Impact of an Adenosine A2A Receptor Agonist and Antagonist on Binding of the Dopamine D2 Receptor Ligand [11C]raclopride in the Rodent Striatum

Adenosine A_2A and dopamine D₂ receptors in the basal ganglia form heterotetrameric structures that are involved in the regulation of motor activity and neuropsychiatric functions. The present study examines the A_2A receptor-mediated modulation of D₂ receptor binding in vivo using positron emission tomography (PET) with the D₂ antagonist tracer [¹¹C]raclopride. Healthy male Wistar rats (n = 8) were scanned (60 min dynamic scan) with [¹¹C]raclopride at baseline and 7 days later following an acute administration of the A_2A agonist CGS21680 (1 mg/kg), using a MicroPET Focus-220 camera. Nondisplaceable binding potential (BP_ND) values were calculated using a simplified reference tissue model (SRTM), with cerebellum as the reference tissue. SRTM analysis did not show any significant changes in [¹¹C]raclopride BP_ND (p = 0.102) in striatum after CGS21680 administration compared to the baseline. As CGS21680 strongly affects hemodynamics, we also used arterial blood sampling and a metabolite-corrected plasma input function for compartment modeling using the reversible two-tissue compartment model (2TCM) to obtain the BP_ND from the k₃/k₄ ratio and from the striatum/cerebellum volume of distribution ratio (DVR) in a second group of animals. These rats underwent dynamic [¹¹C]raclopride scans after pretreatment with a vehicle (n = 5), a single dose of CGS21680 (1 mg/kg, n = 5), or a single dose of the A_2A antagonist KW6002 (1 mg/kg, n = 5). The parent fraction in plasma was significantly higher in the CGS21680-treated group (p = 0.0001) compared to the vehicle-treated group. GCS21680 administration significantly reduced the striatal k₃/k₄ ratio (p < 0.01), but k₃ and k₄ estimates may be less reliable. The BP_ND (DVR-1) decreased from 1.963 ± 0.27 in the vehicle-treated group to 1.53 ± 0.55 (p = 0.080) or 1.961 ± 0.11 (p = 0.993) after the administration of CGS21680 or KW6002, respectively. Our study suggests that the A_2A agonist CGS21680, but not the antagonist KW6002, may reduce the D₂ receptor availability in the striatum.

A new approach to produce [18F]MC225 via one-step synthesis, a PET radiotracer for measuring P-gp function

Background

[¹⁸F]MC225 is a radiotracer for imaging P-glycoprotein (P-gp) function at the blood-brain barrier. The P-gp function can be altered due to different factors, for instance, decreased P-gp function has been described in patients with Alzheimer’s or Parkinson’s Disease. The current applied radiosynthesis of [¹⁸F]MC225 involves 2 steps, including the distillation of the [¹⁸F] fluoroethylbromide intermediate. To develop a more robust synthetic procedure, it is of interest to produce the radiotracer via a 1-step synthesis. The present study describes a new synthetic approach to produce [¹⁸F]MC225 via direct ¹⁸F-fluorination. Moreover, we also provide the appropriate conditions for the automation of the synthesis. A mesylate precursor was synthesized via a multi-step synthetic route and used for the radiolabeling. The nucleophilic substitution of the mesylate group by [¹⁸F] Fluoride was automated in two different synthesis modules: IBA Synthera and Eckert and Ziegler PharmTracer (E&Z).

Results

The mesylate precursor was synthesized in 7 steps starting with 5-hydroxy-1-tetralone (commercially available) in practical yields. The stability of the precursor was improved via mesylate salt formation method. The radiolabeling was done by adding the mesylate precursor dissolved in DMF to the dried [¹⁸F]KF/K_2.2.2 complex and heating at 140 °C for 30 min. Quality control by UPLC confirmed the production of [¹⁸F]MC225 with a molar activity (A_m) higher than 100 GBq/micromole. The synthesis time in Synthera was 106 min and the product was obtained with a radiochemical purity higher than 95% and RCY of 6.5%, while the production in E&Z lasted 120 min and the product had a lower radiochemical purity (91%) and RCY (3.8%).

Conclusions

[¹⁸F]MC225 was successfully produced via a 1-step reaction. The procedure is suitable for automation using commercially available synthesis modules. The automation of the radiosynthesis in the Synthera module allows the production of the [¹⁸F]MC225 by a reliable and simple method.