From Carbon-11-Labeled Amino Acids to Peptides in Positron Emission Tomography: the Synthesis and Clinical Application

11C-Methionine uptake in meningiomas after stereotactic radiotherapy

OBJECTIVE

11C-Methionine positron emission tomography (MET-PET) is used for stereotactic radiotherapy planning in meningioma patients. The role of MET-PET during subsequent follow-up (FU) is unclear. We analyzed the uptake of 11C-Methionine before and after stereotactic radiotherapy (SRT) in patients with a complex meningioma and investigated if there was a difference between patients with progressive disease (PD) and stable disease (SD) during FU.

METHODS

This retrospective study investigates 62 MET-PETs in 29 complex meningioma patients. Standardized uptake value (SUV)max and SUVpeak tumor-to-normal ratios (T/N-ratios) were calculated, comparing the tumor region with both the mirroring intracranial area and the right frontal gray matter. The difference in 11C-Methionine uptake pre- and post-SRT was analyzed, as well as the change in uptake between PD or SD.

RESULTS

Median (IQR) FU duration was 67 months (50.5-91.0). The uptake of 11C-Methionine in meningiomas remained increased after SRT. Neither a statistically significant difference between MET-PETs before and after SRT was encountered, nor a significant difference in one of the four T/N-ratios between patients with SD versus PD with median (IQR) SUVmax T/NR front 2.65 (2.13-3.68) vs 2.97 (1.55-3.54) [p = 0.66]; SUVmax T/Nmirror 2.92 (2.19-3.71) vs 2.95 (1.74-3.60) [p = 0.61]; SUVpeak T/NR front 2.35 (1.64-3.40) vs 2.25 (1.44-3.74) [p = 0.80]; SUVpeak T/Nmirror 2.38 (1.91-3.36) vs 2.35 (1.56-3.72) [p = 0.95].

CONCLUSIONS

Our data do not support use of MET-PET during FU of complex intracranial meningiomas after SRT. MET-PET could not differentiate between progressive or stable disease.

A practical guide for the preparation of C1-labeled α-amino acids using aldehyde catalysis with isotopically labeled CO2

Isotopically carbon-labeled α-amino acids are valuable synthetic targets that are increasingly needed in pharmacology and medical imaging. Existing preparations rely on early stage introduction of the isotopic label, which leads to prohibitive synthetic costs and time-intensive preparations. Here we describe a protocol for the preparation of C1-labeled α-amino acids using simple aldehyde catalysts in conjunction with [*C]CO2 (* = 14, 13, 11). This late-stage labeling strategy is enabled by the one-pot carboxylate exchange of unprotected α-amino acids with [*C]CO2. The protocol consists of three separate procedures, describing the syntheses of (±)-[1-13C]phenylalanine, (±)-[1-11C]phenylalanine and (±)-[1-14C]phenylalanine from unlabeled phenylalanine. Although the delivery of [*C]CO2 is operationally distinct for each experiment, each procedure relies on the same fundamental chemistry and can be executed by heating the reaction components at 50-90 °C under basic conditions in dimethylsulfoxide. Performed on scales of up to 0.5 mmol, this methodology is amenable to C1-labeling of many proteinogenic α-amino acids and nonnatural derivatives, which is a breakthrough from existing methods. The synthesis of (±)-[1-13C]phenylalanine requires ~2 d, with product typically obtained in a 60-80% isolated yield (n = 3, μ = 71, σ = 8.3) with an isotopic incorporation of 70-88% (n = 18, μ = 72, σ = 9.0). Starting from the preformed imino acid (~3 h preparation time), rapid synthesis of (±)-[1-11C]phenylalanine can be completed in ~1 h with an isolated radiochemical yield of 13%. Finally, (±)-[1-14C]phenylalanine can be accessed in ~2 d with a 51% isolated yield and 11% radiochemical yield.

Insights into Peptidoglycan-Targeting Radiotracers for Imaging Bacterial Infections: Updates, Challenges, and Future Perspectives

The unique structural architecture of the peptidoglycan allows for the stratification of bacteria as either Gram-negative or Gram-positive, which makes bacterial cells distinguishable from mammalian cells. This classification has received attention as a potential target for diagnostic and therapeutic purposes. Bacteria's ability to metabolically integrate peptidoglycan precursors during cell wall biosynthesis and recycling offers an opportunity to target and image pathogens in their biological state. This Review explores the peptidoglycan biosynthesis for bacteria-specific targeting for infection imaging. Current and potential radiolabeled peptidoglycan precursors for bacterial infection imaging, their development status, and their performance in vitro and/or in vivo are highlighted. We conclude by providing our thoughts on how to shape this area of research for future clinical translation.

Enantioselective Synthesis of Carbon-11-Labeled Amino Acids and Peptides

Radiolabeled amino acids (AAs), their derivatives, and peptides are essential radiotracers in nuclear imaging. Despite its potential, the preparation of enantiopure radiopharmaceuticals poses several challenges, demanding a great need for rapid and stereocontrolled reactions. This chapter describes a highly stereoselective carbon-11 alkylation of Schiff bases, to obtain radiolabeled AAs and small peptides. The method uses chiral quaternary ammonium salt phase-transfer catalyst with two alkylating agents, namely, [11C]methyl iodide and [11C]benzyl iodide. This methodology allows the radiolabeling of AAs and peptides with excellent regioselectivity and enantiomeric or diastereomeric excess.

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.

30 years of [11C]methyl triflate

Some scientific discoveries are well known only to a core group of researchers working on technical subjects. Nevertheless, they open new research directions, allow existing knowledge to be viewed in entirely new and useful ways, or provide a way to make something that was hard or impossible to make before. Carbon-11 methyl triflate ([¹¹C]MeOTf) is one such advance, facilitating the synthesis of many carbon-11 radio tracers and broadening the range of applications of carbon-11 radiochemistry. The year 2022 marked the 30th anniversary of the original paper in Applied Radiation and Isotopes introducing a simple synthesis of [¹¹C]MeOTf from carbon-11 methyl iodide ([¹¹C]MeI) and it also marked the end of the fruitful career and life of the researcher who developed it, Douglas Jewett. It seems fitting to say a few words on how it came to be and how it has helped advance carbon-11 radiochemistry.

Aldehyde-catalysed carboxylate exchange in α-amino acids with isotopically labelled CO2

The isotopic labelling of small molecules is integral to drug development and for understanding biochemical processes. The preparation of carbon-labelled α-amino acids remains difficult and time consuming, with established methods involving label incorporation at an early stage of synthesis. This explains the high cost and scarcity of C-labelled products and presents a major challenge in 11C applications (11C t1/2 = 20 min). Here we report that aldehydes catalyse the isotopic carboxylate exchange of native α-amino acids with *CO2 (* = 14, 13, 11). Proteinogenic α-amino acids and many non-natural variants containing diverse functional groups undergo labelling. The reaction probably proceeds via the trapping of *CO2 by imine-carboxylate intermediates to generate iminomalonates that are prone to monodecarboxylation. Tempering catalyst electrophilicity was key to preventing irreversible aldehyde consumption. The pre-generation of the imine carboxylate intermediate allows for the rapid and late-stage 11C-radiolabelling of α-amino acids in the presence of [11C]CO2.

Copper-Mediated Radiocyanation of Unprotected Amino Acids and Peptides

This report describes a copper-mediated radiocyanation of aryl halides that is applicable to complex molecules. This transformation tolerates an exceptionally wide range of functional groups, including unprotected amino acids. As such, it enables the site-specific introduction of [¹¹C]CN into peptides at an iodophenylalanine residue. The use of a diamine-ligated copper(I) mediator is crucial for achieving high radiochemical yield under relatively mild conditions, thus limiting racemization and competing side reactions of other amino acid side chains. The reaction has been scaled and automated to deliver radiolabeled peptides, including analogues of adrenocorticotropic hormone 1-27 (ACTH) and nociceptin (NOP). For instance, this Cu-mediated radiocyanation was leveraged to prepare >40 mCi of [¹¹C]cyano-NOP to evaluate biodistribution in a primate using positron emission tomography. This investigation provides preliminary evidence that nociceptin crosses the blood-brain barrier and shows uptake across all brain regions (SUV > 1 at 60 min post injection), consistent with the known distribution of NOP receptors in the rhesus brain.

Synthesis and [*C]CO-labelling of (C,N) gem-dimethylbenzylamine-palladium complexes for potential applications in positron emission tomography

Various aryl-palladium complexes were synthesised from gem-dimethylbenzylamine derivatives by C-H activation under extremely mild conditions. Interestingly, these highly stable structures reacted with [13C]carbon monoxide to produce the desired labelled lactams in 29% to 51% yields over the C-H activation/carbonylation steps. As representative examples, a non-natural amino acid and an estradiol-based conjugate were prepared and labelled in model experiments with [13C]CO in homogeneous or heterogeneous conditions. Especially, the latter was radiolabelled with [11C]CO using a convenient procedure from the resin-supported palladium complex precursor. Thus, these results strongly suggest that cyclometallated palladium complexes obtained from gem-dimethylbenzylamine moieties are promising precursors for the practical synthesis of new [11C]tracers for Positron Emission Tomography.

Production of a broad palette of positron emitting radioisotopes using a low-energy cyclotron: Towards a new success story in cancer imaging?

Over the last several years, positron emission tomography (PET) has matured as an indispensable component of cancer diagnostics. Owing to the large variability observed among the cancer patients and the need to personalize individual patient's diagnosis and treatment, the need for new positron emitting radioisotopes has continued to grow. This mini review opens with a brief introduction to the criteria for radioisotope selection for PET imaging. Subsequently, positron emitting radioisotopes are categorized as: established, emerging and futuristic, based on the stages of their advancement. The production methodologies and the radiochemical separation procedures for obtaining the important radioisotopes in a form suitable for preparation of radiopharmaceuticals for PET imaging are briefly discussed.

Synthesis of L-[5-11 C]Leucine and L-α-[5-11 C]Methylleucine via Pd0 -mediated 11 C-Methylation and Microfluidic Hydrogenation: Potentiality of Leucine PET Probes for Tumor Imaging

The efficient synthesis of L-[5-¹¹ C]leucine and L-α-[5-¹¹ C]methylleucine has been investigated using a continuous two-step sequence of rapid reactions consisting of Pd⁰ -mediated ¹¹ C-methylation and microfluidic hydrogenation. The synthesis of L-[5-¹¹ C]leucine and L-α-[5-¹¹ C]methylleucine was accomplished within 40 min with a decay-corrected radiochemical yield of 15-38 % based on [¹¹ C]CH₃ I, radiochemical purity of 95-99 %, and chemical purity of 95-99 %. The Pd impurities in the injectable solution measured using inductively coupled plasma mass spectrometry met the international criteria for human use. Positron emission tomography scanning after an intravenous injection of L-[5-¹¹ C]leucine or L-α-[5-¹¹ C]methyl leucine in A431 tumor-bearing mice was performed. As a result, L-α-[5-¹¹ C]methylleucine was found to be a potentially useful probe for visualizing the tumor. Tissue distribution analysis showed that the accumulation value of L-α-[5-¹¹ C]methylleucine in tumor tissue was high [12±3% injected dose/g tissue (%ID/g)].

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Preclinical dosimetry models and the prediction of clinical doses of novel positron emission tomography radiotracers

Dosimetry models using preclinical positron emission tomography (PET) data are commonly employed to predict the clinical radiological safety of novel radiotracers. However, unbiased clinical safety profiling remains difficult during the translational exercise from preclinical research to first-in-human studies for novel PET radiotracers. In this study, we assessed PET dosimetry data of six ¹⁸F-labelled radiotracers using preclinical dosimetry models, different reconstruction methods and quantified the biases of these predictions relative to measured clinical doses to ease translation of new PET radiotracers to first-in-human studies. Whole-body PET images were taken from rats over 240 min after intravenous radiotracer bolus injection. Four existing and two novel PET radiotracers were investigated: [¹⁸F]FDG, [¹⁸F]AlF-NOTA-RGDfK, [¹⁸F]AlF-NOTA-octreotide ([¹⁸F]AlF-NOTA-OC), [¹⁸F]AlF-NOTA-NOC, [¹⁸F]ENC2015 and [¹⁸F]ENC2018. Filtered-back projection (FBP) and iterative methods were used for reconstruction of PET data. Predicted and true clinical absorbed doses for [¹⁸F]FDG and [¹⁸F]AlF-NOTA-OC were then used to quantify bias of preclinical model predictions versus clinical measurements. Our results show that most dosimetry models were biased in their predicted clinical dosimetry compared to empirical values. Therefore, normalization of rat:human organ sizes and correction for reconstruction method biases are required to achieve higher precision of dosimetry estimates.

11C- and 18F-labelled tryptophans as PET-tracers for imaging of altered tryptophan metabolism in age-associated disorders

The ageing of the world’s population is the result of increased life expectancy observed in almost all countries throughout the world. Consequently, a rising tide of ageing-associated disorders, like cancer and neurodegenerative diseases, represents one of the main global challenges of the 21st century. The ability of mankind to overcome these challenges is directly dependent on the capability to develop novel methods for therapy and diagnosis of age-associated diseases. One hallmark of age-related pathologies is an altered tryptophan metabolism. Numerous pathological processes including neurodegenerative and neurological diseases like epilepsy, Parkinson’s and Alzheimer’s diseases, cancer and diabetes exhibit marked changes in tryptophan metabolism. Visualization of key processes of tryptophan metabolic pathways, especially using positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI, can be exploited to early detect the aforementioned disorders with considerable accuracy, allowing appropriate and timely treatment of patients. Here we review the published 11C- and 18F-labelled tryptophans with respect to the production and also preclinical and clinical evaluation as PET-tracers for visualization of different branches of tryptophan metabolism.

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Sensing Living Bacteria in Vivo Using d-Alanine-Derived 11C Radiotracers

Incorporation of d-amino acids into peptidoglycan is a unique metabolic feature of bacteria. Since d-amino acids are not metabolic substrates in most mammalian tissues, this difference can be exploited to detect living bacteria in vivo. Given the prevalence of d-alanine in peptidoglycan muropeptides, as well as its role in several antibiotic mechanisms, we targeted this amino acid for positron emission tomography (PET) radiotracer development. d-[3-11C]Alanine and the dipeptide d-[3-11C]alanyl-d-alanine were synthesized via asymmetric alkylation of glycine-derived Schiff-base precursors with [11C]methyl iodide in the presence of a cinchonidinium phase-transfer catalyst. In cell experiments, both tracers showed accumulation by a wide variety of both Gram-positive and Gram-negative pathogens including Staphylococcus aureus and Pseudomonas aeruginosa. In a mouse model of acute bacterial myositis, d-[3-11C]alanine was accumulated by living microorganisms but was not taken up in areas of sterile inflammation. When compared to existing clinical nuclear imaging tools, specifically 2-deoxy-2-[18F]fluoro-d-glucose and a gallium citrate radiotracer, d-alanine showed more bacteria-specific uptake. Decreased d-[3-11C]alanine uptake was also observed in antibiotic-sensitive microbes after antimicrobial therapy, when compared to that in resistant organisms. Finally, prominent uptake of d-[3-11C]alanine uptake was seen in rodent models of discitis-osteomyelitis and P. aeruginosa pneumonia. These data provide strong justification for clinical translation of d-[3-11C]alanine to address a number of important human infections.

Synthesis and preliminary radiopharmacological characterisation of an 11 C-labelled azadipeptide nitrile as potential PET tracer for imaging of cysteine cathepsins

An O-methyltyrosine-containing azadipeptide nitrile was synthesised and investigated for its inhibitory activity towards cathepsins L, S, K, and B. Labelling with carbon-11 was accomplished by reaction of the corresponding phenolic precursor with [¹¹ C]methyl iodide starting from cyclotron-produced [¹¹ C]methane. Radiopharmacological evaluation of the resulting radiotracer in a mouse xenograft model derived from a mammary tumour cell line by small animal PET imaging indicates tumour targeting with complex pharmacokinetics. Radiotracer uptake in the tumour region was considerably lower under treatment with the nonradioactive reference compound and the epoxide-based irreversible cysteine cathepsin inhibitor E64. The in vivo behaviour observed for this radiotracer largely confirms that of the corresponding ¹⁸ F-fluoroethylated analogue and suggests the limited suitability of azadipeptide nitriles for the imaging of tumour-associated cysteine cathepsins despite target-mediated uptake is evidenced.

Synthesis of enantiopure 18F-trifluoromethyl cysteine as a structure-mimetic amino acid tracer for glioma imaging

Although 11C-labelled sulfur-containing amino acids (SAAs) including L-methyl-[11C]methionine and S-[11C]-methyl-L-cysteine, are attractive tracers for glioma positron emission tomography (PET) imaging, their applications are limited by the short half-life of the radionuclide 11C (t1/2 = 20.4 min). However, development of 18F-labelled SAAs (18F, t1/2 = 109.8 min) without significant structural changes or relying on prosthetic groups remains to be a great challenge due to the absence of adequate space for chemical modification. Methods: We herein present 18F-trifluoromethylated D- and L-cysteines which were designed by replacing the methyl group with 18F-trifluoromethyl group using a structure-based bioisosterism strategy. These two enantiomers were synthesized stereoselectively from serine-derived cyclic sulfamidates via a nucleophilic 18F-trifluoromethylthiolation reaction followed by a deprotection reaction. Furthermore, we conducted preliminary in vitro and in vivo studies to investigate the feasibility of using 18F-trifluoromethylated cysteines as PET tracers for glioma imaging. Results: The two-step radiosynthesis provided the desired products in excellent enantiopurity (ee > 99%) with 14% ± 3% of radiochemical yield. In vitro cell study demonstrated that both enantiomers were taken up efficiently by C6 tumor cells and were mainly transported by systems L and ASC. Among them, the D-enantiomer exhibited relatively good stability and high tumor-specific accumulation in the animal studies. Conclusion: Our findings indicate that 18F-trifluoromethylated D-cysteine, a new SAA tracer, may be a potential candidate for glioma imaging. Taken together, our study represents a first step toward developing 18F-trifluoromethylated cysteines as structure-mimetic tracers for PET tumor imaging.