The structure-activity relationships for a series of pyrazine-based A 2B adenosine receptor antagonists are described. From this work, LAS101057 (17), a potent, selective, and orally efficacious A 2B receptor antagonist, was identified as a clinical development candidate. LAS101057 inhibits agonist-induced IL-6 production in human fibroblasts and is active in an ovalbumin (OVA)-sensitized mouse model after oral administration, reducing airway hyperresponsiveness to methacholine, Th2 cytokine production, and OVA-specific IgE levels.KEYWORDS: A 2B adenosine receptor antagonist, ovalbumin mouse model, clinical candidate T he purinergic, G protein-coupled receptor A 2B has a low affinity for the endogenous mediator adenosine.1-3 Evidence published over the past decade has shown that the A 2B receptor mediates the release of inflammatory mediators from several cell types, including many which are relevant for the pathophysiology of asthma, such as mast cells, smooth muscle cells, airways epithelial cells, and fibroblasts.
4,5The A 2B receptor has a pro-inflammatory role in the gastrointestinal tract, 6 and it has been shown to contribute to the development of colitis in a murine A 2B receptor knockout model.
7Some of the initial evidence obtained using A 2B receptor knockout mice suggested an anti-inflammatory role for this receptor. 8,9 However, subsequent studies do not entirely support this concept.10-12 A 2B activates cystic fibrosis transmembrane regulator chloride channel-mediated liquid release in the lungs, 13 and it could play a role in severe watery infectious diarrhea through activation of a chloride secretory response.14 A 2B receptor knockout mice have also shown that A 2B receptor prevents endothelial damage during inflammation and stimulates endothelial cell proliferation and capillary tube formation, mediating the release of the pro-angiogenic factors VEGF, bFGF, and IGF-1, 15 suggesting a possible role in tumorigenesis.16 Expression of A 2B is upregulated in several types of cancer cells. 17,18 A 2B adenosine antagonists that have been or are in clinical trials are the dual A 2B /A 3 antagonist QAF 805 and the selective A 2B antagonist In previous disclosures, 20,21 it was revealed that appropriately substituted N-(5,6-diarylpyridin-2-yl)amides are potent and selective A 2B adenosine receptor antagonists. Although particular compounds from these series demonstrated good oral bioavailability in rat pharmacokinetic studies, in general all compounds tested presented rapid systemic clearance.Herein are detailed further optimization efforts in a structurally related series which ultimately led to the discovery of a clinical candidate LAS101057 (17) intended for use in oral asthma therapy.Studies conducted in parallel to those described above revealed that replacement of the central pyridine scaffold of compounds from the amide-based series by a pyrazine moiety gave compounds with good activity/selectivity profiles ( Table 1).The pharmacokinetic profiles of the pyrazines 1 and 2 in the rat were ...