title: Synthesis and pharmacological characterization of [(125)I]MRS5127, a high affinity, selective agonist radioligand for the A3 adenosine receptor.
abstract: A recently reported selective agonist of the human A(3) adenosine receptor (hA(3)AR), MRS5127 (1'R,2'R,3'S,4'R,5'S)-4'-[2-chloro-6-(3-iodobenzylamino)-purine]-2',3'-O-dihydroxy-bicyclo-[3.1.0]hexane, was radioiodinated and characterized pharmacologically. It contains a rigid bicyclic ring system in place of a 5'-truncated ribose moiety, and was selected for radiolabeling due to its nanomolar binding affinity at both human and rat A(3)ARs. The radioiodination of the N(6)-3-iodobenzyl substituent by iododestannylation of a 3-(trimethylstannyl)benzyl precursor was achieved in 73% yield, measured after purification by HPLC. [(125)I]MRS5127 bound to the human A(3)AR expressed in membranes of stably transfected HEK 293 cells. Specific binding was saturable, competitive, and followed a one-site binding model, with a K(d) value of 5.74+/-0.97nM. At a concentration equivalent to its K(d), non-specific binding comprised 27+/-2% of total binding. In kinetic studies, [(125)I]MRS5127 rapidly associated with the hA(3)AR (t(1/2)=0.514+/-0.014min), and the affinity calculated from association and dissociation rate constants was 3.50+/-1.46nM. The pharmacological profile of ligands in competition experiments with [(125)I]MRS5127 was consistent with the known structure-activity-relationship profile of the hA(3)AR. [(125)I]MRS5127 bound with similar high affinity (K(d), nM) to recombinant A(3)ARs from mouse (4.90+/-0.77), rabbit (2.53+/-0.11), and dog (3.35+/-0.54). For all of the species tested, MRS5127 exhibited A(3)AR agonist activity based on negative coupling to cAMP production. Thus, [(125)I]MRS5127 represents a new species-independent agonist radioligand for the A(3)AR. The major advantage of [(125)I]MRS5127 compared with previously used A(3)AR radioligands is its high affinity, low degree of non-specific binding, and improved A(3)AR selectivity.
authors.affiliation: Department of Pharmacology, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, United States. jauchamp@mcw.edu
authors.forename: John A
authors.initials: JA
authors.lastname: Auchampach
authors.forename: Elizabeth T
authors.initials: ET
authors.lastname: Gizewski
authors.forename: Tina C
authors.initials: TC
authors.lastname: Wan
authors.forename: Sonia
authors.initials: S
authors.lastname: de Castro
authors.forename: Garth G
authors.initials: GG
authors.lastname: Brown
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Jacobson
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000378:metabolism; D058909:Adenosine A3 Receptor Agonists; D000818:Animals; D002478:Cells, Cultured; D004285:Dogs; D006801:Humans; D007457:Iodine Radioisotopes; D051379:Mice; D011817:Rabbits; D011869:Radioligand Assay; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000378:metabolism; D051381:Rats; D013329:Structure-Activity Relationship
chemical_list: C547395:4'-(2-chloro-6-(3-iodobenzylamino)purine)-2',3'-O-dihydroxy-bicyclo(3.1.0)hexane; D058909:Adenosine A3 Receptor Agonists; D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals; D000241:Adenosine