PET radiotracer development for imaging high‐affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine‐18 labeled aminotetralins in rodents

Abstract

Imaging the high-affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of 18F-5-OH-FPPAT, and the newer 18F-5-OH-FHXPAT and 18F-7-OH-FHXPAT. Syntheses of 18F-5-OH-FHXPAT and 18F-7-OH-FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague-Dawley rats were used with the 3 radiotracers (74–111 kBq/cc). Competition with dopamine (1–100 nM) and Gpp(NH)p (10–50 μM) were carried out to demonstrate binding to dopamine D2 and D3 HA-states and binding kinetics of 18F-5-OH-FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with 18F-5-OH-FHXPAT to ascertain HA-state binding. PET/CT imaging in rats and wild type (WT) and D2 knock-out mice were carried out using 18F-7-OH-FHXPAT (2–37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 × 10−2 min−1 for 18F-5-OH-FPPAT. Treatment with Gpp(NH)p significantly reduced 50–80% striatal binding with faster dissociation rates (5.0 × 10−2 min−1), suggesting HA-state binding of 18F-5-OH-FPPAT and 18F-5-OH-FHXPAT. Striatal binding of 18F-5-OH-FHXPAT in ex vivo brain slices were sensitive to Gpp(NH)p, suggesting HA-state binding in vivo. PET binding ratios of 18F-7-OH-FHXPAT in rat brain were ventral striatum/cerebellum=2.09 and dorsal striatum/cerebellum=1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane-bound HA-state of the dopamine receptor.

[18F]F13640, a 5-HT1A Receptor Radiopharmaceutical Sensitive to Brain Serotonin Fluctuations

“…These in vivo data are also in discordance with our in vitro autoradiography results in the same region, i.e., a low level of binding which is not affected by serotonin displacement and not influenced by addition of Gpp(NH)p in buffer. However, for other regions, we observed the same range of decrease produced by uncoupling of receptors as described for the D2/D3 agonist [ 18 F]-5-OH-FPPAT, an average of 50% (Mukherjee et al, 2017). Classical 5-HT 1A radiotracers also show low specific binding in the cerebellum (Mathis et al, 1994;Shiue et al, 1997).…”

New Dopamine D2 Receptor Agonist, [3H]MCL-536, for Detecting Dopamine D2high Receptors in Vivo

“…40 Reports on the aminotetralin radioligands [ 18 F]5-OH-FPPAT, [ 18 F]5-OH-FHXPAT, and [ 18 F]7-OH-FHXPAT describe their development as D2high and D3high ligands, respectively. 41,42 Last year, a series of 18 F-labeled chromanol derivatives were evaluated in rats, and one analogue ([ 18 F]FEt-AMC13) possessed a striatum/cerebellum binding ratio of 2.08, although it exhibited a lower binding potential and it was less sensitive to competition with raclopride in comparison to previously reported agonist PET radioligands. 43,44 Thus, development of an 18 F-labeled D2 agonist which exhibits high selectivity and specificity for the D2high receptor for human use remains an important goal.…”

“…Furthermore, N -alkyl radiofluorinated aporphine derivatives, including [ 18 F]­FNPA and [ 18 F]­FNEA, did not prove to be D2 agonists . Reports on the aminotetralin radioligands [ 18 F]­5-OH-FPPAT, [ 18 F]­5-OH-FHXPAT, and [ 18 F]­7-OH-FHXPAT describe their development as D2high and D3high ligands, respectively. , Last year, a series of 18 F-labeled chromanol derivatives were evaluated in rats, and one analogue ([ 18 F]­FEt-AMC13) possessed a striatum/cerebellum binding ratio of 2.08, although it exhibited a lower binding potential and it was less sensitive to competition with raclopride in comparison to previously reported agonist PET radioligands. , Thus, development of an 18 F-labeled D2 agonist which exhibits high selectivity and specificity for the D2high receptor for human use remains an important goal. Such a radioligand would be an invaluable tool for studying unusual activity of D2 receptor dynamics in the living brain, before extensive neurological changes occur, and before physical symptoms emerge.…”

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

“…Great efforts have been made to develop PET ligands (both agonists or antagonists) for in vivo imaging of D3Rs and D2Rs. Many of them displayed high affinity for both D2Rs and D3Rs, but either their selectivity or physicochemical properties or brain penetration were far from satisfactory [ 162 , 163 , 164 , 165 , 166 ] to differentiate between imaging of D2Rs and D3Rs. Availability of the high affinity and D3R preferring agonist [ 11 C]-(+)-PHNO [ 42 ] has made possible to image D3Rs in the human brain [ 44 , 167 ].…”