LY 186655, a phosphodiesterase inhibitor, inhibits histamine release from human basophils, lung and skin fragments

Cyclic nucleotide phosphodiesterases

Cyclic nucleotide second messengers (cAMP and cGMP) play a central role in signal transduction and regulation of physiologic responses. Their intracellular levels are controlled by the complex superfamily of cyclic nucleotide phosphodiesterase (PDE) enzymes. Continuing advances in our understanding of the molecular pharmacology of these enzymes has led to the development of selective inhibitors as therapeutic agents for disease states ranging from cancer and heart failure to depression and sexual dysfunction. Several PDE types have been identified as therapeutic targets for immune/inflammatory diseases. This article briefly reviews the available in vitro, preclinical, and clinical data supporting the potential for selective PDE inhibitors as immunomodulatory agents.

The effect of selective and non-selective phosphodiesterase inhibitors on allergen- and leukotriene C(4)-induced contractions in passively sensitized human airways

Non-selective inhibitors of cyclic nucleotide phosphodiesterase (PDE) block allergen-induced contraction of passively sensitized human airways in vitro by a dual mechanism involving a direct relaxant effect on smooth muscle and inhibition of histamine and cysteinyl leukotriene (LT) release from airways. We investigated the effects of non-selective PDE inhibitors and selective inhibitors of PDE3 and PDE4 in order to determine the involvement of PDE isoenzymes in the suppression of allergic bronchoconstriction. Macroscopically normal airways from 76 patients were sensitized with IgE-rich sera (>250 u ml(-1)) containing specific antibodies against allergen (Dermatophagoides farinae). Contractile responses of bronchial rings were assessed using standard organ bath techniques. Passive sensitization caused increased contractile responses to allergen, histamine and LTC(4). Non-selective PDE inhibitors (theophylline, 3-isobutyl-1-methylxanthine [IBMX]), a PDE3-selective inhibitor (motapizone), PDE4-selective inhibitors (RP73401, rolipram, AWD 12-281) and a mixed PDE3/4 inhibitor (zardaverine) all significantly relaxed inherent bronchial tone at resting tension and to a similar degree. Theophylline, IBMX, zardaverine and the combination of motapizone and RP73401 inhibited the contractile responses to allergen and LTC(4). Pre-treatment with motapizone, RP73401, rolipram or the methylxanthine adenosine receptor antagonist, 8-phenyltheophylline, did not significantly decrease responses to either allergen or LTC(4). We conclude that combined inhibition of PDE3 and PDE4, but not selective inhibition of either isoenzyme or antagonism of adenosine receptors, is effective in suppressing allergen-induced contractions of passively sensitized human airways. The relationship between allergen- and LTC(4)-induced responses suggests that PDE inhibitors with PDE3 and PDE4 selectivity are likely to act in part through inhibition of mediator release and not simply through direct relaxant actions on airway smooth muscle.

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.

Selective phosphodiesterase inhibitors for the treatment of bronchial asthma and chronic obstructive pulmonary disease

Theophylline is commonly used in the treatment of obstructive airway diseases. The identification and functional characterization of different phosphodiesterase (PDE) isoenzymes has led to the development of various isoenzyme-selective inhibitors as potential anti-asthma drugs. Considering the distribution of isoenzymes in target tissues, with high activity of PDE3 and PDE4 in airway smooth muscle and inflammatory cells, selective inhibitors of these isoenzymes may add to the therapy of chronic airflow obstruction. However, initial data from clinical trials with selective PDE3 and PDE4 inhibitors have been somewhat disappointing and have tempered the expectations considerably since these drugs had limited efficacy and their use was clinically limited through side effects. The improved understanding of the molecular biology of PDEs enabled the synthesis of novel drugs with an improved risk/benefit ratio. These 'second generation' selective drugs have produced more promising clinical results not only for the treatment of bronchial asthma but also for the treatment of chronic obstructive pulmonary disease.

The effects of anti-asthma drugs on mediator release from cultured human mast cells

BACKGROUND

A method for generating human mast cells in vitro was recently established. Little is known about the pharmacological profiles of allergic mediator release from cultured mast cells.

OBJECTIVE

The main objective was to investigate the nature of cultured mast cells from a pharmacological point of view. We examined the effect of anti-asthma drugs on the release of histamine, sulfidoleukotrienes (LTs) and prostaglandin D2 (PGD2) from the cultured mast cells.

METHODS

Using the method established by Saito et al. we cultured cord blood mononuclear cells in the presence of 80 ng/mL stem cell factor (SCF), 50 ng/mL interleukin-6 (IL-6) and 300 nmol/L prostaglandin E2 (PGE2), and obtained almost pure (> 99%) mast cells. We sensitized cultured mast cells with immunoglobulin E (IgE)-rich serum, and then treated them with some anti-asthma drugs before challenge with anti-human IgE. Released histamine, LTs and PGD2 were measured by high-performance liquid chromatography, commercial enzyme-linked immunosorbent assay (ELISA) and enzyme immunoassay (EIA) systems, respectively.

RESULTS

The cultured mast cells released histamine, LTs and PGD2 following immunological stimulation through IgE. The mast cell stabilizing agents disodium cromoglycate (DSCG, 1 mmol/L) and azelastine (100 micromol/L) significantly inhibited the release of these three mediators. The beta-adrenoceptor agonists isoproterenol, salbutamol, and clenbuterol also inhibited all three mediators' release in a concentration-dependent manner. The non-selective and selective phosphodiesterase (PDE) inhibitors theophylline, rolipram, and cilostazol had no significant effect on mediator release at clinically useful concentrations. BAY x 1005 (a 5-lipoxygenase-activating protein inhibitor) inhibited the LTs release, whereas indomethacin (a cyclo-oxygenase I and II inhibitor) and NS-398 (a cyclo-oxygenase II inhibitor) inhibited PGD2 release.

CONCLUSIONS

The present results indicate that cultured mast cells release histamine, LTs and PGD2 following IgE crosslinking. Anti-asthma drugs showed a characteristic suppression of the release of each mediator. The suppressive actions of these drugs are similar to their pharmacological actions on human lung mast cells. These results suggest that cultured mast cells are useful for the analysis of function and pharmacological profiles of lung mast cells.

Effects of theophylline and rolipram on leukotriene C4 (LTC4) synthesis and chemotaxis of human eosinophils from normal and atopic subjects

1. The effects of the non-selective phosphodiesterase (PDE) inhibitor theophylline and the selective PDE4 inhibitor rolipram on leukotriene C4 (LTC4) synthesis and chemotaxis of complement 5a (C5a)- and platelet-activating factor (PAF)-stimulated human eosinophils obtained from normal and atopic donors were investigated. 2. Eosinophils were purified from peripheral venous blood of normal and atopic subjects by an immunomagnetic procedure to a purity > 99%. Eosinophils were stimulated with PAF (0.1 microM) or C5a 0.1 microM for 15 min and LTC4 was measured by radioimmunoassay (RIA). Eosinophil chemotaxis in response to PAF and C5a was assessed with 48-well microchambers (Boyden). 3. Under these conditions substantial amounts of LTC4 (about 300-1000 pg per 10(6) cells) were only detectable in the presence of indomethacin (0.1-10 microM). To explain this finding it was hypothesized that indomethacin reversed the inhibition of LTC4 synthesis by endogenously synthesized prostaglandins, in particular prostaglandin E2 (PGE2). In fact, eosinophils release 23 pg PGE2 per 10(6) cells following PAF stimulation; this PGE2 synthesis was completely inhibited by indomethacin and readdition of PGE2 inhibited eosinophil LTC4 synthesis (IC50 = 3 nM). The following experiments were performed in the presence of 10 microM indomethacin. 4. Theophylline (IC50 approximately 50 microM) and rolipram (IC50 approximately 0.03-0.2 microM) suppressed PAF- and C5a-stimulated LTC4 synthesis. This PDE inhibitor-induced suppression of LTC4 generation is mediated by activation of protein kinase A, since it was reversed by the protein kinase A inhibitor Rp-8-Br-cyclic AMPS. In addition, exogenous arachidonic acid concentration-dependently (0.3 microM-3 microM) reversed the inhibition of LTC4 synthesis by the PDE inhibitors, indicating that theophylline and rolipram suppress the mobilization of arachidonic acid. The beta 2-adrenoceptor agonist salbutamol inhibited eosinophil LTC4 synthesis (IC50 = 0.08 microM). The combination of salbutamol with theophylline (10 microM) or rolipram (3 nM) appeared to be additive. 5. Theophylline (IC50 approximately 40 microM), rolipram (IC50 approximately 0.02 microM [C5a], approximately 0.6 microM [PAF]) and PGE2 (IC50 approximately 3 nM) inhibited C5a- and PAF-stimulated eosinophil chemotaxis. The combination of PGE2 with theophylline resulted in an additive effect. 6. Both C5a- and PAF-stimulated eosinophil chemotaxis and LTC4 generation were significantly elevated in eosinophils from atopic individuals compared to normal subjects. However, eosinophils from normal and atopic individuals were not different with respect to their total cyclic AMP-PDE and PDE4 isoenzyme activities as well as the potencies of theophylline and rolipram to suppress LTC4 generation and chemotaxis.