In Vitro and In Vivo Characterization of Two C-11-Labeled PET Tracers for Vesicular Acetylcholine Transporter

Purpose: The vesicular acetylcholine transporter (VAChT) is a specific biomarker for imaging presynaptic cholinergic neurons. Herein, two potent and selective ¹¹C-labeled VAChT inhibitors were evaluated in rodents and nonhuman primates for imaging VAChT in vivo.

Procedures: For both (−)-[¹¹C]2 and (−)-[¹¹C]6, biodistribution, autoradiography, and metabolism studies were performed in male Sprague Dawley rats. Positron emission tomography (PET) brain studies with (−)-[¹¹C]2 were performed in adult male cynomolgus macaques; 2 h dynamic data was acquired, and the regions of interest were drawn by co-registration of the PET images with the MRI.

Results: The resolved enantiomers (−)-2 and (−)-6 were very potent and selective for VAChT in vitro (K_i < 5 nM for VAChT with >35-fold selectivity for VAChT vs. σ receptors); both radioligands, (−)-[¹¹C]2 and (−)-[¹¹C]6, demonstrated high accumulation in the VAChT-enriched striatum of rats. (−)-[¹¹C]2 had a higher striatum to cerebellum ratio of 2.4-fold at 60 min; at 30 min, striatal uptake reached 0.550 ± 0.086 %ID/g. Uptake was also specific and selective; following pretreatment with (±)-2, striatal uptake of (−)-[¹¹C]2 in rats at 30 min decreased by 50 %, while pretreatment with a potent sigma ligand had no significant effect on striatal uptake in rats. In addition, (−)-[¹¹C]2 displayed favorable in vivo stability in rat blood and brain. PET studies of (−)-[¹¹C]2 in nonhuman primates indicate that it readily crosses the blood-brain barrier (BBB) and provides clear visualization of the striatum; striatal uptake reaches the maximum at 60 min, at which time the target to nontarget ratio reached ~2-fold.

Conclusions: The radioligand (−)-[¹¹C]2 has high potential to be a suitable PET radioligand for imaging VAChT in the brain of living subjects.