Synthesis and pharmacological characterization of [(125)I]MRS5127, a high affinity, selective agonist radioligand for the A3 adenosine receptor

Biochem Pharmacol. 2010 Apr 1;79(7):967-73. doi: 10.1016/j.bcp.2009.11.009. Epub 2009 Nov 14.


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.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adenosine / analogs & derivatives*
  • Adenosine / chemical synthesis
  • Adenosine / metabolism
  • Adenosine A3 Receptor Agonists*
  • Animals
  • Cells, Cultured
  • Dogs
  • Humans
  • Iodine Radioisotopes*
  • Mice
  • Rabbits
  • Radioligand Assay
  • Radiopharmaceuticals / chemical synthesis*
  • Radiopharmaceuticals / metabolism
  • Rats
  • Structure-Activity Relationship


  • 4'-(2-chloro-6-(3-iodobenzylamino)purine)-2',3'-O-dihydroxy-bicyclo(3.1.0)hexane
  • Adenosine A3 Receptor Agonists
  • Iodine Radioisotopes
  • Radiopharmaceuticals
  • Adenosine