A PET Tracer for Brain α_2C Adrenoceptors, ¹¹C-ORM-13070: Radiosynthesis and Preclinical Evaluation in Rats and Knockout Mice

Abstract

We report the development of a PET tracer for α 2C adrenoceptor imaging and its preliminary preclinical evaluation. α 2C 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 11 C-ORM-13070 (1--(3-11 C-methoxymethylpyridin-2-yl)-piperazine) was synthesized by 11 C-methylation of O-desmethyl-ORM-13070 with 11 C-methyl triflate, which was prepared from cyclotron-produced 11 C-methane via 11 C-methyl iodide. Rats and mice were investigated in vivo with PET and ex vivo with autoradiography. The specificity of 11 C-ORM-13070 binding to α 2 adrenoceptors was demonstrated in rats pretreated with atipamezole, an α 2 adrenoceptor antagonist. The α 2C adrenoceptor selectivity of the tracer was determined by comparing tracer binding in wild-type and α 2A -and α 2AC adrenoceptor knockout (KO) mice. 11 C-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 11 C-methane, was 9.6% ± 2.7% (decaycorrected 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 α 2A KO mice-that is, in the brain regions known to contain the highest densities of α 2C adrenoceptors. In rats pretreated with atipamezole and in α 2AC KO mice, 11 C tracer binding in the striatum and olfactory tubercle was low, similar to that of the frontal cortex and thalamus, regions with low densities of α 2C 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 11 C tracers. Conclusion: An efficient method for the radiosynthesis of 11 C-ORM-13070 was developed. 11 C-ORM-13070 emerged as a potential novel radiotracer for in vivo imaging of brain α 2C adrenoceptors.

Validation of [11C]ORM‐13070 as a PET tracer for alpha2c‐adrenoceptors in the human brain

“…Tracer metabolism was taken into account in the PET data analysis as the primary analysis was performed with the ratio method which eliminates possible bias caused by radioactive tracer metabolites—as long as there are no treatment‐related alterations in tracer disposition and metabolism. According to previously collected evidence, there was not anticipated to be any specific binding by the metabolites (Arponen et al, : supplemental material). As the metabolites have not been fully characterized, it is possible that they could cause accumulation of radioactivity in the reference region, which may be more pronounced than their nondisplaceable uptake in the target region.…”

“…Preclinical studies have identified the same fragments and experiments in rats also suggested that one of these compounds (M1), but not the other (M2), can gain access into the brain. The molecular structures of M1 and M2 have not been resolved in spite of extensive efforts involving stable isotopes and mass spectroscopy, but they have been inferred to be volatile, very small molecular weight fragments with no affinity for α 2C ‐ARs (Arponen et al, ).…”

“…The PET tracer [ 11 C]ORM‐13070 was prepared according to the previously described procedures (Arponen et al, ). The specific radioactivity of the tracer at the time of injection was higher than 150 GBq/µmol.…”

“…Venous blood samples were collected at three time points (2, 15, and 30 min) after tracer injection for analysis of the plasma concentrations of [ 11 C]ORM‐13070 and its radioactive metabolites as previously described (Arponen et al ) in order to exclude treatment‐related alterations in tracer disposition and metabolism.…”

Radiosynthesis and Preclinical Evaluation of an α2A-Adrenoceptor Tracer Candidate, 6-[18F]Fluoro-marsanidine

“…In 2014, Arponen et al reported an α 2C -selective PET tracer, [ 11 C]ORM-13070. This tracer was evaluated in mice devoid of α 2A -adrenoceptors and mice lacking both α 2A -and α 2Cadrenoceptors in order to confirm its high subtype selectivity [23] and has since been introduced for human use [24].…”