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

Trophic dilution of rare earth elements along the food chain of the Seine estuary (France)

Society's interest in rare earth elements (REEs) and their increasing use in many fields is leading to enrichments in aquatic environments, such as estuaries. This study of the Seine estuary assessed the distribution of REEs along the food web, including different species from 5 phyla representing different trophic levels. Total REE concentrations, which were higher in algae, mollusks, crustaceans and annelids (4.85-156; 1.59-4.08; 2.48 ± 1.80 and 0.14 ± 0.11 μg/g dw, respectively) than in vertebrates (0.03-0.15 μg/g dw), correlated with δ15N indicated a trophic dilution. REE contributions in the studied species were higher for light REEs than for heavy and medium REEs. Positives anomalies for Eu, Gd, Tb and Lu were highlighted particularly in vertebrates, possibly due to species-dependent bioaccumulation/detoxification or related to anthropogenic inputs. The calculated BAF and BSAF indicated an important partitioning of REEs in organisms compared to the dissolved phase and a limited transfer from sediment to organisms.

Prostate-specific Membrane Antigen: Alpha-labeled Radiopharmaceuticals

Novel prostate-specific membrane antigen (PSMA) ligands labeled with α-emitting radionuclides are sparking a growing interest in prostate cancer treatment. These targeted alpha therapies (TATs) have attractive physical properties that deem them effective in progressive metastatic castrate-resistant prostate cancer (mCRPC). Among the PSMA TAT radiopharmaceuticals, [225Ac]Ac-PSMA has been used extensively on a compassionate basis and is currently undergoing phase I trials. Notably, TAT has the potential to improve quality of life and has favorable antitumor activity and outcomes in multiple scenarios other than in mCRPC. In addition, resistance mechanisms to TAT may be amenable to combination therapies.

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.

Potential Biomedical Applications of Terbium-Based Nanoparticles (TbNPs): A Review on Recent Advancement

The scientific world is increasingly focusing on rare earth metal oxide nanomaterials due to their consequential biological prospects, navigated by breakthroughs in biomedical applications. Terbium belongs to rare earth elements (lanthanide series) and possesses remarkably strong luminescence at lower energy emission and signal transduction properties, ushering in wide applications for diagnostic measurements (i.e., bioimaging, biosensors, fluorescence imaging, etc.) in the biomedical sectors. In addition, the theranostic applications of terbium-based nanoparticles further permit the targeted delivery of drugs to the specific site of the disease. Furthermore, the antimicrobial properties of terbium nanoparticles induced via reactive oxygen species (ROS) cause oxidative damage to the cell membrane and nuclei of living organisms, ion release, and surface charge interaction, thus further creating or exhibiting excellent antioxidant characteristics. Moreover, the recent applications of terbium nanoparticles in tissue engineering, wound healing, anticancer activity, etc., due to angiogenesis, cell proliferation, promotion of growth factors, biocompatibility, cytotoxicity mitigation, and anti-inflammatory potentials, make this nanoparticle anticipate a future epoch of nanomaterials. Terbium nanoparticles stand as a game changer in the realm of biomedical research, proffering a wide array of possibilities, from revolutionary imaging techniques to advanced drug delivery systems. Their unique properties, including luminescence, magnetic characteristics, and biocompatibility, have redefined the boundaries of what can be achieved in biomedicine. This review primarily delves into various mechanisms involved in biomedical applications via terbium-based nanoparticles due to their physicochemical characteristics. This review article further explains the potential biomedical applications of terbium nanoparticles with in-depth significant mechanisms from the individual literature. This review additionally stands as the first instance to furnish a "single-platted" comprehensive acquaintance of terbium nanoparticles in shaping the future of healthcare as well as potential limitations and overcoming strategies that require exploration before being trialed in clinical settings.

Trophic transfer of rare earth elements in the food web of the Loire estuary (France)

The increasing use of rare earth elements (REEs) in many industrial sectors and in medecine, causes discharges into the environment and particularly in estuarine areas subjected to strong anthropogenic pressures. Here, we assessed the distribution of REEs along the food web of the Loire estuary. Several species representative of different trophic levels were sampled: 8 vertebrates, 3 crustaceans, 2 mollusks, 3 annelids and 4 algae, as well as Haploops sp. tubes rather related to sediment. The total REE concentrations measured by ICP-MS were the highest in Haploops sp. tubes (141.1 ± 4.7 μg/g dw), algae (1.5 to 34.5 μg/g dw), mollusks (9.9 to 12.0 μg/g dw), annelids (0.7 to 19.9 μg/g dw) and crustaceans (1.4 to 6.3 μg/g dw) and the lowest in vetebrates (0.1 to 1.6 μg/g dw). The individual contribution of REEs was, however, similar between most studied species with a higher contribution of light REEs (76.7 ± 7.6 %) compared to heavy REEs (14.1 ± 3.7 %) or medium REEs (9.2 ± 5.8 %). Trophic relations were estimated by stable isotope analysis of C and N and the linear regression of δ15N with total REE concentrations highlighted a trophic dilution with a corresponding TMS of -2.0. The tissue-specific bioaccumulation investigated for vertebrates demonstrated a slightly higher REE accumulation in gonads than in the muscle. Finally, positive Eu, Gd, Tb and Lu anomalies were highlighted in the normalized REE patterns of most studied species (especially in fish and crustaceans), which is consistent with results in the dissolved phase for Eu and Gd. These anomalies could either be due to anthropogenic inputs or to various bioaccumulation/elimination processes according to the specific species physiology. This study, including most of the trophic levels of the Loire estuary food web provides new insights on the bioaccumulation and trophic transfer of REEs in natural ecosystems.

Nuclear Cross-Section of Proton-Induced Reactions on Enriched 48Ti Targets for the Production of Theranostic 47Sc Radionuclide, 46cSc, 44mSc, 44gSc, 43Sc, and 48V

The cross-sections of the 48Ti(p,x)47Sc, 46cSc, 44mSc, 44gSc, 43Sc, and 48V nuclear reactions were measured from 18 to 70 MeV, with particular attention to 47Sc production. Enriched 48Ti powder was deposited on an aluminum backing and the obtained targets were characterized via elastic backscattering spectroscopy at the INFN-LNL. Targets were exposed to low-intensity proton irradiation using the stacked-foils technique at the ARRONAX facility. Activated samples were measured using γ-spectrometry; the results were compared with the data int he literature and the theoretical TALYS-based values. A regular trend in the new values obtained from the different irradiation runs was noted, as well as a good agreement with the literature data, for all the radionuclides of interest: 47Sc, 46cSc, 44mSc, 44gSc, 43Sc, and 48V. 47Sc production was also discussed, considering yield and radionuclidic purity, for different 47Sc production scenarios.

Hotspots and frontiers in PSMA research for prostate cancer: a bibliometric and visualization analysis over the past 20 years

BACKGROUND

Prostate-specific membrane antigen (PSMA)-targeted imaging and therapy have significantly changed the management of patients with prostate cancer (PCa) at different disease stages. This advancement has attracted the attention of scholars, leading to a prolific output of scholarly publications. This study comprehensively outlines the knowledge framework associated with PSMA-based diagnosis and treatment of PCa through the application of bibliometric analysis, and discusses the potential research trends and foci.

METHODS

Articles and reviews related to PSMA for prostate cancer from 2003 to 2022 were retrieved from Web of Science Core Collection. VOSviewer, Citespace, and R-bibliometrix were primarily employed to execute and visually represent co-authorship, co-citation, and co-occurrence analysis of countries, institutions, authors, references and keywords in this field.

RESULTS

A total of 3830 papers were included. The papers on the field of PSMA-based PCa therapy and imaging had been continuously increased since 2003, but the rate has slowed from 2020. The United States made the largest contribution in this field, in terms of publications 997 (26.03%), H-index (110) and total citations (53,167 times). We identified the most productive institution were Technical University of Munich, and Australian institutions had become very active in recent years. Journal of Nuclear Medicine was the most prominent journal in this field. Professors Matthias Eiber and Martin G Pomper made great achievements, while Ali Afshar-Oromieh was the most co-cited author. According to the result of keywords and topics analysis, "ga-68 labeled psma ligand", "radiation dosimetry" and "HBED-CC" were major research areas in the near future, while "Extended pelvic lymph node dissection" was considered to be the future research foci.

CONCLUSIONS

The field of psma-based PCa therapy and imaging is in the stage of vigorous development and has a bright prospect. The United States and Germany have achieved outstanding results in this area, while Australia has recently developed rapidly. It is foreseeable that more research foci will be lied in the early detection of pelvic lymph nodes and the multimodal imaging-guided surgery.

Construction of the Bioconjugate Py-Macrodipa-PSMA and Its In Vivo Investigations with Large 132/135La3+ and Small 47Sc3+ Radiometal Ions

To harness radiometals in clinical settings, a chelator forming a stable complex with the metal of interest and targets the desired pathological site is needed. Toward this goal, we previously reported a unique set of chelators that can stably bind to both large and small metal ions, via a conformational switch. Within this chelator class, py-macrodipa is particularly promising based on its ability to stably bind several medicinally valuable radiometals including large 132/135La3+, 213Bi3+, and small 44Sc3+. Here, we report a 10-step organic synthesis of its bifunctional analogue py-macrodipa-NCS, which contains an amine-reactive -NCS group that is amenable for bioconjugation reactions to targeting vectors. The hydrolytic stability of py-macordipa-NCS was assessed, revealing a half-life of 6.0 d in pH 9.0 aqueous buffer. This bifunctional chelator was then conjugated to a prostate-specific membrane antigen (PSMA)-binding moiety, yielding the bioconjugate py-macrodipa-PSMA, which was subsequently radiolabeled with large 132/135La3+ and small 47Sc3+, revealing efficient and quantitative complex formation. The resulting radiocomplexes were injected into mice bearing both PSMA-expressing and PSMA-non-expressing tumor xenografts to determine their biodistribution patterns, revealing delivery of both 132/135La3+ and 47Sc3+ to PSMA+ tumor sites. However, partial radiometal dissociation was observed, suggesting that py-macrodipa-PSMA needs further structural optimization.

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.

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.

Rearmed Bifunctional Chelating Ligand for 225Ac/155Tb Precision-Guided Theranostic Radiopharmaceuticals─H4noneunpaX

Superior bifunctional chelating ligands, which can sequester both α-emitting radionuclides (225Ac, 213Bi) and their diagnostic companions (155Tb, 111In), remain a formidable challenge to translating targeted alpha therapy, with complementary diagnostic imaging, to the clinic. H4noneupaX, a chelating ligand with an unusual diametrically opposed arrangement of pendant donor groups, has been developed to this end. H4noneunpaX preferentially complexes Ln3+ and An3+ ions, forming thermodynamically stable (pLa = 17.8, pLu = 21.3) and kinetically inert complexes─single isomeric species by nuclear magnetic resonance and density functional theory. Metal binding versatility demonstrated in radiolabeling [111In]In3+, [155Tb]Tb3+, [177Lu]Lu3+, and [225Ac]Ac3+ achieved high molar activities under mild conditions. Efficient, scalable synthesis enabled in vivo evaluation of bifunctional H4noneunpaX conjugated to two octreotate peptides targeting neuroendocrine tumors. Single photon emission computed tomography/CT and biodistribution studies of 155Tb-radiotracers in AR42J tumor-bearing mice showed excellent image contrast, good tumor uptake, and high in vivo stability. H4noneunpaX shows significant potential for theranostic applications involving 225Ac/155Tb or 177Lu/155Tb.

Novel radionuclides for use in Nuclear Medicine in Europe: where do we stand and where do we go?

BACKGROUND

In order to support the ongoing research across Europe to facilitate access to novel radionuclides, the PRISMAP consortium (European medical radionuclides programme) was established to offer the broadest catalog of non-conventional radionuclides for medical and translational research. The aim of this article is to introduce readers with current status of novel radionuclides in Europe.

MAIN BODY

A consortium questionnaire was disseminated through the PRISMAP consortium and user community, professional associations and preclinical/clinical end users in Europe and the current status of clinical end-users in nuclear medicine were identified. A total of 40 preclinical/clinical users institutions took part in the survey. Clinical end users currently use the following radionuclides in their studies: 177Lu, 68 Ga, 111In, 90Y, other alpha emitters, 225Ac, 64Cu and Terbium isotopes. Radionuclides that would be of interest for users within the next 2-5 years are 64Cu, Terbium radionuclide "family" and alpha emitters, such as 225Ac.

CONCLUSIONS

Thanks to a questionnaire distributed by the PRISMAP consortium, the current status and needs of clinical end-users in nuclear medicine were identified.

RUNX3 regulates the susceptibility against EGFR-targeted non-small cell lung cancer therapy using 47Sc-conjugated cetuximab

BACKGROUND

Radioimmunotherapy with cetuximab and conjugates with various radioisotopes is a feasible treatment option for different tumor models. Scandium-47 (⁴⁷Sc), one of several β^--particle-emitting radioisotopes, displays favorable physical and chemical properties for conjugation to monoclonal antibodies. However, the therapeutic efficacy of ⁴⁷Sc in preclinical and clinical studies is largely unknown. Given that intrinsic alterations in tumors greatly contribute to resistance to anti-epidermal growth factor receptor (EGFR)-targeted therapy, research on overcoming resistance to radioimmunotherapy using cetuximab is required.

METHODS

⁴⁷Sc was produced by irradiation of a CaCO₃ target at the HANARO research reactor in KAERI (Korea Atomic Energy Research Institute) and prepared by chromatographic separation of the irradiated target. Cetuximab was conjugated with ⁴⁷Sc using the bifunctional chelating agent DTPA. Radiochemical purity was determined using instant thin-layer chromatography. The immunoreactivity of ⁴⁷Sc-DTPA-cetuximab was evaluated using the Lindmo method and an in vitro cell-binding assay. The inhibitory effects of cetuximab and ⁴⁷Sc-DTPA-cetuximab were confirmed using cell growth inhibition and BrdU cell proliferation assays. Differences in protein expression levels between cetuximab- and ⁴⁷Sc-DTPA-cetuximab-treated cells were confirmed using western blotting. Complex formation between RUNX3 and DNA repair components was confirmed using immunoprecipitation and western blotting.

RESULTS

Cetuximab induces cell cycle arrest and cell death in EGFR-overexpressing NSCLC cells. Radiolabeling of cetuximab with ⁴⁷Sc led to increased therapeutic efficacy relative to cetuximab alone. Application of ⁴⁷Sc-DTPA-cetuximab induced DNA damage responses, and activation of RUNX3 significantly enhanced the therapeutic efficacy of ⁴⁷Sc-DTPA-cetuximab. RUNX3 mediated susceptibility to EGFR-targeted NSCLC therapy using ⁴⁷Sc-DTPA-cetuximab via interaction with components of the DNA damage and repair machinery.

CONCLUSIONS

⁴⁷Sc-DTPA-cetuximab promoted cell death in EGFR-overexpressing NSCLC cells by targeting EGFR and inducing DNA damage as a result of β irradiation emitted from the conjugated ⁴⁷Sc. Activation of RUNX3 played a key role in DNA damage and repair processes in response to the ionizing radiation and inhibited cell growth, thus leading to more effective tumor suppression. RUNX3 can potentially moderate susceptibility to ⁴⁷Sc-conjugated cetuximab by modulating DNA damage and repair process mechanisms.

Alpha-induced production and robust radiochemical separation of 43Sc as an emerging radiometal for formulation of PET radiopharmaceuticals

Scandium-43 is an emerging PET radiometal that was produced by α-particle bombardment on natural CaCO₃ target via ^natCa (α,p) ⁴³Sc and ^natCa (α,n) ⁴³Ti→⁴³Sc reactions using K-130 cyclotron at VECC. A robust radiochemical procedure based on selective precipitation of ⁴³Sc as Sc(OH)₃ was developed for separation of the radioisotope from the irradiated target. The overall yield of the separation process was >85% and it was obtained in a form suitable for preparation of target specific radiopharmaceuticals for PET imaging of cancer.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

New Horizons in Radioligand Therapy: 161Tb-PSMA-617 in Advanced mCRPC

ABSTRACT

An 85-year-old man with advanced metastatic castration-resistant prostate cancer and progression after 8 cycles of 177 Lu-prostate-specific membrane antigen (PSMA)-617 radioligand therapy (RLT) received 1 cycle of 161 Tb-PSMA RLT. This one administration of 6.5 GBq 161 Tb-PSMA-617 resulted in impressive partial remission with a PSA decline by 53.4% (from 474 to 221 ng/mL) and a concomitant decrease in tumor burden on PSMA PET/CT imaging. The presented case provides stunning initial evidence of the therapeutic potential of 161 Tb-PSMA RLT in metastatic castration-resistant prostate cancer, even in patients progressing after extensive 177 Lu-based RLT. 161 Tb-labeled PSMA ligands may thus offer a promising alternative to standard PSMA RLT.

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.

Preclinical Evaluation of [155/161Tb]Tb-Crown-TATE—A Novel SPECT Imaging Theranostic Agent Targeting Neuroendocrine Tumours

Terbium radioisotopes (149Tb, 152Tb, 155Tb, 161Tb) offer a unique class of radionuclides which encompass all four medicinally relevant nuclear decay modalities (α, β+, γ, β−/e−), and show high potential for the development of element-matched theranostic radiopharmaceuticals. The goal of this study was to design, synthesise, and evaluate the suitability of crown-TATE as a new peptide-conjugate for radiolabelling of [155Tb]Tb3+ and [161Tb]Tb3+, and to assess the imaging and pharmacokinetic properties of each radiotracer in tumour-bearing mice. [155Tb]Tb-crown-TATE and [161Tb]Tb-crown-TATE were prepared efficiently under mild conditions, and exhibited excellent stability in human serum (>99.5% RCP over 7 days). Longitudinal SPECT/CT images were acquired for 155Tb- and 161Tb- labelled crown-TATE in male NRG mice bearing AR42J tumours. The radiotracers, [155Tb]Tb-crown-TATE and [161Tb]Tb-crown-TATE, showed high tumour targeting (32.6 and 30.0 %ID/g, respectively) and minimal retention in non-target organs at 2.5 h post-administration. Biodistribution studies confirmed the SPECT/CT results, showing high tumour uptake (38.7 ± 8.0 %ID/g and 38.5 ± 3.5 %ID/g, respectively) and favourable tumour-to-background ratios. Blocking studies further confirmed SSTR2-specific tumour accumulation. Overall, these findings suggest that crown-TATE has great potential for element-matched molecular imaging and radionuclide therapy using 155Tb and 161Tb.

An Overview of In Vitro Assays of 64Cu-, 68Ga-, 125I-, and 99mTc-Labelled Radiopharmaceuticals Using Radiometric Counters in the Era of Radiotheranostics

Radionuclides are unstable isotopes that mainly emit alpha (α), beta (β) or gamma (γ) radiation through radiation decay. Therefore, they are used in the biomedical field to label biomolecules or drugs for diagnostic imaging applications, such as positron emission tomography (PET) and/or single-photon emission computed tomography (SPECT). A growing field of research is the development of new radiopharmaceuticals for use in cancer treatments. Preclinical studies are the gold standard for translational research. Specifically, in vitro radiopharmaceutical studies are based on the use of radiopharmaceuticals directly on cells. To date, radiometric β- and γ-counters are the only tools able to assess a preclinical in vitro assay with the aim of estimating uptake, retention, and release parameters, including time- and dose-dependent cytotoxicity and kinetic parameters. This review has been designed for researchers, such as biologists and biotechnologists, who would like to approach the radiobiology field and conduct in vitro assays for cellular radioactivity evaluations using radiometric counters. To demonstrate the importance of in vitro radiopharmaceutical assays using radiometric counters with a view to radiogenomics, many studies based on ⁶⁴Cu-, ⁶⁸Ga-, ¹²⁵I-, and ^99mTc-labeled radiopharmaceuticals have been revised and summarized in this manuscript.

Scandium-44 Radiolabeled Peptide and Peptidomimetic Conjugates Targeting Neuropilin-1 Co-Receptor as Potential Tools for Cancer Diagnosis and Anti-Angiogenic Therapy

Pathological angiogenesis, resulting from an imbalance between anti- and pro-angiogenic factors, plays a pivotal role in tumor growth, development and metastasis. The inhibition of the angiogenesis process by the VEGF/VEGFR-2/NRP-1 pathway raises interest in the search for such interaction inhibitors for the purpose of the early diagnosis and treatment of angiogenesis-dependent diseases. In this work we designed and tested peptide-based radiocompounds that selectively bind to the neuropilin-1 co-receptor and prevent the formation of the pro-angiogenic VEGF-A₁₆₅/NRP-1 complex. Three biomolecules, A7R and retro-inverso ^DR7A peptides, and the branched peptidomimetic Lys(hArg)-Dab-Pro-Arg (K4R), conjugated with macrocyclic chelator through two linkers' types, were labeled with theranostic scandium-44 radionuclide, and studied in vitro as potential targeted radiopharmaceuticals. ELISA (enzyme-linked immunosorbent assay) studies showed no negative effect of the introduced biomolecules' changes and high NRP-1 affinity in the case of A7R- and K4R-radiocompounds and a lack affinity for ^DR7A-radiocompounds. All radiopeptides showed a hydrophilic nature as well as high stability against ligand exchange reactions in cysteine/histidine solutions. Unfortunately, all radiocompounds showed unsatisfactory nano-scale stability in human serum, especially for use as therapeutic radioagents. Further work is ongoing and focused on the search for angiogenesis inhibitors that are more human serum stable.

H3TPAN-Triazole-Bn-NH2: Tripicolinate Clicked-Bifunctional Chelate for [225Ac]/[111In] Theranostics

A new, high-denticity, bifunctional ligand─H₃TPAN-triazole-Bn-NH₂─has been synthesized and studied in complexation with [²²⁵Ac]Ac³⁺ and [¹¹¹In]In³⁺ for radiopharmaceutical applications. The bifunctional chelator is readily synthesized, using a high-yielding four-step prep, which is highly adaptable and allows for straightforward incorporation of different covalent linkers using Cu^I-catalyzed alkyne-azide cycloaddition (click) chemistry. Nuclear magnetic resonance (NMR) studies of H₃TPAN-triazole-Bn-NH₂ with La³⁺ and In³⁺ metal ions show the formation of a single, asymmetric complex with each ion in solution, corroborated by density functional theory (DFT) calculations. Radiolabeling studies with [²²⁵Ac]Ac³⁺ and [¹¹¹In]In³⁺ showed highly effective complexation, achieving quantitative radiochemical conversions at low ligand concentrations (<10^-6 M) under mild conditions (RT, 10 min), which is further accompanied by high stability in human serum. The bioconjugate─H₃TPAN-triazole-Bn-Aoc-Pip-Nle-CycMSH_hex─was prepared for targeting of MC1R-positive tumors, and the corresponding ¹¹¹In-radiolabeled tracer was studied in vivo. SPECT/CT and biodistribution studies in C57BL/6J mice bearing B16-F10 tumors were performed, with the radiotracer showing good in vivo stability; tumor uptake was achieved. This work highlights a new promising and versatile bifunctional chelator, easily prepared and encouraging for ²²⁵Ac/¹¹¹In theranostics.

Synthesis, solid-state, solution, and theoretical characterization of an "in-cage" scandium-NOTA complex

Developing chelators that strongly and selectively bind rare-earth elements (Sc, Y, La, and lanthanides) represents a longstanding fundamental challenge in inorganic chemistry. Solving these challenges is becoming more important because of increasing use of rare-earth elements in numerous technologies, ranging from paramagnets to luminescent materials. Within this context, we interrogated the complexation chemistry of the scandium(III) (Sc³⁺) trication with the hexadentate 1,4,7-triazacyclononane-1,4,7-triacetic acid (H₃NOTA) chelator. This H₃NOTA chelator is often regarded as an underperformer for complexing Sc³⁺. A common assumption is that metalation does not fully encapsulate Sc³⁺ within the NOTA^3- macrocycle, leaving Sc³⁺ on the periphery of the chelate and susceptible to demetalation. Herein, we developed a synthetic approach that contradicted those assumptions. We confirmed that our procedure forced Sc³⁺ into the NOTA^3- binding pocket by using single crystal X-ray diffraction to determine the Na[Sc(NOTA)(OOCCH₃)] structure. Density functional theory (DFT) and ⁴⁵Sc nuclear magnetic resonance (NMR) spectroscopy showed Sc³⁺ encapsulation was retained when the crystals were dissolved. Solution-phase and DFT studies revealed that [Sc(NOTA)(OOCCH₃)]^1- could accommodate an additional H₂O capping ligand. Thermodynamic properties associated with the Sc-OOCCH₃ and Sc-H₂O capping ligand interactions demonstrated that these capping ligands occupied critical roles in stabilizing the [Sc(NOTA)] chelation complex.

Nuclear data for light charged particle induced production of emerging medical radionuclides

Whatever the radionuclide to be used in nuclear medicine, it is essential to know the expected yield during the production process, but also of all the possible radionuclidic impurities coproduced, that can have an impact on the product final quality, as well as in the related waste management. The availability of the majority of emerging radioisotopes, including the theranostic ones or pairs, is mainly limited by the fact that, for most of them, the optimal production route still needs to be strengthened if not defined in some cases. The aim of this work is to present a review on the charged particle induced nuclear cross sections to produce some emerging radionuclides for medical applications to show that all types of projectiles should be considered in the quest of producing medical radionuclides. An accurate analysis of the production routes is presented for some radionuclides (67Cu, 47Sc, 89Zr, 103Pd, 186gRe, 97Ru, 211At) chosen as examples to highlight (i) how the quality of the final product strongly depends on the chosen target/projectile/energy parameters set, (ii) how deuteron production routes may sometimes be more effective than the proton ones or lead to a different impurity profile and (iii) how α-particle beams may allow to bypass the limitations occurring when using Z = 1 beams. An overview of possible advantages and drawbacks of the cited production routes and of potential cross sections that still need to be measured, is also reported.

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.

Improvement of nuclear reaction modeling for the production of 47Sc on natural vanadium targets for medical applications

The proton-induced reaction on natural vanadium targets is studied for the production of the innovative theranostic radionuclide 47Sc as well as of its contaminants, mainly 46Sc. The theoretical excitation functions are calculated using the nuclear reaction code TALYS and are compared with the most recent experimental data. A better agreement between the theoretical curves and the data is achieved with an optimization of the nuclear level density parameters. The obtained improvements represent a useful and important result for accurate evaluations of yields and purities which are needed quantities for subsequent dosimetric studies, in view of the radiopharmaceutical applications of 47Sc. The optimization procedure is explained and shown for 47Sc and 46Sc, and also a comparison among the theoretical and experimental cumulatives is given (for the main contaminant) in addition to an estimation of the production yields for two irradiation conditions for both nuclides.

AAZTA-Derived Chelators for the Design of Innovative Radiopharmaceuticals with Theranostic Applications

With the development of 68Ga and 177Lu radiochemistry, theranostic approaches in modern nuclear medicine enabling patient-centered personalized medicine applications have been growing in the last decade. In conjunction with the search for new relevant molecular targets, the design of innovative chelating agents to easily form stable complexes with various radiometals for theranostic applications has gained evident momentum. Initially conceived for magnetic resonance imaging applications, the chelating agent AAZTA features a mesocyclic seven-membered diazepane ring, conferring some of the properties of both acyclic and macrocyclic chelating agents. Described in the early 2000s, AAZTA and its derivatives exhibited interesting properties once complexed with metals and radiometals, combining a fast kinetic of formation with a slow kinetic of dissociation. Importantly, the extremely short coordination reaction times allowed by AAZTA derivatives were particularly suitable for short half-life radioelements (i.e., 68Ga). In view of these particular characteristics, the scope of this review is to provide a survey on the design, synthesis, and applications in the nuclear medicine/radiopharmacy field of AAZTA-derived chelators.

Radioactive nuclei for β + γ PET and theranostics: selected candidates

Positron emission tomography (PET) is an established medical diagnostic imaging method. Continuous improvements are aimed at refining image reconstruction, reducing the amount of radioactive tracer and combining with targeted therapy. Time-of-flight (TOF)-PET provides the localization of the tracer through improved time resolution, nuclear physics may contribute to this goal via selection of radioactive nuclei emitting additional γ-rays. This additional radiation, when properly detected, localizes the decay of the tracer at the line of response (LoR) determined by two detected 511 keV quanta. Selected candidates are presented. Some are particularly interesting, as they are strong candidates for theranostic applications.

Radiolabeled PSMA Inhibitors

PSMA has shown to be a promising target for diagnosis and therapy (theranostics) of prostate cancer. We have reviewed developments in the field of radio- and fluorescence-guided surgery and targeted photodynamic therapy as well as multitargeting PSMA inhibitors also addressing albumin, GRPr and integrin αvβ3. An overview of the regulatory status of PSMA-targeting radiopharmaceuticals in the USA and Europe is also provided. Technical and quality aspects of PSMA-targeting radiopharmaceuticals are described and new emerging radiolabeling strategies are discussed. Furthermore, insights are given into the production, application and potential of alternatives beyond the commonly used radionuclides for radiolabeling PSMA inhibitors. An additional refinement of radiopharmaceuticals is required in order to further improve dose-limiting factors, such as nephrotoxicity and salivary gland uptake during endoradiotherapy. The improvement of patient treatment achieved by the advantageous combination of radionuclide therapy with alternative therapies is also a special focus of this review.

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.

Global experience with PSMA-based alpha therapy in prostate cancer

PURPOSE

This review discusses the current state of prostate-specific membrane antigen (PSMA)-based alpha therapy of metastatic castration-resistant prostate cancer (mCRPC). With this in-depth discussion on the growing field of PSMA-based alpha therapy (PAT), we aimed to increase the interactions between basic scientists and physician-scientists in order to advance the field.

METHODS

To achieve this, we discuss the potential, current status, and opportunities for alpha therapy and strategies, attempted to date, and important questions that need to be addressed. The paper reviews important concepts, including whom to treat, how to treat, what to expect regarding treatment outcome, and toxicity, and areas requiring further investigations.

RESULTS

There is much excitement about the potential of this field. Much of the potential exists because these therapies utilize unique mechanisms of action, difficult to achieve with other conventional therapies.

CONCLUSION

A better understanding of the strengths and limitations of PAT may help in creating an effective therapy for mCRPC and design a rational combinatorial approach to treatment by targeting different tumor pathways.

Theragnostic radionuclides: a clinical perspective

The concept of theragnostics goes back to the earliest days of nuclear medicine, with [¹²³I/¹³¹I]iodide in thyroid disease and [¹²³I/¹³¹I]MIBG in phaeochromocytoma being examples in long-term use. However, in recent years there has been a great expansion in the application of theragnostics, beginning with [⁶⁸Ga/¹⁷⁷Lu]-labelled somatostatin peptides for evaluation and treatment of neuroendocrine tumors. We are currently seeing the rapid development of [⁶⁸Ga/¹⁷⁷Lu]PSMA theragnostics in metastatic prostate cancer. While these applications are very promising, there are a number of practicalities which must be addressed in the development and introduction of novel theragnostics. The physical half-lives of the diagnostic and therapeutic radionuclides must be appropriate for imaging and delivery of targeted cell killing, respectively. The types of radioactive emissions are critical; beta particles can traverse several millimeters but also risk damaging non-target tissues, while alpha particles deliver their energy over a much shorter path length, a few cell diameters, and must be more directly targeted. It must be practical to produce the therapeutic radionuclide and the final radiopharmaceutical and deliver them to the final user within an appropriate time-frame determined by half-life and stability. The biodistribution of the agent must demonstrate adequate accumulation and retention in the target tissue with clearance from adjacent and/or radio-sensitive normal tissues. The commercial success of recently introduced theragnostics suggests a rosy future for personalized medicine.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Radiolabeled Bombesin Analogs

Simple Summary

Recent medical advancements have strived for a personalized medicine approach to patients, aimed at optimizing therapy outcomes with minimum toxicity. In this respect, nuclear medicine methodologies have been playing increasingly important roles. For example, the overexpression of peptide receptors, such as the gastrin-releasing peptide receptor (GRPR), on tumor cells as opposed to their lack of expression in healthy surrounding tissues can be elegantly exploited with the aid of “smart” peptide carriers, such as the analogs of the amphibian 14-peptide bombesin (BBN). These molecules can bring clinically attractive radionuclides to malignant lesions in prostate, breast, and other human cancers, sparing healthy tissues. Depending upon the radionuclide in question, diagnostic imaging with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) has been pursued, identifying patients who are eligible for peptide radionuclide receptor therapy (PRRT) in an integrated “theranostic” approach. In the present review, we (i) discuss the major steps taken in the development of anti-GRPR theranostic radioligands, with a focus on those selected for clinical testing; (ii) comment on the present status in this field of research; and (iii) reflect on the current limitations as well as on new opportunities for their broader and more successful clinical applications.

Abstract

The gastrin-releasing peptide receptor (GRPR) is expressed in high numbers in a variety of human tumors, including the frequently occurring prostate and breast cancers, and therefore provides the rationale for directing diagnostic or therapeutic radionuclides on cancer lesions after administration of anti-GRPR peptide analogs. This concept has been initially explored with analogs of the frog 14-peptide bombesin, suitably modified at the N-terminus with a number of radiometal chelates. Radiotracers that were selected for clinical testing revealed inherent problems associated with these GRPR agonists, related to low metabolic stability, unfavorable abdominal accumulation, and adverse effects. A shift toward GRPR antagonists soon followed, with safer analogs becoming available, whereby, metabolic stability and background clearance issues were gradually improved. Clinical testing of three main major antagonist types led to promising outcomes, but at the same time brought to light several limitations of this concept, partly related to the variation of GRPR expression levels across cancer types, stages, previous treatments, and other factors. Currently, these parameters are being rigorously addressed by cell biologists, chemists, nuclear medicine physicians, and other discipline practitioners in a common effort to make available more effective and safe state-of-the-art molecular tools to combat GRPR-positive tumors. In the present review, we present the background, current status, and future perspectives of this endeavor.

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.

Terbium Medical Radioisotope Production: Laser Resonance Ionization Scheme Development

Terbium (Tb) is a promising element for the theranostic approach in nuclear medicine. The new CERN-MEDICIS facility aims for production of its medical radioisotopes to support related R&D projects in biomedicine. The use of laser resonance ionization is essential to provide radioisotopic yields of highest quantity and quality, specifically regarding purity. This paper presents the results of preparation and characterization of a suitable two-step laser resonance ionization process for Tb. By resonance excitation via an auto-ionizing level, the high ionization efficiency of 53% was achieved. To simulate realistic production conditions for Tb radioisotopes, the influence of a surplus of Gd atoms, which is a typical target material for Tb generation, was considered, showing the necessity of radiochemical purification procedures before mass separation. Nevertheless, a 10-fold enhancement of the Tb ion beam using laser resonance ionization was observed even with Gd:Tb atomic ratio of 100:1.

Theranostics in neuroendocrine tumors: an overview of current approaches and future challenges

Neuroendocrine neoplasms (NENs) comprise a heterogeneous group of tumors, mainly localized in the gastrointestinal system. What characterizes NENs is the expression of hormone receptors on the tumor cell surface, making them accessible for diagnostic and therapeutic approaches (theranostics) using radiolabelled peptides. Somatostatin receptors subtype-two (SST2) play an important role in NENs since they are overexpressed and homogeneously distributed at the surface of the majority of NENs. Accordingly, targeting SST2 for diagnostic and therapeutic purposes has been established. Current research aims at upregulating its expression by epigenetic treatment or improving its targeting via use of alternative radioligands. In addition, recent data suggest a future role of SST antagonists as a diagnostic tool and a potential therapeutic option. Another promising target is the glucagon-like peptide-1 (GLP-1) receptor. Targeting GLP-1R using exendin-4 (GLP-1 analogue) has a high sensitivity for the localization of the often SST2-negative sporadic insulinomas and insulinomas in the context of multiple endocrine neoplasia type-1. Further options for patients with insufficient expression of SST2 involve metaiodobenzylguanidine (MIBG) and the molecular target C-X-C motif chemokine receptor-4 (CXCR4), which have been evaluated for potential theranostic approach in symptomatic NENs or dedifferentiated tumors. Recently, new targets such as the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the fibroblast activation protein (FAP) have been identified in NENs. Finally, minigastrin - a ligand targeting the cholecystokinin-2 (CCK2) receptors in medullary thyroid carcinoma and foregut neuroendocrine tumors - may improve future management of these diseases with currently limited therapeutic options. This review summarises the current approaches and future challenges of diagnostic and therapeutic evaluations in neuroendocrine neoplasms.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives

Simple Summary

One of the most frequently diagnosed cancer in men is adenocarcinoma of the prostate. Once the disease is metastatic, only very limited treatment options are available, resulting in a very short median survival time of 13 months; however, this reality is gradually changing due to the discovery of prostate-specific membrane antigen (PSMA), a protein that is present in cancerous prostate tissue. Researchers have developed pharmaceuticals specific for PSMA, ranging from antibodies (mAb) to low-molecular weight molecules coupled to beta minus and alpha-emitting radionuclides for their use in targeted radionuclide therapy (TRT). TRT offers the possibility of selectively removing cancer tissue via the emission of radiation or radioactive particles within the tumour. In this article, the major milestones in PSMA ligand research and the therapeutic developments are summarised, together with a future perspective on the enhancement of current therapeutic approaches.

Abstract

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

Emerging chelators for nuclear imaging

Chelators are necessary in nuclear medicine imaging to direct an inorganic radionuclide, a radiometal, to a desired target; unfortunately, there is no 'one-size-fits-all' chelator. As the toolbox of radiometals is expanding, new chelators are required to prevent off-target side effects. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is the current gold standard chelator for several radiometals, but typically, chelation requires harsh conditions, making it unsuitable to label biological vectors. The ideal chelator would allow labelling under mild conditions (near-neutral pH and low temperatures [∼37 °C]) and be both thermodynamically and kinetically stable. Over the past 2-3 years, several exciting chelators have been developed that have superior properties to make them worth investigating for future clinical applications.

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.

Clickable Radiocomplexes With Trivalent Radiometals for Cancer Theranostics: In vitro and in vivo Studies

Pre-targeting approaches based on the inverse-electron-demand Diels-Alder (iEDDA) reaction between strained trans-cyclooctenes (TCO) and electron-deficient tetrazines (Tz) have emerged in recent years as valid alternatives to classic targeted strategies to improve the diagnostic and therapeutic properties of radioactive probes. To explore these pre-targeting strategies based on in vivo click chemistry, a small family of clickable chelators was synthesized and radiolabelled with medically relevant trivalent radiometals. The structure of the clickable chelators was diversified to modulate the pharmacokinetics of the resulting [111In]In-radiocomplexes, as assessed upon injection in healthy mice. The derivative DOTA-Tz was chosen to pursue the studies upon radiolabelling with 90Y, yielding a radiocomplex with high specific activity, high radiochemical yields and suitable in vitro stability. The [90Y]Y-DOTA-Tz complex was evaluated in a prostate cancer PC3 xenograft by ex-vivo biodistribution studies and Cerenkov luminescence imaging (CLI). The results highlighted a quick elimination through the renal system and no relevant accumulation in non-target organs or non-specific tumor uptake. Furthermore, a clickable bombesin antagonist was injected in PC3 tumor-bearing mice followed by the radiocomplex [90Y]Y-DOTA-Tz, and the mice imaged by CLI at different post-injection times (p.i.). Analysis of the images 15 min and 1 h p.i. pointed out an encouraging quick tumor uptake with a fast washout, providing a preliminary proof of concept of the usefulness of the designed clickable complexes for pre-targeting strategies. To the best of our knowledge, the use of peptide antagonists for this purpose was not explored before. Further investigations are needed to optimize the pre-targeting approach based on this type of biomolecules and evaluate its eventual advantages.

Emerging Radionuclides in a Regulatory Framework for Medicinal Products - How Do They Fit?

Recent years have seen the establishment of several radionuclides as medicinal products in particular in the setting of theranostics and PET. [¹⁷⁷Lu]Lutetium Chloride or [⁶⁴Cu]Copper Chloride have received marketing authorization as radionuclide precursor, [⁶⁸Ga]Gallium Chloride has received regulatory approval in the form of different ⁶⁸Ge/⁶⁸Ga generators. This is a formal requirement by the EU directive 2001/83, even though for some of these radionuclide precursors no licensed kit is available that can be combined to obtain a final radiopharmaceuticals, as it is the case for Technetium-99m. In view of several highly promising, especially metallic radionuclides for theranostic applications in a wider sense, the strict regulatory environment poses the risk of slowing down development, in particular for radionuclide producers that want to provide innovative radionuclides for clinical research purposes, which is the basis for their further establishment. In this paper we address the regulatory framework for novel radionuclides within the EU, the current challenges in particular related to clinical translation and potential options to support translational development within Europe and worldwide.

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.