Use of ¹¹C-MPDX and PET to Study Adenosine A₁ Receptor Occupancy by Nonradioactive Agonists and Antagonists

Abstract

Adenosine A 1 receptors (A 1 Rs) 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 A 1 R 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 5 7); pretreated with a centrally active A 1 R agonist, N 6 -cyclopentyladenosine (CPA; 0.25 mg/kg intraperitoneally; dissociation constant, 0.48 nM; n 5 7); pretreated with a moderate dose of caffeine (antagonist for A 1 Rs and adenosine A 2A receptors; 4 mg/kg intraperitoneally; dissociation constant, 11 mM; n 5 6); and pretreated with a high dose of caffeine (40 mg/kg intraperitoneally; n 5 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 5 6) resulted in 65.9% A 1 R occupancy, and a dose of 40 mg/kg (n 5 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. Conclusion: Small-animal PET with 11 C-MPDX can be used to assess antagonist but not agonist binding at A 1 Rs. 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 A 1 R protein having allosteric interactions.

Novel Approach to Repeated Arterial Blood Sampling in Small Animal PET: Application in a Test-Retest Study with the Adenosine A1 Receptor Ligand [11C]MPDX

“…Biodistribution SUV values (acquired after the retest scan) are presented in the supplementary data. These were comparable with previously reported values for [ 11 C]MPDX in our institution [18, 19].

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“…For the sake of clarity, the data are plotted on a logarithmic X -axis. Both curves show a peak about 1 min after the start of the infusion pump followed by a slow wash-out as observed previously [18]. Plasma radioactivity at 0.83 and 1 min appears to be slightly greater in scan 2 than in scan 1, but all other data points overlap and the two curves are not statistically different.…”

“…Lower levels of radioactivity are observed in other brain areas such as cerebral cortex and the olfactory bulb. Similar images were acquired in previous studies from our institution [7, 18, 19]. Visual differences between scan 1 and scan 2 were not detected.…”

Chemical Probes for the Adenosine Receptors

“…One of the most studied derivatives is xanthine derivative [1-methyl- 11 C]8-dicyclopropylmethyl-1-methyl-3-propylxanthine ( 11 C-MPDX, 2). This compound has been utilized for various studies [26][27][28]. For example, Paul and coworkers performed in vivo binding studies in rats hypothesizing that agonists and antagonists may bind in different sites at the A 1 receptor [26].…”

“…This compound has been utilized for various studies [26][27][28]. For example, Paul and coworkers performed in vivo binding studies in rats hypothesizing that agonists and antagonists may bind in different sites at the A 1 receptor [26]. The same derivative was also utilized to evaluate A 1 AR alterations in patients with chronic diffuse axonal injury [27] or to investigate the density of A 1 ARs in patients with early-stage Parkinson's disease [28].…”