1,3,8-trisubstituted xanthines. Effects of substitution pattern upon adenosine receptor A1/A2 affinity

A series of 11 8-substituted xanthines having three different substitution patterns on the 1- and 3-positions [pattern a (R1 = R3 = CH2CH2CH3), b (R1 = CH2CH2CH3, R3 = CH3), and c (R1 = CH3, R3 = CH2CH2CH3)] was prepared. These compounds were assessed for affinity and selectivity in binding to adenosine A1 and A2 receptors. Compounds with greatest affinity at the A1 receptor had the 1,3-substitution pattern a. With one exception, compounds with pattern a also exhibited the most potent binding at the A2 receptor; however, several compounds with pattern c were equipotent at the A2 receptor with those having pattern a. Additionally, the substituents on the 1- and 3-positions of these 8-substituted xanthines were equally important for determining maximum affinity to the A1 receptor, while the substituent at the 3-position is more important than the substituent at the 1-position for potency at the A2 receptor. As a result of this, it is possible to maximize selectivity for the A1 receptor by choice of the 1- and 3-position substituents. However, the R1/R3 substitution pattern required for maximum A1 selectivity is also dependent upon the substituent in the 8-position in a manner which is not fully understood.