cAMP (1 microM), enhanced the excitatory response to both FLP17A and 5-HT. Furthermore, the adenylyl cyclase activator, forskolin (50 nM), significantly enhanced the excitatory response to both FLP17A and 5-HT. The phosphodiesterase inhibitor, ibudilast (10 microM), enhanced the excitatory response to FLP17A. The protein kinase inhibitor, H-9 dihydrochloride (10 microM) significantly reduced the excitatory response to 5-HT. H-9 dihydrochloride also had a direct effect on pharyngeal activity. The effect of FLP17A and 5-HT on two mutants, egl-8 (loss-of-function phospholipase-Cbeta) and egl-30 (loss-of-function Galphaq) was also investigated. Both these mutants have a lower pharyngeal pumping rate than wild-type which has to be considered when interpreting the effects of these mutations on the excitatory responses to FLP17A and 5HT. However, even taking into consideration the lower basal activity of these mutants, it is clear that the percentage increase in pharyngeal pumping rate induced by FLP17A is greatly reduced in both mutants compared to wild-type. In the case of 5-HT, the effect of the mutant backgrounds on the response was less pronounced. Overall, the data support a role for cAMP in modulating the excitatory action of both FLP17A and 5-HT on C. elegans pharyngeal pumping and furthermore implicate an EGL-30 dependent pathway in the regulation of the response to FLP17A.

Ibudilast-induced decreases in cytosolic Ca(2+) level and contraction in rat aorta

The mechanism by which ibudilast induces vasodilation was examined in isolated endothelium-denuded rat aorta. Ibudilast inhibited the contractions induced by phenylephrine (PE) and high K(+) with decrease of [Ca(2+)](i) level in a concentration-dependent manner, to the same degree. 3-Isobutyl-1-methylxanthine (IBMX) inhibited PE-induced contraction and [Ca(2+)](i) level in a concentration-dependent manner, but it inhibited high K(+)-induced contraction without decrease of [Ca(2+)](i) level. In comparison with IBMX, the increases of cAMP and cGMP contents in ibudilast were much smaller than that of muscle tension. Ibudilast did not inhibit 12-deoxyphorbol 13-isobutyrate (DPB)-induced contraction in the presence of verapamil. Treatment with 30 microM ibudilast inhibited the extracellularly added Ca(2+)-induced muscle tension and increases in [Ca(2+)](i) level during high K(+) depolarization. These results suggested that ibudilast inhibited PE- and high K(+)-induced muscle contractions mainly by the inhibition of [Ca(2+)](i) level in endothelium-denuded rat aorta.

Ibudilast (AV-411)

The treatment of neuropathic pain is a major unresolved medical challenge. Present pharmacotherapies only have modest efficacy and numerous side effects. The use of opioid analgesics is additionally coupled with dependence and withdrawal syndromes. Ibudilast (AV-411) is a non-selective phosphodiesterase inhibitor that is also known to suppress glial cell activation. It has been used clinically for other indications with a good safety profile. As glial cell activation is considered to crucially contribute to neuropathic pain as well as opioid dependence and withdrawal, the authors conceived that ibudilast may be useful for treating these conditions. Preclinical data indicate that ibudilast crosses the blood-brain barrier, is well tolerated, is active on oral administration, reduces glial activation and attenuates pain symptoms in diverse rat models of neuropathic pain. In addition, it enhances acute morphine analgesia and attenuates morphine tolerance and withdrawal. Thus ibudilast may improve opioid efficacy and is a promising therapeutic candidate for neuropathic pain, with a novel mechanism of action.

Preferential inhibition of human phosphodiesterase 4 by ibudilast

Ibudilast ophthalmic solution exhibited an improved clinical efficacy over cromoglycate in the treatment of allergic conjunctivitis. To further characterize its principal mode of action, the phosphodiesterase (PDE) inhibitory profile of ibudilast has been examined using human recombinant enzymes. Ibudilast, but not the other commonly used anti-allergic ophthalmic solutions including cromoglycate, ketotifen, tranilast and levocabastine, potently inhibits purified human PDE4A, 4B, 4C and 4D with IC50 values at 54, 65, 239 and 166 nM, respectively. Ibudilast effectively blocks lipopolysaccharide (LPS)-induced tumor necrosis factor (TNFalpha, IC50 = 6.2 microM) and N-formyl-Met-Leu-Phe (fMLP)-induced leukotriene (LT) B4 biosynthesis (IC50 = 2.5 microM) in human whole blood, which are 3 and 6-fold more potent than cilomilast, respectively. The attenuated inflammatory and allergic responses from the potent and preferential PDE4 inhibition of ibudilast may have contributed significantly to its beneficial pharmacological responses and distinguishes ibudilast from the other ophthalmic solutions in the treatment of ocular allergy.

The inhibitory profile of Ibudilast against the human phosphodiesterase enzyme family

Ibudilast is widely used in Japan to treat ischemic stroke and bronchial asthma. Its mode of action is through the inhibition of cyclic nucleotide phosphodiesterases (PDEs). Growing evidence suggests this compound has utility in a range of neurological conditions linked to its ability to elevate cellular cyclic nucleotide concentrations, however limited data exists on Ibudilast's action on individual PDE families. We therefore used an extensive panel of human PDE enzymes to define the PDE inhibitory profile of this compound. Ibudilast preferentially inhibits PDE3A, PDE4, PDE10 and PDE11 with lesser inhibition of a number of other families. The significance of these findings is discussed in relation to Ibudilast's observed effects on certain disease states.

Ibudilast, a phosphodiesterase inhibitor, protects against white matter damage under chronic cerebral hypoperfusion in the rat

Cerebrovascular white matter (WM) lesions, which are frequently observed in vascular cognitive impairment and vascular dementia, can be produced in rats by clipping the common carotid arteries bilaterally. Since TNF-alpha is known to cause the degeneration of myelin, we examined whether these lesions can be ameliorated by ibudilast, a cyclic AMP phosphodiesterase (PDE) inhibitor that suppresses tumor necrosis factor (TNF)-alpha production. After the ligation of both common carotid arteries in 29 rats, 21 rats received a daily oral administration of 10, 30 or 60 mg/kg ibudilast and 8 rats received vehicle for 14 days. The pathological changes in the white matter were quantified in terms of white matter lesions and the emergence of activated microglia immunoreactive for major histocompatibility complex (MHC) antigen. In the vehicle-treated animals, white matter lesions and microglial activation occurred in the optic tract, internal capsule and corpus callosum. A low dose (10 mg/kg) of ibudilast failed to suppress the white matter lesions and microglial activation, whereas a dose of either 30 or 60 mg/kg ibudilast ameliorated these lesions (p<0.001). Without an alterations in laboratory blood data, 60 mg/kg ibudilast exhibited percent reduction of the white matter lesions ranging between 50% and 70%, which was more effective than 30 mg/kg ibudilast (p<0.05). The TNF-alpha immunoreactive glia decreased in number in the 60 mg/kg ibudilast-treated group as compared to the vehicle-treated group (p<0.001). These results indicate a dose-dependent protective effect of ibudilast against cerebrovascular white matter lesions and suggest a potential use for ibudilast in the treatment of vascular dementia.

Ibudilast: a non-selective PDE inhibitor with multiple actions on blood cells and the vascular wall

Ibudilast (3-isobutyryl-2-isopropylpyrazolo[1,5-a]pyridine) is a nonselective inhibitor of cyclic nucleotide phosphodiesterase (PDE). It is widely used in Japan for improving prognosis and relieving symptoms in patients suffering from ischemic stroke or bronchial asthma. These clinical applications are based on the properties of ibudilast that inhibit platelet aggregation, improve cerebral blood flow and attenuate allergic reactions. The inhibition of platelet aggregation and vasodilatation by ibudilast may be due to synergistic elevation of intracellular cyclic nucleotides and release of nitric oxide (NO) or prostacyclin from endothelium, rather than direct inhibition of PDE5 or PDE3. Another important property of ibudilast is its antiinflammatory activity possibly associated with potent inhibition of PDE4. Combined with its relaxing effects on bronchial smooth muscle, antiinflammatory activity of ibudilast could favorably influence pathophysiology of asthma by antagonizing chemical mediators triggering asthmatic attacks. Ibudilast was also reported to significantly attenuate inflammatory cell infiltration in the lumbar spinal cord in an animal model of encephalomyelitis. Future investigations should include effects of ibudilast on inflammatory reactions between endothelium and blood cells, which may initiate the development of atherosclerosis.

Ibudilast attenuates astrocyte apoptosis via cyclic GMP signalling pathway in an in vitro reperfusion model

We examined the effect of 3-isobutyryl-2-isopropylpyrazolo[1,5-a]pyridine (ibudilast), which has been clinically used for bronchial asthma and cerebrovascular disorders, on cell viability induced in a model of reperfusion injury. Ibudilast at 10 - 100 microM significantly attenuated the H(2)O(2)-induced decrease in cell viability. Ibudilast inhibited the H(2)O(2)-induced cytochrome c release, caspase-3 activation, DNA ladder formation and nuclear condensation, suggesting its anti-apoptotic effect. Phosphodiesterase inhibitors such as theophylline, pentoxyfylline, vinpocetine, dipyridamole and zaprinast, which increased the guanosine-3',5'-cyclic monophosphate (cyclic GMP) level, and dibutyryl cyclic GMP attenuated the H(2)O(2)-induced injury in astrocytes. Ibudilast increased the cyclic GMP level in astrocytes. The cyclic GMP-dependent protein kinase inhibitor KT5823 blocked the protective effects of ibudilast and dipyridamole on the H(2)O(2)-induced decrease in cell viability, while the cyclic AMP-dependent protein kinase inhibitor KT5720, the cyclic AMP antagonist Rp-cyclic AMPS, the mitogen-activated protein/extracellular signal-regulated kinase inhibitor PD98059 and the leukotriene D(4) antagonist LY 171883 did not. KT5823 also blocked the effect of ibudilast on the H(2)O(2)-induced cytochrome c release and caspase-3-like protease activation. These findings suggest that ibudilast prevents the H(2)O(2)-induced delayed apoptosis of astrocytes via a cyclic GMP, but not cyclic AMP, signalling pathway.

Ibudilast modulates platelet-endothelium interaction mainly through cyclic GMP-dependent mechanism

3-Isobutyryl-2-isopropylpyrazolo[1,5-a]pyridine (ibudilast) has been widely used in Japanese clinics for its antiasthmatic and antithrombotic effects. We investigated the mechanisms involved in the antiplatelet effects of the agent, specifically focusing on platelet-endothelium interaction. Ibudilast inhibits both phosphodiesterase (PDE) 3 and 5, the two major PDE isoforms of human platelets, with an IC50 of 31 and 2.2 microM, respectively. Cyclic guanosine monophosphate (GMP) accumulation in washed human platelets exposed to ibudilast alone increased significantly only at high concentrations of the agent (100 microM), whereas > or = 1 microM ibudilast enhanced cyclic GMP levels in the platelets cocultured with bovine aorta endothelial cells (ECs). In contrast, ibudilast enhanced cyclic AMP accumulation only at 100 microM, either with or without ECs. The synergistic effect of ibudilast and EC on cyclic nucleotide accumulation also was demonstrated by the inhibitory capability of the drug and the cells on platelet aggregation. The synergism between ibudilast and aspirin-pretreated ECs was more pronounced than that between ibudilast and N(omega)-nitro-L-arginine (L-NNA)-pretreated ECs. Ibudilast affected neither ATP diphosphohydrolase activity nor NO release from EC up to a concentration of 10 microM. We conclude that ibudilast exhibits antiplatelet properties mainly by inhibiting PDE5 to potentiate antiplatelet function of endothelium-derived NO.

Cyclic GMP/cyclic GMP-dependent protein kinase system prevents excitotoxicity in an immortalized oligodendroglial cell line

Previously, we have demonstrated that excitotoxicity of oligodendrocyte-like cells (OLC), differentiated from immortalized rat O-2A progenitor cells (CG-4 cells), is prevented by cyclic AMP-elevating agents. We now report that some agents that elevate cyclic GMP prevent OLC excitotoxicity. Kainate-induced injury was prevented by cyclic GMP analogues (8-bromo-cyclic GMP and dibutyryl cyclic GMP), a guanylate cyclase activator [atrial natriuretic peptide (ANP)], and phosphodiesterase inhibitors [3-isobutyl-1-methylxanthine (IBMX), ibudilast, propentofylline, and rolipram]. When both forskolin and 8-bromo-cyclic GMP were added, kainate-induced injury was additively prevented. There was a strong positive correlation between suppression of kainate-induced Ca2+ influx and prevention of injury by these chemicals. The measurement of intracellular cyclic AMP and cyclic GMP by radioimmunoassay demonstrated the following: an increase of cyclic GMP with treatment with 8-bromo-cyclic GMP, dibutyryl cyclic GMP, and ANP; an increase of cyclic AMP with treatment with ibudilast and rolipram; and an increase of both cyclic AMP and cyclic GMP with treatment with IBMX and propentofylline. Kainate-induced Ca2+ influx was decreased by 8-(4-chlorophenylthiol)-guanosine-3',5'-monophosphate, an activator of cyclic GMP-dependent protein kinase (PKG), or okadaic acid, an inhibitor of protein phosphatases 1 and 2A. RT-PCR and westem blotting of OLC demonstrated transcription of PKG II gene and translation of PKG Ibeta mRNA, but no translation of PKG Ialpha mRNA. Therefore, we concluded that the cyclic GMP/PKG system prevents OLC excitotoxicity.

Phosphodiesterase 4 inhibitors and the treatment of asthma: where are we now and where do we go from here?

Research conducted over the last 20 years has established that inflammation of the airways is central to the airway dysfunction that characterises asthma. Typically, the airway wall is infiltrated by a variety of cells including mast cells, eosinophils and T lymphocytes, which have deviated towards a T(H)2 phenotype. Together, these cells release a plethora of mediators including interleukin (IL)-4, IL-5, granulocyte/macrophage colony-stimulating factor and eotaxin which ultimately cause the histopathology and symptoms of asthma. Glucocorticosteroids are the only drugs currently available that effectively impact upon this inflammation and resolve, to a greater or lesser extent, compromised lung function. However, steroids are nonselective and generally unsuitable for paediatric use. New drugs are clearly required. One group of potential therapeutic agents for asthma are inhibitors of cyclic AMP-specific phosphodiesterase (PDE), of which theophylline may be considered a prototype. It is now known that PDE is a generic term which refers to at least 11 distinct enzyme families that hydrolyse cAMP and/or cGMP. Over the last decade, inhibitors of PDE4 (a cAMP-specific family that negatively regulates the function of almost all pro-inflammatory and immune cells, and exerts widespread anti-inflammatory activity in animal models of asthma) have been developed with the view to reducing the adverse effects profile associated with non-selective inhibitors such as theophylline. Such is the optimism regarding PDE4 as a viable therapeutic target that more than 100 PDE4 inhibitor patent applications have been filed since 1996 by 13 major pharmaceutical companies. This article reviews the progress of PDE4 inhibitors as anti-inflammatory agents, and identifies problems that have been encountered by the pharmaceutical industry in the clinical development of these drugs and what strategies are being considered to overcome them.

Phosphodiesterase 4 (PDE4) inhibitors in asthma and chronic obstructive pulmonary disease (COPD)

Phosphodiesterases (PDE) are a family of enzymes responsible for the metabolism of the intracellular second messengers cyclic AMP and cyclic GMP. PDE4 is a cyclic AMP specific PDE that is the major if not sole cyclic AMP metabolizing enzymes found in inflammatory and immune cells, and contributes significantly to cyclic AMP metabolism in smooth muscles. Based on its cellular and tissue distribution and the demonstration that selective inhibitors of this isozyme reduce bronchoconstriction in animals and suppress the activation of inflammatory cells, PDE4 has become an important molecular target for the development of novel therapies for asthma and COPD. This chapter will review the evidence demonstrating the ability of PDE4 inhibitors to modify airway obstruction, airway inflammation and airway remodelling and hyperreactivity, will present some preliminary findings obtained with theses compounds in clinical trials and and will discuss experimental approaches designed to identify novel compounds that maintain the beneficial activity of the initial selective PDE4 inhibitors but with a reduced tendency of elicit the gastrointestinal side effects observed with this class of compounds.

Ibudilast suppresses TNFalpha production by glial cells functioning mainly as type III phosphodiesterase inhibitor in the CNS

Tumor necrosis factor alpha (TNFalpha) is considered to play a critical role in the development of various pathological processes in the central nervous system (CNS), such as neuronal degeneration, demyelination and HIV-related pathology. In order to search for the agents which suppress TNFalpha production in the CNS for future treatment of these pathological conditions, we examined the effects of ibudilast on TNFalpha production by murine microglia and astrocytes. Some actions of ibudilast are reportedly mediated by inhibition of type IV phosphodiesterase (PDE). Type IV PDE inhibitor has been shown to be the most effective for experimental autoimmune inflammatory demyelination. Therefore, we also determined the subtype of PDE inhibited by ibudilast. Ibudilast significantly and selectively suppressed TNFalpha production by microglia in a dose-dependent manner, without affecting their viability. The inhibition assay indicated that ibudilast is a rather selective inhibitor for type III PDE purified from brain, heart and kidney with moderate inhibitory activity against types I, II and IV PDEs from various tissues. Although it required 10 microM or higher concentrations to effectively suppress TNFalpha production in vitro, the combination of ibudilast with other subtypes of PDE inhibitors synergistically suppressed TNFalpha and nitric oxide production by microglia at 1 microM, a similar concentration that could be obtained in vivo at usual therapeutic dose. Thus, ibudilast, when used in a combination with other PDE inhibitors, will be useful for future strategies to treat intractable neurological diseases in which TNFalpha may play a causative role.

Ibudilast, a phosphodiesterase inhibitor, ameliorates experimental autoimmune encephalomyelitis in Dark August rats

A phosphodiesterase inhibitor (PDEI), Ibudilast, which has been in wide use for the management of bronchial asthma and cerebrovascular disease in Japan, was tested for its clinical efficacy on experimental autoimmune encephalomyelitis (EAE) in Dark August rats. The severity of acute EAE was significantly ameliorated by prophylactic oral treatment with Ibudilast (10 mg/kg per day) starting on the day of immunization, although it did not modify the course of the disease when it was given after the onset of the first clinical sign of EAE. Histologically, inflammatory cell infiltration in the lumbar spinal cord was significantly reduced in Ibudilast-treated animals as compared to control animals. Ibudilast mildly suppressed MBP-induced proliferation of T cells in regional lymph nodes, the secretion of interferon-gamma from T cells activated by MBP in CFA, and the secretion of tumor necrosis factor-alpha from macrophages. While the in vitro studies did not suggest difference between Ibudilast and other PDEIs such as rolipram, the clinical dose of Ibudilast is approximately 200-fold higher than that of rolipram and the effective dose of Ibudilast was relatively close to what has been therapeutically used in patients. Thus, Ibudilast may be a candidate for clinical use for patients with multiple sclerosis. 1999 Elsevier Science B.V. All rights reserved.

Inhibitory effect of ibudilast (KC-404) on the expression of the beta2 integrin family on an eosinophilic cell line (EoL-1)

BACKGROUND

Adhesion molecules are thought to play a key role in inflammatory processes in bronchial asthma. We previously observed an increased expression of the beta2-integrin family on an eosinophilic cell line (EoL-1) by platelet-activating factor (PAF).

OBJECTIVE

In the present study, we examined the effect of ibudilast (KC-404), a novel anti-asthma agent, on beta2 integrin expression induced by PAF.

MATERIAL AND METHODS

EoL-1 cells (1x10(6)/mL) were incubated in the presence or absence of 10(-6) M ibudilast (KC-404), then cells were cultured in the presence or absence of PAF (10(-7) M) for 45 minutes. Flow cytometric analysis for CD11a, CD11b, and CD18 expression was examined.

RESULTS

Ibudilast had an inhibitory effect on beta2 integrin expression induced by PAF [CD11a: 84.8% versus 73.1% (preincubation with ibudilast), CD11b: 35.8% versus 26.2%, CD18 74.9% versus 65.6%].

CONCLUSIONS

Ibudilast (KC-404) has anti-inflammatory activities through its inhibitory effect on the expression of adhesion molecules on eosinophils.

The effect duration of selective phosphodiesterase inhibitors in the guinea pig

The beta-adrenoceptor agonists, isoprenaline, salbutamol and salmeterol, the non-selective phosphodiesterase (PDE) isoenzyme inhibitors, theophylline, trequinsin; the PDE3 isoenzyme inhibitor, milrinone; the PDE3/4 isoenzyme inhibitor, benzafentrine; and the PDE4 isoenzyme inhibitors, denbufylline, nitraquazone, RP 73401, Ro-20-1724, rolipram and tibenelast all induced concentration-dependent reversal of prostaglandin F2alpha-induced contraction of guinea-pig superfused trachea in vitro. The relaxant response of the non-selective PDE isoenzyme inhibitor trequinsin was slow in onset and demonstrated very slow recovery, similar to that observed with the long-acting beta2-adrenoceptor agonist, salmeterol and the PDE4 inhibitor, RP 73401. The relaxant agonists also significantly reversed bombesin-induced bronchospasm in anaesthetised guinea-pigs and there was a highly significant correlation between the ability of drugs to reverse PGF2alpha-induced contraction of guinea-pig isolated trachea in vitro and bombesin-induced bronchoconstriction in vivo. Furthermore, both salmeterol and trequinsin demonstrated long lasting bronchodilator responses consistent with the in vitro data. These results show that PDE isoenzyme inhibitors demonstrate different pharmacodynamic profiles that is not determined by PDE4 inhibitory potency and indicate that other factors may be important in this regard.

Proposal for pharmacologically distinct conformers of PDE4 cyclic AMP phosphodiesterases

cAMP-specific phosphodiesterase inhibitors display a range of activities in vitro and in vivo which suggest they may be useful in the treatment of inflammatory diseases. However, these compounds elicit a number of side-effects which may limit their therapeutic potential. Certain side-effects of PDE4 inhibitors such as emesis and gastric acid secretion are associated with their actions at a high affinity rolipram binding site (HARBS). In contrast, a number of anti-inflammatory actions of PDE4 inhibitors are better correlated with inhibition of PDE4 catalytic activity than with displacement of [3H] rolipram from HARBS. This suggests that native PDE4s in different cell-types can be discriminated pharmacologically. Although known to be associated with PDE4, the nature of HARBS is uncertain. The majority of evidence suggests it represents particular conformational states of PDE subtypes with which rolipram interacts with high potency (KD approximately 2 nM) (High-affinity PDE4, HPDE4). Rolipram is generally moderately or weakly active (IC50-200 nM-2000 nM) in inhibiting catalytic activity of the majority of crude, partially-purified or recombinant PDE4-preparations (Low-affinity PDE4, LPDE4). Solubilization or V/GSH treatment of particulate eosinophil PDE4, cAMP-dependent kinase activation of RNPDE4D3 and membrane association of HSPDE4A4 increase the potencies of some (e.g., rolipram) but not other (e.g., trequinsin) inhibitors. In eosinophils, the changes in enzyme properties brought about by solubilization result in a close correlation between the potency order of compounds in inhibiting cAMP hydrolysis and displacing [3H] rolipram from HARBS. The identification of distinct pharmacological PDE4 forms may have therapeutic consequences since it may be possible to synthesize potent inhibitors of LPDE4 with low affinity for HARBS which should, theoretically, be less emetic. Most inhibitors synthesized to date (rolipram, denbufylline nitraquazone, etc.) display high-affinity for HARBS but are much weaker in inhibiting cAMP hydrolysis. Other compounds (RP 73401, trequinsin, CDP 840) display slightly higher potency against LPDE4 or do not discriminate between the two putative PDE4 forms. Recently, inhibitors have been synthesized which are considerably more active against LPDE4 than HPDE4. Such compounds with appropriate pharmacokinetic properties may retain anti-inflammatory activity but have a reduced capacity to cause nausea and emesis and, consequently, have a wider therapeutic window than compounds currently undergoing clincial evaluation.

Phosphodiesterase inhibitors prevent endothelin-1-induced vasoconstriction, bronchoconstriction, and thromboxane release in perfused rat lung

Endothelin-1 (ET-1) causes broncho- and vasoconstriction in the rat isolated perfused lung and induces the release of thromboxane and prostacyclin. Pharmacological inhibition of phosphodiesterases (PDE) is known to relax airway and vascular smooth muscle and it attenuates the release of pro-inflammatory mediators. Therefore, we examined whether and how rolipram (specific for PDE IV) and motapizone (specific for PDE III) affect ET-1-elicited changes in lung function. 5 microM motapizone attenuated broncho- and vasoconstriction to a greater extent than 5 microM rolipram. Simultaneous pretreatment with both PDE inhibitors protected completely. Thromboxane release was suppressed by rolipram, but not by motapizone. Prostacyclin release was neither influenced by single, nor by combined pretreatment with either compound. We conclude that combined inhibition of PDE III and IV counteracts ET-1-elicited pressor- and inflammatory actions in the lung.

Effects of agents which elevate cyclic AMP on guinea-pig eosinophil homotypic aggregation

1. Eosinophil recruitment and activation in inflamed tissue is thought to play an important role in the pathophysiology of allergic diseases. Experimental evidence suggests that elevating cyclic AMP is an effective means of reducing eosinophil recruitment in vivo and may therefore have therapeutic benefit. In the present study, we have assessed the capacity of cyclic AMP-elevating agents to modulate guinea-pig eosinophil homotypic aggregation, a CD18-dependent process, which may be an important component of eosinophil function in vivo. 2. Prostaglandin E1 (PGE1, 10(-16) to 10(-6) M) inhibited platelet activating-factor (PAF)- and C5a-induced eosinophil aggregation in a concentration-dependent manner. However, PAF-induced responses were more potently and more effectively inhibited by PGE1. The inhibitory effects of PGE1 on PAF-induced aggregation were reversed by pretreatment of eosinophils with the protein kinase A inhibitors H89 and KT5720. 3. The beta 2-adrenoceptor agonists, salbutamol and salmeterol, concentration-dependently inhibited eosinophil aggregation induced by C5a and PAF and, again. PAF-induced responses were more effectively reduced. The inhibitory effect of salmeterol was mediated by beta-adrenoceptors, as assessed by the reversal after pretreatment with propranolol. 4. Rolipram, a selective phosphodiesterase 4 (PDE4) inhibitor, also attenuated PAF- and C5a-induced aggregation and at a low concentration which did not affect aggregation per se, had a synergistic effect with PGE1 and salbutamol to suppress this response. 5. Activation of eosinophils with PAF or C5a induced a small but significant increase in the level of CD18 expression on the eosinophil surface. PGE1 (10(-7) M) decreased PAF- and C5a-induced upregulation of CD18 by 93% and 62%, respectively. 6. These results demonstrate that cyclic AMP-elevating agents effectively inhibit eosinophil aggregation, a CD18-dependent functional response. Because CD18 has been shown to be important for eosinophil recruitment to inflamed tissue in vivo, our findings may be of relevance to the efficacy of cyclic AMP-elevating agents at inhibiting eosinophil trafficking.

Cyclic AMP-elevating agents prolong or inhibit eosinophil survival depending on prior exposure to GM-CSF

1. Purified human eosinophils survived for up to 7 days when cultured in vitro in the presence of 1 ng ml-1 granulocyte-macrophage colony stimulating factor (GM-CSF) with a viability of 73%. In the absence of GM-CSF, eosinophil viability decreased after one day in culture, and only 4% of cells were viable by day 4. 2. Culture of eosinophils with cholera toxin produced a concentration-dependent decrease in GM-CSF-induced survival at 7 days (IC50 = 7 ng ml-1) which was associated with a 6 fold increase in the intracellular cyclic AMP concentration. This inhibition of cell survival could be prevented by the addition of the protein kinase A inhibitor, H89 (10(-6)M). 3. When eosinophils were cultured with dibutyryl cyclic AMP, there was a concentration-dependent inhibition of GM-CSF-induced survival at 7 days with an IC50 of 200 microM. The related cyclic nucleotide analogue, dibutyryl cyclic GMP did not inhibit GM-CSF-induced eosinophil survival over the same concentration range. 4. Culture of eosinophils with forskolin, or with the phosphodiesterase inhibitors, rolipram and SK&F94120, had no effect on GM-CSF-induced eosinophil survival at any concentration examined. 5. After 7 days' culture in the absence of GM-CSF, fractionation of eosinophil DNA on agarose gels demonstrated a 'ladder' pattern characteristic of apoptosis. GM-CSF prevented DNA fragmentation and this protection could be overcome by both cholera toxin and dibutyryl cyclic AMP. 6. GM-CSF did not affect intracellular cyclic AMP concentrations in unstimulated eosinophils or in cells stimulated by cholera toxin. Thus, GM-CSF does not apparently increase eosinophil survival by affecting cyclic AMP levels. 7. In the absence of GM-CSF both cholera toxin and dibutyryl cyclic AMP decreased the rate of eosinophil death, when compared to cells cultured with medium alone. The t1/2 values for cell death were 1.63 +/- 0.3, 2.46 +/- 0.3 and 4.62 +/- 1.0 days for cells cultured in the presence of medium, cholera toxin and dibutyryl cyclic AMP respectively. 8. In conclusion, cyclic AMP exerts opposing effects on eosinophil survival depending on prior exposure of the cells to GM-CSF.

Suppression of eosinophil function by RP 73401, a potent and selective inhibitor of cyclic AMP-specific phosphodiesterase: comparison with rolipram

1. We have investigated the inhibitory potency of RP 73401, a novel, highly selective and potent inhibitor of cyclic AMP-specific phosphodiesterase (PDE IV), against partially-purified PDE isoenzymes from smooth muscle and the particulate PDE IV from guinea-pig eosinophils. The inhibitory effects of RP 73401 on the generation of superoxide (.O2-), major basic protein (MBP) and eosinophil cationic protein (ECP) from guinea-pig eosinophils have also been studied. 2. RP 73401 potently inhibited partially-purified cyclic AMP-specific phosphodiesterase (PDE IV) from pig aortic smooth muscle (IC50 = 1.2 nM); it was similarly potent against the particulate PDE IV from guinea-pig peritoneal eosinophils (IC50 = 0.7 nM). It displayed at least a 19000 fold selectivity for PDE IV compared to its potencies against other PDE isoenzymes. Rolipram was approximately 2600 fold less potent than RP 73401 against pig aortic smooth muscle PDE IV (IC50 = 3162 nM) and about 250 times less potent against eosinophil PDE IV (IC50 = 186 nM). 3. Solubilization of the eosinophil particulate PDE IV increased the potency of rolipram 10 fold but did not markedly affect the potency of RP 73401. A similar (10 fold) increase in the PDE IV inhibitory potency of rolipram, but not RP 73401, was observed when eosinophil membranes were exposed to vanadate/glutathione complex (V/GSH). 4. Reverse transcription polymerase chain reaction (RT-PCR), using primer pairs designed against specific sequences in four distinct rat PDE IV subtype cDNA clones (PDE IVA-D), showed only mRNA for PDE IVD in guinea-pig eosinophils. PDE IVD was also the predominant subtype expressed in pig aortic smooth muscle cells. 5. RP 73401 (Kiapp = 0.4 nM) was 4 fold more potent than (+/-)-rolipram (Kiapp = 1.7 nM) in displacing[3H]-(+/-)-rolipram from guinea-pig brain membranes.6. In intact eosinophils, RP 73401 potentiated isoprenaline-induced cyclic AMP accumulation(EC50 = 79 nM). RP 73401 also inhibited leukotriene B4-induced generation of *02- (IC50 = 25 nM), and the release of major basic protein (ICo = 115 nM) and eosinophil cationic protein (IC50 = 7 nM). Rolipram was 3-14 times less potent than RP 73401.7. Thus RP 73401 is a very potent and selective PDE IV inhibitor which suppresses eosinophil function suggesting that it may be a useful agent for the treatment of inflammatory diseases such as asthma. The greatly different inhibitory potencies of rolipram against PDE IV from smooth muscle and eosinophils(in contrast to the invariable effects of RP 73401) are unlikely to be attributable to diverse PDE IV subtypes but suggest distinct interactions of the two inhibitors with the enzyme.

Anti-inflammatory and bronchodilator properties of RP 73401, a novel and selective phosphodiesterase type IV inhibitor

1. We have investigated the effects of RP 73401, a novel, potent and highly selective cyclic nucleotide phosphodiesterase (PDE) type IV inhibitor, in guinea-pig and rat models of bronchoconstriction and allergic inflammation. In some models, the effects of RP 73401 have been compared with those of the standard PDE type IV inhibitor, rolipram. 2. RP 73401 (0.4-400 micrograms kg-1, intratracheally (i.t.) on lactose) inhibited antigen-induced bronchospasm in previously sensitized conscious guinea-pigs (ID50: 7 +/- 1 micrograms kg-1) and in anaesthetized rats (ID50: 100 +/- 25 micrograms kg-1). Rolipram inhibited the antigen-induced bronchospasm in guinea-pigs with an ID50 of 5 +/- 1 micrograms kg-1. In guinea-pig bronchoalveolar lavage (BAL) fluid, total inflammatory cell and eosinophil numbers were reduced by RP 73401 (ID50s: 3.9 +/- 0.8 micrograms kg-1 and 3.2 +/- 0.7 micrograms kg-1, respectively). In the rat, inflammatory cell numbers are less affected. Only the highest dose of RP 73401 (400 micrograms kg-1) significantly inhibited eosinophil influx (41 +/- 16% inhibition). 3. RP 73401 (0.02-100 micrograms kg-1, i.v.) inhibited PAF-induced bronchial hyperreactivity to bombesin in the anaesthetized guinea-pig (ID50: 0.09 +/- 0.03 micrograms kg-1) and inhibited (0.4-40 micrograms kg-1, i.t.) histamine-induced airway microvascular leakage in the anaesthetized guinea-pig by approximately 60% at all doses. 4. RP 73401 relaxed guinea-pig isolated trachea under basal tone (EC50: 9 nM) and when precontracted with histamine (IC50: 2 nM), methacholine (IC50: 29 nM) or leukotriene D4 (LTD4, IC50: 4 nM). 5. RP 73401 (0.4-100 microg kg-1, i.t.) inhibited bronchospasm induced by histamine (ID.%: 34 +/- 6 microg kg-1), methacholine (ID50: 66 +/- 12 pg kg-1) and LTD4 (ID50: <4 microg kg-1) in the anaesthetized guinea pig.Against these same bronchoconstrictors, rolipram (i.t.) had ID5o values of 44 +/- 4, 72 +/- 18 and<4 pg kg- respectively. RP 73401 (4 and 40 pg kg-, i.t.) increased the magnitude and duration of bronchodilatation produced by salbutamol in the anaesthetized guinea-pig. At doses producing significant bronchodilatation, RP 73401 was without effect on heart rate or blood pressure in the anaesthetized guinea-pig. RP 73401 (0.01 -0.25 mg kg-1, i.v.) did not affect heart rate and produced only a small fall in blood pressure in the anaesthetized rat.6. These data demonstrate that RP 73401 and rolipram inhibit antigen- and mediator-induced bronchospasmin guinea-pigs with the same potency. Furthermore, RP 73401 administered directly into the airways, protects against allergic airway inflammation. These results indicate the importance of PDE IV in regulating smooth muscle and inflammatory cell activity. At doses suppressing the inflammatory response in the lung, RP 73401 had little effect in the cardiovascular system. RP 73401 may have a role as a bronchodilator and, more importantly, as a prophylactic anti-inflammatory agent in the treatment of asthma.