Radiosynthesis and evaluation of new PET ligands for peripheral cannabinoid receptor type 1 imaging

Imaging and Genetic Tools for the Investigation of the Endocannabinoid System in the CNS

As central nervous system (CNS)-related disorders present an increasing cause of global morbidity, mortality, and high pressure on our healthcare system, there is an urgent need for new insights and treatment options. The endocannabinoid system (ECS) is a critical network of endogenous compounds, receptors, and enzymes that contribute to CNS development and regulation. Given its multifaceted involvement in neurobiology and its significance in various CNS disorders, the ECS as a whole is considered a promising therapeutic target. Despite significant advances in our understanding of the ECS's role in the CNS, its complex architecture and extensive crosstalk with other biological systems present challenges for research and clinical advancements. To bridge these knowledge gaps and unlock the full therapeutic potential of ECS interventions in CNS-related disorders, a plethora of molecular-genetic tools have been developed in recent years. Here, we review some of the most impactful tools for investigating the neurological aspects of the ECS. We first provide a brief introduction to the ECS components, including cannabinoid receptors, endocannabinoids, and metabolic enzymes, emphasizing their complexity. This is followed by an exploration of cutting-edge imaging tools and genetic models aimed at elucidating the roles of these principal ECS components. Special emphasis is placed on their relevance in the context of CNS and its associated disorders.

A general procedure for carbon isotope labeling of linear urea derivatives with carbon dioxide

Carbon isotope labeling is a traceless technology, which allows tracking the fate of organic compounds either in the environment or in living organisms. This article reports on a general approach to label urea derivatives with all carbon isotopes, including 14C and 11C, based on a Staudinger aza-Wittig sequence. It provides access to all aliphatic/aromatic urea combinations.

[11C]phosgene: Synthesis and application for development of PET radiotracers

Carbon-11-labeled phosgene ([11C]phosgene, [11C]COCl2) is a useful labeling agent that connects two heteroatoms by inserting [11C]carbonyl (11C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of 11C-labeled PET tracers has been performed using [11C]phosgene as a labeling agent. However, [11C]phosgene has not been frequently used for 11C-labeling because preparation of [11C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [11C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [11C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of 11C-labeled tracers 1-4 times/week. Using [11C]phosgene we have developed and produced many PET radiotracers containing [11C]urea and [11C]carbamate moieties. In this review, we report the performance of our method for preparing [11C]phosgene, including automated synthesis apparatus developed in house, and the application of [11C]phosgene for development and production of 11C-labeled PET tracers.

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.

Fluorinated CRA13 analogues: Synthesis, in vitro evaluation, radiosynthesis, in silico and in vivo PET study

Fluorine is a unique atom that imparts distinct properties to bioactive molecules upon incorporation. Herein, we prepare and study fluorinated derivatives of the nanomolar affine peripherally restricted dual CB₁R/CB₂R agonist; CRA13 and its analogs. Binding affinity evaluation relative to CRA13 proved the stronger binding affinity of compound 7c to CB₁R and CB₂R by 6.95 and 5.64 folds. Physicochemical properties evaluation proved compound 7c improved lipophilicity profile suggesting some enhanced BBB penetration relative to CRA13. Radiosynthesis of ¹⁸F-labeled compound 7c was conducted conveniently affording pure hot ligand. In vivo PET study investigation demonstrated efficient distribution of ¹⁸F-labeled compound 7c in peripheral tissues visualizing peripheral CB₁R/CB₂R generating time-activity-curves showing good standard uptake values. Despite enhanced BBB penetration and increased cannabinoid receptors binding affinity, low brain uptake of 7c was observed. In silico docking study explained the measured binding affinities of compounds 7a-d to CB₁R. While most of previous efforts aimed to develop central cannabinoid PET imaging agents, ¹⁸F-labeled compound 7c might be a promising agent serving as a universal CB₁R/CB₂R PET imaging agents for diagnosis and therapy of various diseases correlated with peripheral cannabinoid system. It might also serve as a lead compound for development of PET imaging of peripheral and central cannabinoid systems.

Preclinical in vivo imaging for brown adipose tissue

Brown adipose tissue (BAT) has multiple functions in the human body, including the production of heat and increasing energy consumption. However, BAT is also related to many kinds of diseases, such as obesity and metabolic disorders. The progression of such diseases occurs at the cellular level, and thus, imaging techniques could prove greatly beneficial for determining optimal therapeutic regimens. Currently, positron-emission tomography (PET) is considered to be the gold standard for assessing the function of activated BAT. However, PET also has inherent disadvantages, and, thus, recent efforts have been focused on exploring, and potentially developing, new imaging techniques to better observe BAT and evaluate its metabolic function. Researchers have already achieved promising success with computed tomography, magnetic resonance approaches, ultrasound, new tracers for use in PET, and other imaging techniques through in vivo and in vitro animal experiments. Since, these studies have shown that BAT may serve as an effective therapeutic target for treatment of metabolic dysfunction diseases, the development of an efficient in vivo BAT imaging technique that is applicable to humans will prove to be of great clinical value. In this review, classical PET imaging technique is highlighted as well as the current status of preclinical imaging methods developed for BAT examination.

Fluorine-18 isotope labeling for positron emission tomography imaging. Direct evidence for DBPR211 as a peripherally restricted CB1 inverse agonist

The [¹⁸F] isotope-labelled CB1 inverse agonist 3 was elaborated and synthesized for positron emission tomography scanning studies. After immediate purification and calibration with its unlabeled counterpart, compound 3 was intravenously injected in mice and revealed that its distribution percentage in brain over 90-min scans among five region of interests, including brain, liver, heart, thigh muscle and kidney was lower than 1%, thus providing direct evidence to justify itself as a peripherally restricted CB1 antagonist.

The future of type 1 cannabinoid receptor allosteric ligands

Allosteric modulation of the type 1 cannabinoid receptor (CB1R) holds great therapeutic potential. This is because allosteric modulators do not possess intrinsic efficacy, but instead augment (positive allosteric modulation) or diminish (negative allosteric modulation) the receptor's response to endogenous ligand. Consequently, CB1R allosteric modulators have an effect ceiling which allows for the tempering of CB1R signaling without the desensitization, tolerance, dependence, and psychoactivity associated with orthosteric compounds. Pain, movement disorders, epilepsy, obesity are all potential therapeutic targets for CB1R allosteric modulation. Several challenges exist for the development of CB1R allosteric modulators, such as receptor subtype specificity, translation to in vivo systems, and mixed allosteric/agonist/inverse agonist activity. Despite these challenges, elucidation of crystal structures of CB1R and compound design based on structure-activity relationships will advance the field. In this review, we will cover recent progress for CB1R allosteric modulators and discuss the future promise of this research.