Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

Solid-phase supported direct 18F-radiofluorination of peptides

The absence of universal and expedient labeling procedures hampers the widespread application of peptides as molecular tracers for positron emission tomography (PET). In this work, we have developed a proof-of-concept direct radiofluorination procedure for peptides using conventional solid-phase peptide synthesis. The method is compatible with all standard amino acids and enables the generation of a range of peptide-based radiopharmaceuticals archieving radiochemical yields of up to 30 % and radiochemical purities above 95 %.

Tackling Prostate Cancer with Theranostic E5B9-Bombesin Target Modules (TMs): From Imaging to Treatment with UniCAR T-Cells

Target modules (TMs), intermediate molecules required for UniCAR T-cell therapy, are promising molecules for immunotheranostic approaches. In the current work, we developed TMs containing a monomeric or dimeric form of the antagonist bombesin peptide (BBN2) and assessed their potential for diagnostic imaging using positron emission tomography (PET) as well as immunotherapy in combination with UniCAR T-cells to target and image GRPR expression in prostate cancer. Synthesized monomeric and dimeric BBN2 TMs retained binding to GRPR in vitro. Both BBN2 TMs specifically activated and redirected UniCAR T-cells to eradicate PC3 and LNCaP cancer cells with high efficiency and in a comparable manner. UniCAR T-cells retained a non-exhausted memory phenotype favorable to their persistence and fitness. The 68Ga-labeled BBN2 TMs showed proof-of-target towards GRPR in PC3 and LNCaP xenografts with similar uptake profiles for both BBN2 TMs in dynamic PET experiments. Clearance occurred exclusively through renal elimination. A tremendously increased in vivo metabolic stability of the BBN2 TMs was observed compared to their counterparts without E5B9. Both monomeric and dimeric BBN2 TMs represent novel and promising immunotheranostic tools for application in prostate cancer with exceptionally high in vivo metabolic stability.

Imaging of Peripheral Intraneural Tumors: A Comprehensive Review for Radiologists

BACKGROUND/OBJECTIVES

Intraneural tumors (INTs) pose a diagnostic challenge, owing to their varied origins within nerve fascicles and their wide spectrum, which includes both benign and malignant forms. Accurate diagnosis and management of these tumors depends upon the skills of the radiologist in identifying key imaging features and correlating them with the patient's clinical symptoms and examination findings.

METHODS

This comprehensive review systematically analyzes the various imaging features in the diagnosis of intraneural tumors, ranging from basic MR to advanced MR imaging techniques such as MR neurography (MRN), diffusion tensor imaging (DTI), and dynamic contrast-enhanced (DCE) MRI.

RESULTS

The article emphasizes the differentiation of benign from malignant lesions using characteristic MRI features, such as the "target sign" and "split-fat sign" for tumor characterization. The role of advanced multiparametric MRI in improving biopsy planning, guiding surgical mapping, and enhancing post-treatment monitoring is also highlighted. The review also underlines the importance of common diagnostic pitfalls and highlights the need for a multi-disciplinary approach to achieve an accurate diagnosis, appropriate treatment strategy, and post-therapy surveillance planning.

CONCLUSIONS

In this review, we illustrate the main imaging findings of intraneural tumors, focusing on specific MR imaging features that are crucial for an accurate diagnosis and the differentiation between benign and malignant lesions.

Ultrasound-Assisted Solid-Phase Affibody Synthesis Using ZEGFR:1907 as an Example-Superior to the Conventional Protocol?

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 ZEGFR: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 ZEGFR: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.

Hetero-Diels-Alder and CuAAC Click Reactions for Fluorine-18 Labeling of Peptides: Automation and Comparative Study of the Two Methods

Radiolabeled peptides are valuable tools for diagnosis or therapies; they are often radiofluorinated using an indirect approach based on an F-18 prosthetic group. Herein, we are reporting our results on the F-18 radiolabeling of three peptides using two different methods based on click reactions. The first one used the well-known CuAAC reaction, and the second one is based on our recently reported hetero-Diels-Alder (HDA) using a dithioesters (thia-Diels-Alder) reaction. Both methods have been automated, and the 18F-peptides were obtained in similar yields and synthesis time (37-39% decay corrected yields by both methods in 120-140 min). However, to obtain similar yields, the CuAAC needs a large amount of copper along with many additives, while the HDA is a catalyst and metal-free reaction necessitating only an appropriate ratio of water/ethanol. The HDA can therefore be considered as a minimalist method offering easy access to fluorine-18 labeled peptides and making it a valuable additional tool for the indirect and site-specific labeling of peptides or biomolecules.

Navigating the landscape of PD-1/PD-L1 imaging tracers: from challenges to opportunities

Immunotherapy targeted to immune checkpoint inhibitors, such as the program cell death receptor (PD-1) and its ligand (PD-L1), has revolutionized cancer treatment. However, it is now well-known that PD-1/PD-L1 immunotherapy response is inconsistent among patients. The current challenge is to customize treatment regimens per patient, which could be possible if the PD-1/PD-L1 expression and dynamic landscape are known. With positron emission tomography (PET) imaging, it is possible to image these immune targets non-invasively and system-wide during therapy. A successful PET imaging tracer should meet specific criteria concerning target affinity, specificity, clearance rate and target-specific uptake, to name a few. The structural profile of such a tracer will define its properties and can be used to optimize tracers in development and design new ones. Currently, a range of PD-1/PD-L1-targeting PET tracers are available from different molecular categories that have shown impressive preclinical and clinical results, each with its own advantages and disadvantages. This review will provide an overview of current PET tracers targeting the PD-1/PD-L1 axis. Antibody, peptide, and antibody fragment tracers will be discussed with respect to their molecular characteristics and binding properties and ways to optimize them.

Next generation radiotheranostics promoting precision medicine

Radiotheranostics is a field of rapid growth with some approved treatments including 131I for thyroid cancer, 223Ra for osseous metastases, 177Lu-DOTATATE for neuroendocrine tumors, and 177Lu-PSMA (prostate-specific membrane antigen) for prostate cancer, and several more under investigation. In this review, we will cover the fundamentals of radiotheranostics, the key clinical studies that have led to current success, future developments with new targets, radionuclides and platforms, challenges with logistics and reimbursement and, lastly, forthcoming considerations regarding dosimetry, identifying the right line of therapy, artificial intelligence and more.

Preclinical Evaluation of 68Ga- and 177Lu-Labeled Integrin αvβ6-Targeting Radiotheranostic Peptides

The integrin αvβ6, an epithelium-specific cell surface receptor, is overexpressed on numerous malignancies, including the highly lethal pancreatic ductal adenocarcinomas. Here, we developed and tested a novel αvβ6-targeting peptide, DOTA-5G (1) radiolabeled with 68Ga, for PET/CT imaging and 177Lu for treatment. With the goal to develop a radiotheranostic, further modifications were made for increased circulation time, renal recycling, and tumor uptake, yielding DOTA-albumin-binding moiety-5G (2). Methods: Peptides 1 and 2 were synthesized on solid phase, and their affinity for αvβ6 was assessed by enzyme-linked immunosorbent assay. The peptides were radiolabeled with 68Ga and 177Lu. In vitro cell binding, internalization, and efflux of 68Ga-1 and 177Lu-2 were evaluated in αvβ6-positive BxPC-3 human pancreatic cancer cells. PET/CT imaging of 68Ga-1 and 68Ga-2 was performed on female nu/nu mice bearing subcutaneous BxPC-3 tumors. Biodistribution was performed for 68Ga-1 (1 and 2 h after injection), 68Ga-2 (2 and 4 h after injection), and 177Lu-1 and 177Lu-2 (1, 24, 48, and 72 h after injection). The 177Lu-2 biodistribution data were extrapolated for human dosimetry data estimates using OLINDA/EXM 1.1. Therapeutic efficacy of 177Lu-2 was evaluated in mice bearing BxPC-3 tumors. Results: Peptides 1 and 2 demonstrated high affinity (<55 nM) for αvβ6 by enzyme-linked immunosorbent assay. 68Ga-1, 68Ga-2, 177Lu-1, and 177Lu-2 were synthesized in high radiochemical purity. Rapid in vitro binding and internalization of 68Ga-1 and 177Lu-2 were observed in BxPC-3 cells. PET/CT imaging and biodistribution studies demonstrated uptake in BxPC-3 tumors. Introduction of the albumin-binding moiety in 177Lu-2 resulted in a 5-fold increase in tumor uptake and retention over time. Based on the extended dosimetry data, the dose-limiting organ for 177Lu-2 is the kidney. Treatment with 177Lu-2 prolonged median survival by 1.5- to 2-fold versus controls. Conclusion: 68Ga-1 and 177Lu-2 demonstrated high affinity for the integrin αvβ6 both in vitro and in vivo, were rapidly internalized into BxPC-3 cells, and were stable in mouse and human serum. Both radiotracers showed favorable pharmacokinetics in preclinical studies, with predominantly renal excretion and good tumor-to-normal-tissue ratios. Favorable human dosimetry data suggest the potential of 177Lu-2 as a treatment for pancreatic ductal adenocarcinoma.

Potential of Nuclear Imaging Techniques to Study the Oral Delivery of Peptides

Peptides are small biomolecules known to stimulate or inhibit important functions in the human body. The clinical use of peptides by oral delivery, however, is very limited due to their sensitive structure and physiological barriers present in the gastrointestinal tract. These barriers can be overcome with chemical and mechanical approaches protease inhibitors, permeation enhancers, and polymeric encapsulation. Studying the success of these approaches pre-clinically with imaging techniques such as fluorescence imaging (IVIS) and optical microscopy is difficult due to the lack of in-depth penetration. In comparison, nuclear imaging provides a better platform to observe the gastrointestinal transit and quantitative distribution of radiolabeled peptides. This review provides a brief background on the oral delivery of peptides and states examples from the literature on how nuclear imaging can help to observe and analyze the gastrointestinal transit of oral peptides. The review connects the fields of peptide delivery and nuclear medicine in an interdisciplinary way to potentially overcome the challenges faced during the study of oral peptide formulations.

CAQK, a peptide associating with extracellular matrix components targets sites of demyelinating injuries

The destruction of the myelin sheath that encircles axons leads to impairments of nerve conduction and neuronal dysfunctions. A major demyelinating disorder is multiple sclerosis (MS), a progressively disabling disease in which immune cells attack the myelin. To date, there are no therapies to target selectively myelin lesions, repair the myelin or stop MS progression. Small peptides recognizing epitopes selectively exposed at sites of injury show promise for targeting therapeutics in various pathologies. Here we show the selective homing of the four amino acid peptide, cysteine-alanine-lysine glutamine (CAQK), to sites of demyelinating injuries in three different mouse models. Homing was assessed by administering fluorescein amine (FAM)-labeled peptides into the bloodstream of mice and analyzing sites of demyelination in comparison with healthy brain or spinal cord tissue. FAM-CAQK selectively targeted demyelinating areas in all three models and was absent from healthy tissue. At lesion sites, the peptide was primarily associated with the fibrous extracellular matrix (ECM) deposited in interstitial spaces proximal to reactive astrocytes. Association of FAM-CAQK was detected with tenascin-C although tenascin depositions made up only a minor portion of the examined lesion sites. In mice on a 6-week cuprizone diet, FAM-CAQK peptide crossed the nearly intact blood-brain barrier and homed to demyelinating fiber tracts. These results demonstrate the selective targeting of CAQK to demyelinating injuries under multiple conditions and confirm the previously reported association with the ECM. This work sets the stage for further developing CAQK peptide targeting for diagnostic and therapeutic applications aimed at localized myelin repair.

Radiolabeled Nanocarriers as Theranostics-Advancement from Peptides to Nanocarriers

Endogenous targeted radiotherapy is emerging as an integral modality to treat a variety of cancer entities. Nevertheless, despite the positive clinical outcome of the treatment using radiolabeled peptides, small molecules, antibodies, and nanobodies, a high degree of hepatotoxicity and nephrotoxicity still persist. This limits the amount of dose that can be injected. In an attempt to mitigate these side effects, the use of nanocarriers such as nanoparticles (NPs), dendrimers, micelles, liposomes, and nanogels (NGs) is currently being explored. Nanocarriers can prolong circulation time and tumor retention, maximize radiation dosage, and offer multifunctionality for different targeting strategies. In this review, the authors first provide a summary of radiation therapy and imaging and discuss the new radiotracers that are used preclinically and clinically. They then highlight and identify the advantages of radio-nanomedicine and its potential in overcoming the limitations of endogenous radiotherapy. Finally, the review points to the ongoing efforts to maximize the use of radio-nanomedicine for efficient clinical translation.

Radiopharmaceuticals for PET and SPECT Imaging: A Literature Review over the Last Decade

Positron emission tomography (PET) uses radioactive tracers and enables the functional imaging of several metabolic processes, blood flow measurements, regional chemical composition, and/or chemical absorption. Depending on the targeted processes within the living organism, different tracers are used for various medical conditions, such as cancer, particular brain pathologies, cardiac events, and bone lesions, where the most commonly used tracers are radiolabeled with 18F (e.g., [18F]-FDG and NA [18F]). Oxygen-15 isotope is mostly involved in blood flow measurements, whereas a wide array of 11C-based compounds have also been developed for neuronal disorders according to the affected neuroreceptors, prostate cancer, and lung carcinomas. In contrast, the single-photon emission computed tomography (SPECT) technique uses gamma-emitting radioisotopes and can be used to diagnose strokes, seizures, bone illnesses, and infections by gauging the blood flow and radio distribution within tissues and organs. The radioisotopes typically used in SPECT imaging are iodine-123, technetium-99m, xenon-133, thallium-201, and indium-111. This systematic review article aims to clarify and disseminate the available scientific literature focused on PET/SPECT radiotracers and to provide an overview of the conducted research within the past decade, with an additional focus on the novel radiopharmaceuticals developed for medical imaging.

Synthesis, proteolytic stability, and in vitro evaluation of DOTA conjugated p160 peptide based radioconjugates: [177Lu]Lu-DOTA-p160

In this work, we describe the synthesis, in vitro stability, and preliminary biological evaluation of [¹⁷⁷Lu]Lu-DOTA-p160 peptide-based radiopharmaceuticals. Our findings highlight that all DOTA-p160-peptide radioconjugates exhibit favorable proteolytic and enzymatic stability with a prolonged half-life in human plasma and serum. Cell uptake studies carried out on MCF-7 cell line revealed saturable binding of the radioconjugates in the nanomolar range, thereby demonstrating their promising potential in the imaging and therapy of breast tumors.

Radioiodine Labeling Reagents and Methods for New Chemical Entities and Biomolecules

Several radioisotopes of iodine (¹²³I, ¹²⁴I, ¹²⁵I, and ¹³¹I) are available for medical use. One of them can be used, depending on the application, for radioiodine labeling of New Chemical Entities (NCEs) and biomolecules (peptides, proteins, protein fragments, monoclonal antibodies, etc.) for the development of novel imaging and therapeutic pharmaceuticals. Direct, using inorganic and organic oxidizing agents and enzyme catalysts, and indirect, using prosthetic groups, radioiodine-labeling methods have been used routinely in the past. In this report, a comprehensive review of the physical properties of various iodine radionuclides, their medical applications, and a summary of various radioiodine labeling reagents and methods for NCEs and biomolecules are provided.

Desferrioxamine B-Mediated Pre-Clinical In Vivo Imaging of Infection by the Mold Fungus Aspergillus fumigatus

Fungal infections are a serious threat, especially for immunocompromised patients. Early and reliable diagnosis is crucial to treat such infections. The bacterially produced siderophore desferrioxamine B (DFO-B) is utilized by a variety of microorganisms for iron acquisition, while mammalian cells lack the uptake of DFO-B chelates. DFO-B is clinically approved for a variety of long-term chelation therapies. Recently, DFO-B-complexed gallium-68 ([68Ga]Ga-DFO-B) was shown to enable molecular imaging of bacterial infections by positron emission tomography (PET). Here, we demonstrate that [68Ga]Ga-DFO-B can also be used for the preclinical molecular imaging of pulmonary infection caused by the fungal pathogen Aspergillus fumigatus in a rat aspergillosis model. Moreover, by combining in vitro uptake studies and the chemical modification of DFO-B, we show that the cellular transport efficacy of ferrioxamine-type siderophores is impacted by the charge of the molecule and, consequently, the environmental pH. The chemical derivatization has potential implications for its diagnostic use and characterizes transport features of ferrioxamine-type siderophores.

PET Imaging Radiotracers of Chemokine Receptors

Chemokines and chemokine receptors have been recognized as critical signal components that maintain the physiological functions of various cells, particularly the immune cells. The signals of chemokines/chemokine receptors guide various leukocytes to respond to inflammatory reactions and infectious agents. Many chemokine receptors play supportive roles in the differentiation, proliferation, angiogenesis, and metastasis of diverse tumor cells. In addition, the signaling functions of a few chemokine receptors are associated with cardiac, pulmonary, and brain disorders. Over the years, numerous promising molecules ranging from small molecules to short peptides and antibodies have been developed to study the role of chemokine receptors in healthy states and diseased states. These drug-like candidates are in turn exploited as radiolabeled probes for the imaging of chemokine receptors using noninvasive in vivo imaging, such as positron emission tomography (PET). Recent advances in the development of radiotracers for various chemokine receptors, particularly of CXCR4, CCR2, and CCR5, shed new light on chemokine-related cancer and cardiovascular research and the subsequent drug development. Here, we present the recent progress in PET radiotracer development for imaging of various chemokine receptors.

[68Ga]Ga-DFO-c(RGDyK): Synthesis and Evaluation of Its Potential for Tumor Imaging in Mice

Angiogenesis has a pivotal role in tumor growth and the metastatic process. Molecular imaging was shown to be useful for imaging of tumor-induced angiogenesis. A great variety of radiolabeled peptides have been developed to target αvβ3 integrin, a target structure involved in the tumor-induced angiogenic process. The presented study aimed to synthesize deferoxamine (DFO)-based c(RGD) peptide conjugate for radiolabeling with gallium-68 and perform its basic preclinical characterization including testing of its tumor-imaging potential. DFO-c(RGDyK) was labeled with gallium-68 with high radiochemical purity. In vitro characterization including stability, partition coefficient, protein binding determination, tumor cell uptake assays, and ex vivo biodistribution as well as PET/CT imaging was performed. [⁶⁸Ga]Ga-DFO-c(RGDyK) showed hydrophilic properties, high stability in PBS and human serum, and specific uptake in U-87 MG and M21 tumor cell lines in vitro and in vivo. We have shown here that [⁶⁸Ga]Ga-DFO-c(RGDyK) can be used for αvβ3 integrin targeting, allowing imaging of tumor-induced angiogenesis by positron emission tomography.

Natural product-based Radiopharmaceuticals:Focus on curcumin and its analogs, flavonoids, and marine peptides

Natural products provide a bountiful supply of pharmacologically relevant precursors for the development of various drug-related molecules, including radiopharmaceuticals. However, current knowledge regarding the importance of natural products in developing new radiopharmaceuticals remains limited. To date, several radionuclides, including gallium-68, technetium-99m, fluorine-18, iodine-131, and iodine-125, have been extensively studied for the synthesis of diagnostic and therapeutic radiopharmaceuticals. The availability of various radiolabeling methods allows the incorporation of these radionuclides into bioactive molecules in a practical and efficient manner. Of the radiolabeling methods, direct radioiodination, radiometal complexation, and halogenation are generally suitable for natural products owing to their simplicity and robustness. This review highlights the pharmacological benefits of curcumin and its analogs, flavonoids, and marine peptides in treating human pathologies and provides a perspective on the potential use of these bioactive compounds as molecular templates for the design and development of new radiopharmaceuticals. Additionally, this review provides insights into the current strategies for labeling natural products with various radionuclides using either direct or indirect methods.

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Potential use of natural products for the development of diagnostic and therapeutic radiopharmaceuticals.

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Profile of potential natural products as molecular templates for the synthesis of new radiopharmaceuticals: Focus on curcumin and its closely related substances, flavonoids, and marine peptides.

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Radiolabeling strategies, challenges, and examples of natural product-based radiopharmaceuticals under investigation.

Pharmacological activities and potential use of bovine colostrum for peptide-based radiopharmaceuticals: A review

Bovine colostrum (BC) is the initial milk produced by cows after giving birth. It has been used to treat human diseases, such as infections, inflammations, and cancers. Accumulating evidence suggests that bovine lactoferrin and bovine antibodies seem to be the most important bioactive constituents in BC. Thus, BC has also been reviewed for its potential to deliver short-term protection against coronavirus disease 2019 (COVID-19). In addition, it can potentially be explored as a precursor for peptide-based radiopharmaceuticals. To date, several bioactive peptides have been isolated from BC, including casocidin-1, casecidin 15 and 17, isracidin, caseicin A, B, and C. Like other peptides, bioactive peptides derived from BC could be used as a valuable precursor for radiopharmaceuticals either for diagnosis or therapy purposes. This review provides bovine colostrum’s biological activities and a perspective on the potential use of peptides from BC for developing radiopharmaceuticals in nuclear medicine.

Radiolabelling an 18F biologic via facile IEDDA "click" chemistry on the GE FASTLab™ platform

The use of biologics in positron emission tomography (PET) imaging is an important area of radiopharmaceutical development and new automated methods are required to facilitate their production. We report an automated radiosynthesis method to produce a radiolabelled biologic via facile inverse electron demand Diels-Alder (IEDDA) "click" chemistry on a single GE FASTLab™ cassette. We exemplified the method by producing a fluorine-18 radiolabelled interleukin-2 (IL2) radioconjugate from a trans-cyclooctene (TCO) modified IL2 precursor. The radioconjugate was produced using a fully automated radiosynthesis on a single FASTLab™ cassette in a decay-corrected radiochemical yield (RCY, d.c.) of 19.8 ± 2.6% in 110 min (from start of synthesis); the molar activity was 132.3 ± 14.6 GBq μmol-1. The in vitro uptake of [18F]TTCO-IL2 correlated with the differential receptor expression (CD25, CD122, CD132) in PC3, NK-92 and activated human PBMCs. The automated method may be adapted for the radiosynthesis of any TCO-modified protein via IEDDA chemistry.

Simple new method for labelling of PSMA-11 with 68Ga in NaHCO3

BACKGROUND

Prostate specific membrane antigen (PSMA) is a type II membrane protein widely expressed on the surface of prostate cancer cells. One of its functions is to act as a receptor mediating the ligand internalization. This PSMA property is employed in the diagnostics and therapy of prostate cancer. Over the years, small molecules with high affinity for PSMA have been developed and labelled with positron emitters (e.g. ⁶⁸Ga, ¹⁸F, ¹¹C, ⁶⁴Cu, or ⁸⁶Y). One of these radiolabelled ligands, [⁶⁸Ga] PSMA-11, is one of the most widespread tracers for PET imaging of the prostate cancer. Many techniques have been proposed and tested for the ⁶⁸Ga labelling of PSMA-11. The aim of our work was to design a labelling method of PSMA-11 that minimizes number of the used chemicals and steps, providing quantitative labelling yield at laboratory temperature and may be easily automated.

METHODOLOGY

A⁶⁸Ge/⁶⁸Ga generator eluate in 0.1 M HCl was loaded on an activated Oasis MCX cartridge, and the cartridge was then thoroughly washed with water. The radionuclide ⁶⁸Ga was eluted from the cartridge with 0.1 M NaHCO₃ (pH = 8.5, n = 36) or with the same solution with pH adjusted to 7.2-9.0 (n = 38). Precursor PSMA-11 was mixed directly with the cartridge eluate of ⁶⁸Ga in 0.1 M NaHCO₃ of given pH. For the stability test, samples of ⁶⁸GaPSMA-11 in 0.1 M NaHCO₃ (pH 8.5) were mixed in ratio 1 : 1 with the following solutions: 0.1 M NaHCO₃ (pH 8.5), human serum, PBS and 0.9% NaCl. In order to estimate an effect of the time elapsed between ⁶⁸Ga elution from the cartridge in 0.1 M NaHCO₃ (pH 8.5) and the labelling onset of PSMA-11, the latter was initiated 0, 5, 10 and 20 min post elution and radiochemical yield was monitored. All the PSMA-11 labelled samples were subjected to radiochemical purity test using HPLC. The whole process starting from generator elution up to HPLC analysis commencement took 10-15 min.

RESULTS

Recovery of ⁶⁸Ga from cartridge Oasis MCX using 0.1 M NaHCO₃ at pH 8.5 was 71.5 ± 1.4%. Thirty six PSMA-11 samples (10 μg in reaction mixture) were labelled at pH 8.5 with total average radiochemical yield of 98 ± 2%. Recovery of ⁶⁸Ga from cartridge Oasis MCX using 0.1 M NaHCO₃ at variable pH of 7.2-9.0 was 62.5 ± 1.8% showing certain decrease with decreasing pH. A total of 138 samples of PSMA-11 were labelled with ⁶⁸ Ga at variable pH (7.2-9.0) and four different amounts of PSMA-11 (1, 2.5, 5 and 10 μg) resulting in the labelling yields of 54.0 ± 5.3%, 88.2 ± 3.2%, 99.4 ± 0.3% and 99.9 ± 0.1%, respectively. Irrespective of the pH, the radiolabelling yield was quantitative for the molar ratio PSMA-11: ⁶⁸Ga > 5000 : 1 in the reaction mixture. Stability tests in 0.1 M NaHCO₃ (pH 8.5), human serum, PBS and 0.9% NaCl revealed no observable release of ⁶⁸Ga from the ⁶⁸Ga-PSMA-11 complex within 3 h. Similarly, the delay between the ⁶⁸Ga elution from the Oasis MCX cartridge in 0.1 M NaHCO₃ (pH 8.5) and start of the labelling of PSMA-11 labelling has no effect on the radiochemical yield.

CONCLUSION

A new method of labelling PSMA-11 ligand with ⁶⁸Ga in 0.1 M NaHCO₃ using Oasis MCX cartridges was proposed, developed and tested. The results demonstrated that it is rapid, simple, reproducible and easy to automate.

Radiochemistry, Production Processes, Labeling Methods, and ImmunoPET Imaging Pharmaceuticals of Iodine-124

Target-specific biomolecules, monoclonal antibodies (mAb), proteins, and protein fragments are known to have high specificity and affinity for receptors associated with tumors and other pathological conditions. However, the large biomolecules have relatively intermediate to long circulation half-lives (>day) and tumor localization times. Combining superior target specificity of mAbs and high sensitivity and resolution of the PET (Positron Emission Tomography) imaging technique has created a paradigm-shifting imaging modality, ImmunoPET. In addition to metallic PET radionuclides, 124I is an attractive radionuclide for radiolabeling of mAbs as potential immunoPET imaging pharmaceuticals due to its physical properties (decay characteristics and half-life), easy and routine production by cyclotrons, and well-established methodologies for radioiodination. The objective of this report is to provide a comprehensive review of the physical properties of iodine and iodine radionuclides, production processes of 124I, various 124I-labeling methodologies for large biomolecules, mAbs, and the development of 124I-labeled immunoPET imaging pharmaceuticals for various cancer targets in preclinical and clinical environments. A summary of several production processes, including 123Te(d,n)124I, 124Te(d,2n)124I, 121Sb(α,n)124I, 123Sb(α,3n)124I, 123Sb(3He,2n)124I, natSb(α, xn)124I, natSb(3He,n)124I reactions, a detailed overview of the 124Te(p,n)124I reaction (including target selection, preparation, processing, and recovery of 124I), and a fully automated process that can be scaled up for GMP (Good Manufacturing Practices) production of large quantities of 124I is provided. Direct, using inorganic and organic oxidizing agents and enzyme catalysis, and indirect, using prosthetic groups, 124I-labeling techniques have been discussed. Significant research has been conducted, in more than the last two decades, in the development of 124I-labeled immunoPET imaging pharmaceuticals for target-specific cancer detection. Details of preclinical and clinical evaluations of the potential 124I-labeled immunoPET imaging pharmaceuticals are described here.

Radiolabeled Peptides and Antibodies in Medicine

Radiolabeled peptides are a relatively new, very specific radiotracer group, which is still expanding. This group is very diverse in terms of peptide size. It contains very small structures containing several amino acids and whole antibodies. Moreover, radiolabeled peptides are diverse in terms of the binding aim and therapeutic or diagnostic applications. The majority of this class of radiotracers is utilized in oncology, where the same structure can be used in therapy and diagnostic imaging by varying the radionuclide. In this study, we collected new reports of radiolabeled peptide applications in diagnosis and therapy in oncology and other fields of medicine. Radiolabeled peptides are also increasingly being used in rheumatology, cardiac imaging, or neurology. The studies collected in this review concern new therapeutic and diagnostic procedures in humans and new structures tested on animals. We also performed an analysis of clinical trials, which concerns application of radiolabeled peptides and antibodies that were reported in the clinicaltrials.gov database between 2008 and 2018.

Identification of a Suitable Peptidic Molecular Platform for the Development of NPY(Y1 )R-Specific Imaging Agents

NPY(Y1 )R (neuropeptide Y receptor subtype 1) is an important target structure for tumor-specific imaging and therapy as this receptor subtype is overexpressed in very high density and incidence especially in human breast cancer. Targeting this receptor with radiolabeled truncated analogues of the endogenous ligand NPY (neuropeptide Y) has, however, not yet resulted in satisfactory imaging results when using positron emission tomography (PET). This can be attributed to the limited stability of these PET imaging agents caused by their fast proteolytic degradation. Although highly promising NPY analogues were developed, their stability has only been investigated in very few cases. In this systematical work, we comparatively determined the stability of the five most promising truncated analogues of NPY that were developed over the last years, showing the highest receptor affinities and subtype selectivities. The stability of the peptides was assessed in human serum as well as in a human liver microsomal stability assay; these gave complementary results, thus demonstrating the necessity to perform both assays and not just conventional serum stability testing. Of the tested peptides, only [Lys(lauroyl)27 ,Pro30 ,Lys(DOTA)31 ,Bip32 ,Leu34 ]NPY27-36 showed high stability against peptidase degradation; thus this is the best-suited truncated NPY analogue for the development of NPY(Y1 )R-specific imaging agents.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

68Ga-labelled desferrioxamine-B for bacterial infection imaging

Purpose

With the increase of especially hospital-acquired infections, timely and accurate diagnosis of bacterial infections is crucial for effective patient care. Molecular imaging has the potential for specific and sensitive detection of infections. Siderophores are iron-specific chelators recognized by specific bacterial transporters, representing one of few fundamental differences between bacterial and mammalian cells. Replacing iron by gallium-68 without loss of bioactivity is possible allowing molecular imaging by positron emission tomography (PET). Here, we report on the preclinical evaluation of the clinically used siderophore, desferrioxamine-B (Desferal®, DFO-B), radiolabelled with ⁶⁸Ga for imaging of bacterial infections.

Methods

In vitro characterization of [⁶⁸Ga]Ga-DFO-B included partition coefficient, protein binding and stability determination. Specific uptake of [⁶⁸Ga]Ga-DFO-B was tested in vitro in different microbial cultures. In vivo biodistribution was studied in healthy mice and dosimetric estimation for human setting performed. PET/CT imaging was carried out in animal infection models, representing the most common pathogens.

Results

DFO-B was labelled with ⁶⁸Ga with high radiochemical purity and displayed hydrophilic properties, low protein binding and high stability in human serum and PBS. The high in vitro uptake of [⁶⁸Ga]Ga-DFO-B in selected strains of Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus agalactiae could be blocked with an excess of iron-DFO-B. [⁶⁸Ga]Ga-DFO-B showed rapid renal excretion and minimal retention in blood and other organs in healthy mice. Estimated human absorbed dose was 0.02 mSv/MBq. PET/CT images of animal infection models displayed high and specific accumulation of [⁶⁸Ga]Ga-DFO-B in both P. aeruginosa and S. aureus infections with excellent image contrast. No uptake was found in sterile inflammation, heat-inactivated P. aeruginosa or S. aureus and Escherichia coli lacking DFO-B transporters.

Conclusion

DFO-B can be easily radiolabelled with ⁶⁸Ga and displayed suitable in vitro characteristics and excellent pharmacokinetics in mice. The high and specific uptake of [⁶⁸Ga]Ga-DFO-B by P. aeruginosa and S. aureus was confirmed both in vitro and in vivo, proving the potential of [⁶⁸Ga]Ga-DFO-B for specific imaging of bacterial infections. As DFO-B is used in clinic for many years and the estimated radiation dose is lower than for other ⁶⁸Ga-labelled radiopharmaceuticals, we believe that [⁶⁸Ga]Ga-DFO-B has a great potential for clinical translation.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04948-y) contains supplementary material, which is available to authorized users.

Sorbitol as a Polar Pharmacological Modifier to Enhance the Hydrophilicity of 99mTc-Tricarbonyl-Based Radiopharmaceuticals

The organometallic technetium-99m tricarbonyl core, [^99mTc][Tc(CO)₃(H₂O)₃]⁺, is a versatile precursor for the development of radiotracers for single photon emission computed tomography (SPECT). A drawback of the ^99mTc-tricarbonyl core is its lipophilicity, which can influence the pharmacokinetic properties of the SPECT imaging probe. Addition of polar pharmacological modifiers to ^99mTc-tricarbonyl conjugates holds the promise to counteract this effect and provide tumor-targeting radiopharmaceuticals with improved hydrophilicities, e.g., resulting in a favorable fast renal excretion in vivo. We applied the "Click-to-Chelate" strategy for the assembly of a novel ^99mTc-tricarbonyl labeled conjugate made of the tumor-targeting, modified bombesin binding sequence [Nle¹⁴]BBN(7-14) and the carbohydrate sorbitol as a polar modifier. The ^99mTc-radiopeptide was evaluated in vitro with PC-3 cells and in Fox-1^nu mice bearing PC-3 xenografts including a direct comparison with a reference conjugate lacking the sorbitol moiety. The glycated ^99mTc-tricarbonyl peptide conjugate exhibited an increased hydrophilicity as well as a retained affinity toward the Gastrin releasing peptide receptor and cell internalization properties. However, there was no significant difference in vivo in terms of pharmacokinetic properties. In particular, the rate and route of excretion was unaltered in comparison to the more lipophilic reference compound. This could be attributed to the intrinsic properties of the peptide and/or its metabolites. We report a novel glycated (sorbitol-containing) alkyne substrate for the "Click-to-Chelate" methodology, which is potentially of general applicability for the development of ^99mTc-tricarbonyl based radiotracers displaying an enhanced hydrophilicity.

Insight into the Development of PET Radiopharmaceuticals for Oncology

While the development of positron emission tomography (PET) radiopharmaceuticals closely follows that of traditional drug development, there are several key considerations in the chemical and radiochemical synthesis, preclinical assessment, and clinical translation of PET radiotracers. As such, we outline the fundamentals of radiotracer design, with respect to the selection of an appropriate pharmacophore. These concepts will be reinforced by exemplary cases of PET radiotracer development, both with respect to their preclinical and clinical evaluation. We also provide a guideline for the proper selection of a radionuclide and the appropriate labeling strategy to access a tracer with optimal imaging qualities. Finally, we summarize the methodology of their evaluation in in vitro and animal models and the road to clinical translation. This review is intended to be a primer for newcomers to the field and give insight into the workflow of developing radiopharmaceuticals.