(89)Zr, a radiometal nuclide with high potential for molecular imaging with PET: chemistry, applications and remaining challenges.

Molecular imaging-and especially Positron Emission Tomography (PET)-is of increasing importance for the diagnosis of various diseases and thus is experiencing increasing dissemination. Consequently, there is a growing demand for appropriate PET tracers which allow for a specific accumulation in the target structure as well as its visualization and exhibit decay characteristics matching their in vivo pharmacokinetics. To meet this demand, the development of new targeting vectors as well as the use of uncommon radionuclides becomes increasingly important. Uncommon nuclides in this regard enable the utilization of various selectively accumulating bioactive molecules such as peptides, antibodies, their fragments, other proteins and artificial structures for PET imaging in personalized medicine. Among these radionuclides, 89Zr (t1/2 = 3.27 days and mean Eβ+ = 0.389 MeV) has attracted increasing attention within the last years due to its favorably long half-life, which enables imaging at late time-points, being especially favorable in case of slowly-accumulating targeting vectors. This review outlines the recent developments in the field of 89Zr-labeled bioactive molecules, their potential and application in PET imaging and beyond, as well as remaining challenges

Synthese, Charakterisierung und Evaluierung von Antikörperkonjugaten mit Dendrimer-basierten Chelator- und Fluoreszenzfarbstoffmultimeren für die Krebsdiagnostik und Therapie

Für eine Verbesserung der radiometallbasierten Diagnostik und Therapie sowie der Diagnostik mittels Fluoreszenzfarbstoffen wurden Chelatbildner- und Fluoreszenz-farbstoff-Multimere auf der Basis von PAMAM-Dendrimeren hergestellt. Diese wurden in den EGF-Rezeptor bindenden Antikörper Matuzumab eingeführt und die synthetisierten Antikörper-Multimer-Konjugate hinsichtlich ihrer Immunreaktivität untersucht. Für die Herstellung der Multimere musste zunächst die Synthese der PAMAM-Dendrimere optimiert werden, da mittels publizierter Methoden nur Dendrimere unzureichender Homogenität erhalten werden konnten. Hierdurch war es möglich, Dendrimere mit einer bis 128 Aminfunktionen in hoher Homogenität herzustellen. Die Dendrimere wurden auf einem Pentaethylenglycol-Linker aufgebaut, der eine Thiol-Funktion für die Umsetzung mit Antikörpern trug. Es wurde eine große Anzahl von Fluoreszenzfarbstoffen auf ihre Multimerisierbarkeit auf diesen Dendrimer-Cores untersucht. Allerdings wiesen nur Dansylchlorid, NBD-chlorid, Coumarin 343, 5(6)-Carboxyfluorescein-pentafluorphenylester und Sulforhodamin B2 Fluorid ausreichend hohe Reaktivitäten mit den Aminfunktionen der PAMAM-Dendrimere auf. Die Untersuchung der Fluoreszenzeigenschaften der hergestellten Farbstoff-Multimere zeigte allerdings, dass sich für eine Multimerisierung zur Verstärkung von Fluoreszenzsignalen einzig Dansylchlorid eignet. Folglich wurden Dansyl-Multimere mit 1 bis 128 Dansylresten synthetisiert und mittels Größenausschlusschromatographie, HPLC, NMR und Massenspektroskopie charakterisiert. Die Dansyl-Multimere Dansyl8-SH, Dansyl16-SH, Dansyl32-SH und Dansyl64-SH wurden in den anti-EGFR Antikörper Matuzumab eingeführt, wobei sich zeigte, dass sich die größeren Dansyl-Multimere Dansyl32-SH und Dansyl64-SH nicht für eine Derivatisierung von Antikörpern eignen, da sie, wahrscheinlich aufgrund ihrer hohen Lipophilie, bei Einführung in das Protein zur Denaturierung des Antikörpers führten. Das Dansyl16-Antikörper-Konjugat war somit das größte herstellbare Konjugat, das isoliert und charakterisiert werden konnte. Diese Verbindung wurde an HT29-Zellen hinsichtlich ihrer Immunreaktivität untersucht und zeigte relativ zum underivatisierten Antikörper eine zu 54% erhaltene Immunreaktivität. Für die Herstellung der Chelat-Multimere wurden zunächst die bekannten Syntheseprotokolle der als Synthon benötigten DOTA-Derivate Tris-tBu-DOTA, Thiol-DOTA und Tris-benzyl-DOTA verbessert. Neben diesen Derivaten wurden weiterhin Tris-allyl-DOTA, DOTA-PFP-ester und DOTA–PNP-ester sowie ein neues Derivat, Tris-4-nitrobenzyl-DOTA, hergestellt und mit den Dendrimeren umgesetzt. Einzig Thiol-DOTA ließ sich quantitativ in die Dendrimere einführen und ergab die gewünschten Chelat-Multimere mit 1 bis 128 DOTAs in hoher Homogenität, die nach ihrer vollständigen Charakterisierung in den Antikörper Matuzumab eingeführt wurden. Dabei wurde sowohl die Anzahl als auch die Größe der eingeführten Multimere variiert, um systematisch zu untersuchen, welchen Einfluss die Größe und die Anzahl der gekoppelten Dendrimere auf die Immunreaktivität der derivatisierten Antikörper haben. Es sollte herausgefunden werden, ob ein Optimum mit größtmöglicher Anzahl von Chelatbildnern und geringst möglicher Verschlechterung der Immunreaktivität erreicht werden kann. Die erhaltenen Antikörper-Multimer-Konjugate wurden hinsichtlich ihrer Immunreaktivität an HT29-Zellen untersucht. Dabei konnte gezeigt werden, dass die Größe des angekoppelten DOTA-Multimers über einen weiten Molekulargewichtsbereich nur einen geringen Einfluss auf die Immunreaktivität des Antikörper-Konjugates hat, dass jedoch die Anzahl der Derivatisierungsstellen pro Antikörper einen drastischen Einfluss auf die Immunreaktivität der Konjugate hat

Silicon-[18F]Fluorine Radiochemistry: Basics, Applications and Challenges

Silicon-[18F]Fluorine (Si-18F) radiochemistry has recently emerged alongside other unconventional approaches such as aluminum-F-18 and boron-F-18 based labeling strategies, reshaping the landscape of modern F-18-radiochemistry. All these novel methodologies are driven by the demand for more convenient F-18-labeling procedures to further disseminate one of the most sophisticated imaging technologies, Positron Emission Tomography (PET). The PET methodology requires special radionuclides such as F-18 (one of the most prominent examples) to be introduced into bioactive molecules. Si-F-18 radiochemistry contributed greatly towards the development of new radiopharmaceuticals for PET imaging. Herein, we describe the radiochemical basics of Si-F-18 bond formation, the application of Si-F-18 tracers for PET imaging, and additionally, the inherent chemical intricacies of this methodology

τBu₂SiF-Derivatized D₂-Receptor Ligands: The First SiFA-Containing Small Molecule Radiotracers for Target-Specific PET-Imaging

The synthesis, radiolabeling and in vitro evaluation of new silicon-fluoride acceptor (SiFA) derivatized D-2-receptor ligands is reported. The SiFA-technology simplifies the introduction of fluorine-18 into target specific biomolecules for Positron-Emission-Tomography (PET). However, one of the remaining challenges, especially for small molecules such as receptor-ligands, is the bulkiness of the SiFA-moiety. We therefore synthesized four Fallypride SiFA-conjugates derivatized either directly at the benzoic acid ring system (SiFA-DMFP, SiFA-FP, SiFA-DDMFP) or at the butyl-side chain (SiFA-M-FP) and tested their receptor affinities. We found D2-receptor affinities for all compounds in the nanomolar range (Ki(SiFA-DMFP) = 13.6 nM, Ki(SiFA-FP) = 33.0 nM, Ki(SiFA-DDMFP) = 62.7 nM and Ki(SiFA-M-FP) = 4.21 nM). The radiofluorination showed highest yields when 10 nmol of the precursors were reacted with F-18]fluoride/TBAHCO(3) in acetonitrile. After a reversed phased cartridge purification the desired products could be isolated as an injectable solution after only 10 min synthesis time with radiochemical yields (RCY) of more than 40% in the case of SiFA-DMFP resulting in specific activities >41 GBq/mu mol (>1,100 Ci/mmol). Furthermore, the radiolabeled products were shown to be stable in the injectable solutions, as well as in human plasma, for at least 90 min

Microfluidics: A Groundbreaking Technology for PET Tracer Production?

Application of microfluidics to Positron Emission Tomography ( PET) tracer synthesis has attracted increasing interest within the last decade. The technical advantages of microfluidics, in particular the high surface to volume ratio and resulting fast thermal heating and cooling rates of reagents can lead to reduced reaction times, increased synthesis yields and reduced by-products. In addition automated reaction optimization, reduced consumption of expensive reagents and a path towards a reduced system footprint have been successfully demonstrated. The processing of radioactivity levels required for routine production, use of microfluidic-produced PET tracer doses in preclinical and clinical imaging as well as feasibility studies on autoradiolytic decomposition have all given promising results. However, the number of microfluidic synthesizers utilized for commercial routine production of PET tracers is very limited. This study reviews the state of the art in microfluidic PET tracer synthesis, highlighting critical design aspects, strengths, weaknesses and presenting several characteristics of the diverse PET market space which are thought to have a significant impact on research, development and engineering of microfluidic devices in this field. Furthermore, the topics of batch- and single-dose production, cyclotron to quality control integration as well as centralized versus de-centralized market distribution models are addressed

Comparison between 68Ga-bombesin (68Ga-BZH3) and the cRGD tetramer 68Ga-RGD4 studies in an experimental nude rat model with a neuroendocrine pancreatic tumor cell line

BACKGROUND: Serotonergic neurons in the rodent hypothalamus are implicated in key neuroendocrine and metabolic functions, including circadian rhythmicity. However, the assessment of the serotonergic system in the human hypothalamus in vivo is difficult as delineation of the hypothalamus is cumbersome with conventional region-of-interest analysis. In the present study, we aimed to develop a method to visualize serotonin transporters (SERT) in the hypothalamus. Additionally, we tested the hypothesis that hypothalamic SERT binding ratios are different between patients with hypothalamic impairment (HI), pituitary insufficiency (PI), and control subjects (C). METHODS: SERT availability was determined in 17 subjects (6 HI, 5 PI, and 6 healthy controls), 2 h after injection of 123I-N-ω-fluoropropyl-2β-carboxymethoxy-3β-(4-iodophenyl) nortropane ([123I]FP-CIT), using single-photon emission computed tomography (performed on a brain-dedicated system) fused with individual magnetic resonance imaging (MRI) scans of the brain. The hypothalamus (representing specific SERT binding) and cerebellum (representing nonspecific binding) were manually delineated on each MRI to assess [123I]FP-CIT binding and specific-to-nonspecific binding ratios. RESULTS: In each healthy subject, [123I]FP-CIT binding was higher in the hypothalamus than in the cerebellum, and the mean hypothalamic binding ratio of SERT was 0.29 ± 0.23. We found no difference in hypothalamic binding ratios between HI, PI, and control subjects (HI 0.16 ± 0.24, PI 0.45 ± 0.39, C 0.29 ± 0.23, p value 0.281). CONCLUSIONS: We were able to demonstrate SERT binding in the human hypothalamus in vivo. However, we did not find altered hypothalamic SERT binding in patients with hypothalamic impairment. TRIAL REGISTRATION: Netherlands Trial Register: NTR2520

Silicon-[18F]Fluorine Radiochemistry: Basics, Applications and Challenges

Silicon-[18F]Fluorine (Si-18F) radiochemistry has recently emerged alongside other unconventional approaches such as aluminum-F-18 and boron-F-18 based labeling strategies, reshaping the landscape of modern F-18-radiochemistry. All these novel methodologies are driven by the demand for more convenient F-18-labeling procedures to further disseminate one of the most sophisticated imaging technologies, Positron Emission Tomography (PET). The PET methodology requires special radionuclides such as F-18 (one of the most prominent examples) to be introduced into bioactive molecules. Si-F-18 radiochemistry contributed greatly towards the development of new radiopharmaceuticals for PET imaging. Herein, we describe the radiochemical basics of Si-F-18 bond formation, the application of Si-F-18 tracers for PET imaging, and additionally, the inherent chemical intricacies of this methodology

Good practices for the automated production of ¹⁸F-SiFA radiopharmaceuticals

Background: The positron emitting isotope fluorine-18 (18F) possesses almost ideal physicochemical properties for the development of radiotracers for diagnostic molecular imaging employing positron emission tomography (PET). 18F in its nucleophilic anionic 18F− form is usually prepared by bombarding an enriched 18O water target with protons of various energies between 5 and 20 MeV depending on the technical specifications of the cyclotron. Large thick-target yields between 5 and 14 GBq/µA can be obtained, enough to prepare large batches of radiotracers capable to serve a considerable contingent of patients (50 + per clinical batch). The overall yield of the radiotracer however depends on the efficiency of the 18F labeling chemistry. The Silicon Fluoride Acceptor chemistry (SiFA) has introduced a convenient and highly efficient way to provide clinical peptide-based 18F-radiotracers in a kit-like procedure matching the convenience of 99mTc radiopharmaceuticals. Main body: A radiotracer’s clinical success primarily hinges on whether its synthesis can be automated. Due to its simplicity, the SiFA chemistry, which is based on isotopic exchange (18F for 19F), does not only work in a manual setup but has been proven to be automatable, yielding large batches of 18F-radiotracers of high molar activity (Am). The production of SiFA radiotracer can be centralized and the radiopharmaceutical be distributed via the “satellite” principle, where one production facility economically serves multiple clinical application sites. Clinically validated tracers such as [18F]SiTATE and [18F]Ga-rhPSMA-7/-7.3 have been synthesized in an automated synthesis unit under good manufacturing practice conditions and used in large patient cohorts. Communication of common guidelines and practices is warranted to further the dissemination of SiFA radiopharmaceuticals and to give easy access to this technology. Conclusion: This current review highlights the most recent achievements in SiFA radiopharmaceutical automation geared towards large batch production for clinical application. Best practice advice and guidance towards a facilitated implementation of the SiFA technology into new and already operating PET tracer production facilities is provided. A brief outlook spotlights the future potential of SiFA radiochemistry within the landscape of non-canonical labeling chemistries.</p

Good practices for the automated production of ¹⁸F-SiFA radiopharmaceuticals

Background: The positron emitting isotope fluorine-18 (18F) possesses almost ideal physicochemical properties for the development of radiotracers for diagnostic molecular imaging employing positron emission tomography (PET). 18F in its nucleophilic anionic 18F− form is usually prepared by bombarding an enriched 18O water target with protons of various energies between 5 and 20 MeV depending on the technical specifications of the cyclotron. Large thick-target yields between 5 and 14 GBq/µA can be obtained, enough to prepare large batches of radiotracers capable to serve a considerable contingent of patients (50 + per clinical batch). The overall yield of the radiotracer however depends on the efficiency of the 18F labeling chemistry. The Silicon Fluoride Acceptor chemistry (SiFA) has introduced a convenient and highly efficient way to provide clinical peptide-based 18F-radiotracers in a kit-like procedure matching the convenience of 99mTc radiopharmaceuticals. Main body: A radiotracer’s clinical success primarily hinges on whether its synthesis can be automated. Due to its simplicity, the SiFA chemistry, which is based on isotopic exchange (18F for 19F), does not only work in a manual setup but has been proven to be automatable, yielding large batches of 18F-radiotracers of high molar activity (Am). The production of SiFA radiotracer can be centralized and the radiopharmaceutical be distributed via the “satellite” principle, where one production facility economically serves multiple clinical application sites. Clinically validated tracers such as [18F]SiTATE and [18F]Ga-rhPSMA-7/-7.3 have been synthesized in an automated synthesis unit under good manufacturing practice conditions and used in large patient cohorts. Communication of common guidelines and practices is warranted to further the dissemination of SiFA radiopharmaceuticals and to give easy access to this technology. Conclusion: This current review highlights the most recent achievements in SiFA radiopharmaceutical automation geared towards large batch production for clinical application. Best practice advice and guidance towards a facilitated implementation of the SiFA technology into new and already operating PET tracer production facilities is provided. A brief outlook spotlights the future potential of SiFA radiochemistry within the landscape of non-canonical labeling chemistries.</p

Good practices for the automated production of ¹⁸F-SiFA radiopharmaceuticals

Background: The positron emitting isotope fluorine-18 (18F) possesses almost ideal physicochemical properties for the development of radiotracers for diagnostic molecular imaging employing positron emission tomography (PET). 18F in its nucleophilic anionic 18F− form is usually prepared by bombarding an enriched 18O water target with protons of various energies between 5 and 20 MeV depending on the technical specifications of the cyclotron. Large thick-target yields between 5 and 14 GBq/µA can be obtained, enough to prepare large batches of radiotracers capable to serve a considerable contingent of patients (50 + per clinical batch). The overall yield of the radiotracer however depends on the efficiency of the 18F labeling chemistry. The Silicon Fluoride Acceptor chemistry (SiFA) has introduced a convenient and highly efficient way to provide clinical peptide-based 18F-radiotracers in a kit-like procedure matching the convenience of 99mTc radiopharmaceuticals. Main body: A radiotracer’s clinical success primarily hinges on whether its synthesis can be automated. Due to its simplicity, the SiFA chemistry, which is based on isotopic exchange (18F for 19F), does not only work in a manual setup but has been proven to be automatable, yielding large batches of 18F-radiotracers of high molar activity (Am). The production of SiFA radiotracer can be centralized and the radiopharmaceutical be distributed via the “satellite” principle, where one production facility economically serves multiple clinical application sites. Clinically validated tracers such as [18F]SiTATE and [18F]Ga-rhPSMA-7/-7.3 have been synthesized in an automated synthesis unit under good manufacturing practice conditions and used in large patient cohorts. Communication of common guidelines and practices is warranted to further the dissemination of SiFA radiopharmaceuticals and to give easy access to this technology. Conclusion: This current review highlights the most recent achievements in SiFA radiopharmaceutical automation geared towards large batch production for clinical application. Best practice advice and guidance towards a facilitated implementation of the SiFA technology into new and already operating PET tracer production facilities is provided. A brief outlook spotlights the future potential of SiFA radiochemistry within the landscape of non-canonical labeling chemistries.</p