We next examined the effects of substituents around

We next examined the effects of substituents around the carboxylic EZLink Sulfo-NHS-SS-Biotin Kit mg moiety on CRTH2 binding (Table 4). The carboxylic acid moiety shared by the representative CRTH2 antagonists and is essential for the CRTH2 activity. Germinal dimethylation of the methylene moiety next to the carboxylic acid resulted in a 25-fold decrease in the binding potency (15-45) compared with that of the original compound 15-40, and similarly, the functional activity of 15-45 was dramatically reduced. Insertion of an oxygen atom between the carboxymethyl moiety and the heteroaryl group resulted in a slight loss in the binding and a significant decrease in functional activity (15-46). These data suggest that the binding site of the CRTH2 (P1), where the acid moiety of the antagonists interacts, is strictly limited. The SAR study of the isoquinoline scaffold demonstrated that with a few exceptions, compounds with high binding affinities generally exhibited highly potent functional activities. One of the most potent antagonists, 15-20 (IC50=19nM), was effective in a chemotaxis assay (IC50=23nM) and exhibited sufficient selectivity in binding to CRTH2 over the DP1 prostanoid receptor (IC50 >1μM) and the COX-1 and COX-2 enzymes (IC50 >10μM).
Conclusion In conclusion, we have identified the novel isoquinoline acetic acid chemotype 15-1 as a potent CRTH2 antagonist. SAR of the scaffold was explored, resulting in the identification of compound 15-20 (TASP0376377), which is a selective functional antagonist of CRTH2. Studies are ongoing to explore the utility of this class of compounds in inflammatory disease models and will be reported in due course.
Experimental
Introduction The global disease burden of allergy remains a significant unmet medical need. For example, asthma impacts approximately 300million individuals and this high prevalence (>10% in developed countries) appears to be increasing by 50% every decade. Mild to moderate disease is well controlled by inhaled therapy, most notably the combination of inhaled corticosteroids and long acting beta-2 agonists and more recently anti-IgE antibody approaches have emerged as a severe asthma treatment option. However, issues with convenience and compliance with these drugs remain. Conversely, the lower clinical efficacy of currently approved oral asthma medications such as leukotriene receptor antagonists has resulted in them being recommended as second-line treatment. Given this context, there remains an unmet need for the development of novel orally delivered anti-inflammatory drugs for asthma and other allergic diseases. Prostaglandin D2 (PGD2), a product of the arachidonic acid cascade, is synthesized primarily by IgE activated mast cells, as well as by macrophages and Th2 lymphocytes. PGD2 has long been associated with the allergic inflammatory response, being found at high concentrations after allergen challenge in the lungs of asthmatic patients, in nasal washings of allergic rhinitis patients and in the skin of patients with atopic dermatitis. PGD2 was initially believed to act through the DP1 receptor which mediates a number of homeostatic functions including vasodilatation. More recently a second receptor for PGD2, known as CRTh2 or DP2 was identified. Interestingly, a well-designed clinical study with a selective DP1 receptor antagonist delivered a negative result in an asthma and allergic rhinitis proof of concept trial. CRTh2 shows minimal homology with DP1 and is expressed on inflammatory cells, in particular showing selective expression on Th2 over Th1 cells. While PGD2 itself is relatively unstable and short-lived in vivo, a number of stable metabolites such as 13,14-dihydro-15-keto-PGD2 (DK-PGD2) display very high levels of selectivity for activation of CRTh2 compared to DP1. Using such specific agonists, it has been demonstrated that the PGD2-mediated activation and migration of eosinophils, basophils and Th2 cells in vitro proceeds selectively via CRTh2 receptor activation. Given the role of these cell types as key orchestrators of the late phase allergic inflammatory response, an emerging body of evidence from the activity of selective CRTh2 antagonists in pre-clinical rodent models supports this mechanism as a new therapeutic modality for treatment of asthma, allergic rhinitis and atopic dematitis. Indeed, a number of selective CRTh2 antagonists such as AZD-1981 and ODC000459 (Fig. 1) (both derived from structure morphing of the CRTh2 agonist indomethacin) have progressed into clinical studies with OC000459 showing efficacy in a human clinical proof of concept trials in allergic asthma and allergic rhinitis.