Finding better therapeutic targets for patients with asthma: adenosine receptors?

Anti-inflammatory effect of a novel locally acting A2A receptor agonist in a rat model of oxazolone-induced colitis

Adenosine represents a powerful modulating factor, which has been shown to orchestrate the scope, duration, and remission of the inflammatory response through the activation of four specific receptors, classified as A₁, A_2A, A_2B, and A₃, all being widely expressed in a variety of immune cells. Several selective A_2A receptor agonists have displayed anti-inflammatory effects, through the suppression of IL-12, TNF, and IFN-γ production by monocytes and lymphocytes, in the setting of chronic intestinal inflammation. However, the therapeutic application of A_2A receptor agonists remains hindered by the risk of serious cardiovascular adverse effects arising from the wide systemic distribution of A_2A receptors. The present study focused on evaluating the anti-inflammatory effects of the novel poorly absorbed A_2A receptor agonist PSB-0777 in a rat model of oxazolone-induced colitis as well as to evaluate its cardiovascular adverse effects, paying particular attention to the onset of hypotension, one of the main adverse effects associated with the systemic pharmacological activation of A_2A receptors. Colitis was associated with decreased body weight, an enhanced microscopic damage score and increased levels of colonic myeloperoxidase (MPO). PSB-0777, but not dexamethasone, improved body weight. PSB-0777 and dexamethasone ameliorated microscopic indexes of inflammation and reduced MPO levels. The beneficial effects of PSB-0777 on inflammatory parameters were prevented by the pharmacological blockade of A_2A receptors. No adverse cardiovascular events were observed upon PSB-0777 administration. The novel A_2A receptor agonist PSB-0777 could represent the base for the development of innovative pharmacological entities able to act in an event-specific and site-specific manner.

Immunoregulatory effects of adenosine

Adenosine, a purine nucleoside, is involved in local and systemic inflammatory responses. Adenosine plays a pivotal role in the modulation of differentiation, maturation and migration of immune cells (neutrophils, macrophages, dendritic cells, lymphocytes, etc.), as well as in the secretion of cytokines and chemokines by activating four G protein-coupled receptors (A1, A2A, A2B, and A3) which are expressed by a variety of immune cells. The development of selective agonists and antagonists of adenosine receptors has advanced the current understanding on the multiple functions of adenosine in immunologic responses and related diseases. This article reviews the metabolism of adenosine and its effects on immune cells and inflammatory diseases via adenosine receptors.

Adenosine receptors as targets for therapeutic intervention in asthma and chronic obstructive pulmonary disease

Asthma and chronic obstructive pulmonary disease (COPD) are pulmonary disorders characterized by various degrees of inflammation and tissue remodeling. Adenosine is a signaling molecule that is elevated in the lungs of patients with asthma and COPD. Adenosine elicits its actions by engaging cell surface adenosine receptors, and substantial preclinical evidence suggests that targeting these receptors will provide novel approaches for the treatment of asthma and COPD. Studies in animal models of airway disease suggest that there may be clinical benefit to the use of A(1), A(3) and A(2B) adenosine receptor antagonists in the treatment of features of asthma and/or COPD, while A(2A) agonists may also prove effective. Several adenosine receptor based pharmacologic agents have entered clinical development for the treatment of asthma and COPD; however, the studies have been limited and the efficacy of such approaches is not yet clear.