Novel ¹⁸F-labelled triazine derivatives for PET imaging of phosphodiesterase 2A

Objectives: Phosphodiesterases (PDEs) are enzymes which degrade the second messengers cAMP and cGMP thereby affecting cellular functions. PDE2A is involved in the pathophysiology of Alzheimer´s disease and cancer. Therefore PDE2A inhibitors are suggested as potential therapeutics. Accordingly we aim to develop an ¹⁸F-labelled radioligand for PET imaging of PDE2A.
Methods: Based on a triazine compound [1] (TA1) novel fluoroalkylated derivatives (TA2-5, see Table 1) were synthesized and their affinity and selectivity towards PDE2A were determined. ¹⁸F-labelling of selected candidates was accomplished by nucleophilic substitution in acetonitrile using tosylate precursors. In vitro autoradiographic studies on rat brain sections were performed with [¹⁸F]TA3 and [¹⁸F]TA4 under control and blocking conditions. For PET/MR studies of [18F]TA3 in mice the radiosynthesis was performed in a TRACERlabTM FX F-N module. In vivo metabolism studies of [18F]TA3 and [¹⁸F]TA4 in mouse plasma and brain samples were carried out by conventional extraction procedure as well as by direct injection of the samples into a micellar HPLC system.
Table 1: Structures and affinity data of TA1-5, autoradiographic image of [¹⁸F]TA3.

IC50PDE2A 4.5 nM 10.4 nM 11.4 nM 7.3 nM 3.0 nM
IC50PDE10A 670 nM 77 nM 318 nM 913 nM > 1000 nM

Results: [¹⁸F]TA3, [¹⁸F]TA4 and [¹⁸F]TA5 were successfully synthesized with labelling yields of 40 - 70%, radiochemical yields of 30 - 45% and specific activities of ≥ 60 GBq/µmol. In vitro autoradiographic experiments showed region-specific accumulation of [¹⁸F]TA3 (see Table 1) and [¹⁸F]TA4 with higher binding density in cortex and striatum than in cerebellum, which is consistent with the distribution pattern of PDE2A in rat brain. PET/MR studies of [¹⁸F]TA3 in mice exhibited a fast wash out of radioactivity from the striatum while constantly increasing uptake was observed in the cerebellum. Metabolism studies of [¹⁸F]TA3 and [¹⁸F]TA4 at 30 minutes p.i. revealed a significant brain concentration of radiometabolites (≥ 40%).
Conclusions: Due to the brain accumulation of radiometabolites, the new radioligands [¹⁸F]TA3 and [¹⁸F]TA4 are exclusively suitable for in vitro imaging of PDE2A. Further in vitro and in vivo characterization of the highly affine and selective PDE2A radioligand [¹⁸F]TA5 is currently in progress.
Reference: [1] Stange et al.: Triazine Derivatives as Inhibitors of Phosphodiesterases; Patent WO2010/054253 A1.