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.