Binding of the Dual-Action Anti-Parkinsonian Drug AG-0029 to Dopamine D2 and Histamine H3 Receptors: A PET Study in Healthy Rats

Abstract

Introduction: Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor dysfunction and a diverse range of nonmotor symptoms. Functional relationships between the dopaminergic and histaminergic systems suggest that dual-action pharmaceuticals like AG-0029 (D₂/D₃ agonist/H₃ antagonist) could ameliorate both the motor and cognitive symptoms of PD. The current study aimed to demonstrate the interaction of AG-0029 with its intended targets in the mammalian brain using positron emission tomography (PET). Methods: Healthy male Wistar rats were scanned with a small-animal PET camera, using either the dopamine D₂/D₃ receptor ligand [¹¹C]raclopride or the histamine H₃ receptor ligand [¹¹C]GSK-189254, before and after treatment with an intravenous, acute, single dose of AG-0029. Dynamic [¹¹C]raclopride PET data (60 min duration) were analyzed using the simplified reference tissue model 2 (SRTM2) with cerebellum as reference tissue and the nondisplaceable binding potential as the outcome parameter. Data from dynamic [¹¹C]GSK-189254 scans (60 min duration) with arterial blood sampling were analyzed using Logan graphical analysis with the volume of distribution (V_T) as the outcome parameter. Receptor occupancy was estimated using a Lassen plot. Results: Dopamine D_2/3 receptor occupancies in the striatum were 22.6 ± 18.0 and 84.0 ± 3.5% (mean ± SD) after administration of 0.1 and 1 mg/kg AG-0029, respectively. In several brain regions, the V_T values of [¹¹C]GSK-189254 were significantly reduced after pretreatment of rats with 1 or 10 mg/kg AG-0029. The H₃ receptor occupancies were 11.9 ± 8.5 and 40.3 ± 11.3% for the 1 and 10 mg/kg doses of AG-0029, respectively. Conclusions: Target engagement of AG-0029 as an agonist at dopamine D₂/D₃ receptors and an antagonist at histamine H₃ receptors could be demonstrated in the rat brain with [¹¹C]raclopride and [¹¹C]GSK-189254 PET, respectively. The measured occupancy values reflect the previously reported high (subnanomolar) affinity of AG-0029 to D₂/D₃ and moderate (submicromolar) affinity to H₃ receptors.

Keywords: Parkinson’s disease; [11C]GSK-189254; [11C]raclopride; anti-Parkinson drug; dopamine D2 receptor; dual-action pharmaceutical; histamine H3 receptor; pharmacokinetic modeling.

Figure 1

Chemical structure of AG-0029.

Figure 2

Dose-dependent effect of AG-0029 administration on heart rate.

Figure 3

Average [¹¹C]raclopride time–activity curves in the striatum (high D₂ receptor expression) and cerebellum (low D₂ receptor expression) at baseline and post-dose. Rats received either (A) a low dose (0.1 mg/kg) or (B) a high dose (1 mg/kg) of AG-0029. Data are plotted as mean ± SD.

Figure 4

Effect of AG-0029 on the influx (R₁) of [¹¹C]raclopride derived from SRTM2.

Figure 5

(A) Graphical comparison between baseline and post-dose [¹¹C]raclopride binding potential (BP_ND) in rats pretreated with either 0.1 or 1 mg/kg of AG-0029 as the test drug. (B) Graphical presentation of the striatal D₂ receptor occupancy versus the AG-0029 dose.

Figure 6

Two sets of BP_ND images of [¹¹C]raclopride scans acquired at baseline (A, C) and after pretreatment with different doses of AG-0029 (B, D).

Figure 7

Kinetics of [¹¹C]GSK-189254. Time–activity curves in (A) blood and (B) plasma at baseline and after administration of 1 or 10 mg/kg of AG-0029. The insets show the shape of the initial peak. (C) Parent fraction in plasma at baseline and post-dose. Error bars indicate standard deviation.

Figure 8

Representative time–activity curves (TACs) of [¹¹C]GSK-182954 in the striatum and the cerebellum (A, B) and frontal cortex and the cerebellum (C, D) after pretreatment with a high (10 mg/kg) dose (B, D) and low (1 mg/kg) dose of AG-0029 (A, C).

Figure 9

Parametric (V_T) images of [¹¹C]GSK-189254 binding (axial, coronal, and sagittal views) of a representative rat brain at baseline (top row) and after administration of 1 mg/kg (middle row), or 10 mg/kg of AG-0029 (bottom row).

Figure 10

Examples of Lassen plots from (A) a rat that received a low dose (1 mg/kg) of AG-0029 and (B) a rat that received a high dose (10 mg/kg) of the drug before the post-dose [¹¹C]GSK-189254 PET scan.