1,3-dialkylxanthine derivatives having high potency as antagonists at human A(2B) adenosine receptors

The structure-activity relationships (SAR) of alkylxanthine derivatives as antagonists at the recombinant human adenosine receptors were explored in o rder to identify selective antagonists of A(2B) receptors. The effects of l engthening alkyl substituents from methyl to butyl at 1- and 3-positions an d additional substitution at the 7- and 8-positions were probed. Ki values, determined in competition binding in membranes of HEK-293 cells expressing A(2B) receptors using I-125-ABOPX (T-125-3-(4-amino-3-iodobenzyl)-8-(pheny l -4-oxyacetate)-1-propylxanthine), were approximately 10 to 100 nM for 8-p henylxanthine functionalized congeners. Xanthines containing 8-aryl, 8-alky l, and 8-cycloalkyl substituents, derivatives of XCC (8-[4-[[carboxy]methyl ]oxylphenyl]-1,3-dipropylxanthine) and XAC (8-[4-[[[[(2-aminoethyl)amino]ca rbonyl]methyl]oxy]phenyl]-1,3-dipropylxanthine), containing various ester a nd amide groups, including L- and D-amino acid conjugates, were included. E nprofylline was 2-fold more potent than theophylline in A(2B) receptor bind ing, and the 2-thio modification was not tolerated. Among the most potent d erivatives examined were XCC, its hydrazide and aminoethyl and fluoroethyl amide derivatives, XAC, N-hydroxyethy-XAC, and the L-citrulline and D-p-ami nophenylalanine conjugates of XAC. An N-hydroxysuccinimide ester of XCC (XC C-NHS, MRS 1204) bound to A2B receptors with a Ki of 9.75 nM and was the mo st selective (at least 20-fold) in this series. In a functional assay of re combinant human A2B receptors, four of these potent xanthines were shown to fully antagonize the effects of NECA-induced stimulation of cyclic AMP acc umulation. Published 1999 Wiley-Liss, Inc.