Introduction Serotonin 5-HT1A receptors have been investigated in various CNS disorders, including epilepsy, mood disorders and neurodegeneration. [18F]Mefway (N-{2-[4-(2'-methoxyphenyl)piperazinyl]ethyl}-N-(2-pyridyl)-N-(cis/trans-4'-[18F]fluoromethylcyclohexane)-carboxamide) has been developed as a suitable positron emission tomography (PET) imaging agent for these receptors. We have now evaluated the suitability of [18F]trans-mefway in rat and mouse models using PET and computerized tomography (CT) imaging and corroborated with ex vivo and in vitro autoradiographic studies. Methods Normal Sprague-Dawley rats and Balb/C mice were used for PET/CT imaging using intravenously injected [18F]trans-mefway. Brain PET data were coregistered with rat and mouse magnetic resonance (MR) imaging template and regional distribution of radioactivity was quantitated. Select animals were used for ex vivo autoradiographic studies in order to confirm regional brain distribution and quantitative measures of binding, using brain region to cerebellum ratios. Binding affinity of trans-mefway and WAY-100635 was measured in rat brain homogenates. Distribution of [18F]trans-4-fluoromethylcyclohexane carboxylate ([18F]FMCHA), a major metabolite of [18F] trans-mefway, was assessed in the rat by PET/CT. Results The inhibition constant, Ki for trans-mefway was 0.84 nM and that for WAY-100635 was 1.07 nM. Rapid brain uptake of [18F]trans-mefway was observed in all rat brain regions and clearance from cerebellum was fast and was used as a reference region in all studies. Distribution of [18F]trans-mefway in various brain regions was consistent in PET and in vitro studies. The dorsal raphe was visualized and quantified in the rat PET but identification in the mouse was difficult. The rank order of binding to the various brain regions was hippocampus>frontal cortex>anterior cingulate cortex>lateral septal nuclei>dorsal raphe nuclei. Conclusion [18F]trans-Mefway appears to be an effective 5-HT1A receptor imaging agent in rodents for studies of various disease models.

The serotonin-1A (5-HT1A) receptor is implicated in an array of neurological and psychiatric disorders. Current PET radioligands targeting 5-HT1A receptors have limitations hindering widespread PET studies of this receptor system. The 5-HT1A specific antagonist radioligand N-{2-[4-(2-methoxyphenyl)piperazinyl]ethyl}-N-(2-pyridyl)-N-(trans-4-18F-fluoromethylcyclohexane)carboxamide (18F-mefway) exhibited promising in vivo properties in rhesus monkeys. The goal of this work was to examine the in vivo cerebral binding profile and metabolism of 18F-mefway in humans. Methods Dynamic 18F-mefway PET data were acquired for six healthy volunteers (4F, 2M; 22–38 years). Scans were initiated with the injection of 192–204 MBq radiotracer and data were acquired for two hours. Venous blood samples were collected and assayed to examine the in vivo metabolism profile of 18F-mefway. To examine the test-retest variability of 18F-mefway, a second PET scan was acquired at least two weeks later for four subjects. Regional binding potentials (BPND) were calculated with MRTM, and voxel-wise BPND maps were calculated with Logan graphical analysis. Regions surrounding the brain were carefully inspected for uptake of radiolabeled species in bone. Results 18F-Mefway uptake in the brain occurred quickly with peak SUVs of 1.7. Rapid washout in the cerebellum resulted in SUVs of 0.2 at 120 minutes, while regions with specific 5-HT1A binding exhibited retention of radioligand yielding SUVs of 0.4–0.9 at 120 minutes. Rapid metabolism of 18F-mefway was observed, with no detected 18F-fluoride ions in plasma. BPND values of 2.4 were measured in the mesial temporal lobe, with values of 1.6 in insular cortex and 0.7–1.0 in other cortical regions. Stable BPND estimates were obtained using 90 minutes of dynamic data. Average test-retest variability was 8%. No evidence of radioactivity uptake in bone was observed. Conclusion 18F-Mefway exhibits favorable in vivo properties for serotonin 5-HT1A receptor measurements in humans. The simple radiosynthesis, high specific binding profile, and absence of PET signal in bone make 18F-mefway an attractive radiotracer for PET experiments examining the 5-HT1A receptor in neuropsychiatric disorders and drug intervention.

We have developed 18F-trans-Mefway (18F-Mefway) for PET imaging studies of serotonin 5-HT1A receptors which are implicated in various brain functions. Translation of imaging the 5-HT1A receptor in animal models to humans will facilitate an understanding of the role of the receptor in human brain disorders. We report comparative brain distribution of 18F-Mefway in normal mice, rats, monkeys and healthy human volunteers. Mefway was found to be very selective with subnanomolar affinity for the serotonin 5-HT1A receptor. Affinities of >55 nM were found for all other human-cloned receptor subtypes tested. Mefway was found to be a poor substrate (>30 μM) for the multidrug resistance 1 protein, suggesting low likelihood of brain uptake being affected by P-glycoprotein. Cerebellum was used as a reference region in all imaging studies across all species due to the low levels of 18F-Mefway binding. Consistent binding of 18F-Mefway in cortical regions, hippocampus and raphe was observed across all species. 18F-Mefway in the human brain regions correlated with the known postmortem distribution of 5-HT1A receptors. Quantitation of raphe was affected by the resolution of the PET scanners in the rodents, while monkeys and humans showed a raphe to cerebellum ratio approximately 3. 18F-Mefway appears to be an effective serotonin 5-HT1A receptor imaging agent in all models including humans. 18F-Mefway therefore may be used to quantify serotonin 5-HT1A receptor distribution in brain regions for the study of various CNS disorders.

18F-Mefway (N-{2-[4-(2′-methoxyphenyl)piperazinyl]ethyl}-N-(2-pyridyl)-N-(4′-18F-fluoro-methylcyclohexane)carboxamide) was developed and evaluated for use as a PET ligand for imaging 5-HT1A receptors. Ongoing studies of 18F-Mefway have shown it to be an effective PET radiotracer. We have synthesized isomers of Mefway by changing the position of the methyl-group in attempts to evaluate stability for imaging purposes. 2-Methyl-, 3-methyl-, and 4-methyl-cyclohexane-1-carboxylic acids and 3-carbomethoxy-, 4-carbomethoxycyclohexane-1-carboxylic acids were coupled with WAY-100634 to provide the methylcyclohexyl derivatives (2-, 3- and 4-methyl). Mefway and 3-Mefway analogs were prepared by reduction of carbomethoxy-derivatives followed by fluorination. In vitro binding affinities for the methylated derivatives in rat brain homogenates was found to be 10.4 nM (2-methyl), 77 nM (3-methyl) and 21.5 nM (4-methyl). Binding affinity of 3-Mefway and 4-Mefway was found to be 17.4 nM and 6.26 nM, respectively. Our results suggest that 3-methyl/3-fluoromethyl substituent has approx. 3-fold lower affinities compared to the 4-methyl/4-fluoromethyl substituent.

The authors would like to correct an error in Fig. 4 (cyclohexyl ketone shown instead of cyclohexyl ester, in structures 11, 12, 13) of the original publication. The corrected Fig. 4 is published with this erratum.The online version of the original article can be found under