Radiosynthesis and evaluation of [11C]EMPA as a potential PET tracer for orexin 2 receptors

Heteroaryl derivatives of suvorexant as OX1R selective PET ligand candidates: Cu-mediated 18F-fluorination of boroxines, in vitro and initial in vivo evaluation

BACKGROUND

The orexin receptor (OXR) plays a role in drug addiction and is aberrantly expressed in colorectal tumors. Subtype-selective OXR PET ligands suitable for in vivo use have not yet been reported. This work reports the development of 18F-labeled OXR PET ligand candidates derived from the OXR antagonist suvorexant and the OX1R-selective antagonist JH112.

RESULTS

Computational analysis predicted that fluorine substitution (1e) and introduction of the fluorobenzothiazole scaffold (1f) would be suitable for maintaining high OX1R affinity. After multi-step synthesis of 1a-1f, in vitro OXR binding studies confirmed the molecular dynamics calculations and revealed single-digit nanomolar OX1R affinities for 1a-f, ranging from 0.69 to 2.5 nM. The benzothiazole 1f showed high OX1R affinity (Ki = 0.69 nM), along with 77-fold subtype selectivity over OX2R. Cu-mediated 18F-fluorination of boroxine precursors allowed for a shortened reaction time of 5 min to provide the non-selective OXR ligand [18F]1c and its selective OX1R congener [18F]1f in activity yields of 14% and 22%, respectively, within a total synthesis time of 52-76 min. [18F]1c and [18F]1f were stable in plasma and serum in vitro, with logD7.4 of 2.28 ([18F]1c) and 2.37 ([18F]1f), and high plasma protein binding of 66% and 77%, respectively. Dynamic PET imaging in rats showed similar brain uptake of [18F]1c (0.17%ID/g) and [18F]1f (0.15%ID/g). However, preinjection of suvorexant did not significantly block [18F]1c or [18F]1f uptake in the rat brain. Pretreatment with cyclosporine A to study the role of P-glycoprotein (P-gp) in limiting brain accumulation moderately increased brain uptake of [18F]1c and [18F]1f. Accordingly, in vitro experiments demonstrated that the P-gp inhibitor zosuquidar only moderately inhibited polarized, basal to apical transport of 1c (p < 0.05) and had no effect on the transport of 1f, indicating that P-gp does not play a relevant role in brain accumulation of [18F]1c and [18F]1f in vivo.

CONCLUSIONS

The in vitro and in vivo results of [18F]1c and [18F]1f provide a solid basis for further development of suitable OXR PET ligands for brain imaging.

Structure-guided discovery of orexin receptor-binding PET ligands

Molecular imaging using positron emission tomography (PET) can serve as a promising tool for visualizing biological targets in the brain. Insights into the expression pattern and the in vivo imaging of the G protein-coupled orexin receptors OX1R and OX2R will further our understanding of the orexin system and its role in various physiological and pathophysiological processes. Guided by crystal structures of our lead compound JH112 and the approved hypnotic drug suvorexant bound to OX1R and OX2R, respectively, we herein describe the design and synthesis of two novel radioligands, [18F]KD23 and [18F]KD10. Key to the success of our structural modifications was a bioisosteric replacement of the triazole moiety with a fluorophenyl group. The 19F-substituted analog KD23 showed high affinity for the OX1R and selectivity over OX2R, while the high affinity ligand KD10 displayed similar Ki values for both subtypes. Radiolabeling starting from the respective pinacol ester precursors resulted in excellent radiochemical yields of 93% and 88% for [18F]KD23 and [18F]KD10, respectively, within 20 min. The new compounds will be useful in PET studies aimed at subtype-selective imaging of orexin receptors in brain tissue.

Synthesis and Characterization of a New Carbon-11 Labeled Positron Emission Tomography Radiotracer for Orexin 2 Receptors Neuroimaging

Purpose

Orexin receptors (OXRs) play a crucial role in modulating various physiological and neuropsychiatric functions within the central nervous system (CNS). Despite their significance, the precise role of OXRs in the brain remains elusive. Positron emission tomography (PET) imaging is instrumental in unraveling CNS functions, and the development of specific PET tracers for OXRs is a current research focus.

Methods

The study investigated MDK-5220, an OX2R-selective agonist with promising binding properties (EC50 on OX2R: 0.023 μM, Ki on hOX2R: 0.14 μM). Synthesized and characterized as an OX2R PET probe, [11C]MDK-5220 was evaluated for its potential as a tracer. Biodistribution studies in mice were conducted to assess OX2R binding selectivity, with particular attention to its interaction with P-glycoprotein (P-gp) on the blood-brain barrier.

Results

[11C]MDK-5220 exhibited promising attributes as an OX2R PET probe, demonstrating robust OX2R binding selectivity in biodistribution studies. However, an observed interaction with P-gp impacted its brain uptake. Despite this limitation, [11C]MDK-5220 presents itself as a potential candidate for further development.

Discussion

The study provides insights into the functionality of the OX system and the potential of [11C]MDK-5220 as an OX2R PET probe. The observed interaction with P-gp highlights a consideration for future modifications to enhance brain uptake. The findings pave the way for innovative tracer development and propel ongoing research on OX systems, contributing to a deeper understanding of their role in the CNS.

Conclusion

[11C]MDK-5220 emerges as a promising OX2R PET probe, despite challenges related to P-gp interaction. This study lays the foundation for further exploration and development of PET probes targeting OXRs, opening avenues for advancing our understanding of OX system functionality within the brain.

Development of Novel 11C-Labeled Selective Orexin-2 Receptor Radioligands for Positron Emission Tomography Imaging

Orexin 2 receptors (OX2R) represent a vital subtype of orexin receptors intricately involved in the regulation of wakefulness, arousal, and sleep-wake cycles. Despite their importance, there are currently no positron emission tomography (PET) tracers available for imaging the OX2R in vivo. Herein, we report [11C]1 ([11C]OX2-2201) and [11C]2 ([11C]OX2-2202) as novel PET ligands. Both compounds 1 (Ki = 3.6 nM) and 2 (Ki = 2.2 nM) have excellent binding affinity activities toward OX2R and target selectivity (OX2/OX1 > 600 folds). In vitro autoradiography in the rat brain suggested good to excellent in vitro binding specificity for [11C]1 and [11C]2. PET imaging in rat brains indicated that the low brain uptake of [11C]2 may be due to P-glycoprotein and/or breast cancer resistance protein efflux interaction and/or low passive permeability. Continuous effort in medicinal chemistry optimization is necessary to improve the brain permeability of this scaffold.

Targeting orexin receptors: Recent advances in the development of subtype selective or dual ligands for the treatment of neuropsychiatric disorders

Orexin-A and orexin-B, also named hypocretin-1 and hypocretin-2, are two hypothalamic neuropeptides highly conserved across mammalian species. Their effects are mediated by two distinct G protein-coupled receptors, namely orexin receptor type 1 (OX1-R) and type 2 (OX2-R), which share 64% amino acid identity. Given the wide expression of OX-Rs in different central nervous system and peripheral areas and the several pathophysiological functions in which they are involved, including sleep-wake cycle regulation (mainly mediated by OX2-R), emotion, panic-like behaviors, anxiety/stress, food intake, and energy homeostasis (mainly mediated by OX1-R), both subtypes represent targets of interest for many structure-activity relationship (SAR) campaigns carried out by pharmaceutical companies and academies. However, before 2017 the research was predominantly directed towards dual-orexin ligands, and limited chemotypes were investigated. Analytical characterizations, including resolved structures for both OX1-R and OX2-R in complex with agonists and antagonists, have improved the understanding of the molecular basis of receptor recognition and are assets for medicinal chemists in the design of subtype-selective ligands. This review is focused on the medicinal chemistry aspects of small molecules acting as dual or subtype selective OX1-R/OX2-R agonists and antagonists belonging to different chemotypes and developed in the last years, including radiolabeled OX-R ligands for molecular imaging. Moreover, the pharmacological effects of the most studied ligands in different neuropsychiatric diseases, such as sleep, mood, substance use, and eating disorders, as well as pain, have been discussed. Poly-pharmacology applications and multitarget ligands have also been considered.

Synthesis and Characterization of Novel Radioiodinated Triazole-Pyrolidine Derivative to Detect Orexin 2 Receptor in the Brain

It is generally accepted that the orexin 2 receptor (OX₂R) plays a critical role in the arousal-promoting function, and in vivo imaging of OX₂R is expected to contribute to elucidation of orexin systems and the development of drugs to treat sleep disorder. In this study, we newly synthesized and characterized a radioiodinated triazole-pyrolidine derivative ([¹²⁵I]TPI) to detect OX₂R in the brain. In vitro studies using OX₁R or OX₂R expression cells showed selective binding of [¹²⁵I]TPI to OX₂R. In addition, in vitro autoradiography using rat brain sections showed high accumulation of radioactivity in the OX₂R expression region. However, [¹²⁵I]TPI showed low brain uptake in normal mice. These results suggest that [¹²⁵I]TPI has a fundamental character to detect OX₂R in vitro, but further structural modification to improve brain pharmacokinetics is required to use it for in vivo detection of OX₂R.

Synthesis and characterization of a new Positron emission tomography probe for orexin 2 receptors neuroimaging

The orexin receptors (OXRs) have been involved in multiple physiological and neuropsychiatric functions. Identification of PET imaging probes specifically targeting OXRs enables us to better understand the OX system. Seltorexant (JNJ-42847922) is a potent OX2R antagonist with the potential to be an OX2R PET imaging probe. Here, we describe the synthesis and characterization of [18F]Seltorexant as an OX2R PET probe. The ex vivo autoradiography studies indicated the good binding specificity of [18F]Seltorexant. In vivo PET imaging of [18F]Seltorexant in rodents showed suitable BBB penetration with the highest brain uptake of %ID/cc = 3.4 at 2 min post-injection in mice. The regional brain biodistribution analysis and blocking studies showed that [18F]Seltorexant had good binding selectivity and specificity. However, pretreatment with unlabelled Seltorexant and P-gp competitor CsA observed significantly increased brain uptake of [18F]Seltorexant, indicating [18F]Seltorexant could interact P-gp at the blood-brain barrier. Our findings demonstrated that [18F]Seltorexant is a potential brain OX2R PET imaging probe, which paves the way for new OX2R PET probes development and OX system investigation.

Positron Emission Tomography in the Neuroimaging of Autism Spectrum Disorder: A Review

Autism spectrum disorder (ASD) is a basket term for neurodevelopmental disorders characterized by marked impairments in social interactions, repetitive and stereotypical behaviors, and restricted interests and activities. Subtypes include (A) disorders with known genetic abnormalities including fragile X syndrome, Rett syndrome, and tuberous sclerosis and (B) idiopathic ASD, conditions with unknown etiologies. Positron emission tomography (PET) is a molecular imaging technology that can be utilized in vivo for dynamic and quantitative research, and is a valuable tool for exploring pathophysiological mechanisms, evaluating therapeutic efficacy, and accelerating drug development in ASD. Recently, several imaging studies on ASD have been published and physiological changes during ASD progression was disclosed by PET. This paper reviews the specific radioligands for PET imaging of critical biomarkers in ASD, and summarizes and discusses the similar and different discoveries in outcomes of previous studies. It is of great importance to identify general physiological changes in cerebral glucose metabolism, cerebral blood flow perfusion, abnormalities in neurotransmitter systems, and inflammation in the central nervous system in ASD, which may provide excellent points for further ASD research.

Synthesis and biological evaluation of novel 18F-labeled phenylbenzofuran-2-carboxamide derivative for detection of orexin 1 receptor in the brain

Although the orexin 1 receptor (OX₁R) in the brain is considered to regulate reward and feeding, the in vivo function of OX₁R has not been fully elucidated. In vivo imaging of OX₁R with positron emission tomography (PET) may be useful to further understand the molecular details of OX₁R. In this study, we newly designed and synthesized a phenylbenzofuran-2-carboxamide (PBC) derivative ([¹⁸F]PBC-1) and evaluated its utility as a PET probe targeting OX₁R in the brain. The results of cell binding assays suggested that [¹⁸F]PBC-1 has affinity for OX₁R. In an in vitro competitive inhibition assay, PBC-1 showed selective binding affinity for OX₁R (IC₅₀ = 19.5 nM) over orexin 2 receptor (IC₅₀ = 456.7 nM). Furthermore, [¹⁸F]PBC-1 displayed sufficient brain uptake for in vivo imaging with PET in a biodistribution study using normal mice, but in vivo instability was observed. These results suggest that further modifications for improvement of the pharmacokinetics are needed, but the PBC scaffold has potential for the development of useful PET probes targeting OX₁R in the brain.

A New Positron Emission Tomography Probe for Orexin Receptors Neuroimaging

The orexin receptor (OX) is critically involved in motivation and sleep−wake regulation and holds promising therapeutic potential in various mood disorders. To further investigate the role of orexin receptors (OXRs) in the living human brain and to evaluate the treatment potential of orexin-targeting therapeutics, we herein report a novel PET probe ([¹¹C]CW24) for OXRs in the brain. CW24 has moderate binding affinity for OXRs (IC₅₀ = 0.253 μM and 1.406 μM for OX₁R and OX₂R, respectively) and shows good selectivity to OXRs over 40 other central nervous system (CNS) targets. [¹¹C]CW24 has high brain uptake in rodents and nonhuman primates, suitable metabolic stability, and appropriate distribution and pharmacokinetics for brain positron emission tomography (PET) imaging. [¹¹C]CW24 warrants further evaluation as a PET imaging probe of OXRs in the brain.

Synthesis and characterization of a novel 18F-labeled 2,5-diarylnicotinamide derivative targeting orexin 2 receptor

Orexin 2 receptor (OX2R) is thought to play an important role in the arousal-promoting function, but its distribution and function in the pathophysiology of orexin-mediated disorders remains to be fully elucidated. In the present study, we synthesized and characterized a novel 18F-labeled 2,5-diarylnicotinamide (DAN) derivative as a potential positron emission tomography (PET) probe for in vivo imaging of OX2R. In in vitro binding experiments, [18F]DAN-1 selectively bound to OX2R. In a biodistribution study using normal mice, [18F]DAN-1 displayed moderate brain uptake (2.10% ID per g at 10 min post-injection). In addition, the radioactivity in the mouse brain at 30 min post-injection was significantly decreased by co-injection with nonradioactive DAN-1, but high nonspecific binding was observed. These results suggested that further structural modifications of [18F]DAN-1 are needed to use it for imaging OX2R in the brain.

A potent and selective C-11 labeled PET tracer for imaging sphingosine-1-phosphate receptor 2 in the CNS demonstrates sexually dimorphic expression

Sphingosine-1-phosphate receptor 2 (S1PR2) plays an essential role in regulating blood-brain barrier (BBB) function during demyelinating central nervous system (CNS) disease. Increased expression of S1PR2 occurs in disease-susceptible CNS regions of female versus male SJL mice and in female multiple sclerosis (MS) patients. Here we reported a novel sensitive and noninvasive method to quantitatively assess S1PR2 expression using a C-11 labeled positron emission tomography (PET) radioligand [(11)C]5a for in vivo imaging of S1PR2. Compound 5a exhibited promising binding potency with IC50 value of 9.52 ± 0.70 nM for S1PR2 and high selectivity over S1PR1 and S1PR3 (both IC50 > 1000 nM). [(11)C]5a was synthesized in ∼40 min with radiochemistry yield of 20 ± 5% (decayed to the end of bombardment (EOB), n > 10), specific activity of 222-370 GBq μmol(-1) (decayed to EOB). The biodistribution study in female SJL mice showed the cerebellar uptake of radioactivity at 30 min of post-injection of [(11)C]5a was increased by Cyclosporin A (CsA) pretreatment (from 0.84 ± 0.04 ID% per g to 2.21 ± 0.21 ID% per g, n = 4, p < 0.01). MicroPET data revealed that naive female SJL mice exhibited higher cerebellar uptake compared with males following CsA pretreatment (standardized uptake values (SUV) 0.58 ± 0.16 vs. 0.48 ± 0.12 at 30 min of post-injection, n = 4, p < 0.05), which was consistent with the autoradiographic results. This data suggested that [(11)C]5a had the capability in assessing the sexual dimorphism of S1PR2 expression in the cerebellum of the SJL mice. The development of radioligands for S1PR2 to identify a clinical suitable S1PR2 PET radiotracer, may greatly contribute to investigating sex differences in S1PR2 expression that contribute to MS subtype and disease progression and it will be very useful for detecting MS in early state and differentiating MS with other patients with neuroinflammatory diseases, and monitoring the efficacy of treating diseases using S1PR2 antagonism.

Selective orexin receptor antagonists

The orexin, or hypocretin, neuropeptides (orexin-A and orexin-B) are produced on neurons in the hypothalamus which project to key areas of the brain that control sleep-wake states, modulation of food intake, panic, anxiety, emotion, reward and addictive behaviors. These neuropeptides exert their effects on a pair of G-protein coupled receptors termed the orexin-1 (OX1) and orexin-2 (OX2) receptors. Emerging biology suggests the involvement of these receptors in psychiatric disorders as they are thought to play a key role in the regulation of multiple systems. This review is intended to highlight key selective OX1 or OX2 small-molecule antagonists.