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

Purpose The alpha(2)-adrenoceptors mediate many effects of norepinephrine and epinephrine, and participate in the regulation of neuronal, endocrine, cardiovascular, vegetative, and metabolic functions. Of the three receptor subtypes, only alpha(2A) and alpha(2C) are found in the brain in significant amounts. Subtype-selective positron emission tomography (PET) imaging of alpha(2)-adrenoceptors has been limited to the alpha(2C) subtype. Here, we report the synthesis of 6-[F-18]fluoro-marsanidine, a subtype-selective PET tracer candidate for alpha(2A)-adrenoceptors, and its preclinical evaluation in rats and mice. Procedures 6-[F-18]Fluoro-marsanidine was synthesized using electrophilic F-18 fluorination with [F-18]Selectfluor bis(triflate). The tracer was evaluated in Sprague Dawley rats and in alpha(2A)-knockout (KO) and wild-type (WT) mice for subtype selectivity. In vivo PET imaging and ex vivo brain autoradiography were performed to determine the tracer distribution in the brain. The specificity of the tracer for the target was determined by pretreatment with the subtype-non-selective alpha(2)-agonist medetomidine. The peripheral biodistribution and extent of metabolism of 6-[F-18]fluoro-marsanidine were also analyzed. Results 6-[F-18]Fluoro-marsanidine was synthesized with [F-18]Selectfluor bis(triflate) in a radiochemical yield of 6.4 +/- 1.7 %. The molar activity was 3.1 to 26.6 GBq/mu mol, and the radiochemical purity was > 99 %. In vivo studies in mice revealed lower uptake in the brains of alpha(2A)-KO mice compared to WT mice. The results for selectivity were confirmed by ex vivo brain autoradiography. Blocking studies revealed reduced uptake in alpha(2A)-adrenoceptor-rich brain regions in pretreated animals, demonstrating the specificity of the tracer. Metabolite analyses revealed very rapid metabolism of 6-[F-18]fluoro-marsanidine with blood-brain barrier-permeable metabolites in both rats and mice. Conclusion 6-[F-18]Fluoro-marsanidine was synthesized and evaluated as a PET tracer candidate for brain alpha(2A)-adrenoceptors. However, rapid metabolism, extensive presence of labeled metabolites in the brain, and high non-specific uptake in mouse and rat brain make 6-[F-18]fluoro-marsanidine unsuitable for alpha(2A)-adrenoceptor targeting in rodents in vivo.