Ultrafast Photoclick Reaction for Selective 18F-Positron Emission Tomography Tracer Synthesis in Flow.

The development of very fast, clean, and selective methods for indirect labeling in PET tracer synthesis is an ongoing challenge. Here we present the development of an ultrafast photoclick method for the synthesis of short-lived 18F-PET tracers based on the photocycloaddition reaction of 9,10-phenanthrenequinones with electron-rich alkenes. The respective precursors are synthetically easily accessible and can be functionalized with various target groups. Using a flow photo-microreactor, the photoclick reaction can be performed in 60 s, and clinically relevant tracers for prostate cancer and bacterial infection imaging were prepared to demonstrate practicality of the method.

Microfluidic Preparation of 89Zr-Radiolabelled Proteins by Flow Photochemistry.

89Zr-radiolabelled proteins functionalised with desferrioxamine B are a cornerstone of diagnostic positron emission tomography. In the clinical setting, 89Zr-labelled proteins are produced manually. Here, we explore the potential of using a microfluidic photochemical flow reactor to prepare 89Zr-radiolabelled proteins. The light-induced functionalisation and 89Zr-radiolabelling of human serum albumin ([89Zr]ZrDFO-PEG3-Et-azepin-HSA) was achieved by flow photochemistry with a decay-corrected radiochemical yield (RCY) of 31.2 ± 1.3% (n = 3) and radiochemical purity >90%. In comparison, a manual batch photoreactor synthesis produced the same radiotracer in a decay-corrected RCY of 59.6 ± 3.6% (n = 3) with an equivalent RCP > 90%. The results indicate that photoradiolabelling in flow is a feasible platform for the automated production of protein-based 89Zr-radiotracers, but further refinement of the apparatus and optimisation of the method are required before the flow process is competitive with manual reactions.

Modular Medical Imaging Agents Based on Azide-Alkyne Huisgen Cycloadditions: Synthesis and Pre-Clinical Evaluation of 18 F-Labeled PSMA-Tracers for Prostate Cancer Imaging.

Since the seminal contribution of Rolf Huisgen to develop the [3+2] cycloaddition of 1,3-dipolar compounds, its azide-alkyne variant has established itself as the key step in numerous organic syntheses and bioorthogonal processes in materials science and chemical biology. In the present study, the copper(I)-catalyzed azide-alkyne cycloaddition was applied for the development of a modular molecular platform for medical imaging of the prostate-specific membrane antigen (PSMA), using positron emission tomography. This process is shown from molecular design, through synthesis automation and in vitro studies, all the way to pre-clinical in vivo evaluation of fluorine-18- labeled PSMA-targeting 'F-PSMA-MIC' radiotracers (t1/2 =109.7 min). Pre-clinical data indicate that the modular PSMA-scaffold has similar binding affinity and imaging properties to the clinically used [68 Ga]PSMA-11. Furthermore, we demonstrated that targeting the arene-binding in PSMA, facilitated through the [3+2]cycloaddition, can improve binding affinity, which was rationalized by molecular modeling. The here presented PSMA-binding scaffold potentially facilitates easy coupling to other medical imaging moieties, enabling future developments of new modular imaging agents.

Fluorine-18 labelled building blocks for PET tracer synthesis.

Positron emission tomography (PET) is an important driver for present day healthcare. Fluorine-18 is the most widely used radioisotope for PET imaging and a thorough overview of the available radiochemistry methodology is a prerequisite for selection of a synthetic approach for new fluorine-18 labelled PET tracers. These PET tracers can be synthesised either by late-stage radiofluorination, introducing fluorine-18 in the last step of the synthesis, or by a building block approach (also called modular build-up approach), introducing fluorine-18 in a fast and efficient manner in a building block, which is reacted further in one or multiple reaction steps to form the PET tracer. This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Design of Radioiodinated Pharmaceuticals: Structural Features Affecting Metabolic Stability towards in Vivo Deiodination.

Radioiodinated pharmaceuticals are convenient tracers for clinical and research investigations because of the relatively long half-lives of radioactive iodine isotopes (i.e., 123I, 124I, and 131I) and the ease of their chemical insertion. Their application in radionuclide imaging and therapy may, however, be hampered by poor in vivo stability of the C-I bond. After an overview of the use of iodine in biology and nuclear medicine, we present here a survey of the catabolic pathways for iodinated xenobiotics, including their biodistribution, accumulation, and biostability. We summarize successful rational improvements in the biostability and conclude with general guidelines for the design of stable radioiodinated pharmaceuticals. It appears to be necessary to consider the whole molecule, rather than the radioiodinated fragment alone. Iodine radionuclides are generally retained in vivo on sp2 carbon atoms in iodoarenes and iodovinyl moieties, but not in iodinated heterocycles or on sp3 carbon atoms. Iodoarene substituents also have an influence, with increased in vivo deiodination in the cases of iodophenols and iodoanilines, whereas methoxylation and difluorination improve biostability.

A universal procedure for the [¹8F]trifluoromethylation of aryl iodides and aryl boronic acids with highly improved specific activity.

Herein, we describe a valuable method for the introduction of the [(18)F]CF3 group into arenes with highly improved specific activity by the reaction of [(18)F]trifluoromethane with aryl iodides or aryl boronic acids. This [(18)F]trifluoromethylation reaction is the first to be described in which the [(18)F]CF3 products are generated in actual trace amounts and can therefore effectively be used as PET tracers. The method shows broad scope with respect to possible aryl iodide and aryl boronic acid substrates, as well as good to excellent conversion. In particular, the [(18)F]trifluoromethylation of boronic acids was found to outperform [(18)F]trifluoromethylation reactions of halogenated aryl precursors with regard to conversion, reaction conditions, and kinetics.

Efficient synthesis of [18F]trifluoromethane and its application in the synthesis of PET tracers.

A new strategy towards [(18)F]trifluoromethyl-containing compounds is developed. [(18)F]trifluoromethane is synthesised in a fast and efficient manner and subsequently used in the reaction with aldehydes and ketones forming [(18)F]trifluoromethyl carbinols in good yields.

Tumour imaging by Positron Emission Tomography using fluorinase generated 5-[¹8F]fluoro-5-deoxyribose as a novel tracer.

INTRODUCTION: 5-[(18)F]Fluoro-5-deoxyribose ([(18)F]FDR) 3 was prepared as a novel monosaccharide radiotracer in a two-step synthesis using the fluorinase, a C-F bond forming enzyme, and a nucleoside hydrolase. The resulting [(18)F]FDR 3 was then explored as a radiotracer for imaging tumours (A431 human epithelial carcinoma) by positron emission tomography in a mice model. METHODS: 5-[(18)F]Fluoro-5-deoxyribose ([(18)F]FDR) 3, was prepared by incubating S-adenosyl-L-methionine (SAM) and [(18)F]fluoride with the fluorinase enzyme, and then incubating the product of this reaction, [(18)F]-5'-fluoro-5'-deoxadenosine ([(18)F]FDA) 2, with an adenosine hydrolase to generate [(18)F]FDR 3. The enzymes were freeze-dried and were used on demand by dissolution in buffer solution. The resulting [(18)F]FDR 3 was then administered to four mice that had tumours induced from the A431 human epithelial carcinoma cell line. RESULTS: The tumour (A431 human epithelial carcinoma) bearing mice were successfully imaged with [(18)F]FDR 3. The radiotracer displayed good tumour imaging resolution. A direct comparison of the uptake and efflux of [(18)F]FDR 3 with 2-[(18)F]fluoro-2-deoxyglucose ([(18)F]FDG) was made, revealing comparative tumour uptake and imaging potential over the first 10-20min. The study revealed however that [(18)F]FDR 3 does not accumulate in the tumour as efficiently as [(18)F]FDG over longer time periods. CONCLUSIONS: [(18)F]FDR 3 can be rapidly synthesised in a two enzyme protocol and used to image tumours in small animal models.

Synthesis, biodistribution and PET studies in rats of (18)F-Labeled bridgehead fluoromethyl analogues of WAY-100635.

INTRODUCTION: In vitro screening of fluoromethyl bridge-fused ring (BFR) analogues of WAY-100635 (5a, 5b and 5c) has shown a high binding affinity and a good selectivity for the 5-HT(1A) receptor. As these compounds were designed to provide PET ligands with high metabolic stability, they are now radiolabeled with fluorine-18 and investigated in vivo. METHODS: BFR precursors were synthesized and reacted with fluorine-18 in dry MeCN in the presence of 2,2,2-kryptofix and K(2)CO(3). In rats, biodistribution and PET studies were performed using [(18)F]5a, [(18)F]5b and [(18)F]5c. The binding specificity was determined by administration of non-labeled WAY-100635 prior to the radiolabeled ligands. RESULTS: [(18)F]5 ligands were synthesized in overall radiochemical yields of 24%-45%, respectively with a radiochemical purity of >98%. Relatively good hippocampus to cerebellum ratios of 5.55, 4.79 and 5.45, respectively were reached at 45 min pi. However, PET studies indicated defluorination of the radioligands by showing high accumulation of radioactivity in the bones in the order of [(18)F]5a≈[(18)F]5b>[(18)F]5c. CONCLUSION: Also in vivo, the radioligands bind preferentially to the 5-HT(1A) receptor. Unfortunately, no metabolic stability with regard to defluorination was observed in rats.

Design, synthesis and in vitro evaluation of bridgehead fluoromethyl analogs of N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-N-(pyridin-2-yl)cyclohexanecarboxamide (WAY-100635) for the 5-HT(1A) receptor.

Fluorinated analogs that are related to the 5-hydroxytryptamine (5-HT(1A)) antagonist, N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-N-(pyridin-2-yl)cyclohexanecarboxamide (WAY-100635), have been synthesized and their binding affinity for the 5-HT(1A) receptor and other neurotransmitter receptors (adrenoceptors, sigma receptors, and dopamine receptors), and serotonin transporters was examined in vitro. These ligands were designed to provide a possible potential positron emission tomography (PET) ligand with high metabolic stability. To this end, the cyclohexyl moiety in WAY-100635 and in O-desmethyl WAY-100635 was replaced by a bridge-fused ring (BFR) such as adamantyl, cubyl, bicyclo[2.2.2]octyl and bicyclo[2.2.1]heptyl to reduce the metabolic rate of the amide bond hydrolysis, while a fluoromethyl group was introduced on the other bridgehead of the BFR to prevent defluorination by HF elimination. All synthesized analogs displayed high affinity in the (sub)nanomolar range for the 5-HT(1A) receptor, comparable to WAY-100635. In addition, 6b, 6c and 6d were reasonably selective to the 5-HT(1A) receptor over the above mentioned receptors. In human hepatocytes, 6b showed a suitable metabolic stability. In conclusion, the obtained data provides a promising starting point for the synthesis of the corresponding (18)F-labeled PET analogs.

Design, synthesis, radiolabeling, and in vitro and in vivo evaluation of bridgehead iodinated analogues of N-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-N-(pyridin-2-yl)cyclohexanecarboxamide (WAY-100635) as potential SPECT ligands for the 5-HT1A receptor.

Here we describe the design, synthesis, and pharmacological profile of 5-HT(1A) receptor ligands related to 1 (WAY-100635). The cyclohexyl moiety in 1 and its O-desmethylated analogue 3 were replaced by the bridgehead iodinated bridge-fused rings: adamantyl, cubyl, bicyclo[2.2.2]octyl, or bicyclo[2.2.1]heptyl. All analogues displayed a (sub)nanomolar affinity for the 5-HT(1A) receptor in vitro. Compounds 6b and 7b appeared to be selective for this receptor over other relevant receptors and could easily be iodinated with radioactive iodine-123. In humane hepatocytes, [(123)I]6b showed a low propensity for amide hydrolysis and a stable carbon-iodine bond. The biodistribution of [(123)I]6b and [(123)I]7b in rats revealed that the carbon-iodine bond was also stable in vivo. Unfortunately, the brain uptake and the specificity for both radioligands were significantly lower than those of the parent molecule 1. In conclusion, the designed tracers are not suitable for SPECT imaging.