Anilide derivatives of an 8-phenylxanthine carboxylic congener are highly potent and selective antagonists at human A(2B) adenosine receptors

No highly selective antagonists of the A(2B) adenosine receptor (AR) have b een reported; however such antagonists have therapeutic potential as antias thmatic agents. Here we report the synthesis of potent and selective A(2B) receptor antagonists. The structure-activity relationships (SAR) of 8-pheny l-1,3-di-(n-propyl)xanthine derivatives in binding to recombinant human A(2 B) ARs in HEK-293 cells (HEK-A(2B)) and at other AR subtypes were explored. Various amide derivatives of 8-[4-[[carboxymethyl]oxy]phenyl]-1,3-di-(n-pr opyl)xanthine, 4a, were synthesized. A comparison of aryl, alkyl, and aralk yl amides demonstrated that simple anilides, particularly those substituted in the para-position with electron-withdrawing groups, such as nitro, cyan o, and acetyl, bind selectively to human A(2B) receptors in the range of 1- 3 nM. The unsubstituted anilide 12 had a K-i value at A(2B) receptors of 1. 48 nM but was only moderately selective versus human A(1)/A(2A) receptors a nd nonselective versus rat A(1) receptors. Highly potent and selective A(2B ) antagonists were a p-aminoacetophenone derivative 20 (K-i value 1.39 nM) and a p-cyanoanilide 27 (K-i value 1.97 nM). Compound 27 was 400-, 245-, an d 123-fold selective for human A(2B) receptors versus human A(1)/A(2A)/A(3) receptors, respectively, and 8.5- and 310-fold selective versus rat A(1)/A (2A) receptors, respectively. Substitution of the 1,3-dipropyl groups with 1,3-diethyl offered no disadvantage for selectivity, and high affinities at A(2B) receptors were maintained. Substitution of the p-carboxymethyloxy gr oup of 4a and its amides with acrylic acid decreased affinity at A(2B) rece ptors while increasing affinity at A(1) receptors. 1,3-Di(cyclohexylmethyl) groups greatly reduced affinity at ARs, although the p-carboxymethyloxy de rivative 9 was moderately selective for A(2B) receptors. Several selective A(2B) antagonists inhibited NECA-stimulated calcium mobilization in HEK-A(2 B) cells.