Identification and Preclinical Development of a 2,5,6-Trisubstituted Fluorinated Pyridine Derivative as a Radioligand for the Positron Emission Tomography Imaging of Cannabinoid Type 2 Receptors

Despite the broad implications of the cannabinoid type 2 receptor (CB2) in neuroinflammatory processes, a suitable CB2-targeted probe is currently lacking in clinical routine. As part of our medicinal chemistry program, we have recently identified the pyridine-based CB2 PET radioligand [¹⁸F]3 with a binding affinity (Ki) of 6 nM for CB2 and a selectivity factor of 696 over the cannabinoid receptor type 1 (CB1). Notwithstanding these promising in vitro properties, [¹⁸F]3 exhibited only moderate in vivo specificity and a fast washout from CB2-rich rat spleen. In order to improve CB2 specificity and selectivity, 15 new derivatives of [¹⁸F]3 were synthesized and tested for their binding affinities towards CB2 and CB1. With a subnanomolar affinity of 0.7 nM (Ki for CB2) and a remarkable selectivity factor of > 12'000 over CB1, target compound 11 (RoSMA-18) exhibited outstanding in vitro performance characteristics and was selected for preclinical evaluation as a PET radioligand. [¹⁸F]RoSMA-18 was synthesized with an average radiochemical yield of 10.6 ± 3.8% (n = 16) and excellent radiochemical purity (> 99%). Molar activities ranged from 52 - 65 GBq/µmol. Exceptional CB2 specificities were achieved in CB2-positive rat spleen by in vitro autoradiography (71 ± 2%) and ex vivo biodistribution (86 ± 2%), which are superior to any previously reported CB2 PET radioligand. This high exceptional specificity was also elegantly corroborated in CB2 knockout mouse spleen. [¹⁸F]RoSMA-18 was sensitive in detecting CB2 upregulation in post-mortem human ALS spinal cord tissue by in vitro autoradiography. PET experiments revealed specific and reversible CB2 binding of [¹⁸F]RoSMA-18 in in the CB2-positive rat spleen. Metabolite studies detected only intact [¹⁸F]RoSMA-18 in the rat brain, however, in vivo defluorination was observed as evidenced by skull uptake, which was circumvented by replacing the hydrogen atoms in the fluoropropyl side chain with deuterium atoms to afford [¹⁸F]RoSMA-18-d₆ in which case no radioactivity accumulation in the skull was observed. RoSMA-18-d₆ showed a Ki value of 0.8 nM for CB2 and > 10 µM for CB1, respectively. Overall, these results suggest that [¹⁸F]RoSMA-18-d₆ is a promising CB2 PET radioligand for clinical translation.