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- 11C-methyl- 3-propylxanthine (11C-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, N6-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 A 2A 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 11C-MPDX binding in the brain. Conclusion: Small-animal PET with 11C-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. Copyright © 2014 by the Society of Nuclear Medicine and Molecular Imaging, Inc.