Pharmacokinetic Modeling of [11C]GSK-189254, PET Tracer Targeting H3 Receptors, in Rat Brain

[Image: see text] The histamine H(3) receptor has been considered as a target for the treatment of various central nervous system diseases. Positron emission tomography (PET) studies with the radiolabeled potent and selective histamine H(3) receptor antagonist [(11)C]GSK-189254 in rodents could be used to examine the mechanisms of action of novel therapeutic drugs or to assess changes of regional H(3) receptor density in animal models of neurodegenerative disease. [(11)C]GSK-189254 was intravenously administered to healthy Wistar rats (n = 10), and a 60 min dynamic PET scan was carried out. Arterial blood samples were obtained during the scan to generate a metabolite-corrected plasma input function. PET data were analyzed using a one-tissue compartment model (1T2k), irreversible (2T3k) or reversible two-tissue compartment models (2T4k), graphical analysis (Logan and Patlak), reference tissue models (SRTM and SRTM2), and standard uptake values (SUVs). The Akaike information criterion and the standard error of the estimated parameters were used to select the most optimal quantification method. This study demonstrated that the 2T4k model with a fixed blood volume fraction and Logan graphical analysis can best describe the kinetics of [(11)C]GSK-189254 in the rat brain. SUV(40–60) and the reference tissue-based measurements DVR(2T4k), BP(ND)(SRTM), and SUV ratio could also be used as a simplified method to estimate H(3) receptor availability in case blood sampling is not feasible

Binding of the Dual-Action Anti-Parkinsonian Drug AG-0029 to Dopamine D-2 and Histamine H-3 Receptors:A PET Study in Healthy Rats

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 (D-2/D-3 agonist/H-3 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 D-2/D-3 receptor ligand [C-11]raclopride or the histamine H-3 receptor ligand [C-11]GSK-189254, before and after treatment with an intravenous, acute, single dose of AG-0029. Dynamic [C-11]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 [C-11]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 [C-11]GSK-189254 were significantly reduced after pretreatment of rats with 1 or 10 mg/kg AG-0029. The H-3 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 D-2/D-3 receptors and an antagonist at histamine H-3 receptors could be demonstrated in the rat brain with [C-11]raclopride and [C-11]GSK-189254 PET, respectively. The measured occupancy values reflect the previously reported high (subnanomolar) affinity of AG-0029 to D-2/D-3 and moderate (submicromolar) affinity to H-3 receptors

Binding of the experimental dual-action antiparkinsonian drug AG-0029 to dopamine D2 and histamine H3 receptors: a PET study in healthy rats

Aim/Introduction: Drugs interacting with multiple targets are under study for the treatment of disorders of the central nervous system. AG-00291 combines very potent agonist affinity (0.08 nM) to the dopamine D2 receptor with moderate antagonist affinity (111 nM) to the histamine H3 receptor. These receptor interactions of AG-0029 were proven by measurement of extracellular dopamine levels in rat striatum and histamine levels in rat prefrontal cortex by microdialysis. AG-0029 showed anti-Parkinson action (contralateral rotation), and a cognition-enhancing effect in the novel object recognition test of 6-OHDA-lesioned rats. This drug may improve both the motor and cognitive symptoms of Parkinson disease. The present study aims to quantify the target engagement of AG-0029. Materials and Methods: Healthy male Wistar rats were scanned in a small animal PET camera (μPET Focus 220), using the dopamine D2/D3 receptor ligand [11C]raclopride or the histamine H3 receptor ligand [11C]GSK189254, before and after treatment with an intravenous, acute, single dose of AG-0029 (0.1 or 1 mg/kg for [11C]raclopride, 1 mg/kg for [11C]GSK189254). Dynamic [11C]raclopride scans (60 min duration) were made without arterial blood sampling, and were analysed using the simplified reference tissue model with cerebellum as reference tissue. Dynamic [11C]GSK189254 scans (90 min duration) were made with arterial blood sampling. Results: Binding potential values of [11C]raclopride in the striatum were dose-dependently reduced after administration of AG-0029 (10 min before tracer injection), from 1.45±0.10 (n=9) to 1.04±0.17 (after a 0.1 mg/kg dose, n=4) and to 0.33±0.18 (after a 1 mg/kg dose, n=5), corresponding to D2/D3 receptor occupancy values of 27±12 and 78±4%. These data (0, 0.1, 1 mg/kg dose) are well-fitted by a one-site model for receptor binding (r2>0.99), half-maximal receptor occupancy being reached at a dose of 0.27 mg/kg and maximal occupancy being 99.3%. Data acquisition and analysis for the [11C] GSK189254 scans is still in progress. Frontal/occipital cortex ratios of radioactivity were reduced from 3.04±0.74 (n=4) at baseline to 2.58±0.13 (n=3) after administration of 1 mg/kg AG-0029, which may correspond to a H3 receptor occupancy of 23%. However, due to high variability of [11C]GSK189254 binding at baseline, the decline of binding potential at this dose was not statistically significant. Conclusion: Dopamine D2/D3 receptor occupancy by the agonist AG-0029 could be reliably measured with [11C]raclopride and small animal PET. Histamine H3 receptor occupancy by AG-0029 may be detectable in PET scans with [11C]GSK189254, but may require a higher dose than 1 mg/kg. References: 1 7-(4-(3-(4-(morpholinomethyl)phenoxy)propyl)piperazin-1- yl)benzo[d]oxazol-2(3H)-one

Measuring H3 receptor occupancy of the experimental anti-Parkinsonian drug AG0029, using [11C]GSK-189254 and PET

Aim/Introduction: Inadequacies of currently designed drugs in treating neurodegenerative disorders have promoted efforts to develop novel drugs interacting with multiple targets. As part of a collaborative drug discovery program, AG-0029 was developed. This compound has agonist affinity to dopamine (0.08 nM) and antagonist affinity (111 nM) to histamine H3 receptors and may enhance both the motor and cognitive symptoms of Parkinson’s disease. Previously, we reported on the occupancy of dopamine D2/D3 receptors in the rodent brain by AG0029 [1]. In the present study, we aimed to quantitatively assess the engagement of AG-0029 with cerebral histamine H3 receptors, using PET. Materials and Methods: Two dynamic PET scans of 60 min with arterial blood sampling were made in 12 healthy male Wistar rats (body weight 373±32 g), at a one-week interval, using 49±8 MBq of the histamine H3 receptor ligand [11C]GSK-189254. The animals were first scanned at baseline and subsequently after administration of either 1 or 10 mg/kg of AG-0029 in saline (i.v., 5 min before tracer injection, n=6 for each drug dose). Results: In baseline scans, the regional distribution of the tracer corresponded to the known distribution of histamine H3 receptors in the rodent brain. Considerable uptake was noticed in frontal-cortex and olfactory-bulb, and low uptake in occipital-cortex and cerebellum. A region-of-interest was manually drawn around the frontal areas with high uptake and a standard elliptical ROI was placed in the cerebellum. Tracer uptake in the cerebellum was not significantly affected by drug pretreatment, in contrast to uptake in the frontal target regions. Frontal-to-cerebellum ratios of radioactivity minus one were plotted as a function of time. The maximum of this curve is an estimation of tracer binding potential (BP). BP values were not reduced after pretreatment with 1 mg/kg AG-0029 (from 2.03±0.24 to 1.97±0.20) but were significantly reduced by 10 mg/kg of the test drug (to 1.33±0.22). Calculated H3 receptor occupancies were 3±10 and 34±11%, respectively. The tracer was slowly metabolized and tracer metabolism was not significantly affected by drug pretreatment. At 62±9 min after injection, 50% of the plasma radioactivity still represented parent compound. Preliminary data analysis using a simplified reference tissue model fit showed the same trends as the ratio method. Conclusion: Our PET data indicated negligible histamine H3 receptor occupancy after administration of 1 mg/kg, but considerable (34%) occupancy after administration of 10 mg/kg AG-0029.References: A. van Waarde, EJNMM, vol. 47, no. Suppl. 1, p. S114, 2020