Development and Evaluation of Two Potential 5‑HT7 Receptor PET Tracers: [18F]ENL09 and [18F]ENL10

The latest addition to the serotonin (5-HT) receptor family is the 5-HT7 receptor (5-HT7R). This receptor has gained interest as a drug target due to its involvement in various disorders such as depression or schizophrenia. There is currently no clinically validated positron emission tomography (PET) tracer for the 5-HT7R available. But, the (arylpiperazinyl-butyl)­oxindole scaffold provides a promising lead structure for this purpose. Here, we synthesized 12 (arylpiperazinyl-butyl)­oxindole derivatives and in vitro affinity screening identified two structures with suitable affinity and selectivity to be radiolabeled and tested as 5-HT7R selective PET tracers. Next, the radiolabeled products [18F]­ENL09 and [18F]­ENL10 were evaluated as PET tracers in rats. Both tracers were found to be P-gp substrates, but after P-gp inhibition the brain uptake showed a regional distribution in line with the known 5-HT7R distribution. The [18F]­ENL10 brain binding was displaceable with a 5-HT7R selective ligand, whereas [18F]­ENL09 was not. We find that [18F]­ENL10 is a promising 5-HT7R selective PET tracer candidate that should be investigated in higher species.