Radiolabelling and biodistribution studies of potential radioligands for PET-Imaging of CB₂

The level of expression of cannabinoid receptors type 2 (CB2) in healthy and diseased brain has not been fully elucidated. Furthermore, there is interest in the ultra-localisation of CB2 in brain. Positron emission tomography (PET) is a technique, which allows monitoring of very low amounts of radiolabelled compounds in living organisms. Here, we attached 18F as radiolabel at different sites of N-aryl-oxadiazolyl-propionamides as potential radioligands and investigated it’s influence on affinity and pharmacology of the compounds.

Methods: Compounds were synthesised according to our previously described route (Rühl et al. Org. Med. Chem. Lett. 2012; 2: 32). Affinities towards CB2 and CB1 were determined via competitive radioligand binding assays with XXX as blocking compound. For labelling of 1, a tosylat moiety as selected as optimal leaving group (LG). Labelling at the aromatic ring in 2 was achieved with trimethylammonium as LG. In vivo organ distribution was investigated on CD-1 mice by injection of ~ 300 kBq of radiotracer (in 200 ml isotonic solution) in the tail vein. At various times, the animals were anesthetized, organs of interest dissected and the percentage of injected dose per gram of wet tissue (% ID/g) calculated. Blocking studies were conducted by intraperitoneal pre-injection of 1 mg/kg SR144,528 dissolved in isotonic solution 10 min prior to the radiotracer. All animals were sacrificed at 60 min p.i., and radioactivity uptake was determined.

Results: 1) In both cases, good labelling yields (XX %) could be achieved.
2) The radiolabelled compounds show the same affinity and specificity towards CB2 as the reference compound. 3) The radiotracers undergo strong metabolism but 4) can cross the blood brain barrier (BBB). After five minutes, approximately 2 to 4% of intact radiotracer was found in the plasma. Nevertheless, 36% of ¹⁸F1 and 80% of ¹⁸F2 could be observed in the brain after 30 minutes. The main metabolite could be identified as the free acid derivative, which has no affinity towards the CB receptors.

Conclusions: The introduction of ¹⁸F into the lead structure does not affect the affinity and the selectivity towards the CB2 receptor. The radiotracers cross the BBB. However, the compounds undergo strong metabolism in plasma.