Species differences in brain adenosine A(1) receptor pharmacology revealed by use of xanthine and pyrazolopyridine based antagonists

1 The pharmacological profile of adenosine A(1) receptors in human, guinea-pig, rat and mouse brain membranes was characterized in a radioligand binding assay by use of the receptor selective antagonist, [H-3]-8-cyclopentyl-1,3-dipropylxanthine ([H-3]-DPCPX). 2 The affinity of [H-3]-DPCPX binding sites in rat cortical and hippocampal membranes was similar. Binding site affinity was higher in rat cortical membranes than in membranes prepared from guinea-pig cortex and hippocampus, mouse cortex and human cortex. pK(D), values (M) were 9.55, 9.44, 8.85, 8.94, 8.67, 9.39 and 8.67, respectively. The binding site density (B-max) was lower in rat cortical membranes than in guinea-pig or human cortical membranes. 3 The rank order of potency of seven adenosine receptor agonists was identical in each species. With the exception of 5'-N-ethylcarboxamidoadenosine (NECA), agonist affinity was 3.5-26.2 fold higher in rat cortical membranes than in human and guinea-pig brain membranes; affinity in rat and mouse brain membranes was similar. While NECA exhibited 9.3 fold higher affinity in rat compared to human cortical membranes, affinity in other species was comparable. The stable GTP analogue. Gpp(NH)p (100 mu M) reduced 2-chloro-N-6-cyclopentyladenosine (CCPA) affinity 7-13.9 fold, whereas the affinity of DPCPX was unaffected. 4 The affinity of six xanthine-based adenosine receptor antagonists was 2.2-15.9 fold higher in rat cortical membranes compared with human or guinea-pig membranes. The rank order of potency was species-independent In contrast, three pyrazolopyridine derivatives, (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-alpyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-alpyridin-3-yl) acryloyl]-piperidin-2-yl acetic acid (FK352) and 6-oxo-3-(2-phenylpyrazolo[1,5-alpyridin-3-yl)-1(6H)-pyridazinebutyric acid (FK838) exhibited similar affinity in human, guinea-pig, rat and mouse brain membranes. pK(t) values (M) for [H-3]-DPCPX binding sites in human cortical membranes were 9.31, 7.52 and 7.92, respectively. 5 Drug affinity-for adenosine A(2A) receptors was determined in a [H-3]-2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine ([H-3]-CGS 21680) binding assay in rat striatal membranes. The pyrazolopyridine derivatives, FK453, FK838 and FK352 exhibited pK(t) values (M) of 5.90, 5.92 and 4.31, respectively, compared with pK(t) values of 9.31, 8.18 and 7.57 determined in the [H-3]-DPCPX binding assay in rat cortical membranes. These novel pyrazolopyridine derivatives therefore represent high affinity, adenosine A(1) receptor selective drugs that, in contrast to xanthine based antagonists, exhibit similar affinity for [H-3]-DPCPX binding sites in human, rat, mouse and guinea-pig brain membranes.