New PET radioligands for imaging of cannabinoid receptors type 2

Objectives: Cannabinoid receptors play an important role in neuroprotection after acute neuronal injury such as traumatic brain injury, stroke, and epilepsy, as well as in chronic neurodegenerative disorders such as multiple sclerosis, Parkinson’s, and Alzheimer’s disease. By means of neuroimaging with positron emission tomography (PET), a better understanding of the involvement of the cannabinoid system in these diseases in living humans will be achieved. Furthermore, neuroimaging with ¹⁸F-labelled CB receptor ligands will be useful for monitoring treatment effects. However, there is a need for radioligands selective for cannabinoid receptors type 2 (CBR₂) which are suitable for PET imaging in humans. Therefore, N-arylamide oxadiazoles, recently described potent, highly selective and orally bio-available CBR₂ agonists [1], were proposed as lead for the development of CBR₂ PET radioligands [2].

Methods: The novel radiotracers [¹⁸F]2, [¹⁸F]4, and [¹⁸F]6 (Figure 1) were synthesized via nucleophilic aromatic substitution of the respective nitro precursor 1 and 3 and the bromine precursor 5 in DMF under microwave conditions (pulse mode, ≤ 150 W, ≤ 132°C, diffuse light). Ki values of precursor compounds 1, 3, and 5 and reference compounds 2, 4, and 6 were determined in competitive radioligand displacement studies on [3H]CP55940-labelled hCB1- and hCB2-CHO cell homogenates.

Results: We present the first examples for labelling of 3-aryl-1,2,4-oxadiazoles by nucleophilic aromatic substitution [3]. The tracers [¹⁸F]2, [¹⁸F]4 and [¹⁸F]6 were prepared with 3%, 3% and 28% radiochemical yield (RCY). The reference compounds 2 and 4 possess the highest CBR₂ affinity (K_iCBR2 ~ 5 nM) and selectivity vs. CBR₁ (K_iCBR1 > 1000 nM) within the structural series investigated (Figure 1). The structure of 1 has been determined by single crystal X-ray diffraction (space group P 1¯ ; R1 = 0.043).

Figure 1. Molecular structure of 1 (50 % ellipsoids) and Ki values of N-arylamide oxadiazole-based CB2 receptor ligands

Conclusions: Due to a high CBR₂ affinity and selectivity, the reference compounds 2 and 4 can be expected to be suitable for imaging studies. The CBR₂ affinity of reference compound 6 is relatively weak probably because of the unsubstituted R1 (R1 = H). The RCY of [¹⁸F]2 and [¹⁸F]4 have to be improved to perform PET studies. Nevertheless, the current RCY of [¹⁸F]2 and [¹⁸F]4 should allow first animal experiments on radiotracer kinetics and metabolism.

Research Support: Work was supported by DFG (Br 1360/12-1).
References: [1] Cheng, Y. et al. (2008), J. Med. Chem. 51, 5019-5034, [2] Evens, N. et al. (2010), Current Topics in Medicinal Chemistry 10, 1527-1543, [3] Rühl, T. et al. (2010), DE 10 2010 063 974.5