Neuroimaging of intracellular signalling: Development of an 18F-labelled quinoline derivative with high affinity for phosphodiesterase 5.

Objectives: The enzyme cyclic nucleotide phosphodiesterase 5 (PDE5) is involved in receptor-mediated signalling processes by regulation of the intracellular levels of the second messenger cGMP. PDE5 is peripherally located in heart, lungs and smooth muscle cells. Within the CNS it is highly expressed in the cerebellum and hippocampus. There is currently a growing interest in using inhibitors of PDE5 such as Sildenafil (Viagra®) as possible drugs in Alzheimer’s and Huntington’s Disease. Moreover, the effects of PDE5 inhibitors on the growth of different tumour cells are reported. Therefore we aimed to develop a 18F-labelled radioligand for imaging of this enzyme particularly in brain.

Methods: A quinoline scaffold (1) was used as basis for the synthesis of a small library of derivatives with fluoro-containing substituents in position 3, 4 and 8 of the quinoline core. The inhibitory activity of the compounds was determined for the human PDE5A1 and a panel of other PDEs by using an enzyme assay with sildenafil as reference. 18F-labelling of the best candidate ICF24075 was performed on an azetidine ring by using a nosylate precursor (Figure). In vitro autoradiography of [18F]ICF24075 was performed on pig brain slices. In vivo metabolism was investigated in plasma and brain samples of mice 30 min p.i. using micellar chromatography.

Figure: 18F-labelling of [18F]ICF24075

Results: The new ligand ICF24075 showed a good inhibitory activity for PDE5 (IC50 = 5.9 nM) and a high selectivity over other PDEs. 18F-labelling of the secondary carbon atom proceeded only with low labelling yields resulting in RCYs of 1.8 ± 0.3% (decay corr.) of the formulated product. In vitro binding studies demonstrated a specific blockade with sildenafil which was highest in the cerebellum. However, in vivo studies in mice revealed the formation of radiometabolites able to cross the blood-brain barrier.

Conclusions: Due to the presence of radiometabolites in the brain, [18F]ICF24075 is not suitable for specific neuroimaging of PDE5. In particular the unexpected fast metabolic degradation of the 18F-labelled azetidine ring is currently further investigated by in vitro studies to confirm our assumption of N-dealkylation.

Reference: (1) Bi, Y. et al; Bioorg. Med. Chem. Lett. 2004, 14, 1577-1580.