Synthesis and Biological Evaluation of Thiophene-Based Cannabinoid Receptor Type 2 Radiotracers for PET Imaging

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 CB₂ upregulation 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 CB₂ receptor in patients suffering from neurodegenerative diseases. Herein we report two novel thiophene-based ¹¹C-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 K_i-values of 3.3 ± 0.5 nM (CB₂) and 1.0 ± 0.2 μM (CB₁) for AAT-015. AAT-778 showed similar K_i-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 CB₂ specificity 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. For in vivo analysis, [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 promising in vitro binding affinities, specific binding of [¹¹C]AAT-015, and [¹¹C]AAT-778 could not be demonstrated.