Imaging Synaptic Vesicle Protein SV2C with <sup>18</sup>F-UCB-F: An In Vitro Autoradiography and In Vivo NHP PET Study.

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The synaptic vesicle protein SV2C, predominantly found in the basal ganglia, has been associated with Parkinson's disease through genetic studies. It plays a crucial role in regulating dopamine release and has been shown to be disrupted in PD animal models and brain tissues from PD patients. In the context of PD-related synaptopathy, SV2C may serve as a potential imaging target for monitoring disease progression and response to treatment. [18F]UCB-F is a radioligand binding to SV2C developed by UCB. Preliminary autoradiography and PET studies in rats showed that [18F]UCB-F displays a brain distribution consistent with the expression of SV2C in vitro but does not display any specific binding in vivo. This study was therefore designed to further investigate the affinity and selectivity of [18F]UCB-F for SV2C and to examine the in vitro and in vivo properties of the radioligand in nonhuman primates. In vitro binding studies were performed to measure the affinity of UCB-F to SV2A, SV2B, and SV2C. In-silico modeling was used to assess the binding mode and energy of UCB-F. Autoradiography studies on rat and nonhuman primate (NHP) brain tissues were performed to confirm that [18F]UCB-F showed similar distribution in rat and NHP tissue. Finally, PET studied in NHPs were performed to examine the in vivo pharmacokinetic properties of [18F]UCB-F. [18F]UCB-F was successfully synthesized from the corresponding precursor with high yield. Autoradiography on brain slices from rats and NHPs demonstrated specific binding of [18F]UCB-F in the pallidum, striatum, substantia nigra, and brainstem, consistent with the known brain expression of SV2C. In NHPs, [18F]UCB-F rapidly crossed the blood-brain barrier, reaching peak uptake values of 2.8%ID in NHP1 and 2.1%ID in NHP2 at 4 min postinjection. The tracer was rapidly washed out from the brain, with no clear regional distribution. Radiometabolite analysis revealed the formation of only more polar radiometabolites, with approximately 15% of unchanged radioligand remaining in plasma at 15 min postinjection. In vitro and in-silico studies demonstrated that the affinity of [18F]UCB-F decreased by approximately one factor of magnitude with increase of temperature from 277 K (4 °C) to 310 K (37 °C). This temperature-related decrease of the affinity for SV2C together with rapid in vivo radiometabolism might explain the discrepancy between in vitro and in vivo performance of [18F]UCB-F. Overall, these findings suggest that [18F]UCB-F is not a suitable PET radioligand for imaging SV2C. Further research is needed to identify alternative candidates with improved in vivo stability and brain retention.

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