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

Production of high purity 47Sc from proton irradiation of natural vanadium targets

BACKGROUND

Scandium-47 is the therapeutic counterpart to the diagnostic radionuclides, 43Sc and 44Sc. Together, these form elementally matched theranostic nuclide pairs, but their incorporation into radiopharmaceuticals requires developing production techniques leading to radioscandium isotopes with high chemical and radionuclidic purity. Previous 47Sc production methods involved expensive, enriched titanium targets that require additional procedures for target recovery. This work investigates the irradiation of natural vanadium targets and the development of purification methods for high-purity 47Sc. Natural vanadium foils were used in cyclotron target configurations. Targets were irradiated with 24 MeV protons at currents of up to 80 µA. A purification method was developed by determining the Kd values of Sc, Cr, and V using MP-50 resin. The final purification method used MP-50 and CM resin columns to isolate the 47Sc from natV and co-produced 51Cr. Inductively Coupled Plasma Mass Spectrometry (ICP-MS), gamma-ray spectroscopy, and a DOTA titration were used to characterize the 47Sc product.

RESULTS

Two cyclotron targets were designed, a small-scale target for developing a purification procedure and a high-power target for scaled-up production. The high-power target maximum current was 80 µA of 24 MeV protons. The yield for an 8 h irradiation at 80 µA of 24 MeV protons, was 128.02 ± 11.1 MBq of 47Sc at End of Bombardment. The radionuclidic purity of 47Sc was 99.5 ± 0.2%. The purification using MP-50 and CM columns resulted in the removal of natV target and 51Cr contaminate in the final 47Sc product, with an average recovery of 72 ± 2.1% and a DOTA apparent molar activity of 7733 ± 155 MBq/µmol. ICP-MS results showed that all top-row transition metals were below the limit of detection (< 1 ppb) with the exception of Zn, which was 64.6 ± 10.3 ppb.

CONCLUSIONS

A high-power cyclotron target capable of withstanding a proton current of 80 µA was developed. A novel separation method was developed for isolating the 47Sc from the vanadium target and the co-produced 51Cr contaminate. The final product characterization resulted in a chemically and radionuclidically pure 47Sc product with high recovery yields.

Emerging role of electrochemistry in radiochemical separation of medically important radiometals: state of the art

Electrochemical separation technology has brought a renaissance in the field of nuclear medicine towards obtaining clinical-grade radiometals for preparation of a wide variety of radiopharmaceuticals. This article is a comprehensive summary of the electrochemical processes developed for the separation of radiometals that could be used for diagnostic or therapeutic applications in nuclear medicine. For using electrochemistry as a tool for the separation of radiometals, intricate knowledge is essential to understand the basic parameters of electrochemical separation processes which include applied potential, selection of electrolyte, choice of the electrode, the temperature of the electrolyte, pH of the electrolyte and time of electrolysis. The advantages of the electrochemical separation approach over the other conventional methodologies such as solvent extraction, column chromatography, sublimation, etc., have also been discussed. The latest research and development from our laboratory on electrochemical methodologies developed for separation of 90Y from 90Sr, 188Re from 188W, 99mTc from 99Mo, 47Sc from 46Ca, 45Ca from 46Sc,153Sm from 154Eu, 169Er from 169Yb, 177Lu from Yb and 132/135La from Ba have been described. In all the cases, the final product is obtained either in a 'no-carrier-added' (NCA) form or free from inextricable impurities and thus found suitable for formulation of radiopharmaceuticals.

Charting the coordinative landscape of the 18F-Sc/44Sc/177Lu triad with the tri-aza-cyclononane (tacn) scaffold

The widely established PET isotope 18F does not have a therapeutic partner. We have recently established that the Sc-F bond can be formed under aqueous, high yielding conditions, paving the way to providing 18F as diagnostic partners to 47Sc and 177Lu radiotherapeutics. Here, we synthesized a library of tacn-based chelators comprised of 10 structurally unique permutations incorporating acetate, methyl-benzylamide and picolinate donor arms. The chelator library encompasses chelators ranging from 6- to 9-dentate, and produces complex changes ranging from +3 to -1. The corresponding Sc-F/Sc and Lu chelate complexes were characterized using computational, spectroscopic and potentiometric methods, followed by optimization of radiolabeling with 18F, 44Sc and 177Lu and concluded by in vivo validation. We identify characterization benchmarks that chart the coordinative landscape of radiochelation approaches for this unusual triad. Our screening identifies two ligand systems, H2L111 and H3L201 as ideal, readily functionalizable constructs for prospective, targeted theranostic applications with 18F/44Sc/177Lu.

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.

Assessing the biocompatibility and stability of CeO2 nanoparticle conjugates with azacrowns for use as radiopharmaceuticals

The application of nanoparticles is promising for the purposes of nuclear medicine due to the possibilities of using them as vectors and transporters of radionuclides. In this study, we have successfully synthesised conjugates of CeO2 nanoparticles and azacrown ligands. Then, the radiolabelling conditions with radionuclides 65Zn, 44Sc and 207Bi were selected and the kinetic stability of the complexes in biologically significant media was evaluated. Optimum conditions for CeO2-APTES-L and CeO2-APTES-DOTA labelling were found: 0.1 g l-1 conjugate and 10-9 M metal cations at 90 °C for complexes with [65Zn]Zn2+, [44Sc]Sc3+ and [207Bi]Bi3+. CeO2-APTES-L-44Sc (radiochemical purity more than 90%) was stable in fetal bovine serum. The obtained results enabled us to choose the most promising complex for biomedical applications for carrying out in vitro and in vivo biodistribution research. Nanoceria and its derivative showed no obvious toxicity to human endothelial cells EA.hy926. Then, the in vivo stability of the studied scandium complex was demonstrated. Taken together, our studies show that functionalised cerium oxide nanoparticles lead to stable radiolabelled nanosystems that may be used for targeted drug delivery, diagnosis and treatment of oncological diseases.

Quantitative SPECT imaging of 155Tb and 161Tb for preclinical theranostic radiopharmaceutical development

BACKGROUND

Element-equivalent matched theranostic pairs facilitate quantitative in vivo imaging to establish pharmacokinetics and dosimetry estimates in the development of preclinical radiopharmaceuticals. Terbium radionuclides have significant potential as matched theranostic pairs for multipurpose applications in nuclear medicine. In particular, 155Tb (t1/2 = 5.32 d) and 161Tb (t1/2 = 6.89 d) have been proposed as a theranostic pair for their respective applications in single photon emission computed tomography (SPECT) imaging and targeted beta therapy. Our study assessed the performance of preclinical quantitative SPECT imaging with 155Tb and 161Tb. A hot rod resolution phantom with rod diameters ranging between 0.85 and 1.70 mm was filled with either 155Tb (21.8 ± 1.7 MBq/mL) or 161Tb (23.6 ± 1.9 MBq/mL) and scanned with the VECTor preclinical SPECT/CT scanner. Image performance was evaluated with two collimators: a high energy ultra high resolution (HEUHR) collimator and an extra ultra high sensitivity (UHS) collimator. SPECT images were reconstructed from photopeaks at 43.0 keV, 86.6 keV, and 105.3 keV for 155Tb and 48.9 keV and 74.6 keV for 161Tb. Quantitative SPECT images of the resolution phantoms were analyzed to report inter-rod contrast, recovery coefficients, and contrast-to-noise metrics.

RESULTS

Quantitative SPECT images of the resolution phantom established that the HEUHR collimator resolved all rods for 155Tb and 161Tb, and the UHS collimator resolved rods ≥ 1.10 mm for 161Tb and ≥ 1.30 mm for 155Tb. The HEUHR collimator maintained better quantitative accuracy than the UHS collimator with recovery coefficients up to 92%. Contrast-to-noise metrics were also superior with the HEUHR collimator.

CONCLUSIONS

Both 155Tb and 161Tb demonstrated potential for applications in preclinical quantitative SPECT imaging. The high-resolution collimator achieves < 0.85 mm resolution and maintains quantitative accuracy in small volumes which is advantageous for assessing sub organ activity distributions in small animals. This imaging method can provide critical quantitative information for assessing and optimizing preclinical Tb-radiopharmaceuticals.

Development of a SnO2-based 44Ti/44Sc generator for medical applications

Towards application of 44Sc for diagnostic nuclear medicine, a 44Ti/44Sc generator based on an inorganic resin has been evaluated. Unlike other radionuclide generators used for medical applications, the long-term retention of the parent 44Ti is vital due to its long half life. Herein, tin dioxide (SnO2), a robust inorganic-based resin, has been synthesized and used as the stationary phase for a 44Ti/44Sc generator. The sorption behavior of 44Ti/44Sc was tested on SnO2 with varying acids, concentrations, and times. Preliminary batch study results showed >88 % 44Ti retention to the resin at lower acid concentrations (0.05 M HNO3 and 0.05 M HCl). A pilot generator was evaluated for a year, demonstrating 85.3 ± 2.8 % 44Sc elution yields and 0.71 ± 0.14 % 44Ti breakthrough in 5 M HNO3. Based on capacity studies, a 7.4 MBq (200 µCi) upscaled generator system was constructed for further evaluation of the SnO2 resin stability and the efficacy of the eluted 44Sc for radiolabeling. 44Sc could be regularly eluted from this generator in 5 M HNO3 with an overall average radiochemical yield 84.7 ± 9.5 %. Post-elution processing of the 44Sc with DGA-normal resin removed all 44Ti present and allowed for high 44Sc-DOTA labeling yields of 94.2 ± 0.5 %. Overall, SnO2 has been shown to be a viable material for a 44Ti/44Sc generator.

In vivoquantitative SPECT imaging of actinium-226: feasibility and proof-of-concept

Objective.225Ac radiopharmaceuticals have tremendous potential for targeted alpha therapy, however,225Ac (t1/2= 9.9 d) lacks direct gamma emissions forin vivoimaging.226Ac (t1/2= 29.4 h) is a promising element-equivalent matched diagnostic radionuclide for preclinical evaluation of225Ac radiopharmaceuticals.226Ac has two gamma emissions (158 keV and 230 keV) suitable for SPECT imaging. This work is the first feasibility study forin vivoquantitative226Ac SPECT imaging and validation of activity estimation.Approach.226Ac was produced at TRIUMF (Vancouver, Canada) with its Isotope Separator and Accelerator (ISAC) facility. [226Ac]Ac3+was radiolabelled with the bioconjugate crown-TATE developed for therapeutic targeting of neuroendocrine tumours. Mice with AR42J tumour xenografts were injected with either 2 MBq of [226Ac]Ac-crown-TATE or 4 MBq of free [226Ac]Ac3+activity and were scanned at 1, 2.5, 5, and 24 h post injection in a preclinical microSPECT/CT. Quantitative SPECT images were reconstructed from the 158 keV and 230 keV photopeaks with attenuation, background, and scatter corrections. Image-based226Ac activity measurements were assessed from volumes of interest within tumours and organs of interest. Imaging data was compared withex vivobiodistribution measured via gamma counter.Main results. We present, to the best of our knowledge, the first everin vivoquantitative SPECT images of226Ac activity distributions. Time-activity curves derived from SPECT images quantify thein vivobiodistribution of [226Ac]Ac-crown-TATE and free [226Ac]Ac3+activity. Image-based activity measurements in the tumours and organs of interest corresponded well withex vivobiodistribution measurements.Significance. Here in, we established the feasibility ofin vivo226Ac quantitative SPECT imaging for accurate measurement of actinium biodistribution in a preclinical model. This imaging method could facilitate more efficient development of novel actinium labelled compounds by providing accurate quantitativein vivopharmacokinetic information essential for estimating toxicities, dosimetry, and therapeutic potency.

Comparison of the dosimetry of scandium-43 and scandium-44 patient organ doses in relation to commonly used gallium-68 for imaging neuroendocrine tumours

BACKGROUND

Several research groups have explored the potential of scandium radionuclides for theragnostic applications due to their longer half-lives and equal or similar coordination chemistry between their diagnostic and therapeutic counterparts, as well as lutetium-177 and terbium-161, respectively. Unlike the gallium-68/lutetium-177 pair, which may show different in-vivo uptake patterns, the use of scandium radioisotopes promises consistent behaviour between diagnostic and therapeutic radiopeptides. An advantage of scandium's longer half-life over gallium-68 is the ability to study radiopeptide uptake over extended periods and its suitability for centralized production and distribution. However, concerns arise from scandium-44's decay characteristics and scandium-43's high production costs. This study aimed to evaluate the dosimetric implications of using scandium radioisotopes with somatostatin analogues against gallium-68 for PET imaging of neuroendocrine tumours.

METHODS

Absorbed dose per injected activity (AD/IA) from the generated time-integrated activity curve (TIAC) were estimated using the radiopeptides [43/44/44mSc]Sc- and [68Ga]Ga-DOTATATE. The kidneys, liver, spleen, and red bone marrow (RBM) were selected for dose estimation studies. The EGSnrc and MCNP6.1 Monte Carlo (MC) codes were used with female (AF) and male (AM) ICRP phantoms. The results were compared to Olinda/EXM software, and the effective dose concentrations assessed, varying composition between the scandium radioisotopes.

RESULTS

Our findings showed good agreement between the MC codes, with - 3 ± 8% mean difference. Kidneys, liver, and spleen showed differences between the MC codes (min and max) in a range of - 4% to 8%. This was observed for both phantoms for all radiopeptides used in the study. Compared to Olinda/EXM the largest observed difference was for the RBM, of 21% for the AF and 16% for the AM for scandium- and gallium-based radiopeptides. Despite the differences, our findings showed a higher absorbed dose on [43/44Sc]Sc-DOTATATE compared to its 68Ga-based counterpart.

CONCLUSION

This study found that [43/44Sc]Sc-DOTATATE delivers a higher absorbed dose to organs at risk compared to [68Ga]Ga-DOTATATE, assuming equal distribution. This is due to the longer half-life of scandium radioisotopes compared to gallium-68. However, calculated doses are within acceptable ranges, making scandium radioisotopes a feasible replacement for gallium-68 in PET imaging, potentially offering enhanced diagnostic potential with later timepoint imaging.

Towards Clinical Development of Scandium Radioisotope Complexes for Use in Nuclear Medicine: Encouraging Prospects with the Chelator 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid (DOTA) and Its Analogues

Scandium (Sc) isotopes have recently attracted significant attention in the search for new radionuclides with potential uses in personalized medicine, especially in the treatment of specific cancer patient categories. In particular, Sc-43 and Sc-44, as positron emitters with a satisfactory half-life (3.9 and 4.0 h, respectively), are ideal for cancer diagnosis via Positron Emission Tomography (PET). On the other hand, Sc-47, as an emitter of beta particles and low gamma radiation, may be used as a therapeutic radionuclide, which also allows Single-Photon Emission Computed Tomography (SPECT) imaging. As these scandium isotopes follow the same biological pathway and chemical reactivity, they appear to fit perfectly into the "theranostic pair" concept. A step-by-step description, initiating from the moment of scandium isotope production and leading up to their preclinical and clinical trial applications, is presented. Recent developments related to the nuclear reactions selected and employed to produce the radionuclides Sc-43, Sc-44, and Sc-47, the chemical processing of these isotopes and the main target recovery methods are also included. Furthermore, the radiolabeling of the leading chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and its structural analogues with scandium is also discussed and the advantages and disadvantages of scandium complexation are evaluated. Finally, a review of the preclinical studies and clinical trials involving scandium, as well as future challenges for its clinical uses and applications, are presented.

Transition and Post-Transition Radiometals for PET Imaging and Radiotherapy

Radiometals are an exciting class of radionuclides because of the large number of metallic elements available that have medically useful isotopes. To properly harness radiometals, they must be securely bound by chelators, which must be carefully matched to the radiometal ion to maximize radiolabeling performance and the stability of the resulting complex. This chapter focuses on practical aspects of radiometallation chemistry including chelator selection, radiolabeling procedures and conditions, radiolysis prevention, purification, quality control, requisite equipment and reagents, and useful tips.

Production and purification of 43Sc and 47Sc from enriched [46Ti]TiO2 and [50Ti]TiO2 targets

The radioscandium isotopes, 43Sc and 47Sc, compose a promising elementally matched theranostic pair that can be used for the development of imaging and therapeutic radiopharmaceuticals with identical structures. This study aimed to investigate the production of high radionuclidic purity 43Sc from enriched [46Ti]TiO2 targets and 47Sc from enriched [50Ti]TiO2 targets and establish a target recycling technique. Enriched [46Ti]TiO2 targets were irradiated with 18 MeV protons, and enriched [50Ti]TiO2 targets were bombarded with 24 MeV protons. 43Sc and 47Sc were purified using ion chromatography attaining recovery yields of 91.7 ± 7.4% and 89.9 ± 3.9%, respectively. The average radionuclidic purity for 43Sc was 98.8 ± 0.3% and for 47Sc 91.5 ± 0.6%, while the average recovery of enriched titanium target material was 96 ± 4.0%. The highest apparent molar activity for [43Sc]Sc-DOTA was 23.2 GBq/µmol and 3.39 GBq/µmol for [47Sc]Sc-DOTA. This work demonstrates the feasibility of using enriched recycled [46Ti]TiO2 and [50Ti]TiO2 targets to produce high purity 43Sc and 47Sc as an elementally matched theranostic isotope pair.

Sc-HOPO: A Potential Construct for Use in Radioscandium-Based Radiopharmaceuticals

Three isotopes of scandium─43Sc, 44Sc, and 47Sc─have attracted increasing attention as potential candidates for use in imaging and therapy, respectively, as well as for possible theranostic use as an elementally matched pair. Here, we present the octadentate chelator 3,4,3-(LI-1,2-HOPO) (or HOPO), an effective chelator for hard cations, as a potential ligand for use in radioscandium constructs with simple radiolabeling under mild conditions. HOPO forms a 1:1 Sc-HOPO complex that was fully characterized, both experimentally and theoretically. [47Sc]Sc-HOPO exhibited good stability in chemical and biological challenges over 7 days. In healthy mice, [43,47Sc]Sc-HOPO cleared the body rapidly with no signs of demetalation. HOPO is a strong candidate for use in radioscandium-based radiopharmaceuticals.

Metal-Ligand Interactions in Scandium Complexes with Radiopharmaceutical Applications

The radioisotopes of scandium (43Sc, 44Sc, and 47Sc) are potential candidates for use in imaging and therapy both separately and as elementally matched pairs for radiotheranostics. In the present study the bonding interactions of Sc3+ with 18 hepta- to decadentate ligands are compared using density functional theory (DFT) calculations. The bonding analysis is based on the natural bond orbital (NBO) model. The main contributions to the bonding were assessed using natural energy decomposition analysis (NEDA). Most of the ligands have anionic character (charges from 2- to 8-); thus the electrical term determines the major differences in the interaction energies. However, interesting features were found in the covalent contributions manifested by the ligand → Sc3+ charge transfer (CT) interactions. Significant differences could be observed in the energetic contributions of the N and O donors to the total CT.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

A combined inorganic-organic titanium-44/scandium-44g radiochemical generator

Scandium-44g (t1/2 = 4.0 h) is an emerging radioisotope for positron emission tomography. It can be produced with a radiochemical generator using its long-lived parent, titanium-44 (t1/2 = 59.1 years). This work presents a new inorganic substrate for 44Ti/44gSc radiochemical generator design based on porous TiO2 microbeads (80 µm and 110 µm particle size, 60 Å pores). Comprehensive evaluation of conditions optimal for generator construction (44Ti loading) and use (44gSc elution) is provided in three steps. For stable 44Ti loading onto titania, heat-treatment at 180 °C for 90 min is shown to be effective while 0.3 M HCl(aq) is identified as the medium of choice for 44gSc elution. Two titania-based 3.6 MBq generators prepared under optimized conditions are characterized with respect to 44gSc recovery and 44Ti breakthrough. Each of these generators employed a different guard substrate to minimize 44Ti breakthrough, TiO2 microbeads and ZR resin. Both are shown to provide comparable 44gSc recoveries close to 50% but differ in 44Ti breakthrough, which is significantly lower with the organic ZR resin guard substrate at 0.0002%. This concept represents a new inorganic-organic approach to 44Ti/44gSc generator design. Benefits of both substrates are exploited: TiO2 has potential for durability necessary for utilizing the long half-life of the 44Ti parent while ZR resin guard segments minimize 44Ti breakthrough.

Porphyrin-Based Brain Tumor-Targeting Agents: [64Cu]Cu-porphyrin and [64Cu]Cu-TDAP

The aim of this study is to evaluate a radioactive metal complex platform for brain tumor targeting. Herein, we introduce a new porphyrin derivative, 5,10,15,20-(tetra-N,N-dimethyl-4-aminophenyl)porphyrin (TDAP), in which four N,N-dimethyl-4-p-phenylenediamine (DMPD) moieties are conjugated to the porphyrin labeled with the radiometal 64Cu. DMPD affected the pharmacokinetics of porphyrin in terms of retention time in vivo and tumor-targeting ability relative to those of unmodified porphyrin. [64Cu]Cu-TDAP showed stronger enhancement than [64Cu]Cu-porphyrin in U87MG glioblastoma cells, especially in the cytoplasm and nucleus, indicating its tumor-targeting properties and potential use as a therapeutic agent. In the subcutaneous and orthotopic models of brain-tumor-bearing mice, [64Cu]Cu-TDAP was clearly visualized in the tumor site via positron emission tomography imaging and showed a tumor-to-brain ratio as high as 13. [64Cu]Cu-TDAP deserves attention as a new diagnostic agent that is suitable for the early diagnosis and treatment of brain tumors.

Radiometals in Imaging and Therapy: Highlighting Two Decades of Research

The present article highlights the important progress made in the last two decades in the fields of molecular imaging and radionuclide therapy. Advancements in radiometal-based positron emission tomography, single photon emission computerized tomography, and radionuclide therapy are illustrated in terms of their production routes and ease of radiolabeling. Applications in clinical diagnostic and radionuclide therapy are considered, including human studies under clinical trials; their current stages of clinical translations and findings are summarized. Because the metalloid astatine is used for imaging and radionuclide therapy, it is included in this review. In regard to radionuclide therapy, both beta-minus (β-) and alpha (α)-emitting radionuclides are discussed by highlighting their production routes, targeted radiopharmaceuticals, and current clinical translation stage.

Liposomes for Cancer Theranostics

Cancer is one of the most well-studied diseases and there have been significant advancements over the last few decades in understanding its molecular and cellular mechanisms. Although the current treatments (e.g., chemotherapy, radiotherapy, gene therapy and immunotherapy) have provided complete cancer remission for many patients, cancer still remains one of the most common causes of death in the world. The main reasons for the poor response rates for different cancers include the lack of drug specificity, drug resistance and toxic side effects (i.e., in healthy tissues). For addressing the limitations of conventional cancer treatments, nanotechnology has shown to be an important field for constructing different nanoparticles for destroying cancer cells. Due to their size (i.e., less than 1 μm), nanoparticles can deliver significant amounts of cancer drugs to tumors and are able to carry moieties (e.g., folate, peptides) for targeting specific types of cancer cells (i.e., through receptor-mediated endocytosis). Liposomes, composed of phospholipids and an interior aqueous core, can be used as specialized delivery vehicles as they can load different types of cancer therapy agents (e.g., drugs, photosensitizers, genetic material). In addition, the ability to load imaging agents (e.g., fluorophores, radioisotopes, MRI contrast media) enable these nanoparticles to be used for monitoring the progress of treatment. This review examines a wide variety of different liposomes for cancer theranostics, with the different available treatments (e.g., photothermal, photodynamic) and imaging modalities discussed for different cancers.

Photonuclear production of 47Ca for 47Ca/47Sc generator from natural CaCO3 targets

This work investigated the indirect production of 47Sc from natural Ca targets via 48Ca(γ,n)47Ca→47Sc + β- + ν‾e with incident electron energies of 30, 35, and 40 MeV. The 47Ca production yields were simulated using the PHITS Monte Carlo simulation code and compared to experimental data. The simulated production rates for all three irradiations are in good agreement with experimental data within uncertainties. As a demonstration of the 47Ca/47Sc generator system, one of the irradiated CaCO3 targets was dissolved in nitric acid, and 47Sc was isolated from the target material using commercially available Eichrom DGA resin. The 47Sc was allowed to grow in, and the purification process was repeated with promising 47Sc and Ca recovery yields.

Design and synthesis of a new bifunctional chelating agent: Application for Al 18F/177Lu complexation

Theranostic and personalized medicine are blooming strategies to improve oncologic patients' health care and facilitate early treatment. While 18F-radiochemistry for theranostic application is attractive due to its imaging properties, combining diagnosis by positron emission tomography (PET) via aluminum-fluoride-18 and β- therapy with lutetium-177 is relevant. Nevertheless, it requires the use of two different chelating agents, which are NOTA and DOTA for aluminum-fluoride-18 and lutetium-177 radiolabeling, respectively. To overcome this issue, we propose herein the synthesis of a new hybrid chelating agent named NO2A-AHM, which can be labeled with different types of emitters (β+, β- and γ) using the mismatched Al18F/177Lu pair. NO2A-AHM, is based on a hydrazine moiety functionalized by a NOTA cycle, a chelating arm, and a linker with a maleimide function. This design is chosen to increase the flexibility and allow the formation of 5 up to 7 coordination bonds with metal ions. Moreover, this agent can be coupled to targeting moieties containing a thiol function, such as peptides, to increase selectivity towards specific cancer cells. Experimental complexation and computational chemistry studies are performed to confirm the capacity of our chelating agent to label both aluminum-fluoride and lutetium using molecular modeling approaches at Density Functional Theory (DFT) level. The proof of concept of the ability of NO2A-AHM to complex both aluminum-fluoride-18, for PET imaging applications, and lutetium-177 for radiotherapy has shown encouraging results which is prominent for the development of a fully consistent theranostic approach.

Accelerator-Based Production of Scandium Radioisotopes for Applications in Prostate Cancer: Toward Building a Pipeline for Rapid Development of Novel Theranostics

In the field of nuclear medicine, the β+ -emitting 43Sc and β- -emitting 47Sc are promising candidates in cancer diagnosis and targeted radionuclide therapy (TRT) due to their favorable decay schema and shared pharmacokinetics as a true theranostic pair. Additionally, scandium is a group-3 transition metal (like 177Lu) and exhibits affinity for DOTA-based chelators, which have been studied in depth, making the barrier to implementation lower for 43/47Sc than for other proposed true theranostics. Before 43/47Sc can see widespread pre-clinical evaluation, however, an accessible production methodology must be established and each isotope's radiolabeling and animal imaging capabilities studied with a widely utilized tracer. As such, a simple means of converting an 18 MeV biomedical cyclotron to support solid targets and produce 43Sc via the 42Ca(d,n)43Sc reaction has been devised, exhibiting reasonable yields. The NatTi(γ,p)47Sc reaction is also investigated along with the successful implementation of chemical separation and purification methods for 43/47Sc. The conjugation of 43/47Sc with PSMA-617 at specific activities of up to 8.94 MBq/nmol and the subsequent imaging of LNCaP-ENZaR tumor xenografts in mouse models with both 43/47Sc-PSMA-617 are also presented.

Engineering a modular 44Ti/44Sc generator: eluate evaluation in preclinical models and estimation of human radiation dosimetry

BACKGROUND

44Sc/47Sc is an attractive theranostic pair for targeted in vivo positron emission tomographic (PET) imaging and beta-particle treatment of cancer. The 44Ti/44Sc generator allows daily onsite production of this diagnostic isotope, which may provide an attractive alternative for PET facilities that lack in-house irradiation capabilities. Early animal and patient studies have demonstrated the utility of 44Sc. In our current study, we built and evaluated a novel clinical-scale 44Ti/44Sc generator, explored the pharmacokinetic profiles of 44ScCl3, [44Sc]-citrate and [44Sc]-NODAGA (1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid) in naïve mice, and estimated the radiation burden of 44ScCl3 in humans.

METHODS

44Ti/44Sc (101.2 MBq) in 6 M HCl solution was utilized to assemble a modular ZR resin containing generator. After assembly, 44Sc was eluted with 0.05 M HCl for further PET imaging and biodistribution studies in female Swiss Webster mice. Based on the biodistribution data, absorbed doses of 44/47ScCl3 in human adults were calculated for 18 organs and tissues using the IDAC-Dose software.

RESULTS

44Ti in 6 M HCl was loaded onto the organic resin generator with a yield of 99.97%. After loading and initial stabilization, 44ScCl3 was eluted with 0.05 M HCl in typical yields of 82.9 ± 5.3% (N = 16), which was normalized to the estimated generator capacity. Estimated generator capacity was computed based on elution time interval and the total amount of 44Ti loaded on the generator. Run in forward and reverse directions, the 44Sc/44Ti ratio from a primary column was significantly improved from 1038 ± 440 to 3557 ± 680 (Bq/Bq) when a secondary, replaceable, ZR resin cartridge was employed at the flow outlet. In vivo imaging and ex vivo distribution studies of the reversible modular generator for 44ScCl3, [44Sc]-citrate and [44Sc]-NODAGA show that free 44Sc remained in the circulation significantly longer than the chelated 44Sc. The dose estimation of 44ScCl3 reveals that the radiation burden is 0.146 mSv/MBq for a 70 kg adult male and 0.179 mSv/MBq for a 57 kg adult female. Liver, spleen and heart wall will receive the highest absorbed dose: 0.524, 0.502, and 0.303 mGy/MBq, respectively, for the adult male.

CONCLUSIONS

A clinical-scale 44Ti/44Sc generator system with a modular design was developed to supply 44ScCl3 in 0.05 M HCl, which is suitable for further radiolabeling and in vivo use. Our data demonstrated that free 44ScCl3 remained in the circulation for extended periods, which resulted in approximately 10 times greater radiation burden than stably chelated 44Sc. Stable 44Sc/47Sc-complexation will be more favorable for in vivo use and for clinical utility.

Activity Measurement of 44Sc and Calibration of Activity Measurement Instruments on Production Sites and Clinics

⁴⁴Sc is a promising radionuclide for positron emission tomography (PET) in nuclear medicine. As a part of the implementation of a production site for ⁴⁴Sc, precise knowledge of the activity of the product is necessary. At the Paul Scherrer Institute (PSI) and the University of Bern (UniBE), ⁴⁴Sc is produced by enriched ⁴⁴CaO-target irradiation with a cyclotron. The two sites use different techniques for activity measurement, namely a dose calibrator at the PSI and a gamma-ray spectrometry system at UniBE and PSI. In this work, the ⁴⁴Sc was produced at the PSI, and samples of the product were prepared in dedicated containers for onsite measurements at PSI, UniBE, and the Institute of Radiation Physics (IRA) in Lausanne for precise activity measurement using primary techniques and for the calibration of the reference ionization chambers. An accuracy of 1% was obtained for the activity measurement, allowing for a precise calibration of the dose calibrator and gamma-ray spectrometry of the two production sites. Each production site now has the capability of measuring ⁴⁴Sc activity with an accuracy of 2%.

Development of purification of no-carrier-added 47Sc of theranostic interest: selective separation study from the natTi(n,p) process

Scandium-47 is one of the most promising medical radioisotopes, and its production and trace separation make it an attractive candidate for theranostic application. In this study, the production of no-carrier-added (NCA) 47Sc through the natTi(n,p) reaction and subsequent purification using liquid–liquid extraction was done for the theranostic application. The comparative separation of NCA 47Sc after the dissolution of an activated Ti target using Di-2-Ethylhexyl Phosphoric Acid (HDEHP) in kerosene was evaluated. The extraction process was optimized in terms of the concentration of extractant, extraction time, pH, and reaction temperature to achieve the maximum possible separation. HDEHP is efficient and promising for rapid extraction and separation of NCA 47Sc from Ti ions at low acidity (pH 0.85) with high extraction percent (>99%), contaminated with 22.3% of Ti ions after 5 min of extraction time. Different stripping reagents were used to separate loaded 47Sc and Ti ions. Firstly, 5 M HCl was enough for stripping the loaded Ti ions. Then the loaded 47Sc was separated with a purity of 100% using 0.05 M NaOH. The obtained results find the HDEHP a promising extractant for efficient separation of 47Sc from irradiated Ti target for preparing the 47Sc radiopharmaceuticals for theranostics applications.

Current State of 44Ti/44Sc Radionuclide Generator Systems and Separation Chemistry

In recent years, there has been an increased interest in 44Ti/44Sc generators as an onsite source of 44Sc for medical applications without needing a proximal cyclotron. The relatively short half-life (3.97 hours) and high positron branching ratio (94.3%) of 44Sc make it a viable candidate for positron emission tomography (PET) imaging. This review discusses current 44Ti/44Sc generator designs, focusing on their chemistry, drawbacks, post-elution processing, and relevant preclinical studies of the 44Sc for potential PET radiopharmaceuticals.

Purification of Carrier-Free 47Sc of Biomedical Interest: Selective Separation Study from natCa(n,γ)

47Sc for theranostic medical applications was produced from the neutron activation of a natural calcium target. Liquid–liquid extraction for separation of the 47Sc radioisotope from 47Ca was carried out with the extractant Cyanex 272 ((2,4,4-trimethylpentyl) phosphinic acid). The effects of various extraction parameters on the extraction efficiency and separation of the two radionuclides were investigated, including the extraction time, pH, metal ion concentrations, extractant concentration, diluent type, and phase ratio. It was shown that the extraction yield of the 47Sc radioisotope with the proposed procedure is about 90%, with a fast separation time of 10 min, at pH 1.8 (0.01 M HCl), and with low E (1%) for 47Ca and high separation factors. The stripping % of the loaded 47Sc isotope was about 99.2% using 0.4 M oxalic acid solution with a purity of 99.9%.

DEVELOPMENT AND ESTIMATION OF HUMAN DOSIMETRY OF A NEW 47SC-RISEDRONATE FOR RADIOPHARMACEUTICAL APPLICATION

Bisphosphonate risedronate (2-(3-pyridinyl)-1-hydroxyethane diphosphonic acid) was radiolabeled with scandium-47 (47Sc) as potential therapeutic radiopharmaceutical for skeletal metastases. Its time-dependent biodistribution in mice was measured and its human dosimetry was derived. The labelling process was performed at 95 °C for 30 min. The stability of the radio-conjugate was tested in human serum at 37 °C and its biodistribution was studied in balb/c mice. The radiochemical yield of ≥90% was obtained corresponding to a specific activity of 277 MBq/mg. The radio-conjugate showed good stability in human serum up to 48 h. A high bone uptake by 48 h post-injection was achieved, which suggests that 47Sc-risedronate may be therapeutically beneficial for the palliation of painful bone metastasis. The estimated absorbed dose coefficient and the time-integrated activity coefficient (ã (rs, TD)) in the bone were 1.35 mGy/MBq and 31.04 (Bq-h/Bq), respectively. The absorbed doses to non-osseous normal organs were much lower than that to the bone.

Cutting edge rare earth radiometals: prospects for cancer theranostics

Background

With recent advances in novel approaches to cancer therapy and imaging, the application of theranostic techniques in personalised medicine has emerged as a very promising avenue of research inquiry in recent years. Interest has been directed towards the theranostic potential of Rare Earth radiometals due to their closely related chemical properties which allow for their facile and interchangeable incorporation into identical bifunctional chelators or targeting biomolecules for use in a diverse range of cancer imaging and therapeutic applications without additional modification, i.e. a “one-size-fits-all” approach. This review will focus on recent progress and innovations in the area of Rare Earth radionuclides for theranostic applications by providing a detailed snapshot of their current state of production by means of nuclear reactions, subsequent promising theranostic capabilities in the clinic, as well as a discussion of factors that have impacted upon their progress through the theranostic drug development pipeline.

Main body

In light of this interest, a great deal of research has also been focussed towards certain under-utilised Rare Earth radionuclides with diverse and favourable decay characteristics which span the broad spectrum of most cancer imaging and therapeutic applications, with potential nuclides suitable for α-therapy (¹⁴⁹Tb), β⁻-therapy (⁴⁷Sc, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁶⁹Er, ¹⁴⁹Pm, ¹⁴³Pr, ¹⁷⁰Tm), Auger electron (AE) therapy (¹⁶¹Tb, ¹³⁵La, ¹⁶⁵Er), positron emission tomography (⁴³Sc, ⁴⁴Sc, ¹⁴⁹Tb, ¹⁵²Tb, ¹³²La, ¹³³La), and single photon emission computed tomography (⁴⁷Sc, ¹⁵⁵Tb, ¹⁵²Tb, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁴⁹Pm, ¹⁷⁰Tm). For a number of the aforementioned radionuclides, their progression from ‘bench to bedside’ has been hamstrung by lack of availability due to production and purification methods requiring further optimisation.

Conclusions

In order to exploit the potential of these radionuclides, reliable and economical production and purification methods that provide the desired radionuclides in high yield and purity are required. With more reactors around the world being decommissioned in future, solutions to radionuclide production issues will likely be found in a greater focus on linear accelerator and cyclotron infrastructure and production methods, as well as mass separation methods. Recent progress towards the optimisation of these and other radionuclide production and purification methods has increased the feasibility of utilising Rare Earth radiometals in both preclinical and clinical settings, thereby placing them at the forefront of radiometals research for cancer theranostics.

Radiochemical separation of 26Al and 41Ca from proton-irradiated vanadium targets for cross-section determination by means of AMS

This work exhibits the very first experimentally determined cross sections for 26Al and 41Ca as proton-induced spallation products of metallic vanadium targets. Additionally, the authors describe a radiochemical separation of 26Al and 41Ca from the vanadium matrix and present the theoretically calculated cross-section values as a reference for the experimental ones.

Synthesis, Physicochemical, Labeling and In Vivo Characterization of 44Sc-Labeled DO3AM-NI as a Hypoxia-Sensitive PET Probe

Hypoxia promotes angiogenesis, which is crucial for tumor growth, and induces malignant progression and increases the therapeutic resistance. Positron emission tomography (PET) enables the detection of the hypoxic regions in tumors using 2-nitroimidazole-based radiopharmaceuticals. We describe here a physicochemical study of the Sc(DO3AM-NI) complex, which indicates: (a) relatively slow formation of the Sc(DO3AM-NI) chelate in acidic solution; (b) lower thermodynamic stability than the reference Sc(DOTA); (c) however, it is substantially more inert and consequently can be regarded as an excellent Sc-binder system. In addition, we report a comparison of ⁴⁴Sc-labeled DO3AM-NI with its known ⁶⁸Ga-labeled analog as a hypoxia PET probe. The in vivo and ex vivo biodistributions of ⁴⁴Sc- and ⁶⁸Ga-labeled DO3AM-NI in healthy and KB tumor-bearing SCID mice were examined 90 and 240 min after intravenous injection. No significant difference was found between the accumulation of ⁴⁴Sc- and ⁶⁸Ga-labeled DO3AM-NI in KB tumors. However, a significantly higher accumulation of [⁶⁸Ga]Ga(DO3AM-NI) was found in liver, spleen, kidney, intestine, lung, heart and brain than for [⁴⁴Sc]Sc(DO3AM-NI), leading to a lower tumor/background ratio. The tumor-to-muscle (T/M) ratio of [⁴⁴Sc]Sc(DO3AM-NI) was approximately 10–15-fold higher than that of [⁶⁸Ga]Ga(DO3AM-NI) at all time points. Thus, [⁴⁴Sc]Sc(DO3AM-NI) allows the visualization of KB tumors with higher resolution, making it a promising hypoxia-specific PET radiotracer.

Porphyrins as Chelating Agents for Molecular Imaging in Nuclear Medicine

Porphyrin ligands, showing a significant affinity for cancer cells, also have the ability to chelate metallic radioisotopes to form potential diagnostic radiopharmaceuticals. They can be applied in single-photon emission computed tomography (SPECT) and positron emission tomography (PET) to evaluate metabolic changes in the human body for tumor diagnostics. The aim of this paper is to present a short overview of the main metallic radionuclides complexed by porphyrin ligands and used in these techniques. These chelation reactions are discussed in terms of the complexation conditions and kinetics and the complex stability.

The Preparation, Biodistribution, and Dosimetry of Encapsulated Radio-Scandium in a Dendrimer for Radio-nano-pharmaceutical Application

This study aimed to investigate the synthesis, characterization, and biodistribution of scandium nanoparticles encapsulated within poly (amidoamine) (PAMAM) dendrimers, as well as to estimate the human absorbed dose. It also aimed to examine, in particular, the amine-terminated PAMAM dendrimers in generation 5. Irradiation of the compound in the nuclear reactor resulted in the formation of Sc-radioactive complex nanoparticles. The compound of the dendrimer-Sc³⁺ was confirmed by the UV-vis spectrometer. The size of the particles was less than 10 nm, and it was assessed using high-resolution transmission electron microscopy (HRTEM) and dynamic light scattering (DLS). The synthesized complex was irradiated by the 3 × 10¹¹ n.cm^-2s^-1 flux of neutron for 2 h. Mice bearing a breast tumor were employed to assess the therapeutic dose that was delivered by the poly scandium-46-nanoparticles. As opposed to the untreated groups, a single injection of poly phosphate-buffered saline to intratumoral in other groups to deliver a dose of 100 µCi resulted in a statistically significant 39.24% reduction in tumor volume 14 days after injection. After applying the biokinetics data in mice, the human’s absorbed dose from scandium-47 encapsulated PAMAM was extrapolated based on animal data. The absorbed doses in critical organs, including the liver, lung, spleen, kidney, and bone, were 0.879, 0.0472, 0.191, 0.107, and 0.155 mGy/MBq, respectively.

Theoretical study of 47Sc production for theranostic applications using proton beams on enriched titanium targets

Recently, scandium-47 has attracted attention in the scientific community thanks to its promising features, making it suitable for targeted radiotherapy and theranostic applications, also in combination with the β+ emitters 43Sc/44Sc. However, in view of possible pre-clinical and clinical studies, finding efficient production routes is still a current research topic. In this work we investigate 47Sc cyclotron production using proton beams on enriched titanium targets. The analysis of the cross sections and yields of both 47Sc (T1/2 = 3.35 d) and its main contaminant 46Sc (T1/2 = 83.79 d) has been performed with the nuclear reaction code Talys (v.1.95). The experimental data (scarce and relatively old) are compared with model calculations and some discrepancies emerge even after the tuning of parameters defining the nuclear level densities, involved in the compound nucleus formation. The 49Ti case allows a more precise cross sections reproduction, conversely the 50Ti case requires further theoretical investigations. Preliminary yields analysis has been carried out for both 47Sc and 46Sc.

Production and subsequent separation of 47Sc of nuclear medicine applications using neutron-induced reactions on different natural targets

47Sc can be produced from different three neutron induced nuclear reactions as 47Ti(n,p)47Sc, 50V(n,α)47Sc and 46Ca(n,γ)47Ca, 47Ca(β−)47Sc using the Egyptian Second Research Reactor. The measured neutron cross-sections (σ) are 62.12 ± 1.93, 4.51 ± 1.27 and 69.36 ± 2.01 mb from the natural targets of TiO2, V2O3 and CaO respectively. The carrier-free 47Sc from 47Ti(n,p), was purified using a composite of Alginate–Carboxymethyl cellulose/di-2-ethylhexyl phosphoric acid. The radiochemical separation of 47Sc with a recovery yield of 90 ± 1.2% was obtained. The eluted 47Sc passed quality control tests (chemical, radionuclide, and radiochemical purities) and was found to be suitable for nuclear medicine applications.

Design and Evaluation of Novel Albumin-Binding Folate Radioconjugates: Systematic Approach of Varying the Linker Entities

Tumor targeting using folate radioconjugates is a promising strategy for theragnostics of folate receptor-positive tumors. The aim of this study was to investigate the impact of structural modifications of folate radioconjugates on their pharmacokinetic properties. Four novel folate radioconjugates ([¹⁷⁷Lu]Lu-OxFol-2, [¹⁷⁷Lu]Lu-OxFol-3, [¹⁷⁷Lu]Lu-OxFol-4, and [¹⁷⁷Lu]Lu-OxFol-5), modified with a lipophilic or hydrophilic linker entity in close proximity to the albumin-binding 4-(p-iodophenyl)butanoate entity or the DOTA chelator, respectively, were designed and evaluated for comparison with the previously developed [¹⁷⁷Lu]Lu-OxFol-1. A hydrophobic 4-(aminomethyl)benzoic acid linker, incorporated in close proximity to the 4-(p-iodophenyl)butanoate entity, enhanced the albumin-binding properties (relative affinity 7.3) of [¹⁷⁷Lu]Lu-OxFol-3 as compared to those of [¹⁷⁷Lu]Lu-OxFol-1 (relative affinity set as 1.0). On the other hand, a hydrophilic d-glutamic acid (d-Glu) linker entity used in [¹⁷⁷Lu]Lu-OxFol-2 compromised the albumin-binding properties. [¹⁷⁷Lu]Lu-OxFol-4 and [¹⁷⁷Lu]Lu-OxFol-5, in which the respective linker entities were incorporated adjacent to the DOTA chelator, showed similar albumin-binding properties (0.6 and 1.0, respectively) as [¹⁷⁷Lu]Lu-OxFol-1. Biodistribution studies in KB tumor-bearing nude mice revealed twofold higher tumor-to-kidney ratios at 4 h and 24 h after injection of [¹⁷⁷Lu]Lu-OxFol-3 (∼1.2) than after injection of [¹⁷⁷Lu]Lu-OxFol-1 (∼0.6). The tumor-to-kidney ratios of [¹⁷⁷Lu]Lu-OxFol-2 were, however, much lower (∼0.2) due to the high kidney retention of this radioconjugate. The tumor-to-kidney ratios of [¹⁷⁷Lu]Lu-OxFol-5 were only slightly increased (∼0.9), and the ratios for [¹⁷⁷Lu]Lu-OxFol-4 (∼0.7) were in the same range as for [¹⁷⁷Lu]Lu-OxFol-1. SPECT/CT imaging studies demonstrated similar tumor uptake of all radioconjugates but a clearly improved tumor-to-kidney ratio for [¹⁷⁷Lu]Lu-OxFol-3 as compared to that for [¹⁷⁷Lu]Lu-OxFol-1. Based on these data, it can be concluded that the linker entity in close proximity to the 4-(p-iodophenyl)butanoate entity affects the radioconjugate's pharmacokinetic profile considerably due to the altered affinity to albumin. Changes in the linker entity, which connects the DOTA chelator with the folate molecule, do not have a major impact on the radioconjugate's tissue distribution profile, however. As a result of these findings, [¹⁷⁷Lu]Lu-OxFol-3 had a comparable therapeutic effect to that of [¹⁷⁷Lu]Lu-OxFol-1 but appeared advantageous in preventing kidney damage. Provided that the kidneys will present the dose-limiting organs in patients, [¹⁷⁷Lu]Lu-OxFol-3 would be the preferred candidate for a clinical translation.

Implications of physics, chemistry and biology for dosimetry calculations using theranostic pairs

Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy (using emitted beta, alpha or Auger-electron particles) for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic (e.g. chemical differences) or extrinsic (e.g. injecting different amounts of each radiopharmaceutical) to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them.

Active Targeted Nanoformulations via Folate Receptors: State of the Art and Future Perspectives

In normal tissues, the expression of folate receptors is low and limited to cells that are important for embryonic development or for folate reabsorption. However, in several pathological conditions some cells, such as cancer cells and activated macrophages, overexpress folate receptors (FRs). This overexpression makes them a potential therapeutic target in the treatment of cancer and inflammatory diseases to obtain a selective delivery of drugs at altered cells level, and thus to improve the therapeutic efficacy and decrease the systemic toxicity of the pharmacological treatments. Two strategies have been used to achieve this folate receptor targeting: (i) the use of ligands with high affinity to FRs (e.g., folic acid or anti-FRs monoclonal antibodies) linked to the therapeutic agents or (ii) the use of nanocarriers whose surface is decorated with these ligands and in which the drug is encapsulated. This manuscript analyzes the use of FRs as a target to develop new therapeutic tools in the treatment of cancer and inflammatory diseases with an emphasis on the nanoformulations that have been developed for both therapeutic and imaging purposes.

New Radionuclides and Technological Advances in SPECT and PET Scanners

Simple Summary

Advances in nuclear medicine are made by technological and radionuclide improvements. Throughout nuclear medicine’s history, these advances were often intertwined and complementary based on different clinical questions, availability and need. This paper covers some of these developments in radionuclides and instrumentation.

Abstract

Developments throughout the history of nuclear medicine have involved improvements in both instrumentation and radionuclides, which have been intertwined. Instrumentation developments always occurred during the search to improving devices’ sensitivity and included advances in detector technology (with the introduction of cadmium zinc telluride and digital Positron Emission Tomography—PET-devices with silicon photomultipliers), design (total body PET) and configuration (ring-shaped, Single-Photon Emission Computed Tomography (SPECT), Compton camera). In the field of radionuclide development, we observed the continual changing of clinically used radionuclides, which is sometimes influenced by instrumentation technology but also driven by availability, patient safety and clinical questions. Some areas, such as tumour imaging, have faced challenges when changing radionuclides based on availability, when this produced undesirable clinical findings with the introduction of unclear focal uptakes and unspecific uptakes. On the other end of spectrum, further developments of PET technology have seen a resurgence in its use in nuclear cardiology, with rubidium-82 from strontium-82/rubidium-82 generators being the radionuclide of choice, moving away from SPECT nuclides thallium-201 and technetium-99m. These continuing improvements in both instrumentation and radionuclide development have helped the growth of nuclear medicine and its importance in the ever-evolving range of patient care options.

Prospects for the production of radioisotopes and radiobioconjugates for theranostics

The development of diagnostic methods in medicine as well as the progress in the synthesis of biologically active compounds allows the use of selected radioisotopes for the simultaneous diagnosis and treatment of diseases, especially cancerous ones, in patients. This approach is called theranostic. This review article includes chemical and physical characterization of chosen theranostic radioisotopes and their compounds that are or could be useful in nuclear medicine.

Branching ratios for the three most intense gamma rays in the decay of 47Ca

A sample of ⁴⁷Ca produced through isotope harvesting at the National Superconducting Cyclotron Laboratory was used to measure branching ratios of 7.17(5)%, 7.11(5)%, and 75.0(5)% for the 489.2, 807.9, and 1297.1 keV characteristic gamma rays, respectively. Based on these updated branching ratios, the ground state to ground state ⁴⁷Ca to ⁴⁷Sc beta decay branching ratio has been indirectly measured as 17.7(5)% and the ground state to 1297.1 keV excited state as 82.2(5)%. These values represent a greatly increased precision for all five branching ratios compared to the currently accepted values (Burrows, 2007). The measurements presented here were made relative to the ingrown ⁴⁷Sc daughter in a⁴⁷Ca sample and the well-established 159.4 keV gamma-ray branching ratio and the half-life for the decay of ⁴⁷Sc (Reher et al., 1986; Meadows and Mode, 1968; Mommsen et al., 1980). These measurements were supported by verifying that the half-lives measured from characteristic gamma-ray peaks over multiple spectra for both ⁴⁷Ca and ⁴⁷Sc were consistent with previously reported values. Additionally, the half-lives of both ⁴⁷Ca and ⁴⁷Sc were independently measured with Liquid Scintillation Counting to reverify the previously reported values used in this study to find updated gamma-ray branching ratio values.

High purity 47Sc production using high-energy photons and natural vanadium targets

Scandium-47 (⁴⁷Sc) is of high value for targeted radiotherapy and theranostics; we report a novel, cost-effective approach to produce high-purity ⁴⁷Sc via photonuclear reactions with natural vanadium. Irradiation at 20 MeV photon end-point energy produces >99.998% pure ⁴⁷Sc, while irradiation at 38 MeV produces 98.8 ± 1.6% pure ⁴⁷Sc. Experimental data suggest producing greater than 100 mCi (3700 MBq) of ⁴⁷Sc using this approach may be feasible. Future research into refinement and scale-up to support pre-clinical research is recommended.