Development, synthesis and F-18 labelling of a fluoroalkylated triazine derivative for PET imaging of phosphodiesterase 2A

Aim:

Phosphodiesterases (PDEs) are a class of enzymes expressed in mammalian cells using the cyclic nucleotides cAMP and/or cGMP as secondary messengers. Inhibitors of PDEs prevent the breakdown of these molecules and thus affect various physiological processes. The PDE2A is highly and specifically expressed in the brain and in certain tumours and is involved in the pathophysiology of related diseases (e.g. neurodegenerative and highly proliferative processes). With this project we aim to develop a radiolabelled inhibitor of PDE2A for PET imaging of this new target.
Materials and Methods:
Starting from a triazine key compound [1] a new fluoropropyl derivative (9-(2-fluoro-5-(3-fluoropropoxy)phenyl)-2-methoxy-7-methylimidazo[5,1-c]pyrido[2,3-e][1,2,4]triazine) as reference structure for in vitro studies and the corresponding tosylate precursor for a one-step radiosynthesis strategy were designed and synthesised. The nucleophilic F-18 labelling was performed in acetonitrile at 80°C for 15 minutes. After isolation by semi-preparative HPLC the radioligand was purified using a Sep-Pak® C18 Plus light cartridge and formulated in 0.9% NaCl. The final product was analysed by radio-TLC and radio-HPLC. In vitro autoradiography was accomplished by incubating sagittal sections of rat brain with 1 MBq of the radioligand for
60 minutes at ambient temperature. Blocking studies were performed by addition of the triazine compound [1] in different concentrations.
Results:
The triazine key compound [1] (IC50 hPDE2A: 4.1 nM), the new fluoropropyl derivative
(IC50 hPDE2A: 11.4 nM) and the tosylate precursor were obtained with overall yields of 70 - 80%. The radioligand was synthesised with a labelling yield of 75.4 ± 4.9% (n = 5), a radiochemical yield of 57.2 ± 0.4% (end of synthesis; n = 3), a specific activity of 60.4 ± 11.6 GBq/µmol (n = 3) and a radiochemical purity of ≥ 99.5%. In first autoradiographic experiments a high and specific accumulation of the radioligand in cortex and striatum and a low uptake in cerebellum was observed, which is consistent with the distribution pattern of PDE2A protein in the brain.
Conclusion:
A new F-18 labelled and highly affine PDE2A inhibitor was successfully developed and synthesised. Further characterisation of this new radioligand is currently performed by small animal PET/MR, metabolism studies and autoradiography.
Reference:
[1] Patent WO2010/054253 A1