Patrik Holm

A PET Tracer for Brain α2C Adrenoceptors, 11C-ORM-13070: Radiosynthesis and Preclinical Evaluation in Rats and Knockout Mice

We report the development of a PET tracer for α2C adrenoceptor imaging and its preliminary preclinical evaluation. α2C adrenoceptors in the human brain may be involved in various neuropsychiatric disorders, such as depression, schizophrenia, and neurodegenerative diseases. PET tracers are needed for imaging of this receptor system in vivo. Methods: High-specific-activity 11C-ORM-13070 (1-[(S)-1-(2,3-dihydrobenzo[1,4]dioxin-2-yl)methyl]-4-(3-11C-methoxymethylpyridin-2-yl)-piperazine) was synthesized by 11C-methylation of O-desmethyl-ORM-13070 with 11C-methyl triflate, which was prepared from cyclotron-produced 11C-methane via 11C-methyl iodide. Rats and mice were investigated in vivo with PET and ex vivo with autoradiography. The specificity of 11C-ORM-13070 binding to α2 adrenoceptors was demonstrated in rats pretreated with atipamezole, an α2 adrenoceptor antagonist. The α2C adrenoceptor selectivity of the tracer was determined by comparing tracer binding in wild-type and α2A- and α2AC adrenoceptor knockout (KO) mice. 11C-ORM-13070 and its radioactive metabolites in rat plasma and brain tissue were analyzed with radio–high-performance liquid chromatography and mass spectroscopy. Human radiation dose estimates were extrapolated from rat biodistribution data. Results: The radiochemical yield, calculated from initial cyclotron-produced 11C-methane, was 9.6% ± 2.7% (decay-corrected to end of bombardment). The specific activity of the product was 640 ± 390 GBq/μmol (decay-corrected to end of synthesis). The radiochemical purity exceeded 99% in all syntheses. The highest levels of tracer binding were observed in the striatum and olfactory tubercle of rats and control and α2A KO mice—that is, in the brain regions known to contain the highest densities of α2C adrenoceptors. In rats pretreated with atipamezole and in α2AC KO mice, 11C tracer binding in the striatum and olfactory tubercle was low, similar to that of the frontal cortex and thalamus, regions with low densities of α2C adrenoceptors. Two radioactive metabolites were found in rat plasma, but only one of them was found in the brain; their identity was not revealed. The estimated effective radiation dose was comparable with the average exposure level in PET studies with 11C tracers. Conclusion: An efficient method for the radiosynthesis of 11C-ORM-13070 was developed. 11C-ORM-13070 emerged as a potential novel radiotracer for in vivo imaging of brain α2C adrenoceptors.