Imaging of the human CB₂ receptors in the brain with PET: Development and biological evaluation of [¹⁸F]JHU94620-d₈

Ziel/Aim The upregulation of cannabinoid receptors type 2 (CB₂) has been reported in association with traumatic brain injury, neurodegeneration and cancer.^[ [ ¹ ] ^] Recently, we reported the development of [¹⁸F]JHU94620. However, this radioligand suffers from low metabolic stability in vivo.^[ [ ² ] ^] Here, we describe the development of the deuterated analogues [¹⁸F]JHU94620-d₄ and -d₈ as well as their biological evaluation.

Methodik/Methods The precursors for radiofluorination were obtained by coupling 4,5-dimethylthiazol-ylidene-2,2,3,3-tetramethylcyclopropane-1-carboxamide with either d₄ or d₈ 1,4-butanediol-bistosylate and radiofluorinated in the presence of Kryptand K2.2.2. and K₂CO₃. The fraction of radiometabolites was quantified in mouse plasma and brain. The CB₂ affinity and specificity was determined by in vitro binding experiments. Additionally, we evaluated the [¹⁸F]JHU94620-d₈ uptake by PET-studies into the spleen of healthy rats and in a rat model overexpressing the hCB₂ in the right striatum^[ [ ³ ] ^] (hCB2-rs).

Ergebnisse/Results [¹⁸F]JHU94620-d₄ and -d₈ were obtained in 10 % radiochemical yield and > 99 % radiochemical purity, with an improved metabolic stability of both deuterated analogues (80 % vs. 36 % for [¹⁸F]JHU94620) in mouse brain 30 min p.i. In vitro evaluation of [¹⁸F]JHU94620-d₈ revealed a specific binding with a K_D(rCB₂) of 0.36 nM and a K_D(hCB₂) of 2.72 nM. PET studies with [¹⁸F]JHU94620-d₈ revealed a rCB₂ specific uptake into the spleen (AUC_0-30min = 33.1 ± 0.8 vs. 16.6 ± 0.5 SUV min after blocking with GW405833). In displacement studies in the hCB2-rs model we could show a reversible and target specific uptake of [¹⁸F]JHU94620-d₈ with an SUV of 6.7 ± 0.3 from 6 to 60 min p.i. and an SUVr (right striatum-to-cerebellum) of 43.0 ± 7.3 at 60 min p.i.

Schlussfolgerungen/Conclusions [¹⁸F]JHU94620-d₈ is a new PET tracer with improved metabolic stability and excellent ability to image the CB₂ receptors in-vivo. Its further evaluation is underway.

• Literatur/References

• 1 Stasiulewicz et al. Int. J. Mol. Sci. 2020; 21: 2778
• 2 Moldovan et al. J. Nucl. Med. 2015; 56: 1048-1048 ; Moldovan et al. J. Med. Chem. 2016, 59, 7840
• 3 Attili et al. Br. J. Pharmacol 2019; 176 , 1481

Publication History

Article published online:
08 April 2021