Radiosynthesis and initial evaluation of [18F]-FEPPA for PET imaging of peripheral benzodiazepine receptors.

INTRODUCTION: A novel [18F]-radiolabelled phenoxyanilide, [18F]-FEPPA, has been synthesized and evaluated, in vitro and ex vivo, as a potential positron emission tomography imaging agent for the peripheral benzodiazepine receptor (PBR).
METHODS: [18F]-FEPPA and two other radiotracers for imaging PBR, namely [11C]-PBR28 and [11C]-PBR28-d3, were synthesised and evaluated in vitro and ex vivo as potential PBR imaging agents.
RESULTS: [18F]-FEPPA is efficiently prepared in one step from its tosylate precursor and [18F]-fluoride in high radiochemical yields and at high specific activity. FEPPA displayed a Ki of 0.07 nM for PBR in rat mitochondrial membrane preparations and a suitable lipophilicity for brain penetration (log P of 2.99 at pH 7.4). Upon intravenous injection into rats, [18F]-FEPPA showed moderate brain uptake [standard uptake value (SUV) of 0.6 at 5 min] and a slow washout (SUV of 0.35 after 60 min). Highest uptake of radioactivity was seen in the hypothalamus and olfactory bulb, regions previously reported to be enriched in PBR in rat brain. Analysis of plasma and brain extracts demonstrated that [18F]-FEPPA was rapidly metabolized, but no lipophilic metabolites were observed in either preparation and only 5% radioactive metabolites were present in brain tissue extracts. Blocking studies to determine the extent of specific binding of [18F]-FEPPA in rat brain were problematic due to large perturbations in circulating radiotracer and the lack of a reference region.
CONCLUSIONS: Further evaluation of the potential of [18F]-FEPPA will require the employment of rigorous kinetic models and/or appropriate animal models.