A binding site model and structure-activity relationships for the rat A3 adenosine receptor

Area of Science:

• Pharmacology
• Medicinal Chemistry
• Molecular Biology

Background:

• The A3 receptor is a novel adenosine receptor subtype.
• Structure-activity relationships (SARs) for A3 receptors are largely unexplored.
• Adenosine receptors play crucial roles in various physiological processes.

Purpose of the Study:

• To systematically investigate the SARs for binding at A3 receptors.
• To identify potent and selective ligands for the A3 receptor.
• To develop a molecular model for the rat A3 receptor.

Main Methods:

• Radioligand binding assays using 125I-N6-2-(4-aminophenyl)ethyladenosine.
• Membranes from Chinese hamster ovary cells expressing rat A3-cDNA.
• Adenylate cyclase inhibition assays.
• Molecular modeling of the rat A3 receptor.

Main Results:

• N6,5'-disubstituted adenosine derivatives showed the highest affinity and selectivity for A3 receptors.
• N6-benzyladenosine-5'-N-ethylcarboxamide demonstrated high potency (Ki, 6.8 nM) and moderate selectivity.
• A3 receptors tolerate hydrophilic substitutions at the N6 position, unlike A1 and A2a receptors.
• Typical A1/A2 antagonists showed no significant affinity for rat A3 receptors.
• A molecular model predicted similarities in ribose binding and identified xanthine-7-ribosides as potential ligands.

Conclusions:

• Novel N6,5'-disubstituted adenosine analogs are potent and selective A3 receptor ligands.
• A His residue in helix 6 and steric bulk contribute to the lack of antagonist binding.
• The developed molecular model provides a framework for designing improved A3 receptor ligands.
• Further studies can utilize this model for site-directed mutagenesis and SAR exploration.