Kinetics modeling and occupancy studies of a novel C-11 PET tracer for VAChT in nonhuman primates

Introduction: Deficits in cholinergic function have been found in the aged brain and in neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). The vesicular acetylcholine transporter (VAChT) is a reliable biomarker for the cholinergic system. We previously reported the initial in vitro and ex vivo characterization of (-)-[¹¹C]TZ659 as a VAChT specific ligand. Here, we report the in vivo specificity, tracer kinetics, and dose-occupancy studies in the nonhuman primate brain. Methods: MicroPET brain imaging of (-)-[¹¹C]TZ659 was performed under baseline conditions in two male macaques. Tracer kinetic modeling was carried out using a two-tissue compartment model (2TCM) and Logan plot with arterial blood input function and using a simplified reference tissue model (SRTM) and Logan plot (LoganREF) without blood input. Specificity for VAChT was demonstrated by pretreatment with (+)-pentazocine, (-)-vesamicol, or S-(-)-eticlopride. Target occupancy (Occ) was calculated following pretreatment with escalating doses of (-)-vesamicol. Results: Baseline PET imaging revealed selective retention in the striatum with rapid clearance from the cerebellar hemispheres as a reference region. Total volume of distribution (V_T) values derived from both 2TCM and Logan analysis with blood input revealed ~3-fold higher levels of (-)-[¹¹C]TZ659 in the striatum than the cerebellar hemispheres. Injection of (-)-vesamicol either as a blocking or displacing agent significantly reduced striatal uptake of (-)-[¹¹C]TZ659. In contrast, pretreatment with the sigma-ligand (+)-pentazocine had no impact. Pretreatment with the S-(-)-eticlopride, a dopamine D₂-like receptor antagonist, increased striatal uptake of (-)-[¹¹C]TZ659. Striatal binding potential (BP_ND, range of 0.33-1.6 with cerebellar hemispheres as the reference region) showed good correlation (r² = 0.97) between SRTM and LoganREF. Occupancy studies found that ~0.0057 mg/kg of (-)-vesamicol produced 50% VAChT occupancy in the striatum. Conclusion: (-)-[¹¹C]TZ659 demonstrated specific and reversible VAChT binding and favorable pharmacokinetic properties for assessing the density of VAChT in the living brain.