Radiosynthesis and biological investigation of an 18F-labeled triazolopyridopyrazine-based inhibitor for imaging of the phosphodiesterase 2A enzyme (PDE2A) in brain


Radiosynthesis and biological investigation of an 18F-labeled triazolopyridopyrazine-based inhibitor for imaging of the phosphodiesterase 2A enzyme (PDE2A) in brain

Wenzel, B.; Fritzsche, S. R.; Deuther-Conrad, W.; Toussaint, M.; Briel, D.; Kopka, K.; Brust, P.; Scheunemann, M.

Objectives: The cyclic nucleotide phosphodiesterase 2A is an intracellular enzyme which hydrolyzes the second messengers cAMP and cGMP and therefore plays an important role in signaling processes. It is highly expressed in distinct brain areas associated with emotion, memory and learning. Therefore, it is assumed that the PDE2A is involved in the pathophysiology of neurodegenerative and neuropsychiatric diseases. To enable a specific imaging of this enzyme in the brain by PET, we are developing fluorine-18 labeled radioligands with high inhibitory potency and selectivity toward other phosphodiesterases, in particular the PDE10A, which is also located in typical PDE2A-rich brain regions.

Methods: Out of a series of 12 tricyclic triazolopyridopyrazine-based derivatives with high inhibitory potency toward PDE2A and selectivity vs. PDE10A [1], the most promising candidate 1 was selected for radiofluorination. The radiolabeling was performed in a two-step one-pot procedure via nucleophilic aromatic substitution of the nitro group (precursor 2) by [18F]fluoride followed by the reduction of the activating ketone function to obtain the desired radiotracer [18F]1 (Figure 1). The product was isolated using semi-preparative HPLC followed by final purification with solid-phase extraction and formulation in isotonic saline containing 10% ethanol. In vitro autoradiography studies with cryosections of rat brain and PET studies in female Sprague-Dawley rats (30 min dynamic PET imaging after intravenous injection, nanoScan® PET/MRI, MEDISO, Budapest, Hungary) were performed. The in vivo metabolism of [18F]1 was investigated by radio-HPLC analysis of extracts obtained from blood plasma and homogenized brain of rats at 30 min p.i.

Results: The new derivative 1 inhibits PDE2A with high potency (IC50 PDE2A3 = 1.99 nM) and superior selectivity against PDE10A (IC50 PDE10A1 = 1910 nM). [18F]1 was obtained with a radiochemical yield of 2.1 ± 0.7% (EOB), molar activities of 10–20 GBq/µmol (EOS) and radiochemical purities of ≥ 99% (n=7). The distribution pattern of [18F]1 detected by in vitro autoradiography corresponds to the anatomical distribution of PDE2A in rat brain with prominent expression in the superficial layers of the somatosensory cortex, in hippocampal structures, and in basal ganglia, and with nearly no expression in the cerebellum. By co-incubation with compound 1 as well as two structurally different well-established PDE2A inhibitors, the binding of [18F]1 was completely inhibited, confirming the high specific interaction with PDE2A. In vivo PET-MR studies revealed a good brain uptake (SUV peak = 1.0); however, a considerable enrichment in PDE2A-specific regions was not detectable (15 min p.i.: SUVcaudate putamen = 0.51 vs. SUVcerebellum = 0.40). Furthermore, in vivo metabolism studies of [18F]1 revealed unneglectable fractions of radiometabolites in blood plasma and brain at 30 min p.i. (parent compound: 50 and 66%, resp.).

Conclusions: [18F]1 is a suitable and high specific probe for the investigation of the PDE2A expression in vitro. However, further work is needed to explore the reasons for the poor in vivo imaging capability of this radiotracer.

Acknowledgment: Deutsche Forschungsgemeinschaft (German Research Foundation, SCHE 1825/3-1).

References: [1] S. R. Fritzsche et al. „Triazolopyridopyrazine-based Inhibitors of Phosphodiesterase 2A – Synthesis and SAR-Exploration“, Annual Meeting of the German Pharmaceutical Society – DPhG 2021, Poster

  • Poster
    24th International Symposium on Radiopharmaceutical Sciences, 29.05.-02.06.2022, Nantes, Frankreich

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Publ.-Id: 33456