Chemogenetic studies with the ligand clozapine N-oxide (CNO) are predicated upon the assumption that CNO is devoid of actions at natural neuroreceptors. However, recent evidence shows that CNO may be converted back to clozapine (CLZ) in vivo , which could yield plasma concentrations that may be sufficient to occupy inter alia dopamine D 2/3 and serotonin 5HT 2A receptors in living brain. To test this phenomenon, we measured striatal dopamine D 2/3 receptor occupancy with [ 18 F]fallypride PET and serotonin 5HT 2A occupancy ex vivo using [ 18 F]MH.MZ. We found a CNO dose-dependent effect on the availability of both neuroreceptor sites. In parallel MR spectroscopy experiments, we found that CNO reduced creatine + phosphcreatine (Cr+PCr) and increased N -acetylaspartate + N -acetylaspartylglutamate (NAA+NAAG) signals in the prefrontal cortex, and also reduced the glutamate signal in dorsal striatum, with peak effect at 2 mg/kg. Thus, our findings suggest that conversion of CNO to CLZ in living rats imparts significant occupancy at endogenous neuroreceptors and significant changes to neurometabolite levels.

Background Positron emission tomography (PET) is a molecular imaging technique that can be used to investigate the in vivo pharmacology of drugs. Initial preclinical evaluation of PET tracers is often conducted in rodents due to the accessibility of disease models as well as economic considerations. Compared to larger species, rodents display a higher expression and/or activity of efflux transporters such as the P-glycoprotein (P-gp). Low brain uptake could, therefore, be species-specific and uptake in rodents not be predictive for that in humans. We hypothesized that a better prediction from rodent data could be achieved when a tracer is evaluated under P-gp inhibition. Consequently, we compared the performance of eight neuroreceptor tracers in rats with and without P-gp inhibition including a specific binding blockade. This data set was then used to predict the binding of these eight tracers in pigs. Methods PET tracers targeting serotonin 5-HT2A receptors ([18F]MH.MZ, [18F]Altanserin, [11C]Cimbi-36, [11C]Pimavanserin), serotonin 5-HT7 receptors ([11C]Cimbi-701, [11C]Cimbi-717 and [11C]BA-10) and dopamine D2/3 receptors ([18F]Fallypride) were used in the study. The brain uptake and target-specific binding of these PET radiotracers were evaluated in rats with and without inhibition of P-gp. Rat data were subsequently compared to the results obtained in pigs. Results Without P-gp inhibition, the amount of target-specific binding in the rat brain was sufficient to justify further translation for three out of eight evaluated tracers. With P-gp inhibition, results for five out of eight tracers justified further translation. The performance in pigs could correctly be predicted for six out of eight tracers when rat data obtained under P-gp inhibition were used, compared to four out of eight tracers without P-gp inhibition. Conclusions P-gp strongly affects the uptake of PET tracers in rodents, but false prediction outcomes can be reduced by evaluating a tracer under P-gp inhibition.

The 5-HT7 receptor (5-HT7R) is involved in a broad range of physiological conditions and disorders. Currently, there is no validated clinical positron emission tomography (PET) tracer available; however, we have recently developed a promising 11C-labeled candidate. In this project, we aimed to further extend our efforts and develop an 18F-labeled derivative, coined [18F]­ENL30. Fluorine-18 has several advantages over carbon-11 especially within the preclinical phase, where a long half-life usually increases evaluation throughput. ENL30 was successfully synthesized in a low albeit sufficient overall yield. Radiolabeling succeeded with a radiochemical yield of approximately 4.5%. Subsequent preclinical PET studies revealed that [18F]­ENL30 binds specifically to the 5-HT7R but suffered from affinity to σ-receptors. Additionally, we identified [18F]­ENL30 to be a P-gp substrate in rats. However, we believe that [18F]­ENL30 may prove to be valuable in higher species that exhibit decreased P-gp dependency. If required, σ-receptor binding could, in such studies, be selectively blocked potentially allowing for selective 5-HT7R imaging.

No abstract

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.