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Objectives: Somatostatin receptor positron emission tomography/computed tomography (SSTR-PET/CT) using [68Ga]-labeled tracers is a widely used imaging modality for neuroendocrine tumors (NET). Recently, [18F]SiTATE, a SiFAlin tagged [Tyr3]-octreotate (TATE) PET tracer, has shown great potential due to favorable clinical characteristics. We aimed to evaluate the reproducibility of Somatostatin Receptor-Reporting and Data System 1.0 (SSTR-RADS 1.0) for structured interpretation and treatment planning of NET using [18F]SiTATE. Methods: Four readers assessed [18F]SiTATE-PET/CT of 95 patients according to the SSTR-RADS 1.0 criteria at two different time points. Each reader evaluated up to five target lesions per scan. The overall scan score and the decision on peptide receptor radionuclide therapy (PRRT) were considered. Inter- and intra-reader agreement was determined using the intraclass correlation coefficient (ICC). Results: The ICC analysis on the inter-reader agreement using SSTR-RADS 1.0 for identical target lesions (ICC ≥ 85%), overall scan score (ICC ≥ 90%), and the decision to recommend PRRT (ICC ≥ 85%) showed excellent agreement. However, significant differences were observed in recommending PRRT among experienced readers (ER) (p = 0.020) and inexperienced readers (IR) (p = 0.004). Compartment-based analysis demonstrated good to excellent inter-reader agreement for most organs (ICC ≥ 74%), except for lymph nodes (ICC ≥ 53%). Conclusion: SSTR-RADS 1.0 represents a highly reproducible and consistent framework system for stratifying SSTR-targeted PET/CT scans, even using the novel SSTR-ligand [18F]SiTATE. Some inter-reader variability was observed regarding the evaluation of uptake intensity prior to PRRT as well as compartment scoring of lymph nodes, indicating that those categories require special attention during further clinical validation and might be refined in a future SSTR-RADS version 1.1. Clinical relevance statement: SSTR-RADS 1.0 is a consistent framework for categorizing somatostatin receptor-targeted PET/CT scans when using [18F]SiTATE. The framework serves as a valuable tool for facilitating and improving the management of patients with NET. Key Points: SSTR-RADS 1.0 is a valuable tool for managing patients with NET. SSTR-RADS 1.0 categorizes patients with showing strong agreement across diverse reader expertise. As an alternative to [68Ga]-labeled PET/CT in neuroendocrine tumor imaging, SSTR-RADS 1.0 reliably classifies [18F]SiTATE-PET/CT. [ABSTRACT FROM AUTHOR]

The article presents the first-in-human F-SiFAlinTATE PET/CT scan of a 69-year-old male patient with metastatic NET CUP. Highlights include overexpression of somatostatin receptors on the cell surface of NET, active surveillance for newly diagnosed cardiac and bone metastases, and tracer uptake of cardiac metastases and the bone metastasis in the left femur.

Abstract: Recently, silicon fluoride building blocks (SiFA) have emerged as valuable and promising tools to overcome challenges in the labeling of peptides and proteins for positron emission tomography (PET). Herein, we report a fully automated synthesis of N-succinimidyl 3-(di-tert-butyl[18F]fluorosilyl)benzoate ([18F]SiFB) by a commercially available Scintomics Hot Box 3 synthesis module, to be used as a prosthetic group for peptide and protein labeling. The drying of K2.2.2./K 18F complex was performed according to the Munich method modified by our group (avoiding azeotropic drying) using oxalic acid to neutralize the base from the 18F− containing QMA eluent. This K2.2.2./K 18F complex was then used for SiFA 18F–19F isotopic exchange followed by a fast purification by a solid-phase-extraction (SPE) to afford [18F]SiFB with an average preparative radiochemical yield (RCY) of 24±1% (non-decay corrected (NDC)) within a synthesis time of 30min. The [18F]SiFB produced by automated synthesis was then used for the 18F-labeling of rat serum albumin (RSA) as a proof of applicability. [Copyright &y& Elsevier]

Abstract: A 64-year-old woman with meningioma presented with left-sided lenticulostriatal ischemia following craniotomy and debulking of a sphenoid wing meningioma. For subsequent radiotherapy planning, an SSTR-targeted PET/CT using the novel ligand 18 F-SiTATE was performed 2.5 months thereafter. The meningioma remnants showed transosseous, intrasellar, and perivascular extension around the internal carotid artery with strong SSTR expression. Moreover, there was focal 18 F-SiTATE uptake in the left caudate and corresponding contrast enhancement due to postischemic blood-brain barrier disruption and reactive SSTR expression. Therefore, increased cortical or subcortical SSTR PET signal may be related to ischemic changes even in the subacute stage after initial stroke., Competing Interests: Conflicts of interest and sources of funding: none declared., (Copyright © 2024 Wolters Kluwer Health, Inc. All rights reserved.)

Purpose: The novel 18 F-labeled somatostatin receptor (SSTR)-directed radiotracer [ 18 F]SiTATE demonstrated promising results for the imaging of various SSTR-expressing tumor types. Although thyroid carcinomas (TC) express SSTR, data on [ 18 F]SiTATE PET/CT imaging in TC are lacking. This study explores the use of [ 18 F]SiTATE PET/CT in a patient cohort with histologically proven TC., Methods: As part of a prospective observational study at a single tertiary cancer center, 21 patients with TC (10 medullary (MTC) and 11 differentiated (DTC)) who underwent at least one [ 18 F]SiTATE PET/CT were included (37 scans in total). Mean SUV max and SUV mean of tumoral lesions, mean total-tumor-volume (TTV), and whole-body (WB)-SUV max and WB-SUV mean on PET with their standard deviations (SDs) were determined. PET parameters were correlated to clinical parameters including tumor marker levels (thyroglobulin for DTC, calcitonin for MTC)., Results: 89 lesions were included in the analysis. Metastases were localized in the bone, lymph nodes, lung, soft tissue, and thyroid bed. Osseous (31 lesions; SUV max 8.6 ± 8.0; SUV mean 5.8 ± 5.4) and nodal (37 lesions; SUV max 8.7 ± 7.8; SUV mean 5.7 ± 5.4) metastases showed the highest uptake. The MTC disease burden on PET significantly correlated with the calcitonin tumor marker level (e.g., TTV: r = 0.771, r 2  = 0.594, p = 0.002). For DTC, no such correlation was present., Conclusion: Our data demonstrate high feasibility of [ 18 F]SiTATE PET/CT in a small cohort of patients with MTC and DTC. The use of [ 18 F]SiTATE may overcome logistical disadvantages of 68 Ga-based tracers and facilitate SSTR-targeted PET/CT imaging of thyroid carcinoma., (© 2024. The Author(s).)

Background: Affibody molecules represent a class of highly specific binders of particular interest for the development of highly affine target-specific radiopharmaceuticals. Their chemical synthesis is, however, intricate due to their considerable length of 58 amino acids; thus, approaches to optimize their preparation are constantly being sought., Methods: As ultrasound assistance has recently been shown to increase the efficiency of amino acid conjugation during solid-phase peptide synthesis (SPPS), the influence of ultrasonication on the outcome of the SPPS-based preparation of the EGFR-specific affibody Z EGFR:1907 was compared to a common protocol relying on mechanical shaking., Results: After the identification of a suitable solid support for the study, the execution of the systematic comparison of both approaches showed that conventional and ultrasound-assisted syntheses yielded equivalent results with analogous composition of the raw products. Further, both approaches produced the affibody in good isolated yields of >20% when applying the same optimal reagent excesses and coupling times for the conjugation of each amino acid. This indicates that, under optimal reaction conditions, the choice of solid support used has a much stronger influence on the outcome of the preparation of Z EGFR:1907 than the application of ultrasound, which did not further improve the synthesis results., Conclusions: Therefore, for the chemical synthesis of affibodies, great attention should be paid to the choice of a suitable solid support, enabling this highly interesting class of biomolecules to be obtained in good yields and to bring them more into the focus of radiopharmaceutical research.

Purpose: This study compares phantom-based variability of extracted radiomics features from scans on a photon counting CT (PCCT) and an experimental animal PET/CT-scanner (Albira II) to investigate the potential of radiomics for translation from animal models to human scans. While oncological basic research in animal PET/CT has allowed an intrinsic comparison between PET and CT, but no 1:1 translation to a human CT scanner due to resolution and noise limitations, Radiomics as a statistical and thus scale-independent method can potentially close the critical gap., Methods: Two phantoms were scanned on a PCCT and animal PET/CT-scanner with different scan parameters and then the radiomics parameters were extracted. A Principal Component Analysis (PCA) was conducted. To overcome the limitation of a small dataset, a data augmentation technique was applied. A Ridge Classifier was trained and a Feature Importance- and Cluster analysis was performed., Results: PCA and Cluster Analysis shows a clear differentiation between phantom types while emphasizing the comparability of both scanners. The Ridge Classifier exhibited a strong training performance with 93% accuracy, but faced challenges in generalization with a test accuracy of 62%., Conclusion: These results show that radiomics has great potential as a translational tool between animal models and human routine diagnostics, especially using the novel photon counting technique. This is another crucial step towards integration of radiomics analysis into clinical practice., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Vellala, Mogler, Haag, Tollens, Rudolf, Pietsch, Wängler, Wängler, Schoenberg, Froelich and Hertel.)

Introduction: The Trk family proteins are membrane-bound kinases predominantly expressed in neuronal tissues. Activated by neurotrophins, they regulate critical cellular processes through downstream signaling pathways. Dysregulation of Trk signaling can drive a range of diseases, making the design and study of Trk inhibitors a vital area of research. This review explores recent advances in the development of type II and III Trk inhibitors, with implications for various therapeutic applications., Areas Covered: Patents covering type II and III inhibitors targeting the Trk family are discussed as a complement of the previous review, Type I inhibitors of tropomyosin receptor kinase (Trk): a 2020-2022 patent update . Relevant patents were identified using the Web of Science database, Google, and Google Patents., Expert Opinion: While type II and III Trk inhibitor development has advanced more gradually compared to their type I counterparts, they hold significant promise in overcoming resistance mutations and achieving enhanced subtype selectivity - a critical factor in reducing adverse effects associated with pan-Trk inhibition. Recent interdisciplinary endeavors have marked substantial progress in the design of subtype selective Trk inhibitors, with impressive success heralded by the type III inhibitors. Notably, the emergence of mutant-selective Trk inhibitors introduces an intriguing dimension to the field, offering precise treatment possibilities.

Fluorine-18 ( 18 F) is undoubtedly one of the most frequently applied radionuclides for the development of new radiotracers for positron emission tomography (PET) in the context of clinical cancer, neurological, and metabolic imaging. Until recently, the available radiochemical methodologies to introduce 18 F into organic molecules ranging from small- to medium- and large-sized compounds were limited to a few applicable protocols. With the advent of late-stage fluorination of small aromatic, nonactivated compounds and various noncanonical labeling strategies geared toward the labeling of peptides and proteins, the molecular toolbox for PET radiotracer development was substantially extended. Especially, the noncanonical labeling methodologies characterized by the formation of Si- 18 F, B- 18 F, and Al- 18 F bonds give access to kit-like 18 F-labeling of complex and side-group unprotected compounds, some of them already in clinical use. This chapter will particularly focus on silicon-fluoride acceptor (SiFA) chemistry and cover the history of its conceptual design and its translation into the clinical practice., (© 2024. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)

The ring-opening Si-fluorination of a variety of azasilole derivatives cyclo-1-(iPr 2 Si)-4-X-C 6 H 3 -2-CH 2 NR (4: R=2,6-iPr 2 C 6 H 3 , X=H; 4 a: R=2,4,6-Me 3 C 6 H 2 , X=H; 9: R=2,6-iPr 2 C 6 H 3 , X=tBuMe 2 SiO; 10: R=2,6-iPr 2 C 6 H 3 , X=OH; 13: R=2,6-iPr 2 C 6 H 3 , X=HCCCH 2 O; 22: R=2,6-iPr 2 C 6 H 3 , X=tBuMe 2 SiCH 2 O) with different 19 F-fluoride sources was studied, optimized and the experience gained was used in a translational approach to create a straightforward 18 F-labelling protocol for the azasilole derivatives [ 18 F]6 and [ 18 F]14. The latter constitutes a potential clickable CycloSiFA prosthetic group which might be used in PET tracer development using Cu-catalysed triazole formation. Based on our findings, CycloSiFA has the potential to become a new entry into non-canonical labelling methodologies for radioactive PET tracer development., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

This paper reports on the development of stable tumor-specific gold nanoparticles (AuNPs) activated by neutron irradiation as a therapeutic option for the treatment of cancer with high tumor angiogenesis. The AuNPs were designed with different mono- or dithiol-ligands and decorated with different amounts of Arg-Gly-Asp (RGD) peptides as a tumor-targeting vector for α v β 3 integrin, which is overexpressed in tissues with high tumor angiogenesis. The AuNPs were evaluated for avidity in vitro and showed favorable properties with respect to tumor cell accumulation. Furthermore, the therapeutic properties of the [ 198 Au]AuNPs were evaluated in vitro on U87MG cells in terms of cell survival, suggesting that these [ 198 Au]AuNPs are a useful basis for future therapeutic concepts.

Background: Reduced expression or impaired signalling of tropomyosin receptor kinases (Trk receptors) are found in a vast spectrum of CNS disorders. [ 18 F]TRACK is the first PET radioligand for TrkB/C with proven in vivo brain penetration and on-target specific signal. Here we report dosimetry data for [ 18 F]TRACK in healthy humans. 6 healthy participants (age 22-61 y, 3 female) were scanned on a General Electric Discovery PET/CT 690 scanner. [ 18 F]TRACK was synthesized with high molar activities (A m  = 250 ± 75 GBq/µmol), and a dynamic series of 12 whole-body scans were acquired after injection of 129 to 147 MBq of the tracer. Images were reconstructed with standard corrections using the manufacturer's OSEM algorithm. Tracer concentration time-activity curves (TACs) were obtained using CT-derived volumes-of-interest. Organ-specific doses and the total effective dose were estimated using the Committee on Medical Internal Radiation Dose equation for adults and tabulated Source tissue values (S values)., Results: Average organ absorbed dose was highest for liver and gall bladder with 6.1E-2 (± 1.06E-2) mGy/MBq and 4.6 (± 1.18E-2) mGy/MBq, respectively. Total detriment weighted effective dose E DW was 1.63E-2 ± 1.68E-3 mSv/MBq. Organ-specific TACs indicated predominantly hepatic tracer elimination., Conclusion: Total and organ-specific effective doses for [ 18 F]TRACK are low and the dosimetry profile is similar to other 18 F-labelled radio tracers currently used in clinical settings., (© 2023. Springer Nature Switzerland AG.)

Background: The positron emitting isotope fluorine-18 ( 18 F) possesses almost ideal physicochemical properties for the development of radiotracers for diagnostic molecular imaging employing positron emission tomography (PET). 18 F in its nucleophilic anionic 18 F - form is usually prepared by bombarding an enriched 18 O 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 18 F labeling chemistry. The Silicon Fluoride Acceptor chemistry (SiFA) has introduced a convenient and highly efficient way to provide clinical peptide-based 18 F-radiotracers in a kit-like procedure matching the convenience of 99m Tc 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 ( 18 F for 19 F), does not only work in a manual setup but has been proven to be automatable, yielding large batches of 18 F-radiotracers of high molar activity (A m ). 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 [ 18 F]SiTATE and [ 18 F]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., (© 2023. Springer Nature Switzerland AG.)

Background: Somatostatin-receptor (SSTR)-targeted PET/CT provides important clinical information in addition to standard imaging in meningioma patients. [ 18 F]SiTATE is a novel, 18 F-labeled SSTR-targeting peptide with superior imaging properties according to preliminary data. We provide the first [ 18 F]SiTATE PET/CT data of a large cohort of meningioma patients., Methods: Patients with known or suspected meningioma undergoing [ 18 F]SiTATE PET/CT were included. Uptake intensity (SUV) of meningiomas, non-meningioma lesions, and healthy organs were assessed using a 50% isocontour volume of interest (VOI) or a spherical VOI, respectively. Also, trans-osseous extension on PET/CT was assessed., Results: A total of 107 patients with 117 [ 18 F]SiTATE PET/CT scans were included. Overall, 231 meningioma lesions and 61 non-meningioma lesions (e.g., post-therapeutic changes) were analyzed. Physiological uptake was lowest in healthy brain tissue, followed by bone marrow, parotid, and pituitary (SUV mean 0.06 ± 0.04 vs. 1.4 ± 0.9 vs. 1.6 ± 1.0 vs. 9.8 ± 4.6; p < 0.001). Meningiomas showed significantly higher uptake than non-meningioma lesions (SUV max 11.6 ± 10.6 vs. 4.0 ± 3.3, p < 0.001). Meningiomas showed significantly higher uptake than non-meningioma lesions (SUVmax 11.6±10.6 vs. 4.0±3.3, p<0.001). 93/231 (40.3%) meningiomas showed partial trans-osseous extension and 34/231 (14.7%) predominant intra-osseous extension. 59/231 (25.6%) meningioma lesions found on PET/CT had not been reported on previous standard imaging., Conclusion: This is the first PET/CT study using an 18 F-labeled SSTR-ligand in meningioma patients: [ 18 F]SiTATE provides extraordinary contrast in meningioma compared to healthy tissue and non-meningioma lesions, which leads to a high detection rate of so far unknown meningioma sites and osseous involvement. Having in mind the advantageous logistic features of 18 F-labeled compared to 68 Ga-labeled compounds (e.g., longer half-life and large-badge production), [ 18 F]SiTATE has the potential to foster a widespread use of SSTR-targeted imaging in neuro-oncology., (© 2023. The Author(s).)

Introduction: Trk inhibitors are significant in the realm of personalized medicine as they target specific genetic alterations, such as NTRK gene fusions, leading to improved treatment outcomes for cancer patients. By tailoring the treatment to the genetic characteristics of the tumor rather than the tumor type, Trk inhibitors offer the potential for more effective and precise therapies, resulting in enhanced response rates and prolonged survival for patients with NTRK fusion-positive cancers., Areas Covered: Patents covering type I inhibitors targeting the Trk family are discussed, building upon our prior review series on Trk inhibitors. Relevant patents were identified through the Web of Science database, Google, and Google Patents., Expert Opinion: The field of Trk inhibitors has evolved significantly, as reflected in the current patent literature, which emphasizes the selective structural refinement of clinical champions. Efforts now concentrate on enhancing efficacy against on-target resistance mechanisms, with modifications made to improve potency, reduce toxicity, and enhance pharmacokinetics. Combination therapies show potential to address off-target resistance mechanisms and improve treatment outcomes. Challenges remain in accurately diagnosing NTRK gene alterations and integrating screening into routine clinical practice. Trk inhibitors have surpassed their conventional role of inhibition and are now seeing new applications in radiopharmaceutical development and as molecular targeting agents.

The human epidermal growth factor receptor (EGFR) is closely related to several cancer-promoting processes and overexpressed on a variety of tumor types, rendering it an important target structure for the imaging and therapy of several malignancies. To date, approaches to develop peptidic radioligands able to specifically address and visualize EGFR-positive tumors have been of limited success. Most of the attempts were based on the lead GE11, as this peptide was previously described to be a highly potent EGFR-specific agent. However, since it has recently been shown that GE11 exhibits an insufficient affinity to the EGFR in monomeric form to be suitable as a basis for the development of tracers based on it, in the present work we investigated which other peptides might be suitable as lead structures for the development of EGFR-specific peptidic radiotracers. For this purpose, we developed 68 Ga-labeled radioligands based on the peptides D4, P1, P2, CPP, QRH, EGBP and Pep11, having been described before as EGFR-specific. In addition, we also tested three truncated versions of the endogenous EGFR ligand hEGF (human epidermal growth factor) with respect to their ability to specifically target the EGFR with high affinity. Therefore, chelator-modified labeling precursors of the mentioned peptides were synthesized, radiolabeled with 68 Ga and the obtained radioligands were evaluated for their hydrophilicity/lipophilicity, stability against degradation by human serum peptidases, in vitro tumor cell uptake, and receptor affinity in competitive displacement experiments on EGFR-positive A431 cells. Although all NODA-GA-modified (NODA-GA: (1,4,7-triazacyclononane-4,7-diyl)diacetic acid-1-glutaric acid) labeling precursors could be obtained more or less efficient in yields between 5 and 74%, the 68 Ga-radiolabeling proved to be unsuccessful for two of the three truncated versions of hEGF ([ 68 Ga]Ga- 8 and [ 68 Ga]Ga- 9 ), producing several side-products. For the other agents [ 68 Ga]Ga- 1 -[ 68 Ga]Ga- 7 , [ 68 Ga]Ga- 10 and [ 68 Ga]Ga- 11 , high radiochemical yields and purities of ≥98% and molar activities of up to 114 GBq/µmol were obtained. In the assay investigating the radiopeptide susceptibilities against serum peptidase degradation, the EGBP-based agent demonstrated a limited stability with a half-life of only 66.4 ± 3.0 min, whereas the other tracers showed considerably higher stabilities of up to an 8000 min half-life. Finally, all radiotracer candidates were evaluated in terms of tumor cell internalization and receptor binding potential on EGFR-positive A431 cell. In these experiments, all developed agents failed to show an EGFR-specific tumor cell uptake or a relevant EGFR-affinity. By contrast, the positive controls tested under identical conditions, [ 125 I]I-hEGF and hEGF demonstrated the expected high EGFR-specific tumor cell uptake (33.6% after 1 h, being reduced to 1.9% under blocking conditions) and affinity (IC 50 value of 15.2 ± 3.3 nM). Thus, these results indicate that none of the previously described peptidic agents developed for EGFR targeting appears to be a reasonable choice as a lead structure for the development of radiopeptides for targeting of EGFR-positive tumors. Likewise, the tested truncated variants of the endogenous hEGF do not seem to be promising alternatives for this purpose.

Purpose: Somatostatin analogues (SSA) are frequently used in the treatment of neuroendocrine tumours. Recently, [ 18 F]SiTATE entered the field of somatostatin receptor (SSR) positron emission tomography (PET)/computed tomography (CT) imaging. The purpose of this study was to compare the SSR-expression of differentiated gastroentero-pancreatic neuroendocrine tumours (GEP-NET) measured by [18F]SiTATE-PET/CT in patients with and without previous treatment with long-acting SSAs to evaluate if SSA treatment needs to be paused prior to [18F]SiTATE-PET/CT., Methods: 77 patients were examined with standardised [18F]SiTATE-PET/CT within clinical routine: 40 patients with long-acting SSAs up to 28 days prior to PET/CT examination and 37 patients without pre-treatment with SSAs. Maximum and mean standardized uptake values (SUVmax and SUVmean) of tumours and metastases (liver, lymphnode, mesenteric/peritoneal and bones) as well as representative background tissues (liver, spleen, adrenal gland, blood pool, small intestine, lung, bone) were measured, SUV ratios (SUVR) were calculated between tumours/metastases and liver, likewise between tumours/metastases and corresponding specific background, and compared between the two groups., Results: SUVmean of liver (5.4 ± 1.5 vs. 6.8 ± 1.8) and spleen (17.5 ± 6.8 vs. 36.7 ± 10.3) were significantly lower (p < 0.001) and SUVmean of blood pool (1.7 ± 0.6 vs. 1.3 ± 0.3) was significantly higher (p < 0.001) in patients with SSA pre-treatment compared to patients without. No significant differences between tumour-to-liver and specific tumour-to-background SUVRs were observed between both groups (all p > 0.05)., Conclusion: In patients previously treated with SSAs, a significantly lower SSR expression ([18F]SiTATE uptake) in normal liver and spleen tissue was observed, as previously reported for 68Ga-labelled SSAs, without significant reduction of tumour-to-background contrast. Therefore, there is no evidence that SSA treatment needs to be paused prior to [18F]SiTATE-PET/CT., Competing Interests: CA has received research contracts Novartis, lecture honorarium Ipsen, Novartis, Advanced Accelerator Applications and honoraria for advisory boards Advanced Accelerator Applications. HI has received research contracts Novartis. LB received honoraria for advisory boards Bayer, Advanced Accelerator Applications and is a Novartis Radiopharmaceuticals GmbH employee since 10/2022. CW has received honoraria from Amgen, Bayer, Chugai, Celgene, Falk, GSK, MSD, Merck, Janssen, Ipsen, Roche, Servier, SIRTeX, Taiho; served on advisory boards for Bayer, BMS, Celgene, Servier, Shire/Baxalta, Rafael Pharmaceuticals, RedHill, Roche, has received travel support by Bayer, Celgene, RedHill, Roche, Servier, Taiho and research grants institutional by Roche. CW serves as an officer for European Society of Medical Oncology ESMO, Deutsche Krebshilfe DKH, Arbeitsgemeinschaft internistische Onkologie AIO. The remaining authors declared that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Eschbach, Hofmann, Späth, Sheikh, Delker, Lindner, Jurkschat, Wängler, Wängler, Schirrmacher, Tiling, Brendel, Wenter, Dekorsy, Zacherl, Todica, Ilhan, Grawe, Cyran, Unterrainer, Rübenthaler, Knösel, Paul, Boeck, Westphalen, Spitzweg, Auernhammer, Bartenstein, Unterrainer and Beyer.)

Radiolabeled heterobivalent peptidic ligands (HBPLs) are a highly promising compound class for the sensitive and specific visualization of tumors as they often exhibit superior properties compared to their monospecific counterparts and are able to concomitantly or complementarily address different receptor types. The combination of two receptor-specific agents targeting the epidermal growth factor receptor (EGFR) and the integrin α v β 3 in one HBPL would constitute a synergistic combination of binding motifs as these two receptor types are concurrently overexpressed on several human tumor types and are closely associated with disease progression and metastasis. Here, we designed and synthesized two heterobivalent radioligands consisting of the EGFR-specific peptide GE11 and α v β 3 -specific cyclic RGD peptides, bearing a (1,4,7-triazacyclononane-4,7-diyl)diacetic acid-1-glutaric acid chelator for efficient radiolabeling and linkers of different lengths between both peptides. Both HBPLs were radiolabeled with 68 Ga 3+ in high radiochemical yields, purities of 96-99%, and molar activities of 36-88 GBq/μmol. [ 68 Ga]Ga- 1 and [ 68 Ga]Ga- 2 were evaluated for their log D (7.4) and stability toward degradation by human serum peptidases, showing a high hydrophilicity for both agents of -3.07 ± 0.01 and -3.44 ± 0.08 as well as a high stability toward peptidase degradation in human serum with half-lives of 272 and 237 min, respectively. Further on, the in vitro receptor binding profiles of both HBPLs to the target EGF and integrin α v β 3 receptors were assessed on EGFR-positive A431 and α v β 3 -positive U87MG cells. Finally, we investigated the in vivo pharmacokinetics of HBPL [ 68 Ga]Ga- 1 by positron emission tomography/computed tomography imaging in A431 tumor-bearing xenograft mice to assess its potential for the receptor-specific visualization of EGFR- and/or α v β 3 -expressing tumors. In these experiments, [ 68 Ga]Ga- 1 demonstrated a tumor uptake of 2.79 ± 1.66% ID/g, being higher than in all other organs and tissues apart from kidneys and blood at 2 h p.i. Receptor blocking studies revealed the observed tumor uptake to be solely mediated by integrin α v β 3 , whereas no contribution of the GE11 peptide sequence to tumor uptake via the EGFR could be determined. Thus, the approach to develop radiolabeled EGFR- and integrin α v β 3 -bispecific HBPLs is in general feasible although another peptide lead structure than GE11 should be used as the basis for the EGFR-specific part of the agents., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

[ 68 Ga]Ga 3+ can be introduced into receptor-specific peptidic carriers via different chelators to obtain radiotracers for Positron Emission Tomography imaging and the chosen chelating agent considerably influences the in vivo pharmacokinetics of the corresponding radiopeptides. A chelator that should be a valuable alternative to established chelating agents for 68 Ga-radiolabeling of peptides would be a backbone-functionalized variant of the chelator CB-DO2A. Here, the bifunctional cross-bridged chelating agent CB-DO2A-GA was developed and compared to the established chelators DOTA, NODA-GA and DOTA-GA. For this purpose, CB-DO2A-GA(tBu) 2 was introduced into the peptide Tyr 3 -octreotate (TATE) and in direct comparison to the corresponding DOTA-, NODA-GA-, and DOTA-GA-modified TATE analogs, CB-DO2A-GA-TATE required harsher reaction conditions for 68 Ga-incorporation. Regarding the hydrophilicity profile of the resulting radiopeptides, a decrease in hydrophilicity from [ 68 Ga]Ga-DOTA-GA-TATE (log D(7.4) of -4.11±0.11) to [ 68 Ga]Ga-CB-DO2A-GA-TATE (-3.02±0.08) was observed. Assessing the stability against metabolic degradation and complex challenge, [ 68 Ga]Ga-CB-DO2A-GA demonstrated a very high kinetic inertness, exceeding that of [ 68 Ga]Ga-DOTA-GA. Therefore, CB-DO2A-GA is a valuable alternative to established chelating agents for 68 Ga-radiolabeling of peptides, especially when the formation of a very stable, positively charged 68 Ga-complex is pursued., (© 2022 The Authors. ChemMedChem published by Wiley-VCH GmbH.)

89Zr represents a highly favorable positron emitter for application in immuno-PET (Positron Emission Tomography) imaging. Clinically, the 89Zr4+ ion is introduced into antibodies by complexation with desferrioxamine B. However, producing complexes of limited kinetic inertness. Therefore, several new chelators for 89Zr introduction have been developed over the last years. Of these, the direct comparison of the most relevant ones for clinical translation, DFO* and 3,4,3-(LI-1,2-HOPO), is still missing. Thus, we directly compared DFO with DFO* and 3,4,3-(LI-1,2-HOPO) immunoconjugates to identify the most suitable agent stable 89Zr-complexation. The chelators were introduced into cetuximab, and an optical analysis method was developed, enabling the efficient quantification of derivatization sites per protein. The cetuximab conjugates were efficiently obtained and radiolabeled with 89Zr at 37 °C within 30 min, giving the [89Zr]Zr-cetuximab derivatives in high radiochemical yields and purities of >99% as well as specific activities of 50 MBq/mg. The immunoreactive fraction of all 89Zr-labeled cetuximab derivatives was determined to be in the range of 86.5−88.1%. In vivo PET imaging and ex vivo biodistribution studies in tumor-bearing animals revealed a comparable and significantly higher kinetic inertness for both [89Zr]Zr-3,4,3-(LI-1,2-HOPO)-cetuximab and [89Zr]Zr-DFO*-cetuximab, compared to [89Zr]Zr-DFO-cetuximab. Of these, [89Zr]Zr-DFO*-cetuximab showed a considerably more favorable pharmacokinetic profile with significantly lower liver and spleen retention than [89Zr]Zr-3,4,3-(LI-1,2-HOPO)-cetuximab. Since [89Zr]Zr-DFO* demonstrates a very high kinetic inertness, paired with a highly favorable pharmacokinetic profile of the resulting antibody conjugate, DFO* currently represents the most suitable chelator candidate for stable 89Zr-radiolabeling of antibodies and clinical translation.

The epidermal growth factor receptor (EGFR) is closely associated with tumor development and progression and thus an important target structure for imaging and therapy of various tumors. As a result of its important role in malignancies of various origins and the fact that antibody-based compounds targeting the EGFR have significant drawbacks in terms of in vivo pharmacokinetics, several attempts have been made within the last five years to develop peptide-based EGFR-specific radioligands based on the GE11 scaffold. However, none of these approaches have shown convincing results so far, which has been proposed to be attributed to different potential challenges associated with the GE11 lead structure: first, an aggregation of radiolabeled peptides, which might prevent their interaction with their target receptor, or second, a relatively low affinity of monomeric GE11, necessitating its conversion into a multimeric or polymeric form to achieve adequate EGFR-targeting properties. In the present work, we investigated if these aforementioned points are indeed critical and if the EGFR-targeting ability of GE11 can be improved by choosing an appropriate hydrophilic molecular design or a peptide multimer system to obtain a promising radiopeptide for the visualization of EGFR-overexpressing malignancies by positron emission tomography (PET). For this purpose, we developed several monovalent 68 Ga-labeled GE11-based agents, a peptide homodimer and a homotetramer to overcome the challenges associated with GE11. The developed ligands were successfully labeled with 68 Ga 3+ in high radiochemical yields of ≥97% and molar activities of 41-104 GBq/μmol. The resulting radiotracers presented log D (7.4) values between -2.17 ± 0.21 and -3.79 ± 0.04 as well as a good stability in human serum with serum half-lives of 112 to 217 min for the monovalent radiopeptides and 84 and 62 min for the GE11 homodimer and homotetramer, respectively. In the following in vitro studies, none of the 68 Ga-labeled radiopeptides demonstrated a considerable EGF receptor-specific uptake in EGFR-positive A431 cells. Moreover, none of the agents was able to displace [ 125 I]I-EGF from the EGFR in competitive displacement assays in the same cell line in concentrations of up to 1 mM, whereas the endogenous receptor ligand hEGF demonstrated a high affinity of 15.2 ± 3.3 nM. These results indicate that it is not the aggregation of the GE11 sequence that seems to be the factor limiting the usefulness of the peptide as basis for radiotracer design but the limited affinity of monovalent and small homomultivalent GE11-based radiotracers to the EGFR. This highlights that the development of small-molecule GE11-based radioligands is not promising., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

In the present study we describe and explain an aberrant behavior in terms of receptor binding profile of a fluorescein-based multimodal imaging agent for gastrin releasing peptide receptor (GRPR) visualization by elucidating a chelating mechanism on sodium ions of its fluorescent dye moiety. This hypothesis is supported by both biological results and spectroscopic analyses of different fluorescein-carrying conjugates and an equally charged set of analogous tartrazine-based GRPR-binding imaging agents. Fluorescein interacts with sodium which reduces the overall negative charge of the dye molecule by one. This reduction in apparent total net charge explains the exceptional behavior found for the fluorescein-based multimodal bioconjugate in the context of the charge-cell binding correlation hypothesis., (© 2022 The Authors. ChemMedChem published by Wiley-VCH GmbH.)

In this work, five different chelating agents, namely DFO, CTH-36, DFO*, 3,4,3-(LI-1,2-HOPO) and DOTA-GA, were compared with regard to the relative kinetic inertness of their corresponding 89 Zr complexes to evaluate their potential for in vivo application and stable 89 Zr complexation. The chelators were identically functionalized with tetrazines, enabling a fully comparable, efficient, chemoselective and biorthogonal conjugation chemistry for the modification of any complementarily derivatized biomolecules of interest. A small model peptide of clinical relevance (TCO-c(RGDfK)) was derivatized via iEDDA click reaction with the developed chelating agents (TCO = trans-cyclooctene and iEDDA = inverse electron demand Diels-Alder). The bioconjugates were labeled with 89 Zr 4+ , and their radiochemical properties (labeling conditions and efficiency), log D , as well as the relative kinetic inertness of the formed complexes, were compared. Furthermore, density functional theory (DFT) calculations were conducted to identify potential influences of chelator modification on complex formation and geometry. The results of the DFT studies showed-apart from the DOTA-GA derivative-no significant influence of chelator backbone functionalization or the conjugation of the chelator tetrazines by iEDDA. All tetrazines could be efficiently introduced into c(RGDfK), demonstrating the high suitability of the agents for efficient and chemoselective bioconjugation. The DFO-, CTH-36- and DFO*-modified c(RGDfK) peptides showed a high radiolabeling efficiency under mild reaction conditions and complete (7.4) , as well as the relative kinetic inertness of the formed complexes, were compared. Furthermore, density functional theory (DFT) calculations were conducted to identify potential influences of chelator modification on complex formation and geometry. The results of the DFT studies showed-apart from the DOTA-GA derivative-no significant influence of chelator backbone functionalization or the conjugation of the chelator tetrazines by iEDDA. All tetrazines could be efficiently introduced into c(RGDfK), demonstrating the high suitability of the agents for efficient and chemoselective bioconjugation. The DFO-, CTH-36- and DFO*-modified c(RGDfK) peptides showed a high radiolabeling efficiency under mild reaction conditions and complete 89 Zr-labeled analogs as homogenous products. In contrast, 3,4,3-(LI-1,2-HOPO)-c(RGDfK) required considerably prolonged reaction times of 5 h for complete radiometal incorporation and yielded several different 89 Zr-labeled analogs as homogenous products. In contrast, 3,4,3-(LI-1,2-HOPO)-c(RGDfK) required considerably prolonged reaction times of 5 h for complete radiometal incorporation and yielded several different 89 Zr-labeled species. The labeling of the DOTA-GA-modified peptide was not successful at all. Compared to [ 89 Zr]Zr-DFO-, [ 89 Zr]Zr-DFO*-c(RGDfK), the corresponding [ 89 Zr]Zr-DFO*-c(RGDfK), the corresponding [ 89 Zr]Zr-3,4,3-(LI-1,2-HOPO) peptide showed a strongly increased lipophilicity. Finally, the relative stability of the 89 Zr complexes against the EDTA challenge was investigated. The [ 89 Zr]Zr-DFO complex showed-as expected-a low kinetic inertness. Unexpectedly, also, the [ 89 Zr]Zr-CTH-36 complex demonstrated a high susceptibility against the challenge, limiting the usefulness of CTH-36 for stable 89 Zr complexation. Only the [ 89 Zr]Zr-DFO* and the [ 89 Zr]Zr-3,4,3-(LI-1,2-HOPO) complexes demonstrated a high inertness, qualifying them for further comparative in vivo investigation to determine the most appropriate alternative to DFO for clinical application.

Background: The chemokine receptor CXCR4 and its ligand CXCL12 have been shown to be a possible imaging and therapeutic target after myocardial infarction (MI). The murine-based and mouse-specific 68 Ga-mCXCL12 PET tracer could be suitable for serial in vivo quantification of cardiac CXCR4 expression in a murine model of MI., Methods and Results: At days 1-6 after MI, mice were intravenously injected with 68 Ga-mCXCL12. Autoradiography was performed and the infarct-to-remote ratio (I/R) was determined. In vivo PET imaging with 68 Ga-mCXCL12 was conducted on days 1-6 after MI and the percentage of the injected dose (%ID/g) of the tracer uptake in the infarct area was calculated. 18 F-FDG-PET was performed for anatomical landmarking. Ex vivo autoradiography identified CXCR4 upregulation in the infarct region with an increasing I/R after 12 hours (1.4 ± 0.3), showing a significant increase until day 2 (4.5 ± 0.6), followed by a plateau phase (day 4) and decrease after 10 days (1.3 ± 1.0). In vivo PET imaging identified similar CXCR4 upregulation in the infarct region which peaked around day 3 post MI (9.7 ± 5.0 %ID/g) and then subsequently decreased by day 6 (2.8 ± 1.0 %ID/g)., Conclusion: Noninvasive molecular imaging of cardiac CXCR4 expression using a novel, murine-based, and specific 68 Ga-mCXCL12 tracer is feasible both ex vivo and in vivo., (© 2020. The Author(s).)

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 ( 68 Ga) and fluorine-18 ( 18 F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[ 18 F]fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a "kit-like" multidose synthesis technology. Nine novel 18 F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [ 125 I]I-TAAG-PSMA in LNCaP cells. IC 50 values ranged from 58-570 nM. Among all compounds, [ 18 F]SiFA-Asp 2 -PEG 3 -PSMA (IC 50 = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV 60min 0.73). A substantial increase in molar activity ( A m ) (from 7.5 ± 0.5 to 86 ± 3 GBq/μmol) led to a significant increase in LNCaP tumor uptake (SUV 60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in -32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a "kit-type" labeling procedure and clinical translation.

Purpose: Radiolabelled somatostatin analogues targeting somatostatin receptors (SSR) are well established for combined positron emission tomography/computer tomography (PET/CT) imaging of neuroendocrine tumours (NET). [ 18 F]SiTATE has recently been introduced showing high image quality, promising clinical performance and improved logistics compared to the clinical reference standard 68 Ga-DOTA-TOC. Here we present the first dosimetry and optimal scan time analysis., Methods: Eight NET patients received a [ 18 F]SiTATE-PET/CT (250 ± 66 MBq) with repeated emission scans (10, 30, 60, 120, 180 min after injection). Biodistribution in normal organs and SSR-positive tumour uptake were assessed. Dosimetry estimates for risk organs were determined using a combined linear-monoexponential model, and by applying 18 F S-values and reference target masses for the ICRP89 adult male or female (OLINDA 2.0). Tumour-to-background ratios were compared quantitatively and visually between different scan times., Results: After 1 h, normal organs showed similar tracer uptake with only negligible changes until 3 h post-injection. In contrast, tracer uptake by tumours increased progressively for almost all types of metastases, thus increasing tumour-to-background ratios over time. Dosimetry resulted in a total effective dose of 0.015 ± 0.004 mSv/MBq. Visual evaluation revealed no clinically relevant discrepancies between later scan times, but image quality was rated highest in 60 and 120 min images., Conclusion: [ 18 F]SiTATE-PET/CT in NET shows overall high tumour-to-background ratios from 60 to 180 min after injection and an effective dose comparable to 68 Ga-labelled alternatives. For clinical use of [ 18 F]SiTATE, the best compromise between image quality and tumour-to-background contrast is reached at 120 min, followed by 60 min after injection., (© 2021. The Author(s).)