Modeling considerations for 11C-CUMI-101, an agonist radiotracer for imaging serotonin 1A receptor in vivo with PET

[18F]F13640: a selective agonist PET radiopharmaceutical for imaging functional 5-HT1A receptors in humans

PURPOSE

F13640 (a.k.a. befiradol, NLX-112) is a highly selective 5-HT_1A receptor ligand that was selected as a PET radiopharmaceutical-candidate based on animal studies. Due to its high efficacy agonist properties, [¹⁸F]F13640 binds preferentially to functional 5-HT_1A receptors, which are coupled to intracellular G-proteins. Here, we characterize brain labeling of 5-HT_1A receptors by [¹⁸F]F13640 in humans and describe a simplified model for its quantification.

METHODS

PET/CT and PET-MRI scans were conducted in a total of 13 healthy male volunteers (29 ± 9 years old), with arterial input functions (AIF) (n = 9) and test-retest protocol (n = 8). Several kinetic models were compared (one tissue compartment model, two-tissue compartment model, and Logan); two models with reference region were also evaluated: simplified reference tissue model (SRTM) and the logan reference model (LREF).

RESULTS

[¹⁸F]F13640 showed high uptake values in raphe nuclei and cortical regions. SRTM and LREF models showed a very high correlation with kinetic models using AIF. As concerns test-retest parameters and the prolonged binding kinetics of [¹⁸F]F13640, better reproducibility, and reliability were found with the LREF method. Cerebellum white matter and frontal lobe white matter stand out as suitable reference regions.

CONCLUSION

The favorable brain labeling and kinetic profile of [¹⁸F]F13640, its high receptor specificity and its high efficacy agonist properties open new perspectives for studying functionally active 5-HT_1A receptors, unlike previous radiopharmaceuticals that act as antagonists. [¹⁸F]F13640's kinetic properties allow injection outside of the PET scanner with delayed acquisitions, facilitating the design of innovative longitudinal protocols in neurology and psychiatry.

TRIAL REGISTRATION

Trial Registration EudraCT 2017-002,722-21.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Brain 5-HT1A Receptor PET Binding, Cortisol Responses to Stress, and the Familial Transmission of Suicidal Behavior

BACKGROUND

The serotonin 1A (5-HT1A) receptor has been implicated in depression and suicidal behavior. Lower resting cortisol levels are associated with higher 5-HT1A receptor binding, and both differentiate suicide attempters with depression. However, it is not clear whether 5-HT1A receptor binding and cortisol responses to stress are related to familial risk and resilience for suicidal behavior.

METHODS

[11C]CUMI-101 positron emission tomography imaging to quantify regional brain 5-HT1A receptor binding was conducted in individuals considered to be at high risk for mood disorder or suicidal behavior on the basis of having a first- or second-degree relative(s) with an early onset mood disorder and history of suicidal behavior. These high-risk individuals were subdivided into the following groups: high risk resilient having no mood disorder or suicidal behavior (n = 29); high risk with mood disorder and no suicidal behavior history (n = 31); and high risk with mood disorder and suicidal behavior (n = 25). Groups were compared with healthy volunteers without a family history of mood disorder or suicidal behavior (n = 34). Participants underwent the Trier Social Stress Task (TSST). All participants were free from psychotropic medications at the time of the TSST and PET scanning.

RESULTS

We observed no group differences in 5-HT1A receptor binding considering all regions simultaneously, nor did we observe heterogeneity of the effect of group across regions. These results were similar across outcome measures (BPND for all participants and BPp in a subset of the sample) and definitions of regions of interest (ROIs; standard or serotonin system-specific ROIs). We also found no group differences on TSST outcomes. Within the high risk with mood disorder and suicidal behavior group, lower BPp binding (β = -0.084, SE = 0.038, P = .048) and higher cortisol reactivity to stress (β = 9.25, 95% CI [3.27,15.23], P = .004) were associated with higher lethality attempts. There were no significant relationships between 5-HT1A binding and cortisol outcomes.

CONCLUSIONS

5-HT1A receptor binding in ROIs was not linked to familial risk or resilience protecting against suicidal behavior or mood disorder although it may be related to lethality of suicide attempt. Future studies are needed to better understand the biological mechanisms implicated in familial risk for suicidal behavior and how hypothalamic-pituitary-adrenal axis function influences such risk.

Quantification of Positron Emission Tomography Data Using Simultaneous Estimation of the Input Function: Validation with Venous Blood and Replication of Clinical Studies

PURPOSE

To determine if one venous blood sample can substitute full arterial sampling in quantitative modeling for multiple positron emission tomography (PET) radiotracers using simultaneous estimation of the input function (SIME).

PROCEDURES

Participants underwent PET imaging with [¹¹C]ABP688, [¹¹C]CUMI-101, and [¹¹C]DASB. Full arterial sampling and additional venous blood draws were performed for quantification with the arterial input function (AIF) and SIME using one arterial or venous (vSIME) sample.

RESULTS

Venous and arterial metabolite-corrected plasma activities were within 6 % of each other at varying time points. vSIME- and AIF-derived outcome measures were in good agreement, with optimal sampling times of 12 min ([¹¹C]ABP688), 90 min ([¹¹C]CUMI-101), and 100 min ([¹¹C]DASB). Simulation-based power analyses revealed that SIME required fewer subjects than the AIF method to achieve statistical power, with significant reductions for [¹¹C]CUMI-101 and [¹¹C]DASB with vSIME. Replication of previous findings and test-retest analyses bolstered the simulation analyses.

CONCLUSIONS

We demonstrate the feasibility of AIF recovery using SIME with one venous sample for [¹¹C]ABP688, [¹¹C]CUMI-101, and [¹¹C]DASB. This method simplifies PET acquisition while allowing for fully quantitative modeling, although some variability and bias are present with respect to AIF-based quantification, which may depend on the accuracy of the single venous blood measurement.

Examining the underpinnings of loudness dependence of auditory evoked potentials with positron emission tomography

Loudness dependence of auditory evoked potentials (LDAEP) has long been considered to reflect central basal serotonin transmission. However, the relationship between LDAEP and individual serotonin receptors and transporters has not been fully explored in humans and may involve other neurotransmitter systems. To examine LDAEP's relationship with the serotonin system, we performed PET using serotonin-1A (5-HT_1A) imaging via [¹¹C]CUMI-101 and serotonin transporter (5-HTT) imaging via [¹¹C]DASB on a mixed sample of healthy controls (n ​= ​4: 4 females, 0 males), patients with unipolar (MDD, n ​= ​11: 4 females, 7 males) and bipolar depression (BD, n ​= ​8: 4 females, 4 males). On these same participants, we also performed electroencephalography (EEG) within a week of PET scanning, using 1000 ​Hz tones of varying intensity to evoke LDAEP. We then evaluated the relationship between LDAEP and 5-HT_1A or 5-HTT binding in both the raphe (5-HT_1A)/midbrain (5-HTT) areas and in the temporal cortex. We found that LDAEP was significantly correlated with 5-HT_1A positively and with 5-HTT negatively in the temporal cortex (p ​< ​0.05), but not correlated with either in midbrain or raphe. In males only, exploratory analysis showed multiple regions in which LDAEP significantly correlated with 5-HT_1A throughout the brain; we did not find this with 5-HTT. This multimodal study partially validates preclinical models of a serotonergic influence on LDAEP. Replication in larger samples is necessary to further clarify our understanding of the role of serotonin in perception of auditory tones.

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.

A Search for CD36 Ligands from Flavor Volatiles in Foods with an Aldehyde Moiety: Identification of Saturated Aliphatic Aldehydes with 9-16 Carbon Atoms as Potential Ligands of the Receptor

Volatile compounds with an aldehyde moiety such as (Z)-9-octadecenal are potential ligands for cluster of differentiation 36 (CD36), a transmembrane receptor that has recently been shown to play a role in mammalian olfaction. In this study, by performing an assay using a peptide mimic of human CD36, we aimed to discover additional ligands for the receptor from volatiles containing a single aldehyde group commonly found in human foods. Straight-chain, saturated aliphatic aldehydes with 9-16 carbons exhibited CD36 ligand activities, albeit to varying degrees. Notably, the activities of tridecanal and tetradecanal were higher than that of oleic acid, the most potent ligand among the fatty acids tested. Among the aldehydes other than aliphatic aldehydes, only phenylacetaldehyde showed a weak activity. These findings make a contribution to our knowledge of recognition mechanisms for flavor volatiles in foods with an aldehyde group.

A hybrid deconvolution approach for estimation of in vivo non-displaceable binding for brain PET targets without a reference region

BACKGROUND AND AIM

Estimation of a PET tracer's non-displaceable distribution volume (VND) is required for quantification of specific binding to its target of interest. VND is generally assumed to be comparable brain-wide and is determined either from a reference region devoid of the target, often not available for many tracers and targets, or by imaging each subject before and after blocking the target with another molecule that has high affinity for the target, which is cumbersome and involves additional radiation exposure. Here we propose, and validate for the tracers [11C]DASB and [11C]CUMI-101, a new data-driven hybrid deconvolution approach (HYDECA) that determines VND at the individual level without requiring either a reference region or a blocking study.

METHODS

HYDECA requires the tracer metabolite-corrected concentration curve in blood plasma and uses a singular value decomposition to estimate the impulse response function across several brain regions from measured time activity curves. HYDECA decomposes each region's impulse response function into the sum of a parametric non-displaceable component, which is a function of VND, assumed common across regions, and a nonparametric specific component. These two components differentially contribute to each impulse response function. Different regions show different contributions of the two components, and HYDECA examines data across regions to find a suitable common VND. HYDECA implementation requires determination of two tuning parameters, and we propose two strategies for objectively selecting these parameters for a given tracer: using data from blocking studies, and realistic simulations of the tracer. Using available test-retest data, we compare HYDECA estimates of VND and binding potentials to those obtained based on VND estimated using a purported reference region.

RESULTS

For [11C]DASB and [11C]CUMI-101, we find that regardless of the strategy used to optimize the tuning parameters, HYDECA provides considerably less biased estimates of VND than those obtained, as is commonly done, using a non-ideal reference region. HYDECA test-retest reproducibility is comparable to that obtained using a VND determined from a non-ideal reference region, when considering the binding potentials BPP and BPND.

CONCLUSIONS

HYDECA can provide subject-specific estimates of VND without requiring a blocking study for tracers and targets for which a valid reference region does not exist.

Autoradiographic Evaluation of [(18)F]FECUMI-101, a High Affinity 5-HT1AR Ligand in Human Brain

[(18)F]FECUMI-101 ([(18)F]1) is a 5HT1AR ligand demonstrating specific binding in brain regions corresponding to the distribution of 5-HT1AR in baboons. However, we detected moderate uptake of [(18)F]1 in baboon thalamus, a brain region lacking 5-HT1AR. We sought to investigate the relative binding of [(18)F]1 to 5-HT1AR, α1R, and 5-HT7R in vitro. Using autoradiography in human brain sections, specific binding of [(18)F]1 to 5-HT1AR was confirmed. However, [(18)F]1 also showed 26% binding to α1R in PFC. The hippocampal formation exhibited 51% and 92% binding of [(18)F]1 to α1R and 5-HT1AR, respectively. Thalamus and cerebellum showed very little binding. There is no measurable specific binding of [(18)F]1 to 5-HT7R and no effect of temperature on [(18)F]1 specific binding to 5-HT1AR or α1R. These results indicate that, while [(18)F]FECUMI-101 is not a completely selective 5-HT1AR ligand for receptor quantification, it may be useful for occupancy measurements of drugs acting at 5-HT1AR in vivo.

Noninvasive blood-free full quantification of positron emission tomography radioligand binding

Full quantification of a positron emission tomography (PET) radioligand binding to its target is preferred because it requires the fewest assumptions, but generally involves measuring the concentration of free radioligand in the arterial plasma by collecting blood samples from the subject's radial artery during the scan, and performing metabolite analysis. This invasive, costly procedure deters subjects' participation, and requires specialized staff and equipment. Simultaneous estimation (SIME) can fully quantify binding using only PET data from multiple brain regions and one individual anchor value, which is based on a single arterial blood sample. Drawing this sample can still be challenging in clinical settings, particularly when using simultaneous PET/magnetic resonance scanners. Here we propose a methodology for full quantification of binding that does not require any blood samples. The methodology substitutes the SIME blood-based anchor with a value predicted using multiple linear regression of noninvasive, easy-to-collect variables related to the radioligand blood concentration, and individual metabolism, such as injected dose, body mass index, or body surface area. As a study case, we show here the methodology in comparison to analysis with full arterial-line blood sampling in a cohort of 23 available scans with [(11)C]CUMI-101, a partial agonist of the serotonin 5-HT1A receptors.

Estimation of in vivo nonspecific binding in positron emission tomography studies without requiring a reference region

Estimation of outcome measures in in vivo neuroreceptor mapping with positron emission tomography (PET) commonly depends on an assumption of uniform nondisplaceable binding throughout the brain. In many cases, this can be estimated based on data from a "reference region," an area of the brain devoid of the receptor of interest. However, often such a region does not exist, as there are some receptors everywhere throughout the brain. Erroneously designating a region as a "reference" can lead to biased estimation, and furthermore, if the level of specific binding in the purported reference region differs between comparison groups, the validity of resulting conclusions may be called into question. We present a method for estimation of all common PET outcome measures that can provide good estimates even when no reference region exists. Our aim is to use information from several regions simultaneously to estimate the information common to all regions. By not requiring specification (or validation) of a reference region, such an approach can provide an automated, objective approach for kinetic modeling of PET data. We illustrate the performance of these methods on simulated data, human [(11)C]WAY-100635 data, and [(11)C]CUMI-101 blocking data in baboons. We show close agreement between estimates obtained by using the proposed method (which does not require a reference region) and estimates based on either a reference region or a blocking study.

(11)C-CUMI-101, a PET radioligand, behaves as a serotonin 1A receptor antagonist and also binds to α(1) adrenoceptors in brain

The PET radioligand (11)C-CUMI-101 was previously suggested as a putative agonist radioligand for the serotonin 1A (5-hydroxytryptamine 1A [5-HT1A]) receptor in recombinant cells expressing human 5-HT1A receptor. However, a recent study showed that CUMI-101 behaved as a potent 5-HT1A receptor antagonist in rat brain. CUMI-101 also has moderate affinity (Ki = 6.75 nM) for α1 adrenoceptors measured in vitro. The current study examined the functional properties and selectivity of CUMI-101, both in vitro and in vivo.

METHODS

The functional assay was performed using (35)S-GTPγS (GTP is guanosine triphosphate) in primate brains. The cross-reactivity of CUMI-101 with α1 adrenoceptors was performed using in vitro radioligand binding studies in rat, monkey, and human brains as well as in vivo PET imaging in mouse, rat, and monkey brains.

RESULTS

CUMI-101 did not stimulate (35)S-GTPγS binding in primate brain, in contrast to 8-OH-DPAT, a potent 5-HT1A receptor agonist. Instead, CUMI-101 behaved as a potent 5-HT1A receptor antagonist by dose-dependently inhibiting 8-OH-DPAT-stimulated (35)S-GTPγS binding. Both in vitro and in vivo studies showed that CUMI-101 had significant α1 adrenoceptor cross-reactivity. On average, across all 3 species examined, cross-reactivity was highest in the thalamus (>45%) and lowest in the neocortex and cerebellum (<10%). PET imaging further confirmed that only preblocking with WAY-100635 plus prazosin decreased (11)C-CUMI-101 brain uptake to that of self-block.

CONCLUSION

CUMI-101 behaves as a 5-HT1A receptor antagonist in primate brain, with significant, regional-dependent α1 adrenoceptor cross-reactivity, limiting its potential use as a PET radioligand in humans.

Synthesis and in vivo evaluation of [(18)F]2-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)-dione ([(18)F]FECUMI-101) as an imaging probe for 5-HT1A receptor agonist in nonhuman primates

The 5-HT1AR partial agonist PET radiotracer, [(11)C]CUMI-101, has advantages over an antagonist radiotracer as it binds preferentially to the high affinity state of the receptor and thereby provides more functionally meaningful information. The major drawback of C-11 tracers is the lack of cyclotron facility in many health care centers thereby limiting widespread clinical or research use. We identified the fluoroethyl derivative, 2-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione (FECUMI-101) (Ki=0.1nM; Emax=77%; EC50=0.65nM) as a partial agonist 5-HT1AR ligand of the parent ligand CUMI-101. FECUMI-101 is radiolabeled with F-18 by O-fluoroethylation of the corresponding desmethyl analogue (1) with [(18)F]fluoroethyltosylate in DMSO in the presence of 1.6equiv of K2CO3 in 45±5% yield (EOS). PET shows [(18)F]FECUMI-101 binds specifically to 5-HT1AR enriched brain regions of baboon. The specificity of [(18)F]FECUMI-101 binding to 5-HT1AR was confirmed by challenge studies with the known 5-HT1AR ligand WAY100635. These findings indicate that [(18)F]FECUMI-101 can be a viable agonist ligand for the in vivo quantification of high affinity 5-HT1AR with PET.

Reference region approaches in PET: a comparative study on multiple radioligands

Reference region (RR) approaches (RRAs) for positron emission tomography (PET) brain studies aim to obtain full quantification without arterial input function (IF) sampling, an invasive and costly procedure. Although they need a reliable RR and are only able to estimate the nondisplaceable binding potential (BPND), RRAs are widely used. If quantitatively reliable, then RRAs can greatly benefit PET investigations, but their suitability can vary widely from radioligand to radioligand. This study compares estimates of BPND both using IF data and several common RRAs on an extensive data set for each of several radioligands. In addition, two new methods, likelihood estimation in graphical analysis with RR and a bootstrapping algorithm for estimating precision, are presented here for the first time. Although many factors contribute to the performance of each RRA, our results suggest that the kinetics in the RR have a role. In particular, RRAs tend to be good when (1) the RR distribution volume is high; (2) the transfer rate constant from plasma to free compartment in the RR is high; and (3) the transfer rate constant from free to plasma compartment in the RR is low.

Autoradiographic evaluation of [3H]CUMI-101, a novel, selective 5-HT1AR ligand in human and baboon brain

[11C]CUMI-101 is the first selective serotonin receptor (5-HT1AR) partial agonist radiotracer for positron emission tomography (PET) tested in vivo in nonhuman primates and humans. We evaluated specific binding of [3H]CUMI-101 by quantitative autoradiography studies in postmortem baboon and human brain sections using the 5-HT1AR antagonist WAY-100635 as a displacer. The regional and laminar distributions of [3H]CUMI-101 binding in baboon and human brain sections matched the known distribution of [3H]8-OH-DPAT and [3H]WAY-100635. Prazosin did not measurably displace [3H]CUMI-101 binding in baboon or human brain sections, thereby ruling out [3H]CUMI-101 binding to α1-adrenergic receptors. This study demonstrates that [11C]CUMI-101 is a selective 5-HT1AR ligand for in vivo and in vitro studies in baboon and human brain.

Transcriptional regulation of the 5-HT1A receptor: implications for mental illness

The serotonin-1A (5-HT(1A)) receptor is an abundant post-synaptic 5-HT receptor (heteroreceptor) implicated in regulation of mood, emotion and stress responses and is the major somatodendritic autoreceptor that negatively regulates 5-HT neuronal activity. Based on animal models, an integrated model for opposing roles of pre- and post-synaptic 5-HT(1A) receptors in anxiety and depression phenotypes and response to antidepressants is proposed. Understanding differential transcriptional regulation of pre- versus post-synaptic 5-HT(1A) receptors could provide better tools for their selective regulation. This review examines the transcription factors that regulate brain region-specific basal and stress-induced expression of the 5-HT(1A) receptor gene (Htr1a). A functional polymorphism, rs6295 in the Htr1a promoter region, blocks the function of specific repressors Hes1, Hes5 and Deaf1, resulting in increased 5-HT(1A) autoreceptor expression in animal models and humans. Its association with altered 5-HT(1A) expression, depression, anxiety and antidepressant response are related to genotype frequency in different populations, sample homogeneity, disease outcome measures and severity. Preliminary evidence from gene × environment studies suggests the potential for synergistic interaction of stress-mediated repression of 5-HT(1A) heteroreceptors, and rs6295-induced upregulation of 5-HT(1A) autoreceptors. Targeted therapeutics to inhibit 5-HT(1A) autoreceptor expression and induce 5-HT(1A) heteroreceptor expression may ameliorate treatment of anxiety and major depression.

Measuring endogenous changes in serotonergic neurotransmission in humans: a [11C]CUMI-101 PET challenge study

Serotonin (5-HT) neurotransmission is implicated in cognitive and emotional processes and a number of neuropsychiatric disorders. The use of positron emission tomography (PET) to measure ligand displacement has allowed estimation of endogenous dopamine release in the human brain; however, applying this methodology to assess central 5-HT release has proved more challenging. The aim of this study was to assess the sensitivity of a highly selective 5-HT(1A) partial agonist radioligand [(11)C]CUMI-101 to changes in endogenous 5-HT levels induced by an intravenous challenge with the selective 5-HT re-uptake inhibitor (SSRI), citalopram, in healthy human participants. We studied 15 healthy participants who underwent PET scanning in conjunction with [(11)C]CUMI-101 after receiving an intravenous infusion of citalopram 10 mg or placebo in a double-blind, crossover, randomized design. Regional estimates of binding potential (BP(ND)) were obtained by calculating total volumes of distribution (V(T)) for presynaptic dorsal raphe nucleus (DRN) and postsynaptic cortical regions. Relative to placebo, citalopram infusion significantly increased [(11)C]CUMI-101 BP(ND) at postsynaptic 5-HT(1A) receptors in several cortical regions, but there was no change in binding at 5-HT(1A) autoreceptors in the DRN. Across the postsynaptic brain regions, citalopram treatment induced a mean 7% in [(11)C]CUMI-101 BP(ND) (placebo 1.3 (0.2); citalopram 1.4 (0.2); paired t-test P=0.003). The observed increase in postsynaptic [(11)C]CUMI-101 availability identified following acute citalopram administration could be attributable to a decrease in endogenous 5-HT availability in cortical terminal regions, consistent with preclinical animal studies, in which acute administration of SSRIs decreases DRN cell firing through activation of 5-HT(1A) autoreceptors to reduce 5-HT levels in postsynaptic regions. We conclude that [(11)C]CUMI-101 may be sensitive to changes in endogenous 5-HT release in humans.

Comparison of high and low affinity serotonin 1A receptors by PET in vivo in nonhuman primates

Serotonin (5-HT) 1A receptors exist in high and low affinity states. Agonist ligands bind preferentially to the high affinity state receptors, providing a more functionally relevant measure than antagonist binding. We now report comparison of 5-HT(1A) binding in vivo using both [¹¹C]CUMI-101 (agonist) and [¹¹C]WAY100635 (antagonist) in nonhuman primates. PET studies show that both tracers bind to known 5-HT(1A) receptor (5-HT(1A)R)-rich regions of baboon brain. The binding (BP(F)) of [¹¹C]CUMI-101 was lower on an average of 55% across the regions of interest (ROIs) compared to [¹¹C]WAY100635. This ratio is consistent with the in vitro binding data of agonist and antagonist 5-HT(1A)R ligands previously reported.

Normative database of the serotonergic system in healthy subjects using multi-tracer PET

The highly diverse serotonergic system with at least 16 different receptor subtypes is implicated in the pathophysiology of most neuropsychiatric disorders including affective and anxiety disorders, obsessive compulsive disorder, post-traumatic stress disorder, eating disorders, sleep disturbance, attention deficit/hyperactivity disorder, drug addiction, suicidal behavior, schizophrenia, Alzheimer, etc. Alterations of the interplay between various pre- and postsynaptic receptor subtypes might be involved in the pathogenesis of these disorders. However, there is a lack of comprehensive in vivo values using standardized procedures. In the current PET study we quantified 3 receptor subtypes, including the major inhibitory (5-HT(1A) and 5-HT(1B)) and excitatory (5-HT(2A)) receptors, and the transporter (5-HTT) in the brain of healthy human subjects to provide a database of standard values. PET scans were performed on 95 healthy subjects (age=28.0 ± 6.9 years; 59% males) using the selective radioligands [carbonyl-(11)C]WAY-100635, [(11)C]P943, [(18)F]altanserin and [(11)C]DASB, respectively. A standard template in MNI stereotactic space served for region of interest delineation. This template follows two anatomical parcellation schemes: 1) Brodmann areas including 41 regions and 2) AAL (automated anatomical labeling) including 52 regions. Standard values (mean, SD, and range) for each receptor and region are presented. Mean cortical and subcortical binding potential (BP) values were in good agreement with previously published human in vivo and post-mortem data. By means of linear equations, PET binding potentials were translated to post-mortem binding (provided in pmol/g), yielding 5.89 pmol/g (5-HT(1A)), 23.5 pmol/g (5-HT(1B)), 31.44 pmol/g (5-HT(2A)), and 11.33 pmol/g (5-HTT) being equivalent to the BP of 1, respectively. Furthermore, we computed individual voxel-wise maps with BP values and generated average tracer-specific whole-brain binding maps. This knowledge might improve our interpretation of the alterations taking place in the serotonergic system during neuropsychiatric disorders.

Presynaptic selectivity of a ligand for serotonin 1A receptors revealed by in vivo PET assays of rat brain

A novel investigational antidepressant with high affinity for the serotonin transporter and the serotonin 1A (5-HT(1A)) receptor, called Wf-516 (structural formula: (2S)-1-[4-(3,4-dichlorophenyl)piperidin-1-yl]-3-[2-(5-methyl-1,3,4-oxadiazol-2-yl)benzo[b]furan-4-yloxy]propan-2-ol monohydrochloride), has been found to exert a rapid therapeutic effect, although the mechanistic basis for this potential advantage remains undetermined. We comparatively investigated the pharmacokinetics and pharmacodynamics of Wf-516 and pindolol by positron emission tomographic (PET) and autoradiographic assays of rat brains in order to elucidate their molecular interactions with presynaptic and postsynaptic 5-HT(1A) receptors. In contrast to the full receptor occupancy by pindolol in PET measurements, the binding of Wf-516 to 5-HT(1A) receptors displayed limited capacity, with relatively high receptor occupancy being achieved in regions predominantly containing presynaptic receptors. This selectivity was further proven by PET scans of neurotoxicant-treated rats deficient in presynaptic 5-HT(1A) receptors. In addition, [(35)S]guanosine 5'-O-[γ-thio]triphosphate autoradiography indicated a partial agonistic ability of Wf-516 for 5-HT(1A) receptors. This finding has lent support to reports that diverse partial agonists for 5-HT(1A) receptors exert high sensitivity for presynaptic components. Thus, the present PET data suggest a relatively high capacity of presynaptic binding sites for partial agonists. Since our in vitro and ex vivo autoradiographies failed to illustrate these distinct features of Wf-516, in vivo PET imaging is considered to be, thus far, the sole method capable of pharmacokinetically demonstrating the unique actions of Wf-516 and similar new-generation antidepressants.

No change in [¹¹C]CUMI-101 binding to 5-HT(1A) receptors after intravenous citalopram in human

The main objective of this study was to determine the sensitivity of [¹¹C]CUMI-101 to citalopram challenge aiming at increasing extracellular 5-HT. CUMI-101 has agonistic properties in human embryonic kidney 293 cells transfected with human recombinant 5-HT(1A) receptors (Hendry et al. [2011] Nucl Med Biol 38:273-277; Kumar et al. [2006] J Med Chem 49:125-134) and has previously been demonstrated to be sensitive to bolus citalopram in monkeys (Milak et al. [2011] J Cereb Blood Flow Metab 31:243-249). We studied six healthy individuals. Two PET-scans were performed on the same day in each individual before and after constant infusion of citalopram (0.15 mg/kg). The imaging data were analyzed using two tissue compartment kinetic modeling with metabolite corrected arterial input and Simplified Reference Tissue Modeling using cerebellum as a reference region. There was no significant difference in regional distribution volume or non-displaceable binding potential values before and after citalopram infusion. The mean receptor occupancy was 0.03 (range -0.14 to 0.17). Our data imply that [¹¹C]CUMI-101 binding is not sensitive to citalopram infusion in humans.

In vivo variation in metabotropic glutamate receptor subtype 5 binding using positron emission tomography and [11C]ABP688

The metabotropic glutamate receptor subtype 5 (mGluR5) has been implicated in the pathophysiology of mood and anxiety disorders. Recently, a positron emission tomography (PET) tracer exhibiting high selectivity and specificity for mGluR5, 3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-O-(11)C-methyl-oxime ([(11)C]ABP688), was developed. In this work, eight healthy adult male humans were imaged twice to assess within-subject [(11)C]ABP688 binding variability using PET. In seven of the eight subjects, significantly higher binding was observed during the second (retest) scan. This binding increase could not be definitively explained by differences in ligand injected mass or dose, or changes in metabolism between scans. In addition, this type of systematic binding increase was not observed in a [(11)C]ABP688 test-retest study performed by our group on anaesthetized baboons. It is therefore possible that the increased binding was because of physiological changes occurring between scans, such as changes in endogenous glutamate levels. If PET imaging with [(11)C]ABP688 could detect such differences, as preliminary evidence suggests, it could be used to help uncover the role of glutamate in the pathophysiology of brain disorders. However, regardless of its ability to detect endogenous glutamate differences, [(11)C]ABP688 binding variability could make accurate assessments of drug occupancy or group differences using this ligand difficult.

Biodistribution, toxicology, and radiation dosimetry of 5-HT1A-receptor agonist positron emission tomography ligand [11C]CUMI-101

Sprague Dawley rats (10/sex/group) were given a single intravenous (iv) dose of CUMI-101 to determine acute toxicity of CUMI-101 and radiation dosimetry estimations were conducted in baboons with [(11)C]CUMI-101. Intravenous administration of CUMI-101 did not produce overt biologically or toxicologically significant adverse effects except transient hypoactivity immediately after dose in the mid- and high-dose groups, which is not considered to be a dose-limiting toxic effect. No adverse effects were observed in the low-dose group. The no observed adverse effect level (NOAEL) is considered to be 44.05 µg/kg for a single iv dose administration in rats. The maximum tolerated dose (MTD) was estimated to be 881 µg/kg for a single iv dose administration. The Medical Internal Radiation Dose (MIRDOSE) estimates indicate the maximum permissible single-study dosage of [(11)C]CUMI-101 in humans is 52 mCi with testes and urinary bladder as the critical organ for males and females, respectively.

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.

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.

Functional neuroimaging in geriatric depression

Abnormalities in specific cerebral networks likely confer vulnerability that increases the susceptibility for development of geriatric depression and affect the course of symptoms. Functional neuroimaging enables the in vivo identification of alterations in cerebral function that characterize disease vulnerability and contribute to variability in depressive symptoms and antidepressant response. Judicious use of functional neuroimaging tools can advance pathophysiologic models of geriatric depression. Furthermore, geriatric depression provides a logical context within which to study the role of specific functional abnormalities in both antidepressant response and key behavioral and cognitive abnormalities of mood disorders.

Ex vivo evaluation of the serotonin 1A receptor partial agonist [³H]CUMI-101 in awake rats

[³H]CUMI-101 is a 5-HT(1A) partial agonist, which has been evaluated for use as a positron emission tracer in baboon and humans. We sought to evaluate the properties of [³H]CUMI-101 ex vivo in awake rats and determine if [³H]CUMI-101 can measure changes in synaptic levels of serotonin after different challenge paradigms. [³H]CUMI-101 shows good uptake and good specific binding ratio (SBR) in frontal cortex 5.18 and in hippocampus 3.18. Binding was inhibited in a one-binding-site fashion by WAY100635 and unlabeled CUMI-101. The ex vivo B(max) of [³H]CUMI-101 in frontal cortex (98.7 fmol/mg) and hippocampus (131 fmol/kg) agree with the ex vivo B(max) of [³H]MPPF in frontal cortex (147.1 fmol/mg) and hippocampus (72.1 fmol/mg) and with in vitro values reported with 8-OH-DPAT. Challenges with citalopram, a selective serotonin reuptake inhibitor, fenfluramine, a serotonin releaser, and 4-chloro-DL-phenylalanine, a serotonin synthesis inhibitor, did not show any effect on the standardized uptake values (SUVs) in any region. Citalopram did alter SBR, but this was due to changes in cerebellar SUVs. Our results indicate that [³H]CUMI-101 is a good radioligand for imaging 5-HT(1A) high-density regions in rats; however, the results from pharmacological challenges remain inconclusive.

In vivo serotonin-sensitive binding of [11C]CUMI-101: a serotonin 1A receptor agonist positron emission tomography radiotracer

Positron emission tomography studies of 5-hydroxytryptamine (5-HT)(1A) receptors have hitherto been limited to antagonist radiotracers. Antagonists do not distinguish high/low-affinity conformations of G protein-coupled receptors and are less likely to be sensitive to intrasynaptic serotonin levels. We developed a novel 5-HT(1A) agonist radiotracer [(11)C]CUMI-101. This study evaluates the sensitivity of [(11)C]CUMI-101 binding to increases in intrasynaptic serotonin induced by intravenous citalopram and fenfluramine. Two Papio anubis were scanned, using [(11)C]CUMI-101 intravenous bolus of 4.5 ± 1.5 mCi. Binding potential (BP(F)=B(avail)/K(D)) was measured before (n=10) and 20 minutes after elevation of intrasynaptic serotonin by intravenous citalopram (2 mg/kg, n=3; 4 mg/kg, n=3) and fenfluramine (2.5 mg/kg, n=3) using a metabolite-corrected arterial input function. Occupancy was also estimated by the Lassen graphical approach. Both citalopram and fenfluramine effects were significant for BP(F) (P=0.031, P=0.049, respectively). The Lassen approach estimated 15.0, 30.4, and 23.7% average occupancy after citalopram 2 mg/kg, 4 mg/kg, and fenfluramine 2.5 mg/kg, respectively. [(11)C]CUMI-101 binding is sensitive to a large increase in intrasynaptic serotonin in response to robust pharmacological challenges. These modest changes in BP(F) may make it unlikely that this ligand will detect changes in intrasynaptic 5-HT under physiologic conditions; future work will focus on evaluating its utility in measuring the responsiveness of the 5-HT system to pharmacological challenges.

In vivo quantification of human serotonin 1A receptor using 11C-CUMI-101, an agonist PET radiotracer

The serotonin (5-hydroxytryptamine, or 5-HT) type 1A receptor (5-HT(1A)R) is implicated in the pathophysiology of numerous neuropsychiatric disorders. We have published the initial evaluation and reproducibility in vivo of [O-methyl-(11)C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione ((11)C-CUMI-101), a novel 5-HT(1A) agonist radiotracer, in Papio anubis. Here, we report the optimal modeling parameters of (11)C-CUMI-101 for human PET studies.

METHODS

PET scans were obtained for 7 adult human volunteers. (11)C-CUMI-101 was injected as an intravenous bolus, and emission data were collected for 120 min in 3-dimensional mode. We evaluated 10 different models using metabolite-corrected arterial input functions or reference region approaches and several outcome measures.

RESULTS

When using binding potential (BP(F) = B(avail)/K(D) [total available receptor concentration divided by the equilibrium dissociation constant]) as the outcome measure, the likelihood estimation in the graphical analysis (LEGA) model performed slightly better than the other methods evaluated at full scan duration. The average test-retest percentage difference was 9.90% ± 5.60%. When using BP(ND) (BP(ND) = f(nd) × B(avail)/K(D); BP(ND) equals the product of BP(F) and f(nd) [free fraction in the nondisplaceable compartment]), the simplified reference tissue method (SRTM) achieved the lowest percentage difference and smallest bias when compared with nondisplaceable binding potential obtained from LEGA using the metabolite-corrected plasma input function (r(2) = 0.99; slope = 0.92). The time-stability analysis indicates that a 120-min scan is sufficient for the stable estimation of outcome measures. Voxel results were comparable to region-of-interest-based analysis, with higher spatial resolution.

CONCLUSION

On the basis of its measurable and stable free fraction, high affinity and selectivity, good blood-brain barrier permeability, and plasma and brain kinetics, (11)C-CUMI-101 is suitable for the imaging of high-affinity 5-HT(1A) binding in humans.

Measuring endogenous 5-HT release by emission tomography: promises and pitfalls

Molecular in vivo neuroimaging techniques can be used to measure regional changes in endogenous neurotransmitters, evoked by challenges that alter synaptic neurotransmitter concentration. This technique has most successfully been applied to the study of endogenous dopamine release using positron emission tomography, but has not yet been adequately extended to other neurotransmitter systems. This review focuses on how the technique has been applied to the study of the 5-hydroxytryptamine (5-HT) system. The principles behind visualising fluctuations in neurotransmitters are introduced, with reference to the dopaminergic system. Studies that aim to image acute, endogenous 5-HT release or depletion at 5-HT receptor targets are summarised, with particular attention to studies in humans. Radiotracers targeting the 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors and the serotonin reuptake transporter have been explored for their sensitivity to 5-HT fluctuations, but with mixed outcomes; tracers for these targets cannot reliably image endogenous 5-HT in humans. Shortcomings in our basic knowledge of the mechanisms underlying changes in binding potential are addressed, and suggestions are made as to how the selection of targets, radiotracers, challenge paradigms, and experimental design might be optimised to improve our chances of successfully imaging endogenous neurotransmitters in the future.

Evaluation of [C]S14506 and [F]S14506 in rat and monkey as agonist PET radioligands for brain 5-HT(1A) receptors

In vitro and ex vivo measurements have shown that the binding of the selective high-affinity agonist, S14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxy-naphthyl)piperazine), to 5-HT(1A) receptors, is similar in affinity (K(d) = 0.79 nM) and extent (B(max)) to that of the antagonist, WAY 100635. We aimed to test whether S14506, labeled with a positron-emitter, might serve as a radioligand for imaging brain 5-HT(1A) receptors in vivo with positron emission tomography (PET). Here we evaluated [(11)C]S14506 and [(18)F]S14506 in rat and rhesus monkey in vivo. After intravenous administration of [(11)C]S14506 into rat, radioactivity entered brain, reaching 210% SUV at 2 min. Radioactivity uptake into brain was higher (~ 350% SUV) in rats pre-treated with the P-glycoprotein (P-gp) inhibitor, cyclosporin A. In rhesus monkey, peak brain uptake of radioactivity after administration of [(11)C]S14506 or [(18)F]S14506 was also moderate and for [(11)C]S14506 increased from ~ 170% SUV after 7 min, to 240% SUV in a monkey pre-treated with the P-gp inhibitor, tariquidar. The ratios of radioactivity in 5-HT(1A) receptor-rich regions, such as cingulate or hippocampus to that in receptor-poor cerebellum reached between 1.35 and 1.5 at 60 min for both [(11)C]S14506 and [(18)F]S14506. [(11)C]S14506 gave one major polar radiometabolite in monkey plasma, and [(18)F]S14506 gave three and two more polar radiometabolites in rat and monkey plasma, respectively. The rat radiometabolites of [(18)F]S14506 did not accumulate in brain. [(18)F]S14506 was not radiodefluorinated in monkey. Thus, despite high-affinity and lack of troublesome brain radiometabolites, both [(11)C]S14506 and [(18)F]S14506 were ineffective for imaging rat or monkey brain 5-HT(1A) receptors in vivo, even under P-gp inhibited conditions. Explanations for the failure of these radioligands are offered.

How have developments in molecular imaging techniques furthered schizophrenia research?

Molecular imaging techniques have led to significant advances in understanding the pathophysiology of schizophrenia and contributed to knowledge regarding potential mechanisms of action of the drugs used to treat this illness. The aim of this article is to provide a review of the major findings related to the application of molecular imaging techniques that have furthered schizophrenia research. This article focuses specifically on neuroreceptor imaging studies with PET and SPECT. After providing a brief overview of neuroreceptor imaging methodology, we consider relevant findings from studies of receptor availability, and dopamine synthesis and release. Results are discussed in the context of current hypotheses regarding neurochemical alterations in the illness. We then selectively review pharmacological occupancy studies and the role of neuroreceptor imaging in drug development for schizophrenia.