Isoquinoline and Quinazoline Urea Analogues as Antagonists for the Human Adenosine A<sub>3</sub> Receptor

Isoquinoline and quinazoline urea derivatives were found to bind to human adenosine A<sub>3</sub> receptors. Series of <i>N</i>-phenyl-<i>N</i>‘-quinazolin-4-ylurea derivatives and <i>N</i>-phenyl-<i>N</i>‘-isoquinolin-1-ylurea derivatives were synthesized and tested in radioligand binding assays on their adenosine receptor affinities. A structure−affinity analysis indicated that on the 2-position of the quinazoline ring or the equivalent 3-position of the isoquinoline ring a phenyl or heteroaryl substituent increased the adenosine A<sub>3</sub> receptor affinity in comparison to unsubstituted or aliphatic derivatives. Furthermore, the structure−affinity relationship of substituted phenylurea analogues was investigated. Substituents such as electron-withdrawing or electron-donating groups were introduced at different positions of the benzene ring to probe electronic and positional effects of substitution. Substitution on the 3- or 4-position of the phenyl ring decreased the adenosine A<sub>3</sub> receptor affinity. Substitution at position 2 with an electron-donating substituent, such as methyl or methoxy, increased human adenosine A<sub>3</sub> receptor affinity, whereas substitution on the 2-position with an electron-withdrawing substituent did not influence affinity. Combination of the optimal substituents in the two series had an additive effect, which led to the potent human adenosine A<sub>3</sub> receptor antagonist <i>N</i>-(2-methoxyphenyl)-<i>N</i>‘-(2-(3-pyridyl)quinazolin-4-yl)urea (VUF5574, <b>10a</b>) showing a <i>K</i><sub>i</sub> value of 4 nM and being at least 2500-fold selective vs A<sub>1</sub> and A<sub>2A</sub> receptors. Compound <b>10a</b> competitively antagonized the effect of an agonist in a functional A<sub>3</sub> receptor assay, i.e., inhibition of cAMP production in cells expressing the human adenosine A<sub>3</sub> receptor; a p<i>A</i><sub>2</sub> value of 8.1 was derived from a Schild plot. In conclusion, compound <b>10a</b> is a potent and selective human adenosine A<sub>3</sub> receptor antagonist and might be a useful tool in further characterization of the human A<sub>3</sub> receptor.