Determination of in vivo Bmax and Kd for 11C-GR103545, an agonist PET tracer for κ-opioid receptors: a study in nonhuman primates

Evaluation of advanced, pathophysiologic new targets for imaging of CNS

The inadequate information about the in vivo pathological, physiological, and neurological impairments, as well as the absence of in vivo tools for assessing brain penetrance and the efficiency of newly designed drugs, has hampered the development of new techniques for the treatment for variety of new central nervous system (CNS) diseases. The searching sites such as Science Direct and PubMed were used to find out the numerous distinct tracers across 16 CNS targets including tau, synaptic vesicle glycoprotein, the adenosine 2A receptor, the phosphodiesterase enzyme PDE10A, and the purinoceptor, among others. Among the most encouraging are [¹⁸ F]FIMX for mGluR imaging, [¹¹ C]Martinostat for Histone deacetylase, [¹⁸ F]MNI-444 for adenosine 2A imaging, [¹¹ C]ER176 for translocator protein, and [¹⁸ F]MK-6240 for tau imaging. We also reviewed the findings for each tracer's features and potential for application in CNS pathophysiology and therapeutic evaluation investigations, including target specificity, binding efficacy, and pharmacokinetic factors. This review aims to present a current evaluation of modern positron emission tomography tracers for CNS targets, with a focus on recent advances for targets that have newly emerged for imaging in humans.

Imaging Kappa Opioid Receptors in the Living Brain with Positron Emission Tomography

Kappa opioid receptor (KOR) neuroimaging using positron emission tomography (PET) has been immensely successful in all phases of discovery and validation in relation to radiotracer development from preclinical imaging to human imaging. There are now several KOR-specific PET radiotracers that can be utilized for neuroimaging, including agonist and antagonist ligands, as well as C-11 and F-18 variants. These technologies will increase KOR PET utilization by imaging centers around the world and have provided a foundation for future studies. In this chapter, I review the advances in KOR radiotracer discovery, focusing on ligands that have been translated into human imaging, and highlight key attributes unique to each KOR PET radiotracer. The utilization of these radiotracers in KOR PET neuroimaging can be subdivided into three major investigational classes: the first, measurement of KOR density; the second, measurement of KOR drug occupancy; the third, detecting changes in endogenous dynorphin following activation or deactivation. Given the involvement of the KOR/dynorphin system in a number of brain disorders including, but not limited to, pain, itch, mood disorders and addiction, measuring KOR density in the living brain will offer insight into the chronic effects of these disorders on KOR tone in humans. Notably, KOR PET has been successful at measuring drug occupancy in the human brain to guide dose selection for maximal therapeutic efficacy while avoiding harmful side effects. Lastly, we discuss the potential of KOR PET to detect changes in endogenous dynorphin in the human brain, to elucidate neural mechanisms and offer critical insight into disease-modifying therapeutics. We conclude with comments on other translational neuroimaging modalities such as MRI that could be used to study KOR-dynorphin tone in the living human brain.

Dynorphin/Kappa-Opioid Receptor System Modulation of Cortical Circuitry

Cortical circuits control a plethora of behaviors, from sensation to cognition. The cortex is enriched with neuropeptides and receptors that play a role in information processing, including opioid peptides and their cognate receptors. The dynorphin (DYN)/kappa-opioid receptor (KOR) system has been implicated in the processing of sensory and motivationally-charged emotional information and is highly expressed in cortical circuits. This is important as dysregulation of DYN/KOR signaling in limbic and cortical circuits has been implicated in promoting negative affect and cognitive deficits in various neuropsychiatric disorders. However, research investigating the role of this system in controlling cortical circuits and computations therein is limited. Here, we review the (1) basic anatomy of cortical circuits, (2) anatomical architecture of the cortical DYN/KOR system, (3) functional regulation of cortical synaptic transmission and microcircuit function by the DYN/KOR system, (4) regulation of behavior by the cortical DYN/KOR system, (5) implications for the DYN/KOR system for human health and disease, and (6) future directions and unanswered questions for the field. Further work elucidating the role of the DYN/KOR system in controlling cortical information processing and associated behaviors will be of importance to increasing our understanding of principles underlying neuropeptide modulation of cortical circuits, mechanisms underlying sensation and perception, motivated and emotional behavior, and cognition. Increased emphasis in this area of study will also aid in the identification of novel ways to target the DYN/KOR system to treat neuropsychiatric disorders.

Kappa Opioid Receptors in the Pathology and Treatment of Major Depressive Disorder

The kappa opioid receptor (KOR) is thought to regulate neural systems associated with anhedonia and aversion and mediate negative affective states that are associated with a number of psychiatric disorders, but especially major depressive disorder (MDD). Largely because KOR antagonists mitigate the effects of stress in preclinical studies, KOR antagonists have been recommended as novel drugs for treating MDD. The purpose of this review is to examine the role of KORs and its endogenous ligand dynorphins (DYNs) in the pathology and treatment of MDD derived from different types of clinical studies. Evidence pertaining to the role of KOR and MDD will be reviewed from (1) post mortem mRNA expression patterns in MDD, (2) the utility of KOR neuroimaging agents and serum biomarkers in MDD, and (3) evidence from the recent Fast Fail clinical trial that established KOR antagonism as a potential therapeutic strategy for the alleviation of anhedonia, a core feature of MDD. These findings are compared with a focused evaluation of stress-induced alterations in OPRK and PDYN mRNA expression. Finally, the current status of the effects of KOR antagonists on behavioral phenotypes of stress in preclinical studies related to MDD is summarized.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

A Survey of Molecular Imaging of Opioid Receptors

The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as μ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-¹¹C]morphine, -codeine and -heroin did not show significant binding in vivo. [¹¹C]Diprenorphine ([¹¹C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the μ-preferring agonist [¹¹C]carfentanil ([¹¹C]Caf). These two tracers have the property of being displaceable by endogenous opioid peptides in living brain, thus potentially serving in a competition-binding model. Indeed, many clinical PET studies with [¹¹C]DPN or [¹¹C]Caf affirm the release of endogenous opioids in response to painful stimuli. Numerous other PET studies implicate μ-OR signaling in aspects of human personality and vulnerability to drug dependence, but there have been very few clinical PET studies of μORs in neurological disorders. Tracers based on naltrindole, a non-peptide antagonist of the δ-preferring endogenous opioid enkephalin, have been used in PET studies of δORs, and [¹¹C]GR103545 is validated for studies of κORs. Structures such as [¹¹C]NOP-1A show selective binding at ORL-1 receptors in living brain. However, there is scant documentation of δ-, κ-, or ORL1 receptors in healthy human brain or in neurological and psychiatric disorders; here, clinical PET research must catch up with recent progress in radiopharmaceutical chemistry.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

Novel Kappa Opioid Receptor Agonist as Improved PET Radiotracer: Development and in Vivo Evaluation

The kappa opioid receptor (KOR) is involved in depression, alcoholism, and drug abuse. The current agonist radiotracer ¹¹C-GR103545 is not ideal for imaging KOR due to its slow tissue kinetics in human. The aim of our project was to develop novel KOR agonist radiotracers with improved imaging properties. A novel compound FEKAP ((( R))-4-(2-(3,4-dichlorophenyl)acetyl)-3-((ethyl(2-fluoroethyl)amino)methyl) piperazine-1-carboxylate) was designed, synthesized, and assayed for in vitro binding affinities. It was then radiolabeled and evaluated in rhesus monkeys. Baseline and blocking scans were conducted on a Focus-220 scanner to assess binding specificity and selectivity. Metabolite-corrected arterial activities over time were measured and used as input functions to analyze the brain regional time-activity curves and derive kinetic and binding parameters with kinetic modeling. FEKAP displayed high KOR binding affinity ( K_i = 0.43 nM) and selectivity (17-fold over mu opioid receptor and 323-fold over delta opioid receptor) in vitro. ¹¹C-FEKAP was prepared in high molar activity (mean of 718 GBq/μmol, n = 19) and >99% radiochemical purity. In monkeys, ¹¹C-FEKAP metabolized fairly fast, with ∼31% of intact parent fraction at 30 min post-injection. In the brain, it exhibited fast and reversible kinetics with good uptake. Pretreatment with the nonselective opioid receptor antagonist naloxone (1 mg/kg) decreased uptake in high binding regions to the level in the cerebellum, and the selective KOR antagonist LY2456302 (0.02 and 0.1 mg/kg) reduced ¹¹C-FEKAP specific binding in a dose-dependent manner. As a measure of specific binding signals, the mean binding potential ( BP_ND) values of ¹¹C-FEKAP derived from the multilinear analysis-1 (MA1) method were greater than 0.5 for all regions, except for the thalamus. The novel KOR agonist tracer ¹¹C-FEKAP demonstrated binding specificity and selectivity in vivo and exhibited attractive properties of fast tissue kinetics and high specific binding.

Development and In Vivo Evaluation of a κ-Opioid Receptor Agonist as a PET Radiotracer with Superior Imaging Characteristics

Studies have shown κ-opioid receptor (KOR) abnormalities in addictive disorders, other central nervous system diseases, and Alzheimer's disease. We have developed the first set of agonist ¹¹C-GR103545 and antagonist ¹¹C-LY2795050 radiotracers for PET imaging of KOR in humans. Nonetheless, ¹¹C-GR103545 displays protracted uptake kinetics and is not an optimal radiotracer. Here, we report the development and evaluation of ¹¹C-methyl-(R)-4-(2-(3,4-dichlorophenyl)acetyl)-3-((diethylamino)methyl)piperazine-1-carboxylate (¹¹C-EKAP) and its comparison with ¹¹C-GR103545. Methods: EKAP was synthesized and assayed for in vitro binding affinities and then radiolabeled. PET studies were performed on rhesus monkeys. Blocking studies were performed with naloxone and the selective KOR antagonists LY2795050 and LY2456302. Arterial input functions were generated for use in kinetic modeling. Brain TACs were analyzed with multilinear analysis 1 to derive binding parameters. Results: EKAP has high KOR affinity (inhibition constant, 0.28 nM) and good selectivity in vitro. ¹¹C-EKAP was prepared in good radiochemical purity. ¹¹C-EKAP rapidly metabolized in plasma and displayed fast and reversible kinetics in brain, with peak uptake at less than 20 min after injection. Preblocking with naloxone (1 mg/kg) or LY2795050 (0.2 mg/kg) produced 84%-89% receptor occupancy, whereas LY2456302 (0.05 and 0.3 mg/kg) dose-dependently reduced ¹¹C-EKAP-specific binding, thus demonstrating its binding specificity and selectivity in vivo. Mean multilinear analysis 1-derived nondisplaceable binding potential values were 1.74, 1.79, 1.46, 0.80, and 0.77 for cingulate cortex, globus pallidus, insula, striatum, and frontal cortex, respectively, consistent with the known KOR distribution in primate brains. Conclusion: We have successfully developed ¹¹C-EKAP as a KOR agonist tracer with dual attractive imaging properties of fast uptake kinetics and high specific binding in vivo.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high‐affinity state postulates that a certain subset of G‐protein‐coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high‐affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high‐affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high‐affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high‐affinity state in vivo are discussed.

Kappa opioid receptor binding in major depression: A pilot study

Endogenous kappa opioids mediate pathological responses to stress in animal models. However, the relationship of the kappa opioid receptor (KOR) to life stress and to psychopathology in humans is not well described. This pilot study sought, for the first time, to quantify KOR in major depressive disorder (MDD) in vivo in humans using positron emission tomography (PET). KOR binding was quantified in vivo by PET imaging with the [11 C]GR103545 radiotracer in 13 healthy volunteers and 10 participants with current MDD. We examined the relationship between regional [11 C]GR103545 total volume of distribution (VT ) and diagnosis, childhood trauma, recent life stress, and, in a subsample, salivary cortisol levels during a modified Trier Social Stress Test (mTSST), amygdala, hippocampus, ventral striatum and raphe nuclei. Whole-brain voxel-wise analyses were also performed. [11 C]GR103545 VT did not differ significantly between MDD participants and healthy volunteers in the four a priori ROIs (p = 0.50). [11 C]GR103545 VT was unrelated to reported childhood adversity (p = 0.17) or recent life stress (p = 0.56). A trend-level inverse correlation was observed between [11 C]GR103545 VT and cortisol area-under-the curve with respect to ground during the mTSST (p = 0.081). No whole-brain voxel-wise contrasts were significant. Regional [11 C]GR103545 VT , a measure of in vivo KOR binding, does not differentiate MDD from healthy volunteers in this pilot sample. Future studies may examine KOR binding in subgroups of depressed individuals at increased risk for KOR abnormalities, including co-occurring mood and substance use disorders, as well as depression with psychotic features.

Novel 18F-Labeled κ-Opioid Receptor Antagonist as PET Radiotracer: Synthesis and In Vivo Evaluation of 18F-LY2459989 in Nonhuman Primates

The κ-opioid receptor (KOR) has been implicated in depression, addictions, and other central nervous system disorders and, thus, is an important target for drug development. We previously developed several ¹¹C-labeled PET radiotracers for KOR imaging in humans. Here we report the synthesis and evaluation of ¹⁸F-LY2459989 as the first ¹⁸F-labeled KOR antagonist radiotracer in nonhuman primates and its comparison with ¹¹C-LY2459989. Methods: The novel radioligand ¹⁸F-LY2459989 was synthesized by ¹⁸F displacement of a nitro group or an iodonium ylide. PET scans in rhesus monkeys were obtained on a small-animal scanner to assess the pharmacokinetic and in vivo binding properties of the ligand. Metabolite-corrected arterial activity curves were measured and used as input functions in the analysis of brain time-activity curves and the calculation of binding parameters. Results: With the iodonium ylide precursor, ¹⁸F-LY2459989 was prepared at high radiochemical yield (36% ± 7% [mean ± SD]), radiochemical purity (>99%), and mean molar activity (1,175 GBq/μmol; n = 6). In monkeys, ¹⁸F-LY2459989 was metabolized at a moderate rate, with a parent fraction of approximately 35% at 30 min after injection. Fast and reversible kinetics were observed, with a regional peak uptake time of less than 20 min. Pretreatment with the selective KOR antagonist LY2456302 (0.1 mg/kg) decreased the activity level in regions with high levels of binding to that in the cerebellum, thus demonstrating the binding specificity and selectivity of ¹⁸F-LY2459989 in vivo. Regional time-activity curves were well fitted by the multilinear analysis 1 kinetic model to derive reliable estimates of regional distribution volumes. With the cerebellum as the reference region, regional binding potentials were calculated and ranked as follows: cingulate cortex > insula > caudate/putamen > frontal cortex > temporal cortex > thalamus, consistent with the reported KOR distribution in the monkey brain. Conclusion: The evaluation of ¹⁸F-LY2459989 in nonhuman primates demonstrated many attractive imaging properties: fast tissue kinetics, specific and selective binding to the KOR, and high specific binding signals. A side-by-side comparison of ¹⁸F-LY2459989 and ¹¹C-LY2459989 indicated similar kinetic and binding profiles for the 2 radiotracers. Taken together, the results indicated that ¹⁸F-LY2459989 appears to be an excellent PET radiotracer for the imaging and quantification of the KOR in vivo.

Intervention effects of five cations and their correction on hemolytic activity of tentacle extract from the jellyfish Cyanea capillata

Cations have generally been reported to prevent jellyfish venom-induced hemolysis through multiple mechanisms by spectrophotometry. Little attention has been paid to the potential interaction between cations and hemoglobin, potentially influencing the antagonistic effect of cations. Here, we explored the effects of five reported cations, La³⁺, Mn²⁺, Zn²⁺, Cu²⁺ and Fe²⁺, on a hemolytic test system and the absorbance of hemoglobin, which was further used to measure their effects on the hemolysis of tentacle extract (TE) from the jellyfish Cyanea capillata. All the cations displayed significant dose-dependent inhibitory effects on TE-induced hemolysis with various dissociation equilibrium constant (K_d) values as follows: La³⁺ 1.5 mM, Mn²⁺ 93.2 mM, Zn²⁺ 38.6 mM, Cu²⁺ 71.9 μM and Fe²⁺ 32.8 mM. The transparent non-selective pore blocker La³⁺ did not affect the absorbance of hemoglobin, while Mn²⁺ reduced it slightly. Other cations, including Zn²⁺, Cu²⁺ and Fe²⁺, greatly decreased the absorbance with K_d values of 35.9, 77.5 and 17.6 mM, respectively. After correction, the inhibitory K_d values were 1.4 mM, 45.8 mM, 128.5 μM and 53.1 mM for La³⁺, Zn²⁺, Cu²⁺ and Fe²⁺, respectively. Mn²⁺ did not inhibit TE-induced hemolysis. Moreover, the inhibitory extent at the maximal given dose of all cations except La³⁺ was also diminished. These corrected results from spectrophotometry were further confirmed by direct erythrocyte counting under microscopy. Our results indicate that the cations, except for La³⁺, can interfere with the absorbance of hemoglobin, which should be corrected when their inhibitory effects on hemolysis by jellyfish venoms are examined. The variation in the inhibitory effects of cations suggests that the hemolysis by jellyfish venom is mainly attributed to the formation of non-selective cation pore complexes over other potential mechanisms, such as phospholipases A2 (PLA2), polypeptides, protease and oxidation. Blocking the pore-forming complexes may be a primary strategy to improve the in vivo damage and mortality from jellyfish stings due to hemolytic toxicity.

Fluorine-18-Labeled Antagonist for PET Imaging of Kappa Opioid Receptors

Kappa opioid receptor (KOR) antagonists are potential drug candidates for diseases such as treatment-refractory depression, anxiety, and addictive disorders. PET imaging radiotracers for KOR can be used in occupancy study to facilitate drug development, and to investigate the roles of KOR in health and diseases. We have previously developed two ¹¹C-labeled antagonist radiotracers with high affinity and selectivity toward KOR. What is limiting their wide applications is the short half-life of ¹¹C. Herein, we report the synthesis of a first ¹⁸F-labeled KOR antagonist radiotracer and the initial PET imaging study in a nonhuman primate.

N-[18F]-FluoropropylJDTic for κ-opioid receptor PET imaging: Radiosynthesis, pre-clinical evaluation, and metabolic investigation in comparison with parent JDTic

INTRODUCTION

To image kappa opioid receptor (KOR) for preclinical studies, N-fluoropropylJDTic 9 derived from the best-established KOR antagonist JDTic, was labeled with fluorine-18.

METHODS

Radiosynthesis of [¹⁸F]9 was achieved according to an automated two-step procedure from [¹⁸F]-fluoride. Peripheral and cerebral distributions were determined by ex vivo experiments and by PET imaging in mouse. Radiometabolism studies were performed both in vivo in mice and in vitro in mouse and human liver microsomes. Identification of the major metabolic fragmentations was carried out by UPLC-MS analysis of enzymatic cleavage of non-radioactive ligand 9. Microsomal metabolic degradation of parent JDTic was also achieved for comparison.

RESULTS

The radiotracer [¹⁸F]9 was produced after 140±5min total synthesis time (2.2±0.4% not decay corrected radiochemical yield) with a specific activity of 41-89GBq/μmol (1.1-2.4Ci/μmol). Peripheral and regional brain distributions of [¹⁸F]9 were consistent with known KOR locations but no significant specific binding in brain was shown. [¹⁸F]9 presented a typical hepatobiliary and renal elimination, and was rapidly metabolized. The in vivo and in vitro radiometabolic profiles of [¹⁸F]9 were similar. Piperidine 12 was identified as the major metabolic fragment of the non-radioactive ligand 9. JDTic 7 was found to be much more stable than 9.

CONCLUSION

Although the newly proposed radioligand [¹⁸F]9 was concluded to be not suitable for KOR PET imaging due to the formation of brain penetrating radiometabolites, our findings highlight the metabolic stability of JDTic and may help in the design of novel JDTic derivatives for in vivo applications.

Practical PERCIST: A Simplified Guide to PET Response Criteria in Solid Tumors 1.0

Positron Emission Tomography (PET) Response Criteria in Solid Tumors (PERCIST 1.0) describes in detail methods for controlling the quality of fluorine 18 fluorodeoxyglucose PET imaging conditions to ensure the comparability of PET images from different time points to allow quantitative expression of the changes in PET measurements and assessment of overall treatment response in PET studies. The steps for actual application of PERCIST are summarized. Several issues from PERCIST 1.0 that appear to require clarification, such as measurement of size and definition of unequivocal progression, also are addressed. (©) RSNA, 2016.

Immediate and Persistent Effects of Salvinorin A on the Kappa Opioid Receptor in Rodents, Monitored In Vivo with PET

Monitoring changes in opioid receptor binding with positron emission tomography (PET) could lead to a better understanding of tolerance and addiction because altered opioid receptor dynamics following agonist exposure has been linked to tolerance mechanisms. We have studied changes in kappa opioid receptor (KOR) binding availability in vivo with PET following kappa opioid agonist administration. Male Sprague-Dawley rats (n=31) were anesthetized and treated with the (KOR) agonist salvinorin A (0.01-1.8 mg/kg, i.v.) before administration of the KOR selective radiotracer [(11)C]GR103545. When salvinorin A was administered 1 min prior to injection of the radiotracer, [(11)C]GR103545 binding potential (BPND) was decreased in a dose-dependent manner, indicating receptor binding competition. In addition, the unique pharmacokinetics of salvinorin A (half-life ~8 min in non-human primates) allowed us to study the residual impact on KOR after the drug had eliminated from the brain. Salvinorin A was administered up to 5 h prior to [(11)C]GR103545, and the changes in BPND were compared with baseline, 2.5 h, 1 h, and 1 min pretreatment times. At lower doses (0.18 mg/kg and 0.32 mg/kg) we observed no prolonged effect on KOR binding but at 0.60 mg/kg salvinorin A induced a sustained decrease in KOR binding (BPND decreased by 40-49%) which persisted up to 2.5 h post administration, long after salvinorin A had been eliminated from the brain. These data point towards an agonist-induced adaptive response by KOR, the dynamics of which have not been previously studied in vivo with PET.

Determining pharmacological selectivity of the kappa opioid receptor antagonist LY2456302 using pupillometry as a translational biomarker in rat and human

BACKGROUND

Selective kappa opioid receptor antagonism is a promising experimental strategy for the treatment of depression. The kappa opioid receptor antagonist, LY2456302, exhibits ~30-fold higher affinity for kappa opioid receptors over mu opioid receptors, which is the next closest identified pharmacology.

METHODS

Here, we determined kappa opioid receptor pharmacological selectivity of LY2456302 by assessing mu opioid receptor antagonism using translational pupillometry in rats and humans.

RESULTS

In rats, morphine-induced mydriasis was completely blocked by the nonselective opioid receptor antagonist naloxone (3mg/kg, which produced 90% mu opioid receptor occupancy), while 100 and 300 mg/kg LY2456302 (which produced 56% and 87% mu opioid receptor occupancy, respectively) only partially blocked morphine-induced mydriasis. In humans, fentanyl-induced miosis was completely blocked by 50mg naltrexone, and LY2456302 dose-dependently blocked miosis at 25 and 60 mg (minimal-to-no blockade at 4-10mg).

CONCLUSIONS

We demonstrate, for the first time, the use of translational pupillometry in the context of receptor occupancy to identify a clinical dose of LY2456302 achieving maximal kappa opioid receptor occupancy without evidence of significant mu receptor antagonism.

Test-retest reproducibility of binding parameters in humans with 11C-LY2795050, an antagonist PET radiotracer for the κ opioid receptor

(11)C-LY2795050 is a new antagonist PET radioligand for the κ opioid receptor (KOR). In this study, we assessed the reproducibility of the binding parameters of (11)C-LY2795050 in healthy human subjects.

METHODS

Sixteen healthy subjects (11 men and 5 women) underwent 2 separate 90-min PET scans with arterial input function and plasma free fraction (fP) measurements. The 2-tissue-compartment model and multilinear analysis-1 were applied to calculate 5 outcome measures in 14 brain regions: distribution volume (VT), VT normalized by fP (VT/fP), and 3 binding potentials (nondisplaceable binding potential, binding potential relative to total plasma concentration, and binding potential relative to free plasma concentration: BPND, BPP, BPF, respectively). Since KOR is distributed ubiquitously throughout the brain, there are no suitable reference regions. We used a fixed fraction of individual cerebellar VT value (VT,CER) as the nondisplaceable VT (VND) (VND = VT,CER/1.17). The relative and absolute test-retest variability and intraclass correlation coefficient were evaluated for the outcome measures of (11)C-LY2795050.

RESULTS

The test-retest variability of (11)C-LY2795050 for VT was no more than 10% in any region and was 12% in the amygdala. For binding potential (BPND and BPP), the test-retest variability was good in regions of moderate and high KOR density (BPND > 0.4) and poor in regions of low density. Correction by fP (VT/fP or BPF) did not improve the test-retest performance.

CONCLUSION

Our results suggest that quantification of (11)C-LY2795050 imaging is reproducible and reliable in regions with moderate and high KOR density. Therefore, we conclude that this first antagonist radiotracer is highly useful for PET studies of KOR.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Kinetic modeling of (11)C-LY2795050, a novel antagonist radiotracer for PET imaging of the kappa opioid receptor in humans

(11)C-LY2795050 is a novel kappa opioid receptor (KOR) antagonist tracer for positron emission tomography (PET) imaging. The purpose of this first-in-human study was to determine the optimal kinetic model for analysis of (11)C-LY2795050 imaging data. Sixteen subjects underwent baseline scans and blocking scans after oral naltrexone. Compartmental modeling and multilinear analysis-1 (MA1) were applied using the arterial input functions. Two-tissue compartment model and MA1 were found to be the best models to provide reliable measures of binding parameters. The rank order of (11)C-LY2795050 distribution volume (VT) matched the known regional KOR densities in the human brain. Blocking scans with naltrexone indicated no ideal reference region for (11)C-LY2795050. Three methods for calculation of the nondisplaceable distribution volume (VND) were assessed: (1) individual VND estimated from naltrexone occupancy plots, (2) mean VND across subjects, and (3) a fixed fraction of cerebellum VT. Approach (3) produced the lowest intersubject variability in the calculation of binding potentials (BPND, BPF, and BPP). Therefore, binding potentials of (11)C-LY2795050 can be determined if the specific binding fraction in the cerebellum is presumed to be unchanged by diseases and experimental conditions. In conclusion, results from the present study show the suitability of (11)C-LY2795050 to image and quantify KOR in humans.

An Improved Antagonist Radiotracer for the κ-Opioid Receptor: Synthesis and Characterization of (11)C-LY2459989

The κ-opioid receptors (KORs) are implicated in several neuropsychiatric diseases and addictive disorders. PET with radioligands provides a means to image the KOR in vivo and investigate its function in health and disease. The purpose of this study was to develop the selective KOR antagonist (11)C-LY2459989 as a PET radioligand and characterize its imaging performance in nonhuman primates.

METHODS

LY2459989 was synthesized and assayed for in vitro binding to opioid receptors. Ex vivo studies in rodents were conducted to assess its potential as a tracer candidate. (11)C-LY2459989 was synthesized by reaction of its iodophenyl precursor with (11)C-cyanide, followed by partial hydrolysis of the resulting (11)C-cyanophenyl intermediate. Imaging experiments with (11)C-LY2459989 were performed in rhesus monkeys with arterial input function measurement. Imaging data were analyzed with kinetic models to derive in vivo binding parameters.

RESULTS

LY2459989 is a full antagonist with high binding affinity and selectivity for KOR (0.18, 7.68, and 91.3 nM, respectively, for κ, μ, and δ receptors). Ex vivo studies in rats indicated LY2459989 as an appropriate tracer candidate with high specific binding signals and confirmed its KOR binding selectivity in vivo. (11)C-LY2459989 was synthesized in high radiochemical purity and good specific activity. In rhesus monkeys, (11)C-LY2459989 displayed a fast rate of peripheral metabolism. Similarly, (11)C-LY2459989 displayed fast uptake kinetics in the brain and an uptake pattern consistent with the distribution of KOR in primates. Pretreatment with naloxone (1 mg/kg, intravenously) resulted in a uniform distribution of radioactivity in the brain. Further, specific binding of (11)C-LY2459989 was dose-dependently reduced by the selective KOR antagonist LY2456302 and the unlabeled LY2459989. Regional binding potential values derived from the multilinear analysis-1 (MA1) method, as a measure of in vivo specific binding signal, were 2.18, 1.39, 1.08, 1.04, 1.03, 0.59, 0.51, and 0.50, respectively, for the globus pallidus, cingulate cortex, insula, caudate, putamen, frontal cortex, temporal cortex, and thalamus.

CONCLUSION

The novel PET radioligand (11)C-LY2459989 displayed favorable pharmacokinetic properties, a specific and KOR-selective binding profile, and high specific binding signals in vivo, thus making it a promising PET imaging agent for KOR.

Evaluation of the agonist PET radioligand [¹¹C]GR103545 to image kappa opioid receptor in humans: kinetic model selection, test-retest reproducibility and receptor occupancy by the antagonist PF-04455242

INTRODUCTION

Kappa opioid receptors (KOR) are implicated in several brain disorders. In this report, a first-in-human positron emission tomography (PET) study was conducted with the potent and selective KOR agonist tracer, [(11)C]GR103545, to determine an appropriate kinetic model for analysis of PET imaging data and assess the test-retest reproducibility of model-derived binding parameters. The non-displaceable distribution volume (V(ND)) was estimated from a blocking study with naltrexone. In addition, KOR occupancy of PF-04455242, a selective KOR antagonist that is active in preclinical models of depression, was also investigated.

METHODS

For determination of a kinetic model and evaluation of test-retest reproducibility, 11 subjects were scanned twice with [(11)C]GR103545. Seven subjects were scanned before and 75 min after oral administration of naltrexone (150 mg). For the KOR occupancy study, six subjects were scanned at baseline and 1.5 h and 8 h after an oral dose of PF-04455242 (15 mg, n=1 and 30 mg, n=5). Metabolite-corrected arterial input functions were measured and all scans were 150 min in duration. Regional time-activity curves (TACs) were analyzed with 1- and 2-tissue compartment models (1TC and 2TC) and the multilinear analysis (MA1) method to derive regional volume of distribution (V(T)). Relative test-retest variability (TRV), absolute test-retest variability (aTRV) and intra-class coefficient (ICC) were calculated to assess test-retest reproducibility of regional VT. Occupancy plots were computed for blocking studies to estimate occupancy and V(ND). The half maximal inhibitory concentration (IC50) of PF-04455242 was determined from occupancies and drug concentrations in plasma. [(11)C]GR103545 in vivo K(D) was also estimated.

RESULTS

Regional TACs were well described by the 2TC model and MA1. However, 2TC VT was sometimes estimated with high standard error. Thus MA1 was the model of choice. Test-retest variability was ~15%, depending on the outcome measure. The blocking studies with naltrexone and PF-04455242 showed that V(T) was reduced in all regions; thus no suitable reference region is available for the radiotracer. V(ND) was estimated reliably from the occupancy plot of naltrexone blocking (V(ND)=3.4±0.9 mL/cm(3)). The IC50 of PF-04455242 was calculated as 55 ng/mL. [(11)C]GR103545 in vivo K(D) value was estimated as 0.069 nmol/L.

CONCLUSIONS

[(11)C]GR103545 PET can be used to image and quantify KOR in humans, although it has slow kinetics and variability of model-derived kinetic parameters is higher than desirable. This tracer should be suitable for use in receptor occupancy studies, particularly those that target high occupancy.

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.