A1 adenosine receptor antagonists, 3-aryl[1,2,4]triazino[4,3-a]benzimidazol-4-(10H)-ones (ATBIs) and N-alkyl and N-acyl-(7-substituted-2-phenylimidazo[1,2-a][1,3,5]triazin-4-yl)amines (ITAs): different recognition of bovine and human binding sites

3-Aryl[1,2,4]triazino[4,3-a]benzimidazol-4-(10H)-ones (ATBIs) 1 and N-alkyl and N-acyl-(7-substituted-2-phenylimidazo[1,2-a][1,3,5]triazin-4-yl)amines (ITAs) 2, recently reported by us as two novel classes of selective A 1 adenosine receptor (A1AR) antagonists using bovine cerebral membranes [Da Settimo et al., 2001, Novellino et al., 2002], were reevaluated for their affinity at human A1, A2A, and A3ARs expressed in CHO cells. All the ATBI derivatives 1 showed similar Ki values in the nanomolar range at both hA1AR and bA1AR, with a good selectivity for the hA1AR over the hA2AAR and hA3AR. The excellent correlation between Ki values at hA1AR and bA1AR in the ATBI series suggests that these compounds interact similarly at the binding sites of the two receptors. Unlike findings for the ATBIs 1, the Ki values of ITAs 2 at the hA 1AR and bA1AR do not correlate very well. Several compounds (2g, 2i, 2j, 2m, 2n, and 2r) that bind with a nanomolar affinity to the bA1AR were found to be significantly less potent, or even inactive, at the hA1AR. A comparison of the amino acids in the TM regions involved in the recognition of the ligands at the bA1AR and hA1AR failed to provide a clear explanation of the great differences in ligand affinities between bA1AR and hA1AR. Some hypotheses are advanced to account for these differences