Multi-patient Dose Synthesis of [18F]Flumazenil Via a Copper-mediated F-fluorination

Background: Flumazenil (FMZ) is a functionally silent imidazobenzodiazepine which binds to the benzodiazepine binding site of approximately 75% of the brain γ-aminobutyric acid-A receptors (GABARs). Positron Emission Tomography (PET) imaging of the GABAARs with [C]FMZ has been used to evidence alterations in neuronal density, to assess target engagement of novel pharmacological agents, and to study disorders such as epilepsy and Huntington's disease. Despite the potential of FMZ PET imaging the short half-life (t) of carbon-11 (20 min) has limited the more widespread clinical use of [C]FMZ. The fluorine-18 (F) isotopologue with a longer t (110 min) is ideally suited to address this drawback. However, the majority of current radiochemical methods for the synthesis of [F]FMZ are non-trivial and low yielding. We report a robust, automated protocol that is good manufacturing practice (GMP) compatible, and yields multi-patient doses of [F]FMZ.

Results: The fully automated synthesis was developed on the Trasis AllinOne (AIO) platform using a single-use cassette. [F]FMZ was synthesized in a one-step procedure from [F]fluoride, via a copper-mediated F-fluorination of a boronate ester precursor. Purification was performed by semi-preparative radio-HPLC and the collected fraction formulated directly into the final product vial. The overall process from start of synthesis to delivery of product is approximately 55 min. Starting with an initial activity of 23.6 ± 5.8 GBq (n = 3) activity yields of [F]FMZ were 8.0 ± 1 GBq (n = 3). The synthesis was successfully reproduced at two independent sites, where the product passed quality control release criteria in line with the European Pharmacopoeia standards and ICH Q3D(R1) guidelines to be suitable for human use.

Conclusion: Reported is a fully automated cassette-based synthesis of [F]FMZ that is Good Manufacturing Practice (GMP) compatible and produces multi-patient doses of [F]FMZ.