Promising Prospects for 44Sc-/47Sc-Based Theragnostics: Application of 47Sc for Radionuclide Tumor Therapy in Mice

CERN-MEDICIS: A Review Since Commissioning in 2017

The CERN-MEDICIS (MEDical Isotopes Collected from ISolde) facility has delivered its first radioactive ion beam at CERN (Switzerland) in December 2017 to support the research and development in nuclear medicine using non-conventional radionuclides. Since then, fourteen institutes, including CERN, have joined the collaboration to drive the scientific program of this unique installation and evaluate the needs of the community to improve the research in imaging, diagnostics, radiation therapy and personalized medicine. The facility has been built as an extension of the ISOLDE (Isotope Separator On Line DEvice) facility at CERN. Handling of open radioisotope sources is made possible thanks to its Radiological Controlled Area and laboratory. Targets are being irradiated by the 1.4 GeV proton beam delivered by the CERN Proton Synchrotron Booster (PSB) on a station placed between the High Resolution Separator (HRS) ISOLDE target station and its beam dump. Irradiated target materials are also received from external institutes to undergo mass separation at CERN-MEDICIS. All targets are handled via a remote handling system and exploited on a dedicated isotope separator beamline. To allow for the release and collection of a specific radionuclide of medical interest, each target is heated to temperatures of up to 2,300°C. The created ions are extracted and accelerated to an energy up to 60 kV, and the beam steered through an off-line sector field magnet mass separator. This is followed by the extraction of the radionuclide of interest through mass separation and its subsequent implantation into a collection foil. In addition, the MELISSA (MEDICIS Laser Ion Source Setup At CERN) laser laboratory, in service since April 2019, helps to increase the separation efficiency and the selectivity. After collection, the implanted radionuclides are dispatched to the biomedical research centers, participating in the CERN-MEDICIS collaboration, for Research & Development in imaging or treatment. Since its commissioning, the CERN-MEDICIS facility has provided its partner institutes with non-conventional medical radionuclides such as Tb-149, Tb-152, Tb-155, Sm-153, Tm-165, Tm-167, Er-169, Yb-175, and Ac-225 with a high specific activity. This article provides a review of the achievements and milestones of CERN-MEDICIS since it has produced its first radioactive isotope in December 2017, with a special focus on its most recent operation in 2020.

Novel Synthetic Strategies Enable the Efficient Development of Folate Conjugates for Cancer Radiotheranostics

The folate receptor (FR) is an interesting target for radiotheranostics due to its overexpression in several tumor types. The progress in developing novel folate radioconjugates is, however, slow due to the synthetic challenges that folate chemistry presents. The goal of this study was, thus, to establish versatile solid-phase synthetic strategies for a convenient preparation of novel folate conjugates. Two approaches were established based on an orthogonal fluorenylmethyloxycarbonyl (Fmoc)-protection strategy to enable a modular buildup of an albumin-binding DOTA conjugate (known as OxFol-1) using folic acid (oxidized folate version) as a targeting agent. The main difference between the two approaches was the sequence of conjugating the single structural units. The approach that introduced the folate entity as the last unit appeared particularly useful for the preparation of conjugates based on 6R- or 6S-5-methyltetrahydrofolic acid (5-MTHF; a reduced folate version) as targeting entity. Three types of folate conjugates were synthesized either with a p-iodophenyl-based albumin binder (OxFol-1, 6R-RedFol-1, and 6S-RedFol-1) or without an albumin-binding entity (OxFol-14, 6R-RedFol-14, and 6S-RedFol-14). All six conjugates were obtained with high chemical purity (>98%) after 9-13 synthesis steps and a single final HPLC purification. Radiolabeling with lutetium-177 was feasible at high molar activity, and the resulting radioconjugates were stable over at least 24 h. Biodistribution and SPECT/CT imaging studies confirmed the favorable effect of an albumin-binding entity to increase the tumor uptake and reduce kidney retention of folate radioconjugates. The increased tumor-to-kidney ratios obtained with [¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 as compared to [¹⁷⁷Lu]Lu-OxFol-1 indicated that 5-MTHF is the preferred FR-targeting agent for albumin-binding radioconjugates. This was, however, not the case for folate radioconjugates without an albumin binder. Thanks to the established synthesis strategy, the preparation of further folate radioconjugates will be facilitated, potentially enabling the optimization of the tissue distribution characteristics even more.

Radiotheranostic Targeting Cancer Stem Cells in Human Colorectal Cancer Xenografts

PURPOSE

The aim of this study was to perform radiotheranostics with radioiodinated monoclonal antibodies (mAbs) for targeting cancer stem cells (CSCs) in human colorectal cancer xenografts and evaluate the relative advantage of a cocktail containing both [¹³¹I]CD133 mAb and [¹³¹I]CD44 mAb.

PROCEDURES

Tumor-bearing mice were randomly divided into eight groups: [¹³¹I]CD133mAb, [¹³¹I]CD44 mAb, [¹³¹I]IgG isotype control, radioiodinated mAb cocktail, CD133 mAb, CD44 mAb, unradioiodinated mAb cocktail, and saline groups. In vivo single photon emission computed tomography (SPECT) imaging was used to monitor dynamically changes in the CSC population after treatment. The radioactivity uptake of tumors was quantified ex vivo. The expression of CD133 and CD44 after treatment was also assessed by immunohistochemistry and western blot. Tumor growth curves and survival curves were generated to assess treatment efficacy. Cell apoptosis and proliferation in xenografts 30 days after treatment were measured by TdT-mediated dUTP-biotin nick end labeling (aka, TUNEL) and Ki67 staining. The expression levels of biomarkers in xenografts 30 days after treatment were measured by flow cytometry.

RESULTS

The radioimmunoimaging (RII) with in vivo SPECT showed that the CSC-targeting radioimmunotherapy (RIT) groups ([¹³¹I]CD133 mAb, [¹³¹I]CD44 mAb, and radioiodinated mAb cocktail groups) had intense accumulations of radiolabeled agents in the tumor areas. The ex vivo biodistribution confirmed these findings. In the CSC-targeting RIT groups, immunohistochemistry and western blot indicated significant reduction of specific target expression in the xenografts. The tumor growth curves and survival curves showed that the CSC-targeting RIT significantly inhibited tumor growth and prolonged mean survival, respectively. Significantly increased apoptosis and decreased proliferation in xenografts further confirmed the therapeutic efficacy of CSC-targeting RIT. Flow cytometry showed that the decreases in CSCs correlated with the presence of the corresponding antibodies.

CONCLUSIONS

Our results suggest that the CSC-targeting RIT can effectively reduce CSCs which consequently inhibits tumor development. The radioiodinated mAb cocktail may generate enhanced CSC-targeting specificity.

Production of scandium radionuclides for theranostic applications: towards standardization of quality requirements

In the frame of "precision medicine", the scandium radionuclides have recently received considerable interest, providing personalised adjustment of radiation characteristics to optimize the efficiency of medical care or therapeutic benefit for particular groups of patients. Radionuclides of scandium, namely scandium-43 and scandium-44 (43/44Sc) as positron emitters and scandium-47 (47Sc), beta-radiation emitter, seem to fit ideally into the concept of theranostic pair. This paper aims to review the work on scandium isotopes production, coordination chemistry, radiolabeling, preclinical studies and the very first clinical studies. Finally, standardized procedures for scandium-based radiopharmaceuticals have been proposed as a basis to pave the way for elaboration of the Ph.Eur. monographs for perspective scandium radionuclides.

Aqueous chemistry of the smallest rare earth: Comprehensive characterization of radioactive and non-radioactive scandium complexes for biological applications

The aqueous chemistry of scandium(III) is of emerging interest for biological applications, specifically in nuclear medicine, as radioactive isotopes of scandium are becoming more readily accessible. In contrast to other rare earths, Sc³⁺ has no d or f electrons, limiting characterization of corresponding coordination complexes to spectroscopic techniques that do not rely on the characteristic electronic transitions of f-elements or transition metal ions. Herein, we provide a comprehensive overview on characterization techniques suitable to elucidate the solution behavior of small and macromolecular complexes of the smallest rare earth.

Preliminary dosimetric analysis of DOTA-folate radiopharmaceutical radiolabelled with 47Sc produced through natV(p,x)47Sc cyclotron irradiation

⁴⁷Sc is one of the most promising theranostic radionuclides, thanks to its low energy γ-ray emission (159 keV), suitable for single photon emission computed tomography imaging and its intense β ^- emission, useful for tumour treatment. Despite promising preclinical results, the translation of ⁴⁷Sc-therapeutic agents to the clinic is hampered by its limited availability. Among different ⁴⁷Sc-production routes currently being investigated, the ^natV(p,x)⁴⁷Sc reaction has proved to be of particular interest, thanks to the low-cost and easy availability on the market of ^natV material and the diffusion of medium energy proton cyclotrons. However, the cross section of this specific nuclear reaction is quite low and small amounts of Sc-contaminants are co-produced at energies E _P ≤ 45 MeV, namely ⁴⁸Sc and ⁴⁶Sc. The main concern with these Sc-contaminants is their contribution to the patient absorbed dose. For such a reason, the absorbed dose contributions to healthy organs and the effective dose contributions by the three radioisotopes, ⁴⁸Sc, ⁴⁷Sc and ⁴⁶Sc, were evaluated using DOTA-folate conjugate (cm10) as an example of radiopharmaceutical product. Considering as acceptable the limits of 99% for the radionuclidic purity and 10% for the contribution of radioactive Sc-contaminants to the total effective dose after ⁴⁷Sc-cm10 injection, it was obtained that proton beam energies below 35 MeV must be used to produce ⁴⁷Sc through irradiation of a ^natV target.

Homologous Structural, Chemical, and Biological Behavior of Sc and Lu Complexes of the Picaga Bifunctional Chelator: Toward Development of Matched Theranostic Pairs for Radiopharmaceutical Applications

The radioactive isotopes scandium-44/47 and lutetium-177 are gaining relevance for radioimaging and radiotherapy, resulting in a surge of studies on their coordination chemistry and subsequent applications. Although the trivalent ions of these elements are considered close homologues, dissimilar chemical behavior is observed when they are complexed by large ligand architectures due to discrepancies between Lu(III) and Sc(III) ions with respect to size, chemical hardness, and Lewis acidity. Here, we demonstrate that Lu and Sc complexes of 1,4-bis(methoxycarbonyl)-7-[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H₃mpatcn) and its corresponding bioconjugate picaga-DUPA can be employed to promote analogous structural features and, subsequently, biological properties for coordination complexes of these ions. The close homology was evidenced using potentiometric methods, computational modeling, variable temperature mass spectrometry, and pair distribution function analysis of X-ray scattering data. Radiochemical labeling, in vitro stability, and biodistribution studies with Sc-47 and Lu-177 indicate that the 7-coordinate ligand environment of the bifunctional picaga ligand is compatible with biological applications and the future investigation of β-emitting, picaga-chelated Sc and Lu isotopes for radiotherapy.

Production, Collection, and Purification of 47Ca for the Generation of 47Sc through Isotope Harvesting at the National Superconducting Cyclotron Laboratory

An experiment was performed at the National Superconducting Cyclotron Laboratory using a 140 MeV/nucleon ⁴⁸Ca beam and a flowing-water target to produce ⁴⁷Ca for the first time with this production route. A production rate of 0.020 ± 0.004 ⁴⁷Ca nuclei per incoming beam particle was measured. An isotope harvesting system attached to the target was used to collect radioactive cationic products, including ⁴⁷Ca, from the water on a cation-exchange resin. The ⁴⁷Ca collected was purified using three separation methods optimized for this work: (1) DGA extraction chromatography resin with HNO₃ and HCl, (2) AG MP-50 cation-exchange resin with an increasing concentration gradient of HCl, and (3) AG MP-50 cation-exchange resin with a methanolic HCl gradient. These methods resulted in ≥99 ± 2% separation yield of ⁴⁷Ca with 100% radionuclidic purity within the limits of detection for HPGe measurements. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to identify low levels of stable ions in the water of the isotope harvesting system during the irradiation and in the final purified solution of ⁴⁷Ca. For the first time, this experiment demonstrated the feasibility of the production, collection, and purification of ⁴⁷Ca through isotope harvesting for the generation of ⁴⁷Sc for nuclear medicine applications.

A comparative PET imaging study of 44gSc- and 68Ga-labeled bombesin antagonist BBN2 derivatives in breast and prostate cancer models

INTRODUCTION

Radiolabeled peptides play a central role in nuclear medicine as radiotheranostics for targeted imaging and therapy of cancer. We have recently proposed the use of metabolically stabilized GRPR antagonist BBN2 for radiolabeling with 18F and 68Ga and subsequent PET imaging of GRPRs in prostate cancer. The present work studied the impact of 44gSc- and 68Ga-labeled DOTA complexes attached to GRPR antagonist BBN2 on the in vitro GRPR binding affinity, and their biodistribution and tumor uptake profiles in MCF7 breast and PC3 prostate cancer models.

METHODS

DOTA-Ava-BBN2 was radiolabeled with radiometals 68Ga and 44gSc. Gastrin-releasing peptide receptor (GRPR) affinities of peptides were assessed in PC3 prostate cancer cells. GRPR expression profiles were studied in human breast cancer tissue samples and MCF7 breast cancer cells. PET imaging of 68Ga- and 44gSc-labeled peptides was performed in MCF7 and PC3 xenografts as breast and prostate cancer models.

RESULTS

Radiopeptides [68Ga]Ga-DOTA-Ava-BBN2 and [44gSc]Sc-DOTA-Ava BBN2 were prepared in radiochemical yields of 70-80% (decay-corrected), respectively. High binding affinities were found for both peptides (IC50 = 15 nM (natGa) and 5 nM (natSc)). Gene expression microarray analysis revealed high GRPR mRNA expression levels in estrogen receptor (ER)-positive breast cancer, which was further confirmed with Western blot and immunohistochemistry. However, PET imaging showed only low tumor uptake of both radiotracers in MCF7 xenografts ([68Ga]Ga-DOTA-BBN2 (SUV60min 0.27 ± 0.06); [44gSc]Sc-DOTA-BBN2 (SUV60min 0.20 ± 0.03)). In contrast, high tumor uptake and retention were found for both radiopeptides in PC3 tumors ([68Ga]Ga-DOTA-BBN2 (SUV60min 0.46 ± 0.07); [44gSc]Sc-DOTA-BBN2 (SUV60min 0.51 ± 0.11)).

CONCLUSIONS

Comparison of 68Ga- and 44gSc-labeled DOTA-Ava-BBN2 peptides revealed slight but noticeable differences of the radiometal with an impact on the in vitro GRPR receptor binding properties in PC3 cells. No differences were found in their in vivo biodistribution profiles in MCF7 and PC3 xenografts. Radiopeptides [68Ga]Ga-DOTA-Ava-BBN2 and [44gSc]Sc-DOTA-Ava-BBN2 displayed comparable tumor uptake and retention profiles with rapid blood and renal clearance profiles in both tumor models.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The favorable PET imaging performance of [44gSc]Sc-DOTA-Ava-BBN2 in prostate cancer should warrant the development of an [43Sc]Sc-DOTA-Ava-BBN2 analog for clinical translation which comes with a main γ-line of much lower energy and intensity compared to 44gSc.

Preclinical evaluation of 5-methyltetrahydrofolate-based radioconjugates-new perspectives for folate receptor-targeted radionuclide therapy

Purpose

The folate receptor (FR) is frequently overexpressed in a variety of tumor types and, hence, an interesting target for radionuclide therapy. The aim of this study was to evaluate a new class of albumin-binding radioconjugates comprising 5-methyltetrahydrofolate (5-MTHF) as a targeting agent and to compare their properties with those of the previously established folic acid-based [¹⁷⁷Lu]Lu-OxFol-1.

Methods

[¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 were investigated in vitro using FR-positive KB tumor cells. Biodistribution studies were performed in KB tumor-bearing mice, and the areas under the curve (AUC_{0 → 120h}) were determined for the uptake in tumors and kidneys. [¹⁷⁷Lu]Lu-6R-RedFol-1 was compared with [¹⁷⁷Lu]Lu-OxFol-1 in a therapy study over 8 weeks using KB tumor-bearing mice.

Results

Both radioconjugates demonstrated similar in vitro properties as [¹⁷⁷Lu]Lu-OxFol-1; however, the tumor uptake of [¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 was significantly increased in comparison with [¹⁷⁷Lu]Lu-OxFol-1. In the case of [¹⁷⁷Lu]Lu-6S-RedFol-1, also the kidney uptake was increased; however, renal retention of [¹⁷⁷Lu]Lu-6R-RedFol-1 was similar to that of [¹⁷⁷Lu]Lu-OxFol-1. This led to an almost 4-fold increased tumor-to-kidney AUC_{0 → 120h} ratio of [¹⁷⁷Lu]Lu-6R-RedFol-1 as compared with [¹⁷⁷Lu]Lu-6S-RedFol-1 and [¹⁷⁷Lu]Lu-OxFol-1. At equal activity, the therapeutic effect of [¹⁷⁷Lu]Lu-6R-RedFol-1 was better than that of [¹⁷⁷Lu]Lu-OxFol-1, reflected by a slower tumor growth and, consequently, an increased median survival time (49 days vs. 34 days).

Conclusion

This study demonstrated the promising potential of 5-MTHF-based radioconjugates for FR-targeting. Application of [¹⁷⁷Lu]Lu-6R-RedFol-1 resulted in unprecedentedly high tumor-to-kidney ratios and, as a consequence, a superior therapeutic effect as compared with [¹⁷⁷Lu]Lu-OxFol-1. These findings, together with the absence of early side effects, make [¹⁷⁷Lu]Lu-6R-RedFol-1 attractive in view of a future clinical translation.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04980-y) contains supplementary material, which is available to authorized users.

Developments toward the Implementation of 44Sc Production at a Medical Cyclotron

⁴⁴Sc has favorable properties for cancer diagnosis using Positron Emission Tomography (PET) making it a promising candidate for application in nuclear medicine. The implementation of its production with existing compact medical cyclotrons would mean the next essential milestone in the development of this radionuclide. While the production and application of ⁴⁴Sc has been comprehensively investigated, the development of specific targetry and irradiation methods is of paramount importance. As a result, the target was optimized for the ⁴⁴Ca(p,n)⁴⁴Sc nuclear reaction using CaO instead of CaCO₃, ensuring decrease in target radioactive degassing during irradiation and increased radionuclidic yield. Irradiations were performed at the research cyclotron at the Paul Scherrer Institute (~11 MeV, 50 µA, 90 min) and the medical cyclotron at the University of Bern (~13 MeV, 10 µA, 240 min), with yields varying from 200 MBq to 16 GBq. The development of targetry, chemical separation as well as the practical issues and implications of irradiations, are analyzed and discussed. As a proof-of-concept study, the ⁴⁴Sc produced at the medical cyclotron was used for a preclinical study using a previously developed albumin-binding prostate-specific membrane antigen (PSMA) ligand. This work demonstrates the feasibility to produce ⁴⁴Sc with high yields and radionuclidic purity using a medical cyclotron, equipped with a commercial solid target station.

Coordination chemistry of [Y(pypa)]- and comparison immuno-PET imaging of [44Sc]Sc- and [86Y]Y-pypa-phenyl-TRC105

Both scandium-44 and yttrium-86 are popular PET isotopes with appropriate half-lives for immuno-positron emission tomography (immuno-PET) imaging. Herein, a new bifunctional H4pypa ligand, H4pypa-phenyl-NCS, is synthesized, conjugated to a monoclonal antibody, TRC105, and labeled with both radionuclides to investigate the long-term in vivo stability of each complex. While the 44Sc-labeled radiotracer exhibited promising pharmacokinetics and stability in 4T1-xenograft mice (n = 3) even upon prolonged interactions with blood serum proteins, the progressive bone uptake of the 86Y-counterpart indicated in vivo demetallation, obviating H4pypa as a suitable chelator for Y3+ ion in vivo. The solution chemistry of [natY(pypa)]- was studied in detail and the complex found to be thermodynamically stable in solution with a pM value 22.0, ≥3 units higher than those of the analogous DOTA- and CHX-A''-DTPA-complexes; the 86Y-result in vivo was therefore most unexpected. To explore further this in vivo lability, Density Functional Theory (DFT) calculation was performed to predict the geometry of [Y(pypa)]- and the results were compared with those for the analogous Sc- and Lu-complexes; all three adopted the same coordination geometry (i.e. distorted capped square antiprism), but the metal-ligand bonds were much longer in [Y(pypa)]- than in [Lu(pypa)]- and [Sc(pypa)]-, which could indicate that the size of the binding cavity is too small for the Y3+ ion, but suitable for both the Lu3+ and Sc3+ ions. Considered along with results from [86Y][Y(pypa-phenyl-TRC105)], it is noted that when matching chelators with radionuclides, chemical data such as the thermodynamic stability and in vitro inertness, albeit useful and necessary, do not always translate to in vivo inertness, especially with the prolonged blood circulation of the radiotracer bound to a monoclonal antibody. Although H4pypa is a nonadentate chelator, which theoretically matches the coordination number of the Y3+ ion, we show herein that its binding cavity, in fact, favors smaller metal ions such as Sc3+ and Lu3+ and further exploitation of the Sc-pypa combination is desired.

Preclinical Evaluation of a High-Affinity Sarcophagine-Containing PSMA Ligand for 64Cu/67Cu-Based Theranostics in Prostate Cancer

The application of small molecules targeting prostate-specific membrane antigen (PSMA) has emerged as a highly promising clinical strategy for visualization and treatment of prostate cancer. Ligands that integrate the ability to both quantify the distribution of radioactivity and treat disease through the use of a matched pair of radionuclides have particular value in clinical and regulatory settings. In this study, we describe the development and preclinical evaluation of RPS-085, a ligand that binds PSMA and serum albumin and exploits the ^64/67Cu radionuclide pair for prostate cancer theranostics. RPS-085 was synthesized by conjugation of a PSMA-targeting moiety, an N^ε-(2-(4-iodophenyl)acetyl)lysine albumin binding group, and a bifunctionalized MeCOSar chelator. The IC₅₀ of the metal-free RPS-085 was determined in a competitive binding assay. The affinity for human serum albumin of the radiolabeled compound was determined by high-performance affinity chromatography. Radiolabeling was performed in NH₄OAc buffer at 25 °C. The stability of the radiolabeled compounds was assessed in vitro and in vivo. The biodistribution of [^64/67Cu]Cu-RPS-085 was determined following intravenous administration to male BALB/c mice bearing LNCaP tumor xenografts. The radiochemical yields of [^64/67Cu]Cu-RPS-085 were nearly quantitative after 20 min. The metal-free complex is a potent inhibitor of PSMA (IC₅₀ = 29 ± 2 nM), and the radiolabeled compound has moderate affinity for human serum albumin (K_d = 9.9 ± 1.7 μM). Accumulation of the tracer in mice was primarily evident in tumor and kidneys. Activity in all other tissues, including blood, was negligible, and the radiolabeled compounds demonstrated high stability in vitro and in vivo. Tumor activity reached a maximum at 4 h post injection (p.i.) and cleared gradually over a period of 96 h. By contrast, activity in the kidney cleared rapidly from 4 to 24 h p.i. As a consequence, by 24 h p.i., the tumor-to-kidney ratio exceeds 2, and the predicted dose to tumors is significantly greater than the dose to kidneys. [⁶⁴Cu]Cu-RPS-085 combines rapid tissue distribution and clearance with prolonged retention in LNCaP tumor xenografts. The pharmacokinetics should enable radioligand therapy using [⁶⁷Cu]Cu-RPS-085. By virtue of its rapid kidney clearance, the therapeutic index of [⁶⁷Cu]Cu-RPS-085 likely compares favorably to its parent structure, [¹⁷⁷Lu]Lu-RPS-063, a highly avid PSMA-targeting compound. On this basis, [^64/67Cu]Cu-RPS-085 show great promise as PSMA-targeting theranostic ligands for prostate cancer imaging and therapy.

Image quality analysis of 44Sc on two preclinical PET scanners: a comparison to 68Ga

Background

⁴⁴Sc has been increasingly investigated as a potential alternative to ⁶⁸Ga in the development of tracers for positron emission tomography (PET). The lower mean positron energy of ⁴⁴Sc (0.63 MeV) compared to ⁶⁸Ga (0.83 MeV) can result in better spatial image resolutions. However, high-energy γ-rays (1157 keV) are emitted at high rates (99.9%) during ⁴⁴Sc decay, which can reduce image quality. Therefore, we investigated the impact of these physical properties and performed an unbiased performance evaluation of ⁴⁴Sc and ⁶⁸Ga with different imaging phantoms (image quality phantom, Derenzo phantom, and three-rod phantom) on two preclinical PET scanners (Mediso nanoScan PET/MRI, Siemens microPET Focus 120).

Results

Despite the presence of high-energy γ-rays in ⁴⁴Sc decay, a higher image resolution of small structures was observed with ⁴⁴Sc when compared to ⁶⁸Ga. Structures as small as 1.3 mm using the Mediso system, and as small as 1.0 mm using the Siemens system, could be visualized and analyzed by calculating full width at half maximum. Full widths at half maxima were similar for both isotopes. For image quality comparison, we calculated recovery coefficients in 1–5 mm rods and spillover ratios in either air, water, or bone-equivalent material (Teflon). Recovery coefficients for ⁴⁴Sc were significantly higher than those for ⁶⁸Ga. Despite the lower positron energy, ⁴⁴Sc-derived spillover ratio (SOR) values were similar or slightly higher to ⁶⁸Ga-derived SOR values. This may be attributed to the higher background caused by the additional γ-rays. On the Siemens system, an overestimation of scatter correction in the central part of the phantom was observed causing a virtual disappearance of spillover inside the three-rod phantom.

Conclusion

Based on these findings, ⁴⁴Sc appears to be a suitable alternative to ⁶⁸Ga. The superior image resolution makes it an especially strong competitor in preclinical settings. The additional γ-emissions have a small impact on the imaging resolution but cause higher background noises and can effect an overestimation of scatter correction, depending on the PET system and phantom.

A Step-by-Step Guide for the Novel Radiometal Production for Medical Applications: Case Studies with 68Ga, 44Sc, 177Lu and 161Tb

The production of novel radionuclides is the first step towards the development of new effective radiopharmaceuticals, and the quality thereof directly affects the preclinical and clinical phases. In this review, novel radiometal production for medical applications is briefly elucidated. The production status of the imaging nuclide 44Sc and the therapeutic β--emitter nuclide 161Tb are compared to their more established counterparts, 68Ga and 177Lu according to their targetry, irradiation process, radiochemistry, and quality control aspects. The detailed discussion of these significant issues will help towards the future introduction of these promising radionuclides into drug manufacture for clinical application under Good Manufacturing Practice (GMP).

[nat/44Sc(pypa)]-: Thermodynamic Stability, Radiolabeling, and Biodistribution of a Prostate-Specific-Membrane-Antigen-Targeting Conjugate

⁴⁴Sc is an attractive positron-emitting radionuclide for PET imaging; herein, a new complex of the Sc³⁺ ion with nonmacrocyclic chelator H₄pypa was synthesized and characterized with high-resolution electrospray-ionization mass spectrometry (HR-ESI-MS), as well as different nuclear magnetic resonance (NMR) spectroscopic techniques (¹H, ¹³C, ¹H-¹³C HSQC, ¹H-¹³C HMBC, COSY, and NOESY). In aqueous solution (pH = 7), [Sc(pypa)]^- presented two isomeric forms, the structures of which were predicted using density functional theory (DFT) calculation with a small energy difference of 22.4 kJ/mol, explaining their coexistence. [Sc(pypa)]^- was found to have superior thermodynamic stability (pM = 27.1) compared to [Sc(AAZTA)]^- (24.7) and [Sc(DOTA)]^- (23.9). In radiolabeling, [⁴⁴Sc][Sc(pypa)]^- formed efficiently at RT in 15 min over a range of pH (2-5.5), resulting in a complex that is highly stable (>99%) in mouse serum over at least six half-lives of scandium-44. Similar labeling efficiency was observed with the PSMA (prostate-specific membrane antigen)-targeting H₄pypa-C7-PSMA617 at pH = 5.5 (RT, 15 min), confirming negligible disturbance from the bifunctionalization on scandium-44 scavenging. Moreover, the kinetic inertness of the radiocomplex was proved in vivo. Surprisingly, the molar activity was found to have profound influence on the pharmacokinetics of the radiotracers where lower molar activity drastically reduced the background accumulations, particularly, kidney, and thus, yielded a much higher tumor-to-background contrast.

Chelation with a twist: a bifunctional chelator to enable room temperature radiolabeling and targeted PET imaging with scandium-44

Scandium-44 has emerged as an attractive, novel PET radioisotope with ideal emission properties and half-life (t 1/2 = 3.97 h, E mean β+ = 632 keV) well matched to the pharmacokinetics of small molecules, peptides and small biologics. Conjugates of the current gold-standard chelator for 44Sc, 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetraacetic acid (DOTA), require heating to achieve radiochemical complexation, limiting application of this isotope in conjunction with temperature-sensitive biologics. To establish Sc(iii) isotopes as broadly applicable tools for nuclear medicine, development of alternative bifunctional chelators is required. To address this need, we characterized the Sc(iii)-chelation properties of the small-cavity triaza-macrocycle-based, picolinate-functionalized chelator H3mpatcn. Spectroscopic and radiochemical studies establish the [Sc(mpatcn)] complex as kinetically inert and appropriate for biological applications. A proof-of-concept bifunctional conjugate targeting the prostate-specific membrane antigen (PSMA), picaga-DUPA, chelates 44Sc to form 44Sc(picaga)-DUPA at room temperature with an apparent molar activity of 60 MBq μmol-1 and formation of inert RRR-Λ and SSS-Δ-twist isomers. Sc(picaga)-DUPA exhibits a K i of 1.6 nM for PSMA, comparable to the 18F-based imaging probe DCFPyL (K i = 1.1 nM) currently in phase 3 clinical trials for imaging prostate cancer. Finally, we successfully employed 44Sc(picaga)-DUPA to image PSMA-expressing tumors in a preclinical mouse model, establishing the picaga bifunctional chelator as an optimal choice for the 44Sc PET nuclide.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Comparison of scandium-44 g with other PET radionuclides in pre-clinical PET phantom imaging

PURPOSE

The decay characteristics of radionuclides in PET studies can impact image reconstruction. ^44gSc has been the topic of recent research due to potential theranostic applications and is a promising radiometal for PET imaging. In this study, the reconstructed images from phantom measurements with scandium in a small-animal PET scanner are compared with ¹⁸F and two prominent radiometals: ⁶⁴Cu and ⁶⁸Ga METHODS: Three phantoms filled with ¹⁸F, ⁶⁴C, ⁶⁸Ga, and ^44gSc were imaged in the Siemens Inveon PET scanner. The NEMA image quality phantom was used to determine the recovery coefficients (RCs), spill-over ratios (SORs), and noise (%SD) under typical pre-clinical imaging conditions. Image contrast was determined using a Derenzo phantom, while the coincidence characteristics were investigated using an NEC phantom. Three reconstruction algorithms were used, namely filtered back projection (FBP), ordered subset expectation maximization (OSEM), and maximum a-posteriori (MAP).

RESULTS

Image quality parameters were measured for ¹⁸F, ⁶⁴Cu, ⁶⁸Ga, and ^44gSc respectively; using FBP, the %SD are 5.65, 5.88, 7.28, and 7.70; the RCs for the 5-mm rod are 0.849, 1.01, 0.615, and 0.825; the SORs in water are 0.0473, 0.0595, 0.141, 0.0923; and the SORs in air are 0.0589, 0.0484, 0.0525, and 0.0509. The contrast measured in the 2.5-mm rods are 0.674, 0.637, 0.196, and 0.347. The NEC rate with ^44gSc increased at a slower rate than ¹⁸F and ⁶⁸Ga as a function of activity in the field of view.

CONCLUSION

^44gSc demonstrates intermediate behavior relative to ¹⁸F and ⁶⁸Ga with regard to RC and contrast measurements. It is a promising radionuclide for preclinical imaging.

New 55Co-labeled Albumin-Binding Folate Derivatives as Potential PET Agents for Folate Receptor Imaging

Overexpression of folate receptors (FRs) on different tumor types (e.g., ovarian, lung) make FRs attractive in vivo targets for directed diagnostic/therapeutic agents. Currently, no diagnostic agent suitable for positron emission tomography (PET) has been adopted for clinical FR imaging. In this work, two ⁵⁵Co-labeled albumin-binding folate derivatives-[⁵⁵Co]Co-cm10 and [⁵⁵Co]Co-rf42-with characteristics suitable for PET imaging have been developed and evaluated. High radiochemical yields (≥95%) and in vitro stabilities (≥93%) were achieved for both compounds, and cell assays demonstrated FR-mediated uptake. Both ⁵⁵Co-labeled folate conjugates demonstrated high tumor uptake of 17% injected activity per gram of tissue (IA/g) at 4 h in biodistribution studies performed in KB tumor-bearing mice. Renal uptake was similar to other albumin-binding folate derivatives, and liver uptake was lower than that of previously reported [⁶⁴Cu]Cu-rf42. Small animal PET/CT images confirmed the biodistribution results and showed the clear delineation of FR-expressing tumors.

Therapeutic Potential of 47Sc in Comparison to 177Lu and 90Y: Preclinical Investigations

Targeted radionuclide therapy with ¹⁷⁷Lu- and ⁹⁰Y-labeled radioconjugates is a clinically-established treatment modality for metastasized cancer. ⁴⁷Sc is a therapeutic radionuclide that decays with a half-life of 3.35 days and emits medium-energy β⁻-particles. In this study, ⁴⁷Sc was investigated, in combination with a DOTA-folate conjugate, and compared to the therapeutic properties of ¹⁷⁷Lu-folate and ⁹⁰Y-folate, respectively. In vitro, ⁴⁷Sc-folate demonstrated effective reduction of folate receptor-positive ovarian tumor cell viability similar to ¹⁷⁷Lu-folate, but ⁹⁰Y-folate was more potent at equal activities due to the higher energy of emitted β⁻-particles. Comparable tumor growth inhibition was observed in mice that obtained the same estimated absorbed tumor dose (~21 Gy) when treated with ⁴⁷Sc-folate (12.5 MBq), ¹⁷⁷Lu-folate (10 MBq), and ⁹⁰Y-folate (5 MBq), respectively. The treatment resulted in increased median survival of 39, 43, and 41 days, respectively, as compared to 26 days in untreated controls. There were no statistically significant differences among the therapeutic effects observed in treated groups. Histological assessment revealed no severe side effects two weeks after application of the radiofolates, even at double the activity used for therapy. Based on the decay properties and our results, ⁴⁷Sc is likely to be comparable to ¹⁷⁷Lu when employed for targeted radionuclide therapy. It may, therefore, have potential for clinical translation and be of particular interest in tandem with ⁴⁴Sc or ⁴³Sc as a diagnostic match, enabling the realization of radiotheragnostics in future.

Radiometals for imaging and theranostics, current production, and future perspectives

The aim of this review is to make the reader familiar with currently available radiometals, their production modes, capacities, and quality concerns related to their medical use, as well as new emerging radiometals and irradiation technologies from the perspective of their diagnostic and theranostic applications. Production methods of ¹⁷⁷ Lu serve as an example of various issues related to the production yield, specific activity, radionuclidic and chemical purity, and production economy. Other radiometals that are currently used or explored for potential medical applications, with particular focus on their theranostic value, are discussed. Using radiometals for diagnostic imaging and therapy is on the rise. The high demand for radiometals for medical use prompts investigations towards using alternative irradiation reactions, while using existing nuclear reactors and accelerator facilities. This review discusses these production capacities and what is necessary to cover the growing demand for theranostic nuclides.

Feasibility study on production of Sc-47 from neutron irradiated Ca target for cancer theranostics applications

Scandium-47 is one of the most useful radioisotopes which is gaining great importance in cancer theranostics applications due to its favorable nuclear and chemical properties. MCNPX2.7.0 code was used to simulate the neutron activation of natural calcium target positioned at a thermal neutron flux of 1.8 × 1014 n cm−2 s−1 in the Egypt Second Research Reactor (ETRR-2). The burn card was used to calculate 47Ca and 47Sc radioactivities during 3 days irradiation and 20 days post-irradiation. The undesirable impurities generated during this period were also calculated. The obtained calculations were found to be in agreement with the experimental measurements. The distribution coefficient value (Kd) of 47Sc(III) as well as 47Ca(II) ions was determined using the commercially available ion-exchanger Chelex 100 in HNO3 and/or HCl media. Radiochemical separation of 47Sc(III) from 47Ca(II) was studied using HNO3 and HCl solutions and the results showed that HNO3 is a better medium than HCl for complete retention and recovery of 47Sc(III), where the recovery yields were 85 ± 1.2 and 95 ± 0.87 % using 1 M HCl and 1 M HNO3 solutions, respectively. The recovery yield obtained in our work was higher than in the reported procedures. Radionuclidic, radiochemical and chemical purities were investigated to ensure the suitability of 47Sc(III) for nuclear medicine applications.

From Bench to Bedside-The Bad Berka Experience With First-in-Human Studies

Precision oncology is being driven by rapid advances in novel diagnostics and therapeutic interventions, with treatments targeted to the needs of individual patients on the basis of genetic, biomarker, phenotypic, or psychosocial characteristics that distinguish a given patient from other patients with similar clinical presentations. Inherent in the theranostics paradigm is the assumption that diagnostic test results can precisely determine whether an individual is likely to benefit from a specific treatment. As part and integral in the current era of precision oncology, theranostics in the context of nuclear medicine aims to identify the appropriate molecular targets in neoplasms (diagnostic tool), so that the optimal ligands and radionuclides (therapeutic tool) with favorable labeling chemistry can be selected for personalized management of a specific disease, taking into consideration the specific patient, and subsequently monitor treatment response. Over the past two decades, the use of gallium-68 labeled peptides for somatostatin receptor (SSTR)-targeted PET/CT (or PET/MRI) imaging followed by lutetium-177 and yttrium-90 labeled SSTR-agonist for peptide receptor radionuclide therapy has demonstrated remarkable success in the management of neuroendocrine neoplasms, and paved the way to other indications of theranostics. Rapid advances are being made in the development of other peptide-based radiopharmaceuticals, small molecular-weight ligands and with newer radioisotopes with more favorable kinetics, potentially useful for theranostics strategies for the clinical application. The present review features the Bad Berka experience with first-in-human studies of new radiopharmaceuticals, for example, prostate-specific membrane antigen ligand, gastrin-releasing peptide receptor, neurotensin receptor 1 ligand, novel SSTR-targeting peptides and nonpeptide, and bone-seeking radiopharmaceuticals. Also new radioisotopes, for example, actinium (²²⁵Ac), copper (⁶⁴Cu), scandium (⁴⁴Sc), and terbium (¹⁵²Tb/¹⁶¹Tb) will be discussed briefly demonstrating the development from basic science to precision oncology in the clinical setting.

Investigations of proton and deuteron induced nuclear reactions on natural and enriched Titanium, Calcium and Vanadium targets, with special reference to the production of 47Sc

⁴⁷Sc could be used in SPECT imaging and also suitable for targeted therapy of small tumors. The excitation functions for the production of ⁴⁷Sc and accompanying impurities via proton and deuteron bombardment of Calcium, Titanium and Vanadium targets were evaluated by three nuclear codes, ALICE, TALYS and EMPIRE. Therefrom, integral yields of ⁴⁷Sc and also ^46gSc as a main impurity were calculated. The various production routes of ⁴⁷Sc were compared together. The results consistency with available experimental data was checked for each reaction. Based on the results, the ⁴⁶Ca(d,n)⁴⁷Sc reaction can leads to the high purity ⁴⁷Sc with the moderate yield.

Photonuclear production, chemistry, and in vitro evaluation of the theranostic radionuclide 47Sc

Background

In molecular imaging and nuclear medicine, theranostic agents that integrate radionuclide pairs are successfully being used for individualized care, which has led to rapidly growing interest in their continued development. These compounds, which are radiolabeled with one radionuclide for imaging and a chemically identical or similar radionuclide for therapy, may improve patient-specific treatment and outcomes by matching the properties of different radionuclides with a targeting vector for a particular tumor type. One proposed theranostic radionuclide is scandium-47 (⁴⁷Sc, T_1/2 = 3.35 days), which can be used for targeted radiotherapy and may be paired with the positron emitting radionuclides, ⁴³Sc (T_1/2 = 3.89 h) and ⁴⁴Sc (T_1/2 = 3.97 h) for imaging. The aim of this study was to investigate the photonuclear production of ⁴⁷Sc via the ⁴⁸Ti(γ,p)⁴⁷Sc reaction using an electron linear accelerator (eLINAC), separation and purification of ⁴⁷Sc, the radiolabeling of somatostatin receptor-targeting peptide DOTATOC with ⁴⁷Sc, and in vitro receptor-mediated binding of [⁴⁷Sc]Sc-DOTATOC in AR42J somatostatin receptor subtype two (SSTR2) expressing rat pancreatic tumor cells.

Results

The rate of ⁴⁷Sc production in a stack of natural titanium foils (n = 39) was 8 × 10⁷ Bq/mA·h (n = 3). Irradiated target foils were dissolved in 2.0 M H₂SO₄ under reflux. After dissolution, trivalent ⁴⁷Sc ions were separated from natural Ti using AG MP-50 cation exchange resin. The recovered ⁴⁷Sc was then purified using CHELEX 100 ion exchange resin. The average decay-corrected two-step ⁴⁷Sc recovery (n = 9) was (77 ± 7)%. A radiolabeling yield of > 99.9% of [⁴⁷Sc]Sc-DOTATOC (0.384 mg in 0.3 mL) was achieved using 1.7 MBq of ⁴⁷Sc. Blocking studies using Octreotide illustrated receptor-mediated uptake of [⁴⁷Sc]Sc-DOTATOC in AR42J cells.

Conclusions

⁴⁷Sc can be produced via the ⁴⁸Ti(γ,p)⁴⁷Sc reaction and separated from natural Ti targets with a yield and radiochemical purity suitable for radiolabeling of peptides for in vitro studies. The data in this work supports the potential use of eLINACs for studies of photonuclear production of medical radionuclides and the future development of high-intensity eLINAC facilities capable of producing relevant quantities of carrier-free radionuclides currently inaccessible via routine production pathways or limited due to costly enriched targets.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0515-8) contains supplementary material, which is available to authorized users.

Porous Silicon as a Platform for Radiation Theranostics Together with a Novel RIB-Based Radiolanthanoid

Mesoporous silicon (PSi) is biocompatible and tailorable material with high potential in drug delivery applications. Here, we report of an evaluation of PSi as a carrier platform for theranostics by delivering a radioactive ion beam- (RIB-) based radioactive lanthanoid into tumors in a mouse model of prostate carcinoma. Thermally hydrocarbonized porous silicon (THCPSi) wafers were implanted with 159Dy at the facility for radioactive ion beams ISOLDE located at CERN, and the resulting [159Dy]THCPSi was postprocessed into particles. The particles were intratumorally injected into mice bearing prostate cancer xenografts. The stability of the particles was studied in vivo, followed by ex vivo biodistribution and autoradiographic studies. We showed that the process of producing radionuclide-implanted PSi particles is feasible and that the [159Dy]THCPSi particles stay stable and local inside the tumor over seven days. Upon release of 159Dy from the particles, the main site of accumulation is in the skeleton, which is in agreement with previous studies on the biodistribution of dysprosium. We conclude that THCPSi particles are a suitable platform together with RIB-based radiolanthanoids for theranostic purposes as they are retained after administration inside the tumor and the radiolanthanoid remains embedded in the THCPSi.

Mini-review: Targeted radiopharmaceuticals incorporating reversible, low molecular weight albumin binders

The combination of low molecular weight, reversible human serum albumin (HSA) binders with targeted radiopharmaceuticals in dual-targeted radioconjugates holds great promise, in particular for endoradiotherapy. Attachment of HSA-binders to radiopharmaceuticals extends their blood circulation time and results in an enhanced tumour uptake as well as often in an improved pharmacokinetic profile. In this mini-review, an overview of currently pursued approaches of this novel strategy is provided.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Promising Scandium Radionuclides for Nuclear Medicine: A Review on the Production and Chemistry up to In Vivo Proofs of Concept

Scandium radionuclides have been identified in the late 1990s as promising for nuclear medicine applications, but have been set aside for about 20 years. Among the different isotopes of scandium, ⁴³Sc and ⁴⁴Sc are interesting for positron emission tomography imaging, whereas ⁴⁷Sc is interesting for therapy. The ⁴⁴Sc/⁴⁷Sc or ⁴³Sc/⁴⁷Sc pairs could be thus envisaged as true theranostic pairs. Another interesting aspect of scandium is that its chemistry is governed by the trivalent ion, Sc³⁺. When combined with its hardness and its size, it gives this element a lanthanide-like behavior. It is then also possible to use it in a theranostic approach in combination with ¹⁷⁷Lu or other lanthanides. This article aims to review the progresses that have been made over the last decade on scandium isotope production and coordination chemistry. It also reviews the radiolabeling aspects and the first (pre) clinical studies performed.

Production of Sc medical radioisotopes with proton and deuteron beams

Proton and deuteron beams (15.3 and 6.8 MeV, respectively) extracted from the PETtrace medical cyclotron at the Radiopharmaceuticals Production and Research Centre in the University of Warsaw, Heavy Ion Laboratory, 28 MeV protons from the C30 cyclotron at the National Centre for Nuclear Research, Świerk, near Warsaw and 33 MeV protons from the ARRONAX accelerator, Nantes were used to produce and investigate the medically interesting Sc radioisotopes. Both natural and isotopically enriched CaCO₃ and TiO₂ targets were used (⁴²Ca, ⁴³Ca, ⁴⁴Ca, ⁴⁸Ca, ⁴⁸Ti). The production efficiency and isotopic purity were determined and are reported here for the highest commercially available enrichments of the target material. The Thick Target Yield, Activities at the End of Bombardment (EOB) and the relative activities of produced impurities at EOB are reported for ⁴³Sc, ^44gSc, ^44mSc and ⁴⁷Sc produced with particle energies below 33 MeV.

Novel Structural Modification Based on Evans Blue Dye to Improve Pharmacokinetics of a Somastostatin-Receptor-Based Theranostic Agent

The development of somastatin (SS) peptide analogues for the detection and treatment of neuroendocrine tumors has been successful with the recent FDA approval of ⁶⁸Ga-DOTA-TATE and ¹⁷⁷Lu-DOTA-TATE. The structure of these peptide constructs contains the peptide binding motif that binds to the receptor with high affinity, a chelator to complex the radioactive metal, and a linker between the peptide and chelator. However, these constructs suffer from rapid blood clearance, which limits their tumor uptake. In this study, this design has been further improved by incorporating a modification to control the in vivo pharmacokinetics. Adding a truncated Evans Blue (EB) dye molecule into the construct provides a prolonged half-life in blood as a result of its low micromolar affinity to albumin. We compared ¹⁷⁷Lu-DOTA-TATE to the modified ¹⁷⁷Lu Evans Blue compound (¹⁷⁷Lu-DMEB-TATE), in vitro and in vivo in mice bearing A427-7 xenografts. The tumor uptake of ¹⁷⁷Lu-DMEB-TATE was significantly greater than the uptake of ¹⁷⁷Lu-DOTA-TATE in the biodistribution and SPECT-imaging studies. The therapeutic effect of the ¹⁷⁷Lu-DMEB-TATE construct was superior to the that of the ¹⁷⁷Lu-DOTA-TATE construct at the doses evaluated.

Production of the positron-emitting radionuclide 68Ga: the radiochemical scheme of radionuclide generator 68Ge → 68Ga

68Ga (T1 / 2 = 68 min) in complexes with peptides is used in positron emission tomography for diagnostics of neuroendocrine tumors. The most promising strategy for 68Ga production is usage of the radionuclide generator 68Ge → 68Ga. In this research, the sorption behavior of Ge(IV) and Ga (III) has been studied. The distribution coefficients (Kd) of Ge(IV) on the anion exchange (Dowex 1×8) and cation exchange (Dowex 50×8) resins in various ethanedioic and hydrochloric acid solutions were determined. For each ion exchange resin, four series of measurements were carried out, in which the concentration of oxalic acid was fixed (0.001 M, 0.003 M, 0.005 M, 0.01 M), and the concentrations of hydrochloric acid ranged from 0 to 3 M. Based on the distribution coefficients, the chemical scheme of the radionuclide generator 68Ge → 68Ga has been developed. The chemical system is based on the anion exchange resin Dowex 1×8 and mixture of 0.005 M C2H2O4 / 0.33 M HCl. Several types of the generators with direct and reverse mode of elution were tested and the optimal scheme was determined. Elution of the generators was performed once a day with 8 ml of 0.005 M C2H2O4 / 0.33 M HCl solution. The 68Ga yield and the 68Ge breakthrough are comparable for all the systems.

Evaluation of a chloride-based 89Zr isolation strategy using a tributyl phosphate (TBP)-functionalized extraction resin

INTRODUCTION

The remarkable stability of the ⁸⁹Zr-DOTA complex has been shown in recent literature. The formation of this complex appears to require ⁸⁹Zr-chloride as the complexation precursor rather than the more conventional ⁸⁹Zr-oxalate. In this work we present a method for the direct isolation of ⁸⁹Zr-chloride from irradiated ^natY foils.

METHODS

⁸⁹Zr, ⁸⁸Zr, and ⁸⁸Y were prepared by 16 MeV proton irradiation of ^natY foils and used for batch-extraction based equilibrium coefficient measurements for TBP and UTEVA resin. Radionuclidically pure ⁸⁹Zr was prepared by 14 MeV proton-irradiation of ^natY foils. These foils were dissolved in concentrated HCl, trapped on columns of TBP or UTEVA resin, and ⁸⁹Zr-chloride was eluted in <1 mL of 0.1 M HCl. For purposes of comparison, conventionally-isolated ⁸⁹Zr-oxalate was converted to ⁸⁹Zr-chloride by trapping, rinsing, and elution from a QMA cartridge into 1 M HCl. Trace metal analysis was performed on the resulting ⁸⁹Zr products.

RESULTS

Equilibrium coefficients for Y and Zr were similar between UTEVA and TBP resins across all HCl concentrations. K_d values of <10^-1 mL/g were observed for Y across all HCl concentrations. K_d values of >10³ mL/g were observed at HCl concentrations >9 M for Zr, falling to K_d values of <10⁰ mL/g at low HCl concentrations. ⁸⁹Zr-chloride was recovered from small columns of TBP in <1 mL of 0.1 M HCl with an overall recovery efficiency of 89 ± 3% (n = 3). An average Y/Zr separation factor of 1.5 × 10⁵ (n = 3) was obtained. Trace metal impurities, notably Fe, were higher in TBP-isolated ⁸⁹Zr-chloride compared with ⁸⁹Zr-chloride prepared using the conventional two-step procedure.

CONCLUSION

TBP-functionalized resin appears promising for the direct isolation of ⁸⁹Zr-chloride from irradiated ^natY targets. Excellent ⁸⁹Zr recovery efficiencies were obtained, and chemical purity was sufficient for proof-of-concept chelation studies.

Scandium and terbium radionuclides for radiotheranostics: current state of development towards clinical application

Currently, different radiometals are in use for imaging and therapy in nuclear medicine: ⁶⁸Ga and ¹¹¹In are examples of nuclides for positron emission tomography (PET) and single photon emission computed tomography (SPECT), respectively, while ¹⁷⁷Lu and ²²⁵Ac are used for β⁻- and α-radionuclide therapy. The application of diagnostic and therapeutic radionuclides of the same element (radioisotopes) would utilize chemically-identical radiopharmaceuticals for imaging and subsequent treatment, thereby enabling the radiotheranostic concept. There are two elements which are of particular interest in this regard: Scandium and Terbium. Scandium presents three radioisotopes for theranostic application. ⁴³Sc (T_1/2 = 3.9 h) and ⁴⁴Sc (T_1/2 = 4.0 h) can both be used for PET, while ⁴⁷Sc (T_1/2 = 3.35 d) is the therapeutic match—also suitable for SPECT. Currently, ⁴⁴Sc is most advanced in terms of production, as well as with pre-clinical investigations, and has already been employed in proof-of-concept studies in patients. Even though the production of ⁴³Sc may be more challenging, it would be advantageous due to the absence of high-energetic γ-ray emission. The development of ⁴⁷Sc is still in its infancy, however, its therapeutic potential has been demonstrated preclinically. Terbium is unique in that it represents four medically-interesting radioisotopes. ¹⁵⁵Tb (T_1/2 = 5.32 d) and ¹⁵²Tb (T_1/2 = 17.5 h) can be used for SPECT and PET, respectively. Both radioisotopes were produced and tested preclinically. ¹⁵²Tb has been the first Tb isotope that was tested (as ¹⁵²Tb-DOTATOC) in a patient. Both radionuclides may be of interest for dosimetry purposes prior to the application of radiolanthanide therapy. The decay properties of ¹⁶¹Tb (T_1/2 = 6.89 d) are similar to ¹⁷⁷Lu, but the coemission of Auger electrons make it attractive for a combined β⁻/Auger electron therapy, which was shown to be effective in preclinical experiments. ¹⁴⁹Tb (T_1/2 = 4.1 h) has been proposed for targeted α-therapy with the possibility of PET imaging. In terms of production, ¹⁶¹Tb and ¹⁵⁵Tb are most promising to be made available at the large quantities suitable for future clinical translation. This review article is dedicated to the production routes, the methods of separating the radioisotopes from the target material, preclinical investigations and clinical proof-of-concept studies of Sc and Tb radionuclides. The availability, challenges of production and first (pre)clinical application, as well as the potential of these novel radionuclides for future application in nuclear medicine, are discussed.