Synthesis of a [6-Pyridinyl-18F]-labelled fluoro derivative of WAY-100635 as a candidate radioligand for brain 5-HT1A receptor imaging with PET

Abstract

In recent years, considerable effort has been spent on the design, synthesis and pharmacological characterization of radiofluorinated derivatives of the 5-HT1A receptor antagonist, WAY-100635, for the in vivo study of these receptors in human brain with PET. (Pyridinyl-6)-fluoro- and (pyridinyl-5)-fluoro-analogues of WAY-100635 (6-fluoro and 5-fluoro-WAY-100635, 5a/6a) were synthesized as well as the corresponding chloro-, bromo- and nitro-derivatives as precursors for labelling (5b–d and 6b–d). Comparative radiolabelling of these precursors with fluorine-18 (positron-emitting isotope, 109.8min half-life) clearly demonstrated that only ortho-fluorination in this pyridine series, and not meta-fluorination, is of interest for the preparation of a radioligand by nucleophilic heteroaromatic substitution. 6-[18F]Fluoro-WAY-100635 ([18F]5a) can be efficiently synthesized in one step, either from the corresponding 6-bromo precursor (using conventional heating at 145°C for 10min) or from the corresponding 6-nitro precursor (using microwave activation at 100 W for 1min). Typically, 15–25mCi (0.55–0.92GBq) of 6-[18F]fluoro-WAY-100635 ([18F]5a, 1–2Ci/μmol or 37–72GBq/μmol) were obtained in 50–70min starting from a 100mCi (3.7GBq) aliquot of a batch of cyclotron-produced [18F]fluoride. This 18F-labelled radioligand is now being evaluated in PET studies.