Synthesis and Preclinical Evaluation of 11C-UCB-J as a PET Tracer for Imaging the Synaptic Vesicle Glycoprotein 2A in the Brain

The synaptic vesicle glycoprotein 2A (SV2A) is found in secretory vesicles in neurons and endocrine cells. PET with a selective SV2A radiotracer will allow characterization of drugs that modulate SV2A (e.g., antiepileptic drugs) and potentially could be a biomarker of synaptic density (e.g., in neurodegenerative disorders). Here we describe the synthesis and characterization of the SV2A PET radiotracer C-11-UCB-J ((R)-1-((3-(C-11-methyl-C-11)pyridin-4-yl)methyl)4-(3,4,5-trifluorophenyl)-pyrrolidin-2-one) in nonhuman primates, including whole-body biodistribution. Methods: C-11-UCB-J was prepared by C-C-11-methylation of the 3-pyridyl trifluoroborate precursor with C-11-methyl iodide via the Suzuki-Miyaura cross-coupling method. Rhesus macaques underwent multiple scans including coinjection with unlabeled UCB-J (17, 50, and 150 mu g/kg) or preblocking with the antiepileptic drug levetiracetam at 10 and 30 mg/kg. Scans were acquired for 2 h with arterial sampling and metabolite analysis to measure the input function. Regional volume of distribution (V-T) was estimated using the 1-tissue-compartment model. Target occupancy was assessed using the occupancy plot; the dissociation constant (K-d) was determined by fitting self-blocking occupancies to a 1-site model, and the maximum number of receptor binding sites (B-max) values were derived from baseline V-T and from the estimated K-d and the nondisplaceable distribution volume (VND). Results: C-11-UCB-J was synthesized with greater than 98% purity. C-11-UCB-J exhibited high free fraction (0.46 +/- 0.02) and metabolized at a moderate rate (39% +/- 5% and 24% +/- 3% parent remaining at 30 and 90 min) in plasma. In the monkey brain, C-11-UCB-J displayed high uptake and fast kinetics. V-T was high (similar to 25-55 mL/cm(3)) in all gray matter regions, consistent with the ubiquitous expression of SV2A. Preblocking with 10 and 30 mg/kg of levetiracetam resulted in approximately 60% and 90% occupancy, respectively. Analysis of the self-blocking scans yielded a K-d estimate of 3.4 nM and B-max of 125-350 nM, in good agreement with the in vitro inhibition constant (K-i) of 6.3 nM and regional B-max in humans. Whole-body biodistribution revealed that the liver and the brain are the dose-limiting organs for males and females, respectively. Conclusion: C-11-UCB-J exhibited excellent characteristics as an SV2A PET radiotracer in nonhuman primates. The radiotracer is currently undergoing first-in-human evaluation.