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Past, Present, and Future of Radiochemical Synthesis

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Medicinal Chemistry".

Deadline for manuscript submissions: closed (15 July 2020) | Viewed by 51817

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Guest Editor
Nuklearmedicinsk Afdeling—Hobrovej 18-22, Postboks 365, 9100 Aalborg, Denmark
Interests: radiochemistry; radiopharmacy; GMP-production; drug degradation; receptor kinetics; analytical methods; chelation; medicinal chemistry
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Special Issue Information

Dear Colleagues,

In line with the two previous Special Issues, which I have guest edited for Molecules, I would like this issue to be about radiochemistry and related topics. I came up with a title “Past, Present, and Future of Radiochemical Synthesis”, which I hope will inspire authors.

We are, for example, looking for reviews that focus on the development of theranostics (therapy + diagnosis) for a specific target or a specific disease, or reviews looking into how radiopharmaceuticals in use today have reached this status.

We are also interested in original research papers within the areal of radiochemistry. These could address how a certain radioactive compound has been prepared and why, how a radioactive compound has taken our understanding of a biological system to a different level, how a radiopharmaceutical can be used for treatment, and/or how a radioactive compound has improved our understanding of a certain drug (binding pattern and/or metabolization of the drug).   

The purpose of this Special Issue is to host research and review papers on the past, present, and future of radiochemical synthesis.

Dr. Svend Borup Jensen
Guest Editor

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Keywords

  • Radiochemisty
  • Radiopharmacy
  • PET
  • SPECT
  • Radioaktiv labeling
  • ADME (absorption, distribution, metabolism, and excretion), but only for radioactive compounds

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Published Papers (11 papers)

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Research

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15 pages, 2963 KiB  
Article
Attempts to Target Staphylococcus aureus Induced Osteomyelitis Bone Lesions in a Juvenile Pig Model by Using Radiotracers
by Pia Afzelius, Aage Kristian Olsen Alstrup, Ole Lerberg Nielsen, Karin Michaelsen Nielsen and Svend Borup Jensen
Molecules 2020, 25(18), 4329; https://doi.org/10.3390/molecules25184329 - 21 Sep 2020
Cited by 4 | Viewed by 2908
Abstract
Background [18F]FDG Positron Emission Tomography cannot differentiate between sterile inflammation and infection. Therefore, we, aimed to develop more specific radiotracers fitted for differentiation between sterile and septic infection to improve the diagnostic accuracy. Consequently, the clinicians can refine the treatment of, [...] Read more.
Background [18F]FDG Positron Emission Tomography cannot differentiate between sterile inflammation and infection. Therefore, we, aimed to develop more specific radiotracers fitted for differentiation between sterile and septic infection to improve the diagnostic accuracy. Consequently, the clinicians can refine the treatment of, for example, prosthesis-related infection. Methods: We examined different target points; Staphylococcus aureus biofilm (68Ga-labeled DOTA-K-A9 and DOTA-GSGK-A11), bone remodeling ([18F]NaF), bacterial cell membranes ([68Ga]Ga-Ubiquicidin), and leukocyte trafficking ([68Ga]Ga-DOTA-Siglec-9). We compared them to the well-known glucose metabolism marker [18F]FDG, in a well-established juvenile S. aureus induced osteomyelitis (OM) pig model. Results: [18F]FDG accumulated in the OM lesions seven days after bacterial inoculation, but disappointingly we were not able to identify any tracer accumulation in OM with any of the supposedly more specific tracers. Conclusion: These negative results are, however, relevant to report as they may save other research groups from conducting the same animal experiments and provide a platform for developing and evaluating other new potential tracers or protocol instead. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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16 pages, 1132 KiB  
Article
Evaluation of a New 177Lu-Labeled Somatostatin Analog for the Treatment of Tumors Expressing Somatostatin Receptor Subtypes 2 and 5
by Rosalba Mansi, Guillaume Pierre Nicolas, Luigi Del Pozzo, Karim Alexandre Abid, Eric Grouzmann and Melpomeni Fani
Molecules 2020, 25(18), 4155; https://doi.org/10.3390/molecules25184155 - 11 Sep 2020
Cited by 2 | Viewed by 3012
Abstract
Targeted radionuclide therapy of somatostatin receptor (SST)-expressing tumors is only partially addressed by the established somatostatin analogs having an affinity for the SST subtype 2 (SST2). Aiming to target a broader spectrum of tumors, we evaluated the bis-iodo-substituted somatostatin analog ST8950 ((4-amino-3-iodo)-d [...] Read more.
Targeted radionuclide therapy of somatostatin receptor (SST)-expressing tumors is only partially addressed by the established somatostatin analogs having an affinity for the SST subtype 2 (SST2). Aiming to target a broader spectrum of tumors, we evaluated the bis-iodo-substituted somatostatin analog ST8950 ((4-amino-3-iodo)-d-Phe-c[Cys-(3-iodo)-Tyr-d-Trp-Lys-Val-Cys]-Thr-NH2), having subnanomolar affinity for SST2 and SST5, labeled with [177Lu]Lu3+ via the chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Human Embryonic Kidney (HEK) cells stably transfected with the human SST2 (HEK-SST2) and SST5 (HEK-SST5) were used for in vitro and in vivo evaluation on a dual SST2- and SST5-expressing xenografted mouse model. natLu-DOTA-ST8950 showed nanomolar affinity for both subtypes (IC50 (95% confidence interval): 0.37 (0.22–0.65) nM for SST2 and 3.4 (2.3–5.2) for SST5). The biodistribution of [177Lu]Lu-DOTA-ST8950 was influenced by the injected mass, with 100 pmol demonstrating lower background activity than 10 pmol. [177Lu]Lu-DOTA-ST8950 reached its maximal uptake on SST2- and SST5-tumors at 1 h p.i. (14.17 ± 1.78 and 1.78 ± 0.35%IA/g, respectively), remaining unchanged 4 h p.i., with a mean residence time of 8.6 and 0.79 h, respectively. Overall, [177Lu]Lu-DOTA-ST8950 targets SST2-, SST5-expressing tumors in vivo to a lower extent, and has an effective dose similar to clinically used radiolabeled somatostatin analogs. Its main drawbacks are the low uptake in SST5-tumors and the persistent kidney uptake. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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13 pages, 1352 KiB  
Article
[99mTc]Tc-DB1 Mimics with Different-Length PEG Spacers: Preclinical Comparison in GRPR-Positive Models
by Panagiotis Kanellopoulos, Emmanouil Lymperis, Aikaterini Kaloudi, Marion de Jong, Eric P. Krenning, Berthold A. Nock and Theodosia Maina
Molecules 2020, 25(15), 3418; https://doi.org/10.3390/molecules25153418 - 28 Jul 2020
Cited by 10 | Viewed by 2413
Abstract
Background: The frequent overexpression of gastrin-releasing peptide receptors (GRPRs) in human cancers provides the rationale for delivering clinically useful radionuclides to tumor sites using peptide carriers. Radiolabeled GRPR antagonists, besides being safer for human use, have often shown higher tumor uptake and [...] Read more.
Background: The frequent overexpression of gastrin-releasing peptide receptors (GRPRs) in human cancers provides the rationale for delivering clinically useful radionuclides to tumor sites using peptide carriers. Radiolabeled GRPR antagonists, besides being safer for human use, have often shown higher tumor uptake and faster background clearance than agonists. We herein compared the biological profiles of the GRPR-antagonist-based radiotracers [99mTc]Tc-[N4-PEGx-DPhe6,Leu-NHEt13]BBN(6-13) (N4: 6-(carboxy)-1,4,8,11-tetraazaundecane; PEG: polyethyleneglycol): (i) [99mTc]Tc-DB7 (x = 2), (ii) [99mTc]Tc-DB13 (x = 3), and (iii) [99mTc]Tc-DB14 (x = 4), in GRPR-positive cells and animal models. The impact of in situ neprilysin (NEP)-inhibition on in vivo stability and tumor uptake was also assessed by treatment of mice with phosphoramidon (PA). Methods: The GRPR affinity of DB7/DB13/DB14 was determined in PC-3 cell membranes, and cell binding of the respective [99mTc]Tc-radioligands was assessed in PC-3 cells. Each of [99mTc]Tc-DB7, [99mTc]Tc-DB13, and [99mTc]Tc-DB14 was injected into mice without or with PA coinjection and 5 min blood samples were analyzed by HPLC. Biodistribution was conducted at 4 h postinjection (pi) in severe combined immunodeficiency disease (SCID) mice bearing PC-3 xenografts without or with PA coinjection. Results: DB7, -13, and -14 displayed single-digit nanomolar affinities for GRPR. The uptake rates of [99mTc]Tc-DB7, [99mTc]Tc-DB13, and [99mTc]Tc-DB14 in PC-3 cells was comparable and consistent with a radioantagonist profile. The radiotracers were found to be ≈70% intact in mouse blood and >94% intact after coinjection of PA. Treatment of mice with PA enhanced tumor uptake. Conclusions: The present study showed that increase of PEG-spacer length in the [99mTc]Tc-DB7–[99mTc]Tc-DB13–[99mTc]Tc-DB14 series had little effect on GRPR affinity, specific uptake in PC-3 cells, in vivo stability, or tumor uptake. A significant change in in vivo stability and tumor uptake was observed only after treatment of mice with PA, without compromising the favorably low background radioactivity levels. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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19 pages, 2674 KiB  
Article
Design, Synthesis, Computational, and Preclinical Evaluation of natTi/45Ti-Labeled Urea-Based Glutamate PSMA Ligand
by Kristina Søborg Pedersen, Christina Baun, Karin Michaelsen Nielsen, Helge Thisgaard, Andreas Ingemann Jensen and Fedor Zhuravlev
Molecules 2020, 25(5), 1104; https://doi.org/10.3390/molecules25051104 - 2 Mar 2020
Cited by 27 | Viewed by 4926
Abstract
Despite promising anti-cancer properties in vitro, all titanium-based pharmaceuticals have failed in vivo. Likewise, no target-specific positron emission tomography (PET) tracer based on the radionuclide 45Ti has been developed, notwithstanding its excellent PET imaging properties. In this contribution, we present liquid–liquid extraction [...] Read more.
Despite promising anti-cancer properties in vitro, all titanium-based pharmaceuticals have failed in vivo. Likewise, no target-specific positron emission tomography (PET) tracer based on the radionuclide 45Ti has been developed, notwithstanding its excellent PET imaging properties. In this contribution, we present liquid–liquid extraction (LLE) in flow-based recovery and the purification of 45Ti, computer-aided design, and the synthesis of a salan-natTi/45Ti-chelidamic acid (CA)-prostate-specific membrane antigen (PSMA) ligand containing the Glu-urea-Lys pharmacophore. The compound showed compromised serum stability, however, no visible PET signal from the PC3+ tumor was seen, while the ex vivo biodistribution measured the tumor accumulation at 1.1% ID/g. The in vivo instability was rationalized in terms of competitive citrate binding followed by Fe(III) transchelation. The strategy to improve the in vivo stability by implementing a unimolecular ligand design is presented. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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9 pages, 1963 KiB  
Communication
Revisiting the Radiosynthesis of [18F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer’s Disease
by Cassis Varlow, Emily Murrell, Jason P. Holland, Alina Kassenbrock, Whitney Shannon, Steven H. Liang, Neil Vasdev and Nickeisha A. Stephenson
Molecules 2020, 25(4), 982; https://doi.org/10.3390/molecules25040982 - 22 Feb 2020
Cited by 11 | Viewed by 3697
Abstract
[18F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [18F]FPEB has been an ongoing challenge. Herein, five metal-free [...] Read more.
[18F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [18F]FPEB has been an ongoing challenge. Herein, five metal-free precursors for the radiofluorination of [18F]FPEB were compared, namely, a chloro-, nitro-, sulfonium salt, and two spirocyclic iodonium ylide (SCIDY) precursors bearing a cyclopentyl (SPI5) and a new adamantyl (SPIAd) auxiliary. The chloro- and nitro-precursors resulted in a low radiochemical yield (<10% RCY), whereas both SCIDY precursors and the sulfonium salt precursor produced [18F]FPEB in the highest RCYs of 25% and 36%, respectively. Preliminary PET/CT imaging studies with [18F]FPEB were conducted in a transgenic model of Alzheimer’s Disease (AD) using B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J (APP/PS1) mice, and data were compared with age-matched wild-type (WT) B6C3F1/J control mice. In APP/PS1 mice, whole brain distribution at 5 min post-injection showed a slightly higher uptake (SUV = 4.8 ± 0.4) than in age-matched controls (SUV = 4.0 ± 0.2). Further studies to explore mGluR5 as an early biomarker for AD are underway. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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18 pages, 2632 KiB  
Article
Radiosynthesis of [18F]-Labelled Pro-Nucleotides (ProTides)
by Alessandra Cavaliere, Katrin C. Probst, Stephen J. Paisey, Christopher Marshall, Abdul K. H. Dheere, Franklin Aigbirhio, Christopher McGuigan and Andrew D. Westwell
Molecules 2020, 25(3), 704; https://doi.org/10.3390/molecules25030704 - 6 Feb 2020
Cited by 6 | Viewed by 4716
Abstract
Phosphoramidate pro-nucleotides (ProTides) have revolutionized the field of anti-viral and anti-cancer nucleoside therapy, overcoming the major limitations of nucleoside therapies and achieving clinical and commercial success. Despite the translation of ProTide technology into the clinic, there remain unresolved in vivo pharmacokinetic and pharmacodynamic [...] Read more.
Phosphoramidate pro-nucleotides (ProTides) have revolutionized the field of anti-viral and anti-cancer nucleoside therapy, overcoming the major limitations of nucleoside therapies and achieving clinical and commercial success. Despite the translation of ProTide technology into the clinic, there remain unresolved in vivo pharmacokinetic and pharmacodynamic questions. Positron Emission Tomography (PET) imaging using [18F]-labelled model ProTides could directly address key mechanistic questions and predict response to ProTide therapy. Here we report the first radiochemical synthesis of [18F]ProTides as novel probes for PET imaging. As a proof of concept, two chemically distinct radiolabelled ProTides have been synthesized as models of 3′- and 2′-fluorinated ProTides following different radiosynthetic approaches. The 3′-[18F]FLT ProTide was obtained via a late stage [18F]fluorination in radiochemical yields (RCY) of 15–30% (n = 5, decay-corrected from end of bombardment (EoB)), with high radiochemical purities (97%) and molar activities of 56 GBq/μmol (total synthesis time of 130 min.). The 2′-[18F]FIAU ProTide was obtained via an early stage [18F]fluorination approach with an RCY of 1–5% (n = 7, decay-corrected from EoB), with high radiochemical purities (98%) and molar activities of 53 GBq/μmol (total synthesis time of 240 min). Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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9 pages, 2309 KiB  
Article
The Radiolabeling of a Gly-Sar Dipeptide Derivative with Flourine-18 and Its Use as a Potential Peptide Transporter PET Imaging Agent
by Andrei Molotkov, John W. Castrillon, Sreevidya Santha, Paul E. Harris, David K. Leung, Akiva Mintz and Patrick Carberry
Molecules 2020, 25(3), 643; https://doi.org/10.3390/molecules25030643 - 2 Feb 2020
Cited by 2 | Viewed by 2615
Abstract
We have developed a novel fluorine-18 radiotracer, dipeptide 1, radiolabeled in two steps from mesylate 3. The initial radiolabeling is achieved in a short reaction time (10 min) and purified through solid-phase extraction (SPE) with modest radiochemical yields (rcy = 10 [...] Read more.
We have developed a novel fluorine-18 radiotracer, dipeptide 1, radiolabeled in two steps from mesylate 3. The initial radiolabeling is achieved in a short reaction time (10 min) and purified through solid-phase extraction (SPE) with modest radiochemical yields (rcy = 10 ± 2%, n = 5) in excellent radiochemical purity (rcp > 99%, n = 5). The de-protection of the tert-butyloxycarbonyl (Boc) and trityl group was achieved with mild heating under acidic conditions to provide 18F-tagged dipeptide 1. Preliminary analysis of 18F-dipeptide 1 was performed to confirm uptake by peptide transporters (PepTs) in human pancreatic carcinoma cell lines Panc1, BxPC3, and ASpc1, which are reported to express the peptide transporter 1 (PepT1). Furthermore, we confirmed in vivo uptake of 18F-dipeptide tracer 1 using microPET/CT in mice harboring subcutaneous flank Panc1, BxPC3, and Aspc1 tumors. In conclusion, we have established the radiolabeling of dipeptide 1 with fluoride-18, and demonstrated its potential as an imaging agent which may have clinical applications for the diagnosis of pancreatic carcinomas. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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9 pages, 443 KiB  
Communication
Late-Stage Copper-Catalyzed Radiofluorination of an Arylboronic Ester Derivative of Atorvastatin
by Gonçalo S. Clemente, Tryfon Zarganes-Tzitzikas, Alexander Dömling and Philip H. Elsinga
Molecules 2019, 24(23), 4210; https://doi.org/10.3390/molecules24234210 - 20 Nov 2019
Cited by 18 | Viewed by 4120
Abstract
There is an unmet need for late-stage 18F-fluorination strategies to label molecules with a wide range of relevant functionalities to medicinal chemistry, in particular (hetero)arenes, aiming to obtain unique in vivo information on the pharmacokinetics/pharmacodynamics (PK/PD) using positron emission tomography (PET). In [...] Read more.
There is an unmet need for late-stage 18F-fluorination strategies to label molecules with a wide range of relevant functionalities to medicinal chemistry, in particular (hetero)arenes, aiming to obtain unique in vivo information on the pharmacokinetics/pharmacodynamics (PK/PD) using positron emission tomography (PET). In the last few years, Cu-mediated oxidative radiofluorination of arylboronic esters/acids arose and has been successful in small molecules containing relatively simple (hetero)aromatic groups. However, this technique is sparsely used in the radiosynthesis of clinically significant molecules containing more complex backbones with several aromatic motifs. In this work, we add a new entry to this very limited database by presenting our recent results on the 18F-fluorination of an arylboronic ester derivative of atorvastatin. The moderate average conversion of [18F]F (12%), in line with what has been reported for similarly complex molecules, stressed an overview through the literature to understand the radiolabeling variables and limitations preventing consistently higher yields. Nevertheless, the current disparity of procedures reported still hampers a consensual and conclusive output. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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12 pages, 2423 KiB  
Article
Fluorine-18 Labeled Fluorofuranylnorprogesterone ([18F]FFNP) and Dihydrotestosterone ([18F]FDHT) Prepared by “Fluorination on Sep-Pak” Method
by Falguni Basuli, Xiang Zhang, Burchelle Blackman, Margaret E. White, Elaine M. Jagoda, Peter L. Choyke and Rolf E. Swenson
Molecules 2019, 24(13), 2389; https://doi.org/10.3390/molecules24132389 - 28 Jun 2019
Cited by 9 | Viewed by 4150
Abstract
To further explore the scope of our recently developed “fluorination on Sep-Pak” method, we prepared two well-known positron emission tomography (PET) tracers 21-[18F]fluoro-16α,17α-[(R)-(1′-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione furanyl norprogesterone ([18F]FFNP) and 16β-[18F]fluoro-5α-dihydrotestosterone ([18F]FDHT). Following the “fluorination on Sep-Pak” method, [...] Read more.
To further explore the scope of our recently developed “fluorination on Sep-Pak” method, we prepared two well-known positron emission tomography (PET) tracers 21-[18F]fluoro-16α,17α-[(R)-(1′-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione furanyl norprogesterone ([18F]FFNP) and 16β-[18F]fluoro-5α-dihydrotestosterone ([18F]FDHT). Following the “fluorination on Sep-Pak” method, over 70% elution efficiency was observed with 3 mg of triflate precursor of [18F]FFNP. The overall yield of [18F]FFNP was 64–72% (decay corrected) in 40 min synthesis time with a molar activity of 37–81 GBq/µmol (1000–2200 Ci/mmol). Slightly lower elution efficiency (~55%) was observed with the triflate precursor of [18F]FDHT. Fluorine-18 labeling, reduction, and deprotection to prepare [18F]FDHT were performed on Sep-Pak cartridges (PS-HCO3 and Sep-Pak plus C-18). The overall yield of [18F]FDHT was 25–32% (decay corrected) in 70 min. The molar activity determined by using mass spectrometry was 63–148 GBq/µmol (1700–4000 Ci/mmol). Applying this quantitative measure of molar activity to in vitro assays [18F]FDHT exhibited high-affinity binding to androgen receptors (Kd~2.5 nM) providing biological validation of this method. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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Review

Jump to: Research

21 pages, 1229 KiB  
Review
Huntington’s Disease: A Review of the Known PET Imaging Biomarkers and Targeting Radiotracers
by Klaudia Cybulska, Lars Perk, Jan Booij, Peter Laverman and Mark Rijpkema
Molecules 2020, 25(3), 482; https://doi.org/10.3390/molecules25030482 - 23 Jan 2020
Cited by 21 | Viewed by 5833
Abstract
Huntington’s disease (HD) is a fatal neurodegenerative disease caused by a CAG expansion mutation in the huntingtin gene. As a result, intranuclear inclusions of mutant huntingtin protein are formed, which damage striatal medium spiny neurons (MSNs). A review of Positron Emission Tomography (PET) [...] Read more.
Huntington’s disease (HD) is a fatal neurodegenerative disease caused by a CAG expansion mutation in the huntingtin gene. As a result, intranuclear inclusions of mutant huntingtin protein are formed, which damage striatal medium spiny neurons (MSNs). A review of Positron Emission Tomography (PET) studies relating to HD was performed, including clinical and preclinical data. PET is a powerful tool for visualisation of the HD pathology by non-invasive imaging of specific radiopharmaceuticals, which provide a detailed molecular snapshot of complex mechanistic pathways within the brain. Nowadays, radiochemists are equipped with an impressive arsenal of radioligands to accurately recognise particular receptors of interest. These include key biomarkers of HD: adenosine, cannabinoid, dopaminergic and glutamateric receptors, microglial activation, phosphodiesterase 10 A and synaptic vesicle proteins. This review aims to provide a radiochemical picture of the recent developments in the field of HD PET, with significant attention devoted to radiosynthetic routes towards the tracers relevant to this disease. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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30 pages, 35703 KiB  
Review
Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals
by Sajid Mushtaq, Seong-Jae Yun and Jongho Jeon
Molecules 2019, 24(19), 3567; https://doi.org/10.3390/molecules24193567 - 2 Oct 2019
Cited by 57 | Viewed by 12188
Abstract
In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for [...] Read more.
In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions. Full article
(This article belongs to the Special Issue Past, Present, and Future of Radiochemical Synthesis)
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