Identification and Development of an Irreversible Monoacylglycerol Lipase (MAGL) Positron Emission Tomography (PET) Radioligand with High Specificity

Preclinical Evaluation of the Reversible Monoacylglycerol Lipase PET Tracer (R)-[11C]YH132: Application in Drug Development and Neurodegenerative Diseases

Monoacylglycerol lipase (MAGL) plays a crucial role in the degradation of 2-arachidonoylglycerol (2-AG), one of the major endocannabinoids in the brain. Inhibiting MAGL could lead to increased levels of 2-AG, which showed beneficial effects on pain management, anxiety, inflammation, and neuroprotection. In the current study, we report the characterization of an enantiomerically pure (R)-[11C]YH132 as a novel MAGL PET tracer. It demonstrates an improved pharmacokinetic profile compared to its racemate. High in vitro MAGL specificity of (R)-[11C]YH132 was confirmed by autoradiography studies using mouse and rat brain sections. In vivo, (R)-[11C]YH132 displayed a high brain penetration, and high specificity and selectivity toward MAGL by dynamic PET imaging using MAGL knockout and wild-type mice. Pretreatment with a MAGL drug candidate revealed a dose-dependent reduction of (R)-[11C]YH132 accumulation in WT mouse brains. This result validates its utility as a PET probe to assist drug development. Moreover, its potential application in neurodegenerative diseases was explored by in vitro autoradiography using brain sections from animal models of Alzheimer's disease and Parkinson's disease.

Preclinical Evaluation of Novel Positron Emission Tomography (PET) Probes for Imaging Leucine-Rich Repeat Kinase 2 (LRRK2)

Parkinson's disease (PD) is one of the most highly debilitating neurodegenerative disorders, which affects millions of people worldwide, and leucine-rich repeat kinase 2 (LRRK2) mutations have been involved in the pathogenesis of PD. Developing a potent LRRK2 positron emission tomography (PET) tracer would allow for in vivo visualization of LRRK2 distribution and expression in PD patients. In this work, we present the facile synthesis of two potent and selective LRRK2 radioligands [11C]3 ([11C]PF-06447475) and [18F]4 ([18F]PF-06455943). Both radioligands exhibited favorable brain uptake and specific bindings in rodent autoradiography and PET imaging studies. More importantly, [18F]4 demonstrated significantly higher brain uptake in the transgenic LRRK2-G2019S mutant and lipopolysaccharide (LPS)-injected mouse models. This work may serve as a roadmap for the future design of potent LRRK2 PET tracers.

Imaging Leucine-Rich Repeat Kinase 2 In Vivo with 18F-Labeled Positron Emission Tomography Ligand

Leucine-rich repeat kinase 2 (LRRK2) has been demonstrated to be closely involved in the pathogenesis of Parkinson's disease (PD), and pharmacological blockade of LRRK2 represents a new opportunity for therapeutical treatment of PD and other related neurodegenerative conditions. The development of an LRRK2-specific positron emission tomography (PET) ligand would enable a target occupancy study in vivo and greatly facilitate LRRK2 drug discovery and clinical translation as well as provide a molecular imaging tool for studying physiopathological changes in neurodegenerative diseases. In this work, we present the design and development of compound 8 (PF-06455943) as a promising PET radioligand through a PET-specific structure-activity relationship optimization, followed by comprehensive pharmacology and ADME/neuroPK characterization. Following an efficient 18F-labeling method, we have confirmed high brain penetration of [18F]8 in nonhuman primates (NHPs) and validated its specific binding in vitro by autoradiography in postmortem NHP brain tissues and in vivo by PET imaging studies.

Radiolabelling small and biomolecules for tracking and monitoring

Radiolabelling small molecules with beta-emitters has been intensively explored in the last decades and novel concepts for the introduction of radionuclides continue to be reported regularly. New catalysts that induce carbon/hydrogen activation are able to incorporate isotopes such as deuterium or tritium into small molecules. However, these established labelling approaches have limited applicability for nucleic acid-based drugs, therapeutic antibodies, or peptides, which are typical of the molecules now being investigated as novel therapeutic modalities. These target molecules are usually larger (significantly >1 kDa), mostly multiply charged, and often poorly soluble in organic solvents. However, in preclinical research they often require radiolabelling in order to track and monitor drug candidates in metabolism, biotransformation, or pharmacokinetic studies. Currently, the most established approach to introduce a tritium atom into an oligonucleotide is based on a multistep synthesis, which leads to a low specific activity with a high level of waste and high costs. The most common way of tritiating peptides is using appropriate precursors. The conjugation of a radiolabelled prosthetic compound to a functional group within a protein sequence is a commonly applied way to introduce a radionuclide or a fluorescent tag into large molecules. This review highlights the state-of-the-art in different radiolabelling approaches for oligonucleotides, peptides, and proteins, as well as a critical assessment of the impact of the label on the properties of the modified molecules. Furthermore, applications of radiolabelled antibodies in biodistribution studies of immune complexes and imaging of brain targets are reported.

Quantification of monoacylglycerol lipase and its occupancy by an exogenous ligand in rhesus monkey brains using [18F]T-401 and PET

Monoacylglycerol lipase (MAGL) is a cytosolic serine hydrolase that cleaves monoacylglycerols into fatty acids and is a potential target for the novel treatment of CNS disorders related to the endocannabinoid system and neuroinflammation. We have developed [¹⁸F]T-401 as a selective Positron emission tomography (PET) imaging agent for MAGL. In this study, we determined an analytical method to quantify MAGL availability and its occupancy by an exogenous inhibitor in rhesus monkey brains using [¹⁸F]T-401-PET. In rhesus monkeys, regional time-activity curves were described well when using an extended 2-tissue compartment model that accommodated the formation of a radiometabolite in the brain. This model yielded reliable estimates of the total distribution volume (V_T), and the rank order of V_T was consistent with known regional activity of MAGL enzyme in primates. The pretreatment of monkeys with JW642 resulted in a dose-dependent reduction of [¹⁸F]T-401 retentions in the brain, and V_T. Lassen's graphical analysis indicated a V_ND of 0.69 mL/cm³ and a plasma JW642 concentration of 126 ng/mL for inhibiting the specific binding by 50%. [¹⁸F]T-401 and the method established can be used for quantification of MAGL in healthy brain and in disease conditions, and is suitable for evaluations of target engagement at cerebral MAGL.

Target occupancy study and whole-body dosimetry with a MAGL PET ligand [11C]PF-06809247 in non-human primates

Background

Monoacylglycerol lipase (MAGL) is a key serine hydrolase which terminates endocannabinoid signaling and regulates arachidonic acid driven inflammatory responses within the central nervous system. To develop [¹¹C]PF-06809247 into a clinically usable MAGL positron emission tomography (PET) radioligand, we assessed the occupancy of MAGL by an inhibitor in the non-human primate (NHP) brain. Additionally, we measured the whole-body distribution of [¹¹C]PF-06809247 in NHP and estimated human effective radiation doses.

Methods

Seven cynomolgus monkeys were enrolled for brain PET measurements. Two PET measurements along with arterial blood sampling were performed in each NHP: one baseline and one pretreatment condition with intravenous administration of PF-06818883, a pro-drug of a selective MAGL inhibitor (total of seven doses between 0.01 and 1.27 mg/kg). Kinetic parameters K₁, k₂ and k₃ were estimated by a two tissue compartment (2TC) model using metabolite corrected plasma radioactivity as the input function. k₄ was set as 0 according to the irreversible binding of [¹¹C]PF-06809247. K_i by 2TC and Patlak analysis were calculated as the influx constant. The target occupancy was calculated using K_i at baseline and pretreatment conditions. Two cynomolgus monkeys were enrolled for whole-body PET measurements. Estimates of the absorbed radiation dose in humans were calculated with OLINDA/EXM 1.1 using the adult male reference model.

Results

Radioactivity retention was decreased in all brain regions following pretreatment with PF-06818883. Occupancy was measured as 25.4–100.5% in a dose dependent manner. Whole-body PET showed high radioactivity uptake values in the liver, small intestine, kidney, and brain. The effective dose of [¹¹C]PF-06809247 was calculated as 4.3 μSv/MBq.

Conclusions

[¹¹C]PF-06809247 is a promising PET ligand for further studies of MAGL in the human brain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-022-00882-2.

A review on structurally diversified synthesized molecules as monoacylglycerol lipase inhibitors and their therapeutic uses

Monoacylglycerol is a metabolic key serine hydrolase, engaged in the regulation of signalling network system of endocannabinoids, which is associated with various physiological processes like pain, inflammation, feeding cognition and neurodegenerative diseases like Alzheimer, Parkinson's disease. The monoacylglycerol also found to act as a regulator and the free fatty acid provider in the proliferation of cancer cells, numerous aggressive tumours such as colorectal cancer, neuroblastoma and nasopharyngeal carcinoma. It also played an important role in increasing the concentration of specific lipids derived from free fatty acids like phosphatidic acid, lysophosphatidic acid, sphingosine-1-phosphate and prostaglandin E2. These signalling lipids are associated with cell proliferation, survival, tumour cell migration, contributing to tumour development, maturation and metastases. In the present study here, we are presenting a review on structurally diverse MAGL inhibitors, their development and their evaluation for different pharmacological activities.

Development of High Brain-Penetrant and Reversible Monoacylglycerol Lipase PET Tracers for Neuroimaging

Monoacylglycerol lipase (MAGL) is one of the key enzymes in the endocannabinoid system. Inhibition of MAGL has been proposed as an attractive approach for the treatment of various diseases. In this study, we designed and successfully synthesized two series of piperazinyl pyrrolidin-2-one derivatives as novel reversible MAGL inhibitors. (R)-[¹⁸F]13 was identified through the preliminary evaluation of two carbon-11-labeled racemic structures [¹¹C]11 and [¹¹C]16. In dynamic positron-emission tomography (PET) scans, (R)-[¹⁸F]13 showed a heterogeneous distribution and matched the MAGL expression pattern in the mouse brain. High brain uptake and brain-to-blood ratio were achieved by (R)-[¹⁸F]13 in comparison with previously reported reversible MAGL PET radiotracers. Target occupancy studies with a therapeutic MAGL inhibitor revealed a dose-dependent reduction of (R)-[¹⁸F]13 accumulation in the mouse brain. These findings indicate that (R)-[¹⁸F]13 ([¹⁸F]YH149) is a highly promising PET probe for visualizing MAGL non-invasively in vivo and holds great potential to support drug development.

Novel Reversible-Binding PET Ligands for Imaging Monoacylglycerol Lipase Based on the Piperazinyl Azetidine Scaffold

Monoacylglycerol lipase (MAGL) is a 33 kDa serine protease primarily responsible for hydrolyzing 2-arachidonoylglycerol into the proinflammatory eicosanoid precursor arachidonic acid in the central nervous system. Inhibition of MAGL constitutes an attractive therapeutic concept for treating psychiatric disorders and neurodegenerative diseases. Herein, we present the design and synthesis of multiple reversible MAGL inhibitor candidates based on a piperazinyl azetidine scaffold. Compounds 10 and 15 were identified as the best-performing reversible MAGL inhibitors by pharmacological evaluations, thus channeling their radiolabeling with fluorine-18 in high radiochemical yields and favorable molar activity. Furthermore, evaluation of [¹⁸F]10 and [¹⁸F]15 ([¹⁸F]MAGL-2102) by autoradiography and positron emission tomography (PET) imaging in rodents and nonhuman primates demonstrated favorable brain uptakes, heterogeneous radioactivity distribution, good specific binding, and adequate brain kinetics, and [¹⁸F]15 demonstrated a better performance. In conclusion, [¹⁸F]15 was found to be a suitable PET radioligand for the visualization of MAGL, harboring potential for the successful translation into humans.

Discovery of Isatin-Based Carbohydrazones as Potential Dual Inhibitors of Fatty Acid Amide Hydrolase and Monoacylglycerol Lipase

Using ligand-based design strategy, a set of isatin-3-carbohydrazones was designed, synthesized and evaluated for dual fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) inhibition properties. Compound 5-chloro-N'-(5-chloro-2-oxoindolin-3-ylidene)-2-hydroxybenzohydrazide (13 b) emerged as a potent MAGL inhibitor with nanomolar activity (IC₅₀ =3.33 nM), while compound 5-chloro-N'-(1-(4-fluorobenzyl)-2-oxoindolin-3-ylidene)-2-hydroxybenzohydrazide (13 j) was the most potent selective FAAH inhibitor (IC₅₀ =37 nM). Compound 5-chloro-N'-(6-chloro-2-oxoindolin-3-ylidene)-2-hydroxybenzohydrazide (13 c) showed dual FAAH-MAGL inhibitory activity with an IC₅₀ of 31 and 29 nM respectively. Enzyme kinetics studies revealed that the isatin-based carbohydrazones are reversible inhibitors for both FAAH and MAGL. Further, blood-brain permeability assay confirmed that the lead compounds (13 b, 13 c, 13 g, 13 m and 13 q) are suitable as CNS candidates. Molecular dynamics simulation studies revealed the putative binding modes and key interactions of lead inhibitors within the enzyme active sites. The lead dual FAAH-MAGL inhibitor 13 c showed significant antioxidant activity and neuroprotection in the cell-based cytotoxicity assay. In summary, the study yielded three potent FAAH/MAGL inhibitor compounds (13 b, 13 c and 13 j) with acceptable pharmacokinetic profile and thus can be considered as promising candidates for treating neurological and mood disorders.

Development of a highly-specific 18F-labeled irreversible positron emission tomography tracer for monoacylglycerol lipase mapping

As a serine hydrolase, monoacylglycerol lipase (MAGL) is principally responsible for the metabolism of 2-arachidonoylglycerol (2-AG) in the central nervous system (CNS), leading to the formation of arachidonic acid (AA). Dysfunction of MAGL has been associated with multiple CNS disorders and symptoms, including neuroinflammation, cognitive impairment, epileptogenesis, nociception and neurodegenerative diseases. Inhibition of MAGL provides a promising therapeutic direction for the treatment of these conditions, and a MAGL positron emission tomography (PET) probe would greatly facilitate preclinical and clinical development of MAGL inhibitors. Herein, we design and synthesize a small library of fluoropyridyl-containing MAGL inhibitor candidates. Pharmacological evaluation of these candidates by activity-based protein profiling identified 14 as a lead compound, which was then radiolabeled with fluorine-18 via a facile S_NAr reaction to form 2-[¹⁸F]fluoropyridine scaffold. Good blood–brain barrier permeability and high in vivo specific binding was demonstrated for radioligand [¹⁸F]14 (also named as [¹⁸F]MAGL-1902). This work may serve as a roadmap for clinical translation and further design of potent ¹⁸F-labeled MAGL PET tracers.

Molecular imaging of NAD+ -dependent deacetylase SIRT1 in the brain

INTRODUCTION

Aging is an inevitable physiological process and the biggest risk factor of Alzheimer's disease (AD). Developing an imaging tracer to visualize aging-related changes in the brain may provide a useful biomarker in elucidating neuroanatomical mechanisms of AD.

METHODS

We developed and characterized a new tracer that can be used to visualize SIRT1 in brains related to aging and AD by positron emission tomography imaging.

RESULTS

The SIRT1 tracer displayed desirable brain uptake and selectivity, as well as stable metabolism and proper kinetics and distribution in rodent and nonhuman primate brains. This new tracer was further validated by visualizing SIRT1 in brains of AD transgenic mice, compared to nontransgenic animals.

DISCUSSION

Our SIRT1 tracer not only enables, for the first time, the demonstration of SIRT1 in animal brains, but also allows visualization and recapitulation of AD-related SIRT1 neuropathological changes in animal brains.

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.

Druggable Targets in Endocannabinoid Signaling

Cannabis and cannabinoid-based extracts have long been utilized for their perceived therapeutic value, and support for the legalization of cannabis for medicinal purposes continues to increase worldwide. Since the discovery of Δ⁹-tetrahydrocannabinol (THC) as the primary psychoactive component of cannabis over 50 years ago, substantial effort has been directed toward detection of endogenous mediators of cannabinoid activity. The discovery of anandamide and 2-arachidonoylglycerol as two endogenous lipid mediators of cannabinoid-like effects (endocannabinoids) has inspired exponential growth in our understanding of this essential pathway, as well as the pathological conditions that result from dysregulated endocannabinoid signaling. This review examines current knowledge of the endocannabinoid system including metabolic enzymes involved in biosynthesis and degradation and their receptors, and evaluates potential druggable targets for therapeutic intervention.

Monoacylglycerol lipase reprograms lipid precursors signaling in liver disease

Dietary oversupply of triglycerides represent the hallmark of obesity and connected complications in the liver such as non-alcoholic fatty liver disease and non-alcoholic steatohepatitis, which eventually progress to cirrhosis and hepatocellular carcinoma. Monoacylglycerol lipase is the last enzymatic step in the hydrolysis of triglycerides, generating glycerol and fatty acids (FAs), which are signaling precursors in physiology and disease. Notably, monoacylglycerol lipase (MGL) also hydrolyzes 2-arachidonoylglycerol, which is a potent ligand within the endocannabinoid system, into arachidonic acid - a precursor for prostaglandin synthesis; thus representing a pivotal substrates provider in multiple organs for several intersecting biological pathways ranging from FA metabolism to inflammation, pain and appetite. MGL inhibition has been shown protective in limiting several liver diseases as FAs may drive hepatocyte injury, fibrogenesis and de- activate immune cells, however the complexity of MGL network system still needs further and deeper understanding. The present review will focus on MGL function and FA partitioning in the horizons of liver disease.

Design, Synthesis, and Evaluation of 18F-Labeled Monoacylglycerol Lipase Inhibitors as Novel Positron Emission Tomography Probes

Dysfunction of monoacylglycerol lipase (MAGL) is associated with several psychopathological disorders, including drug addiction and neurodegenerative diseases. Herein we design, synthesize, and evaluate several irreversible fluorine-containing MAGL inhibitors for positron emission tomography (PET) ligand development. Compound 6 (identified from a therapeutic agent) was advanced for ¹⁸F-labeling via a novel spirocyclic iodonium ylide (SCIDY) strategy, which demonstrated high brain permeability and excellent specific binding. This work supports further development of novel ¹⁸F-labeled MAGL PET probes.