Inhibition of eosinophil cyclic nucleotide PDE activity and opsonised zymosan-stimulated respiratory burst by 'type IV'-selective PDE inhibitors

The Complexity and Multiplicity of the Specific cAMP Phosphodiesterase Family: PDE4, Open New Adapted Therapeutic Approaches

Cyclic nucleotides (cAMP, cGMP) play a major role in normal and pathologic signaling. Beyond receptors, cyclic nucleotide phosphodiesterases; (PDEs) rapidly convert the cyclic nucleotide in its respective 5′-nucleotide to control intracellular cAMP and/or cGMP levels to maintain a normal physiological state. However, in many pathologies, dysregulations of various PDEs (PDE1-PDE11) contribute mainly to organs and tissue failures related to uncontrolled phosphorylation cascade. Among these, PDE4 represents the greatest family, since it is constituted by 4 genes with multiple variants differently distributed at tissue, cellular and subcellular levels, allowing different fine-tuned regulations. Since the 1980s, pharmaceutical companies have developed PDE4 inhibitors (PDE4-I) to overcome cardiovascular diseases. Since, they have encountered many undesired problems, (emesis), they focused their research on other PDEs. Today, increases in the knowledge of complex PDE4 regulations in various tissues and pathologies, and the evolution in drug design, resulted in a renewal of PDE4-I development. The present review describes the recent PDE4-I development targeting cardiovascular diseases, obesity, diabetes, ulcerative colitis, and Crohn’s disease, malignancies, fatty liver disease, osteoporosis, depression, as well as COVID-19. Today, the direct therapeutic approach of PDE4 is extended by developing allosteric inhibitors and protein/protein interactions allowing to act on the PDE interactome.

Discovery of a novel orally active PDE-4 inhibitor effective in an ovalbumin-induced asthma murine model

Phosphodiesterase-4 (PDE-4) is responsible for metabolizing adenosine 3',5'-cyclic monophosphate that reduces the activation of a wide range of inflammatory cells including eosinophils. PDE-4 inhibitors are under development for the treatment of respiratory diseases such as asthma and chronic obstructive pulmonary disease. Herein, we report a novel PDE-4 inhibitor, PDE-423 (3-[1-(3-cyclopropylmethoxy-4-difluoromethoxybenzyl)-1H-pyrazol-3-yl]-benzoic acid), which shows good in vitro and in vivo oral activities. PDE-423 exhibited in vitro IC(50)s of 140 nM and 550 nM in enzyme assay and cell-based assay, respectively. In vivo study using ovalbumin-induced asthmatic mice revealed that PDE-423 reduced methacholine-stimulated airway hyperreactivity in a dose-dependent manner by once daily oral administration (ED(50)=18.3 mg/kg), in parallel with decreased eosinophil peroxidase activity and improved lung histology. In addition, PDE-423 was effective in diminishing lipopolysaccharide-induced neutrophilia in vivo as well as in vitro. Oral administration of PDE-423 (100 mg/kg) had no effect on the duration of xylazine/ketamine-induced anesthesia and did not induce vomiting incidence in ferrets up to the dose of 1000 mg/kg. The present study indicates that a novel PDE-4 inhibitor, PDE-423, has good pharmacological profiles implicating this as a potential candidate for the development of a new anti-asthmatic drug.

Phosphodiesterase inhibitors: history of pharmacology

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

Aspirin attenuates the anti-inflammatory effects of theophylline via inhibition of cAMP production in mice with non-eosinophilic asthma

Theophylline is commonly used to treat severe asthma and chronic obstructive pulmonary disease (COPD) characterized by non-eosinophilic inflammation. Acetyl salicylic acid (ASA) is one of the most widely used medications worldwide, but up to 20% of patients with asthma experience aggravated respiratory symptoms after taking ASA. Here we evaluated the adverse effect of ASA on the therapeutic effect of theophylline in mice with non-eosinophilic asthma. A non-eosinophilic asthma mouse model was induced by airway sensitization with lipopolysaccharide-containing allergen and then challenged with allergen alone. Therapeutic intervention was performed during allergen challenge. Theophylline inhibited lung inflammation partly induced by Th1 immune response. ASA attenuated the beneficial effects of theophylline. However, co-administration of the ASA metabolite salicylic acid (SA) showed no attenuating effect on theophylline treatment. The therapeutic effect of theophylline was associated with increase in cAMP levels, which was blocked by co-treatment of theophylline and ASA. ASA co-treatment also attenuated the anti-inflammatory effects of a specific phosphodiesterase 4 inhibitor. These results demonstrate that ASA reverses anti-inflammatory effects of theophylline, and that ASA exerts its adverse effects through the inhibition of cAMP production. Our data suggest that ASA reverses lung inflammation in patients taking theophylline, although clinical evidence will be needed.

Phosphodiesterase type 4 inhibitors: potential in the treatment of multiple sclerosis?

Phosphodiesterases (PDEs) are involved in the regulation of intracellular levels of the second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). These enzymes hydrolyse the cyclic nucleotides to the corresponding nucleoside 5'-monophosphates. Nine PDE subtypes have been identified; these differ in their substrate specificity and mode of activation. The type 4 PDE (PDE(4)) hydrolyses cAMP, is activated by elevated levels of cAMP, and is inhibited by rolipram. Inhibition of enzyme activity has been shown to modulate the activity of cells of the immune system. The production of tumour necrosis factor (TNF)(alpha) by activated monocytes and macrophages is inhibited, and cytokine secretion and proliferation of type 1 T helper cells are suppressed. Both immune cell activation and their concomitant induction of cytokine secretion are implicated in multiple sclerosis (MS), which is the major demyelinating disease of the central nervous system. Studies with the selective PDE(4) inhibitor rolipram in experimental autoimmune encephalomyelitis (an animal model of MS) in mice, rats and nonhuman primates have demonstrated the efficacy of the compound in this disease model, suggesting that PDE(4) inhibitors could ameliorate the clinical course of MS. Unfortunately, clinical trials with PDE(4) inhibitors revealed the major adverse effects of these drugs, namely nausea and vomiting. However, novel PDE(4) inhibitors, which target only a subpopulation of PDE(4) enzymes, may provoke fewer adverse effects. The efficacy of a PDE(4) inhibitor in MS still needs to be demonstrated in a well designed clinical trial.

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

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

Changes in intracellular cyclic adenosine monophosphate levels in peripheral blood leukocytes during immunotherapy with vespid venom

BACKGROUND

The allergen-induced release of mediators from basophils is known to be decreased after rush venom immunotherapy (VIT) compared with pretreatment values. A rise in the intracellular cyclic adenosine monophosphate (cAMP) level is known to inhibit mediator release.

OBJECTIVES

To determine changes in cAMP levels in peripheral blood leukocytes (PBLs) during rush VIT and to evaluate their relation to allergen-specific reactivity of basophils.

METHODS

Ten patients allergic to vespid venom (VV) were investigated before rush VIT and after reaching the maintenance dose. Five VV-allergic patients not undergoing VIT served as controls. Patients' PBLs were incubated with VV, and allergen-induced histamine and leukotriene release from basophils was measured. Levels of cAMP were determined in PBLs and in plasma.

RESULTS

Immediately after rush VIT, VV-induced histamine release (P = .04) and VV-induced leukotriene release (P = .01) were significantly reduced. Intracellular cAMP levels increased significantly (P = .047). However, 6 months after VIT, mediator responses in basophils were comparable with pre-VIT values. No significant changes were found in the control group.

CONCLUSIONS

An increase in intracellular cAMP levels might account for the decreased reactivity of basophils to allergen after 1 week of VIT. However, similar to the decreased mediator release after 1 week of VIT, this is not a long-term effect as values returned to baseline after 6 months.

Phosphodiesterase inhibitors

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

Simultaneous determination of YM-64227, a phosphodiesterase type 4 inhibitor, and its ?ve metabolites in dog plasma by high-performance liquid chromatography with ?uorescence detection

We developed and validated a reversed-phase high-performance liquid chromatographic method with fluorescence detection for the simultaneous determination of YM-64227 [4-cyclohexyl-1-ethyl-7-methylpyrido(2,3-d)pyrimidin-2-(1H)-one], a novel and selective phosphodiesterase type 4 inhibitor, and its fi ve hydroxylated metabolites in dog plasma. The plasma samples were extracted with tert-butyl methyl ether under alkali conditions. The analytes were well separated on a phenyl ethyl column (5 microm, 250 x 4.6 mm i.d.), opreating at 40 degrees C and using an acetonitrile-acetic acid gradient at a fl ow rate of 1.0 mL/min. The fluorescence signal was monitored at an excitation and emission wavelength of 330 and 400 nm, respectively. No interfering peak was observed at the retention time of YM-64227, its metabolites or the internal standard. The validated quantitation range of the method was 0.4-200 ng/mL for all analytes using 0.5 mL of the plasma sample. The recovery of analytes in the extraction process was more than 65.5%. The intra- and inter-assay precision was less than 5.1 and 12.6%, respectively, and the intra- and inter-assay accuracy ranged from -8.1 to 11.8% and -8.0 to 9.9%, respectively. Using this assay, the plasma concentration of YM-64227 and metabolites can be determined after the oral administration of YM-64227 to beagle dogs.

Relaxant effect of YM976, a novel phosphodiesterase 4 inhibitor, on bovine tracheal smooth muscle

Effects of 4-(3-chlorophenyl)-1,7-diethylpyrido[2,3-d]pyrimidin-2(1H)-one (YM976), a novel and selective phosphodiesterase type 4 inhibitor, on tension and adenosine 3',5'-cyclic monophosphate (cAMP) content of bovine tracheal smooth muscle were compared with those of rolipram and theophylline. YM976, rolipram and theophylline relaxed the tracheal preparations contracted with histamine in a concentration-dependent manner. The relaxant effects of YM976 and rolipram were more potent than those of theophylline. These phosphodiesterase inhibitors-induced relaxations were dramatically diminished when tracheal smooth muscle was contracted with methacholine instead of histamine. Pretreatment of the tracheal preparations with YM976 (10 microM) or rolipram (10 microM), but not with theophylline (1 mM), shifted the concentration-response curves for contractile responses to histamine; however, the same procedure failed to affect concentration-response relationships for methacholine-induced contractions. At 1 and 10 microM, both YM976 and rolipram increased the tissues cAMP content. These results suggest that YM976 relaxes tracheal smooth muscle, probably through the cAMP-dependent mechanism.

Substituted 4-(2,2-diphenylethyl)pyridine-N-oxides as phosphodiesterase-4 inhibitors: SAR study directed toward the improvement of pharmacokinetic parameters

A detailed SAR study directed toward the optimization of pharmacokinetic parameters for analogues of L-791,943 is reported. The introduction of a soft metabolic site on this structure permitted the identification of L-826,141 as a potent phosphodiesterase type 4 (PDE4) inhibitor that is well absorbed and that presents a shorter half-life than L-791,943 in a variety of animal species. The efficacy of L-826,141 is also demonstrated in different in vivo models.

Bronchodilatory and anti-inflammatory properties of inhaled selective phosphodiesterase inhibitors in a guinea pig model of allergic asthma

In a guinea pig model of allergic asthma, we investigated the effects of the selective phosphodiesterase inhibitors rolipram (phosphodiesterase 4-selective), Org 9935 (phosphodiesterase 3-selective) and Org 20241 (dual phosphodiesterase 4/phosphodiesterase 3-selective), administered by aerosol inhalation in approximately equipotent bronchodilatory doses, on allergen-induced early and late asthmatic reactions, airway hyperreactivity and airway inflammation. Using ovalbumin-sensitized non-challenged animals, different nebulizer concentrations of each inhibitor were tested for their protective effects against histamine-induced bronchoconstriction. Inhalation of 2.5 mM rolipram, 100 mM 4,5-dihydro-6-(5,6-dimethoxybenzo[b]thien-2-yl-5-methyl-3(2H)pyridazinone (Org 9935) and 10 and 100 mM N-hydroxy-4-(3,4-dimethoxyphenyl)-thiazole-2-carboximidamide HCl (Org 20241) provided a similar, 1.8-fold (P<0.01), 2.0-fold (P<0.05), and 1.8- and 1.9-fold (P<0.05) protection, respectively. The duration of these bronchoprotective effects were different, the rate of decline being faster with rolipram and the lower Org 20241 concentration than with Org 9935 and the higher concentration of Org 20241. All compounds strongly protected against the immediate allergen-induced bronchoconstriction and significantly (P<0.05) diminished the overall early asthmatic reaction from 0 to 6 h following allergen-provocation. The severity of the late asthmatic reaction was also significantly inhibited by rolipram (P<0.05) and Org 9935 (P<0.05). Allergen-induced airway hyperreactivity to inhaled histamine after the early reaction, at 6 h after ovalbumin challenge, was strongly reduced by rolipram (P<0.05) and completely prevented by the two other phosphodiesterase inhibitors; in addition, airway hyperreactivity after the late asthmatic reaction, at 24 h, was abolished in all treatment groups. Bronchoalveolar lavage performed at 24 h after allergen challenge revealed no inhibition of eosinophil infiltration in the rolipram-treated animals, whereas inhalation of Org 9935 and the higher-but not the lower-concentration of Org 20241 strongly reduced the influx of these cells. Eosinophil peroxidase activity in the lavage fluid tended to be diminished in all treatment groups but significance was not reached with the exception of the lower concentration of Org 20241. Infiltration of lymphocytes and macrophages was significantly inhibited by Org 9935 only (P<0.05 and P<0.01, respectively), whereas neutrophil influx was not significantly affected. The results indicate that inhalation of phosphodiesterase 3-, phosphodiesterase 4- and dual phosphodiesterase 3/phosphodiesterase 4-selective inhibitors afford protection against acute histamine- and allergen-induced bronchoconstriction and prevent the development of airway hyperreactivity both after the early and late asthmatic reaction predominantly through inhibition of phosphodiesterase 4; in contrast, for significant reduction of eosinophil infiltration, both phosphodiesterase 3 and phosphodiesterase 4 inhibition seems to be required.

Prostaglandin D2 is a potent chemoattractant for human eosinophils that acts via a novel DP receptor

Prostaglandin D2 (PGD2) is released following exposure of asthmatics to allergen and acts via the adenylyl cyclase-coupled receptor for PGD2 (DP receptor). In this study, it is reported that human eosinophils possess this receptor, which would be expected to inhibit their activation. In contrast, it was found that prostaglandin D2 is a potent stimulator of eosinophil chemotaxis, actin polymerization, CD11b expression, and L-selectin shedding. These responses are specific for eosinophils, as neutrophils display little or no response to prostaglandin D2. They were not due to interaction with receptors for other prostanoids, as prostaglandins E2 and F(2alpha), U46619 (a thromboxane A2 analogue), and carbaprostacyclin (a prostacyclin analogue) displayed little or no activity. Furthermore, they were not shared by the selective DP receptor agonist BW245C and were not prevented by the selective DP receptor antagonist BWA868C, indicating that they were not mediated by DP receptors. In contrast, the prostaglandin D2 metabolite 13,14-dihydro-15-oxoprostaglandin D2 induced eosinophil activation but did not stimulate DP receptor-mediated adenosine 3',5'-cyclic monophosphate (cAMP) formation. These results indicate that in addition to the classic inhibitory DP1 receptor, eosinophils possess a second, novel DP2 receptor that is associated with PGD2-induced cell activation. These 2 receptors appear to interact to regulate eosinophil responses to PGD2, as blockade of DP1 receptor-mediated cAMP production by BWA868C resulted in enhanced DP2 receptor-mediated stimulation of CD11b expression. The balance between DP1 and DP2 receptors could determine the degree to which prostaglandin D2 can activate eosinophils and may play a role in eosinophil recruitment in asthma.

Involvement of A(3) receptors in the potentiation by adenosine of the inhibitory effect of theophylline on human eosinophil degranulation: possible novel mechanism of the anti-inflammatory action of theophylline

The current use of theophylline in asthma is based on both the bronchodilatory and the anti-inflammatory effects. The exact mechanism of these actions is still controversial and may include the inhibition of adenosine 3',5'-monophosphate phosphodiesterase enzyme (PDE) and antagonism of adenosine receptors. In this study, the mechanism of the anti-inflammatory action was investigated by studying the inhibition by theophylline of complement C5a (C5a)-induced degranulation of human eosinophils and its interaction with adenosine. Theophylline (10-1000 microM) inhibited C5a-induced release of eosinophil peroxidase (EPO) in a concentration-dependent manner with an IC(50) of 233.5 microM and a maximal inhibition of 90.3 +/- 3.0%. In contrast, the PDE4 inhibitor rolipram (up to 50 microM) had no effect. The adenosine A(3) receptor agonist N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) also inhibited release (IC(50) = 7.5 microM), but neither adenosine itself nor the selective A(1) and A(2) agonists and antagonists had any significant effect, even at 100 microM. The inhibition produced by clinically relevant concentration of theophylline (50 microM) was potentiated by ineffective concentrations of exogenous adenosine and additive to that produced by IB-MECA. The potent and selective A(3) antagonist MRS 1220, but not the A(1) or A(2) antagonists, significantly reversed the inhibitory effect of theophylline. These results suggest that therapeutic concentrations of theophylline inhibit human eosinophil partly by acting as an A(3) agonist. Together with the potentiation of theophylline action by adenosine, perhaps via the A(3) receptors, these novel actions may, at least in part, contribute to the mechanism of the anti-inflammatory action of this drug in vivo.

Effects of PDE4 inhibitors on lipopolysaccharide-induced priming of superoxide anion production from human mononuclear cells

AIMS

Phosphodiesterase 4 (PDE4) inhibitors have been described as potent anti-inflammatory compounds, involving an increase in intracellular levels of cyclic 3',5'-adenosine monophosphate (AMP). The aim of this study was to compare the effects of selective PDE4 inhibitors, rolipram and RP 73-401 with the cell permeable analogue of cyclic AMP, dibutyryl-cyclic AMP (db-cAMP) and the anti-inflammatory cytokine interleukin-10 (IL-10) on superoxide anion production from peripheral blood mononuclear cells preincubated with lipopolysaccharide (LPS).

MAJOR FINDINGS

We report that, after incubation of the cells with LPS, a large increase in superoxide anion production was observed. Rolipram or RP 73-401 (10(-8) to 10(-5) M) induced significant reductions of fMLP-induced superoxide anion production in cells incubated with or without LPS. The db-cAMP (10(-5) to 10(-3) M) also elicited dose-dependent inhibitions of the fMLP-induced superoxide anion production. In contrast, IL-10 (1 or 10 ng/ml) did not elicit a reduction in fMLP-induced superoxide anion production in both conditions.

PRINCIPAL CONCLUSION

These results suggest that the inhibitory activity of PDE4 inhibitors on fMLP-induced production of superoxide anion production is mediated by db-cAMP rather than IL-10.

Requirement of additional adenylate cyclase activation for the inhibition of human eosinophil degranulation by phosphodiesterase IV inhibitors

Human eosinophils contain predominantly phosphodiesterase type IV, but selective inhibitors of this isoenzyme fail to inhibit certain eosinophil responses such as degranulation. In this study, the effect of activation of adenylate cyclase on the ability of several highly selective PDE IV inhibitors to inhibit complement C5a-induced O2- release and degranulation of human eosinophils in vitro was investigated. All four selective PDE IV inhibitors, N-(3,5-dichloropyrid-4-yl)-3-cyclopentyl-oxy-4-methoxybenzamide (RP 73401), rolipram, N-(3,5-dichloropyrid-4-yl)-[1-(4-fluorobenzyl)-5-hydroxy-indol-3-yl]glyoxylacidamide (AWD 12-281) and c-4-cyano-4-(3-cyclopentyloxy-4-methoxyphenyl-r-1-cyclohexane carboxylic acid) (SB 207499) potently inhibited C5a-induced O2- generation (IC50 = 0.03, 0.42, 0.55 and 0.86 microM, respectively), but generally failed to inhibit degranulation. The only exception was AWD 12-281, which inhibited degranulation (IC50 = 16.2 microM). In the presence of different AC activators (histamine, salbutamol, prostaglandin E2 and forskolin), the PDE IV inhibitors became potent inhibitors of degranulation. The interaction between the PDE IV inhibitors and the AC activators resulted in a synergistic increase in intracellular levels of adenosine 3', 5'-monophosphate (cAMP). These results show that PDE IV inhibitors generally require an additional cAMP signal to be able to inhibit eosinophil degranulation, and that this signal can be generated via both membrane receptors and direct AC activation. This may be relevant to the in vivo effectiveness of PDE IV inhibitors in eosinophilic inflammation.

Effects of the phosphodiesterase 4 inhibitor RPR 73401 in a model of immunological inflammation

The study was performed to investigate effects of the phosphodiesterase 4 inhibitor RPR 73401 [N-(3, 5-dichloropyrid-4-yl)-3-cyclopentyl-oxy-4-methoxybenzamid] on an allergic skin reaction. To simulate an immunological inflammation, BALB/c mice were sensitized to dinitrochlorobenzene or toluenediisocyanate. At first, the abdominal skin was shaved and 50 microliter Freund's adjuvant were injected intracutaneously once. Then, the horny layer was removed by adhesive tape stripping and 100 microliter 0.5% dinitrochlorobenzene or 5% toluenediisocyanate were administered on the epidermis for 4 days. After repeated local treatment of the ear skin with 20 microliter 3% RPR 73401 or intraperitoneal administration of 1 and 5 mg/kg RPR 73401, 20 microliter 1% dinitrochlorobenzene or 0.5% toluenediisocyanate were given topically as a challenge. The vehicle controls showed a high increase in ear thickness over 48 h after challenge, whereas RPR 73401 administered on either route reduced this increase significantly. Nevertheless after topical administration, RPR 73401 had a longer lasting effect. These and other results may point to an indication for RPR 73401 in immunological dermatitis.

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.

Suppression of human inflammatory cell function by subtype-selective PDE4 inhibitors correlates with inhibition of PDE4A and PDE4B

1. Of the four major phosphodiesterase 4 (PDE4) subtypes, PDE4A, PDE4B and PDE4D are widely expressed in human inflammatory cells, including monocytes and T lymphocytes. We explored the functional role of these subtypes using ten subtype-selective PDE4 inhibitors, each belonging to one of two classes: (i) dual PDE4A/PDE4B inhibitors or (ii) PDE4D inhibitors. 2. These compounds were evaluated for their ability to inhibit antigen-stimulated T-cell proliferation and bacterial lipopolysaccharide (LPS)-stimulated tumour necrosis factor alpha (TNFalpha) release from peripheral blood monocytes. 3. All compounds inhibited T-cell proliferation in a concentration-dependent manner; with IC50 values distributed over an approximately 50 fold range. These compounds also inhibited TNFalpha release concentration-dependently, with a wider ( approximately 1000 fold) range of IC50 values. 4. In both sets of experiments, mean IC50 values were significantly correlated with compound potency against the catalytic activity of recombinant human PDE4A or PDE4B when analysed by either linear regression of log IC50 values or by Spearman's rank-order correlation. The correlation between inhibition of inflammatory cell function and inhibition of recombinant PDE4D catalytic activity was not significant in either analysis. 5. These results suggest that PDE4A and/or PDE4B may play the major role in regulating these two inflammatory cell functions but do not rule out PDE4D as an important mediator of other activities in mononuclear leukocytes and other immune and inflammatory cells. Much more work is needed to establish the functional roles of the PDE4 subtypes across a broader range of cellular functions and cell types.

A comparison of the inhibitory activity of PDE4 inhibitors on leukocyte PDE4 activity in vitro and eosinophil trafficking in vivo

1. Phosphodiesterase (PDE) 4 inhibitors have been shown to inhibit eosinophil PDE4 activity in vitro and accumulation of eosinophils in experimental airways inflammation. However, direct effects on eosinophil trafficking have not been studied in detail and it is not known if activity in vitro translates into efficacy in vivo. In the present study, we compared the activity of five PDE4 inhibitors in vitro and against trafficking of (111)In-eosinophils in cutaneous inflammation in the guinea-pig. 2. The rank order of potency for inhibition of PDE4 activity in guinea-pig eosinophil, neutrophil and macrophage, and human neutrophil lysates was RP73401 > SB207499 >CDP840 > rolipram > LAS31025. On TNFalpha production by human PBMC, all inhibitors with the exception of rolipram showed potency similar to their effect on neutrophil lysates. 3. In a brain cerebellum binding assay, the rank order of potency at displacing [3H]-rolipram was RP73401 > rolipram > SB207499 > CDP840 > LAS30125. 4. Trafficking of (111)In-eosinophils to skin sites injected with PAF, ZAP or antigen in sensitized sites was inhibited by oral administration of all PDE4 inhibitors. The rank order of potency was RP73401 = rolipram > LAS31025 > SB207499 > CDP840. 5. With the exception was RP73401, which was the most potent compound in all assays, there was no clear relationship between activity of PDE4 inhibitors in vitro and capacity to inhibit eosinophil trafficking in vivo. Thus, we conclude that in vitro activity of PDE4 inhibitors does not predict in vivo efficacy in an experimental model of eosinophil trafficking.

Phosphodiesterase 4 inhibitors as airways smooth muscle relaxant agents: synthesis and biological activities of triazine derivatives

A series of triazine derivatives was synthesized. The compounds were evaluated for tracheal smooth muscle relaxant and type 4 phosphodiesterase inhibitory activities. A highly significant correlation was observed between the two effects. Two compounds exhibited potent relaxant activity (EC50: 17 and 24 nM) and might be useful for the treatment of asthma.

Substituted furans as inhibitors of the PDE4 enzyme

The synthesis and in vitro activity of a series of substituted furans as a novel structural class of PDE4 inhibitors is described. Comparison of emetic threshold with known PDE4 inhibitors is presented.

Emesis induced by inhibitors of type IV cyclic nucleotide phosphodiesterase (PDE IV) in the ferret

Emesis induced by inhibitors of type IV cyclic nucleotide phosphodiesterase (PDE IV) has been investigated in the ferret. The PDE IV inhibitors studied were: RS14203, R-rolipram and CT-2450 (i.e. (R)-N-[4-[1-(3-cyclopentyloxy-4-methoxyphenyl)-2-(4-pyridyl)ethyl]phenyl ]N'-ethylurea), in addition to the less active enantiomers S-rolipram and CT-3405. Following oral administrations, different emetic profiles were observed with time. Emesis induced by RS14203 exhibited a dose-response relationship but no such relationship was seen for R-rolipram or CT-2450. The incidence of emesis was positively influenced by the dose of PDE IV inhibitors administered, allowing a rank order of potency: RS14203 > R-rolipram > S-rolipram > CT-2450 > CT-3405. PDE IV inhibitor-induced emesis was abolished by the tachykinin NK1 receptor antagonist, CP-99,994. No peripheral release of substance P by PDE IV inhibitors seems to be involved in triggering the emetic reflex since L-743,310, which only has peripheral NK1 receptor antagonist activity, was without effect. The implication of 5-HT3 receptors in PDE IV inhibitor-induced emesis was variable. Our results suggest that the PDE IV inhibitors studied are mixed peripheral-central emetogens. PDE IV inhibition itself could be plausible mechanism of action of these agents. However, whether emesis is mediated via a specific isoform of PDE IV remains to be established.

Regulation of human mast cell and basophil function by cAMP

1. Mast cells and basophils are important in mediating allergic disorders such as asthma. Activation of these cells results in the release of a wide variety of mediators that can promote inflammatory responses. 2. Receptor-mediated activators of adenylate cyclase such as the beta-adrenoceptor agonist, isoprenaline, and prostaglandin E2 (PGE2) are effective at inhibiting mediator release from human lung mast cells (HLMC) but not basophils. In HLMC, both isoprenaline and PGE2 elevate and sustain increases in cyclic adenosine 3',5'-monophosphate (cAMP) whereas in basophils, both compounds cause transient increases in cAMP. 3. Non-selective inhibitors of phosphodiesterase (PDE) such as theophylline and 3-isobutyl-1-methylxanthine are effective inhibitors of mediator release from both HLMC and basophils and both compounds cause elevations of cAMP that are sustained in both cell types. 4. Studies with selective inhibitors of PDE indicate that the cAMP-specific PDE, PDE 4, regulates the activity of basophils but not HLMC. The nature of the PDE regulating HLMC responses is uncertain. 5. These data indicate that agents that induce and sustain elevations in intracellular cAMP attenuate the stimulated release of mediators from mast cells and basophils. However, the responsiveness of HLMC and basophils to selected cAMP-active agents differs.

Phosphodiesterase expression in human epithelial cells

Epithelial cells play a critical role in airway inflammation and have the capacity to produce many inflammatory mediators, including bioactive lipids and proinflammatory cytokines. Intracellular levels of cAMP and cGMP are important in the control of inflammatory cell function. These cyclic nucleotides are inactivated via a family of phosphodiesterase (PDE) enzymes, providing a possible site for drug intervention in chronic inflammatory conditions. We studied the expression of PDE activity in an epithelial cell line (A549) and in primary human airway epithelial cells (HAECs). We measured PDE function using specific inhibitors to identify the PDE families present and used RT-PCR to elucidate the expression of PDE isogenes. Both A549 cells and HAECs predominantly expressed PDE4 activity, with lesser PDE1, PDE3, and PDE5 activity. RT-PCR identified HSPDE4A5 and HSPDE4D3 together with HSPDE7. Inhibition of PDE4 and PDE3 reduced secretion by these cells. Epithelial PDE may be an important target for PDE4 inhibitors in the development of the control of asthmatic inflammation, particularly when delivered via the inhaled route.

Signal transduction and activation of the NADPH oxidase in eosinophils

Activation of eosinophil NADPH oxidase and the subsequent release of toxic oxygen radicals has been implicated in the mechanism of parasite killing and inflammation. At present, little is known of the signal transduction pathway that govern agonist-induced activation of the respiratory burst and is the subject of this review. In particular, we focus on the ability of leukotrine B4 to activate the NADPH oxidase in guinea-pig peritoneal eosinophils which can be obtained in sufficient number and purity for detailed biochemical experiments to be performed.

Characterization of phosphodiesterase 4 in guinea-pig macrophages: multiple activities, association states and sensitivity to selective inhibitors

1. The cyclic AMP phosphodiesterases (PDE) in guinea-pig peritoneal macrophages were isolated, partially characterized and their role in regulating the cyclic AMP content in intact cells evaluated. 2. Differential centrifugation of macrophage lysates revealed that approximately 90% of the PDE activity was membrane-bound and exclusively hydrolyzed cyclic AMP. This activity was not removed by KCl (200 mM) but was readily solubilized by the non-ionic detergent, Triton X-100 (1% v/v). Greater than 80% of the hydrolytic activity was suppressed by the PDE4 inhibitors, R-rolipram and nitraquazone with IC50s of 240 and 540 nM, respectively. 3. Anion-exchange chromatography of the total protein extracted from macrophages resolved two major peaks of cyclic AMP PDE activity that were insensitive to cyclic GMP (10 microM), calmodulin (50 units plus 2 mM CaCl2) and a PDE3 inhibitor, SK&F 95654 (10 microM), but were markedly suppressed by RS-rolipram (10 microM). The two peaks of PDE activity were arbitrarily designated CPPDE4alpha and CPPDE4beta with respect to the order from which they were eluted from the column where the prefix, CP, refers to the species, Cavia porcellus. 4. The hydrolysis of cyclic AMP catalyzed by CPPDE4alpha and CPPDE4beta conformed to Michaelis-Menten kinetic behaviour with similar K(m)s (13.4 and 6.4 microM, respectively). 5. Thermal denaturation of membrane-bound PDE4 at 50 degrees C followed bi-exponential kinetics with t1/2 values of 1.5 and 54.7 min for the first and second components, respectively. In contrast, CPPDE4alpha and CPPDE4beta each decayed mono-exponentially with significantly different thermostabilities (t1/2 = 2.77 and 1.15 min, respectively). 6. Gel filtration of CPPDE4beta separated two peaks of rolipram-sensitive PDE activity. The main peak eluted at a volume indicative of a approximately 180 kDa protein but was preceded by a much larger form of the enzyme that had an estimated weight of 750 kDa. Size exclusion chromatography of CPPDE4alpha resolved a broad peak of activity with molecular weights spanning 50 to 200 kDa. 7. Of ten PDE inhibitors examined, none distinguished CPPDE4alpha from CPPDE4beta with respect to their IC50 values or their rank order of potency. RS-rolipram acted as a purely competitive inhibitor of cyclic AMP hydrolysis with K(i)s of 2 microM and 1.5 microM for CPPDE4alpha and CPPDE4beta, respectively. In contrast to the membrane-associated enzyme(s), R-rolipram and nitraquazone were 4 to 19 fold less potent as inhibitors of CPPDE4alpha and CPPDE4beta. 8. In intact macrophages, Ro 20-1724 and RS-rolipram potentiated isoprenaline-induced cyclic AMP accumulation under conditions where a PDE3 inhibitor, SK&F 94120, was essentially inactive. 9. These data demonstrate that the predominant cyclic AMP hydrolyzing activity in guinea-pig macrophages is a PDE4. Moreover, thermostability studies and size exclusion chromatography indicates the possible expression of two intrinsic, membrane-associated isoenzymes which can regulate the cyclic AMP content in intact cells. The finding that soluble and particulate forms of the same enzyme exhibit different sensitivities to rolipram and nitraquazone implies that PDE4 can change conformation. Finally, the identification of multiple molecular weight species of CPPDE4 suggests that this enzyme(s) might form multimeric complexes of variable association states.

PDE4 inhibition and a corticosteroid in chronically antigen exposed conscious guinea-pigs

BACKGROUND

The physiological and pharmacological consequences of repeated aero-allergen challenge have not been previously characterized in conscious, sensitized guinea-pigs.

OBJECTIVES

This study was undertaken to compare the effects of two anti-inflammatory compounds, dexamethasone and Ro 20- 1724, on an acute and chronic airway inflammation, in terms of airway function, reactivity and leucocyte infiltration.

METHODS

Sensitized guinea-pigs received eight saline or ovalbumin (OvA) inhalation exposures over 4 weeks and either vehicle, the type 4 PDE inhibitor, Ro 20-1724 (3 mgkg(-1)), or dexamethasone (1.5 mg/kg(-1)), 30 min before and 6 h after each challenge. Airway function of the conscious animal (sGaw) was monitored over the duration of the first and final OvA challenge. Airway reactivity to the thromboxane mimetic, U46619, was also determined following the final OvA exposure as was the leucocyte infiltration.

RESULTS

The first antigen challenge induced a large early (0-3h) and smaller late (17-24h) bronchoconstrictor response. Neither phase was affected by the drug treatments. The final OvA challenge induced early and late phase bronchoconstrictor responses but of similar magnitude. The late phase was also significantly prolonged. Ro 20-1724 and dexamethasone significantly attenuated both phases. Airway reactivity to the inhaled thromboxane mimetic, U46619, was also significantly enhanced at 120h after the final OvA exposure in contrast to the saline challenged group. This hyperreactivity was attenuated by Ro 20-1724 and dexamethasone. Bronchoalveolar lavage after repeated OvA exposures revealed eosinophilia which was attenuated by Ro 20-1724 and dexamethasone.

CONCLUSIONS

This model demonstrates differential airway responses to acute and chronic antigen challenge. Repeated administration of dexamethasone and Ro 20-1724 with each OvA exposure attenuated all of the chronic inflammatory responses: early and late phase responses, hyperreactivity and eosinophilia.

Correlations of PDE-4 inhibition between enzymes of smooth muscle and inflammatory cell sources

The sensitivities of PDE-4 enzymes from smooth muscle and inflammatory cell sources from different species to a range of structurally diverse compounds were compared. All inflammatory cell PDE-4 sources displayed good crosscorrelations in their sensitivity to inhibition by these compounds. Similarly, PDE-4 enzymes from smooth muscle sources were well-correlated; however, there was no crosscorrelation between PDE-4 from smooth muscle sources and those of inflammatory cell sources, possibly reflecting differences in subcellular location of enzymes as well as subtype expression. The present study concludes that PDE-4 preparations from smooth muscle sources as well as those from inflammatory cell sources may be used to model the potential smooth muscle cell relaxing properties and anti-inflammatory properties of a compound in relation to human asthma.

Functional and biochemical evidence for diazepam as a cyclic nucleotide phosphodiesterase type 4 inhibitor

1. The responses of the electrically-driven right ventricle strip of the guinea-pig heart to diazepam were recorded in the absence and in the presence of different selective cyclic nucleotide phosphodiesterase (PDE) inhibitors. 2. Diazepam, at concentrations ranging from 1 microM to 100 microM, was devoid of effect on the contractile force in this preparation. 3. Conversely, diazepam (5 microM-100 microM) produced a consistent positive inotropic response in the presence of a concentration (1 microM), that was without effect in the absence of diazepam, of either of the selective PDE 3 inhibitors milrinone or SK&F 94120, but not in the presence of the selective PDE 4 inhibitor rolipram. 4. This effect of diazepam was not gamma-aminobutyric acid (GABA)-dependent, since it was neither mimicked nor potentiated by GABA, and was not affected by either a high concentration (5 microM) of the antagonists of the benzodiazepine/GABA/channel chloride receptor complex, picrotoxin, flumazenil and beta-carboline-3-carboxylic acid methyl ester (betaCCMe), or by the inverse agonists, beta-carboline-3-carboxylic acid N-methylamide (betaCCMa) and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM, 0.1 microM). Furthermore, a specific antagonist of the peripheral benzodiazepine receptors, PK 11195 (5 microM), did not influence the effect of diazepam. 5. Biochemical studies with isolated PDEs, confirmed that diazepam selectively inhibits type 4 PDE from guinea-pig right ventricle rather than the other PDEs present in that tissue. The compound inhibited this enzyme in a non-competitive manner. Diazepam was also able to inhibit PDE 5, the cyclic GMP specific PDE absent from cardiac muscle, with a potency close to that shown for PDE 4. 6. Diazepam displaced the selective type 4 PDE inhibitor, rolipram from its high affinity binding site in rat brain cortex membranes, and also potentiated the rise in cyclic AMP levels induced by isoprenaline in guinea-pig eosinophils, where only type 4 PDE is present. 7. The PDE inhibitory properties of diazepam were shared, although with lower potency, by other structurally-related benzodiazepines, that also displaced [3H]-rolipram from its high affinity binding site. The order of potency found for these compounds in these assays was not related to their potencies as modulators of the GABA receptor through its benzodiazepine binding site. 8. The pharmacological and biochemical data presented in this study indicate that diazepam behaves as a selective type 4 PDE inhibitor in cardiac tissue and this effect seems neither to be mediated by the benzodiazepine/GABA/channel chloride receptor complex nor by peripheral type benzodiazepine receptors.

Effects of cAMP-phosphodiesterase isozyme inhibitor on cytokine production by lipopolysaccharide-stimulated human peripheral blood mononuclear cells

1. The effects of cAMP-phosphodiesterase (PDE) isozyme inhibitors on the production of tumor necrosis factor alpha (TNF-alpha), and interleukins 1 beta 8 (IL-1 beta and IL-8) by lipopolysaccharide (LPS)-stimulated human peripheral blood mononuclear cells (PBMC) were evaluated. In addition, we investigated the effects of dibutyryl cAMP (dbcAMP) and beta-adrenergic receptor agonist on the production of these cytokines. 2. Type IV PDE inhibitors were more effective at inhibiting the production of TNF-alpha and IL-1 beta by LPS-stimulated PBMC than a nonselective, type III or type III/IV inhibitor. In contrast, these agents had no effect on IL-8 production. 3. Increasing concentrations of dbcAMP progressively reduced the production of TNF-alpha and IL-1 beta but not IL-8. 4. The addition of beta-agonist increased the inhibitory effect of PDE inhibitors tested on the production of TNF-alpha and IL-1 beta. 5. Type IV PDE inhibitors could be potent pharmacological agents for the treatment of diseases in which TNF-alpha and IL-1 beta are important etiological factors.

Human phosphodiesterase 4A: characterization of full-length and truncated enzymes expressed in COS cells

The type 4 phosphodiesterase (PDE) family comprises four enzymes (4A, 4B, 4C and 4D) that are characterized by their specificity for cAMP and selective inhibition by the anti-depressant drug rolipram (4-[3-(cyclopentoxyl)-4-methoxyphenyl]2-pyrrolidone). In common with other PDEs, they consist of a central conserved domain associated with catalytic activity in addition to two N-terminal upstream conserved regions (UCR1 and UCR2) that are unique to the type 4 enzymes. We have isolated a 2 kb cDNA encoding a full-length type 4A PDE{HSPDE4A4B[Bolger, Michaeli, Martins, St.John, Steiner, Rodgers, Riggs, Wigler and Ferguson (1993) Mol. Cell. Biol. 13, 6558-6571]} from a human frontal cortex cDNA library. Northern blot analysis showed that the major PDE4A mRNA of 4.5 kb was widely distributed in different human tissues. The recombinant PDE4A expressed in COS cells had a molecular mass of approx. 117 kDa as revealed by SDS/PAGE/Western blotting with a PDE4A-specific antibody and was specific for cAMP with a Km of 4.8 microM. The enzyme activity was potently inhibited by R-rolipram (IC50 204 nM) and showed a 2.7-fold stereoselectivity over the S enantiomer. Analysis of the kinetics of inhibition indicated that R-rolipram did not behave as a simple competitive inhibitor. Dixon replots suggested that there was more than one mode of interaction consistent with the detection in the enzyme of a high-affinity binding site for R-rolipram with a Kd of 2.3 nM. Truncation of the PDE4A enzyme by deletion mutagenesis showed that neither of the UCRs was required for catalytic activity and identified an approx. 71 kDa core enzyme with a K(m) for cAMP of 3.3 microM. In contrast with the full-length PDE4A, R-rolipram behaved as a simple competitive inhibitor of this form of the enzyme with decreased potency (IC50 1022 nM) and no stereoselectivity. In addition, no high-affinity rolipram-binding site was detected in the truncated enzyme, indicating that this interaction involves sequences upstream of the catalytic domain of the enzyme.

Phosphodiesterase (PDE)4 inhibitors: anti-inflammatory drugs of the future?

Phosphodiesterase type 4 (PDE4) plays a major role in modulating the activity of virtually all cells involved in the inflammatory process. Inhibitors of this enzyme family display impressive anti-inflammatory and disease-modifying effects in a variety of experimental models. In this review, Mauro Teixeira, Robert Gristwood, Nicola Cooper and Paul Hellewell examine the capacity of PDE4 inhibitors to exert anti-inflammatory actions in vivo and discuss the potential of this class of drugs to take their place as novel therapeutic agents for a variety of inflammatory diseases.

Effects of phosphodiesterase inhibitors on human lung mast cell and basophil function

1. The non-hydrolysable cyclic AMP analogue, dibutyryl (Bu2)-cyclic AMP, inhibited the stimulated release of histamine from both basophils and human lung mast cells (HLMC) in a dose-dependent manner. The concentrations required to inhibit histamine release by 50% (IC50) were 0.8 and 0.7 mM in basophils and HLMC, respectively. The cyclic GMP analogue, Bu2-cyclic GMP, was ineffective as an inhibitor of histamine release in basophils and HLMC. 2. The non-selective phosphodiesterase (PDE) inhibitors, theophylline and isobutyl-methylxanthine (IBMX) inhibited the IgE-mediated release of histamine from both human basophils and HLMC in a dose-dependent fashion. IBMX and theophylline were more potent inhibitors in basophils than HLMC. IC50 values for the inhibition of histamine release were, 0.05 and 0.2 mM for IBMX and theophylline, respectively, in basophils and 0.25 and 1.2 mM for IBMX and theophylline in HLMC. 3. The PDE 4 inhibitor, rolipram, attenuated the release of both histamine and the generation of sulphopeptidoleukotrienes (sLT) from activated basophils at sub-micromolar concentrations but was ineffective at inhibiting the release of histamine and the generation of both sLT and prostaglandin D2 (PGD2) in HLMC. Additional PDE 4 inhibitors, denbufylline, Ro 20-1724, RP 73401 and nitraquazone, were all found to be effective inhibitors of mediator release in basophils but were ineffective in HLMC unless high concentrations (1 mM) were employed. 4. Neither 8-methoxymethyl IBMX (PDE 1 inhibitor), zaprinast (PDE 5 inhibitor) nor a range of PDE 3 inhibitors (siguazodan, SKF 94120, SKF 95654) were effective inhibitors of mediator release from either basophils or HLMC. 5. In basophils, rolipram acted to potentiate the inhibitory effects of the adenylate cyclase activator, forskolin, whereas in HLMC, rolipram failed to potentiate the inhibitory effects of forskolin. 6. Extracts of purified HLMC and basophils hydrolysed cyclic AMP. IBMX (100 microM) inhibited the PDE activity in basophil extracts by 67 +/- 7% (P < 0.0001) and in HLMC extracts by 63 +/- 9% (P < 0.0005). The hydrolysis of cyclic AMP by basophil extracts was inhibited by the selective PDE inhibitors (all at 10 microM), rolipram (56 +/- 8%, P < 0.0001) and the mixed PDE 3/4 inhibitor, Org 30029 (47 +/- 9%, P < 0.01), whereas 8-methoxymethyl IBMX, siguazodan and zaprinast were ineffective. In HLMC, rolipram, Org 30029, 8-methoxymethyl IBMX, siguazodan and zaprinast all inhibited the hydrolysis of cyclic AMP by extracts to a significant (P < 0.05) and similar extent (approximately 25% inhibition at 10 microM). 7. In total, these data suggest that modulation of the PDE 4 isoform can regulate basophil responses whereas an association of the PDE 4 isoform with the regulation of HLMC function remains uncertain.

Effects of inhibitors of phosphodiesterase, on antigen-induced bronchial hyperreactivity in conscious sensitized guinea-pigs and airway leukocyte infiltration

1. The aim of this study was to determine the effects of inhibitors of phosphodiesterase (PDE) on the early and late phase bronchoconstriction in sensitized, conscious guinea-pigs and the subsequent development of acute airway hyperreactivity to the inhaled thromboxane mimetic, U46619, and leukocyte infiltration following ovalbumin (OvA) challenge. 2. Following an inhalation challenge with OvA, there was an early bronchoconstriction which peaked at 15 min with recovery after 3-4 h. A late phase bronchoconstriction occurred between 17 and 24 h after challenge. The PDE 4 inhibitors, Ro 20-1724 (3 mg kg-1, i.p.) and rolipram (1 mg kg-1, i.p.) administered 30 min before and 6 h after antigen challenge (double dosing regimen), did not affect the development of the early or late phase responses. 3. Seventeen to twenty four hours following an acute OvA or saline challenge, a consistently greater bronchoconstrictor response to inhaled U46619 was observed in the OvA challenged group. This increase in responsiveness was significantly attenuated by the administration of Ro 20-1724 and rolipram 30 min before and 6 h after antigen challenge (P < 0.05); this was not attributable to a residual bronchodilator effect of these compounds. There was a trend towards inhibition of the hyperreactivity to U46619 by aminophylline but not by the PDE3 inhibitors, siguazodan or SKF 95654. 4. Aminophylline, rolipram and Ro 20-1724 when administered as the double dose regimen attenuated the rise in macrophages, eosinophils and neutrophils recovered in bronchial lavage fluid 17 to 24 h after antigen challenge. 5. The dose of Ro 20-1724 given at 6 h post challenge was essential for attenuation of airway hyperreactivity and to protect against leukocyte influx. 6. In summary, aminophylline, rolipram and Ro 20-1724 have anti-inflammatory effects against antigen-induced airway leukocyte infiltration. Rolipram and Ro 20-1724 additionally attenuated the development of acute airway hyperreactivity, effects which are probably mediated through inhibition of PDE type 4. A dose of PDE inhibitor 6 h after the antigen challenge appears to be essential to achieve this protection. Inhibitors of PDE type 3 were generally without effect. However, there was no effect of rolipram or Ro 20-1724 on the development of either the early or late phase type responses.