Preclinical PET Neuroimaging of [11C]Bexarotene

Automated production of 11C-labeled carboxylic acids and esters via "in-loop" 11C-carbonylation using GE FX synthesis modules

An in-loop 11C-carbonylation process for the radiosynthesis of 11C-carboxylic acids and esters from halide precursors has been developed. The reaction proceeds at room temperature under mild conditions and enables 11C-carbonylation of both electron deficient and electron rich (hetero)aromatic halides to provide 11C-carboxylic acids and esters in good to excellent radiochemical yields, high radiochemical purity, and excellent molar activity. The process has been fully automated using commercial radiochemistry synthesis modules, and application to clinical production is demonstrated via validated cGMP radiosyntheses of [11C]bexarotene and [11C]acetoacetic acid.

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.

Nuclear Receptor Imaging In Vivo-Clinical and Research Advances

Nuclear receptors are transcription factors that function in normal physiology and play important roles in diseases such as cancer, inflammation, and diabetes. Noninvasive imaging of nuclear receptors can be achieved using radiolabeled ligands and positron emission tomography (PET). This quantitative imaging approach can be viewed as an in vivo equivalent of the classic radioligand binding assay. A main clinical application of nuclear receptor imaging in oncology is to identify metastatic sites expressing nuclear receptors that are targets for approved drug therapies and are capable of binding ligands to improve treatment decision-making. Research applications of nuclear receptor imaging include novel synthetic ligand and drug development by quantifying target drug engagement with the receptor for optimal therapeutic drug dosing and for fundamental research into nuclear receptor function in cells and animal models. This mini-review provides an overview of PET imaging of nuclear receptors with a focus on radioligands for estrogen receptor, progesterone receptor, and androgen receptor and their use in breast and prostate cancer.

Copper(I)-Mediated 11C-Carboxylation of (Hetero)arylstannanes

A novel copper-mediated carboxylation strategy of aryl- and heteroaryl-stannanes is described. The method serves as a mild (i.e., 1 atm) carboxylation method using stable carbon dioxide and is transferable as a radiosynthetic approach for carbon-11-labeled aromatic and heteroaromatic carboxylic acids using sub-stoichiometric quantities of [¹¹C]CO₂. The methodology was applied to the radiosynthesis of the retinoid X receptor agonist, [¹¹C]bexarotene, with a decay-corrected radiochemical yield of 32 ± 5% and molar activity of 38 ± 23 GBq/μmol (n = 3).

Fluorine-18 (18F)-labeled retinoid x receptor (RXR) partial agonist whose tissue transferability is affected by other RXR ligands

Bexarotene (1), a retinoid X receptor (RXR) agonist approved for the treatment of cutaneous T cell lymphoma (CTCL), was reported to migrate into baboon brain based on findings obtained by positron emission tomography (PET) with a ¹¹C-labeled tracer. However, co-administration of non-radioactive 1 had no effect on the distribution of [¹¹C]1, probably due to non-specific binding of 1 as a result of its high lipophilicity. Here, we report a fluorine-18 (¹⁸F)-labeled PET tracer [¹⁸F]6 derived from RXR partial agonist CBt-PMN (2), which has lower lipophilicity and weaker RXR-binding ability than [¹¹C]1. The concomitant administration of 1 or 2 with [¹⁸F]6 with resulted in decreased accumulation of [¹⁸F]6 in liver, together with increased brain uptake and increased accumulation in kidney and muscle, as visualized by PET. A plausible explanation of these findings is the inhibition of [¹⁸F]6 uptake into the liver by concomitantly administered 1 or 2, leading to an increase in blood concentration of [¹⁸F]6 followed by increased accumulation in other tissues.

Development of Positron Emission Tomography Radiotracers for the GABA Transporter 1

In vivo positron emission tomography (PET) imaging of the γ-aminobutyric acid (GABA) receptor complex has been accomplished using radiolabeled benzodiazepine derivatives, but development of specific presynaptic radioligands targeting the neuronal membrane GABA transporter type 1 (GAT-1) has been less successful. The availability of new structure-activity studies of GAT-1 inhibitors and the introduction of a GAT-1 inhibitor (tiagabine, Gabatril) into clinical use prompted us to reinvestigate the syntheses of PET ligands for this transporter. Initial synthesis and rodent PET studies of N-[¹¹C]methylnipecotic acid confirmed the low brain uptake of that small and polar molecule. The common design approach to improve blood-brain barrier permeability of GAT-1 inhibitors is the attachment of a large lipophilic substituent. We selected an unsymmetrical bis-aromatic residue attached to the ring nitrogen by a vinyl ether spacer from a series recently reported by Wanner and coworkers. Nucleophilic aromatic substitution of an aryl chloride precursor with [¹⁸F]fluoride was used to prepare the desired candidate radiotracer ( R, E/ Z)-1-(2-((4-fluoro-2-(4-[¹⁸F]fluorobenzoyl)styryl)oxy)ethyl)piperidine-3-carboxylic acid (( R, E/ Z)-[¹⁸F]10). PET studies in rats showed no brain uptake, which was not altered by pretreatment of animals with the P-glycoprotein inhibitor cyclosporine A, indicating efflux by Pgp was not responsible. Subsequent PET imaging studies of ( R, E/ Z)-[¹⁸F]10 in rhesus monkey brain showed very low brain uptake. Finally, to test if the free carboxylic acid group was the likely cause of poor brain uptake, PET studies were done using the ethyl ester derivative of ( R, E/ Z)-[¹⁸F]10. Rapid and significant monkey brain uptake of the ester was observed, followed by a slow washout over 90 min. The blood-brain barrier permeability of the ester supports a hypothesis that the free acid function limits brain uptake of nipecotic acid-based GAT-1 radioligands, and future radiotracer efforts should investigate the use of carboxylic acid bioisosteres.

Boron reagents for divergent radiochemistry

This review discusses boron reagents as precursors for divergent radiolabelling with a focus on carbon-11, fluorine-18 and iodine-123, -125, -131.

Discovery of PET radiopharmaceuticals at the academia-industry interface

Project-specific collaborations between academia and pharmaceutical partners are a growing phenomenon within molecular imaging and in particular in the positron emission tomography (PET) radiopharmaceutical community. This cultural shift can be attributed in part to decreased public funding in academia in conjunction with the increased reliance on outsourcing of chemistry, radiochemistry, pharmacology and molecular imaging studies by the pharmaceutical industry. This account highlights some of our personal experiences working with industrial partners to develop new PET radiochemistry methodologies for drug discovery and neuro-PET research studies. These symbiotic academic-industrial partnerships have not only led to novel radiotracers for new targets but also to the application of new carbon-11 and fluorine-18 labeling methodologies and technologies to label previously unprecedented compounds for in vivo evaluations.