title: Use of 11C-MPDX and PET to study adenosine A1 receptor occupancy by nonradioactive agonists and antagonists.
abstract: Adenosine A1 receptors (A1Rs) in human and rodent brains can be visualized with the radioligand 8-dicyclopropylmethyl-1-(11)C-methyl-3-propylxanthine ((11)C-MPDX) and PET. Here we investigated whether A1R occupancy by nonradioactive agonists and antagonists can be assessed with this technique.
  
  
  METHODS
  Small-animal PET scans with arterial blood sampling were obtained for 4 groups of isoflurane-anesthetized Wistar rats: controls (n = 7); pretreated with a centrally active A1R agonist, N(6)-cyclopentyladenosine (CPA; 0.25 mg/kg intraperitoneally; dissociation constant, 0.48 nM; n = 7); pretreated with a moderate dose of caffeine (antagonist for A1Rs and adenosine A2A receptors; 4 mg/kg intraperitoneally; dissociation constant, 11 μM; n = 6); and pretreated with a high dose of caffeine (40 mg/kg intraperitoneally; n = 6).
  
  
  RESULTS
  The administration of CPA resulted in a strong reduction (>50%) in the heart rate, and caffeine administration resulted in a small increase (10%-15%). A caffeine dose of 4 mg/kg (n = 6) resulted in 65.9% A1R occupancy, and a dose of 40 mg/kg (n = 6) resulted in 98.5% occupancy (calculated from a modified Lassen plot). However, the administration of CPA resulted in an increase in (11)C-MPDX binding in the brain.
  
  
  CONCLUSIONS
  Small-animal PET with (11)C-MPDX can be used to assess antagonist but not agonist binding at A1Rs. Changes in tracer uptake after the administration of CPA resembled previously reported changes induced by treatment of rats with ethanol and an adenosine kinase inhibitor (ABT702). Thus, the administration of an exogenous agonist or increasing the level of an endogenous agonist have similar effects. Agonists and antagonists may bind to different sites on the A1R protein having allosteric interactions.
authors.affiliation: Nuclear Medicine and Molecular Imaging, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands.
authors.forename: Soumen
authors.initials: S
authors.lastname: Paul
authors.forename: Shivashankar
authors.initials: S
authors.lastname: Khanapur
authors.forename: Jurgen W
authors.initials: JW
authors.lastname: Sijbesma
authors.forename: Kiichi
authors.initials: K
authors.lastname: Ishiwata
authors.forename: Philip H
authors.initials: PH
authors.lastname: Elsinga
authors.forename: Peter
authors.initials: P
authors.lastname: Meerlo
authors.forename: Rudi A
authors.initials: RA
authors.lastname: Dierckx
authors.forename: Aren
authors.initials: A
authors.lastname: van Waarde
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000737:chemistry; D000495:Allosteric Site; D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging; D002110:Caffeine / Q000737:chemistry; D002250:Carbon Radioisotopes; D000431:Ethanol / Q000737:chemistry; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D009025:Morpholines / Q000737:chemistry; D049268:Positron-Emission Tomography / Q000379:methods*; D011743:Pyrimidines / Q000737:chemistry; D019275:Radiopharmaceuticals; D051381:Rats; D017208:Rats, Wistar; D043682:Receptor, Adenosine A1 / Q000378:metabolism*; D014970:Xanthines
chemical_list: C451639:1-methyl-8-dicyclopropylmethyl-1-methyl-3-propylxanthine; D002250:Carbon Radioisotopes; D009025:Morpholines; D011743:Pyrimidines; D019275:Radiopharmaceuticals; D043682:Receptor, Adenosine A1; D014970:Xanthines; C416933:ABT 702; D002110:Caffeine; D000431:Ethanol; C048599:N(6)-cyclopentyladenosine; D000241:Adenosine
keywords: N6-cyclopentyladenosine; adenosine A1 receptor; brain; caffeine; occupancy