Synthesis and 18F-Radiolabelling of Novel Benzoimidazotriazines for Imaging of Phosphodiesterase 2A (PDE2A)

1. Introduction
Cyclic nucleotide phosphodiesterases (PDEs) are a class of intracellular enzymes that inactivate the secondary messenger molecules cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Thus, PDEs regulate the signaling cascades mediated by these cyclic nucleotides and affect fundamental cellular processes, such as proliferation, differentiation, migration, survival, and apoptosis. Accordingly, they are promising therapeutic targets. Since PDE2A was found to be related to a variety of tumors, it is our aim to synthesize novel PDE2A inhibitors based on the benzoimidazotriazine (BIT) moiety that might be a prospective lead compound for the development of an F-18 labelled ligand for PDE2A imaging with PET.

Fig A. BIT key intermediates, B. Radiosynthesis of [18F]BIT1

2. Materials & Methods
Based on BIT key intermediates (Fig. A), a small series of novel fluorinated BIT derivatives was successfully prepared (overall in 7-10 steps) and the affinities towards PDE2A and other PDE subtypes were estimated. The most promising compound, BIT1, was radiolabelled by using the corresponding nitro precursor. The reaction was optimized by choosing different solvents, amounts of precursor, modes of heating (conventional or microwave), temperatures, and reaction times. Afterwards, best conditions (Fig. B) were transferred to an automated synthesis module (TracerLab FX2 N, GE Healthcare). The radiotracer was isolated by semi-preparative HPLC (Reprosil-Pur AQ column, 25010mm, 46 % ACN/aqu. 20 mM NH4OAc, flow 5.5 ml/min) followed by purification with a Sep-Pak C18 Plus light cartridge and formulation in isotonic saline containing 10% ethanol.

3. Results
BIT1 showed a high affinity towards PDE2A (IC50 PDE2A3 = 3.33 nM) and selectivity over other PDE subtypes. [18F]BIT1 was successfully synthesized with a radiochemical yield of 51.9 ± 1.3 % (n=3), molar activities between 46 – 100 GBq/µmol and radiochemical purities of ≥ 99%.

4. Discussion & Conclusion
Radiofluorination of a novel PDE2A ligand [18F]BIT1 was obtained with appropriate radiochemical yield and molar activity. First biological investigations are planned to estimate the potential of [18F]BIT1 as imaging agent for PDE2A.

Acknowledgement
1. Deutsche Forschungsgemeinschaft (German Research Foundation, Project Number: SCHE 1825/3-1).
2. Scholarship Program for Research and Innovation in Science and Technology Project (RISET-PRO)-Indonesia Ministry of Research, Technology and Higher Education.