In vivo kinetics of [F-18]MEFWAY: a comparison with [C-11]WAY100635 and [F-18]MPPF in the nonhuman primate

Evidence For Cannabidiol Modulation of Serotonergic Transmission in a Model of Osteoarthritis via in vivo PET Imaging and Behavioral Assessment

Background

Preclinical studies indicate that cannabidiol (CBD), the primary nonaddictive component of cannabis, has a wide range of reported pharmacological effects such as analgesic and anxiolytic actions; however, the exact mechanisms of action for these effects have not been examined in chronic osteoarthritis (OA). Similar to other chronic pain syndromes, OA pain can have a significant affective component characterized by mood changes. Serotonin (5-HT) is a neurotransmitter implicated in pain, depression, and anxiety. Pain is often in comorbidity with mood and anxiety disorders in patients with OA. Since primary actions of CBD are analgesic and anxiolytic, in this first in vivo positron emission tomography (PET) imaging study, we investigate the interaction of CBD with serotonin 5-HT1A receptor via a combination of in vivo neuroimaging and behavioral studies in a well-validated OA animal model.

Methods

The first aim of this study was to evaluate the target involvement, including the evaluation of modulation by acute administration of CBD, or a specific target antagonist/agonist intervention, in control animals. The brain 5-HT1A activity/availability was assessed via in vivo dynamic PET imaging (up to 60 min) using a selective 5-HT1A radioligand ([18F]MeFWAY). Tracer bindings of 17 ROIs were evaluated based on averaged SUVR values over the last 10 min using CB as the reference region. We subsequently examined the neurochemical and behavioral alterations in OA animals (induction with monosodium iodoacetate (MIA) injection), as compared to control animals, via neuroimaging and behavioral assessment. Further, we examined the effects of repeated low-dose CBD treatment on mechanical allodynia (von Frey tests) and anxiety-like (light/dark box tests, L/D), depressive-like (forced swim tests, FST) behaviors in OA animals, as compared to after vehicle treatment.

Results

The tracer binding was significantly reduced in control animals after an acute dose of CBD administered intravenously (1.0 mg/kg, i.v.), as compared to that for baseline. This binding specificity to 5-HT1A was further confirmed by a similar reduction of tracer binding when a specific 5-HT1A antagonist WAY1006235 was used (0.3 mg/kg, i.v.). Mice subjected to the MIA-induced OA for 13-20 days showed a decreased 5-HT1A tracer binding (25% to 41%), consistent with the notion that 5-HT1A plays a role in the modulation of pain in OA. Repeated treatment with CBD administered subcutaneously (5 mg/kg/day, s.c., for 16 days after OA induction) increased 5-HT1A tracer binding, while no significant improvement was observed after vehicle. A trend of increased anxiety or depressive-like behavior in the light/dark box or forced swim tests after OA induction, and a decrease in those behaviors after repeated low-dose CBD treatment, are consistent with the anxiolytic action of CBD through 5HT1A receptor activation. There appeared to be a sex difference: females seem to be less responsive at the baseline towards pain stimuli, while being more sensitive to CBD treatment.

Conclusion

This first in vivo PET imaging study in an OA animal model has provided evidence for the interaction of CBD with the serotonin 5-HT1A receptor. Behavioral studies with more pharmacological interventions to support the target involvement are needed to further confirm these critical findings.

Synthesis, in vitro and in vivo evaluation of 11C-O-methylated arylpiperazines as potential serotonin 1A (5-HT1A) receptor antagonist radiotracers

Background

Serotonin 1A (5-HT_1A) receptors are implicated in the pathogenesis of several psychiatric and neurodegenerative disorders motivating the development of suitable radiotracers for in vivo positron emission tomography (PET) neuroimaging. The gold standard PET imaging agent for this target is [carbonyl-¹¹C]WAY-100635, labeled via a technically challenging multi-step reaction that has limited its widespread use. While several antagonist and agonist-based PET radiotracers for 5-HT _1A receptors have been developed, their clinical translation has been hindered by methodological challenges and/or and non-specific binding. As a result, there is continued interest in the development of new and more selective 5-HT_1A PET tracers having a relatively easier and reliable radiosynthesis process for routine production and with favorable metabolism to facilitate tracer-kinetic modeling. The purpose of the current study was to develop and characterize a radioligand with suitable characteristics for imaging 5-HT_1A receptors in the brain. The current study reports the in vitro characterization and radiosyntheses of three candidate 5-HT_1A receptor antagonists, DF-100 (1), DF-300 (2) and DF-400 (3), to explore their suitability as potential PET radiotracers.

Results

Syntheses of 1–3 and corresponding precursors for radiolabeling were achieved from isonicotinic, picolinic acid or picolino nitrile. In vitro binding studies demonstrated nanomolar affinity of the compounds for 5-HT_1A receptors. Binding of 1–3 for other biogenic amines, neurotransmitter receptors, and transporters was negligible with the exception of moderate affinities for α₁-adrenergic receptors (4–6-fold less potent than that for 5-HT_1A receptor). Radioligands [¹¹C]1–3 were efficiently prepared by ¹¹C-O-methylation of the corresponding phenolic precursor in non-decay corrected radiochemical yields of 7–11% with > 99% chemical and radiochemical purities. Dynamic PET studies in rats demonstrated negligible brain uptake of [¹¹C]1 and [¹¹C]2. In contrast, significant brain uptake of [¹¹C]3 was observed with an early peak SUV of 4–5. However, [¹¹C]3 displayed significant off-target binding attributed to α₁-adrenergic receptors based on regional distribution (thalamus>hippocampus) and blocking studies.

Conclusion

Despite efficient radiolabeling, results from PET imaging experiments limit the application of [¹¹C]3 for in vivo quantification of 5-HT_1A receptors. Nevertheless, derivatives of compound 3 may provide a scaffold for alternative PET radiotracers with improved selectivity for 5-HT _1A receptors or α₁-adrenergic receptors.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.

P-Glycoprotein, not BCRP, Limits the Brain Uptake of [(18)F]Mefway in Rodent Brain

PURPOSE

The aim of this study was to determine whether the brain uptake of [(18)F]Mefway is influenced by the action of P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) in rodents.

PROCEDURES

[(18)F]Mefway was applied to rats pharmacologically inhibited with tariquidar (TQD) and to genetically disrupted mice.

RESULTS

Pretreatment of TQD results in 160% higher hippocampal uptake compared with control rats. In genetically disrupted mice, a maximal brain uptake value of 3.2 SUV in the triple knockout mice (tKO, Mdr1a/b((-/-))Bcrp1((-/-))) was comparable to that of the double knockout mice (dKO, Mdr1a/b((-/-))) and 2-fold those of the wild-type and Bcrp1((-/-)) knockout mice. The differences of binding values were statistically insignificant between control and experimental groups. The brain-to-plasma ratios for tKO mice were also two to five times higher than those for other groups.

CONCLUSIONS

[(18)F]Mefway is modulated by P-gp, and not by Bcrp in rodents.

Marmoset Serotonin 5-HT1A Receptor Mapping with a Biased Agonist PET Probe 18F-F13714: Comparison with an Antagonist Tracer 18F-MPPF in Awake and Anesthetized States

BACKGROUND

In vivo mapping by positron emission tomography of the serotonin 1A receptors has been hindered by the lack of suitable agonist positron emission tomography probes. ¹⁸F-labeled F13714 is a recently developed biased agonist positron emission tomography probe that preferentially targets subpopulations of serotonin 1A receptors in their "active state," but its brain labeling pattern in nonhuman primate has not been described. In addition, a potential confound in the translatability of PET data between nonhuman animal and human arise from the use of anesthetics that may modify the binding profiles of target receptors.

METHODS

Positron emission tomography scans were conducted in a cohort of common marmosets (n=4) using the serotonin 1A receptor biased agonist radiotracer, ¹⁸F-F13714, compared with a well-characterized ¹⁸F-labeled antagonist radiotracer, ¹⁸F-MPPF. Experiments on each animal were performed under both consciousness and isoflurane-anesthesia conditions.

RESULTS

¹⁸F-F13714 binding distribution in marmosets by positron emission tomography differs markedly from that of the ¹⁸F-MPPF. Whereas ¹⁸F-MPPF showed highest binding in hippocampus and amygdala, ¹⁸F-F13714 showed highest labeling in other regions, including insular and cingulate cortex, thalamus, raphe, caudate nucleus, and putamen. The binding potential values of ¹⁸F-F13714 were about one-third of those observed with ¹⁸F-MPPF, with marked individual- and region-specific differences under isoflurane-anesthetized vs conscious conditions.

CONCLUSIONS

These findings highlight the importance of investigating the brain imaging of serotonin 1A receptors using agonist probes such as ¹⁸F-F13714, which may preferentially target subpopulations of serotonin 1A receptors in specific brain regions of nonhuman primate as a biased agonist.

In Vivo Monitoring of Sevoflurane-induced Adverse Effects in Neonatal Nonhuman Primates Using Small-animal Positron Emission Tomography

Background

Animals exposed to sevoflurane during development sustain neuronal cell death in their developing brains. In vivo micro-positron emission tomography (PET)/computed tomography imaging has been utilized as a minimally invasive method to detect anesthetic-induced neuronal adverse effects in animal studies.

Methods

Neonatal rhesus monkeys (postnatal day 5 or 6, 3 to 6 per group) were exposed for 8 h to 2.5% sevoflurane with or without acetyl-l-carnitine (ALC). Control monkeys were exposed to room air with or without ALC. Physiologic status was monitored throughout exposures. Depth of anesthesia was monitored using quantitative electroencephalography. After the exposure, microPET/computed tomography scans using 18F-labeled fluoroethoxybenzyl-N-(4-phenoxypyridin-3-yl) acetamide (FEPPA) were performed repeatedly on day 1, 1 and 3 weeks, and 2 and 6 months after exposure.

Results

Critical physiologic metrics in neonatal monkeys remained within the normal range during anesthetic exposures. The uptake of [18F]-FEPPA in the frontal and temporal lobes was increased significantly 1 day or 1 week after exposure, respectively. Analyses of microPET images recorded 1 day after exposure showed that sevoflurane exposure increased [18F]-FEPPA uptake in the frontal lobe from 0.927 ± 0.04 to 1.146 ± 0.04, and in the temporal lobe from 0.859 ± 0.05 to 1.046 ± 0.04 (mean ± SE, P < 0.05). Coadministration of ALC effectively blocked the increase in FEPPA uptake. Sevoflurane-induced adverse effects were confirmed by histopathologic evidence as well.

Conclusions

Sevoflurane-induced general anesthesia during development increases glial activation, which may serve as a surrogate for neurotoxicity in the nonhuman primate brain. ALC is a potential protective agent against some of the adverse effects associated with such exposures.

Development of a Fluorinated Class-I HDAC Radiotracer Reveals Key Chemical Determinants of Brain Penetrance

Despite major efforts, our knowledge about many brain diseases remains remarkably limited. Epigenetic dysregulation has been one of the few leads toward identifying the causes and potential treatments of psychiatric disease over the past decade. A new positron emission tomography radiotracer, [(11)C]Martinostat, has enabled the study of histone deacetylase in living human subjects. A unique property of [(11)C]Martinostat is its profound brain penetrance, a feature that is challenging to engineer intentionally. In order to understand determining factors for the high brain-uptake of Martinostat, a series of compounds was evaluated in rodents and nonhuman primates. The study revealed the major structural contributors to brain uptake, as well as a more clinically relevant fluorinated HDAC radiotracer with comparable behavior to Martinostat, yet longer half-life.

Comparative assessment of (18) F-Mefway as a serotonin 5-HT1A receptor PET imaging agent across species: Rodents, nonhuman primates, and humans

We have developed (18) F-trans-Mefway ((18) F-Mefway) for positron emission tomography (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 (18) F-Mefway in normal mice, rats, monkeys, and healthy human volunteers. Mefway was found to be very selective, with subnanomolar affinity for the 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 (18) F-Mefway binding. Consistent binding of (18) F-Mefway in cortical regions, hippocampus, and raphe was observed across all species. (18) F-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 rodents, whereas monkeys and humans showed a raphe to cerebellum ratio of approximately 3. (18) F-Mefway appears to be an effective 5-HT1A receptor imaging agent in all models, including humans. (18) F-Mefway therefore may be used to quantify 5-HT1A receptor distribution in brain regions for the study of various CNS disorders. J. Comp. Neurol. 524:1457-1471, 2016. © 2015 Wiley Periodicals, Inc.

Optimal timing of [¹⁸F]Mefway PET for imaging the serotonin 1A receptor in healthy male subjects

To determine the optimal acquisition time of [(18)F]Mefway PET, we examined the regional specific-to-nonspecific binding ratios and evaluated the relationship between distribution volume ratios (DVRs) and standardized uptake value ratios (SUVRs) in various time windows. The specific-to-nonspecific binding ratios peaked after 40 min and there was a strong correlation between DVR and SUVR in the 60-80 min. Therefore, we recommend the use of a single time point between 60 and 80 min for [(18)F]Mefway static PET.

Methods to Increase the Metabolic Stability of (18)F-Radiotracers

The majority of pharmaceuticals and other organic compounds incorporating radiotracers that are considered foreign to the body undergo metabolic changes in vivo. Metabolic degradation of these drugs is commonly caused by a system of enzymes of low substrate specificity requirement, which is present mainly in the liver, but drug metabolism may also take place in the kidneys or other organs. Thus, radiotracers and all other pharmaceuticals are faced with enormous challenges to maintain their stability in vivo highlighting the importance of their structure. Often in practice, such biologically active molecules exhibit these properties in vitro, but fail during in vivo studies due to obtaining an increased metabolism within minutes. Many pharmacologically and biologically interesting compounds never see application due to their lack of stability. One of the most important issues of radiotracers development based on fluorine-18 is the stability in vitro and in vivo. Sometimes, the metabolism of ¹⁸F-radiotracers goes along with the cleavage of the C-F bond and with the rejection of [¹⁸F]fluoride mostly combined with high background and accumulation in the skeleton. This review deals with the impact of radiodefluorination and with approaches to stabilize the C-F bond to avoid the cleavage between fluorine and carbon.

18F-Mefway PET imaging of serotonin 1A receptors in humans: a comparison with 18F-FCWAY

Introduction

The purpose of this research is to evaluate the prospects for the use of 4-(trans-¹⁸F-fluoranylmethyl)-N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexane-1-carboxamide (¹⁸F-Mefway) in comparison to ¹⁸F-trans-4-fluoro-N-2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide (¹⁸F-FCWAY) for the quantification of 5-HT_1A receptors in human subjects.

Method

Five healthy male controls were included for two positron emission tomography (PET) studies: ¹⁸F-FCWAY PET after the pretreatment with 500 mg of disulfiram and two months later, ¹⁸F-Mefway PET without disulfiram. Regional time-activity curves (TACs) were extracted from nine cortical and subcortical regions in dynamic PET images. Using cerebellar cortex without vermis as reference tissue, in vivo kinetics for both radioligands were compared based on the distribution volume ratio (DVR) calculated by non-invasive Logan graphical analysis and area under the curve ratio of the TACs (AUC ratio).

Result

Although the pattern of regional uptakes in the ¹⁸F-Mefway PET was similar to that of the ¹⁸F-FCWAY PET (highest in the hippocampus and lowest in the cerebellar cortex), the amount of regional uptake in ¹⁸F-Mefway PET was almost half of that in ¹⁸F-FCWAY PET. The skull uptake in ¹⁸F-Mefway PET was only 25% of that in ¹⁸F-FCWAY PET with disulfiram pretreatment. The regional DVR values and AUC ratio values for ¹⁸F-Mefway were 17—40% lower than those of ¹⁸F-FCWAY. In contrast to a small overestimation of DVR values by AUC ratio values (< 10%) in ¹⁸F-FCWAY PET, the overestimation bias of AUC ratio values was much higher (up to 21%) in ¹⁸F-Mefway PET.

Conclusion

As ¹⁸F-Mefway showed lower DVR values and greater overestimation bias of AUC ratio values, ¹⁸F-Mefway may appear less favorable than ¹⁸F-FCWAY. However, in contrast to ¹⁸F-FCWAY, the resistance to in vivo defluorination of ¹⁸F-Mefway obviates the need for the use of a defluorination inhibitor. Thus, ¹⁸F-Mefway may be a good candidate PET radioligand for 5-HT_1A receptor imaging in human.

Synthesis and Evaluation of Mefway Analogs as Ligands for Serotonin 5HT1A Receptors

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-HT_1A 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.

First-in-human evaluation of 18F-mefway, a PET radioligand specific to serotonin-1A receptors

The serotonin-1A (5-HT1A; 5-HT is 5-hydroxytryptamine) receptor is implicated in an array of neurologic 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-(18)F-fluoromethylcyclohexane)carboxamide ((18)F-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 (18)F-mefway in humans.

METHODS

Dynamic (18)F-mefway PET data were acquired for 6 healthy volunteers (4 women, 2 men; age, 22-38 y). Scans were initiated with the injection of 192-204 MBq of radiotracer, and data were acquired for 2 h. Venous blood samples were collected and assayed to examine the in vivo metabolism profile of (18)F-mefway. To examine the test-retest variability of (18)F-mefway, a second PET scan was acquired at least 2 wk later for 4 subjects. Regional binding potentials (BPNDs) were calculated with the multilinear reference tissue model, and voxelwise BPND maps were calculated with Logan graphical analysis. Regions surrounding the brain were carefully inspected for uptake of radiolabeled species in bone.

RESULTS

(18)F-mefway uptake in the brain occurred quickly, with a peak standardized uptake value (SUV) of 1.7. Rapid washout in the cerebellum resulted in SUVs of 0.2 at 120 min, whereas regions with specific 5-HT1A binding exhibited retention of radioligand, yielding SUVs of 0.4-0.9 at 120 min. Rapid metabolism of (18)F-mefway was observed, with no detected (18)F-fluoride ions in plasma. BPND values of 2.4 were measured in the mesial temporal lobe, with values of 1.6 in the insular cortex and 0.7-1.0 in other cortical regions. Stable BPND estimates were obtained using 90 min of dynamic data. Average test-retest variability was 8%. No evidence of radioactivity uptake in bone was observed.

CONCLUSION

(18)F-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 (18)F-mefway an attractive radiotracer for PET experiments examining the 5-HT1A receptor in neuropsychiatric disorders and drug intervention.

The effects of chronic alcohol self-administration on serotonin-1A receptor binding in nonhuman primates

BACKGROUND

Previous studies have found interrelationships between the serotonin system and alcohol self-administration. The goal of this work was to directly observe in vivo effects of chronic ethanol self-administration on serotonin 5-HT1A receptor binding with [(18)F]mefway PET neuroimaging in rhesus monkeys. Subjects were first imaged alcohol-naïve and again during chronic ethanol self-administration to quantify changes in 5-HT1A receptor binding.

METHODS

Fourteen rhesus monkey subjects (10.7-12.8 years) underwent baseline [(18)F]mefway PET scans prior to alcohol exposure. Subjects then drank gradually increasing ethanol doses over four months as an induction period, immediately followed by at least nine months ad libidum ethanol access. A post [(18)F]mefway PET scan was acquired during the final three months of ad libidum ethanol self-administration. 5-HT1A receptor binding was assayed with binding potential (BPND) using the cerebellum as a reference region. Changes in 5-HT1A binding during chronic ethanol self-administration were examined. Relationships of binding metrics with daily ethanol self-administration were also assessed.

RESULTS

Widespread increases in 5-HT1A binding were observed during chronic ethanol self-administration, independent of the amount of ethanol consumed. A positive correlation between 5-HT1A binding in the raphe nuclei and average daily ethanol self-administration was also observed, indicating that baseline 5-HT1A binding in this region predicted drinking levels.

CONCLUSIONS

The increase in 5-HT1A binding levels during chronic ethanol self-administration demonstrates an important modulation of the serotonin system due to chronic alcohol exposure. Furthermore, the correlation between 5-HT1A binding in the raphe nuclei and daily ethanol self-administration indicates a relationship between the serotonin system and alcohol self-administration.

Initial in vivo PET imaging of 5-HT1A receptors with 3-[(18)F]mefway

4-trans-[(18)F]Mefway is a PET radiotracer with high affinity for 5-HT1A receptors. Our preliminary work indicated the positional isomer, 3-[(18)F]mefway, would be suitable for PET imaging of 5-HT1A receptors. We now compare the in vivo behaviour of 3-mefway with 4-mefway to evaluate 3-[(18)F]mefway as a potential 5-HT1A PET radiotracer. Two male rhesus macaques were given bolus injections of both 3- and 4-trans-[(18)F]mefway in separate experiments. 90 minute dynamic PET scans were acquired. TACs were extracted in the mesial temporal lobe (MTL) and caudal anterior cingulate gyrus (cACg). The cerebellum (CB) was used as a reference region. In vivo behavior of the radiotracers in the CB was compared based upon the ratio of normalized PET uptake for 3- and 4-trans-[(18)F]mefway. Specific binding was compared by examining MTL/CB and cACg/CB ratios. The subject-averaged ratio of 3-[(18)F]mefway to 4-trans-[(18)F]mefway in the cerebellum was 0.96 for 60-90 minutes. MTL/CB reached plateaus of ~2.7 and ~6 by 40 minutes and 90 minutes for 3- and 4-trans-[(18)F]mefway, respectively. cACg/CB reached plateaus of ~2.5 and ~6 by 40 minutes and 70 minutes for 3- and 4-trans-[(18)F]mefway, respectively. The short pseudoequilibration times and sufficient uptake of 3-[(18)F]mefway may be useful in studies requiring short scan times. Furthermore, the similar nondisplaceable clearance in the CB to 4-trans-[(18)F]mefway suggests the lower BPND of 3-[(18)F]mefway is due to a lower affinity. The lower affinity of 3-[(18)F]mefway may make it useful for measuring changes in endogenous 5-HT levels, however, this remains to be ascertained.

N-(4-[(18)F]-fluoropyridin-2-yl)-N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}carboxamides as analogs of WAY100635. New PET tracers of serotonin 5-HT(1A) receptors

N-(4-[(18)F]-Fluoropyridin-2-yl)-N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-carboxamides were prepared by labeling their 4-nitropyridin-2-yl precursors through nitro substitution by the (18)F anion. In vitro and in vivo tests showed that the cyclohexanecarboxamide derivative is a reversible, selective and high affinity 5-HT1A receptor antagonist (IC50 = 0.29 nM, ki = 0.18 nM) with high brain uptake, slow brain clearance and stability to defluorination when compared with conventional standards. This PET radioligand is a promising candidate for an improved in vivo quantification of 5-HT1A receptors in neuropsychiatric disorders.

Translational possibility of [18 F]Mefway to image serotonin 1A receptors in humans: Comparison with [18 F]FCWAY in rodents

PURPOSE

To compare the cerebral uptake and binding potential of [¹⁸ F]FCWAY and [¹⁸ F]Mefway in the rodent to assess their potential for imaging serotonin 1A (5-HT_1A ) receptors.

MATERIALS AND METHODS

In vitro liver microsomal studies were performed to evaluate the degree of defluorination. Dynamic positron emission tomography (PET) studies were then conducted for 2 h with or without an anti-defluorination agent. The regions of interest were the hippocampus and frontal cortex (5-HT_1A target regions) and the cerebellum (5-HT_1A nontarget region). The in vivo kinetics of the radioligands were compared based on the brain uptake values and target-to-nontarget ratio. We also performed a comparison of binding potential (BP_ND ) as a steady-state binding parameter. Finally, binding affinities to 5-HT_1A receptors were assessed in Chinese hamster ovary cells (CHO-K1) cells expressing human recombinant 5-HT_1A receptors.

RESULTS

The radiochemical yield of [¹⁸ F]Mefway was slightly higher than that of [¹⁸ F]FCWAY (19 vs. 15%). With regard to metabolic stability against defluorination, both compounds exhibited similar stability in rat liver microsomes, but [¹⁸ F]Mefway displayed higher stability in the human microsome (defluorination ratio at 30 min: 32 vs. 29 in rat liver microsomes, 31 vs. 64 in human liver microsomes for [¹⁸ F]Mefway and [¹⁸ F]FCWAY, respectively). There were no significant differences in brain uptake, the target-to-nontarget ratios, and the BP_ND (at hippocampus, peak brain uptakes: 6.9 vs. 8.5, target-to-nontarget ratios: 6.9 vs. 8.5, BP_ND : 5.2 vs. 6.2 for [¹⁸ F]Mefway and [¹⁸ F]FCWAY). The binding affinity of [¹⁸ F]Mefway was considerably higher than that of [¹⁸ F]FCWAY (IC₅₀ : 1.5 nM vs. 2.2 nM).

CONCLUSION

[¹⁸ F]Mefway exhibits favorable characteristics compared to [¹⁸ F]FCWAY in rodents, and may be a promising radioligand for use in human subjects. Synapse 68:595-603, 2014. © 2014 Wiley Periodicals, Inc.

Acute physical stress induces the alteration of the serotonin 1A receptor density in the hippocampus

Stress affects the serotonergic system, which is associated with depression. Previous research has showed that chronic stress causes the deactivation of the limbic system. However, the influence of the acute physical stress on the serotonergic system in vivo was primarily unclear. The purpose of this research is to elucidate the effects of the acute physical stress in vivo using PET. For quantification of the 5-HT1A receptors in the brain, we measured [(18)F]Mefway uptake in the two experiment groups (control and despair rats). The despair group was subjected to the external stressful situation (i.e., forced swimming) and total duration time of immobility, refers to the despair severity, and was analyzed. In the intercomparison experiment, the resulting PET images of [(18)F]Mefway in the despair rat displayed a significant reduction of radioactivity in the hippocampus (HP) compared with the control. The nondisplaceable binding potential (BPND ) refers to the ratio of the concentration of radioligand in the receptor-rich region (i.e., HP) to the concentration of that in the receptor-free region (i.e., cerebellum). The hippocampal uptake and the BPND in the despair group were respectively about 25 and 18% lower than those of the control group. The ratio of specific binding to nonspecific binding in the despair group was 18% lower than that of the control. In the intracomparison experiments, the BPND and immobility in the despair group showed a strong negative correlation. Taken together, the data illustrates that an acute physical stress induces the change in the serotonergic system that correlates with the behavioral despair.

Evaluation of [18F]Mefway biodistribution and dosimetry based on whole-body PET imaging of mice

PURPOSE

[(18)F]Mefway is a novel radiotracer specific to the serotonin 5-HT1A receptor class. In preparation for using this tracer in humans, we have performed whole-body PET studies in mice to evaluate the biodistribution and dosimetry of [(18)F]Mefway.

METHODS

Six mice (three females and three males) received IV injections of [(18)F]Mefway and were scanned for 2 h in an Inveon-dedicated PET scanner. Each animal also received a high-resolution CT scan using an Inveon CT. The CT images were used to draw volume of interest on the following organs: the brain, large intestine, stomach, heart, kidneys, liver, lungs, pancreas, bone, spleen, testes, thymus, gallbladder, uterus, and urinary bladder. All organ time-activity curves without decay correction were normalized to the injected activity. The area under the normalized curves was then used to compute the residence times in each organ. Data were analyzed using PMOD and Matlab software. The absorbed doses in mouse organs were computed using the RAdiation Dose Assessment Resource animal models for dose assessment. The residence times in mouse organs were converted to human values using scale factors based on differences between organ and body weights. OLINDA/EXM 1.1 software was used to compute the absorbed human doses in multiple organs for both female and male phantoms.

RESULTS

The highest mouse residence times were found in the liver, urinary bladder, and kidneys. The largest doses in mice were found in the urinary bladder (critical organ), kidney, and liver for both females and males, indicating primary elimination via urinary system. The projected human effective doses were 1.21E - 02 mSv/MBq for the adult female model and 1.13E - 02 mSv/MBq for the adult male model. The estimated human biodistribution of [(18)F]Mefway was similar to that of [(11)C]WAY 100,635, a 5-HT1A tracer for which dosimetry has been evaluated in humans.

CONCLUSIONS

The elimination of radiotracer was primarily via the kidney and urinary bladder with the urinary bladder being the critical organ. Whole-body mouse imaging can be used as a preclinical tool to provide initial estimates of the absorbed doses of [(18)F]Mefway in humans.

5-HT1A sex based differences in Bmax, in vivo KD, and BPND in the nonhuman primate

Serotonin (5-HT) dysfunction has been implicated in neuropsychiatric illnesses and may play a pivotal role in the differential prevalence of depression between the sexes. Previous PET studies have revealed sex-based differences in 5-HT1A binding potential (BPND). The binding potential is a function of the radioligand-receptor affinity (1/KDapp), and receptor density (Bmax). In this work, we use a multiple-injection (MI) PET protocol and the 5-HT1A receptor antagonist, [(18)F]mefway, to compare sex-based differences of in vivo affinity, Bmax, and BPND in rhesus monkeys.

METHODS

PET [(18)F]mefway studies were performed on 17 (6m, 11f) rhesus monkeys using a 3-injection protocol that included partial saturation injections of mefway. Compartmental modeling was performed using a model to account for non-tracer doses of mefway for the estimation of KDapp and Bmax. BPND estimates were also acquired from the first injection (high specific activity [(18)F]mefway, 90-minute duration) for comparison using the cerebellum (CB) as a reference region. Regions of interest were selected in 5-HT1A binding regions of the hippocampus (Hp), dorsal anterior cingulate cortex (dACC), amygdala (Am), and raphe nuclei (RN).

RESULTS

Female subjects displayed significantly (*p<0.05) lower KDapp in the Hp (-32%), Am (-38%), and RN (-37%). Only the Hp displayed significant differences in Bmax with females having a Bmax of -29% compared to males. Male subjects demonstrated significantly lower BPND measurements in the Am (14%) and RN (29%).

CONCLUSION

These results suggest that the higher BPND values found in females are the result of lower [(18)F]mefway KDapp. Although a more experimentally complex measurement, separate assay of KDapp and Bmax provides a more sensitive measure than BPND to identify the underlying differences between females and males in 5-HT1A function.

Evaluation of serotonin 5-HT(1A) receptors in rodent models using [¹⁸F]mefway PET

INTRODUCTION

Serotonin 5-HT(1A) 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 imaging template and regional distribution of radioactivity was quantitated. Selected animals were used for ex vivo autoradiographic studies 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, K(i) 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-HT(1A) receptor imaging agent in rodents for studies of various disease models.

Serotonin transporter genotype affects serotonin 5-HT1A binding in primates

Disruption of the serotonin system has been implicated in anxiety and depression and a related genetic variation has been identified that may predispose individuals for these illnesses. The relationship of a functional variation of the serotonin transporter promoter gene (5-HTTLPR) on serotonin transporter binding using in vivo imaging techniques have yielded inconsistent findings when comparing variants for short (s) and long (l) alleles. However, a significant 5-HTTLPR effect on receptor binding at the 5-HT(1A) receptor site has been reported in humans, suggesting the 5-HTTLPR polymorphism may play a role in serotonin (5-HT) function. Rhesus monkeys possess a 5-HTTLPR length polymorphism similar to humans and serve as an excellent model for studying the effects of this orthologous genetic variation on behaviors and neurochemical functions related to the 5-HT system. In this study, PET imaging of [(18)F]mefway was performed on 58 rhesus monkeys (33 l/l, 25 s-carriers) to examine the relation between 5-HT(1A) receptor-specific binding and 5-HTTLPR genotypes. Significantly lower 5-HT(1A) binding was found in s-carrier subjects throughout both cortical brain regions and the raphe nuclei. These results demonstrate that the underlying 5-HT neurochemical system is influenced by this functional polymorphism and illustrate the strong potential for extending the nonhuman primate model into investigating the role of this genetic variant on behavior and gene-environment interactions.

MicroPET/CT Imaging of [18F]-FEPPA in the Nonhuman Primate: A Potential Biomarker of Pathogenic Processes Associated with Anesthetic-Induced Neurotoxicity

Background. The inhalation anesthetics nitrous oxide (N2O) and isoflurane (ISO) are used in surgical procedures for human infants. Injury to the central nervous system is often accompanied by localization of activated microglia or astrocytosis at the site of injury. The tracer that targets to the peripheral benzodiazepine receptor (PBR), [18F]N-2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide ([18F]-FEPPA), has been reported as a sensitive biomarker for the detection of neuronal damage/inflammation. Methods. On postnatal day (PND) 5 or 6 rhesus monkey neonates were exposed to a mixture of N2O/oxygen and ISO for 8 hours and control monkeys were exposed to room air. MicroPET/CT images with [18F]-FEPPA were obtained for each monkey 1 day, one week, three weeks, and 6 months after the anesthetic exposure. Results. The radiotracer quickly distributed into the brains of both treated and control monkeys on all scan days. One day after anesthetic exposure, the uptake of [18F]-FEPPA was significantly increased in the temporal lobe. One week after exposure, the uptake of [18F]-FEPPA in the frontal lobe of treated animals was significantly greater than that in controls. Conclusions. These findings suggest that microPET imaging is capable of dynamic detection of inhaled anesthetic-induced brain damage in different brain regions of the nonhuman primate.

Measurement of 5-HT(1A) receptor density and in-vivo binding parameters of [(18)F]mefway in the nonhuman primate

The goal of this work was to characterize the in-vivo behavior of [(18)F]mefway as a suitable positron emission tomography (PET) radiotracer for the assay of 5-hydroxytryptamine(1A) (5-HT(1A)) receptor density (B(max)). Six rhesus monkeys were studied using a multiple-injection (M-I) protocol consisting of three sequential bolus injections of [(18)F]mefway. Injection times and amounts of unlabeled mefway were optimized for the precise measurement of B(max) and specific binding parameters k(off) and k(on) for estimation of apparent K(D). The PET time series were acquired for 180 minutes with arterial sampling performed throughout. Compartmental analysis using the arterial input function was performed to obtain estimates for K(1), k(2), k(off), B(max), and K(Dapp) in the cerebral cortex and raphe nuclei (RN) using a model that accounted for nontracer doses of mefway. Averaged over subjects, highest binding was seen in the mesial temporal and dorsal anterior cingulate cortices with B(max) values of 42±8 and 36±8 pmol/mL, respectively, and lower values in the superior temporal cortex, RN, and parietal cortex of 24±4, 19±4, and 13±2 pmol/mL, respectively. The K(Dapp) of mefway for the 5-HT(1A) receptor sites was 4.3±1.3 nmol/L. In conclusion, these results show that M-I [(18)F]mefway PET experiments can be used for the in-vivo measurement of 5-HT(1A) receptor density.

Discrepancies in the P-glycoprotein-mediated transport of (18)F-MPPF: a pharmacokinetic study in mice and non-human primates

PURPOSE

Several in vivo studies have found that the 5-HT(1A) PET radioligand (18)F-MPPF is a substrate of rodent P-glycoprotein (P-gp). However, in vitro assays suggest that MPPF is not a substrate of human P-gp. We have now tested the influence of inhibiting P-gp on the brain kinetics of (18)F-MPPF in mice and non-human primates.

METHODS

We measured the peripheral kinetics (arterial input function, metabolism, free fraction in plasma (f(P))) during (18)F-MPPF brain PET scanning in baboons with or without cyclosporine A (CsA) infusion. We measured (3)H-MPPF transport at the mouse BBB using in situ brain perfusion in P-gp/Bcrp deficient mice and after inhibiting P-gp with PSC833.

RESULTS

There was an unexpected 1.9-fold increase in brain area under the curve in CsA-treated baboons (n = 4), with no change in radiometabolite-corrected arterial input. However, total volume of distribution corrected for f(P) (V(T)/f(P)) remained unchanged. In situ brain perfusion showed that P-gp restricted the permeability of the mouse BBB to (3)H-MPPF while Bcrp did not.

CONCLUSION

These and previous in vitro results suggest that P-gp may not influence the permeability of human BBB to (18)F-MPPF. However, CsA treatment increased (18)F-MPPF free fraction, which is responsible for a misleading, P-gp unrelated enhanced brain uptake.

Serotonin and molecular neuroimaging in humans using PET

The serotonergic system is one of the most important modulatory neurotransmitter systems in the human brain. It plays a central role in major physiological processes and is implicated in a number of psychiatric disorders. Along with the dopaminergic system, it is also one of the phylogenetically oldest human neurotransmitter systems and one of the most diverse, with 14 different receptors identified up to this day, many of whose function remains to be understood. The system's functioning is even more diverse than the number of its receptors, since each is implicated in a number of different processes. This review aims at illustrating the distribution and summarizing the main functions of the serotonin (5-hydroxytryptamin, 5-HT) receptors as well as the serotonin transporter (SERT, 5-HTT), the vesicular monoamine transporter 2, monoamine oxidase type A and 5-HT synthesis in the human brain. Recent advances in in vivo quantification of these different receptors and enzymes that are part of the serotonergic system using positron emission tomography are described.

An in vivo comparison of cis- and trans-[18F]mefway in the nonhuman primate

INTRODUCTION

[(18)F]Mefway is a serotonin 5-HT(1A) PET radiotracer with high specificity and favorable in vivo imaging properties. The chemical structure of [(18)F]mefway permits (18)F labeling in either the cis or trans positions at the 4-cyclohexyl site. We have previously reported on the in vivo kinetics of trans-[(18)F]mefway in the nonhuman primate. In this work, we compare the in vivo binding of cis-[(18)F]mefway and trans-[(18)F]mefway to evaluate the properties of cis-[(18)F]mefway for 5-HT(1A) PET imaging.

METHODS

The cis- and trans-[(18)F]mefway tracers were synthesized via nucleophilic substitution with their respective tosyl precursors. Two monkeys (one male, one female) were given bolus injections of both cis- and trans-labeled [(18)F]mefway in separate experiments. Dynamic scans were acquired for 90 min with a microPET P4 scanner. Time-activity curves were extracted in the areas of the mesial temporal cortex (MTC), anterior cingulate gyrus (aCG), insular cortex (IC), raphe nuclei (RN) and cerebellum (CB). The in vivo behavior of the radiotracers was compared based upon the nondisplaceable binding potential (BP(ND)) using the CB as a reference region.

RESULTS

Averaged over the two subjects, BP(ND) values were as follows: MTC: 7.7, 0.58; aCG: 4.95, 0.32; IC: 3.27, 0.2; and RN: 3.05, 0.13, for trans-[(18)F]mefway and cis-[(18)F]mefway, respectively.

CONCLUSION

The cis-labeled [(18)F]mefway tracer has low specific binding throughout the 5-HT(1A) regions of the brain compared to trans-[(18)F]mefway, suggesting that the target-to-background binding of cis-[(18)F]mefway may limit its use for in vivo assessment of 5-HT(1A) binding.

5-HT radioligands for human brain imaging with PET and SPECT

The serotonergic system plays a key modulatory role in the brain and is the target for many drug treatments for brain disorders either through reuptake blockade or via interactions at the 14 subtypes of 5-HT receptors. This review provides the history and current status of radioligands used for positron emission tomography (PET) and single photon emission computerized tomography (SPECT) imaging of human brain serotonin (5-HT) receptors, the 5-HT transporter (SERT), and 5-HT synthesis rate. Currently available radioligands for in vivo brain imaging of the 5-HT system in humans include antagonists for the 5-HT(1A), 5-HT(1B), 5-HT(2A), and 5-HT(4) receptors, and for SERT. Here we describe the evolution of these radioligands, along with the attempts made to develop radioligands for additional serotonergic targets. We describe the properties needed for a radioligand to become successful and the main caveats. The success of a PET or SPECT radioligand can ultimately be assessed by its frequency of use, its utility in humans, and the number of research sites using it relative to its invention date, and so these aspects are also covered. In conclusion, the development of PET and SPECT radioligands to image serotonergic targets is of high interest, and successful evaluation in humans is leading to invaluable insight into normal and abnormal brain function, emphasizing the need for continued development of both SPECT and PET radioligands for human brain imaging.