No abstract

The adenosine A(2B) receptor is the least well characterized of the four known adenosine receptor subtypes because of the absence of potent, selective agonists. Here, we present five non-adenosine agonists. Among them, 2-amino-4-(4-hydroxyphenyl)-6-(1H-imidazol-2-ylmethylsulfanyl)pyridine-3,5-dicarbonitrile, 17, LUF5834, is a high-efficacy partial agonist with EC(50) = 12 nM and 45-fold selectivity over the adenosine A(3) receptor but lacking selectivity versus the A(1) and A(2A) subtypes. Compound 18, LUF5835, the 3-hydroxyphenyl analogue, is a full agonist with EC(50) = 10 nM.

Many G protein-coupled receptors (GPCRs), including the adenosine A(1) receptor (A(1)AR), have been shown to be allosterically modulated by small molecule ligands. So far, in the absence of structural information, the exact location of the allosteric site on the A(1)AR is not known. We synthesized a series of bivalent ligands (4) with an increasing linker length between the orthosteric and allosteric pharmacophores and used these as tools to search for the allosteric site on the A(1)AR. The compounds were tested in both equilibrium radioligand displacement and functional assays in the absence and presence of a reference allosteric enhancer, (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone, PD81,723 (1). Bivalent ligand N(6)-[2-amino-3-(3,4-dichlorobenzoyl)-4,5,6,7-tetrahydrothieno[2,3-c]pyridin-6-yl-9-nonyloxy-4-phenyl]-adenosine 4h (LUF6258) with a 9 carbon atom spacer did not show significant changes in affinity or potency in the presence of 1, indicating that this ligand bridged both sites on the receptor. Furthermore, 4h displayed an increase in efficacy, but not potency, compared to the parent, monovalent agonist 2. From molecular modeling studies, we speculate that the allosteric site of the A(1)AR is located in the proximity of the orthosteric site, possibly within the boundaries of the second extracellular loop of the receptor.

Adenosine receptor antagonists usually possess a bi- or tricyclic heteroaromatic structure at their core with varying substitution patterns to achieve selectivity and/or greater affinity. Taking into account molecular modeling results from a series of potent adenosine A1 receptor antagonists, a pharmacophore was derived from which we show that a monocyclic core can be equally effective. To achieve a compound that may act at the CNS we propose imposing a restriction related to its polar surface area (PSA). In consequence, we have synthesized two novel series of pyrimidines, possessing good potency at the adenosine A1 receptor and desirable PSA values. In particular, compound 30 (LUF 5735) displays excellent A1 affinity (Ki = 4 nM) and selectivity (< or =50% displacement of 1 muM concentrations of the radioligand at the other three adenosine receptors) and has a PSA value of 53 A2.

A 2A adenosine receptor antagonists usually have bi- or tricyclic N aromatic systems with varying substitution patterns to achieve desired receptor affinity and selectivity. Using a pharmacophore model designed by overlap of nonxanthine type of previously known A 2A antagonists, we synthesized a new class of compounds having a 2-amino nicotinonitrile core moiety. From our data, we conclude that the presence of at least one furan group rather than phenyl is beneficial for high affinity on the A 2A adenosine receptor. Compounds 39 (LUF6050) and 44 (LUF6080) of the series had K i values of 1.4 and 1.0 nM, respectively, with reasonable selectivity toward the other adenosine receptor subtypes, A 1, A 2B, and A 3. The high affinity of 44 was corroborated in a cAMP second messenger assay, yielding subnanomolar potency for this compound.