Structure-Activity Studies of 1 H -Imidazo[4,5- c ]quinolin-4-amine Derivatives as A 3 Adenosine Receptor Positive Allosteric Modulators.

We previously reported 1 H -imidazo[4,5- c ]quinolin-4-amines as A 3 adenosine receptor (A 3 AR) positive allosteric modulators (PAMs). A 3 AR agonists, but not PAMs, are in clinical trials for inflammatory diseases and liver conditions. We synthesized new analogues to distinguish 2-cyclopropyl antagonist 17 (orthosteric interaction demonstrated by binding and predicted computationally) from PAMs (derivatives with large 2-alkyl/cycloalkyl/bicycloalkyl groups). We predicted PAM binding at a hydrophobic site on the A 3 AR cytosolic interface. Although having low Caco-2 permeability and high plasma protein binding, hydrophobic 2-cyclohept-4-enyl- N -3,4-dichlorophenyl, MRS7788 18 , and 2-heptan-4-yl- N -4-iodophenyl, MRS8054 39 , derivatives were orally bioavailable in rat. 2-Heptan-4-yl- N -3,4-dichlorophenyl 14 and 2-cyclononyl- N -3,4-dichlorophenyl 20 derivatives and 39 greatly enhanced Cl-IB-MECA-stimulated [ 35 S]GTPγS binding E max , with only 12b trending toward decreasing the agonist EC 50 . A feasible route for radio-iodination at the p- position of a 4-phenylamino substituent suggests a potential radioligand for allosteric site binding. Herein, we advanced an allosteric approach to developing A 3 AR-activating drugs that are potentially event- and site-specific in action.