¹¹C-ORM-13070, a novel PET ligand for brain α₂C-adrenoceptors: radiometabolism, plasma pharmacokinetics, whole-body distribution and radiation dosimetry in healthy men

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

Application of the PET ligand [11C]ORM-13070 to examine receptor occupancy by the α2C-adrenoceptor antagonist ORM-12741: translational validation of target engagement in rat and human brain

Background

Availability of the α_2C-adrenoceptor (α_2C-AR) positron emission tomography (PET) tracer, [¹¹C]ORM-13070, and the α_2C-AR antagonist ORM-12741 allows probing of the roles of this G-protein coupled receptor subtype in brain function, both in healthy humans and in patients with various brain disorders. This translational study employed [¹¹C]ORM-13070 autoradiography and PET to determine α_2C-AR occupancy by ORM-12741 in rat and human brain, respectively.

Results

ORM-12741 has high affinity (K_i: 0.08 nM) and potent antagonist activity (K_b: 0.04 nM) as well as selectivity (K_i estimates for the human α_2A-AR and α_2B-AR were 8.3 nM and 0.8 nM, respectively) for the human α_2C-AR subtype. [¹¹C]ORM-13070 had highest uptake in the basal ganglia of rat and human brain. Pretreatment with ORM-12741 inhibited [¹¹C]ORM-13070 binding in rat striatum in a time- and dose-dependent manner at 10 and 50 µg/kg (s.c.) with an EC₅₀ estimate of 1.42 ng/mL in rat plasma, corresponding to protein-free drug concentration of 0.23 nM. In the living human brain, time- and dose-related α_2C-AR occupancy was detected with EC₅₀ estimates of 24 ng/mL and 31 ng/mL for the caudate nucleus and putamen, respectively, corresponding to protein-free concentrations in plasma of 0.07 nM and 0.1 nM. Modelling-based maximum α_2C-AR occupancy estimates were 63% and 52% in the caudate nucleus and the putamen, respectively.

Conclusions

ORM-12741 is a selective α_2C-AR antagonist which penetrates the rat and human brain to occupy α_2C-ARs in a manner consistent with its receptor pharmacology.

Trialregistrationnumberanddateofregistration: ClinicalTrial.cov NCT00829907. Registered 11 December 2008. https://clinicaltrials.gov/.

PET Radiotracers for CNS-Adrenergic Receptors: Developments and Perspectives

Epinephrine (E) and norepinephrine (NE) play diverse roles in our body’s physiology. In addition to their role in the peripheral nervous system (PNS), E/NE systems including their receptors are critical to the central nervous system (CNS) and to mental health. Various antipsychotics, antidepressants, and psychostimulants exert their influence partially through different subtypes of adrenergic receptors (ARs). Despite the potential of pharmacological applications and long history of research related to E/NE systems, research efforts to identify the roles of ARs in the human brain taking advantage of imaging have been limited by the lack of subtype specific ligands for ARs and brain penetrability issues. This review provides an overview of the development of positron emission tomography (PET) radiotracers for in vivo imaging of AR system in the brain.

Selective adrenergic alpha2C receptor antagonist ameliorates acute phencyclidine-induced schizophrenia-like social interaction deficits in rats

RATIONALE

Social withdrawal is a core feature of the negative symptoms of schizophrenia. Currently available pharmacotherapies have only limited efficacy towards the negative symptoms, i.e., there is a significant unmet medical need in the treatment of these symptoms.

OBJECTIVE

We wanted to confirm whether selective adrenergic α_2C receptor (AR) antagonist therapy could ameliorate acute phencyclidine (PCP)-induced schizophrenia-like social interaction deficits in rats, and to compare the effects of an α_2C AR antagonist to another putative therapeutic alternative, an α₇ nicotinic acetylcholine receptor (nAChR) partial agonist, as well against three commonly used atypical antipsychotics.

METHODS

Here, we used acute PCP administration and modified a protocol for testing social interaction deficits in male Wistar rats and then used this model to compare the effects of an α_2C AR antagonist (ORM-13070 0.3 and 1.0 mg/kg s.c.) with an α₇ nAChR partial agonist (EVP-6124 0.3 mg/kg s.c.) and three atypical antipsychotics (clozapine 2.5 mg/kg i.p., risperidone 0.04 and 0.08 mg/kg s.c., olanzapine 0.125 and 0.5 mg/kg s.c.) on social interaction behavior.

RESULTS

Acute PCP (1.5 mg/kg s.c.) produced robust and reproducible deficits in social interaction behavior without affecting locomotor activity. The selective α_2C AR antagonist significantly ameliorated PCP-induced social interaction deficits. In contrast, neither the partial α₇ nAChR agonist nor any of the three atypical antipsychotics were able to reverse the behavioral deficits at the selected doses.

CONCLUSION

Our findings confirm that α_2C AR antagonism is a potential mechanism for the treatment of the negative symptoms of schizophrenia.

Less Exploited GPCRs in Precision Medicine: Targets for Molecular Imaging and Theranostics

Precision medicine relies on individually tailored therapeutic intervention taking into account individual variability. It is strongly dependent on the availability of target-specific drugs and/or imaging agents that recognize molecular targets and patient-specific disease mechanisms. The most sensitive molecular imaging modalities, Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET), rely on the interaction between an imaging radioprobe and a target. Moreover, the use of target-specific molecular tools for both diagnostics and therapy, theranostic agents, represent an established methodology in nuclear medicine that is assuming an increasingly important role in precision medicine. The design of innovative imaging and/or theranostic agents is key for further accomplishments in the field. G-protein-coupled receptors (GPCRs), apart from being highly relevant drug targets, have also been largely exploited as molecular targets for non-invasive imaging and/or systemic radiotherapy of various diseases. Herein, we will discuss recent efforts towards the development of innovative imaging and/or theranostic agents targeting selected emergent GPCRs, namely the Frizzled receptor (FZD), Ghrelin receptor (GHSR-1a), G protein-coupled estrogen receptor (GPER), and Sphingosine-1-phosphate receptor (S1PR). The pharmacological and clinical relevance will be highlighted, giving particular attention to the studies on the synthesis and characterization of targeted molecular imaging agents, biological evaluation, and potential clinical applications in oncology and non-oncology diseases. Whenever relevant, supporting computational studies will be also discussed.

The importance of small polar radiometabolites in molecular neuroimaging: A PET study with [11C]Cimbi-36 labeled in two positions

[11C]Cimbi-36, a 5-HT2A receptor agonist PET radioligand, contains three methoxy groups amenable to [11C]-labeling. In pigs, [11C]Cimbi-36 yields a polar (M1) and a less polar (M2) radiometabolite fraction, while changing the labeling to [11C]Cimbi-36_5 yields only the M1 fraction. We investigate whether changing the labeling position of [11C]Cimbi-36 eliminates M2 in humans, and if this changes the signal-to-background ratio. Six healthy volunteers each underwent two dynamic PET scans; after injection of [11C]Cimbi-36, both the M1 and M2 fraction appeared in plasma, whereas only the M1 appeared after [11C]Cimbi-36_5 injection. [11C]Cimbi-36_5 generated higher uptake than [11C]Cimbi-36 in both neocortex and cerebellum. With the simplified reference tissue model mean neocortical non-displaceable binding potential for [11C]Cimbi-36 was 1.38 ± 0.07, whereas for [11C]Cimbi-36_5, it was 1.18 ± 0.14. This significant difference can be explained by higher non-displaceable binding caused by demethylation products in the M1 fraction such as [11C]formaldehyde and/or [11C]carbon dioxide/bicarbonate. Although often considered without any impact on binding measures, we show that small polar radiometabolites can substantially decrease the signal-to-background ratio of PET radioligands for neuroimaging. Further, we find that [11C]Cimbi-36 has a better signal-to-background ratio than [11C]Cimbi-36_5, and thus will be more sensitive to changes in 5-HT2A receptor levels in the brain.

Therapeutic Potential of Selectively Targeting the α2C-Adrenoceptor in Cognition, Depression, and Schizophrenia-New Developments and Future Perspective

α_2A- and α_2C-adrenoceptors (ARs) are the primary α₂-AR subtypes involved in central nervous system (CNS) function. These receptors are implicated in the pathophysiology of psychiatric illness, particularly those associated with affective, psychotic, and cognitive symptoms. Indeed, non-selective α₂-AR blockade is proposed to contribute toward antidepressant (e.g., mirtazapine) and atypical antipsychotic (e.g., clozapine) drug action. Both α_2C- and α_2A-AR share autoreceptor functions to exert negative feedback control on noradrenaline (NA) release, with α_2C-AR heteroreceptors regulating non-noradrenergic transmission (e.g., serotonin, dopamine). While the α_2A-AR is widely distributed throughout the CNS, α_2C-AR expression is more restricted, suggesting the possibility of significant differences in how these two receptor subtypes modulate regional neurotransmission. However, the α_2C-AR plays a more prominent role during states of low endogenous NA activity, while the α_2A-AR is relatively more engaged during states of high noradrenergic tone. Although augmentation of conventional antidepressant and antipsychotic therapy with non-selective α₂-AR antagonists may improve therapeutic outcome, animal studies report distinct yet often opposing roles for the α_2A- and α_2C-ARs on behavioral markers of mood and cognition, implying that non-selective α₂-AR antagonism may compromise therapeutic utility both in terms of efficacy and side-effect liability. Recently, several highly selective α_2C-AR antagonists have been identified that have allowed deeper investigation into the function and utility of the α_2C-AR. ORM-13070 is a useful positron emission tomography ligand, ORM-10921 has demonstrated antipsychotic, antidepressant, and pro-cognitive actions in animals, while ORM-12741 is in clinical development for the treatment of cognitive dysfunction and neuropsychiatric symptoms in Alzheimer's disease. This review will emphasize the importance and relevance of the α_2C-AR as a neuropsychiatric drug target in major depression, schizophrenia, and associated cognitive deficits. In addition, we will present new prospects and future directions of investigation.

Tolerability of ORM-12741 and effects on episodic memory in patients with Alzheimer's disease

Introduction

ORM-12741 is a novel selective antagonist of alpha-2C adrenoceptors. This trial evaluated the safety and efficacy of ORM-12741 in patients with Alzheimer's disease (AD).

Methods

A randomized, double-blind, placebo-controlled, exploratory phase 2a trial was conducted in 100 subjects with AD and neuropsychiatric symptoms. Participants were randomized to receive one of two flexible doses of ORM-12741 (30–60 mg or 100–200 mg) or placebo b.i.d. for 12 weeks in addition to standard therapy with cholinesterase inhibitors. Efficacy was assessed primarily with the Cognitive Drug Research (CDR) computerized assessment system and secondarily with the Neuropsychiatric Inventory (NPI).

Results

A statistically significant treatment effect was seen in one of the four primary CDR system end points, Quality of Episodic Memory (P = .030; not adjusted for multiple comparisons), favoring ORM-12741 over placebo. NPI caregiver distress scores also favored ORM-12741 (P = .034). ORM-12741 was well tolerated.

Discussion

This is the first clinical trial providing evidence on an acceptable safety profile for ORM-12741 in patients with AD and neuropsychiatric symptoms. In addition, the trial provided hints of potential therapeutic benefit, primarily on episodic memory, in this patient population.

Plasma radiometabolite correction in dynamic PET studies: Insights on the available modeling approaches

Full kinetic modeling of dynamic PET images requires the measurement of radioligand concentrations in the arterial plasma. The unchanged parent radioligand must, however, be separated from its radiometabolites by chromatographic methods. Thus, only few samples can usually be analyzed and the resulting measurements are often noisy. Therefore, the measurements must be fitted with a mathematical model. This work presents a comprehensive analysis of the different models proposed in the literature to describe the plasma parent fraction (PPf) and of the alternative approaches for radiometabolite correction. Finally, we used a dataset of [(11)C]PBR28 brain PET data as a case study to guide the reader through the PPf model selection process.

Sensitivity of [(11)C]ORM-13070 to increased extracellular noradrenaline in the CNS - a PET study in human subjects

RATIONALE

No validated methods have been available for studying brain noradrenergic neurotransmission in vivo in humans. Positron emission tomography (PET) radiotracers are widely used in clinical drug development targeted to brain receptors and can also in some cases be employed to monitor extracellular (synaptic) neurotransmitter concentrations.

OBJECTIVES

The objective of this study is to test the sensitivity of [(11)C]ORM-13070 uptake to increased concentrations of extracellular (synaptic) noradrenaline in the human brain.

METHODS

Eight subjects underwent a control PET scan with [(11)C]ORM-13070, a subtype-selective α2C-adrenoceptor antagonist radioligand, and two PET scans after two different noradrenaline challenges, i.e. during ketamine infusion and after a dose of atomoxetine combined with cold stimulation. Tracer uptake in the caudate nucleus and putamen was described with AUC values in scan time windows of 10-20 and 5-30 min post injection and quantified with the ratio method. Voxel-based analysis was performed with average bound per free (B/F) ratio images.

RESULTS

Both noradrenaline challenges were consistently associated with 10-20 % (p < 0.05) reductions in tracer uptake in the dorsal striatum, as determined with region-of-interest-based analysis. Voxel-based analysis revealed significant reductions in B/F ratios in the dorsal striatum, in the brain stem and in several cortical areas. Reductions of 24 and 23 % were detected in the peak putamen clusters with ketamine and atomoxetine + cold, respectively.

CONCLUSION

Direct experimental support was gained for the suitability of [(11)C]ORM-13070 for imaging of brain noradrenergic neurotransmission.

Amphetamine decreases α2C-adrenoceptor binding of [11C]ORM-13070: a PET study in the primate brain

BACKGROUND

The neurotransmitter norepinephrine has been implicated in psychiatric and neurodegenerative disorders. Examination of synaptic norepinephrine concentrations in the living brain may be possible with positron emission tomography (PET), but has been hampered by the lack of suitable radioligands.

METHODS

We explored the use of the novel α2C-adrenoceptor antagonist PET tracer [(11)C]ORM-13070 for measurement of amphetamine-induced changes in synaptic norepinephrine. The effect of amphetamine on [(11)C]ORM-13070 binding was evaluated ex vivo in rat brain sections and in vivo with PET imaging in monkeys.

RESULTS

Microdialysis experiments confirmed amphetamine-induced elevations in rat striatal norepinephrine and dopamine concentrations. Regional [(11)C]ORM-13070 receptor binding was high in the striatum and low in the cerebellum. After injection of [(11)C]ORM-13070 in rats, mean striatal specific binding ratios, determined using cerebellum as a reference region, were 1.4±0.3 after vehicle pretreatment and 1.2±0.2 after amphetamine administration (0.3mg/kg, subcutaneous). Injection of [(11)C]ORM-13070 in non-human primates resulted in mean striatal binding potential (BP ND) estimates of 0.65±0.12 at baseline. Intravenous administration of amphetamine (0.5 and 1.0mg/kg, i.v.) reduced BP ND values by 31-50%. Amphetamine (0.3mg/kg, subcutaneous) increased extracellular norepinephrine (by 400%) and dopamine (by 270%) in rat striata.

CONCLUSIONS

Together, these results indicate that [(11)C]ORM-13070 may be a useful tool for evaluation of synaptic norepinephrine concentrations in vivo. Future studies are required to further understand a potential contribution of dopamine to the amphetamine-induced effect.

Validation of [(11) C]ORM-13070 as a PET tracer for alpha2c -adrenoceptors in the human brain

This study explored the use of the α2C -adrenoceptor PET tracer [(11) C]ORM-13070 to monitor α2C -AR occupancy in the human brain. The subtype-nonselective α2 -AR antagonist atipamezole was administered to eight healthy volunteer subjects to determine its efficacy and potency (Emax and EC50 ) at inhibiting tracer uptake. We also explored whether the tracer could reveal changes in the synaptic concentrations of endogenous noradrenaline in the brain, in response to several pharmacological and sensory challenge conditions. We assessed occupancy from the bound-to-free ratio measured during 5-30 min post injection. Based on extrapolation of one-site binding, the maximal extent of inhibition of striatal [(11) C]ORM-13070 uptake (Emax ) achievable by atipamezole was 78% (95% CI 69-87%) in the caudate nucleus and 65% (53-77%) in the putamen. The EC50 estimates of atipamezole (1.6 and 2.5 ng/ml, respectively) were in agreement with the drug's affinity to α2C -ARs. These findings represent clear support for the use of [(11) C]ORM-13070 for monitoring drug occupancy of α2C -ARs in the living human brain. Three of the employed noradrenaline challenges were associated with small, approximately 10-16% average reductions in tracer uptake in the dorsal striatum (atomoxetine, ketamine, and the cold pressor test; P < 0.05 for all), but insulin-induced hypoglycemia did not affect tracer uptake. The tracer is suitable for studying central nervous system receptor occupancy by α2C -AR ligands in human subjects. [(11) C]ORM-13070 also holds potential as a tool for in vivo monitoring of synaptic concentrations of noradrenaline, but this remains to be further evaluated in future studies.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.