STRUCTURE-ACTIVITY-RELATIONSHIPS OF 9-ALKYLADENINE AND RIBOSE-MODIFIED ADENOSINE DERIVATIVES AT RAT A(3) ADENOSINE RECEPTORS

9-Alkyladenine derivatives and ribose-modified N-6-benzyladenosine derivatives were synthesized in an effort to identify selective ligands for the rat A(3) adenosine receptor and leads for the development of antagonists. The derivatives contained structural features previously determined to be important for A(3) selectivity in adenosine derivatives, such as an N-6-(3-iodobenzyl) moiety, and were further substituted at the 2-position with halo, amino, or thio groups. Affinity was determined in radioligand binding assays at rat brain A(3) receptors stably expressed in Chinese hamster ovary (CHO) cells, using [I-125]]AB-MECA (N-6-(4-amino-3-iodobenzyl)adenosine-5'-(N-methyluronamid)), and at rat brain A(1) and A(2a) receptors using [H-3]-N-6-PIA ((R)-N-6-phenylisopropyladenosine) and [H-3]CGS 21680 (2-[[[4-(2-carboxyethyl)-phenyl]ethyl]amino]-5'-(N-ethylcarbamoyl)adenosine), respectively. A series of N-6-(3-iodobenzyl) 2-amino derivatives indicated that a small 2-alkylamino group, e.g., methylamino, was favored at A(3) receptors. N-6-(3-Iodobenzyl)-9-methyl-2-(methylthio)adenine was 61-fold more potent than the corresponding 2-methoxy ether at A(3) receptors and of comparable affinity at A(1) and A(2a) receptors, resulting in a 3-6-fold selectivity for A(3) receptors. A pair of chiral N-6-(3-iodobenzyl) 9-(2,3-dihydroxypropyl) derivatives showed stereoselectivity, with the R-enantiomer favored at A(3) receptors by 5.7-fold. 2-Chloro-9-(beta-D-erythrofuranosyl)-N-6-(3-iodobenzyl)adenine had a K-i value at A(3) receptors of 0.28 mu M. 2-Chloro-9-[2-amino-2,3-dideoxy-beta-D-5-(methylcarbamoyl)-arabinofuranosyl]-N-6-(3-iodobenzyl)adenine was moderately selective for A(1) and A(3) VS A(2a) receptors. A 3'-deoxy analogue of a highly A(3)-selective adenosine derivative retained selectivity in binding and was a full agonist in the inhibition of adenylyl cyclase mediated via cloned rat A(3) receptors expressed in CHO cells. The 3'-OH and 4'-CH2OH groups of adenosine are not required for activation at A(3) receptors. A number of 2',3'-dideoxyadenosines and 9-acyclic-substituted adenines appear to inhibit adenylyl cyclase at the allosteric ''P'' site.