Derivatives of the triazoloquinazoline adenosine antagonist (CGS 15943) having high potency at the human A2B and A3 receptor subtypes

The adenosine antagonist 9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine (CGS 15943) binds nonselectively to human A(1), A(2A), and A(3) receptors with high affinity. Acylated derivatives and one alkyl derivative of the 5-amino group and other modifications were prepared in an effort to enhance A(2B) or A(3) subtype potency. In general, distal modifications of the N-5-substituent were highly modulatory to potency and selectivity at adenosine receptors, as determined in radioligand binding assays at rat brain A(1) and A(2A) receptors and at recombinant human A(3) receptors. In Chinese hamster ovary cells stably transfected with human A(2B) receptor cDNA, inhibition of agonist-induced cyclic AMP production was measured. An N-5-(2-iodophenyl)acetyl derivative was highly selective for A(2A) receptors. An (R)-N-5-alpha-methyl-(phenylacetyl) derivative was the most potent derivative at A(3) receptors, with a K-i value of 0.36 nM. A bulky N-5-diphenylacetyl derivative, 13, displayed a K-i value of 0.59 nM at human A(3) receptors and was moderately selective for that subtype. Thus, a large, nondiscriminating hydrophobic region occurs in the Ag receptor in proximity to the N-5-substituent. A series of straight-chain N-5-aminoalkylacyl derivatives demonstrated that for A(2B) receptors the optimal chain length occurs with three methylene groups, i.e., the N-5-gamma-aminobutyryl derivative 27 which had a pA(2) value of 8.0 but was not selective for A(2B) receptors. At A(1), A(2A), and A(3) receptors however the optimum occurs with four methylene groups. An N-5-pivaloyl derivative, which was less potent than 27 at A(1), A(2A), and A(3) receptors, retained moderate potency at A(2B) receptors. A molecular model of the 27-A(2B) receptor complex based on the structure of rhodopsin utilizing a "cross-docking" procedure was developed in order to visualize the environment of the ligand binding site.