An Overview of PET Radiochemistry, Part 1: The Covalent Labels 18F, 11C, and 13N

[18F]-Radiolabelled Nanoplatforms: A Critical Review of Their Intrinsic Characteristics, Radiolabelling Methods, and Purification Techniques

A wide range of nano-objects is found in many applications of our everyday life. Recognition of their peculiar properties and ease of functionalization has prompted their engineering into multifunctional platforms that are supposed to afford efficient tools for the development of biomedical applications. However, bridging the gap between bench to bedside cannot be expected without a good knowledge of their behaviour in vivo, which can be obtained through non-invasive imaging techniques, such as positron emission tomography (PET). Their radiolabelling with [18F]-fluorine, a technique already well established and widely used routinely for PET imaging, with [18F]-FDG for example, and in preclinical investigation using [18F]-radiolabelled biological macromolecules, has, therefore, been developed. In this context, this review highlights the various nano-objects studied so far, the reasons behind their radiolabelling, and main in vitro and/or in vivo results obtained thereof. Then, the methods developed to introduce the radioelement are presented. Detailed indications on the chemical steps involved are provided, and the stability of the radiolabelling is discussed. Emphasis is then made on the techniques used to purify and analyse the radiolabelled nano-objects, a point that is rarely discussed despite its technical relevance and importance for accurate imaging. The pros and cons of the different methods developed are finally discussed from which future work can develop.

Advances of radiolabeled GRPR ligands for PET/CT imaging of cancers

GRPR is a type of seven-transmembrane G-protein coupled receptor that belongs to the bombesin protein receptor family. It is highly expressed in various cancers, including prostate cancer, breast cancer, lung cancer, gastrointestinal cancer, and so on. As a result, molecular imaging studies have been conducted using radiolabeled GRPR ligands for tumor diagnosis, as well as monitoring of recurrence and metastasis. In this paper, we provided a comprehensive overview of relevant literature from the past two decades, with a specific focus on the advancements made in radiolabeled GRPR ligands for imaging prostate cancer and breast cancer.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Pharmacokinetic Evaluation of New Drugs Using a Multi-Labelling Approach and PET Imaging: Application to a Drug Candidate with Potential Application in Neuromuscular Disorders

BACKGROUND AND OBJECTIVE

The determination of pharmacokinetic properties of new chemical entities is a key step in the process of drug development. Positron emission tomography (PET) is an ideal technique to obtain both biodistribution and pharmacokinetic parameters of new compounds over a wide range of chemical modalities. Here, we use a multi-radionuclide/multi-position labelling approach to investigate distribution, elimination, and metabolism of a triazole-based FKBP12 ligand (AHK2) with potential application in neuromuscular disorders.

METHODS

Target engagement and stabilizing capacity of the drug candidate (AHK2) towards FKBP12-RyR was evaluated using competitive ligand binding and proximity ligation assays, respectively. Subsequently, AHK2 was labelled either with the positron emitter carbon-11 (¹¹C) via ¹¹C-methylation to yield both [¹¹C]AHK2.1 and [¹¹C]AHK2.2, or by palladium-catalysed reduction of the corresponding 5-iodotriazole derivative using ³H gas to yield [³H]AHK2. Metabolism was first investigated in vitro using liver microsomes. PET imaging studies in rats after intravenous (IV) administration at different doses (1 µg/Kg and 5 mg/Kg) were combined with determination of arterial blood time-activity curves (TACs) and analysis of plasma samples by high performance liquid chromatography (HPLC) to quantify radioactive metabolites. Arterial TACs were obtained in continuous mode by using an in-house developed system that enables extracorporeal blood circulation and continuous measurement of radioactivity in the blood. Pharmacokinetic parameters were determined by non-compartmental modelling of the TACs.

RESULTS

In vitro studies indicate that AHK2 binds to FKBP12 at the rapamycin-binding pocket, presenting activity as a FKBP12/RyR stabilizer. [¹¹C]AHK2.1, [¹¹C]AHK2.2 and [³H]AHK2 could be obtained in overall non-decay corrected radiochemical yields of 14 ± 2%, 15 ± 2% and 0.05%, respectively. Molar activities were 60-110 GBq/µmol, 68-122 GBq/µmol and 0.4-0.5 GBq/μmol, respectively. In vitro results showed that oxidation of the thioether group into sulfoxide, demethylation of the CH₃O-Ar residue and demethylation of -N(CH₃)₂ were the main metabolic pathways. Fast metabolism was observed in vivo. Pharmacokinetic parameters obtained from metabolite-corrected arterial blood TACs showed a short half-life (12.6 ± 3.3 min). Dynamic PET imaging showed elimination via urine when [¹¹C]AHK2.2 was administered, probably reflecting the biodistribution of [¹¹C]methanol as the major metabolite. Contrarily, accumulation in the gastrointestinal track was observed after administration of [¹¹C]AKH2.1.

CONCLUSIONS

AHK2 binds to FKBP12 at the rapamycin-binding pocket, presenting activity as a FKBP12/RyR stabilizer. Studies performed with the ³H- and ¹¹C-labelled FKBP12/RyR stabilizer AHK2 confirm fast blood clearance, linear pharmacokinetics and rapid metabolism involving oxidation of the sulfide and amine moieties and oxidative demethylation of the CH₃-O-Ar and tertiary amine groups as the main pathways. PET studies suggest that knowledge about metabolic pathways is paramount to interpret images.

On the dissociation pathways of copper complexes relevant as PET imaging agents

Several bifunctional chelators have been synthesized in the last years for the development of new ⁶⁴Cu-based PET agents for in vivo imaging. When designing a metal-based PET probe, it is important to achieve high stability and kinetic inertness once the radioisotope is coordinated. Different competitive assays are commonly used to evaluate the possible dissociation mechanisms that may induce Cu(II) release in the body. Among them, acid-assisted dissociation tests or transchelation challenges employing EDTA or SOD are frequently used to evaluate both solution thermodynamics and the kinetic behavior of potential metal-based systems. Despite of this, the Cu(II)/Cu(I) bioreduction pathway that could be promoted by the presence of bioreductants still remains little explored. To fill this gap we present here a detailed spectroscopic study of the kinetic behavior of different macrocyclic Cu(II) complexes. The complexes investigated include the cross-bridge cyclam derivative [Cu(CB-TE1A)]⁺, whose structure was determined using single-crystal X-ray diffraction. The acid-assisted dissociation mechanism was investigated using HClO₄ and HCl to analyse the effect of the counterion on the rate constants. The complexes were selected so that the effects of complex charge and coordination polyhedron could be assessed. Cyclic voltammetry experiments were conducted to investigate whether the reduction to Cu(I) falls within the window of common bioreducing agents. The most striking behavior concerns the [Cu(NO2Th)]²⁺ complex, a 1,4,7-triazacyclononane derivative containing two methylthiazolyl pendant arms. This complex is extremely inert with respect to dissociation following the acid-catalyzed mechanism, but dissociates rather quickly in the presence of a bioreductant like ascorbic acid.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

If It Works, Don't Touch It? A Cell-Based Approach to Studying 2-[18F]FDG Metabolism

The glucose derivative 2-[¹⁸F]fluoro-2-deoxy-D-glucose (2-[¹⁸F]FDG) is still the most used radiotracer for positron emission tomography, as it visualizes glucose utilization and energy demand. In general, 2-[¹⁸F]FDG is said to be trapped intracellularly as 2-[¹⁸F]FDG-6-phosphate, which cannot be further metabolized. However, increasingly, this dogma is being questioned because of publications showing metabolism beyond 2-[¹⁸F]FDG-6-phosphate and even postulating 2-[¹⁸F]FDG imaging to depend on the enzyme hexose-6-phosphate dehydrogenase in the endoplasmic reticulum. Therefore, we aimed to study 2-[¹⁸F]FDG metabolism in the human cancer cell lines HT1080, HT29 and Huh7 applying HPLC. We then compared 2-[¹⁸F]FDG metabolism with intracellular tracer accumulation, efflux and the cells’ metabolic state and used a graphical Gaussian model to visualize metabolic patterns. The extent of 2-[¹⁸F]FDG metabolism varied considerably, dependent on the cell line, and was significantly enhanced by glucose withdrawal. However, the metabolic pattern was quite conserved. The most important radiometabolites beyond 2-[¹⁸F]FDG-6-phosphate were 2-[¹⁸F]FDMannose-6-phosphate, 2-[¹⁸F]FDG-1,6-bisphosphate and 2-[¹⁸F]FD-phosphogluconolactone. Enhanced radiometabolite formation under glucose reduction was accompanied by reduced efflux and mirrored the cells’ metabolic switch as assessed via extracellular lactate levels. We conclude that there can be considerable metabolism beyond 2-[¹⁸F]FDG-6-phosphate in cancer cell lines and a comprehensive understanding of 2-[¹⁸F]FDG metabolism might help to improve cancer research and tumor diagnosis.

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.

Advances in the In Vivo Molecular Imaging of Invasive Aspergillosis

Invasive pulmonary aspergillosis (IPA) is a life-threatening infection of immunocompromised patients with Aspergillus fumigatus, a ubiquitous environmental mould. While there are numerous functioning antifungal therapies, their high cost, substantial side effects and fear of overt resistance development preclude permanent prophylactic medication of risk-patients. Hence, a fast and definitive diagnosis of IPA is desirable, to quickly identify those patients that really require aggressive antimycotic treatment and to follow the course of the therapeutic intervention. However, despite decades of research into this issue, such a diagnostic procedure is still not available. Here, we discuss the array of currently available methods for IPA detection and their limits. We then show that molecular imaging using positron emission tomography (PET) combined with morphological computed tomography or magnetic imaging is highly promising to become a future non-invasive approach for IPA diagnosis and therapy monitoring, albeit still requiring thorough validation and relying on further acceptance and dissemination of the approach. Thereby, our approach using the A. fumigatus-specific humanized monoclonal antibody hJF5 labelled with ⁶⁴Cu as PET-tracer has proven highly effective in pre-clinical models and hence bears high potential for human application.

Research progress of 18F labeled small molecule positron emission tomography (PET) imaging agents

With the development of positron emission tomography (PET) technology, a variety of PET imaging agents labeled with radionuclide ¹⁸F have been developed and widely used in the diagnosis and treatment of various clinical diseases in recent years. For example, they have showed a great value of study in the field of tumor detection, tumor treatment and evaluation of tumor therapy in a non-invasive, qualitative and quantitative way. In this review, we highlight the recent development in chemical synthesis, structure and characterization, imaging characterization, and potential applications of these ¹⁸F labeled small molecule PET imaging agents for the past five years. The development and application of ¹⁸F labeled small molecules will expand our knowledge of the function and distribution of diseases-related molecular targets and shed light on the diagnosis and treatment of various diseases including tumors.

Cellular and molecular imaging for stem cell tracking in neurological diseases

Stem cells (SCs) are cells with strong proliferation ability, multilineage differentiation potential and self-renewal capacity. SC transplantation represents an important therapeutic advancement for the treatment strategy of neurological diseases, both in the preclinical experimental and clinical settings. Innovative and breakthrough SC labelling and tracking technologies are widely used to monitor the distribution and viability of transplanted cells non-invasively and longitudinally. Here we summarised the research progress of the main tracers, labelling methods and imaging technologies involved in current SC tracking technologies for various neurological diseases. Finally, the applications, challenges and unresolved problems of current SC tracing technologies were discussed.

[18F]AZD2461, an Insight on Difference in PARP Binding Profiles for DNA Damage Response PET Imaging

BACKGROUND

Poly (ADP-ribose) polymerase (PARP) inhibitors are extensively studied and used as anti-cancer drugs, as single agents or in combination with other therapies. Most radiotracers developed to date have been chosen on the basis of strong PARP1-3 affinity. Herein, we propose to study AZD2461, a PARP inhibitor with lower affinity towards PARP3, and to investigate its potential for PARP targeting in vivo.

METHODS

Using the Cu-mediated 18F-fluorodeboronation of a carefully designed radiolabelling precursor, we accessed the 18F-labelled isotopologue of the PARP inhibitor AZD2461. Cell uptake of [18F]AZD2461 in vitro was assessed in a range of pancreatic cell lines (PSN-1, PANC-1, CFPAC-1 and AsPC-1) to assess PARP expression and in vivo in xenograft-bearing mice. Blocking experiments were performed with both olaparib and AZD2461.

RESULTS

[18F]AZD2461 was efficiently radiolabelled via both manual and automated procedures (9 % ± 3 % and 3 % ± 1 % activity yields non-decay corrected). [18F]AZD2461 was taken up in vivo in PARP1-expressing tumours, and the highest uptake was observed for PSN-1 cells (7.34 ± 1.16 %ID/g). In vitro blocking experiments showed a lesser ability of olaparib to reduce [18F]AZD2461 binding, indicating a difference in selectivity between olaparib and AZD2461.

CONCLUSION

Taken together, we show the importance of screening the PARP selectivity profile of radiolabelled PARP inhibitors for use as PET imaging agents.

Rapidly (and Successfully) Translating Novel Brain Radiotracers From Animal Research Into Clinical Use

The advent of preclinical research scanners for in vivo imaging of small animals has added confidence into the multi-step decision-making process of radiotracer discovery and development. Furthermore, it has expanded the utility of imaging techniques available to dissect clinical questions, fostering a cyclic interaction between the clinical and the preclinical worlds. Significant efforts from medicinal chemistry have also made available several high-affinity and selective compounds amenable for radiolabeling, that target different receptors, transporters and enzymes in vivo. This substantially increased the range of applications of molecular imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT). However, the process of developing novel radiotracers for in vivo imaging of the human brain is a multi-step process that has several inherent pitfalls and technical difficulties, which often hampers the successful translation of novel imaging agents from preclinical research into clinical use. In this paper, the process of radiotracer development and its relevance in brain research is discussed; as well as, its pitfalls, technical challenges and future promises. Examples of successful and unsuccessful translation of brain radiotracers will be presented.

In vivo Imaging Technologies to Monitor the Immune System

The past two decades have brought impressive advancements in immune modulation, particularly with the advent of both cancer immunotherapy and biologic therapeutics for inflammatory conditions. However, the dynamic nature of the immune response often complicates the assessment of therapeutic outcomes. Innovative imaging technologies are designed to bridge this gap and allow non-invasive visualization of immune cell presence and/or function in real time. A variety of anatomical and molecular imaging modalities have been applied for this purpose, with each option providing specific advantages and drawbacks. Anatomical methods including magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound provide sharp tissue resolution, which can be further enhanced with contrast agents, including super paramagnetic ions (for MRI) or nanobubbles (for ultrasound). Conjugation of the contrast material to an antibody allows for specific targeting of a cell population or protein of interest. Protein platforms including antibodies, cytokines, and receptor ligands are also popular choices as molecular imaging agents for positron emission tomography (PET), single-photon emission computerized tomography (SPECT), scintigraphy, and optical imaging. These tracers are tagged with either a radioisotope or fluorescent molecule for detection of the target. During the design process for immune-monitoring imaging tracers, it is important to consider any potential downstream physiologic impact. Antibodies may deplete the target cell population, trigger or inhibit receptor signaling, or neutralize the normal function(s) of soluble proteins. Alternatively, the use of cytokines or other ligands as tracers may stimulate their respective signaling pathways, even in low concentrations. As in vivo immune imaging is still in its infancy, this review aims to describe the modalities and immunologic targets that have thus far been explored, with the goal of promoting and guiding the future development and application of novel imaging technologies.

[PET imaging for better understanding of normal and pathological neurotransmission]

Positron emission tomography imaging is still an expanding field of preclinical and clinical investigations exploring the brain and its normal and pathological functions. In addition to technological improvements in PET scanners, the availability of suitable radiotracers for unexplored pharmacological targets is a key factor in this expansion. Many radiotracers (or radiopharmaceuticals, when administered to humans) have been developed by multidisciplinary teams to visualize and quantify a growing numbers of brain receptors, transporters, enzymes and other targets. The development of new PET radiotracers still represents an exciting challenge, given the large number of neurochemical functions that remain to be explored. In this article, we review the development context of the first preclinical radiotracers and their passage to humans. The main current contributions of PET radiotracers are described in terms of imaging neuronal metabolism, quantification of receptors and transporters, neurodegenerative and neuroinflammatory imaging. The different approaches to functional imaging of neurotransmission are also discussed. Finally, the contributions of PET imaging to the research and development of new brain drugs are described.

Design and Challenges of Radiopharmaceuticals

This review describes general concepts with regard to radiopharmaceuticals for diagnostic or therapeutic applications that help to understand the specific challenges encountered during the design, (radio)synthesis, in vitro and in vivo evaluation and clinical translation of novel radiopharmaceuticals. The design of a radiopharmaceutical requires upfront decisions with regard to combining a suitable vector molecule with an appropriate radionuclide, considering the type and location of the molecular target, the desired application, and the time constraints imposed by the relatively short half-life of radionuclides. Well-designed in vitro and in vivo experiments allow nonclinical validation of radiotracers. Ultimately, in combination with a limited toxicology package, the radiotracer becomes a radiopharmaceutical for clinical evaluation, produced in compliance with regulatory requirements for medicines for intravenous (IV) injection.