title: Synthesis and Biological Evaluation of Thiophene-Based Cannabinoid Receptor Type 2 Radiotracers for PET Imaging.
abstract: Over the past two decades, our understanding of the endocannabinoid system has greatly improved due to the wealth of results obtained from exploratory studies. Currently, two cannabinoid receptor subtypes have been well-characterized. The cannabinoid receptor type 1 (CB1) is widely expressed in the central nervous system, while the levels of the cannabinoid receptor type 2 (CB2) in the brain and spinal cord of healthy individuals are relatively low. However, recent studies demonstrated a CB2upregulation on activated microglia upon neuroinflammation, an indicator of neurodegeneration. Our research group aims to develop a suitable positron emission tomography (PET) tracer to visualize the CB2receptor in patients suffering from neurodegenerative diseases. Herein we report two novel thiophene-based11C-labeled PET ligands designated [11C]AAT-015 and [11C]AAT-778. The reference compounds were synthesized using Gewald reaction conditions to obtain the aminothiophene intermediates, followed by amide formation. Saponification of the esters provided their corresponding precursors. Binding affinity studies revealed Ki-values of 3.3 ± 0.5 nM (CB2) and 1.0 ± 0.2 μM (CB1) for AAT-015. AAT-778 showed similar Ki-values of 4.3 ± 0.7 nM (CB2) and 1.1 ± 0.1 μM (CB1). Radiosynthesis was carried out under basic conditions using [11C]iodomethane as methylating agent. After semi-preparative HPLC purification both radiolabeled compounds were obtained in 99% radiochemical purity and the radiochemical yields ranged from 12 to 37%. Specific activity was between 96 and 449 GBq/μmol for both tracers. In order to demonstrate CB2specificity of [11C]AAT-015 and [11C]AAT-778, we carried out autoradiography studies using CB2-positive mouse/rat spleen tissues. The obtained results revealed unspecific binding in spleen tissue that was not blocked by an excess of CB2-specific ligand GW402833. Forin vivoanalysis, [11C]AAT-015 was administered to healthy rats via tail-vein injection. Evaluation of the CB2-positive spleen, however, showed no accumulation of the radiotracer. Despite the promisingin vitrobinding affinities, specific binding of [11C]AAT-015, and [11C]AAT-778 could not be demonstrated.
authors: Department of Chemistry and Applied Biology, Institute of Pharmaceutical Sciences, Swiss Federal Institute of Technology Zurich, Switzerland.
authors: Achi
authors: A
authors: Haider
authors: Department of Chemistry and Applied Biology, Institute of Pharmaceutical Sciences, Swiss Federal Institute of Technology Zurich, Switzerland.
authors: Adrienne
authors: A
authors: Müller Herde
authors: Department of Molecular and Medical Pharmacology, University of California, Los Angeles Los Angeles, CA, USA.
authors: Roger
authors: R
authors: Slavik
authors: Neuromuscular Diseases Unit/ALS Clinic, Kantonsspital St. Gallen St. Gallen, Switzerland.
authors: Markus
authors: M
authors: Weber
authors: Department of Biotechnology Chemistry and Pharmacy, University of Siena Siena, Italy.
authors: Claudia
authors: C
authors: Mugnaini
authors: Institute of Biomolecular Chemistry, National Research Counsil of Italy Naples, Italy.
authors: Alessia
authors: A
authors: Ligresti
authors: Department of Chemistry and Applied Biology, Institute of Pharmaceutical Sciences, Swiss Federal Institute of Technology Zurich, Switzerland.
authors: Roger
authors: R
authors: Schibli
authors: Department of Nuclear Medicine, University Hospital Zurich Zurich, Switzerland.
authors: Linjing
authors: L
authors: Mu
authors: Department of Chemistry and Applied Biology, Institute of Pharmaceutical Sciences, Swiss Federal Institute of Technology Zurich, Switzerland.
authors: Simon
authors: S
authors: Mensah Ametamey
keywords: cannabinoid receptor type 2; neurodegenerative disorders; neuroinflammation; positron emission tomography; thiophene-based structures