Abstract
The structure-activity relationships (SAR) of alkylxanthine derivatives as antagonists at the recombinant human adenosine receptors were explored in order to identify selective antagonists of A(2B) receptors. The effects of lengthening alkyl substituents from methyl to butyl at 1- and 3-positions and additional substitution at the 7- and 8-positions were probed. K(i) values, determined in competition binding in membranes of HEK-293 cells expressing A(2B) receptors using (125)I-ABOPX ((125)I-3-(4-amino-3-iodobenzyl)-8-(phenyl-4-oxyacetate)-1-propylxanthine), were approximately 10 to 100 nM for 8-phenylxanthine functionalized congeners. Xanthines containing 8-aryl, 8-alkyl, and 8-cycloalkyl substituents, derivatives of XCC (8-[4-[[[carboxy]methyl]oxy]phenyl]-1,3-dipropylxanthine) and XAC (8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]-oxy]phenyl]-1,3-dipropylxanthine), containing various ester and amide groups, including L- and D-amino acid conjugates, were included. Enprofylline was 2-fold more potent than theophylline in A(2B) receptor binding, and the 2-thio modification was not tolerated. Among the most potent derivatives examined were XCC, its hydrazide and aminoethyl and fluoroethyl amide derivatives, XAC, N-hydroxyethyl-XAC, and the L-citrulline and D-p-aminophenylalanine conjugates of XAC. An N-hydroxysuccinimide ester of XCC (XCC-NHS, MRS 1204) bound to A(2B) receptors with a K(i) of 9.75 nM and was the most selective (at least 20-fold) in this series. In a functional assay of recombinant human A(2B) receptors, four of these potent xanthines were shown to fully antagonize the effects of NECA-induced stimulation of cyclic AMP accumulation.