Radiosynthesis and preclinical evaluation of [ C]Cimbi-701 - Towards the imaging of cerebral 5-HT receptors.

The serotonin 7 (5-HT ) receptor is suggested to be involved in a broad variety of CNS disorders, but very few in vivo tools exist to study this important target. Molecular imaging with positron emission tomography (PET) would enable an in vivo characterization of the 5-HT receptor. However, no clinical PET radiotracer exists for this receptor, and thus we aimed to develop such a tracer. In this study, we present the preclinical evaluation of [ C]Cimbi-701. Cimbi-701 was synthesized in a one-step procedure starting from SB-269970. Its selectivity profile was determined using an academic screening platform (NIMH Psychoactive Drug Screening Program). Successful radiolabeling of [ C]Cimbi-701 and subsequent in vivo evaluation was conducted in rats, pigs and baboon. In vivo specificity was investigated by 5-HT and σ receptor blocking studies. P-gp efflux transporter dependency was investigated using elacridar. [ C]Cimbi-701 could successfully be synthesized. Selectivity profiling revealed high affinity for the 5-HT (K = 18 nM), σ-1 (K = 9.2 nM) and σ-2 (K = 1.6 nM) receptors. In rats, [ C]Cimbi-701 acted as a strong P-gp substrate. After P-gp inhibition, rat brain uptake could specifically be blocked by 5-HT and σ receptor ligands. In pig, high brain uptake and specific 5-HT and σ-receptor binding was found for [ C]Cimbi-701 without P-gp inhibition. Finally, low brain uptake was found in baboons. Both the specific σ-receptor binding and the low brain uptake of [ C]Cimbi-701 displayed in baboon discouraged further translation to humans. Instead, we suggest exploration of this structural class as results indicate that selective 5-HT receptor imaging might be possible when more selective non-P-gp substrates are identified.