Proton-induced production and radiochemical isolation of 44Ti from scandium metal targets for 44Ti/44Sc generator development

Adapting Solid Phase Radiometalation Photorelease to the Synthesis of 44Sc and 177Lu Radiopharmaceuticals

Synthetic methods that simplify and streamline radiopharmaceutical synthesis help expand utility and access of radiopharmaceuticals to greater patient populations. As radiochemical synthesis is inherently limited by the isotope's half-life, methods that shorten and simplify radiosynthesis and formulation, while also minimizing degradation prior to administration to the patient, are needed. Recently, we introduced solid phase radiometalation photorelease (SPRP) as a new strategy for the synthesis of 68Ga3+ and 64Cu-labeled radiopharmaceuticals. Herein, we expand SPRP to 44Sc3+ and 177Lu3+ and demonstrate its utility in synthesizing two targeted radiopharmaceuticals. Employing a series of model peptide constructs linked to the chelator AAZTA, which has been extensively validated for 44Sc3+, 177Lu3+, and more recently for 68Ga3+, we optimized radiochemical labeling conditions and photorelease with 44Sc3+ and 177Lu3+. Specifically, we show that radionuclide capture on resin is robust and high-yielding following 24-72 h of storage on solid-phase immobilized chelate (177Lu3+) and in the presence of excess target separation impurities such 1 mM calcium (target material for the cyclotron-production of 44Sc3+). The photochemical release of 177Lu3+ and 44Sc-labeled tracers was optimized by addition of ascorbate, an FDA-approved radical quencher, producing 40-60% nondecay-corrected, radiochemical conversion yields and >98% radiochemical purity. Finally, a proof of concept radiolabeling and subsequent preclinical PET-CT study with two targeted radiopharmaceuticals, 44Sc-AAZTA-Glu-PSMA-617 and 44Sc-DOTA-Lys-PSMA-617, successfully demonstrate the compatibility of SPRP with preclinically and clinically relevant rare earth isotopes.

Improved titanium-44 purification process for establishing a high apparent molar activity titanium-44/scandium-44 generator

44Sc-radiopharmaceuticals are gaining more interest but still lack availability. The proof of principle of a44Ti/44Sc generator, which can produce 44Sc daily, has been established but with some limitations and drawbacks. Despite recent advances, separation of 44Ti from massive quantities of scandium target material is still cumbersome. In this work, the improved radiochemical separation of 44Ti from residual scandium target material was carried out by precipitation of Sc with fluoride ions. Furthermore, two approaches were used to set up a high apparent molar activity small-scale generator. The first method relied on extraction chromatography for fine purification using a DGA resin, followed by loading of the purified 44Ti onto a ZR resin column. In the second method, 44Ti was loaded on the ZR resin directly after the precipitation step. This second method was used to set up a generator of 370 kBq and evaluate by radiolabeling. An apparent molar activity of 2 MBq/nmol was obtained for the radiolabeling with DOTA, the most common and suitable chelate for scandium. This result is comparable with previously published data on 44 m/44Sc.

Scandium Radioisotopes-Toward New Targets and Imaging Modalities

The concept of theranostics uses radioisotopes of the same or chemically similar elements to label biological ligands in a way that allows the use of diagnostic and therapeutic radiation for a combined diagnosis and treatment regimen. For scandium, radioisotopes -43 and -44 can be used as diagnostic markers, while radioisotope scandium-47 can be used in the same configuration for targeted therapy. This work presents the latest achievements in the production and processing of radioisotopes and briefly characterizes solutions aimed at increasing the availability of these radioisotopes for research and clinical practice.

Cyclotron production of 43Sc and 44gSc from enriched 42CaO, 43CaO, and 44CaO targets

Introduction: 43Sc and 44gSc are both positron-emitting radioisotopes of scandium with suitable half-lives and favorable positron energies for clinical positron emission tomography (PET) imaging. Irradiation of isotopically enriched calcium targets has higher cross sections compared to titanium targets and higher radionuclidic purity and cross sections than natural calcium targets for reaction routes possible on small cyclotrons capable of accelerating protons and deuterons. Methods: In this work, we investigate the following production routes via proton and deuteron bombardment on CaCO3 and CaO target materials: 42Ca(d,n)43Sc, 43Ca(p,n)43Sc, 43Ca(d,n)44gSc, 44Ca(p,n)44gSc, and 44Ca(p,2n)43Sc. Radiochemical isolation of the produced radioscandium was performed with extraction chromatography using branched DGA resin and apparent molar activity was measured with the chelator DOTA. The imaging performance of 43Sc and 44gSc was compared with 18F, 68Ga, and 64Cu on two clinical PET/CT scanners. Discussion: The results of this work demonstrate that proton and deuteron bombardment of isotopically enriched CaO targets produce high yield and high radionuclidic purity 43Sc and 44gSc. Laboratory capabilities, circumstances, and budgets are likely to dictate which reaction route and radioisotope of scandium is chosen.

Peptide-based positron emission tomography probes: current strategies for synthesis and radiolabelling

In medical imaging, techniques such as magnetic resonance imaging, contrast-enhanced computerized tomography, and positron emission tomography (PET) are extensively available and routinely used for disease diagnosis and treatment. Peptide-based targeting PET probes are usually small peptides with high affinity and specificity to specific cellular and tissue targets opportunely engineered for acting as PET probes. For instance, either the radioisotope (e.g., 18F, 11C) can be covalently linked to the peptide-probe or another ligand that strongly complexes the radioisotope (e.g., 64Cu, 68Ga) through multiple coordinative bonds can be chemically conjugated to the peptide delivery moiety. The main advantages of these probes are that they are cheaper than classical antibody-based PET tracers and can be efficiently chemically modified to be radiolabelled with virtually any radionuclide making them very attractive for clinical use. The goal of this review is to report and summarize recent technologies in peptide PET-based molecular probes synthesis and radiolabelling with the most used radioisotopes in 2022.

Production of zirconium-88 via proton irradiation of metallic yttrium and preparation of target for neutron transmission measurements at DICER

A process for the production of tens to hundreds of GBq amounts of zirconium-88 (⁸⁸Zr) using proton beams on yttrium was developed. For this purpose, yttrium metal targets (≈20 g) were irradiated in a ~16 to 34 MeV proton beam at a beam current of 100-200 µA at the Los Alamos Isotope Production Facility (IPF). The ⁸⁸Zr radionuclide was produced and separated from the yttrium targets using hydroxamate resin with an elution yield of 94(5)% (1σ). Liquid DCl solution in D₂O was selected as a suitable ⁸⁸Zr sample matrix due to the high neutron transmission of deuterium compared to hydrogen and an even distribution of ⁸⁸Zr in the sample matrix. The separated ⁸⁸Zr was dissolved in DCl and 8 µL of the obtained solution was transferred to a tungsten sample can with a 1.2 mm diameter hole using a syringe and automated filling station inside a hot cell. Neutron transmission of the obtained ⁸⁸Zr sample was measured at the Device for Indirect Capture Experiments on Radionuclides (DICER).

Studies on production of 43,44,44mSc from 12C+natCl reactions up to 64 MeV projectile energy

For the first-time production of ^43,44,44mSc radionuclides via ¹²C + ^natCl reaction have been reported. Production yield and experimental cross sections of ^natCl(¹²C,xn)^43,44,44mSc up to 64 MeV have been reported. Experimental cross sections have been found comparable with the theoretically evaluated data using PACE4 and EMPIRE3.2.2 codes.

Production of a broad palette of positron emitting radioisotopes using a low-energy cyclotron: Towards a new success story in cancer imaging?

Over the last several years, positron emission tomography (PET) has matured as an indispensable component of cancer diagnostics. Owing to the large variability observed among the cancer patients and the need to personalize individual patient's diagnosis and treatment, the need for new positron emitting radioisotopes has continued to grow. This mini review opens with a brief introduction to the criteria for radioisotope selection for PET imaging. Subsequently, positron emitting radioisotopes are categorized as: established, emerging and futuristic, based on the stages of their advancement. The production methodologies and the radiochemical separation procedures for obtaining the important radioisotopes in a form suitable for preparation of radiopharmaceuticals for PET imaging are briefly discussed.

State of the Art in Radiolabeling of Antibodies with Common and Uncommon Radiometals for Preclinical and Clinical Immuno-PET

Inert and stable radiolabeling of monoclonal antibodies (mAb), antibody fragments, or antibody mimetics with radiometals is a prerequisite for immuno-PET. While radiolabeling is preferably fast, mild, efficient, and reproducible, especially when applied for human use in a current Good Manufacturing Practice compliant way, it is crucial that the obtained radioimmunoconjugate is stable and shows preserved immunoreactivity and in vivo behavior. Radiometals and chelators have extensively been evaluated to come to the most ideal radiometal–chelator pair for each type of antibody derivative. Although PET imaging of antibodies is a relatively recent tool, applications with ⁸⁹Zr, ⁶⁴Cu, and ⁶⁸Ga have greatly increased in recent years, especially in the clinical setting, while other less common radionuclides such as ⁵²Mn, ⁸⁶Y, ⁶⁶Ga, and ⁴⁴Sc, but also ¹⁸F as in [¹⁸F]AlF are emerging promising candidates for the radiolabeling of antibodies. This review presents a state of the art overview of the practical aspects of radiolabeling of antibodies, ranging from fast kinetic affibodies and nanobodies to slow kinetic intact mAbs. Herein, we focus on the most common approach which consists of first modification of the antibody with a chelator, and after eventual storage of the premodified molecule, radiolabeling as a second step. Other approaches are possible but have been excluded from this review. The review includes recent and representative examples from the literature highlighting which radiometal–chelator–antibody combinations are the most successful for in vivo application.

Improved Sc-44 production in a siphon-style liquid target on a medical cyclotron

In order to use new and promising radiometals for molecular imaging, it is important that they can be obtained as inexpensively and easily as possible. This often requires a cyclotron with solid target hardware or a radionuclide generator, which are not widely available for rarely used radionuclides. Here, we investigate the improved production of ⁴⁴Sc with a siphon-style liquid target system and compare to our previous work with a simple liquid target. A metal salt solution with a high concentration of natural abundance Ca(NO₃)₂ (0.14 g/cm³) was irradiated with a medical cyclotron (12 MeV protons; 20 μA). ⁴⁴Sc was produced via the ^natCa(p,x)⁴⁴Sc reaction. As the pressure increase during irradiation was reduced in the siphon-style target, it was possible to irradiate with a higher proton beam current (20 μA) than with the simple liquid target system (7.9 μA). In addition, the saturation yield per μA of ⁴⁴Sc was increased by a factor of 3.18 ± 0.05 (6.2 ± 0.1 MBq/μA with the siphon target versus 1.94 ± 0.08 MBq/μA with the simple target). This results in an overall increase in ⁴⁴Sc activity by a factor of 11.

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.

Synthesis, radiolabeling, and pre-clinical evaluation of [44Sc]Sc-AAZTA conjugate PSMA inhibitor, a new tracer for high-efficiency imaging of prostate cancer

PURPOSE

The aim of this work was to demonstrate the suitability of AAZTA conjugated to PSMA inhibitor (B28110) labeled with scandium-44 as a new PET tracer for diagnostic imaging of prostate cancer.

BACKGROUND

Nowadays, scandium-44 has received significant attention as a potential radionuclide with favorable characteristics for PET applications. A polyaminopolycarboxylate heptadentate ligand based on a 1,4-diazepine scaffold (AAZTA) has been thoroughly studied as chelator for Gd³⁺ ions for MRI applications. The excellent results of the equilibrium, kinetic, and labeling studies led to a preliminary assessment of the in vitro and in vivo behavior of [⁴⁴Sc][Sc-(AAZTA)]^- and two derivatives, i.e., [⁴⁴Sc][Sc (CNAAZTA-BSA)] and [⁴⁴Sc][Sc (CNAAZTA-cRGDfK)].

RESULTS

B28110 was synthesized by hybrid approach, combining solid-phase peptide synthesis (SPPS) and solution chemistry to obtain high purity (97%) product with an overall yield of 9%. Subsequently, the radioactive labeling was performed with scandium-44 produced from natural calcium target in cyclotron, in good radiochemical yields (RCY) under mild condition (pH 4, 298 K). Stability study in human plasma showed good RCP% of [⁴⁴Sc]Sc-B28110 up to 24 h (94.32%). In vivo PET/MRI imaging on LNCaP tumor-bearing mice showed high tracer accumulation in the tumor regions as early as 20 min post-injection. Ex vivo biodistribution studies confirmed that the accumulation of ⁴⁴Sc-PSMA-617 was two-fold lower than that of the radiolabeled B28110 probes.

CONCLUSIONS

This work demonstrated the suitability of B28110 for the complexation with scandium-44 at room temperature and the high performance of the resulting new tracer based on AAZTA chelator for the diagnosis of prostate cancer using PET.

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β+-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

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.

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.

Electron linear accelerator production and purification of scandium-47 from titanium dioxide targets

The photonuclear production of no-carrier-added (NCA) ⁴⁷Sc from solid ^NatTiO₂ and the subsequent chemical processing and purification have been developed. Scandium-47 was produced by the ⁴⁸Ti(γ,p)⁴⁷Sc reaction with Bremsstrahlung photons produced from the braking of electrons in a high-Z (W or Ta) convertor. Production yields were simulated with the PHITS code (Particle and Heavy Ion Transport-code System) and compared to experimental results. Irradiated TiO₂ targets were dissolved in fuming H₂SO₄ in the presence of Na₂SO₄ and ⁴⁷Sc was purified using the commercially available Eichrom DGA resin. Typical ⁴⁷Sc recovery yields were >90% with excellent specific activity for small batches (<185 MBq batches).