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 [C-11]HC070 in rodents and nonhuman primates. Procedures [C-11]HC070 was radiolabeled utilizing the corresponding precursor and [C-11]CH3I via N-methylation protocol. Ex vivo biodistribution study of [C-11]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 [C-11]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 [C-11]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/mu mol, 8.6 +/- 0.2 Ci/mu mol) at the end of bombardment (EOB). Radiotracer [C-11]HC070 has good solubility in the aqueous dose solution. The ex vivo biodistribution study showed that [C-11]HC070 had a quick rat brain clearance. Autoradiography demonstrated that [C-11]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 [C-11]HC070 has good brain uptake with maximum SUV of 2.2 in the macaque brain, followed by rapid clearance. Conclusions Our data showed that [C-11]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.