Synthesis and Preclinical Evaluation of¹¹C-UCB-J as a PET Tracer for Imaging the Synaptic Vesicle Glycoprotein 2A in the Brain

Abstract

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 11 C-UCB-J ((R)-1-((3-( 11 C-methyl-11 C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one) in nonhuman primates, including whole-body biodistribution. Methods: 11 C-UCB-J was prepared by C-11 C-methylation of the 3-pyridyl trifluoroborate precursor with 11 C-methyl iodide via the Suzuki-Miyaura cross-coupling method. Rhesus macaques underwent multiple scans including coinjection with unlabeled UCB-J (17, 50, and 150 μg/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 (V ND ). Results: 11 C-UCB-J was synthesized with greater than 98% purity. 11 C-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, 11 C-UCB-J displayed high uptake and fast kinetics. V T was high (∼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: 11 C-UCB-J exhibited excellent characteristics as an SV2A PET radiotracer in nonhuman primates. The radiotracer is currently undergoing first-in-human evaluation.

Synaptic Vesicle Glycoprotein 2A Ligands in the Treatment of Epilepsy and Beyond

“…Other PET tracers based on 11 C-labeling have been also recently developed in preclinical species, e.g. [ 11 C]UCB-A [77] and [ 11 C]UCB-J [78]. As with UCB-H, UCB-J [( R )-1-((3-(methyl- 11 C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one] is a heterocyclic non-acetamide compound [74].…”

“…As with UCB-H, UCB-J [( R )-1-((3-(methyl- 11 C)pyridin-4-yl)methyl)-4-(3,4,5-trifluorophenyl)pyrrolidin-2-one] is a heterocyclic non-acetamide compound [74]. Its whole body biodistribution was studied in non-human primates, demonstrating high uptake and fast kinetics in the brain [78]. Preblocking with levetiracetam 10 and 30 mg/kg resulted in approximately 60 and 90 % occupancy, respectively.…”

“…Preblocking with levetiracetam 10 and 30 mg/kg resulted in approximately 60 and 90 % occupancy, respectively. The recent evaluation demonstrated that [ 11 C]-UCB-J has exceptional imaging qualities for PET measurements of SV2A in the human brain, including high brain uptake, rapid kinetics appropriate for the 11 C radiolabel, a high and reliably measurable free fraction in plasma, and suitable regional time–activity curves [78]. [ 11 C]UCB-J displays rapid metabolism in human subjects and excellent brain uptake, which is highest in the striatum and the cortex, while being moderate in the cerebellum and the thalamus (Fig.…”

A single‐center, open‐label positron emission tomography study to evaluate brivaracetam and levetiracetam synaptic vesicle glycoprotein 2A binding in healthy volunteers

“…Positron emission tomography (PET) can be used noninvasively to determine drug target engagement and occupancy in living subjects. Several PET radioligands have recently been developed for imaging of SV2A, with 11 C‐UCB‐J being considered best in class thus far . We previously demonstrated that 11 C‐UCB‐J binding could be displaced by intravenous infusion of BRV (5 mg/kg) or LEV (30 mg/kg) in nonhuman primates.…”

Automated radiosynthesis of [11C]UCB‐J for imaging synaptic density by positron emission tomography

“…This is achieved by radiotracers selective to synaptic vesicle glycoprotein SV2A, a transmembrane protein expressed ubiquitously in secretory vesicles in all brain areas . Arising from several structural scaffolds related to the antiepileptic drug levetiracetam (LEV, 1 ; Figure ) the carbon‐11 radiolabelled UCB‐J analogue ([ 11 C] 2 ; Figure ) has been shown to have excellent in vitro and in vivo properties for imaging SV2A from both preclinical and clinical studies …”