Translational PET imaging research

Spatial Distribution of Brain PET Tracers by MALDI Imaging

Evaluating tissue distribution of Positron Emission Tomography (PET) tracers during their development conventionally involves autoradiography techniques, where radioactive compounds are used for ex vivo visualization and quantification in tissues during preclinical development stages. Mass Spectrometry Imaging (MSI) offers a potential alternative, providing spatial information without the need for radioactivity with a similar spatial resolution. This study aimed to optimize a MSI sample preparation protocol for assessing PET tracer candidates ex vivo with a focus on two compounds: UCB-J and UCB2400. We tested different matrices and introduced washing steps to improve PET tracer detection. Tissue homogenates were prepared to construct calibration curves for quantification. The incorporation of a washing step into the MSI sample preparation protocol enhanced the signal of both PET tracers. Our findings highlight MSI's potential as a cost-effective and efficient method for the evaluation of PET tracer distribution. The optimized approach offered here can provide a protocol that enhances the signal and minimizes ion suppression effect, which can be valuable for future evaluation of PET tracers in MSI studies.

PDE4D inhibitors: Opening a new era of PET diagnostics for Alzheimer's disease

As the incidence of Alzheimer's disease (AD) continues to rise, the need for an effective PET radiotracer to facilitate early diagnosis has become more pressing than ever before in modern medicine. Phosphodiesterase (PDE) is closely related to cognitive impairment and neuroinflammatory processes in AD. Current research progress shows that specific PDE4D inhibitors radioligands can bind specifically to the PDE4D enzyme in the brain, thereby showing pathology-related signal enhancement in AD animal models, indicating the potential of these ligands as effective radiotracers. At the same time, we need to pay attention to the important role computer aided drug design (CADD) plays in advancing AD drug design and PET imaging. Future research will verify the potential of these ligands in clinical applications through computer simulation techniques, providing patients with timely intervention and treatment, which is of great significance.

Radiotracers for Molecular Imaging of Angiotensin-Converting Enzyme 2

Angiotensin-converting enzymes (ACE) are well-known for their roles in both blood pressure regulation via the renin-angiotensin system as well as functions in fertility, immunity, hematopoiesis, and many others. The two main isoforms of ACE include ACE and ACE-2 (ACE2). Both isoforms have similar structures and mediate numerous effects on the cardiovascular system. Most remarkably, ACE2 serves as an entry receptor for SARS-CoV-2. Understanding the interaction between the virus and ACE2 is vital to combating the disease and preventing a similar pandemic in the future. Noninvasive imaging techniques such as positron emission tomography and single photon emission computed tomography could noninvasively and quantitatively assess in vivo ACE2 expression levels. ACE2-targeted imaging can be used as a valuable tool to better understand the mechanism of the infection process and the potential roles of ACE2 in homeostasis and related diseases. Together, this information can aid in the identification of potential therapeutic drugs for infectious diseases, cancer, and many ACE2-related diseases. The present review summarized the state-of-the-art radiotracers for ACE2 imaging, including their chemical design, pharmacological properties, radiochemistry, as well as preclinical and human molecular imaging findings. We also discussed the advantages and limitations of the currently developed ACE2-specific radiotracers.

Advances and Insights in Positron Emission Tomography Tracers for Metabotropic Glutamate Receptor 4 Imaging

Positron emission tomography (PET) imaging employs positron-emitting radioisotopes to visualize biological processes in living subjects with high sensitivity and quantitative accuracy. As the most translational molecular imaging modality, PET can detect and image a wide range of radiotracers with minimal or no modification to parent drugs or targeting molecules. This Perspective provides a comprehensive analysis of developing PET radioligands using allosteric modulators for the metabotropic glutamate receptor subtype 4 (mGluR4) as a therapeutic target for neurological disorders. We focus on the selection of lead compounds from various chemotypes of mGluR4 positive allosteric modulators (PAMs) and discuss the challenges and systematic characterization required in developing brain-penetrant PET tracers specific for mGluR4. Through this analysis, we offer insights into the development and evaluation of PET ligands. Our review concludes that further research and development in this field hold great promise for discovering effective treatments for neurological disorders.

Characterization of (R)- and (S)-[18F]OF-NB1 in Rodents as Positron Emission Tomography Probes for Imaging GluN2B Subunit-Containing N-Methyl-d-Aspartate Receptors

The N-methyl-d-aspartate receptor (NMDAR) subtype 2B (GluN1/2B) is implicated in various neuropathologies. Given the lack of a validated radiofluorinated positron emission tomography (PET) probe for the imaging of GluN1/2B receptors, we comprehensively investigated the enantiomers of [18F]OF-NB1 in rodents. Particularly, the (R)- and (S)- enantiomers were evaluated using in silico docking, in vitro autoradiography, in vivo PET imaging, and ex vivo biodistribution studies. A select panel of GluN1/2B antagonists (CP-101,606, CERC-301, and eliprodil) and the off-target sigma-1 receptor ligands (fluspidine and SA4503) were used to determine the specificity and selectivity of the tested enantiomers. Additionally, a nonmetal-mediated radiofluorination strategy was devised that harnesses the potential of diaryliodoniums in the nucleophilic radiofluorination of nonactivated aromatic compounds. Both enantiomers exhibited known GluN1/2B binding patterns; however, the R-enantiomer showed higher GluN1/2B-specific accumulation in rodent autoradiography and higher brain uptake in PET imaging experiments compared to the S-enantiomer. Molecular simulation studies provided further insights with respect to the difference in binding, whereby a reduced ligand-receptor interaction was observed for the S-enantiomer. Nonetheless, both enantiomers showed dose dependency when two different doses (1 and 5 mg/kg) of the GluN1/2B antagonist, CP-101,606, were used in the PET imaging study. Taken together, (R)-[18F]OF-NB1 appears to exhibit the characteristics of a suitable PET probe for imaging of GluN2B-containing NMDARs in clinical studies.

Evaluation of [18F]RoSMA-18-d6 as a CB2 PET Radioligand in Nonhuman Primates

The cannabinoid type 2 receptor (CB2) has been implicated in a variety of central and peripheral inflammatory diseases, prompting significant interest in the development of CB2-targeted diagnostic and therapeutic agents. A validated positron emission tomography (PET) radioligand for imaging CB2 in the living human brain as well as in peripheral tissues is currently lacking. As part of our research program, we have recently identified the trisubstituted pyridine, [18F]RoSMA-18-d6, which proved to be highly suitable for in vitro and in vivo mapping of CB2 in rodents. The aim of this study was to assess the performance characteristics of [18F]RoSMA-18-d6 in nonhuman primates (NHPs) to pave the way for clinical translation. [18F]RoSMA-18-d6 was synthesized from the respective tosylate precursor according to previously reported procedures. In vitro autoradiograms with NHP spleen tissue sections revealed a high binding of [18F]RoSMA-18-d6 to the CB2-rich NHP spleen, which was significantly blocked by coincubation with the commercially available CB2 ligand, GW405833 (10 μM). In contrast, no specific binding was observed by in vitro autoradiography with NHP brain sections, which was in agreement with the notion of a CB2-deficient healthy mammalian brain. In vitro findings were corroborated by PET imaging experiments in NHPs, where [18F]RoSMA-18-d6 uptake in the spleen was dose-dependently attenuated with 1 and 5 mg/kg GW405833, while no specific brain signal was observed. Remarkably, we observed tracer uptake and retention in the NHP spinal cord, which was reduced by GW405833 blockade, pointing toward a potential utility of [18F]RoSMA-18-d6 in probing CB2-expressing cells in the bone marrow. If these observations are substantiated in NHP models of enhanced leukocyte proliferation in the bone marrow, [18F]RoSMA-18-d6 may serve as a valuable marker for hematopoietic activity in various pathologies. In conclusion, [18F]RoSMA-18-d6 proved to be a suitable PET radioligand for imaging CB2 in NHPs, supporting its translation to humans.

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.

Experimental Medicine Approaches in Early-Phase CNS Drug Development

Traditionally, Phase 1 clinical trials were largely conducted in healthy normal volunteers and focused on collection of safety, tolerability, and pharmacokinetic data. However, in the CNS therapeutic area, with more drugs failing in later phase development, Phase 1 trials have undergone an evolution that includes incorporation of novel approaches involving novel study designs, inclusion of biomarkers, and early inclusion of patients to improve the pharmacologic understanding of novel CNS-active compounds early in clinical development with the hope of improving success in later phase pivotal trials. In this chapter, the authors will discuss the changing landscape of Phase 1 clinical trials in CNS, including novel trial methodology, inclusion of pharmacodynamic biomarkers, and experimental medicine approaches to inform early decision-making in clinical development.

Evaluation of (rac)-, (R)-, and (S)-18F-OF-NB1 for Imaging GluN2B Subunit-Containing N-Methyl-d-Aspartate Receptors in Nonhuman Primates

Despite 2 decades of research, no N-methyl-d-aspartate (NMDA) glutamate receptor (GluN) subtype 2B (GluN1/2B) radioligand is yet clinically validated. Previously, we reported on (rac)-18F-OF-NB1 as a promising GluN1/2B PET probe in rodents and its successful application for the visualization of GluN2B-containing NMDA receptors in postmortem brain tissues of patients with amyotrophic lateral sclerosis. In the current work, we report on the in vivo characterization of (rac)-, (R)-, and (S)-18F-OF-NB1 in nonhuman primates. Methods: PET scans were performed on rhesus monkeys. Plasma profiling was used to obtain the arterial input function. Regional brain time-activity curves were generated and fitted with the 1- and 2-tissue-compartment models and the multilinear analysis 1 method, and the corresponding regional volumes of distribution were calculated. Blocking studies with the GluN1/2B ligand Co 101244 (0.25 mg/kg) were performed for the enantiopure radiotracers. Receptor occupancy, nonspecific volume of distribution, and regional binding potential (BP ND) were obtained. Potential off-target binding toward σ1 receptors was assessed for (S)-18F-OF-NB1 using the σ1 receptor ligand FTC-146. Results: Free plasma fraction was moderate, ranging from 12% to 16%. All radiotracers showed high and heterogeneous brain uptake, with the highest levels in the cortex. (R)-18F-OF-NB1 showed the highest uptake and slowest washout kinetics of all tracers. The 1-tissue-compartment model and multilinear analysis 1 method fitted the regional time-activity curves well for all tracers and produced reliable regional volumes of distribution, which were higher for (R)- than (S)-18F-OF-NB1. Receptor occupancy by Co 101244 was 85% and 96% for (S)-18F-OF-NB1 and (R)-18F-OF-NB1, respectively. Pretreatment with FTC-146 at both a low (0.027 mg/kg) and high (0.125 mg/kg) dose led to a similar reduction (48% and 49%, respectively) in specific binding of (S)-18F-OF-NB1. Further, pretreatment with both Co 101244 and FTC-146 did not result in a further reduction in specific binding compared with Co 101244 alone in the same monkey (82% vs. 81%, respectively). Regional BP ND values ranged from 1.3 in the semiovale to 3.4 in the cingulate cortex for (S)-18F-OF-NB1. Conclusion: Both (R)- and (S)-18F-OF-NB1 exhibited high binding specificity to GluN2B subunit-containing NMDA receptors. The fast washout kinetics, good regional BP ND values, and high plasma free fraction render (S)-18F-OF-NB1 an attractive radiotracer for clinical translation.

PET Imaging of the Neuropeptide Y System: A Systematic Review

Neuropeptide Y (NPY) is a vastly studied biological peptide with numerous physiological functions that activate the NPY receptor family (Y₁, Y₂, Y₄ and Y₅). Moreover, these receptors are correlated with the pathophysiology of several diseases such as feeding disorders, anxiety, metabolic diseases, neurodegenerative diseases, some types of cancers and others. In order to deepen the knowledge of NPY receptors' functions and molecular mechanisms, neuroimaging techniques such as positron emission tomography (PET) have been used. The development of new radiotracers for the different NPY receptors and their subsequent PET studies have led to significant insights into molecular mechanisms involving NPY receptors. This article provides a systematic review of the imaging biomarkers that have been developed as PET tracers in order to study the NPY receptor family.

Performance evaluation of the IRIS XL-220 PET/CT system, a new camera dedicated to non-human primates

BACKGROUND

Non-human primates (NHP) are critical in biomedical research to better understand the pathophysiology of diseases and develop new therapies. Based on its translational and longitudinal abilities along with its non-invasiveness, PET/CT systems dedicated to non-human primates can play an important role for future discoveries in medical research. The aim of this study was to evaluate the performance of a new PET/CT system dedicated to NHP imaging, the IRIS XL-220 developed by Inviscan SAS. This was performed based on the National Electrical Manufacturers Association (NEMA) NU 4-2008 standard recommendations (NEMA) to characterize the spatial resolution, the scatter fraction, the sensitivity, the count rate, and the image quality of the system. Besides, the system was evaluated in real conditions with two NHP with ¹⁸F-FDG and (-)-[¹⁸F]FEOBV which targets the vesicular acetylcholine transporter, and one rat using ¹⁸F-FDG.

RESULTS

The full width at half maximum obtained with the 3D OSEM algorithm ranged between 0.89 and 2.11 mm in the field of view. Maximum sensitivity in the 400-620 keV and 250-750 keV energy windows were 2.37% (22 cps/kBq) and 2.81% (25 cps/kBq), respectively. The maximum noise equivalent count rate (NEC) for a rat phantom was 82 kcps at 75 MBq and 88 kcps at 75 MBq for energy window of 250-750 and 400-620 keV, respectively. For the monkey phantom, the maximum NEC was 18 kcps at 126 MBq and 19 kcps at 126 MBq for energy window of 250-750 and 400-620 keV, respectively. The IRIS XL provided an excellent quality of images in non-human primates and rats using ¹⁸F-FDG. The images acquired using (-)-[¹⁸F]FEOBV were consistent with those previously reported in non-human primates.

CONCLUSIONS

Taken together, these results showed that the IRIS XL-220 is a high-resolution system well suited for PET/CT imaging in non-human primates.

The PET tracer [11C]MK-6884 quantifies M4 muscarinic receptor in rhesus monkeys and patients with Alzheimer's disease

[Figure: see text].

Hybrid total-body pet scanners-current status and future perspectives

Purpose Since the 1990s, PET has been successfully combined with MR or CT systems. In the past years, especially PET systems have seen a trend towards an enlarged axial field of view (FOV), up to a factor of ten. Methods Conducting a thorough literature research, we summarize the status quo of contemporary total-body (TB) PET/CT scanners and give an outlook on possible future developments. Results Currently, three human TB PET/CT systems have been developed: The PennPET Explorer, the uExplorer, and the Biograph Vision Quadra realize aFOVs between 1 and 2 m and show a tremendous increase in system sensitivity related to their longer gantries. Conclusion The increased system sensitivity paves the way for short-term, low-dose, and dynamic TB imaging as well as new examination methods in almost all areas of imaging.

Pharmacodynamic measures within tumors expose differential activity of PD(L)-1 antibody therapeutics

Macromolecules such as monoclonal antibodies (mAbs) are likely to experience poor tumor penetration because of their large size, and thus low drug exposure of target cells within a tumor could contribute to suboptimal responses. Given the challenge of inadequate quantitative tools to assess mAb activity within tumors, we hypothesized that measurement of accessible target levels in tumors could elucidate the pharmacologic activity of a mAb and could be used to compare the activity of different mAbs. Using positron emission tomography (PET), we measured the pharmacodynamics of immune checkpoint protein programmed-death ligand 1 (PD-L1) to evaluate pharmacologic effects of mAbs targeting PD-L1 and its receptor programmed cell death protein 1 (PD-1). For PD-L1 quantification, we first developed a small peptide-based fluorine-18-labeled PET imaging agent, [¹⁸F]DK222, which provided high-contrast images in preclinical models. We then quantified accessible PD-L1 levels in the tumor bed during treatment with anti-PD-1 and anti-PD-L1 mAbs. Applying mixed-effects models to these data, we found subtle differences in the pharmacodynamic effects of two anti-PD-1 mAbs (nivolumab and pembrolizumab). In contrast, we observed starkly divergent target engagement with anti-PD-L1 mAbs (atezolizumab, avelumab, and durvalumab) that were administered at equivalent doses, correlating with differential effects on tumor growth. Thus, we show that measuring PD-L1 pharmacodynamics informs mechanistic understanding of therapeutic mAbs targeting PD-L1 and PD-1. These findings demonstrate the value of quantifying target pharmacodynamics to elucidate the pharmacologic activity of mAbs, independent of mAb biophysical properties and inclusive of all physiological variables, which are highly heterogeneous within and across tumors and patients.

Update on PET Tracer Development for Muscarinic Acetylcholine Receptors

The muscarinic cholinergic system regulates peripheral and central nervous system functions, and, thus, their potential as a therapeutic target for several neurodegenerative diseases is undoubted. A clinically applicable positron emission tomography (PET) tracer would facilitate the monitoring of disease progression, elucidate the role of muscarinic acetylcholine receptors (mAChR) in disease development and would aid to clarify the diverse natural functions of mAChR regulation throughout the nervous system, which still are largely unresolved. Still, no mAChR PET tracer has yet found broad clinical application, which demands mAChR tracers with improved imaging properties. This paper reviews strategies of mAChR PET tracer design and summarizes the binding properties and preclinical evaluation of recent mAChR tracer candidates. Furthermore, this work identifies the current major challenges in mAChR PET tracer development and provides a perspective on future developments in this area of research.

Positron Emission Tomography Imaging of the Endocannabinoid System: Opportunities and Challenges in Radiotracer Development

The endocannabinoid system (ECS) is involved in a wide range of biological functions and comprises cannabinoid receptors and enzymes responsible for endocannabinoid synthesis and degradation. Over the past 2 decades, significant advances toward developing drugs and positron emission tomography (PET) tracers targeting different components of the ECS have been made. Herein, we summarized the recent development of PET tracers for imaging cannabinoid receptors 1 (CB1R) and 2 (CB2R) as well as the key enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), particularly focusing on PET neuroimaging applications. State-of-the-art PET tracers for the ECS will be reviewed including their chemical design, pharmacological properties, radiolabeling, as well as preclinical and human PET imaging. In addition, this review addresses the current challenges for ECS PET biomarker development and highlights the important role of PET ligands to study disease pathophysiology as well as to facilitate drug discovery.

Striatal Dopamine D2 Receptor Occupancy Induced by Daily Application of Blonanserin Transdermal Patches: Phase II Study in Japanese Patients With Schizophrenia

BACKGROUND

Transdermal antipsychotic patch formulations offer potential benefits, including improved adherence. This study investigated the striatal dopamine D2 receptor occupancy with daily blonanserin transdermal patch application.

METHODS

This open-label, phase II study enrolled 18 Japanese outpatients (20 to <65 years) with schizophrenia (DSM-IV-TR criteria; total Positive and Negative Syndrome Scale score <120 at screening) treated with blonanserin 8-mg or 16-mg tablets. Patients continued tablets for 2-4 weeks at their current dose and were then assigned to once-daily blonanserin patches (10/20/40/60/80 mg daily) for 2-4 weeks based on the oral dose. [11C]raclopride positron emission tomography scanning determined blonanserin striatal dopamine D2 receptor occupancy (primary endpoint). Secondary endpoints included assessment of receptor occupancy by dose, changes in Positive and Negative Syndrome Scale and Clinical Global Impressions-Severity of Illness-Severity scores, patient attitudes towards adherence, and patch adhesiveness.

RESULTS

Of 18 patients who started the blonanserin tablet treatment period, 14 patients completed treatment. Mean D2 receptor occupancy for blonanserin tablets 8 mg/d (59.2%, n = 5) and 16 mg/d (66.3%, n = 9) was within the values for blonanserin patches: 10 mg/d (33.3%, n = 3), 20 mg/d (29.9%, n = 2), 40 mg/d (61.2%, n = 3), 60 mg/d (59.0%, n = 3), and 80 mg/d (69.9%, n = 3). Occupancy generally increased with increasing blonanserin dose for both formulations with the half maximal receptor occupancy for tablets and patches associated with doses of 6.9 mg/d and 31.9 mg/d, respectively. Diurnal variability in occupancy was lower during transdermal patch treatment than during tablet treatment. Blonanserin transdermal patches were well tolerated with no major safety concerns.

CONCLUSIONS

Blonanserin patches (40/80 mg/d) have lower diurnal variability in occupancy than blonanserin tablets (8/16 mg/d), and patches at doses of 40 mg/d and 80 mg/d appear to be a suitable alternative for blonanserin tablets at doses of 8 mg/d and 16 mg/d, respectively. Blonanserin patches represent a potential new treatment option for patients with schizophrenia.

TRIAL REGISTRY

JAPIC Clinical Trials Information registry (www.clinicaltrials.jp; JapicCTI-No: JapicCTI-121914).

Conductive Hearing Loss Aggravates Memory Decline in Alzheimer Model Mice

The study of cognitive impairment associated with hearing loss has recently garnered considerable interest. Epidemiological data have demonstrated that hearing loss is a risk factor for cognitive decline as a result of aging. However, no previous study has examined the effect of hearing loss in patients with cognitive problems such as Alzheimer’s disease. Therefore, we investigated the effect of conductive hearing loss in an Alzheimer’s mouse model. Positron emission tomography (PET) and magnetic resonance imaging (MRI) were used to evaluate changes in glucose metabolism and gray matter concentrations in the 5xFAD Alzheimer’s Disease (AD) transgenic mouse model with and without conductive hearing loss (HL). Conductive hearing loss was induced using chronic perforation of the tympanic membrane. Behavioral data from the Y-maze and passive avoidance tests revealed greater memory deficits in the AD with HL (AD-HL) group than in the AD group. Following induction of hearing loss, lower cerebral glucose metabolism in the frontal association cortex was observed in the AD-HL group than in the AD group. Although lower glucose metabolism in the hippocampus and cerebellum was found in the AD-HL group than in the AD group at 3 months, the gray matter concentrations in these regions were not significantly different between the groups. Furthermore, the gray matter concentrations in the simple lobule, cingulate/retrosplenial cortex, substantia nigra, retrosigmoid nucleus, medial geniculate nucleus, and anterior pretectal nucleus at 7 months were significantly lower in the AD-HL group than in the AD group. Taken together, these results indicate that even partial hearing loss can aggravate memory impairment in Alzheimer’s disease.

Synthesis and evaluation of NLRP3-inhibitory sulfonylurea [11C]MCC950 in healthy animals

The diaryl sulfonylurea MCC950/CRID3 is a potent NLRP3 inhibitor (IC₅₀ = 8 nM) and, in animal models, MCC950 protects against numerous NLRP3-related neurodegenerative disorders. To evaluate the brain uptake and investigate target engagement of MCC950, we synthesised [¹¹C-urea]MCC950 via carrier added [¹¹C]CO₂ fixation chemistry (activity yield = 237 MBq; radiochemical purity >99%; molar activity = 7 GBq/µmol; radiochemical yield (decay-corrected from [¹¹C]CO₂) = 1.1%; synthesis time from end-of-bombardment = 31 min; radiochemically stable for >1 h). Despite preclinical efficacy in neurodegeneration studies, preclinical positron emission tomography (PET) imaging studies in mouse, rat and rhesus monkey revealed poor brain uptake of low molar activity [¹¹C]MCC950 and rapid washout. In silico prediction tools suggest efflux transporter liabilities for MCC950 at microdoses, and this information should be taken into account when developing next generation NLRP3 inhibitors and/or PET radiotracers.

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.

Target engagement approaches for pharmacological evaluation in animal models

The field of chemical biology has introduced several approaches, typically using chemical probes, to measure the direct binding interaction of a small molecule with its biological target in cells. The use of these direct target engagement assays in pharmaceutical development can support mechanism of action hypothesis testing, rank ordering of compounds, and iterative improvements of chemical matter. This Feature Article highlights a newer application of these approaches: the quantification of target engagement in animal models to support late stage preclinical development and the nomination of a drug candidate to clinical trials. Broadly speaking, these efforts can be divided between compounds that covalently and reversibly interact with protein targets; recent examples for both categories are discussed for a range of targets, along with their limitations. New, promising technologies are also highlighted, in addition to the application of target engagement determination to new therapeutic modalities.

Mind the Gaps: Ontogeny of Human Brain P-gp and Its Impact on Drug Toxicity

Available data on human brain P-glycoprotein ontogeny during infancy and childhood are limited. This review discusses the current body of data relating to maturation of human brain P-glycoprotein including transporter expression levels in post-mortem human brain samples, in vivo transporter activity using probe substrates, surrogate marker endpoints, and extrapolations from animal models. Overall, the data tend to confirm that human brain P-glycoprotein activity keeps developing after birth, although with a developmental time frame that remains unclear. This knowledge gap is a concern given the critical role of brain P-glycoprotein in drug safety and efficacy, and the vulnerable nature of the pediatric population. Future research could include the measurement of brain P-glycoprotein activity across age groups using positron emission tomography or central pharmacodynamic responses. For now, caution is advised when extrapolating adult data to children aged younger than 2 years for drugs with P-glycoprotein-dependent central nervous system activity.

Labelled chemical probes for demonstrating direct target engagement in living systems

Demonstrating target engagement in living systems can help drive successful drug discovery. Target engagement and occupancy studies in cells confirm direct binding of a ligand to its intended target protein and provide the binding affinity. Combined with biomarkers to measure the functional consequences of target engagement, these experiments can increase confidence in the relationship between in vitro pharmacology and observed biological effects. In this review, we focus on chemically and radioactively labelled probes as key reagents for performing such experiments. Using recent examples, we examine how the labelled probes have been employed in combination with unlabelled ligands to quantify target engagement in cells and in animals. Finally, we consider future developments of this emerging methodology.

Analysis of PK11195 concentrations in rodent whole blood and tissue samples by rapid and reproducible chromatographic method to support target-occupancy PET studies

In Positron Emission Tomography (PET) research, it is important to assess not only pharmacokinetics of a radiotracer in vivo, but also of the drugs used in blocking/displacement PET studies. Typically, pharmacokinetic/pharmacodynamic (PK/PD) analyses of drugs used in rodent PET studies are based on population average pharmacokinetic profiles of the drugs due to limited blood volume withdrawal while simultaneously maintaining physiological homeostasis. This likely results in bias of PET data quantification, including unknown bias of target occupancy (TO) measurements. This study aimed to develop a High Performance Liquid Chromatography (HPLC) method for PK/PD quantification of drugs used in preclinical rodent PET research, specifically the translocator 18 kDa protein (TSPO) selective drug, PK11195, that used sub-millilitre blood volumes. The lowest detection limit for the proposed HPLC method ranged between 7.5 and 10 ng/mL depending on the method used to calculate the limit of detection, and the measured average relative standard deviation for intermediate precision was equal to 17.2%. Most importantly, we were able to demonstrate a significant difference between calculated PK11195 concentrations at 0.5, 1, 2, 3, 5, 15 and 30 min post-administration and individually measured whole blood levels (significance level range from p < 0.05 to p < 0.001; one-way ANOVA, Dunnet's post hoc test, p < 0.05). The HPLC method developed here uses sub-millilitre sample volumes to reproducibly assess PK/PD of PK11195 in rodent blood. This study highlights the importance of individually measured PK/PD drug concentrations when quantifying the TO from blocking/displacement rodent PET experiments.

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.

Development and optimization of a novel automated loop method for production of [11C]nicotine

A novel, rapid, and automated loop method for the synthesis of [¹¹C]nicotine was developed and optimized. The method involves, a reaction of the precursor, (+) nornicotine or (-) nornicotine, with a gas-phase produced [¹¹C]CH₃I in an 800 µL loop at 75 °C for 5 min followed by a semi-preparatory Reversed-Phase High-Performance Liquid Chromatography (RP-HPLC) purification. The optimized synthesis and purification process was complete in < 30 min and produced [¹¹C]nicotine with > 99.9% Radiochemical Purity (RCP), no [¹¹C]CH₃I, no (+) nornicotine, 105 mCi/µmole specific activity, 7.0 - 7.2 pH, and 16.6% ethanol. The current method can be optimized, to reduce the ethanol content (<10%), and can be translated to a cGMP production of [¹¹C]nicotine for human clinical trials.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

CETSA quantitatively verifies in vivo target engagement of novel RIPK1 inhibitors in various biospecimens

The proof of target engagement (TE) is a key element for evaluating potential investment in drug development. The cellular thermal shift assay (CETSA) is expected to facilitate direct measurement of intracellular TE at all stages of drug development. However, there have been no reports of applying this technology to comprehensive animal and clinical studies. This report demonstrates that CETSA can not only quantitatively evaluate the drug-TE in mouse peripheral blood, but also confirm TE in animal tissues exemplified by using the receptor interacting protein 1 kinase (RIPK1) lead compound we have developed. Our established semi-automated system allows evaluation of the structure-activity relationship using native RIPK1 in culture cell lines, and also enables estimation of drug occupancy ratio in mouse peripheral blood mononuclear cells. Moreover, optimized tissue homogenisation enables monitoring of the in vivo drug-TE in spleen and brain. Our results indicate that CETSA methodology will provide an efficient tool for preclinical and clinical drug development.

Instantaneous Conversion of [11 C]CO2 to [11 C]CO via Fluoride-Activated Disilane Species

The development of a fast and novel methodology to generate carbon-11 carbon monoxide ([11 C]CO) from cyclotron-produced carbon-11 carbon dioxide ([11 C]CO2 ) mediated by a fluoride-activated disilane species is described. This methodology allows up to 74 % conversion of [11 C]CO2 to [11 C]CO using commercially available reagents, readily available laboratory equipment and mild reaction conditions (room temperature). As proof of utility, radiochemically pure [carbonyl-11 C]N-benzylbenzamide was successfully synthesized from produced [11 C]CO in up to 74 % radiochemical yield (RCY) and >99 % radiochemical purity (RCP) in ≤10 min from end of [11 C]CO2 delivery.

18F-FPP: A PET Ligand for the 5-HT2C Receptor?

In the current issue of ACS Chemical Neuroscience, Kim et al. report on the early characterization of 4-(3-[¹⁸F] fluorophenethoxy)pyrimidine (¹⁸F-FPP) as a new positron emission tomography radiotracer for imaging brain 5-HT_2C receptors ( Kim, J., et al. ( 2017 ) A potential PET radiotracer for the 5-HT2c receptor: Synthesis and in vivo evaluation of 4-(3-[¹⁸F]Fluorophenethoxy)pyrimidine. ACS Chem. Neurosci.

, DOI

10.1021/acschemneuro.6b00445 ). At the present time, the tracer properties of ¹⁸F-FPP have only been reported in rats. If ¹⁸F-FPP is indeed shown to be suitable as a 5-HT_2C receptor PET tracer in humans, it will very likely have an important impact both in the development of any new chemical entities (NCEs) targeted to 5-HT_2C receptors, as well as a tool to advance understanding of 5-HT_2C receptor function both in normal and abnormal brain states.

Possibility of Predicting Serotonin Transporter Occupancy From the In Vitro Inhibition Constant for Serotonin Transporter, the Clinically Relevant Plasma Concentration of Unbound Drugs, and Their Profiles for Substrates of Transporters

Accurate prediction of target occupancy facilitates central nervous system drug development. In this review, we discuss the predictability of serotonin transporter (SERT) occupancy in human brain estimated from in vitro K_i values for human SERT and plasma concentrations of unbound drug (C_u,plasma), as well as the impact of drug transporters in the blood-brain barrier. First, the geometric means of in vitro K_i values were compared with the means of in vivo K_i values (K_i,u,plasma) which were calculated as C_u,plasma values at 50% occupancy of SERT obtained from previous clinical positron emission tomography/single photon emission computed tomography imaging studies for 6 selective serotonin transporter reuptake inhibitors and 3 serotonin norepinephrine reuptake inhibitors. The in vitro K_i values for 7 drugs were comparable to their in vivo K_i,u,plasma values within 3-fold difference. SERT occupancy was overestimated for 5 drugs (P-glycoprotein substrates) and underestimated for 2 drugs (presumably uptake transporter substrates, although no evidence exists as yet). In conclusion, prediction of human SERT occupancy from in vitro K_i values and C_u,plasma was successful for drugs that are not transporter substrates and will become possible in future even for transporter substrates, once the transporter activities will be accurately estimated from in vitro experiments.

Quantitating drug-target engagement in single cells in vitro and in vivo

Quantitation of drug target engagement in single cells has proven to be difficult, often leaving unanswered questions in the drug development process. We found that intracellular target engagement of unlabeled new therapeutics can be quantitated using polarized microscopy combined with competitive binding of matched fluorescent companion imaging probes. We quantitated the dynamics of target engagement of covalent BTK inhibitors, as well as reversible PARP inhibitors, in populations of single cells using a single companion imaging probe for each target. We then determined average in vivo tumor concentrations and found marked population heterogeneity following systemic delivery, revealing single cells with low target occupancy at high average target engagement in vivo.

Imaging in Central Nervous System Drug Discovery

The discovery and development of central nervous system (CNS) drugs is an extremely challenging process requiring large resources, timelines, and associated costs. The high risk of failure leads to high levels of risk. Over the past couple of decades PET imaging has become a central component of the CNS drug-development process, enabling decision-making in phase I studies, where early discharge of risk provides increased confidence to progress a candidate to more costly later phase testing at the right dose level or alternatively to kill a compound through failure to meet key criteria. The so called "3 pillars" of drug survival, namely; tissue exposure, target engagement, and pharmacologic activity, are particularly well suited for evaluation by PET imaging. This review introduces the process of CNS drug development before considering how PET imaging of the "3 pillars" has advanced to provide valuable tools for decision-making on the critical path of CNS drug development. Finally, we review the advances in PET science of biomarker development and analysis that enable sophisticated drug-development studies in man.

A Modular Probe Strategy for Drug Localization, Target Identification and Target Occupancy Measurement on Single Cell Level

Late stage failures of candidate drug molecules are frequently caused by off-target effects or inefficient target engagement in vivo. In order to address these fundamental challenges in drug discovery, we developed a modular probe strategy based on bioorthogonal chemistry that enables the attachment of multiple reporters to the same probe in cell extracts and live cells. In a systematic evaluation, we identified the inverse electron demand Diels-Alder reaction between trans-cyclooctene labeled probe molecules and tetrazine-tagged reporters to be the most efficient bioorthogonal reaction for this strategy. Bioorthogonal biotinylation of the probe allows the identification of drug targets in a chemoproteomics competition binding assay using quantitative mass spectrometry. Attachment of a fluorescent reporter enables monitoring of spatial localization of probes as well as drug-target colocalization studies. Finally, direct target occupancy of unlabeled drugs can be determined at single cell resolution by competitive binding with fluorescently labeled probe molecules. The feasibility of the modular probe strategy is demonstrated with noncovalent PARP inhibitors.

TASP0434299: A Novel Pyridopyrimidin-4-One Derivative as a Radioligand for Vasopressin V1B Receptor

A novel pyridopyrimidin-4-one derivative, N-tert-butyl-2-[2-(3-methoxyphenyl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d]pyrimidin-3(4H)-yl]acetamide (TASP0434299), was characterized as a radioligand candidate for arginine vasopressin 1B (V1B) receptor. TASP0434299 exhibited high binding affinities for human and rat V1B receptors with IC50 values of 0.526 and 0.641 nM, respectively, and potent antagonistic activity at the human V1B receptor with an IC50 value of 0.639 nM without apparent binding affinities for other molecules at 1 μM. [(3)H]TASP0434299 bound to membranes expressing the human V1B receptor as well as those prepared from the rat anterior pituitary in a saturable manner. The binding of [(3)H]TASP0434299 to the membranes was dose-dependently displaced by several ligands for the V1B receptor. In addition, the intravenous administration of [(3)H]TASP0434299 to rats produced a saturable radioactive accumulation in the anterior pituitary where the V1B receptor is enriched, and it was dose-dependently blocked by the oral administration of 2-[2-(3-chloro-4-fluorophenyl)-6-[3-(morpholin-4-yl)propoxy]-4-oxopyrido[2,3-d]pyrimidin-3(4H)-yl]-N-isopropylacetamide hydrochloride, a V1B receptor antagonist, indicating that [(3)H]TASP0434299 can be used as an in vivo radiotracer to measure the occupancy of the V1B receptor. Finally, the intravenous administration of [(11)C]TASP0434299 provided positron emission tomographic images of the V1B receptor in the pituitary in an anesthetized monkey, and the signal was blocked by pretreatment with an excess of unlabeled TASP0434299. These results indicate that radiolabeled TASP0434299 is the first radioligand to be capable of quantifying the V1B receptor selectively in both in vitro and in vivo studies and will provide a clinical biomarker for determining the occupancy of the V1B receptor during drug development or for monitoring the levels of the V1B receptor in diseased conditions.

Organ-specific physiological responses to acute physical exercise and long-term training in humans

Virtually all tissues in the human body rely on aerobic metabolism for energy production and are therefore critically dependent on continuous supply of oxygen. Oxygen is provided by blood flow, and, in essence, changes in organ perfusion are also closely associated with alterations in tissue metabolism. In response to acute exercise, blood flow is markedly increased in contracting skeletal muscles and myocardium, but perfusion in other organs (brain and bone) is only slightly enhanced or is even reduced (visceral organs). Despite largely unchanged metabolism and perfusion, repeated exposures to altered hemodynamics and hormonal milieu produced by acute exercise, long-term exercise training appears to be capable of inducing effects also in tissues other than muscles that may yield health benefits. However, the physiological adaptations and driving-force mechanisms in organs such as brain, liver, pancreas, gut, bone, and adipose tissue, remain largely obscure in humans. Along these lines, this review integrates current information on physiological responses to acute exercise and to long-term physical training in major metabolically active human organs. Knowledge is mostly provided based on the state-of-the-art, noninvasive human imaging studies, and directions for future novel research are proposed throughout the review.

Fluorine-18 radiochemistry, labeling strategies and synthetic routes

Fluorine-18 is the most frequently used radioisotope in positron emission tomography (PET) radiopharmaceuticals in both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.7 min half-life, 635 keV positron energy), along with high specific activity and ease of large scale production, make it an attractive nuclide for radiochemical labeling and molecular imaging. Versatile chemistry including nucleophilic and electrophilic substitutions allows direct or indirect introduction of (18)F into molecules of interest. The significant increase in (18)F radiotracers for PET imaging accentuates the need for simple and efficient (18)F-labeling procedures. In this review, we will describe the current radiosynthesis routes and strategies for (18)F labeling of small molecules and biomolecules.

A philosophy for CNS radiotracer design

Decades after its discovery, positron emission tomography (PET) remains the premier tool for imaging neurochemistry in living humans. Technological improvements in radiolabeling methods, camera design, and image analysis have kept PET in the forefront. In addition, the use of PET imaging has expanded because researchers have developed new radiotracers that visualize receptors, transporters, enzymes, and other molecular targets within the human brain.

However, of the thousands of proteins in the central nervous system (CNS), researchers have successfully imaged fewer than 40 human proteins. To address the critical need for new radiotracers, this Account expounds on the decisions, strategies, and pitfalls of CNS radiotracer development based on our current experience in this area.

We discuss the five key components of radiotracer development for human imaging: choosing a biomedical question, selection of a biological target, design of the radiotracer chemical structure, evaluation of candidate radiotracers, and analysis of preclinical imaging. It is particularly important to analyze the market of scientists or companies who might use a new radiotracer and carefully select a relevant biomedical question(s) for that audience. In the selection of a specific biological target, we emphasize how target localization and identity can constrain this process and discuss the optimal target density and affinity ratios needed for binding-based radiotracers. In addition, we discuss various PET test–retest variability requirements for monitoring changes in density, occupancy, or functionality for new radiotracers.

In the synthesis of new radiotracer structures, high-throughput, modular syntheses have proved valuable, and these processes provide compounds with sites for late-stage radioisotope installation. As a result, researchers can manage the time constraints associated with the limited half-lives of isotopes. In order to evaluate brain uptake, a number of methods are available to predict bioavailability, blood–brain barrier (BBB) permeability, and the associated issues of nonspecific binding and metabolic stability. To evaluate the synthesized chemical library, researchers need to consider high-throughput affinity assays, the analysis of specific binding, and the importance of fast binding kinetics. Finally, we describe how we initially assess preclinical radiotracer imaging, using brain uptake, specific binding, and preliminary kinetic analysis to identify promising radiotracers that may be useful for human brain imaging. Although we discuss these five design components separately and linearly in this Account, in practice we develop new PET-based radiotracers using these design components nonlinearly and iteratively to develop new compounds in the most efficient way possible.

PET/SPECT imaging agents for neurodegenerative diseases

Single photon emission computed tomography (SPECT) or positron emission computed tomography (PET) imaging agents for neurodegenerative diseases have a significant impact on clinical diagnosis and patient care. The examples of Parkinson's Disease (PD) and Alzheimer's Disease (AD) imaging agents described in this paper provide a general view on how imaging agents, i.e. radioactive drugs, are selected, chemically prepared and applied in humans. Imaging the living human brain can provide unique information on the pathology and progression of neurodegenerative diseases, such as AD and PD. The imaging method will also facilitate preclinical and clinical trials of new drugs offering specific information related to drug binding sites in the brain. In the future, chemists will continue to play important roles in identifying specific targets, synthesizing target-specific probes for screening and ultimately testing them by in vitro and in vivo assays.

Medications development for substance-use disorders: contextual influences (dis)incentivizing pharmaceutical-industry positioning

INTRODUCTION

The significant contribution of substance-use disorders (SUDs) to the global-disease burden and associated unmet medical needs has not engendered a commensurate level of pharma-industry research and development (R&D) for novel SUD therapeutics invention. Analysis of contextual factors shaping this position suggests potential routes toward incentivizing R&D commitment for that purpose.

AREAS COVERED

This article considers multiple primary factors that have consorted to disincentivize pharma industry's operating in the SUD space: ill-understood pathology; variegated treatments and patient profiles; involved clinical trials; and - with particular reference to SUDs-negative cultural/business stigmas and shallow commercial precedent. Industry incentivization for SUD drug innovation requires progress on several fronts, including: translational experimental data and systems; personalized, holistic SUD treatment approaches; interactions among pharma, nonindustry constituencies, and the medical profession with vested interests in countering negative stereotypes and expanding SUD treatment options; and public-private alliances focused on improving SUD pharmacotherapy.

EXPERT OPINION

Given the well-entrenched business stance whereby the prospect of future profits in major markets largely determines drug-company R&D investment trajectory, strategic initiatives offering substantial reductions in the risks and opportunity (i.e., time and money) costs associated with SUD drug discovery are likely to be the most potent drivers for encouraging mainstream industry positioning in this therapeutic area. Such initiatives could originate from front-loaded R&D operational and back-loaded patent, regulatory, marketing and health-care policy reforms. These may be too involved and protracted for the turbulent pharmaceutical industry to entertain amid its recent retrenchment from psychiatric/CNS diseases and intense pressures to increase productivity and shareholder value.

Radiosynthesis and ex vivo evaluation of [(11)C-carbonyl]carbamate- and urea-based monoacylglycerol lipase inhibitors

INTRODUCTION

Monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) are the two primary enzymes that regulate the tone of endocannabinoid signaling. Although new PET radiotracers have been discovered for imaging FAAH in vivo, no such radiotracer exists for imaging MAGL. Here we report the radiosynthesis of five candidate MAGL radiotracers and their ex vivo evaluations in mice and rats.

METHODS

Candidate carbamate and urea MAGL inhibitors were radiolabeled at the carbonyl position by [(11)C]CO2 fixation. Radiotracers were administered (tail-vein injection) to rodents and brain uptake of radioactivity measured at early and late time points ex vivo. Specificity of uptake was explored by pretreatment with unlabeled inhibitors (2 mg/kg, ip) 30 min prior to radiotracer administration.

RESULTS

All five candidate MAGL radiotracers were prepared in high specific activity (>65 GBq/μmol) and radiochemical purity (>98%). Moderate brain uptake (0.2-0.8 SUV) was observed for each candidate while pretreatment did not reduce uptake for four of the five tested. For two candidates ([(11)C]12 and [(11)C]14), high retention of radioactivity was observed in the blood (ca. 10 and 4 SUV at 40 min) which was blocked by pretreatment with unlabeled inhibitors. The most promising candidate, [(11)C]18, demonstrated moderate brain uptake (ca. 0.8 SUV) which showed circa 50% blockade by pretreatment with unlabeled 18.

CONCLUSION

One putative and four reported potent and selective MAGL inhibitors have been radiolabeled via [(11)C]CO2 fixation as radiotracers for this enzyme. Despite the promising in vitro pharmacological profile, none of the five candidate radiotracers exhibited in vivo behavior suitable for PET neuroimaging.