The next generation of PDE4 inhibitors

Harnessing C-O Bonds in Stereoselective Cross-Coupling and Cross-Electrophile Coupling Reactions

Herein, we discuss our laboratory's research in the activation of alcohol derivatives in cross-coupling and cross-electrophile coupling reactions. Our developed methods enable the use of secondary alcohols to afford tertiary stereogenic centers, which we applied to the synthesis of pharmaceutically relevant compounds and substructures. We first discuss the synthesis of bioactive compounds via stereospecific Kumada cross-coupling reactions, followed by a discussion on the development of our stereoselective cross-electrophile coupling reaction to synthesize cyclopropanes.

PAN-selective inhibition of cAMP-phosphodiesterase 4 (PDE4) induces gastroparesis in mice

Inhibitors of cAMP-phosphodiesterase 4 (PDE4) exert a number of promising therapeutic benefits, but adverse effects, in particular emesis and nausea, have curbed their clinical utility. Here, we show that PAN-selective inhibition of PDE4, but not inhibition of PDE3, causes a time- and dose-dependent accumulation of chow in the stomachs of mice fed ad libitum without changing the animals' food intake or the weight of their intestines, suggesting that PDE4 inhibition impairs gastric emptying. Indeed, PDE4 inhibition induced gastric retention in an acute model of gastric motility that traces the passage of a food bolus through the stomach over a 30 minutes time period. In humans, abnormal gastric retention of food is known as gastroparesis, a syndrome predominated by nausea (>90% of cases) and vomiting (>80% of cases). We thus explored the abnormal gastric retention induced by PDE4 inhibition in mice under the premise that it may represent a useful correlate of emesis and nausea. Delayed gastric emptying was produced by structurally distinct PAN-PDE4 inhibitors including Rolipram, Piclamilast, Roflumilast, and RS25344, suggesting that it is a class effect. PDE4 inhibitors induced gastric retention at similar or below doses commonly used to induce therapeutic benefits (e.g., 0.04 mg/kg Rolipram), thus mirroring the narrow therapeutic window of PDE4 inhibitors in humans. YM976, a PAN-PDE4 inhibitor that does not efficiently cross the blood-brain barrier, induced gastroparesis only at significantly higher doses (≥1 mg/kg). This suggests that PDE4 inhibition may act in part through effects on the autonomic nervous system regulation of gastric emptying and that PDE4 inhibitors that are not brain-penetrant may have an improved safety profile. The PDE4 family comprises four subtypes, PDE4A, B, C, and D. Selective ablation of any of these subtypes in mice did not induce gastroparesis per se, nor did it protect from PAN-PDE4 inhibitor-induced gastroparesis, indicating that gastric retention may result from the concurrent inhibition of multiple PDE4s. Thus, potentially, any of the four PDE4 subtypes may be targeted individually for therapeutic benefits without inducing nausea or emesis. Acute gastric retention induced by PDE4 inhibition is alleviated by treatment with the widely used prokinetic Metoclopramide, suggesting a potential of this drug to alleviate the side effects of PDE4 inhibitors. Finally, given that the cause of gastroparesis remains largely idiopathic, our findings open the possibility that a physiologic or pathophysiologic downregulation of PDE4 activity/expression may be causative in a subset of patients.

Enantioselective Intermolecular C-O Bond Formation in the Desymmetrization of Diarylmethines Employing a Guanidinylated Peptide-Based Catalyst

We report a series of enantioselective C-O bond cross-coupling reactions based on remote symmetry breaking processes in diarylmethine substrates. The key to the chemistry is multifunctional guanidinylated peptide-based ligands that allow highly selective, intermolecular Cu-catalyzed cross-coupling of phenolic nucleophiles. The scope of the process is explored, demonstrating efficiency for substrates with a range of electronic and steric perturbations to the nucleophile. Scope and limitations are also reported for variation of the diarylmethine. While the presence of an intervening tBu group is found to be optimal for maximum enantioselectivity, several other substituents may also be present such that appreciable selectivity can be achieved, providing an uncommon level of scope for diarylmethine desymmetrizations. In addition, chemoselective reactions are possible when there are phenolic hydroxyl groups within substrates that contain a second reactive site, setting the stage for applications in diverse complex molecular settings.

Assessing protein-ligand binding modes with computational tools: the case of PDE4B

In a first step in the discovery of novel potent inhibitor structures for the PDE4B family with limited side effects, we present a protocol to rank newly designed molecules through the estimation of their IC[Formula: see text] values. Our protocol is based on reproducing the linear relationship between the logarithm of experimental IC[Formula: see text] values [[Formula: see text](IC[Formula: see text])] and their calculated binding free energies ([Formula: see text]). From 13 known PDE4B inhibitors, we show here that (1) binding free energies obtained after a docking process by AutoDock are not accurate enough to reproduce this linear relationship; (2) MM-GB/SA post-processing of molecular dynamics (MD) trajectories of the top ranked AutoDock pose improves the linear relationship; (3) by taking into account all representative structures obtained by AutoDock and by averaging MM-GB/SA computations on a series of 40 independent MD trajectories, a linear relationship between [Formula: see text](IC[Formula: see text]) and the lowest [Formula: see text] is achieved with [Formula: see text].

Modulation of Compartmentalised Cyclic Nucleotide Signalling via Local Inhibition of Phosphodiesterase Activity

Cyclic nucleotide phosphodiesterases (PDEs) are the only enzymes that degrade the cyclic nucleotides cAMP and cGMP, and play a key role in modulating the amplitude and duration of the signal delivered by these two key intracellular second messengers. Defects in cyclic nucleotide signalling are known to be involved in several pathologies. As a consequence, PDEs have long been recognized as potential drug targets, and they have been the focus of intense research for the development of therapeutic agents. A number of PDE inhibitors are currently available for the treatment of disease, including obstructive pulmonary disease, erectile dysfunction, and heart failure. However, the performance of these drugs is not always satisfactory, due to a lack of PDE-isoform specificity and their consequent adverse side effects. Recent advances in our understanding of compartmentalised cyclic nucleotide signalling and the role of PDEs in local regulation of cAMP and cGMP signals offers the opportunity for the development of novel strategies for therapeutic intervention that may overcome the current limitation of conventional PDE inhibitors.

Discovery and modelling studies of natural ingredients from Gaultheria yunnanensis (FRANCH.) against phosphodiesterase-4

Phosphodiesterase-4 (PDE4) is an anti-inflammatory target for treatment of asthma and chronic obstructive pulmonary disease (COPD). Here, we report the isolation and characterization of 13 compounds (G1-G13) by bioassay-guided fractionation of the ethyl acetate extraction of Gaultheria yunnanensis (FRANCH.), one of which pentacyclic triterpene (G1) has never been reported. Four of them (G1, G2, G4, and G5) inhibit PDE4 with the IC50 values < 20 μM and G1 is the most potent ingredient with an IC50 of 245 nM and moderate selectivity over other PDE families. Molecular dynamics simulations suggest that G1 forms a hydrogen bond with Asn362, in addition to the hydrogen bond with Gln369 and π-π interactions with Phe372, which are commonly observed in the binding of most PDE4 inhibitors. The calculated binding free energies for the interactions of PDE4-G1 and PDE4-G2 are -19.4 and -18.8 kcal/mol, in consistence with the bioassay that G1 and G2 have IC50 of 245 nM and 542 nM, respectively. The modelling results of these active compounds may aid the rational design of novel PDE4 inhibitors as anti-inflammatory agents.

Fluorine in medicinal chemistry

Since its first use in the steroid field in the late 1950s, the use of fluorine in medicinal chemistry has become commonplace, with the small electronegative fluorine atom being a key part of the medicinal chemist's repertoire of substitutions used to modulate all aspects of molecular properties including potency, physical chemistry and pharmacokinetics. This review will highlight the special nature of fluorine, drawing from a survey of marketed fluorinated pharmaceuticals and the medicinal chemistry literature, to illustrate key concepts exploited by medicinal chemists in their attempts to optimize drug molecules. Some of the potential pitfalls in the use of fluorine will also be highlighted.

Selective inhibitors of phosphodiesterases: therapeutic promise for neurodegenerative disorders

PDE inhibitors: significant contributors to the treatment of neurodegenerative diseases.

The role of phosphodiesterase inhibitors in the management of pulmonary vascular diseases

Phosphodiesterase inhibitors (PDE) can be used as therapeutic agents for various diseases such as dementia, depression, schizophrenia and erectile dysfunction in men, as well as congestive heart failure, chronic obstructive pulmonary disease, rheumatoid arthritis, other inflammatory diseases, diabetes and various other conditions. In this review we will concentrate on one type of PDE, mainly PDE5 and its role in pulmonary vascular diseases.

A General, Simple Catalyst for Enantiospecific Cross Couplings of Benzylic Ammonium Triflates and Boronic Acids: No Phosphine Ligand Required

Highly improved conditions for the enantiospecific cross coupling of benzylic ammonium triflates with boronic acids are reported. This method relies on the use of Ni(cod)₂ without ancillary phosphine or N-heterocyclic carbene ligands as catalyst. These conditions enable the coupling of new classes of boronic acids and benzylic ammonium triflates. In particular, both heteroaromatic and vinyl boronic acids are well tolerated as coupling partners. In addition, these conditions enable the use of ammonium triflates with a variety of substituents at the benzylic stereocenter. Further, naphthyl-substitution is not required on the benzylic ammonium triflate; ammonium triflates with simple aromatic substituents also undergo this coupling. Good to high yields and levels of stereochemical fidelity are observed. This new catalyst system greatly expands the utility of enantiospecific cross couplings of these amine-derived substrates for the preparation of highly enantioenriched products.

Nickel-catalyzed cross-couplings of benzylic pivalates with arylboroxines: stereospecific formation of diarylalkanes and triarylmethanes

We have developed a stereospecific nickel-catalyzed cross-coupling of benzylic pivalates with arylboroxines. The success of this reaction relies on the use of Ni(cod)2 as the catalyst and NaOMe as a uniquely effective base. This reaction has broad scope with respect to the arylboroxine and benzylic pivalate, enabling the synthesis of a variety of diarylalkanes and triarylmethanes in good to excellent yields and ee's.

Nickel-catalyzed cross couplings of benzylic ammonium salts and boronic acids: stereospecific formation of diarylethanes via C-N bond activation

We have developed a nickel-catalyzed cross coupling of benzylic ammonium triflates with aryl boronic acids to afford diarylmethanes and diarylethanes. This reaction proceeds under mild reaction conditions and with exceptional functional group tolerance. Further, it transforms branched benzylic ammonium salts to diarylethanes with excellent chirality transfer, offering a new strategy for the synthesis of highly enantioenriched diarylethanes from readily available chiral benzylic amines.

A biphasic and brain-region selective down-regulation of cyclic adenosine monophosphate concentrations supports object recognition in the rat

BACKGROUND

We aimed to further understand the relationship between cAMP concentration and mnesic performance.

METHODS AND FINDINGS

Rats were injected with milrinone (PDE3 inhibitor, 0.3 mg/kg, i.p.), rolipram (PDE4 inhibitor, 0.3 mg/kg, i.p.) and/or the selective 5-HT4R agonist RS 67333 (1 mg/kg, i.p.) before testing in the object recognition paradigm. Cyclic AMP concentrations were measured in brain structures linked to episodic-like memory (i.e. hippocampus, prefrontal and perirhinal cortices) before or after either the sample or the testing phase. Except in the hippocampus of rolipram treated-rats, all treatment increased cAMP levels in each brain sub-region studied before the sample phase. After the sample phase, cAMP levels were significantly increased in hippocampus (1.8 fold), prefrontal (1.3 fold) and perirhinal (1.3 fold) cortices from controls rat while decreased in prefrontal cortex (∼0.83 to 0.62 fold) from drug-treated rats (except for milrinone+RS 67333 treatment). After the testing phase, cAMP concentrations were still increased in both the hippocampus (2.76 fold) and the perirhinal cortex (2.1 fold) from controls animals. Minor increase were reported in hippocampus and perirhinal cortex from both rolipram (respectively, 1.44 fold and 1.70 fold) and milrinone (respectively 1.46 fold and 1.56 fold)-treated rat. Following the paradigm, cAMP levels were significantly lower in the hippocampus, prefrontal and perirhinal cortices from drug-treated rat when compared to controls animals, however, only drug-treated rats spent longer time exploring the novel object during the testing phase (inter-phase interval of 4 h).

CONCLUSIONS

Our results strongly suggest that a "pre-sample" early increase in cAMP levels followed by a specific lowering of cAMP concentrations in each brain sub-region linked to the object recognition paradigm support learning efficacy after a middle-term delay.

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.

Stereospecific nickel-catalyzed cross-coupling reactions of alkyl ethers: enantioselective synthesis of diarylethanes

Secondary benzylic ethers undergo stereospecific substitution reactions with Grignard reagents in the presence of nickel catalysts. Reactions proceed with inversion of configuration and high stereochemical fidelity. This reaction allows for facile enantioselective synthesis of biologically active diarylethanes from readily available optically enriched carbinols.

A review of the herbal phosphodiesterase inhibitors; future perspective of new drugs

Phosphodiesterase inhibitors (PDEIs) are a class of drugs that are widely used because of their various pharmacological properties including cardiotonic, vasodilator, smooth muscle relaxant, antidepressant, antithrombotic, bronchodilator, antiinflammatory and enhancer of cognitive function. In the recent years, interest in drugs of plant origin has been progressively increased. Some pharmacologically active substances that come from plants demonstrate PDEI activity. They mainly belong to alkaloids, flavonoids, and saponins. In this review, studies on herbal PDEI were reviewed and their possible therapeutic applications were discussed. Screening plants for PDE inhibitory activity may help to develop standardized phytotherapeutic products or find new sources for new lead structures with PDEI pharmacological activity. The studies discussed in this paper are mainly in vitro and for more reasonable and conclusive results, it is required to conduct in vivo and finally human and clinical tests.

Phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes

Cyclic AMP is part of an endogenous mechanism that downregulates inflammatory response, and its intracellular concentration is regulated chiefly by cyclic nucleotide phosphodiesterases type 4. The goal of the present study was to determine whether phosphodiesterases 4 are involved in the inflammatory response of astrocytes mediated by Toll-like receptors. Astrocyte cultures established from newborn rat brain were challenged with lipoteichoic acid, a ligand of Toll-like receptor 2, polyinosinic-polycytidylic acid, a ligand of Toll-like receptor 3, or lipopolysaccharide, a ligand of Toll-like receptor 4. After 24 h the expression of genes encoding phosphodiesterase 4A, phosphodiesterase 4B and phosphodiesterase 4D was determined by real time reverse transcription polymerase chain reaction. The challenge of astrocytes with the ligands profoundly up-regulated expression of the phosphodiesterase 4B mRNA, while the phosphodiesterase 4A and 4D mRNA was either unaffected or downregulated. Moreover, Toll-like receptor ligation specifically up-regulated expression of the phosphodiesterase 4B2 transcriptional variant. Thus, polyinosinic-polycytidylic acid, lipopolysaccharide and lipoteichoic acid induced approximately 7-, 5- and 4-fold up-regulation of the message, respectively. Toll-like receptor ligation also led to an over 2-fold increase in the protein level of phosphodiesterase 4B2 as revealed by immunoblot analysis. The inactivation of Rho proteins by pretreatment with toxin B form C. difficile enhanced ligation-induced up-regulation of the phosphodiesterase 4B2 message by 4-9-fold. However, in spite of this increase in the message abundance, there was no increase in the protein level compared to cells challenged with the ligands alone. These results demonstrate that the phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes, and that its up-regulation at the protein level is controlled by complex mechanisms.

Exploration and optimization of substituted triazolothiadiazines and triazolopyridazines as PDE4 inhibitors

An expansion of structure-activity studies on a series of substituted 7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazine PDE4 inhibitors and the introduction of a related [1,2,4]triazolo[4,3-b]pyridazine based inhibitor of PDE4 is presented. The development of SAR included strategic incorporation of known substituents on the critical catachol diether moiety of the 6-phenyl appendage on each heterocyclic core. From these studies, (R)-3-(2,5-dimethoxyphenyl)-6-(4-methoxy-3-(tetrahydrofuran-3-yloxy)phenyl)-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazine (10) and (R)-3-(2,5-dimethoxyphenyl)-6-(4-methoxy-3-(tetrahydrofuran-3-yloxy)phenyl)-[1,2,4]triazolo[4,3-b]pyridazine (18) were identified as highly potent PDE4A inhibitors. Each of these analogues was submitted across a panel of 21 PDE family members and was shown to be highly selective for PDE4 isoforms (PDE4A, PDE4B, PDE4C, PDE4D). Both 10 and 18 were then evaluated in divergent cell-based assays to assess their relevant use as probes of PDE4 activity. Finally, docking studies with selective ligands (including 10 and 18) were undertaken to better understand this chemotypes ability to bind and inhibit PDE4 selectively.

Reduced adiposity and high-fat diet-induced adipose inflammation in mice deficient for phosphodiesterase 4B

The concept that obesity is an inflammatory state has changed our understanding of this condition and suggested that pharmacological interventions targeting inflammation may be useful strategies to improve metabolic complications of obesity. Phosphodiesterase 4 (PDE4) inhibitors exhibit profound antiinflammatory effects, but whether PDE4 inhibition suppresses obesity-induced inflammation is unknown. Among PDE4 isoforms, PDE4B is the major species mediating inflammatory responses. We therefore examined obesity-related phenotypes in mice deficient for PDE4B. Compared with wild-type littermates, PDE4B-null mice were leaner, with lower fat pad weights, smaller adipocytes, and decreased serum leptin levels on both chow and high-fat diets (HFDs). PDE4B deficiency suppressed TNF-alpha mRNA levels and macrophage infiltration in white adipose tissue in mice on HFD, but insulin sensitivity was unaltered. PDE4B-null mice on HFDs had increased locomotor activity. These results suggest a previously unappreciated role for PDE4B in the regulation of energy balance and that PDE4B inhibitors could have utility in treatment of obesity and for suppression of obesity-induced inflammation in white adipose tissue.

The role of the PDE4D cAMP phosphodiesterase in the regulation of glucagon-like peptide-1 release

BACKGROUND AND PURPOSE

Increases in intracellular cyclic AMP (cAMP) augment the release/secretion of glucagon-like peptide-1 (GLP-1). As cAMP is hydrolysed by cAMP phosphodiesterases (PDEs), we determined the role of PDEs and particularly PDE4 in regulating GLP-1 release.

EXPERIMENTAL APPROACH

GLP-1 release, PDE expression and activity were investigated using rats and GLUTag cells, a GLP-1-releasing cell line. The effects of rolipram, a selective PDE4 inhibitor both in vivo and in vitro and stably overexpressed catalytically inactive PDE4D5 (D556A-PDE4D5) mutant in vitro on GLP-1 release were investigated.

KEY RESULTS

Rolipram (1.5 mg x kg(-1) i.v.) increased plasma GLP-1 concentrations approximately twofold above controls in anaesthetized rats and enhanced glucose-induced GLP-1 release in GLUTag cells (EC(50) approximately 1.2 nmol x L(-1)). PDE4D mRNA transcript and protein were detected in GLUTag cells using RT-PCR with gene-specific primers and Western blotting with a specific PDE4D antibody respectively. Moreover, significant PDE activity was inhibited by rolipram in GLUTag cells. A GLUTag cell clone (C1) stably overexpressing the D556A-PDE4D5 mutant, exhibited elevated intracellular cAMP levels and increased basal and glucose-induced GLP-1 release compared with vector-transfected control cells. A role for intracellular cAMP/PKA in enhancing GLP-1 release in response to overexpression of D556A-PDE4D5 mutant was demonstrated by the finding that the PKA inhibitor H89 reduced both basal and glucose-induced GLP-1 release by 37% and 39%, respectively, from C1 GLUTag cells.

CONCLUSIONS AND IMPLICATIONS

PDE4D may play an important role in regulating intracellular cAMP linked to the regulation of GLP-1 release.

Novel drugs and therapeutic targets for severe mood disorders

Monoaminergic-based drugs remain the primary focus of pharmaceutical industry drug discovery efforts for mood disorders, despite serious limitations regarding their ability to achieve remission. The quest for novel therapies for unipolar depression and bipolar disorder has generally centered on two complementary approaches: (1) understanding the presumed therapeutically relevant biochemical targets of currently available medications, and using that knowledge to design new drugs directed at both direct biochemical targets and downstream targets that are regulated by chronic drug administration; and (2) developing pathophysiological models of the illness to design therapeutics to attenuate or prevent those pathological processes. This review describes several promising drugs and drug targets for mood disorders using one or both of these approaches. Agents interacting with non-catecholamine neurotransmitter systems with particular promise for unipolar and bipolar depression include excitatory amino acid neurotransmitter modulators (eg, riluzole, N-methyl-D-aspartate antagonists, and AMPA receptor potentiators) and neuropeptide antagonists (targeting corticotropin releasing factor-1 and neurokinin receptors). Potential antidepressant and mood-stabilizing agents targeting common intracellular pathways of known monoaminergic agents and lithium/mood stabilizers are also reviewed, such as neurotrophic factors, extracellular receptor-coupled kinase (ERK) mitogen-activated protein (MAP) kinase and the bcl-2 family of proteins, and inhibitors of phosphodiesterase, glycogen synthase kinase-3, and protein kinase C. A major thrust of drug discovery in mood disorders will continue efforts to identify agents with rapid and sustained onsets of action (such as intravenous administration of ketamine), as well as identify drugs used routinely in non-psychiatric diseases for their antidepressant and mood-stabilizing properties.

Identification of a potent new chemotype for the selective inhibition of PDE4

A series of substituted 3,6-diphenyl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines were prepared and analyzed as inhibitors of phosphodiesterase 4 (PDE4). Synthesis, structure-activity relationships, and the selectivity of a highly potent analogue against related phosphodiesterase isoforms are presented.

Evaluation of PDE4 inhibition for COPD

Targeting type 4 phosphodiesterase (PDE4) for treatment of COPD has multilevel benefits to patients by reducing inflammation, relieving bronchoconstriction, and improving pulmonary circulation. The isoenzyme-specific narrow spectrum PDE4 inhibitors such as cilomilast and roflumilast may have limited clinical efficacy in managing severe and very severe COPD. Development of dual therapy by combining PDE4 inhibition with Ca²⁺ channel antagonism may introduce an effective novel armory for physicians to manage patients with severe COPD.

PDE4 associates with different scaffolding proteins: modulating interactions as treatment for certain diseases

cAMP is an ubiquitous second messenger that is crucial to many cellular processes. The sole means of terminating the cAMP signal is degradation by cAMP phosphodiesterases (PDEs). The PDE4 family is of particular interest because PDE4 inhibitors have therapeutic potential for the treatment of various inflammatory and auto-immune diseases and also have anti-depressant and memory-enhancing effects. The subcellular targeting of PDE4 isoforms is fundamental to the compartmentalization of cAMP signaling pathways and is largely achieved via proteinprotein interactions. Increased knowledge of these protein-protein interactions and their regulatory properties could aid in the design of novel isoform-specific inhibitors with improved efficacy and fewer prohibitive side effects.

Identification of genomic targets downstream of p38 mitogen-activated protein kinase pathway mediating tumor necrosis factor-alpha signaling

Inhibition of p38 MAPK suppresses the expression of proinflammatory cytokines such as TNF-alpha and IL-1 beta in macrophages and fibroblast-like synoviocytes (FLS). However, there have been no genomewide studies on the gene targets of p38 MAPK signaling in synoviocytes. Microarray technology was applied to generate a comprehensive analysis of all genes regulated by the p38 MAPK signaling pathway in FLS. Gene expression levels were measured with Agilent oligonucleotide microarrays. Four independent sets of mRNA modulated by TNF-alpha and vehicle were used to measure the change of gene expression due to TNF-alpha, and three experiments were done to ascertain the effect of SB-203580, a p38 MAPK inhibitor, on TNF-alpha-induced genes. Microarray data were validated by RT-quantitative polymerase chain reaction. One hundred forty-one significantly expressed genes were more than twofold upregulated by TNF-alpha. Thirty percent of these genes were downregulated by the p38 inhibitor SB-203580, whereas 67% of these genes were not significantly changed. The SB-203580-inhibited genes include proinflammatory cytokines such as interleukins and chemokines, proteases including matrix metallopeptidases, metabolism-related genes such as cyclooxygenases and phosphodiesterase, genes involved in signal transduction, and genes encoding for transcription factors, receptors, and transporters. Approximately one-third of the TNF-alpha-induced genes in FLS are regulated by the p38 MAPK signal pathway, showing that p38 MAPK is a possible target for suppressing proinflammatory gene expressions in rheumatoid arthritis.

PDE4B5, a novel, super-short, brain-specific cAMP phosphodiesterase-4 variant whose isoform-specifying N-terminal region is identical to that of cAMP phosphodiesterase-4D6 (PDE4D6)

The cAMP-specific phosphodiesterase-4 (PDE4) gene family is the target of several potential selective therapeutic inhibitors. The four PDE4 genes generate several distinct protein-coding isoforms through the use of alternative promoters and 5'-coding exons. Using mouse transcripts, we identified a novel, super-short isoform of human PDE4B encoding a novel 5' terminus, which we label PDE4B5. The protein-coding region of the novel 5' exon is conserved across vertebrates, chicken, zebrafish, and fugu. Reverse-transcription-polymerase chain reaction (PCR) and quantitative (PCR) measurements show that this isoform is brain-specific. The novel protein is 58 +/- 2 kDa; it has cAMP hydrolyzing enzymatic activity and is inhibited by PDE4-selective inhibitors rolipram and cilomilast (Ariflo). Confocal and subcellular fractionation analyses show that it is distributed predominantly and unevenly within the cytosol. The 16 novel N-terminal residues of PDE4B5 are identical to the 16 N-terminal residues of the super-short isoform of PDE4D (PDE4D6), which is also brain-specific. PDE4B5 is able to bind the scaffold protein DISC1, whose gene has been linked to schizophrenia. Microarray expression profiling of the PDE4 gene family shows that specific PDE4 genes are enriched in muscle and blood fractions; however, only by monitoring the individual isoforms is the brain specificity of the super-short PDE4D and PDE4B isoforms revealed. Understanding the distinct tissue specificity of PDE4 isoforms will be important for understanding phosphodiesterase biology and opportunities for therapeutic intervention.

Mapping binding sites for the PDE4D5 cAMP-specific phosphodiesterase to the N- and C-domains of beta-arrestin using spot-immobilized peptide arrays

Beta2-ARs (beta2-adrenoceptors) become desensitized rapidly upon recruitment of cytosolic beta-arrestin. PDE4D5 (family 4 cAMP-specific phosphodiesterase, subfamily D, isoform 5) can be recruited in complex with beta-arrestin, whereupon it regulates PKA (cAMP-dependent protein kinase) phosphorylation of the beta2-AR. In the present study, we have used novel technology, employing a library of overlapping peptides (25-mers) immobilized on cellulose membranes that scan the entire sequence of beta-arrestin 2, to define the interaction sites on beta-arrestin 2 for binding of PDE4D5 and the cognate long isoform, PDE4D3. We have identified a binding site in the beta-arrestin 2 N-domain for the common PDE4D catalytic unit and two regions in the beta-arrestin 2 C-domain that confer specificity for PDE4D5 binding. Alanine-scanning peptide array analysis of the N-domain binding region identified severely reduced interaction with PDE4D5 upon R26A substitution, and reduced interaction upon either K18A or T20A substitution. Similar analysis of the beta-arrestin 2 C-domain identified Arg286 and Asp291, together with the Leu215-His220 region, as being important for binding PDE4D5, but not PDE4D3. Transfection with wild-type beta-arrestin 2 profoundly decreased isoprenaline-stimulated PKA phosphorylation of the beta2-AR in MEFs (mouse embryo fibroblasts) lacking both beta-arrestin 1 and beta-arrestin 2. This effect was negated using either the R26A or the R286A mutant form of beta-arrestin 2 or a mutant with substitution of an alanine cassette for Leu215-His220, which showed little or no PDE4D5 binding, but was still recruited to the beta2-AR upon isoprenaline challenge. These data show that the interaction of PDE4D5 with both the N- and C-domains of beta-arrestin 2 are essential for beta2-AR regulation.

Differential effects of phosphodiesterase PDE-3/PDE-4-specific inhibitors on vasoconstriction and cAMP-dependent vasorelaxation following balloon angioplasty

It is known that cAMP and cGMP are important for vasorelaxation, and cyclic nucleotide phosphodiesterases (PDEs) regulate their levels. Balloon angioplasty (BAL) is associated with reduced cAMP and cGMP levels, and inhibition of PDE-3 reduces restenosis. In this study, we found that BAL increased PDE-3 activity, which affected vasoreactivity of rat aortic rings 24-h post-BAL; these were compared with intact (INT) and ex vivo endothelium-denuded rings (RUB) from sham rats. In BAL and RUB rings, vasorelaxant responses to ACh were abolished. The EC(50) for phenylephrine (PE) was 1.8-fold less in RUB than in INT or BAL, whereas the maximal contractile effect of PE was greater in BAL than in INT or RUB. PDE-3 inhibitors reduced the maximal response to PE by >65% in BAL compared with 10-30% in INT and RUB; the reduction of the maximal response to U-46619 was 37% in BAL compared with 8% in INT with no reduction in RUB. PDE-4 inhibitors reduced PE-induced tone by <30% in an endothelium-dependent manner. Vasorelaxant responses to agonists that utilize cAMP were greatly impaired in BAL and RUB rings, and inhibition of PDE-3 enhanced the vasorelaxant responses in BAL or RUB. Inhibition of PDE-4 increased vasorelaxant responses to isoproterenol (ISO) to a much lesser degree. Thus PDE-3 and PDE-4 inhibitors exhibited differential effects on PE-induced tone and vasorelaxant responses to ISO. Inhibition of PDE-3 also produced a greater increase in cAMP in BAL than INT or RUB rings. These results suggest that increased PDE-3 activity after BAL may promote a vasospastic state and that the reduction in cAMP may, possibly, influence vessel remodeling.

Nasal cytotoxic and carcinogenic activities of systemically distributed organic chemicals

Toxicity and carcinogenicity in the mucosa of the nasal passages in rodents has been produced by a variety of organic chemicals which are systemically distributed. In this review, 14 such chemicals or classes were identified that produced rodent nasal cytotoxicity, but not carcinogenicity, and 11 were identified that produced nasal carcinogenicity. Most chemicals that affect the nasal mucosa were either concentrated in that tissue or readily activated there, or both. All chemicals with effects in the nasal mucosa that were DNA-reactive, were also carcinogenic, if adequately tested. None of the rodent nasal cytotoxins has been identified as a human systemic nasal toxin. This may reflect the lesser biotransformation activity of human nasal mucosa compared to rodent and the much lower levels of human exposures. None of the rodent carcinogens lacking DNA reactivity has been identified as a nasal carcinogen or other cancer hazard to humans. Some DNA-reactive rodent carcinogens that affect the nasal mucosa, as well as other tissues, have been associated with cancer at various sites in humans, but not the nasal cavity. Thus, findings in only the rodent nasal mucosa do not necessarily predict either a toxic or carcinogenic hazard to that tissue in humans.

Oxidative stress employs phosphatidyl inositol 3-kinase and ERK signalling pathways to activate cAMP phosphodiesterase-4D3 (PDE4D3) through multi-site phosphorylation at Ser239 and Ser579

RAW macrophages, which express the PDE4D3 and PDE4D5 cAMP phosphodiesterase isoforms, exhibited increased PDE4 activity when challenged with H2O2 in a fashion that was negated by treatment with the cell permeant antioxidant, N-acetyl cysteine and by diphenyleneiodonium chloride, an inhibitor of NADPH oxidase. In Cos1 cells transfected to express PDE4D3, challenge with H2O2 caused a rapid increase in both the activity and phosphorylation of PDE4D3. Lysates from H2O2-treated COS cells caused the phosphorylation of purified, recombinant PDE4D3 at two sites. One was the established ERK phosphorylation site at Ser579, located at the extreme C-terminus of the catalytic unit, and the other was a novel site at Ser239, located at the extreme N-terminus of the catalytic unit. Double Ser239Ala:Ser579Ala mutation of PDE4D3 prevented its H2O2-dependent phosphorylation both in vitro and in intact COS cells. Phosphorylation of PDE4D3 at Ser579 was ablated by treating COS cells with the MEK inhibitor, PD98059, which also negated activation. The activity of the Ser239Ala:Ser579Ala double mutant, and the Ser579Ala single PDE4D3 mutant was unaffected by H2O2 challenge of COS cells, whilst the Ser239Ala mutant was inhibited. Wortmannin inhibited the H2O2-dependent phosphorylation of PDE4D3 in COS cells by around 50%, whilst it fully ablated phosphorylation at Ser239 as well as ablating activation of PDE4D3. Neither immunodepletion of p70S6 kinase nor siRNA-mediated knockdown of mTor inhibited the H2O2-dependent phosphorylation of PDE4D3 at Ser239. Activation of PDE4D3 by challenge with H2O2 was not additive with activation through protein kinase A (PKA)-mediated phosphorylation of PDE4D3. Challenge with H2O2 did not alter PKA-mediated phosphorylation of PDE4D3 at Ser54. H2O2 dependent phosphorylation of PDE4D3, at Ser239 and Ser579, did not alter the sensitivity of PDE4D3 to inhibition by the selective PDE4 inhibitor, rolipram. An unknown protein kinase acting downstream of phosphatidyl inositol 3-kinase phosphorylates PDE4D3 at Ser239. This switches the effect of phosphorylation by ERK at Ser579 from inhibition to activation. We propose that phosphorylation at Ser239 attenuates interaction between either UCR2 or the UCR1/UCR2 module and the PDE4 catalytic unit so as to re-programme the functional outcome effect of phosphorylation by ERK. We identify a novel process through which reactive oxygen species activate long PDE4 isoforms so as to reduce cAMP levels and thereby promote inflammatory responses.

Pharmacological profile of a novel phosphodiesterase 7A and -4 dual inhibitor, YM-393059, on acute and chronic inflammation models

YM-393059 is a novel phosphodiesterase (PDE) 7A and PDE4 dual inhibitor that inhibits both Th1 [interleukin (IL)-2 and interferon-gamma] and Th2 (IL-4) cytokines in vitro [Yamamoto, S., Sugahara, S., Naito, R., Ichikawa, A., Ikeda, K., Yamada, T., Shimizu, Y., 2006. The effects of a novel phosphodiesterase 7A and -4 dual inhibitor, YM-393059, on T-cell-related cytokine production in vitro and in vivo. Eur. J. Pharmacol. 541, 106-114]. To characterize the pharmacological profile of YM-393059, its effects on several acute and chronic inflammation models were examined. In acute inflammation models, YM-393059 significantly suppressed the delayed-type hypersensitivity reaction to sheep red blood cells in mice with an ED(50) value of 17.1 mg/kg. YM-393059 failed to suppress paw edema in the carrageenin-induced edema model in rats. These pharmacological effects were similar to those of cyclosporine, a typical T-cell immunosuppressant. However, YM-393059, but not cyclosporine, significantly inhibited zymosan-induced neutrophil accumulation in mice with an ED(50) value of 25.7 mg/kg. In mouse toluene-2,4-diisocyanate-induced contact dermatitis, a chronic inflammation model, YM-393059 and cyclosporine significantly suppressed ear edema at doses of 30 and 20 mg/kg, respectively. In this model, YM-393059 also tended to reduce the serum immunoglobulin E antibody level, whereas cyclosporine dramatically potentiated it. These results suggest that YM-393059 inhibits both Th1- and Th2-cell-dependent reactions and also the function of neutrophils.

Scanning peptide array analyses identify overlapping binding sites for the signalling scaffold proteins, beta-arrestin and RACK1, in cAMP-specific phosphodiesterase PDE4D5

The cAMP-specific phosphodiesterase PDE4D5 can interact with the signalling scaffold proteins RACK (receptors for activated C-kinase) 1 and beta-arrestin. Two-hybrid and co-immunoprecipitation analyses showed that RACK1 and beta-arrestin interact with PDE4D5 in a mutually exclusive manner. Overlay studies with PDE4D5 scanning peptide array libraries showed that RACK1 and beta-arrestin interact at overlapping sites within the unique N-terminal region of PDE4D5 and at distinct sites within the conserved PDE4 catalytic domain. Screening scanning alanine substitution peptide arrays, coupled with mutagenesis and truncation studies, allowed definition of RACK1 and beta-arrestin interaction sites. Modelled on the PDE4D catalytic domain, these form distinct well-defined surface-exposed patches on helices-15-16, for RACK1, and helix-17 for beta-arrestin. siRNA (small interfering RNA)-mediated knockdown of RACK1 in HEK-293 (human embryonic kidney) B2 cells increased beta-arrestin-scaffolded PDE4D5 approx. 5-fold, increased PDE4D5 recruited to the beta2AR (beta2-adrenergic receptor) upon isoproterenol challenge approx. 4-fold and severely attenuated (approx. 4-5 fold) both isoproterenol-stimulated PKA (protein kinase A) phosphorylation of the beta2AR and activation of ERK (extracellular-signal-regulated kinase). The ability of a catalytically inactive form of PDE4D5 to exert a dominant negative effect in amplifying isoproterenol-stimulated ERK activation was ablated by a mutation that blocked the interaction of PDE4D5 with beta-arrestin. In the present study, we show that the signalling scaffold proteins RACK1 and beta-arrestin compete to sequester distinct 'pools' of PDE4D5. In this fashion, alterations in the level of RACK1 expression may act to modulate signal transduction mediated by the beta2AR.

cAMP phosphodiesterase-4A1 (PDE4A1) has provided the paradigm for the intracellular targeting of phosphodiesterases, a process that underpins compartmentalized cAMP signalling

Specificity of cAMP signalling pathways has shown that the intracellular targeting of the individual components confers a three-dimensional context to the signalling paradigms in which they can exquisitely control the specificity of the outcome of the signal. Pivotal to this paradigm is degradation of cAMP by sequestered PDEs (phosphodiesterases). cAMP rapidly diffuses within cells and, without the action of spatially confined PDE populations, cAMP gradients could not be formed and shaped within cells so as to regulate targeted effector proteins. Of particular importance in regulating compartmentalized cAMP signalling are isoforms of the PDE4 family, which are individually defined by unique N-terminal regions. We have developed and pioneered the concept that a major function of this N-terminal region is to confer intracellular targeting of particular PDE4 isoforms on specific signalling complexes and intracellular locations. The paradigm for this concept developed from our original studies on the PDE4A1 (RD1) isoform. The N-terminal region unique to PDE4A1 consists of two well-defined helical regions separated by a mobile hinge region. Helix-2 provides the core membrane-insertion module, with helix-1 facilitating membrane association and fidelity of targeting in living cells. The irreversible, Ca(2+)-dependent insertion of the N-terminal region of PDE4A1 into membranes provides 'long-term' memory of cell activation.

Local anti-inflammatory effect and behavioral studies on new PDE4 inhibitors

Phosphodiesterase 4 (PDE4) inhibitors are effective anti-inflammatory drugs, although some adverse effects are observed in animals and humans. These effects have forced researchers to find new PDE4 inhibitors with less adverse effects. We recently reported the synthesis of novel heterocyclic-fused pyridazinones that inhibit PDE4. As a first step in the study of the anti-inflammatory properties of these compounds, we studied the effects of local administration of these pyridazinone derivatives in a mouse model of acute inflammation. We found that 6-Benzyl-3-methyl-4-phenylpyrazolo[3,4-d]pyridazin-7(6H)-one (CC4), ethyl 6,7-dihydro-6-ethyl-3-methyl-7-oxo-4-phenyl-thieno[2,3-d]pyridazine-2-carboxylate (CC6) and ethyl 6,7-dihydro-6-ethyl-3-methyl-4-phenyl-1H-pyrrolo[2,3-d]pyridazine-2-carboxylate (CC12) reduced the paw edema induced by zymosan in mice as rolipram (the PDE4 inhibitor prototype with anti-inflammatory activity) and indomethacin did. It is well known that rolipram locally administered induces some adverse effects such as hyperalgesia. Thus, we studied this effect after local administration of CC4, CC6 and CC12 in the formalin test. We found that CC6 induced hyperalgesic effects, whereas CC4 and CC12 did not change the nociceptive threshold. Furthermore, we found that rolipram and CC6 reduced locomotor activity, whereas CC4 and CC12 did not change locomotor performance of the mice. Since CC4 and CC12 neither affected the nociceptive threshold nor changed the locomotor performance of mice, they appear more suitable than CC6 for future studies on animals and could be developed as an anti-inflammatory drug for humans.

Hypoxia-induced remodelling of PDE4 isoform expression and cAMP handling in human pulmonary artery smooth muscle cells

Human pulmonary artery smooth muscle cells (hPASM cells) express PDE4A10, PDE4A11, PDE4B2, PDE4C and PDE4D5 isoforms. Hypoxia causes a transient up-regulation of PDE4B2 that reaches a maximum after 7 days and sustained up-regulation of PDE4A10/11 and PDE4D5 over 14 days in hypoxia. Seven days in hypoxia increases both intracellular cAMP levels, protein kinase A (PKA) activity and activated, phosphorylated extracellular signal regulated kinase (pERK) but does not alter either PKA isoform expression or total cAMP phosphodiesterase-4 (PDE4) activity or cAMP phosphodiesterase-3 (PDE3) activity. Both the cyclooxygenase inhibitor, indomethacin and the ERK inhibitors, UO126 and PD980589 reverse the hypoxia-induced increase in intracellular cAMP levels back to those seen in normoxic hPASM cells. Challenge of normoxic hPASM cells with prostaglandin E(2) (PGE(2)) elevates cAMP to levels comparable to those seen in hypoxic cells but fails to increase intracellular cAMP levels in hypoxic hPASM cells. The adenylyl cyclase activator, forskolin increases cAMP levels in both normoxic and hypoxic hPASM cells to comparable elevated levels. Challenge of hypoxic hPASM cells with indomethacin attenuates total PDE4 activity whilst challenge with UO126 increases total PDE4 activity. We propose that the hypoxia-induced activation of ERK initiates a phospholipase A(2)/COX-driven autocrine effect whereupon PGE(2) is generated, causing the activation of adenylyl cyclase and increase in intracellular cAMP. Despite the hypoxia-induced increases in the expression of PDE4A10/11, PDE4B2 and PDE4D5 and activation of certain of these long PDE4 isoforms through PKA phosphorylation, we suggest that the failure to see any overall increase in PDE4 activity is due to ERK-mediated phosphorylation and inhibition of particular PDE4 long isoforms. Such hypoxia-induced increase in expression of PDE4 isoforms known to interact with certain signalling scaffold proteins may result in alterations in compartmentalised cAMP signalling. The hypoxia-induced increase in cAMP may represent a compensatory protective mechanism against hypoxia-induced mitogens such as endothelin-1 and serotonin.

The effects of a novel phosphodiesterase 7A and -4 dual inhibitor, YM-393059, on T-cell-related cytokine production in vitro and in vivo

YM-393059, (+/-)-N-(4,6-dimethylpyrimidin-2-yl)-4-[2-(4-methoxy-3-methylphenyl)-5-(4-methylpiperazin-1-yl)-4,5,6,7-tetrahydro-1H-indol-1-yl]benzenesulfonamide difumarate, is a novel phosphodiesterase (PDE) inhibitor that inhibited the PDE7A isoenzyme with a high potency (IC50=14 nM) and PDE4 with a moderate potency (IC50=630 nM). In a cell-based assay, YM-393059 was found to inhibit anti-CD3 antibody, Staphylococcal enterotoxin B, and phytohaemagglutinin-induced interleukin (IL)-2 production in mouse splenocytes with IC50 values ranging from 0.48 to 1.1 microM. It also inhibited anti-CD3 antibody-induced interferon (IFN)-gamma and IL-4 production in splenocytes with IC50 values of 1.8 and 2.8 microM, respectively. YM-393059's inhibition of anti-CD3 antibody-stimulated cytokine (IL-2, IFN-gamma, and IL-4) production was 20- to 31-fold weaker than that of YM976, a selective PDE4 inhibitor. However, orally administered YM-393059 and YM976 inhibited anti-CD3 antibody-induced IL-2 production equipotently in mice. In addition, YM-393059 inhibited lipopolysaccharide-induced tumor necrosis factor-alpha production in vivo more potently than IL-2 (ED50 values of 2.1 mg/kg and 74 mg/kg). In contrast to YM976, YM-393059 did not shorten the duration of alpha2-adrenoceptor agonist-induced sleep in mice, which is a model for the assessment of the typical side effects caused by PDE4 inhibitors, nausea and emesis. YM-393059 is a novel and attractive compound for the treatment of a wide variety of T-cell-mediated diseases.

Multiple Conformations of Phosphodiesterase-5

Phosphodiesterase-5 (PDE5) is the target for sildenafil, vardenafil, and tadalafil, which are drugs for treatment of erectile dysfunction and pulmonary hypertension. We report here the crystal structures of a fully active catalytic domain of unliganded PDE5A1 and its complexes with sildenafil or icarisid II. These structures together with the PDE5A1-isobutyl-1-methylxanthine complex show that the H-loop (residues 660-683) at the active site of PDE5A1 has four different conformations and migrates 7-35A upon inhibitor binding. In addition, the conformation of sildenafil reported herein differs significantly from those in the previous structures of chimerically hybridized or almost inactive PDE5. Mutagenesis and kinetic analyses confirm that the H-loop is particularly important for substrate recognition and that invariant Gly(659), which immediately precedes the H-loop, is critical for optimal substrate affinity and catalytic activity.

Cellular plasticity cascades: targets for the development of novel therapeutics for bipolar disorder

For a number of patients with bipolar disorder, current pharmacotherapy is generally insufficient. Despite adequate treatment, patients continue to have recurrent mood episodes, residual symptoms, functional impairment, psychosocial disability, and significant medical and psychiatric comorbidity. Drug development for bipolar disorder may occur through one of two approaches: the first is by understanding the therapeutically relevant biochemical targets of currently effective medications. Two promising direct targets of lithium and valproate are glycogen synthase kinase-3 and histone deacetylase. The second path results from our understanding that severe mood disorders, although not classical neurodegenerative disorders, are associated with regional impairments of structural plasticity and cellular resilience. This suggests that effective treatments will need to provide both trophic and neurochemical support, which serves to enhance and maintain normal synaptic connectivity, thereby allowing the chemical signal to reinstate the optimal functioning of critical circuits necessary for normal affective functioning. For many refractory patients, drugs mimicking "traditional" strategies, which directly or indirectly alter monoaminergic levels, may be of limited benefit. Newer "plasticity enhancing" strategies that may have utility in the treatment of mood disorders include inhibitors of glutamate release, N-methyl-D-aspartate antagonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid potentiators, cyclic adenosine monophosphate phosphodiesterase inhibitors, and glucocorticoid receptor antagonists.

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.

Enantiomer discrimination illustrated by the high resolution crystal structures of type 4 phosphodiesterase

Type 4 phosphodiesterase (PDE4) inhibitors are emerging as new treatments for a number of disorders including asthma and chronic obstructive pulmonary disease. Here we report the biochemical characterization on the second generation inhibitor (+)-1 (L-, IC50=0.4 nM) and its enantiomer (-)-1 (L-, IC50=43 nM) and their cocrystal structures with PDE4D at 2.0 A resolution. Despite the 107-fold affinity difference, both enantiomers interact with the same sets of residues in the rigid active site. The weaker (-)-1 adopts an unfavorable conformation to preserve the pivotal interactions between the Mg-bound waters and the N-oxide of pyridine. These structures support a model in which inhibitors are anchored by the invariant glutamine at one end and the metal-pocket residues at another end. This model provides explanations for most of the observed structure-activity relationship and the metal ion dependency of the catechol-ether based inhibitors and should facilitate their further design.

Phosphodiesterase-4 influences the PKA phosphorylation status and membrane translocation of G-protein receptor kinase 2 (GRK2) in HEK-293beta2 cells and cardiac myocytes

Membrane-recruitment of GRK2 (G-protein receptor kinase 2) provides a fundamental step in the desensitization process controlling GPCRs (G-protein-coupled receptors), such as the beta2AR (beta2-adrenergic receptor). In the present paper, we show that challenge of HEK-293beta2 [human embryonic kidney cells stably overexpressing the FLAG-tagged beta2AR-GFP (green fluorescent protein)] cells with the beta-adrenoceptor agonist, isoprenaline, causes GRK2 to become phosphorylated by PKA (cAMP-dependent protein kinase). This action is facilitated when cAMP-specific PDE4 (phosphodiesterase-4) activity is selectively inactivated, either chemically with rolipram or by siRNA (small interfering RNA)-mediated knockdown of PDE4B and PDE4D. PDE4-selective inhibition by rolipram facilitates the isoprenaline-induced membrane translocation of GRK2, phosphorylation of the beta2AR by GRK2, membrane translocation of beta-arrestin and internalization of beta2ARs. PDE4-selective inhibition also enhances the ability of isoprenaline to trigger the PKA phosphorylation of GRK2 in cardiac myocytes. In the absence of isoprenaline, rolipram-induced inhibition of PDE4 activity in HEK-293beta2 cells acts to stimulate PKA phosphorylation of GRK2, with consequential effects on GRK2 membrane recruitment and GRK2-mediated phosphorylation of the beta2AR. We propose that a key role for PDE4 enzymes is: (i) to gate the action of PKA on GRK2, influencing the rate of GRK2 phosphorylation of the beta2AR and consequential recruitment of beta-arrestin subsequent to beta-adrenoceptor agonist challenge, and (ii) to protect GRK2 from inappropriate membrane recruitment in unstimulated cells through its phosphorylation by PKA in response to fluctuations in basal levels of cAMP.