Studies on anti-allergic action of AH 21-132, a novel isozyme-selective phosphodiesterase inhibitor in airways

PDE4 inhibitors as new anti-inflammatory drugs: effects on cell trafficking and cell adhesion molecules expression

Phosphodiesterase 4 (PDE4) is a major cyclic AMP-hydrolyzing enzyme in inflammatory and immunomodulatory cells. The wide range of inflammatory mechanisms under control by PDE4 points to this isoenzyme as an attractive target for new anti-inflammatory drugs. Selective inhibitors of PDE4 have demonstrated a broad spectrum of anti-inflammatory activities including the inhibition of cellular trafficking and microvascular leakage, cytokine and chemokine release from inflammatory cells, reactive oxygen species production, and cell adhesion molecule expression in a variety of in vitro and in vivo experimental models. The initially detected side effects, mainly nausea and emesis, appear at least partially overcome by the 'second generation' PDE4 inhibitors, some of which like roflumilast and cilomilast are in the later stages of clinical development for treatment of chronic obstructive pulmonary disease. These new drugs may also offer opportunities for treatment of other inflammatory diseases.

Administration of pentoxifylline during allergen sensitization dissociates pulmonary allergic inflammation from airway hyperresponsiveness

Asthma, a chronic inflammatory disease characterized by intermittent, reversible airflow obstruction and airway hyperresponsiveness (AHR), is classically characterized by an excess of Th2 cytokines (IL-13, IL-4) and depletion of Th1 cytokines (IFN-gamma, IL-12). Recent studies indicating an important role for Th1 immunity in the development of AHR with allergic inflammation suggest that Th1/Th2 balance may be important in determining the association of AHR with allergic inflammation. We hypothesized that administration of pentoxifylline (PTX), a phosphodiesterase inhibitor known to inhibit Th1 cytokine production, during allergen (OVA) sensitization and challenge would lead to attenuation of AHR in a murine model of allergic pulmonary inflammation. We found that PTX treatment led to attenuation of AHR when administered at the time of allergen sensitization without affecting other hallmarks of pulmonary allergic inflammation. Attenuation of AHR with PTX treatment was found in the presence of elevated bronchoalveolar lavage fluid levels of the Th2 cytokine IL-13 and decreased levels of the Th1 cytokine IFN-gamma. PTX treatment during allergen sensitization leads to a divergence of AHR and pulmonary inflammation following allergen challenge.

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 inhibition of phosphodiesterase type IV suppresses the chemotactic responsiveness of rat eosinophils in vitro

Previous studies demonstrated that the selective inhibition of phosphodiesterase type IV suppresses antigen-induced eosinophil infiltration and also downregulates certain eosinophil functions assessed in vitro. In the current study, we compared the effect of selective inhibitors of phosphodiesterase IV with the effect of phosphodiesterase III and V inhibitors, focusing on eosinophil chemotaxis stimulated by platelet-activating factor (PAF) and leukotriene B4 in a modified Boyden chamber. The effect of beta 2-adrenoceptor agonists and forskolin as well as the analogue N6-2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (Bt2 cyclic AMP) was also determined. For this purpose eosinophils were obtained by lavage of the peritoneal cavity of normal Wistar rats and purified on Percoll gradients to 85-95% purity. Our results showed that PAF and leukotriene B4 (0.001-10 microM) elicited a concentration-dependent increase in eosinophil migration with maximal responses observed at 1 microM and 0.1 microM respectively. Pre-incubation with the type IV phosphodiesterase inhibitor, rolipram (1-100 microM), suppressed the chemotactic response triggered by PAF and leukotriene B4, in association with elevation of eosinophil cyclic AMP, whereas the compounds milrinone and SK&F 94836 (type III selective) as well as zaprinast (type V selective) were ineffective. The beta 2-adrenoceptor agonists salbutamol and salmeterol (1-100 microM) did not alter the intracellular levels of cyclic AMP and also failed to inhibit the eosinophil response. Moreover, incubation of eosinophils with the adenylate cyclase activator forskolin (1-100 microM), while inducing a discrete increase in cyclic AMP, markedly inhibited PAF- and leukotriene B4-induced eosinophil chemotaxis. Eosinophils treated with a combination of individually inactive amounts of forskolin plus rolipram significantly inhibited the eosinophil migration elicited by PAF and leukotriene B4, but did not change cyclic AMP baseline levels. Though only at the highest concentration tested (100 microM), the analogue Bt2 cyclic AMP abolished the eosinophil chemotaxis. Thus we conclude that the direct inhibitory effect of phosphodiesterase IV inhibitors on eosinophil chemotaxis may account for their suppressive activity on tissue eosinophil accumulation following antigen challenge.