Special Issue "Targets, Tracers and Translation – Novel Radiopharmaceuticals Boost Nuclear Medicine"

A special issue of Pharmaceuticals (ISSN 1424-8247).

Deadline for manuscript submissions: closed (14 January 2019)

Special Issue Editor

Guest Editor
PD Dr. Gerald Reischl

Universitat Tubingen, Department of Preclinical Imaging and Radiopharmacy, Tubingen, Germany
Website | E-Mail
Interests: Radiopharmaceutical Sciences; PET isotope production methods; PET tracer development; small molecules; antibodies; imaging of hypoxia/inflammation/infectious diseases; preclinical evaluation and translation into clinical application

Special Issue Information

Dear Colleagues,

This is the 4th Special Issue in Pharmaceuticals within the last five years dealing with aspects of Radiopharmaceutical Sciences. It demonstrates a significant interest and increasing relevance to ameliorate Nuclear Medicine imaging with PET or SPECT, and also radiotherapeutical procedures.

Numerous targets and mechanisms have been identified and have been under investigation over the previous years, covering many fields of medical and clinical research. Oncology still is the most important area. In addition to tumor hypoxia, angiogenesis, apoptosis or inflammation, more recently, tumor senescence or targets like PSMA, chemokine receptors or tyrosine kinases are a focus of attention. In neuro imaging, especially imaging of neurodegenerative diseases, which previously focused on amyloid plaques, now Tau proteins, alpha-synuclein or TSPO as targets are being evaluated. Last but not least, in immunology imaging of inflammation or infectious diseases (bacterial or fungal) gain increasing interest.

In addition to the classical PET isotopes (18F, 11C, 13N, 15O) further nuclides are available (e.g., 68Ga, 64Cu, 89Zr, 44Sc). There are also new labeling techniques under development to aim for new highly specific tracers (radiopharmaceuticals) to investigate the targets mentioned.

In this context, the large variety of accessible animal and cell models for the preclinical evaluation of targets, mechanisms and tracers have to be highlighted.

Mandatory for successful developments in this multi-disciplinary environment are close collaborations among the researchers from medicine, biology, biochemistry, chemistry and pharmacy. Most of the members of the community of the radiopharmaceutical sciences work in a clinical periphery. All our basic research in radiochemistry and/or preclinical radiopharmaceutical development finds its justification and needs the perspective of a consequent translation of the results into clinical application in nuclear medicine. The statement “a mouse is not a man” is trivial and translation “from bench to bedside” must be the driving force. Regulatory requirements and especially GMP have become a challenge to be faced and handled, but nevertheless cannot be showstoppers.

Possible contributions of radiopharmaceutical sciences and nuclear medicine imaging to a personalized medicine have been shown in the past and even seem to be broader in the future. Imaging sciences today are regarded as an integral part of clinical research. With new therapies becoming more complex and specific, imaging together with, e.g., metabolomics procedures, histology and advanced data analysis is a most valuable tool in such multi-modal approaches, providing molecular and functional profiles for therapy planning and monitoring.

You are cordially invited to contribute original research, as well as review articles, to this Special Issue, “Targets, Tracers and Translation—Novel Radiopharmaceuticals Boost Nuclear Medicine”.

Areas of interest include, but are not limited to:

  • New targets for imaging
  • Tracer developments towards translation
  • Multi-modal imaging
  • Studies of tumor pathways
  • Imaging for therapy monitoring

PD Dr. Gerald Reischl
Guest Editor

Manuscript Submission Information

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Keywords

  • Tracer development
  • PET and SPECT tracers
  • Therapy monitoring
  • Endoradiotherapy
  • Translational approaches
  • Nuclear medicine

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Published Papers (14 papers)

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Research

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Open AccessArticle Localization of 99mTc-GRP Analogs in GRPR-Expressing Tumors: Effects of Peptide Length and Neprilysin Inhibition on Biological Responses
Pharmaceuticals 2019, 12(1), 42; https://doi.org/10.3390/ph12010042
Received: 26 February 2019 / Revised: 16 March 2019 / Accepted: 18 March 2019 / Published: 20 March 2019
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Abstract
The overexpression of gastrin-releasing peptide receptors (GRPRs) in frequently occurring human tumors has provided the opportunity to use bombesin (BBN) analogs as radionuclide carriers to cancer sites for diagnostic and therapeutic purposes. We have been alternatively exploring human GRP motifs of higher GRPR [...] Read more.
The overexpression of gastrin-releasing peptide receptors (GRPRs) in frequently occurring human tumors has provided the opportunity to use bombesin (BBN) analogs as radionuclide carriers to cancer sites for diagnostic and therapeutic purposes. We have been alternatively exploring human GRP motifs of higher GRPR selectivity compared to frog BBN sequences aiming to improve pharmacokinetic profiles. In the present study, we compared two differently truncated human endogenous GRP motifs: GRP(14–27) and GRP(18–27). An acyclic tetraamine was coupled at the N-terminus to allow for stable binding of the SPECT radionuclide 99mTc. Their biological profiles were compared in PC-3 cells and in mice without or with coinjection of phosphoramidon (PA) to induce transient neprilysin (NEP) inhibition in vivo. The two 99mTc-N4-GRP(14/18–27) radioligands displayed similar biological behavior in mice. Coinjection of PA exerted a profound effect on in vivo stability and translated into notably improved radiolabel localization in PC-3 experimental tumors. Hence, this study has shown that promising 99mTc-radiotracers for SPECT imaging may indeed derive from human GRP sequences. Radiotracer bioavailability was found to be of major significance. It could be improved during in situ NEP inhibition resulting in drastically enhanced uptake in GRPR-expressing lesions. Full article
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Open AccessArticle 2-Nitroimidazole-Furanoside Derivatives for Hypoxia Imaging—Investigation of Nucleoside Transporter Interaction, 18F-Labeling and Preclinical PET Imaging
Pharmaceuticals 2019, 12(1), 31; https://doi.org/10.3390/ph12010031
Received: 3 December 2018 / Revised: 4 February 2019 / Accepted: 12 February 2019 / Published: 15 February 2019
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Abstract
The benefits of PET imaging of tumor hypoxia in patient management has been demonstrated in many examples and with various tracers over the last years. Although, the optimal hypoxia imaging agent has yet to be found, 2-nitroimidazole (azomycin) sugar derivatives—mimicking nucleosides—have proven their [...] Read more.
The benefits of PET imaging of tumor hypoxia in patient management has been demonstrated in many examples and with various tracers over the last years. Although, the optimal hypoxia imaging agent has yet to be found, 2-nitroimidazole (azomycin) sugar derivatives—mimicking nucleosides—have proven their potential with [18F]FAZA ([18F]fluoro-azomycin-α-arabinoside) as a prominent representative in clinical use. Still, for all of these tracers, cellular uptake by passive diffusion is postulated with the disadvantage of slow kinetics and low tumor-to-background ratios. We recently evaluated [18F]fluoro-azomycin-β-deoxyriboside (β-[18F]FAZDR), with a structure more similar to nucleosides than [18F]FAZA and possible interaction with nucleoside transporters. For a deeper insight, we comparatively studied the interaction of FAZA, β-FAZA, α-FAZDR and β-FAZDR with nucleoside transporters (SLC29A1/2 and SLC28A1/2/3) in vitro, showing variable interactions of the compounds. The highest interactions being for β-FAZDR (IC50 124 ± 33 µM for SLC28A3), but also for FAZA with the non-nucleosidic α-configuration, the interactions were remarkable (290 ± 44 µM {SLC28A1}; 640 ± 10 µM {SLC28A2}). An improved synthesis was developed for β-FAZA. For a PET study in tumor-bearing mice, α-[18F]FAZDR was synthesized (radiochemical yield: 15.9 ± 9.0% (n = 3), max. 10.3 GBq, molar activity > 50 GBq/µmol) and compared to β-[18F]FAZDR and [18F]FMISO, the hypoxia imaging gold standard. We observed highest tumor-to-muscle ratios (TMR) for β-[18F]FAZDR already at 1 h p.i. (2.52 ± 0.94, n = 4) in comparison to [18F]FMISO (1.37 ± 0.11, n = 5) and α-[18F]FAZDR (1.93 ± 0.39, n = 4), with possible mediation by the involvement of nucleoside transporters. After 3 h p.i., TMR were not significantly different for all 3 tracers (2.5–3.0). Highest clearance from tumor tissue was observed for β-[18F]FAZDR (56.6 ± 6.8%, 2 h p.i.), followed by α-[18F]FAZDR (34.2 ± 7.5%) and [18F]FMISO (11.8 ± 6.5%). In conclusion, both isomers of [18F]FAZDR showed their potential as PET hypoxia tracers. Differences in uptake behavior may be attributed to a potential variable involvement of transport mechanisms. Full article
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Open AccessArticle Comparative Study of Two Oxidizing Agents, Chloramine T and Iodo-Gen®, for the Radiolabeling of β-CIT with Iodine-131: Relevance for Parkinson’s Disease
Pharmaceuticals 2019, 12(1), 25; https://doi.org/10.3390/ph12010025
Received: 30 November 2018 / Revised: 18 January 2019 / Accepted: 31 January 2019 / Published: 5 February 2019
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Abstract
Parkinson’s disease (PD) is a neurodegenerative disease characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, leading to alteration of the integrity of dopaminergic transporters (DATs). In recent years, some radiopharmaceuticals have been used in the clinic to [...] Read more.
Parkinson’s disease (PD) is a neurodegenerative disease characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, leading to alteration of the integrity of dopaminergic transporters (DATs). In recent years, some radiopharmaceuticals have been used in the clinic to evaluate the integrity of DATs. These include tropane derivatives such as radiolabeled β-CIT and FP-CIT with iodine-123 (123I), and TRODAT-1 with metastable technetium-99 (99mTc). Radiolabeling of β-CIT with radioactive iodine is based on electrophilic radioiodination using oxidizing agents, such as Chloramine T or Iodo-Gen®. For the first time, the present work performed a comparative study of the radiolabeling of β-CIT with iodine-131 (131I), using either Chloramine T or Iodo-Gen® as oxidizing agents, in order to improve the radiolabeling process of β-CIT and to choose the most advantageous oxidizing agent to be used in nuclear medicine. Both radiolabeling methods were similar and resulted in high radiochemical yield (> 95%), with suitable 131I-β-CIT stability up to 72 h. Although Chloramine T is a strong oxidizing agent, it was as effective as Iodo-Gen® for β-CIT radiolabeling with 131I, with the advantage of briefer reaction time and solubility in aqueous medium. Full article
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Open AccessArticle [177Lu]Lu-PSMA-617 Salivary Gland Uptake Characterized by Quantitative In Vitro Autoradiography
Pharmaceuticals 2019, 12(1), 18; https://doi.org/10.3390/ph12010018
Received: 30 November 2018 / Revised: 21 December 2018 / Accepted: 10 January 2019 / Published: 24 January 2019
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Abstract
Irradiation of salivary glands remains the main dose-limiting side effect of therapeutic PSMA-inhibitors, especially when using alpha emitters. Thus, further advances in radiopharmaceutical design and therapy strategies are needed to reduce salivary gland uptake, thereby allowing the administration of higher doses and potentially [...] Read more.
Irradiation of salivary glands remains the main dose-limiting side effect of therapeutic PSMA-inhibitors, especially when using alpha emitters. Thus, further advances in radiopharmaceutical design and therapy strategies are needed to reduce salivary gland uptake, thereby allowing the administration of higher doses and potentially resulting in improved response rates and better tumor control. As the uptake mechanism remains unknown, this work investigates the salivary gland uptake of [177Lu]Lu-PSMA-617 by autoradiography studies on pig salivary gland tissue and on PSMA-overexpressing LNCaP cell membrane pellets. Displacement studies were performed with non-labeled PSMA-617 and 2-PMPA, respectively. The uptake of [177Lu]Lu-PSMA-617 in glandular areas was determined to be partly PSMA-specific, with a high non-specific uptake fraction. The study emphasizes that [177Lu]Lu-PSMA-617 accumulation in pig salivary glands can be attributed to a combination of both specific and non-specific uptake mechanisms. The observation is of high impact for future design of novel radiopharmaceuticals addressing the dose-limiting salivary gland irradiation of current alpha endoradiotherapy in prostate cancer. Full article
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Open AccessArticle Cholecystokinin-2 Receptor Targeting with Novel C-terminally Stabilized HYNIC-Minigastrin Analogs Radiolabeled with Technetium-99m
Pharmaceuticals 2019, 12(1), 13; https://doi.org/10.3390/ph12010013
Received: 29 November 2018 / Revised: 8 January 2019 / Accepted: 10 January 2019 / Published: 15 January 2019
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Abstract
The high overexpression of cholecystokinin-2 receptors (CCK2R) in tumors, such as medullary thyroid carcinoma, allows for highly specific diagnostic and therapeutic targeting with radiolabeled peptide probes derived from natural ligands for the receptor. Based on the ideal imaging characteristics, high availability and low [...] Read more.
The high overexpression of cholecystokinin-2 receptors (CCK2R) in tumors, such as medullary thyroid carcinoma, allows for highly specific diagnostic and therapeutic targeting with radiolabeled peptide probes derived from natural ligands for the receptor. Based on the ideal imaging characteristics, high availability and low cost of technetium-99m (99mTc)-labeled radiopharmaceuticals we have developed two hydrazinonicotinic acid (HYNIC) conjugated minigastrin analogs allowing labeling at high specific activity. The CCK2R targeting peptide conjugates show specific amino acid substitutions in the C-terminal receptor-specific sequence with the aim to increase stability and tumor targeting. The CCK2R affinity and the cell uptake of the new radioligands were analyzed using A431 human epidermoid carcinoma cells stably transfected with human CCK2R and mock transfected cells. Metabolic studies in BALB/c mice revealed a high resistance against enzymatic degradation for both radioligands. Biodistribution studies in tumor-xenografted athymic BALB/c nude mice at 1 h and 4 h p.i. showed that the two 99mTc-labeled compounds showed varying uptake in receptor expressing organs, stomach and pancreas (1.3–10.4% IA/g), as well as kidneys, the main route of excretion (7.8–19.9% IA/g). The tumor uptake in A431-CCK2R xenografts was 24.75 ± 4.38% IA/g for [99mTc]Tc-HYNIC-MGS5 and 42.48 ± 6.99% IA/g for [99mTc]Tc-HYNIC-MGS11 at 4 h p.i., whereas the tumor-to-kidney ratio was comparable (2.6–3.3). On demand availability and potential application for radioguided surgery of a 99mTc-labeled minigastrin analog support the further evaluation of these highly promising new compounds. Full article
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Open AccessArticle Optimization of the Automated Synthesis of [11C]mHED—Administered and Apparent Molar Activities
Pharmaceuticals 2019, 12(1), 12; https://doi.org/10.3390/ph12010012
Received: 30 November 2018 / Revised: 27 December 2018 / Accepted: 10 January 2019 / Published: 14 January 2019
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Abstract
The tracer [11C]meta-Hydroxyephedrine ([11C]mHED) is one of the most applied PET tracers for cardiac imaging, whose radiosynthesis was already reported in 1990. While not stated in the literature, separation difficulties and an adequate formulation of [...] Read more.
The tracer [11C]meta-Hydroxyephedrine ([11C]mHED) is one of the most applied PET tracers for cardiac imaging, whose radiosynthesis was already reported in 1990. While not stated in the literature, separation difficulties and an adequate formulation of the product are well known challenges in its production. Furthermore, the precursor (metaraminol) is also a substrate for the norepinephrine transporter, and can therefore affect the image quality. This study aims at optimizing the synthetic process of [11C]mHED and investigating the effect of the apparent molar activity (sum of mHED and metaraminol) in patients and animals. The main optimization was the improved separation through reverse phase-HPLC by a step gradient and subsequent retention of the product on a weakly-cationic ion exchange cartridge. The µPET/µCT was conducted in ten rats (ischemic model) and the apparent molar activity was correlated to the VOI- and SUV-ratio of the myocardium/intra-ventricular blood pool. Moreover, nine long-term heart transplanted and five Morbus Fabry patients underwent PET and MRI imaging for detection of changes in the sympathetic innervation. In summary, the fully-automated synthesis and optimized purification method of [11C]mHED is easily applicable and reproducible. Moreover, it was shown that the administered apparent molar activities had a negligible effect on the imaging quality. Full article
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Open AccessArticle Development of [131I]I-EOE-TPZ and [131I]I-EOE-TPZMO: Novel Tirapazamine (TPZ)-Based Radioiodinated Pharmaceuticals for Application in Theranostic Management of Hypoxia
Pharmaceuticals 2019, 12(1), 3; https://doi.org/10.3390/ph12010003
Received: 29 November 2018 / Revised: 18 December 2018 / Accepted: 20 December 2018 / Published: 1 January 2019
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Abstract
Introduction: Benzotriazine-1,4-dioxides (BTDOs) such as tirapazamine (TPZ) and its derivatives act as radiosensitizers of hypoxic tissues. The benzotriazine-1-monoxide (BTMO) metabolite (SR 4317, TPZMO) of TPZ also has radiosensitizing properties, and via unknown mechanisms, is a potent enhancer of the radiosensitizing effects of [...] Read more.
Introduction: Benzotriazine-1,4-dioxides (BTDOs) such as tirapazamine (TPZ) and its derivatives act as radiosensitizers of hypoxic tissues. The benzotriazine-1-monoxide (BTMO) metabolite (SR 4317, TPZMO) of TPZ also has radiosensitizing properties, and via unknown mechanisms, is a potent enhancer of the radiosensitizing effects of TPZ. Unlike their 2-nitroimidazole radiosensitizer counterparts, radiolabeled benzotriazine oxides have not been used as radiopharmaceuticals for diagnostic imaging or molecular radiotherapy (MRT) of hypoxia. The radioiodination chemistry for preparing model radioiodinated BTDOs and BTMOs is now reported. Hypothesis: Radioiodinated 3-(2-iodoethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (I-EOE-TPZ), a novel bioisosteric analogue of TPZ, and 3-(2-iodoethoxyethyl)-amino-1,2,4-benzotriazine-1-oxide (I-EOE-TPZMO), its monoxide analogue, are candidates for in vivo and in vitro investigations of biochemical mechanisms in pathologies that develop hypoxic microenvironments. In theory, both radiotracers can be prepared from the same precursors. Methods: Radioiodination procedures were based on classical nucleophilic [131I]iodide substitution on Tos-EOE-TPZ (P1) and by [131I]iodide exchange on I-EOE-TPZ (P2). Reaction parameters, including temperature, reaction time, solvent and the influence of pivalic acid on products’ formation and the corresponding radiochemical yields (RCY) were investigated. Results: The [131I]iodide labeling reactions invariably led to the synthesis of both products, but with careful manipulation of conditions the preferred product could be recovered as the major product. Radioiodide exchange on P2 in ACN at 80 ± 5 °C for 30 min afforded the highest RCY, 89%, of [131I]I-EOE-TPZ, which upon solid phase purification on an alumina cartridge gave 60% yield of the product with over 97% of radiochemical purity. Similarly, radioiodide exchange on P2 in ACN at 50 ± 5 °C for 30 min with pivalic acid afforded the highest yield, 92%, of [131I]I-EOE-TPZMO exclusively with no trace of [131I]I-EOE-TPZ. In both cases, extended reaction times and/or elevated temperatures resulted in the formation of at least two additional radioactive reaction products. Conclusions: Radioiodination of P1 and P2 with [131I]iodide leads to the facile formation of [131I]I-EOE-TPZMO. At 80 °C and short reaction times, the facile reduction of the N-4-oxide moiety was minimized to afford acceptable radiochemical yields of [131I]I-EOE-TPZ from either precursor. Regeneration of [131I]I-EOE-TPZ from [131I]I-EOE-TPZMO is impractical after reaction work-up. Full article
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Open AccessArticle Synthesis and Initial In Vivo Evaluation of [11C]AZ683—A Novel PET Radiotracer for Colony Stimulating Factor 1 Receptor (CSF1R)
Pharmaceuticals 2018, 11(4), 136; https://doi.org/10.3390/ph11040136
Received: 31 October 2018 / Revised: 6 December 2018 / Accepted: 11 December 2018 / Published: 13 December 2018
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Abstract
Positron emission tomography (PET) imaging of Colony Stimulating Factor 1 Receptor (CSF1R) is a new strategy for quantifying both neuroinflammation and inflammation in the periphery since CSF1R is expressed on microglia and macrophages. AZ683 has high affinity for CSF1R (Ki = 8 [...] Read more.
Positron emission tomography (PET) imaging of Colony Stimulating Factor 1 Receptor (CSF1R) is a new strategy for quantifying both neuroinflammation and inflammation in the periphery since CSF1R is expressed on microglia and macrophages. AZ683 has high affinity for CSF1R (Ki = 8 nM; IC50 = 6 nM) and >250-fold selectivity over 95 other kinases. In this paper, we report the radiosynthesis of [11C]AZ683 and initial evaluation of its use in CSF1R PET. [11C]AZ683 was synthesized by 11C-methylation of the desmethyl precursor with [11C]MeOTf in 3.0% non-corrected activity yield (based upon [11C]MeOTf), >99% radiochemical purity and high molar activity. Preliminary PET imaging with [11C]AZ683 revealed low brain uptake in rodents and nonhuman primates, suggesting that imaging neuroinflammation could be challenging but that the radiopharmaceutical could still be useful for peripheral imaging of inflammation. Full article
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Open AccessArticle Evaluation of Radiolabeled Girentuximab In Vitro and In Vivo
Pharmaceuticals 2018, 11(4), 132; https://doi.org/10.3390/ph11040132
Received: 26 October 2018 / Revised: 21 November 2018 / Accepted: 26 November 2018 / Published: 28 November 2018
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Abstract
Girentuximab (cG250) targets carbonic anhydrase IX (CAIX), a protein which is expressed on the surface of most renal cancer cells (RCCs). cG250 labeled with 177Lu has been used in clinical trials for radioimmunotherapy (RIT) of RCCs. In this work, an extensive characterization [...] Read more.
Girentuximab (cG250) targets carbonic anhydrase IX (CAIX), a protein which is expressed on the surface of most renal cancer cells (RCCs). cG250 labeled with 177Lu has been used in clinical trials for radioimmunotherapy (RIT) of RCCs. In this work, an extensive characterization of the immunoconjugates allowed optimization of the labeling conditions with 177Lu while maintaining immunoreactivity of cG250, which was then investigated in in vitro and in vivo experiments. cG250 was conjugated with S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA(SCN)) by using incubation times between 30 and 90 min and characterized by mass spectrometry. Immunoconjugates with five to ten DOTA(SCN) molecules per cG250 molecule were obtained. Conjugates with ratios less than six DOTA(SCN)/cG250 had higher in vitro antigen affinity, both pre- and postlabeling with 177Lu. Radiochemical stability increased, in the presence of sodium ascorbate, which prevents radiolysis. The immunoreactivity of the radiolabeled cG250 tested by specific binding to SK-RC-52 cells decreased when the DOTA content per conjugate increased. The in vivo tumor uptake was < 10% ID/g and independent of the total amount of protein in the range between 5 and 100 µg cG250 per animal. Low tumor uptake was found to be due to significant necrotic areas and heterogeneous CAIX expression. In addition, low vascularity indicated relatively poor accessibility of the CAIX target. Full article
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Open AccessFeature PaperArticle Pretargeted Imaging with Gallium-68—Improving the Binding Capability by Increasing the Number of Tetrazine Motifs
Pharmaceuticals 2018, 11(4), 102; https://doi.org/10.3390/ph11040102
Received: 3 September 2018 / Revised: 8 October 2018 / Accepted: 9 October 2018 / Published: 11 October 2018
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Abstract
The inverse electron-demand Diels-Alder reaction between 1,2,4,5-tetrazine (Tz) and trans-cyclooct-2-ene (TCO) has gained increasing attraction among extensive studies on click chemistry due to its exceptionally fast reaction kinetics and high selectivity for in vivo pretargeting applications including PET imaging. The facile two-step [...] Read more.
The inverse electron-demand Diels-Alder reaction between 1,2,4,5-tetrazine (Tz) and trans-cyclooct-2-ene (TCO) has gained increasing attraction among extensive studies on click chemistry due to its exceptionally fast reaction kinetics and high selectivity for in vivo pretargeting applications including PET imaging. The facile two-step approach utilizing TCO-modified antibodies as targeting structures has not made it into clinics yet. An increase in the blood volume of humans in comparison to mice seems to be the major limitation. This study aims to show if the design of multimeric Tz-ligands by chelator scaffolding can improve the binding capacity and may lead to enhanced PET imaging with gallium-68. We utilized for this purpose the macrocyclic siderophore Fusarinine C (FSC) which allows conjugation of up to three Tz-residues due to three primary amines available for site specific modification. The resulting mono- di- and trimeric conjugates were radiolabelled with gallium-68 and characterized in vitro (logD, protein binding, stability, binding towards TCO modified rituximab (RTX)) and in vivo (biodistribution- and imaging studies in normal BALB/c mice using a simplified RTX-TCO tumour surrogate). The 68Ga-labelled FSC-based Tz-ligands showed suitable hydrophilicity, high stability and high targeting specificity. The binding capacity to RTX-TCO was increased according to the grade of multimerization. Corresponding in vivo studies showed a multimerization typical profile but generally suitable pharmacokinetics with low accumulation in non-targeted tissue. Imaging studies in RTX-TCO tumour surrogate bearing BALB/c mice confirmed this trend and revealed improved targeting by multimerization as increased accumulation in RTX-TCO positive tissue was observed. Full article
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Open AccessArticle Ketamine and Ceftriaxone-Induced Alterations in Glutamate Levels Do Not Impact the Specific Binding of Metabotropic Glutamate Receptor Subtype 5 Radioligand [18F]PSS232 in the Rat Brain
Pharmaceuticals 2018, 11(3), 83; https://doi.org/10.3390/ph11030083
Received: 24 July 2018 / Revised: 23 August 2018 / Accepted: 25 August 2018 / Published: 29 August 2018
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Abstract
Several studies showed that [11C]ABP688 binding is altered following drug-induced perturbation of glutamate levels in brains of humans, non-human primates and rats. We evaluated whether the fluorinated derivative [18F]PSS232 can be used to assess metabotropic glutamate receptor 5 (mGluR5) [...] Read more.
Several studies showed that [11C]ABP688 binding is altered following drug-induced perturbation of glutamate levels in brains of humans, non-human primates and rats. We evaluated whether the fluorinated derivative [18F]PSS232 can be used to assess metabotropic glutamate receptor 5 (mGluR5) availability in rats after pharmacological challenge with ketamine, known to increase glutamate, or ceftriaxone, known to decrease glutamate. In vitro autoradiography was performed on rat brain slices with [18F]PSS232 to prove direct competition of the drugs for mGluR5. One group of rats were challenged with a bolus injection of either vehicle, racemic ketamine, S-ketamine or ceftriaxone followed by positron emission tomography PET imaging with [18F]PSS232. The other group received an infusion of the drugs during the PET scan. Distribution volume ratios (DVRs) were calculated using a reference tissue model. In vitro autoradiography showed no direct competition of the drugs with [18F]PSS232 for the allosteric binding site of mGluR5. DVRs of [18F]PSS232 binding in vivo did not change in any brain region neither after bolus injection nor after infusion. We conclude that [18F]PSS232 has utility for measuring mGluR5 density or occupancy of the allosteric site in vivo, but it cannot be used to measure in vivo fluctuations of glutamate levels in the rat brain. Full article
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Open AccessArticle Design and Synthesis of 99mTcN-Labeled Dextran-Mannose Derivatives for Sentinel Lymph Node Detection
Pharmaceuticals 2018, 11(3), 70; https://doi.org/10.3390/ph11030070
Received: 15 June 2018 / Revised: 11 July 2018 / Accepted: 12 July 2018 / Published: 16 July 2018
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Abstract
Background: New approaches based on the receptor-targeted molecular interaction have been recently developed with the aim to investigate specific probes for sentinel lymph nodes. In particular, the mannose receptors expressed by lymph node macrophages became an attractive target and different multifunctional mannose derivate [...] Read more.
Background: New approaches based on the receptor-targeted molecular interaction have been recently developed with the aim to investigate specific probes for sentinel lymph nodes. In particular, the mannose receptors expressed by lymph node macrophages became an attractive target and different multifunctional mannose derivate ligands for the labeling with 99mTc have been developed. In this study, we report the synthesis of a specific class of dextran-based, macromolecular, multifunctional ligands specially designed for labeling with the highly stable [99mTc≡N]2+ core. Methods: The ligands have been obtained by appending to a macromolecular dextran scaffold pendant arms bearing a chelating moiety for the metallic group and a mannosyl residue for allowing the interaction of the resulting macromolecular 99mTc conjugate with specific receptors on the external membrane of macrophages. Two different chelating systems have been selected, S-methyl dithiocarbazate [H2N‒NH‒C(=S)SCH3=HDTCZ] and a sequence of two cysteine residues, that in combination with a monophosphine coligand, are able to bind the [99mTc≡N]2+ core. Conclusions: High-specific-activity labeling has been obtained by simple mixing and heating of the [99mTc≡N]2+ group with the new mannose-dextran derivatives. Full article
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Open AccessArticle Design, Synthesis, In Vitro, and Initial In Vivo Evaluation of Heterobivalent Peptidic Ligands Targeting Both NPY(Y1)- and GRP-Receptors—An Improvement for Breast Cancer Imaging?
Pharmaceuticals 2018, 11(3), 65; https://doi.org/10.3390/ph11030065
Received: 18 June 2018 / Revised: 29 June 2018 / Accepted: 3 July 2018 / Published: 4 July 2018
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Abstract
Heterobivalent peptidic ligands (HBPLs), designed to address two different receptors independently, are highly promising tumor imaging agents. For example, breast cancer has been shown to concomitantly and complementarily overexpress the neuropeptide Y receptor subtype 1 (NPY(Y1)R) as well as the gastrin-releasing [...] Read more.
Heterobivalent peptidic ligands (HBPLs), designed to address two different receptors independently, are highly promising tumor imaging agents. For example, breast cancer has been shown to concomitantly and complementarily overexpress the neuropeptide Y receptor subtype 1 (NPY(Y1)R) as well as the gastrin-releasing peptide receptor (GRPR). Thus, radiolabeled HBPLs being able to bind these two receptors should exhibit an improved tumor targeting efficiency compared to monospecific ligands. We developed here such bispecific HBPLs and radiolabeled them with 68Ga, achieving high radiochemical yields, purities, and molar activities. We evaluated the HBPLs and their monospecific reference peptides in vitro regarding stability and uptake into different breast cancer cell lines and found that the 68Ga-HBPLs were efficiently taken up via the GRPR. We also performed in vivo PET/CT imaging and ex vivo biodistribution studies in T-47D tumor-bearing mice for the most promising 68Ga-HBPL and compared the results to those obtained for its scrambled analogs. The tumors could easily be visualized by the newly developed 68Ga-HBPL and considerably higher tumor uptakes and tumor-to-background ratios were obtained compared to the scrambled analogs in and ex vivo. These results demonstrate the general feasibility of the approach to use bispecific radioligands for in vivo imaging of breast cancer. Full article
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Review

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Open AccessReview Radioligands for Tropomyosin Receptor Kinase (Trk) Positron Emission Tomography Imaging
Pharmaceuticals 2019, 12(1), 7; https://doi.org/10.3390/ph12010007
Received: 27 November 2018 / Revised: 17 December 2018 / Accepted: 17 December 2018 / Published: 3 January 2019
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Abstract
The tropomyosin receptor kinases family (TrkA, TrkB, and TrkC) supports neuronal growth, survival, and differentiation during development, adult life, and aging. TrkA/B/C downregulation is a prominent hallmark of various neurological disorders including Alzheimer’s disease (AD). Abnormally expressed or overexpressed full-length or oncogenic fusion [...] Read more.
The tropomyosin receptor kinases family (TrkA, TrkB, and TrkC) supports neuronal growth, survival, and differentiation during development, adult life, and aging. TrkA/B/C downregulation is a prominent hallmark of various neurological disorders including Alzheimer’s disease (AD). Abnormally expressed or overexpressed full-length or oncogenic fusion TrkA/B/C proteins were shown to drive tumorigenesis in a variety of neurogenic and non-neurogenic human cancers and are currently the focus of intensive clinical research. Neurologic and oncologic studies of the spatiotemporal alterations in TrkA/B/C expression and density and the determination of target engagement of emerging antineoplastic clinical inhibitors in normal and diseased tissue are crucially needed but have remained largely unexplored due to the lack of suitable non-invasive probes. Here, we review the recent development of carbon-11- and fluorine-18-labeled positron emission tomography (PET) radioligands based on specifically designed small molecule kinase catalytic domain-binding inhibitors of TrkA/B/C. Basic developments in medicinal chemistry, radiolabeling and translational PET imaging in multiple species including humans are highlighted. Full article
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