In vivo performance of different ¹⁸F-labelled cannabinoid receptor 2 radioligands

Aim: Cannabinoid receptor 2 (CB2) expression in healthy brain is very low and the upregulation is associated with inflammation, traumatic brain injury, neurodegeneration and cancer[1]. In view of the increasing interest in CB2-targeted therapies, PET offers an attractive strategy to quantify the availability of CB2 in the diseased brain. To achieve this goal, we developed a number of 18F-labelled CB2 ligands and biologically evaluated them in rodents. Here we present a comparative overview of the obtained results.
Methods: Structure-activity-relationships-driven target compound identification, organic synthesis and radiofluorination was performed for compounds of the thiazole ([¹⁸F]JHU94620[2] and [¹⁸F]LUZ5[3]), naphthyridin-2-one ([¹⁸F]LU14[4] and [¹⁸F]LU13[5]) and indole ([¹⁸F]RM365) families. The new radioligands were assessed in vitro by binding experiments using CHO(hCB2) cells and rat spleen homogenates and by autoradiography on cryosections of rodent spleen. Furthermore, the radioligands were examined for their metabolic stability and biodistribution by PET. Furthermore, for selected radioligands the binding to highly expressed hCB2 in a rat model overexpressing hCB2(D80N) in the right striatum (AAV-hCB2)[6] was investigated.
Results: The low- to subnanomolar hCB2 affinities of the presented radioligands were demonstrated in vitro by Kd values ranging from 0.4 to 2.9 nM with a hCB2 selectivity against hCB1 of >1000-fold. The highest metabolic stability was observed for [18F]RM365 with 55% and 90% and the lowest for [¹⁸F]JHU94620 with 7% and 36% of the initial fraction in plasma and brain 30 min p.i., respectively. The in vivo experiments confirmed the in vitro autoradiographic results, and showed high uptake for [¹⁸F]JHU94620, but low or non-displaceable uptake for [¹⁸F]LU14, [¹⁸F]LUZ5 and [¹⁸F]RM365 in spleen. Contrary to the low uptake of [¹⁸F]LUZ5 in the brain of naïve Wistar rats, target-specific and displaceable uptake for [¹⁸F]LU14 and [¹⁸F]RM365 was demonstrated in the AAV-hCB2 rat model, with the highest signal-to-noise ratio determined for [¹⁸F]RM365 expressed as SUVR of 20 and lowest for [¹⁸F]LU14 expressed as SUVR of 6.
Conclusion: A novel series of CB2 receptor radioligands has been developed and preliminarily evaluated in rodents. Binding affinity to the CB2 varies between species, however PET scans with an AAV-hCB2 rat model revealed a high brain uptake and target specificity for hCB2 with excellent signal-to-noise ratios and displaceable binding.
References: [1] Stasiulewicz et al. IJMS, 2020, 21, 2778; [2] Moldovan et al. JMC, 2016, 59, 17; [3] Ueberham et al. JMC, 2023; [4] Teodoro et al. IJMS, 2021, 22, 15; [5] Gündel et al., JMC 2022, 65, 13; [6] Attili et al. BJP, 2019, 176, 1481