Classics in Neuroimaging: Imaging the Cholinergic System with Positron Emission Tomography

An improved synthesis of [18 F]VAT and its precursor

The vesicular acetylcholine transporter (VAChT) in the brain is an important presynaptic cholinergic biomarker, and neuroimaging studies of VAChT may provide in vivo information about psychiatric and neurologic conditions including Alzheimer's disease that are not accessible by other methods. The 18 F-labeled radiotracer, ((-)-(1-(-8-(2-[18 F]fluoroethoxy)-3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)piperidin-4-yl)(4-fluorophenyl)-methanone ([18 F]VAT, 1), was reported as a selective and high affinity ligand for the in vivo imaging of VAChT. The synthesis of [18 F]VAT has been reported in a two-step procedure with total 140 min, which includes preparation of 2-[18 F]fluoroethyltosylate and alkylation of benzovesamicol (-)-5 precursor with this radiosynthon using two different automated production modules consecutively. A multiple step synthetic route was employed for the synthesis of stereospecific precursor benzovesamicol (-)-5, which is difficult to be adapted for scale-up. To make the production of this tracer more amenable for clinical imaging, we present an improved total synthesis protocol to attain [18 F]VAT: (1) a tosylethoxy group being pre-installed tosylate precursor (-)-8 is synthesized to render a simple one-step radiofluorination under mild conditions; (2) The key optically active intermediate benzovesamicol (-)-5 was obtained via the regio- and enantio-enriched ring-opening amination of meso-epoxide 3 with 4-phenylpiperidine derivative 2 under catalysis of a chiral salenCo(III) catalyst 4b, which dramatically simplifies the synthetic route of the tosylate precursor (-)-8. [18 F]VAT 1 was prepared within ~65 min with desired chemical and radiochemical purities, via a fully automated procedure, using a commercial PET tracer production module. The final drug product was obtained as a sterile, pyrogen-free solution that conforms United States Pharmacopeia (USP) <823> requirements.

New Advances in the Exploration of Esterases with PET and Fluorescent Probes

Esterases are hydrolases that catalyze the hydrolysis of esters into the corresponding acids and alcohols. The development of fluorescent probes for detecting esterases is of great importance due to their wide spectrum of biological and industrial applications. These probes can provide a rapid and sensitive method for detecting the presence and activity of esterases in various samples, including biological fluids, food products, and environmental samples. Fluorescent probes can also be used for monitoring the effects of drugs and environmental toxins on esterase activity, as well as to study the functions and mechanisms of these enzymes in several biological systems. Additionally, fluorescent probes can be designed to selectively target specific types of esterases, such as those found in pathogenic bacteria or cancer cells. In this review, we summarize the recent fluorescent probes described for the visualization of cell viability and some applications for in vivo imaging. On the other hand, positron emission tomography (PET) is a nuclear-based molecular imaging modality of great value for studying the activity of enzymes in vivo. We provide some examples of PET probes for imaging acetylcholinesterases and butyrylcholinesterases in the brain, which are valuable tools for diagnosing dementia and monitoring the effects of anticholinergic drugs on the central nervous system.

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

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

Concise Review: Recent advances in neuroimaging techniques to assist clinical trials on cell-based therapies in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are progressive disorders for which a curative therapy is still lacking. Cell-based therapy aims at replacing dysfunctional cellular populations by repairing damaged tissue and by enriching the microenvironment of selective brain areas, and thus constitutes a promising disease-modifying treatment of neurodegenerative diseases. Scientific research has engineered a wide range of human-derived cellular populations to help overcome some of the logistical, safety, and ethical issues associated with this approach. Open-label studies and clinical trials in human participants have employed neuroimaging techniques, such as positron emission tomography (PET) and magnetic resonance imaging (MRI), to assess the success of the transplantation, to evaluate the functional integration of the implanted tissue into the host environment and to understand the pathophysiological changes associated with the therapy. Neuroimaging has constituted an outcome measure of large, randomized clinical trials, and has given answers to clarify the pathophysiology underlying some of the complications linked with this therapy. Novel PET radiotracers and MRI sequences for the staging of neurodegenerative diseases and to study alterations at molecular level significantly expands the translational potential of neuroimaging to assist pre-clinical and clinical research on cell-based therapy in these disorders. This concise review summarizes the current use of neuroimaging in human studies of cell-based replacement therapy and focuses on future application of PET and MRI techniques to evaluate the pathophysiology and treatment efficacy, as well as to aid patient selection and as an outcome measure to improve treatment success.