Evaluation of [¹⁸F]RoSMA-18-d₆ as a CB2 PET Radioligand in Nonhuman Primates

The cannabinoid type 2 receptor (CB2) has been implicated in a variety of central and peripheral inflammatory diseases, prompting significant interest in the development of CB2-targeted diagnostic and therapeutic agents. A validated positron emission tomography (PET) radioligand for imaging CB2 in the living human brain as well as in peripheral tissues is currently lacking. As part of our research program, we have recently identified the trisubstituted pyridine, [¹⁸F]RoSMA-18-d₆, which proved to be highly suitable for in vitro and in vivo mapping of CB2 in rodents. The aim of this study was to assess the performance characteristics of [¹⁸F]RoSMA-18-d₆ in nonhuman primates (NHPs) to pave the way for clinical translation. [¹⁸F]RoSMA-18-d₆ was synthesized from the respective tosylate precursor according to previously reported procedures. In vitro autoradiograms with NHP spleen tissue sections revealed a high binding of [¹⁸F]RoSMA-18-d₆ to the CB2-rich NHP spleen, which was significantly blocked by coincubation with the commercially available CB2 ligand, GW405833 (10 μM). In contrast, no specific binding was observed by in vitro autoradiography with NHP brain sections, which was in agreement with the notion of a CB2-deficient healthy mammalian brain. In vitro findings were corroborated by PET imaging experiments in NHPs, where [¹⁸F]RoSMA-18-d₆ uptake in the spleen was dose-dependently attenuated with 1 and 5 mg/kg GW405833, while no specific brain signal was observed. Remarkably, we observed tracer uptake and retention in the NHP spinal cord, which was reduced by GW405833 blockade, pointing toward a potential utility of [¹⁸F]RoSMA-18-d₆ in probing CB2-expressing cells in the bone marrow. If these observations are substantiated in NHP models of enhanced leukocyte proliferation in the bone marrow, [¹⁸F]RoSMA-18-d₆ may serve as a valuable marker for hematopoietic activity in various pathologies. In conclusion, [¹⁸F]RoSMA-18-d₆ proved to be a suitable PET radioligand for imaging CB2 in NHPs, supporting its translation to humans.