Radiosynthesis and Evaluation of a C-11 Radiotracer for Transient Receptor Potential Canonical 5 in the Brain

Purpose: TRPC5 belongs to the mammalian superfamily of transient receptor potential (TRP) Ca²⁺-permeable cationic channels and it has been implicated in various CNS disorders. As part of our ongoing interest in the development of a PET radiotracer for imaging TRPC5, herein, we explored the radiosynthesis, and in vitro and in vivo evaluation of a new C-11 radiotracer [¹¹C]HC070 in rodents and nonhuman primates. Procedures: [¹¹C]HC070 was radiolabeled utilizing the corresponding precursor and [¹¹C]CH₃I via N-methylation protocol. Ex vivo biodistribution study of [¹¹C]HC070 was performed in Sprague–Dawley rats. In vitro autoradiography study was conducted for the rat brain sections to characterize the radiotracer distribution in the brain regionals. MicroPET brain imaging studies of [¹¹C]HC070 were done for 129S1/SvImJ wild-type mice and 129S1/SvImJ TRPC5 knockout mice for 0–60-min dynamic data acquisition after intravenous administration of the radiotracer. Dynamic PET scans (0–120 min) for the brain of cynomolgus male macaques were performed after the radiotracer injection. Results: [¹¹C]HC070 was efficiently prepared with good radiochemical yield (45 ± 5%, n = 15), high chemical and radiochemical purity (> 99%), and high molar activity (320.6 ± 7.4 GBq/μmol, 8.6 ± 0.2 Ci/μmol) at the end of bombardment (EOB). Radiotracer [¹¹C]HC070 has good solubility in the aqueous dose solution. The ex vivo biodistribution study showed that [¹¹C]HC070 had a quick rat brain clearance. Autoradiography demonstrated that [¹¹C]HC070 specifically binds to TRPC5-enriched regions in rat brain. MicroPET study showed the peak brain uptake (SUV value) was 0.63 in 129S1/SvImJ TRPC5 knockout mice compared to 1.13 in 129S1/SvImJ wild-type mice. PET study showed that [¹¹C]HC070 has good brain uptake with maximum SUV of ~ 2.2 in the macaque brain, followed by rapid clearance. Conclusions: Our data showed that [¹¹C]HC070 is a TRPC5-specific radiotracer with high brain uptake and good brain washout pharmacokinetics in both rodents and nonhuman primates. The radiotracer is worth further investigating of its suitability to be a PET radiotracer for imaging TRPC5 in animals and human subjects in vivo.