[¹⁸F]JHU94620-d₈ – A new radiotracer for the noninvasive in vivo assessment of the CB₂ receptor in brain and peripheral tissues by PET

Introduction
An upregulation of cannabinoid receptors type 2 (₂) has been reported in association with inflammation processes, traumatic brain injury, neurodegeneration and cancer.[1] The activation of ₂ leads to an anti-inflammatory action. Therefore, the non-invasive assessment of the ₂ availability with PET can support decisions for ₂-directed therapies. Recently, we have reported on the development of the PET radiotracer [¹⁸F]JHU94620. This radioligand suffers from low metabolic stability in vivo.[2] Here, we describe the deuterated analogue [¹⁸F]JHU94620-d₈ and its biological evaluation.
Methods
The precursor for radiofluorination was obtained by coupling 4,5-dimethylthiazol-ylidene-2,2,3,3-tetramethylcyclopropane-1-carboxamide with 1,4-butandiol-1,1,2,2,3,3,4,4-d₈ bis(4-methylbenzenesulfonate) and radiofluorinated in the presence of the Kryptand K2.2.2. and K2CO3. The fraction of radiometabolites after injection of 27 to 50 MBq i.v. (AM ≈ 150 GBq/µmol) was quantified in mice plasma and brain 30 min p.i. The ₂ binding affinity (KD) and specificity (Ki) of [¹⁸F]JHU94620-d₈ was determined in vitro. Additionally, PET studies (injection of 23 ± 5 MBq i.v.) were performed to evaluate the [¹⁸F]JHU94620-d₈ uptake into the spleen of adult healthy Wistar rats and in rats overexpressing the functional inactive h₂(D𔕐N) in the right striatum[3] (h₂-rs).
Results/Discussion
[¹⁸F]JHU94620-d₈ was obtained in 10% radiochemical yield and >99% radiochemical purity, showing an improved metabolic stability of the deuterated analogue (80% vs. 36% for [¹⁸F]JHU94620, determined in the brain 30 min p.i.). It revealed a KD on rat ₂ of 0.36 nM and on human ₂ of 2.72 nM, as well as a Ki(hCB1) > 1 µM and Ki(h₂) of 0.9 nM. PET studies revealed a ₂-specific uptake of [¹⁸F]JHU94620-d₈ into the rat spleen (AUC0-30min = 33 vs. 17 SUV × min after blocking with GW405833). In the h₂-rs rats we could show a reversible and target-specific uptake of [¹⁸F]JHU94620-d₈ with an SUVmean of 6.7 ± 0.3 from 6 to 60 min p.i. and an SUVr (right striatum-to-cerebellum) of 43 ± 7 at 60 min p.i.
Conclusions
[¹⁸F]JHU94620-d₈ is a new PET tracer with improved metabolic stability as compared with the non-deuterated version, thus indicating an excellent ability to image the ₂ receptors in vivo.
Acknowledgement
This research was funded by Deutsche Forschungsgemeinschaft (DFG), grant number MO2677/4-1
Disclosure
A German patent application was filed Nr. 10 2020118 255.4
References
[1] Stasiulewicz, A., Znajdek, K., et al. 2020, ‘A Guide to Targeting the Endocannabinoid System in Drug Design’, IJMS, 2020, 21, 2778; doi:10.3390/ijms21082778
[2] Moldovan, R.-P., Deuther-Conrad, W., et al., 2015, ‘¹⁸F-JHU94620, a high affinity PET radioligand for imaging of cannabinoid subtype 2 receptors (₂R)’, J. Nucl. Med., 56, 1048
[3] Attili, B., Celen, S., et al., 2019, ‘Preclinical evaluation of [¹⁸F]MA3: a ₂ receptor agonist radiotracer for PET’, Br. J. Pharmacol., 176, 1481-1491