N⁶-Substituted 9-methyladenines: A new class of adenosine receptor antagonists

A series of 15 N⁶-substituted 9-methyladenines have been assessed as antagonists of A₂-adenosine receptor-mediated stimulation of adenylate cyclase in membranes of human platelets and rat PC12 cells and of A₁-adenosine receptor-mediated inhibition of adenylate cyclases in membranes of rat fat cells and as inhibitors of binding of N⁶-R-[³H]phenylisopropyladenosine to A₁-adenosine receptors in rat brain membranes. N⁶ substitution can markedly increase the potency of 9-methyladenine at A₁ receptors, while having lesser effects or even decreasing potency at. A₂ receptors. Effects of N⁶ substituents on adenosine receptor activity of the 9-methyladenines are reminiscent of effects of N⁶ substituents on activity of adenosine, suggesting that N⁶ substituted 9-methyladenines bind to adenosine receptors in the same orientation as do N⁶-substituted adenosines. N⁶-Cyclopentyl-9-methyladenine with K_i values at the A₁ receptors of 1.3 μM (fat cells) and 0.5 μM (brain) is at least 100-fold more potent than 9-methyladenine (K_i 100 μM, both receptors), while at the A₂ receptors K_B values of 5 μM (platelets) and 25 μM (PC12 cells) make it 5-fold more potent and equipotent, respectively, compared to 9-methyladenine (K_B 24 μM, both receptors). N⁶-Cyclopentyl and several other N⁶-alkyl and N⁶-cycloalkyl analogs are selective for A₁ receptors while 9-methyladenine is the most A₂ receptor selective antagonist. The N⁶-R- and N⁶-S-(1-phenyl-2-propyl)-9-methyladenines, analogous to N⁶-R- and N⁶-S-phenylisopropyladenosines, exhibit stereoselectivity at both A₁ and A₂ receptors. Marked differences in potency of certain N⁶-substituted 9-methyladenines at the A₁ receptors of human platelets and rat PC12 cells provide evidence that these are not identical receptors.