Comparison of in Silico, Electrochemical, in Vitro and in Vivo Metabolism of a Homologous Series of (Radio)fluorinated σ Receptor Ligands Designed for Positron Emission Tomography.

The imaging of σ receptors in the brain by fluorinated radiotracers will be used for the validation of σ receptors as drug targets as well as for differential diagnosis of diseases in the central nervous system. The biotransformation of four homologous fluorinated PET tracers 1'-benzyl-3-(ω-fluoromethyl to ω-fluorobutyl)-3H-spiro[2]benzofuran-1,4'-piperidine] ([ F]1-4) was investigated. In silico studies using fast metabolizer (FAME) software, electrochemical oxidations, in vitro studies with rat liver microsomes, and in vivo metabolism studies after application of the PET tracers [ F]1-4 to mice were performed. Combined liquid chromatography and mass spectrometry (HPLC-MS) analysis allowed structural identification of non-radioactive metabolites. Radio-HPLC and radio-TLC provided information about the presence of unchanged parent radiotracers and their radiometabolites. Radiometabolites were not found in the brain after application of [ F]2-4, but liver, plasma, and urine samples contained several radiometabolites. Less than 2 % of the injected dose of [ F]4 reached the brain, rendering [ F]4 less appropriate as a PET tracer than [ F]2 and [ F]3. Compounds [ F]2 and [ F]3 possess the most promising properties for imaging of σ receptors in the brain. High σ affinity (K =0.59 nm), low lipophilicity (logD =2.57), high brain penetration (4.6 % of injected dose after 30 min), and the absence of radiometabolites in the brain favor the fluoroethyl derivative [ F]2 slightly over the fluoropropyl derivative [ F]3 for human use.