There is considerable evidence to suggest that adenosine, through activation of A_2B receptors on mast cells and smooth muscle cells, is involved in the pathophysiology of asthma (Holgate, 2005). Here we describe the pharmacology of a novel selective A_2B receptor antagonist AS-16 (2-(4-benzyloxy-phenyl)-N-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yl]-acetamide) which may have therapeutic utility in asthma and respiratory disease. In radioligand binding studies utilizing the selective A_2B receptor ligand [3H]MRE-2029, AS-16 has an affinity of 22 nM for the human recombinant A_2B receptors expressed in HEK-293 cells. It is selective over the other adenosine receptor subtypes, and monoamine receptors in general, having an affinity greater than 1,000 nM at these targets. In functional assays, using recombinant A_2B receptors in CHO cells and endogenous A2B receptors in the human mast cell line HMC-1, AS-16 demonstrates an affinity (K_)B of 75 nM and 87 nM respectively. The in vivo efficacy of AS-16 has been examined in sheep which are naturally sensitive to the allergen Ascaris suum. In these animals antigen-induced bronchospasm has two phases similar to what occurs with allergen challenge studies in patients, an immediate increase in airflow resistance (early airway response), followed several hours later by a second increase in airway resistance (late airway response). AS-16 (2 mg/kg) was dosed orally twice-daily for 2 days prior to antigen challenge. This dosing regime gave a consistent plasma concentration (70-100 ng/ml) during the experimental phase of the studies. AS-16 reduced the early airway response by approximately 20 % and almost completely abolished the late airway response. A further feature of the sheep model, which is also consistent with studies in patients, is that antigen challenge induces airway hyperresponsiveness (AHR). AS-16 (2 mg/kg p.o.) completely prevented the AHR to inhaled carbachol, assessed 24 h after allergen challenge. Finally, AS-16 (2 mg/kg p.o.) was able to completely block adenosine-induced bronchospasm, suggesting that in sheep adenosine effects are mediated via the A_2B receptor. Thus, AS-16 is a novel, selective A_2B receptor antagonist which will be a useful tool to explore the pathophysiology of asthma and respiratory disease, and may represent a novel therapy for the treatment of these chronic diseases.