GABAA Receptor Specific Pyrazolopyrimidines as Potential Imaging Agents: In Vivo Characteristics of a New 18F-labelled Indiplon Derivative

The design of radioligands selective for particular GABAA receptor isoforms is challenging because of their great potential for the diagnostic imaging of neuropsychiatric diseases such as epilepsy and sleep disorders. A subtype specificity higher than that of the classical benzodiazepines makes the novel pyrazolopyrimidine indiplon an attractive candidate structure. In the present work, N-(3-[18F]fluoro-propyl)-N-{3-[3-(thiophene-2-carbonyl)-pyrazolo[1,5-a]pyrimidine- 7-yl]-phenyl}-acetamide ([18F]fluoro-propyl-indiplon; [18F]FPI) was characterized and validated as a candidate tracer for positron emission tomography imaging of α1-GABAA receptors. In vitro, receptor affinity and autoradiography of [18F]FPI were assessed in cerebellar homogenates and sagittal brain slices of rats, respectively. The regional brain and organ uptake was investigated in NMRI mice at various times after i.v. application of [18F]FPI by ex vivo autoradiography and dissection, respectively. Radiometabolites of [18F]FPI were determined in blood, brain, and urine samples by highperformance liquid chromatography. [18F]FPI binds to cerebellar receptors with high affinity (KD 2.93 nM), and the in vitro labelling pattern appears to be more selective for α1-containing GABAA receptors than [3H]flunitrazepam. Intravenous application of [18F]FPI revealed (i) a strong biotransformation, (ii) low brain-to-plasma ratios of 18F (<0.4 at all times after tracer injection), (iii) homogenous and non-specific 18F distribution in the rat brain, and (iv) high radioactivity uptake in liver and femur. Thus, despite the promising results obtained by in vitro evaluation, [18F]FPI is excluded from further development as PET tracer.