An Overview of Targeted Alpha Therapy with 225Actinium and 213Bismuth

Design of 225Ac-PSMA for targeted alpha therapy in prostate cancer

The first alpha emitting radiopharmaceutical, 223RaCl2, radium dichloride, was approved 10 years ago into the clinical armament of treating bone metastases in metastatic castration-resistant prostate cancer (mCRPC). In addition to this, the first beta-emitting radionuclide Lu-177 chelated with a prostate-specific membrane antigen (PSMA) compound, got last year its marketing approval for the third line treatment of mCRPC. Therefore, there is great excitement about combining alpha-emitters and prostate cancer targeting PSMA compounds. This review describes the clinical history of alpha-emitting PSMA in treating mCRPC. Here, we present the potential, current status, and opportunities for 225Ac-PSMA therapy. The work reviews the basic concepts, current treatment outcome, and toxicity, and areas requiring further investigations such as dosimetric aspects in clinical studies covering more than 400 patients. In general, approximately two-thirds of the patients benefit from this third-line therapy. There is also successful evidence of using 225Ac-PSMA in the second-line of prostate cancer management. The future potential of 225Ac-PSMA therapy and targeted alpha therapy (TAT) of cancer in general is enormous. According to our overview the clinical experience with 225Ac-PSMA therapy to date has shown great benefit and physicians dedicated to theragnostics are anxiously waiting for new applications. Hopefully, this review helps in deeper understanding of the strengths and limitations of TAT and may help in creating effective therapy protocols.

A Technique to Quantify Very Low Activities in Regions of Interest With a Collimatorless Detector

We present a new method to measure sub-microcurie activities of photon-emitting radionuclides in organs and lesions of small animals in vivo. Our technique, named the collimator-less likelihood fit, combines a very high sensitivity collimatorless detector with a Monte Carlo-based likelihood fit in order to estimate the activities in previously segmented regions of interest along with their uncertainties. This is done directly from the photon projections in our collimatorless detector and from the region of interest segmentation provided by an x-ray computed tomography scan. We have extensively validated our approach with 225Ac experimentally in spherical phantoms and mouse phantoms, and also numerically with simulations of a realistic mouse anatomy. Our method yields statistically unbiased results with uncertainties smaller than 20% for activities as low as ~111Bq (3nCi) and for exposures under 30 minutes. We demonstrate that our method yields more robust recovery coefficients when compared to SPECT imaging with a commercial pre-clinical scanner, specially at very low activities. Thus, our technique is complementary to traditional SPECT/CT imaging since it provides a more accurate and precise organ and tumor dosimetry, with a more limited spatial information. Finally, our technique is specially significant in extremely low-activity scenarios when SPECT/CT imaging is simply not viable.

Actinium-225 as an example for monitoring of internal exposure of occupational intakes of radionuclides in face of new nuclear-medical applications for short-lived alpha emitting particles

Monitoring of internal exposure to short-lived alpha-emitting radionuclides such as actinium-225 (225Ac), which are becoming increasingly important in nuclear medicine, plays an important role in the radiation protection of occupationally exposed persons. After having tested gamma spectrometry, liquid scintillation counting and alpha spectrometry for monitoring of internal exposure, the focus of the present study was on solid phase extraction of 225Ac from urine in combination with alpha spectrometry. The development of the method was based on recent findings from the literature on this topic. The method was used in a pilot phase to monitor internal exposure of four workers who were directly or indirectly involved in the manufacture and/or use of 225Ac. The monitoring protocol allowed a relatively short 24-hour urine sample analysis with excellent recovery of the internal standard, but it did not allow for a detection limit of less than 1 mBq nor a sufficient yield of 225Ac. Based on these results it is concluded that an in vitro excretion analysis alone is not appropriate for monitoring internal exposure to 225Ac. Instead, different radiation monitoring techniques have to be combined to ensure the radiation protection of employees.

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.

Navigating The Prostate Cancer Frontier: A Bibliometric and Altmetric Analysis of [225Ac]Ac-PSMA Therapy

The main aim of this study was to evaluate the current state of bibliometric and altmetric research output of [225Ac]Ac-Prostate specific membrane antigen (PSMA) and its implications for prostate cancer (PC). Both PubMed and Scopus digital libraries were systematically explored to retrieve relevant data on the topic of interest. The study of various bibliometric and altmetric indices was facilitated through the use of Microsoft Excel, Stata (Version 17.0), and VOSviewer (Version 1.6) Softwares. The parameters included in this study comprised the examination of published articles, annual trends, countries, institutions, authors, journals, and co-occurring keywords. From 2014 to 2024, our study examined a total of 100 publications within the given domain. The studies that received the highest citations primarily centered on the crucial topic of metastatic castration-resistant prostate cancer, with a particular emphasis on evaluating the safety and effectiveness of [225Ac]Ac-PSMA therapy. Moreover, much scholarly inquiry has been devoted to examining the [225Ac]Ac-PSMA adverse effects. Three high prolific countries (namely, Germany, United States, and South Africa) dominated the research render in terms of publications and citations. Finally, A strong correlation was observed between altmetric score and citation number (P < 0.001). The observed surge in scholarly research output and altmetric indicators associated with [225Ac]Ac-PSMA signifies a shift in emphasis towards embracing alpha targeted therapy in PC.

Radio theranostics in paragangliomas and pheochromocytomas

This continuing education aims to present in a clear and easy-to-understand manner the biology of paragangliomas and pheochromocytomas (PPGLs), the functional imaging studies available for their diagnosis and therapeutic planning, the requirements necessary to administer radioligand therapy (RLT) and the characteristics of these treatments (inclusion criteria, administration protocols, adverse effects and future perspectives). In this pathology we have two RLT options: [131I]MIBG and [177Lu]Lu-DOTA-TATE. The indication for treatment is determined by the expression of its therapeutic target in functional imaging studies, allowing precision and personalized medicine. Although most of the results we have for both treatments have as origin small retrospective series, RLT is presented as a safe and well-tolerated therapeutic option in PPGLs with slow-moderate progression or with uncontrollable symptoms, obtaining high disease control rates.

Phage-encoded bismuth bicycles enable instant access to targeted bioactive peptides

Genetically encoded libraries play a crucial role in discovering structurally rigid, high-affinity macrocyclic peptide ligands for therapeutic applications. Bicyclic peptides with metal centres like bismuth were recently developed as a new type of constrained peptide with notable affinity, stability and membrane permeability. This study represents the genetic encoding of peptide-bismuth and peptide-arsenic bicycles in phage display. We introduce bismuth tripotassium dicitrate (gastrodenol) as a water-soluble bismuth(III) reagent for phage library modification and in situ bicyclic peptide preparation, eliminating the need for organic co-solvents. Additionally, we explore arsenic(III) as an alternative thiophilic element that is used analogously to our previously introduced bicyclic peptides with a bismuth core. The modification of phage libraries and peptides with these elements is instantaneous and entirely biocompatible, offering an advantage over conventional alkylation-based methods. In a pilot display screening campaign aimed at identifying ligands for the biotin-binding protein streptavidin, we demonstrate the enrichment of bicyclic peptides with dissociation constants two orders of magnitude lower than those of their linear counterparts, underscoring the impact of structural constraint on binding affinity.

Development of 225Ac-doped biocompatible nanoparticles for targeted alpha therapy

Targeted alpha therapy (TAT) relies on chemical affinity or active targeting using radioimmunoconjugates as strategies to deliver α-emitting radionuclides to cancerous tissue. These strategies can be affected by transmetalation of the parent radionuclide by competing ions in vivo and the bond-breaking recoil energy of decay daughters. The retention of α-emitting radionuclides and the dose delivered to cancer cells are influenced by these processes. Encapsulating α-emitting radionuclides within nanoparticles can help overcome many of these challenges. Poly(lactic-co-glycolic acid) (PLGA) nanoparticles are a biodegradable and biocompatible delivery platform that has been used for drug delivery. In this study, PLGA nanoparticles are utilized for encapsulation and retention of actinium-225 ([225Ac]Ac3+). Encapsulation of [225Ac]Ac3+ within PLGA nanoparticles (Zave = 155.3 nm) was achieved by adapting a double-emulsion solvent evaporation method. The encapsulation efficiency was affected by both the solvent conditions and the chelation of [225Ac]Ac3+. Chelation of [225Ac]Ac3+ to a lipophilic 2,9-bis-lactam-1,10-phenanthroline ligand ([225Ac]AcBLPhen) significantly decreased its release (< 2%) and that of its decay daughters (< 50%) from PLGA nanoparticles. PLGA nanoparticles encapsulating [225Ac]AcBLPhen significantly increased the delivery of [225Ac]Ac3+ to murine (E0771) and human (MCF-7 and MDA-MB-231) breast cancer cells with a concomitant increase in cell death over free [225Ac]Ac3+ in solution. These results demonstrate that PLGA nanoparticles have potential as radionuclide delivery platforms for TAT to advance precision radiotherapy for cancer. In addition, this technology offers an alternative use for ligands with poor aqueous solubility, low stability, or low affinity, allowing them to be repurposed for TAT by encapsulation within PLGA nanoparticles.

Efficacy and Safety of 225 Ac-DOTATATE in the Treatment of Neuroendocrine Neoplasms With High SSTR Expression

PURPOSE

We aimed to evaluate the efficacy and safety of 225 Ac-DOTATATE targeted α therapy (TAT) in various neuroendocrine neoplasms (NENs) with high somatostatin receptor (SSTR) expression.

PATIENTS AND METHODS

This single-center prospective study included 10 patients with histologically diagnosed NENs that exhibited increased SSTR expression on 68 Ga-DOTATATE PET/CT imaging. All patients received 225 Ac-DOTATATE TAT. The primary end points were molecular imaging-based response and disease control rate (DCR), measured using the slightly modified Positron Emission Tomography Response Criteria in Solid Tumors 1.0. The secondary end points were adverse event profiles and clinical responses. The adverse event profile was determined according to the Common Terminology Criteria for Adverse Events version 5.0. Clinical response was assessed using the EORTC QLQ-C30 v3.0 (European Organization for Research and Treatment of Cancer Core Quality of Life questionnaire version 3.0).

RESULTS

A molecular imaging-based partial response was observed in 40% of all patients, SD in 40%, PD in 20%, and DCR in 80%. The DCR was 83.3% (5/6) in patients who were previously treated with 177 Lu-DOTATATE. According to the EORTC QLQ-C30 v3.0 score, most symptoms improved after 225 Ac-DOTATATE treatment, with only diarrhea showing no improvement. Grade III/IV hematological, kidney, and liver toxicities were not observed. The median follow-up time was 14 months (7-22 months), and no deaths were reported.

CONCLUSIONS

This initial study suggests that 225 Ac-DOTATATE is a potentially promising option for treating NENs with elevated SSTR expression, with an acceptable toxicity profile and well-tolerated adverse effects.

Actinium-225 targeted alpha particle therapy for prostate cancer

Targeted alpha particle therapy (TAT) has emerged as a promising strategy for the treatment of prostate cancer (PCa). Actinium-225 (225Ac), a potent alpha-emitting radionuclide, may be incorporated into targeting vectors, causing robust and in some cases sustained antitumor responses. The development of radiolabeling techniques involving EDTA, DOTA, DOTPA, and Macropa chelators has laid the groundwork for advancements in this field. At the forefront of clinical trials with 225Ac in PCa are PSMA-targeted TAT agents, notably [225Ac]Ac-PSMA-617, [225Ac]Ac-PSMA-I&T and [225Ac]Ac-J591. Ongoing investigations spotlight [225Ac]Ac-hu11B6, [225Ac]Ac-YS5, and [225Ac]Ac-SibuDAB, targeting hK2, CD46, and PSMA, respectively. Despite these efforts, hurdles in 225Ac production, daughter redistribution, and a lack of suitable imaging techniques hinder the development of TAT. To address these challenges and additional advantages, researchers are exploring alpha-emitting isotopes including 227Th, 223Ra, 211At, 213Bi, 212Pb or 149Tb, providing viable alternatives for TAT.

Development of Novel Bimodal Agents Based on Near-Infrared BODIPY-Conjugated Hoechst Derivatives for Combined Use in Auger Electron and Photodynamic Cancer Therapy

Auger electron therapy and photodynamic therapy (PDT) have attracted attention as powerful anticancer modalities. Herein, we report the development of novel bimodal agents for Auger electron therapy and PDT, and their application to combination therapy. [125I]NBH-1/NBH-1 and [125I]NBH-2/NBH-2, composing Hoechst and iodostyryl-BODIPY, were synthesized and evaluated regarding their usefulness as bimodal agents. [125I]NBH-1 showed significantly higher nuclear uptake than [125I]NBH-2 and radioactivity-dependent cytotoxicity induced by Auger electrons. In addition, NBH-1 exhibited photoinduced cytotoxicity. Combination therapy using [125I]NBH-1 and NBH-1 with light irradiation induced a superior cytotoxicity to these treatments alone. In tumor-bearing mice injected with NBH-1 or [125I]NBH-1/NBH-1 under light irradiation, significant tumor growth inhibition was observed compared with that of the control group. Especially, [125I]NBH-1/NBH-1 under light irradiation showed the strongest therapeutic effects among all treatments. These results suggest that [125I]NBH-1/NBH-1 is a potent bimodal agent for Auger therapy and PDT and that combination therapy using [125I]NBH-1 and NBH-1 shows enhanced therapeutic efficacy.

Exploration of Coincidence Detection of Cascade Photons to Enhance Preclinical Multi-Radionuclide SPECT Imaging

We proposed a technique of coincidence detection of cascade photons (CDCP) to enhance preclinical SPECT imaging of therapeutic radionuclides emitting cascade photons, such as Lu-177, Ac-225, Ra-223, and In-111. We have carried out experimental studies to evaluate the proposed CDCP-SPECT imaging of low-activity radionuclides using a prototype coincidence detection system constructed with large-volume cadmium zinc telluride (CZT) imaging spectrometers and a pinhole collimator. With In-111 in experimental studies, the CDCP technique allows us to improve the signal-to-contamination in the projection (Projection-SCR) by ~53 times and reduce ~98% of the normalized contamination. Compared to traditional scatter correction, which achieves a Projection-SCR of 1.00, our CDCP method boosts it to 15.91, showing enhanced efficacy in reducing down-scattered contamination, especially at lower activities. The reconstructed images of a line source demonstrated the dramatic enhancement of the image quality with CDCP-SPECT compared to conventional and triple-energy-window-corrected SPECT data acquisition. We also introduced artificial energy blurring and Monte Carlo simulation to quantify the impact of detector performance, especially its energy resolution and timing resolution, on the enhancement through the CDCP technique. We have further demonstrated the benefits of the CDCP technique with simulation studies, which shows the potential of improving the signal-to-contamination ratio by 300 times with Ac-225, which emits cascade photons with a decay constant of ~0.1 ns. These results have demonstrated the potential of CDCP-enhanced SPECT for imaging a super-low level of therapeutic radionuclides in small animals.

Development of 225Ac/213Bi generator based on α-ZrP-PAN composite for targeted alpha therapy

BACKGROUND

Radioligand therapy using alpha emitters has gained more and more prominence in the last decade. Despite continued efforts to identify new appropriate radionuclides, the combination of 225Ac/213Bi remains among the most promising. Bismuth-213 has been employed in clinical trials in combination with appropriate vectors to treat patients with various forms of cancer, such as leukaemia, bladder cancer, neuroendocrine tumours, melanomas, gliomas, or lymphomas. However, the half-life of 213Bi (T½ = 46 min) implies that its availability for clinical use is limited to hospitals possessing a 225Ac/213Bi radionuclide generator, which is still predominantly scarce. We investigated a new Ac/Bi generator system based on using the composite sorbent α-ZrP-PAN (zirconium(IV) phosphate as active component and polyacrylonitrile as matrix). The developed 225Ac/213Bi generator was subjected to long-term testing after its development. The elution profile was determined and the elution yield, the contamination of the eluate with the parent 225Ac and the contamination of the eluate with the column material were monitored over time.

RESULTS

The high activity (75 MBq of parent 225Ac) generator with a length of 75 mm and a diameter of 4 mm containing the composite sorbent α-ZrP-PAN with a particle size of 0.8 to 1.0 mm as the stationary phase, eluted with a mixture of 10 mM DTPA in 5 mM nitric acid, provided 213Bi with yields ranging from 77 % to 96 % in 2.8 mL of eluate, with parent 225Ac contamination in the order of 10-3 %, up to twenty days of use.

CONCLUSION

All the results of the monitored parameters indicate that the composite sorbent α-ZrP-PAN based separation system for the elution of 213Bi is a very promising and functional solution.

Simultaneous Separation of Protactinium-230 and Uranium-230 Isotopes from a Proton-Irradiated Thorium Matrix

The use of radioisotopes in cancer therapy is becoming increasingly important. As a potential candidate for targeted alpha therapy, 230U (t1/2 = 20.8 d), the decay daughter of 230Pa (t1/2 = 17.4 d) is currently being investigated for cancer treatment. For radioisotopes to be used in biomedicine, they must be radiochemically pure and free from carrier interference. This can be challenging given their short half-life. Thus, radiological separation methods for harvesting isotopes for use in biomedicine must be simple, fast, and capable of obtaining the required purity levels. Herein, we demonstrate a new rapid method for the separation of 230U and 230Pa from a proton-irradiated 232Th metal target and from coproduced fission products with high recovery and purity. A dual-column approach was used, in which the first column was an anion exchange (AX) column and the second column was a DGA column. The MP-1 AX resin was used for the primary separation of the three major components (U, Pa, and Th, as well as fission products), and the DGA column was used for further purification of the separated 230U. The method is simple and allows for straightforward separation of U, Pa, Th, and fission products; also, the entire process from target dissolution to shipment of 230U could be completed in 3 days with an overall 230U recovery yield of 96 ± 3%. Additionally, separated 230Pa fractions can be used to harvest ingrown 230U after initial separation.

Biodistribution study of 211Pb progeny released from intravenously applied 223Ra labelled TiO2 nanoparticles in a mouse model

BACKGROUND

Targeted alpha therapy is one of the most powerful therapeutical modalities available in nuclear medicine. It's therapeutic potency is based on the nuclides that emit one or several alpha particles providing strong and highly localized therapeutic effects. However, some of these radionuclides, like e.g.223Ra or 225Ac decay in cascades, where the radioactive progeny originating from the consecutive alpha-decays may leave the original vector and cause unwanted irradiation of non-target organs. This progeny, even if partially retained in target tissues by internalization processes, typically do not follow the fate of originally targeted radiopharmaceutical and potentially spread over body following their own biodistribution. In this study we aimed to estimate 211Pb/211Bi progeny fate from the 223Ra surface-labelled TiO2 nanoparticles in vitro and the fate of 211Pb in vivo in a mice model.

RESULTS

In vitro stability studies have shown significant differences between the release of the mother 223Ra and its progeny (211Pb, 211Bi) in all the biological matrices that have been tested. The lowest released activities were measured in saline, resulting in less than 5 % of released activity for all nuclides. Contrary to that, the highest released activity of 223Ra of up to 10 % within 48 h was observed in 5 % solution of albumin. The released activity of its progeny; the 211Pb and 211Bi was in the range of 20-40 % in this test medium. Significantly higher released activities of 211Pb and 211Bi compared to 223Ra by at least 10 % was observed in each biological medium, except saline, where no significant differences were observed. The in vivo biodistribution studies results in a mice model, show similar pattern, where it was found that even after accumulation of nanoparticles in target tissues, approximately 10 % of 211Pb is continuously released into the blood stream within 24 h, followed by its natural accumulation in kidneys.

CONCLUSION

This study confirms our assumption that the progeny formed in a chain alpha decay of a certain nuclide, in this case the 223Ra, can be released from its original vector, leave the target tissue, relocate and could be deposited in non-target organs. We did not observe complete progeny wash-out from its original target tissues in our model. This indicates strong dependence of the progeny hot atom fate after its release from the original radiopharmaceutical preparation on multiple factors, like their internalization and retention in cells, cell membranes, extracellular matrices, protein binding, etc. We hypothesize, that also the primary tumour or metastasis size, their metabolic activity may significantly influence progeny fate in vivo, directly impacting the dose delivered to non-target tissues and organs. Therefore a bottom-up approach should be followed and detailed pre-/clinical studies on the release and biodistribution of radioactive progeny originating from the chain alpha emitters should be preferably performed.

Advancements in the development of radiopharmaceuticals for nuclear medicine applications in the treatment of bone metastases

Bone metastases are a painful and complex condition that overwhelmingly impacts the prognosis and quality of life of cancer patients. Over the years, nuclear medicine has made remarkable progress in the diagnosis and management of bone metastases. This review aims to provide a comprehensive overview of the recent advancements in nuclear medicine for the diagnosis and management of bone metastases. Furthermore, the review explores the role of targeted radiopharmaceuticals in nuclear medicine for bone metastases, focusing on radiolabeled molecules that are designed to selectively target biomarkers associated with bone metastases, including osteocytes, osteoblasts, and metastatic cells. The applications of radionuclide-based therapies, such as strontium-89 (Sr-89) and radium-223 (Ra-223), are also discussed. This review also highlights the potential of theranostic approaches for bone metastases, enabling personalized treatment strategies based on individual patient characteristics. Importantly, the clinical applications and outcomes of nuclear medicine in osseous metastatic disease are discussed. This includes the assessment of treatment response, predictive and prognostic value of imaging biomarkers, and the impact of nuclear medicine on patient management and outcomes. The review identifies current challenges and future perspectives on the role of nuclear medicine in treating bone metastases. It addresses limitations in imaging resolution, radiotracer availability, radiation safety, and the need for standardized protocols. The review concludes by emphasizing the need for further research and advancements in imaging technology, radiopharmaceutical development, and integration of nuclear medicine with other treatment modalities. In summary, advancements in nuclear medicine have significantly improved the diagnosis and management of osseous metastatic disease and future developements in the integration of innovative imaging modalities, targeted radiopharmaceuticals, radionuclide production, theranostic approaches, and advanced image analysis techniques hold great promise in improving patient outcomes and enhancing personalized care for individuals with bone metastases.

Using VARSKIN+v1.2 to estimate dose from direct skin contamination with radionuclides 223 Ra, 212 Pb and 225 Ac; considerations for Nuclear Medicine staff and associated Personal Protective Equipment (PPE)

OBJECTIVE

To calculate depth-weighted doses for 223 Ra, 212 Pb and 225 Ac for the skin sites of trunk, arms/legs, face, wrist, back of hand, fingertip, back and side of fingers using VARSKIN+v1.2.

METHODS

Published depth distribution histograms of the basal cells were used with dose averaging in VARSKIN+v1.2. A density correction factor was applied for the 1 g/cc within VARSKIN. Results were compared to the regulatory 70 µm depth and to average depth values for the skin sites.

RESULTS

223 Ra has no alpha component at the regulatory 70 µm. This dose is exceeded by the depth-weighted dose rates for all sites (except the fingertip) with factors ×74 (back of finger) to x3600 (trunk). 212 Pb and 225 Ac have alpha contributions at 70 µm. . For 212 Pb, this dose value is greater by over ×2 than the depth-weighted dose rate for the wrist, back of hand, and finger sites, and underestimates dose rates for the other sites. For 225 Ac, the 70µm dose rate is exceeded by the depth-weighted dose rates for the trunk, face, arms/legs by factors of ×4-10. Using fixed depth values, the depth-weighted dose rates are larger for all sites except the fingertip. The skin dose is also calculated for biological half-lives of 1, 3 and 6 h. Using the depth-weighted dose rates and a 3 h biological half-life, the activity for 500 mSv is in the range 9-177 Bq for the trunk, face, arms/legs, wrist and hand for all three radionuclides.

CONCLUSION

For alpha-emitting radionuclides a depth-weighted calculation gives more representative dose values. The very low activity values for 500 mSv skin dose to be exceeded have implications for appropriate staff PPE and training.

Separation of lead-212 from natural thorium solution utilizing novel sulfonamide dibenzo-18-crown-6

The extraction of lead-212 (212Pb) from radioactive thorium (Th) waste is immensely important, as it serves to mitigate environmental risks associated with radioactive waste and provides a vital source for medical isotopes. To economically extract 212Pb from thorium, we synthesized a novel extractant known as 2,13-disulfonyldiethylamine dibenzo-18-crown-6 (DSADB18C6). We assessed its performance in isolating Pb(II) by employing stable lead and optimizing the parameters of the extraction system. The results showcased an exceptional ability to extract Pb(II) efficiently. Within 10 minutes, using 20 mmol of DSADB18C6 and under conditions of 10 mg L-1 lead concentration (HNO3 = 0.5 mol L-1), the extraction efficiency reached up to 96.1%. Even after four rounds of stripping with 0.4 mol L-1 ammonium citrate, the efficiency remained at 90.2%. Moreover, the extraction stoichiometry and thermodynamics revealed that DSADB18C6 exhibited superior extraction performance compared to the commercial extractant 4',4'',(5'')-di-(tert-butyldicyclohexano)-18-crown-6 (DtBuDC18C6), in line with the density functional theory (DFT) calculation result. Furthermore, we successfully separated 212Pb from the thorium nitrate solution, maintaining radioactive equilibrium with its progeny. Gamma-spectroscopy confirmed a recovery yield of extraction exceeding 85.7%. This study presents a viable approach, underscoring the potential of DSADB18C6 as a promising extractant for the effective separation of 212Pb from radioactive thorium sources.

Implementing Ac-225 labelled radiopharmaceuticals: practical considerations and (pre-)clinical perspectives

BACKGROUND

In the past years, there has been a notable increase in interest regarding targeted alpha therapy using Ac-225, driven by the observed promising clinical anti-tumor effects. As the production and technology has advanced, the availability of Ac-225 is expected to increase in the near future, making the treatment available to patients worldwide.

MAIN BODY

Ac-225 can be labelled to different biological vectors, whereby the success of developing a radiopharmaceutical depends heavily on the labelling conditions, purity of the radionuclide source, chelator, and type of quenchers used to avoid radiolysis. Multiple (methodological) challenges need to be overcome when working with Ac-225; as alpha-emission detection is time consuming and highly geometry dependent, a gamma co-emission is used, but has to be in equilibrium with the mother-nuclide. Because of the high impact of alpha emitters in vivo it is highly recommended to cross-calibrate the Ac-225 measurements for used quality control (QC) techniques (radio-TLC, HPLC, HP-Ge detector, and gamma counter). More strict health physics regulations apply, as Ac-225 has a high toxicity, thereby limiting practical handling and quantities used for QC analysis.

CONCLUSION

This overview focuses specifically on the practical and methodological challenges when working with Ac-225 labelled radiopharmaceuticals, and underlines the required infrastructure and (detection) methods for the (pre-)clinical application.

Evaluation of the tolerability and safety of [225Ac]Ac-PSMA-I&T in patients with metastatic prostate cancer: a phase I dose escalation study

BACKGROUND

Life expectancy of patients with metastatic castration-resistant prostate cancer (mCRPC) is still limited despite several systemic treatments. Within five years after diagnosis of primary prostate cancer, 10-20% of the patients have mCRPC and curation is not an option. Radionuclide therapy (RNT) targeted against prostate-specific membrane antigen (PSMA) emerged as a new treatment option and showed effective results in patients with mCRPC. Survival benefit after [177Lu]Lu-PSMA RNT has already been demonstrated in several clinical trials. However, [225Ac]Ac-PSMA (225Ac-PSMA) appears to be an even more promising radiopharmaceutical for the treatment of mCRPC. The use of alpha emitting radionuclides offers advantages over beta emitting radionuclides due to the high linear energy transfer effective for killing tumor cells and the limited range to reduce the radiation effects on the healthy tissue. However, these results are based on retrospective data and safety data of 225Ac-PSMA are still limited. Therefore, a prospective trial is needed to determine the optimal amount of activity that can be administered.

METHODS

The 225Ac-PSMA-Imaging & Therapy (I&T) trial is an investigator-initiated phase I, single-center, open label, repeated dose-escalation and expansion trial. Patient with PSMA-positive mCRPC after at least one line of chemotherapy and/or one line of nonsteroidal antiandrogen will be treated with 225Ac-PSMA-I&T in increasing amount of activity per cycle. Dose-escalation following an accelerated 3 + 3 design which allows to open the next dose-level cohort in the absence of dose limiting toxicity while the previous one is still ongoing. Up to 4 treatment cohorts will be explored including up to 3 dose-escalation cohorts and one expansion cohort where patients will be administered with the recommended dose. A total of up to 30 patients will be enrolled in this trial. All patients will be evaluated for safety. Additionally, dosimetry was performed for the patients in the dose-escalation cohorts after the first 225Ac-PSMA-I&T administration.

DISCUSSION

This trial will assess the safety and tolerability of 225Ac-PSMA-I&T in patients with mCRPC to recommend the optimal dose for the phase II trial.

TRIAL REGISTRATION

ClinicalTrials.gov, (NCT05902247). Retrospectively registered 13 June 2023.

Development of CD46 targeted alpha theranostics in prostate cancer using 134Ce/225Ac-Macropa-PEG4-YS5

Rationale: 225Ac, a long-lived α-emitter with a half-life of 9.92 days, has garnered significant attention as a therapeutic radionuclide when coupled with monoclonal antibodies and other targeting vectors. Nevertheless, its clinical utility has been hampered by potential off-target toxicity, a lack of optimized chelators for 225Ac, and limitations in radiolabeling methods. In a prior study evaluating the effectiveness of CD46-targeted radioimmunotherapy, we found great therapeutic efficacy but also significant toxicity at higher doses. To address these challenges, we have developed a radioimmunoconjugate called 225Ac-Macropa-PEG4-YS5, incorporating a stable PEGylated linker to maximize tumoral uptake and increase tumor-to-background ratios. Our research demonstrates that this conjugate exhibits greater anti-tumor efficacy while minimizing toxicity in prostate cancer 22Rv1 tumors. Methods: We synthesized Macropa.NCS and Macropa-PEG4/8-TFP esters and prepared Macropa-PEG0/4/8-YS5 (with nearly ~1:1 ratio of macropa chelator to antibody YS5) as well as DOTA-YS5 conjugates. These conjugates were then radiolabeled with 225Ac in a 2 M NH4OAc solution at 30 °C, followed by purification using YM30K centrifugal purification. Subsequently, we conducted biodistribution studies and evaluated antitumor activity in nude mice (nu/nu) bearing prostate 22Rv1 xenografts in both single-dose and fractionated dosing studies. Micro-PET imaging studies were performed with 134Ce-Macropa-PEG0/4/8-YS5 in 22Rv1 xenografts for 7 days. Toxicity studies were also performed in healthy athymic nude mice. Results: As expected, we achieved a >95% radiochemical yield when labeling Macropa-PEG0/4/8-YS5 with 225Ac, regardless of the chelator ratios (ranging from 1 to 7.76 per YS5 antibody). The isolated yield exceeded 60% after purification. Such high conversions were not observed with the DOTA-YS5 conjugate, even at a higher ratio of 8.5 chelators per antibody (RCY of 83%, an isolated yield of 40%). Biodistribution analysis at 7 days post-injection revealed higher tumor uptake for the 225Ac-Macropa-PEG4-YS5 (82.82 ± 38.27 %ID/g) compared to other conjugates, namely 225Ac-Macropa-PEG0/8-YS5 (38.2 ± 14.4/36.39 ± 12.4 %ID/g) and 225Ac-DOTA-YS5 (29.35 ± 7.76 %ID/g). The PET Imaging of 134Ce-Macropa-PEG0/4/8-YS5 conjugates resulted in a high tumor uptake, and tumor to background ratios. In terms of antitumor activity, 225Ac-Macropa-PEG4-YS5 exhibited a substantial response, leading to prolonged survival compared to 225Ac-DOTA-YS5, particularly when administered at 4.625 kBq doses, in single or fractionated dose regimens. Chronic toxicity studies observed mild to moderate renal toxicity at 4.625 and 9.25 kBq doses. Conclusions: Our study highlights the promise of 225Ac-Macropa-PEG4-YS5 for targeted alpha particle therapy. The 225Ac-Macropa-PEG4-YS5 conjugate demonstrates improved biodistribution, reduced off-target binding, and enhanced therapeutic efficacy, particularly at lower doses, compared to 225Ac-DOTA-YS5. Incorporating theranostic 134Ce PET imaging further enhances the versatility of macropa-PEG conjugates, offering a more effective and safer approach to cancer treatment. Overall, this methodology has a high potential for broader clinical applications.

Current status of PSMA-targeted imaging and therapy

Currently, the incidence of prostate cancer is increasing, and it has become a great threat to men's health. The detection, staging, and follow-up of prostate cancer patients are inseparable from morphology or magnetic resonance imaging (MRI). However, these do not fully meet the needs of diagnosis and patient management. In particular, owing to the late diagnosis, metastatic castration-resistant prostate cancer (mCRPC) patients usually have poor survival and few options for further effective treatment. Prostate-specific membrane antigen (PSMA), because of its overexpression on prostate cancer cells, has gained interest due to its application in the imaging and theranostics field. Several PSMA radioligands have been developed for imaging and treating prostate cancer. Many clinical trials have assessed the efficacy and safety profiles of these radionuclide agents and show promise in patients who have exhausted other standard treatment options. To date, several small compounds for targeting PSMA have been developed, and 68Ga-PSMA-11 and 18F-DCFPyL have been approved by the United States (US) Food and Drug Administration (FDA) for imaging of prostate cancer. 111In- or 99mTc-labeled PSMA-ligand can guide surgeons searching for radioactive metastatic lymph nodes, and 177Lu- or 225Ac-labeled PSMA-ligand can be used for internal radiotherapy. Moreover, some molecules for therapeutic application are undergoing different stages of clinical trials. In this review, we present current perspectives on the use of PSMA-targeted imaging and theranostics in prostate cancer. As PSMA-targeted imaging and therapeutics are becoming the standard of care for prostate cancer patients, we emphasize the importance of integrating nuclear medicine physicians into multidisciplinary oncology teams.

Reining in Radium for Nuclear Medicine: Extra-Large Chelator Development for an Extra-Large Ion

Targeted α therapy (TAT) of soft-tissue cancers using the α particle-emitting radionuclide 223Ra holds great potential because of its favorable nuclear properties, adequate availability, and established clinical use for treating metastatic prostate cancer of the bone. Despite these advantages, the use of 223Ra has been largely overshadowed by other α emitters due to its challenging chelation chemistry. A key criterion that needs to be met for a radionuclide to be used in TAT is its stable attachment to a targeting vector via a bifunctional chelator. The low charge density of Ra2+ arising from its large ionic radius weakens its electrostatic binding interactions with chelators, leading to insufficient complex stability in vivo. In this study, we synthesized and evaluated macropa-XL as a novel chelator for 223Ra. It bears a large 21-crown-7 macrocyclic core and two picolinate pendent groups, which we hypothesized would effectively saturate the large coordination sphere of the Ra2+ ion. The structural chemistry of macropa-XL was first established with the nonradioactive Ba2+ ion using X-ray diffraction and X-ray absorption spectroscopy, which revealed the formation of an 11-coordinate complex in a rare anti pendent-arm configuration. Subsequently, the stability constant of the [Ra(macropa-XL)] complex was determined via competitive cation exchange with 223Ra and 224Ra radiotracers and compared with that of macropa, the current state-of-the-art chelator for Ra2+. A moderate log KML value of 8.12 was measured for [Ra(macropa-XL)], which is approximately 1.5 log K units lower than the stability constant of [Ra(macropa)]. This relative decrease in Ra2+ complex stability for macropa-XL versus macropa was further probed using density functional theory calculations. Additionally, macropa-XL was radiolabeled with 223Ra, and the kinetic stability of the resulting complex was evaluated in human serum. Although macropa-XL could effectively bind 223Ra under mild conditions, the complex appeared to be unstable to transchelation. Collectively, this study sheds additional light on the chelation chemistry of the exotic Ra2+ ion and contributes to the small, but growing, number of chelator development efforts for 223Ra-based TAT.

Targeted Alpha Therapy: All We Need to Know about 225Ac's Physical Characteristics and Production as a Potential Theranostic Radionuclide

The high energy of α emitters, and the strong linear energy transfer that goes along with it, lead to very efficient cell killing through DNA damage. Moreover, the degree of oxygenation and the cell cycle state have no impact on these effects. Therefore, α radioisotopes can offer a treatment choice to individuals who are not responding to β- or gamma-radiation therapy or chemotherapy drugs. Only a few α-particle emitters are suitable for targeted alpha therapy (TAT) and clinical applications. The majority of available clinical research involves 225Ac and its daughter nuclide 213Bi. Additionally, the 225Ac disintegration cascade generates γ decays that can be used in single-photon emission computed tomography (SPECT) imaging, expanding the potential theranostic applications in nuclear medicine. Despite the growing interest in applying 225Ac, the restricted global accessibility of this radioisotope makes it difficult to conduct extensive clinical trials for many radiopharmaceutical candidates. To boost the availability of 225Ac, along with its clinical and potential theranostic applications, this review attempts to highlight the fundamental physical properties of this α-particle-emitting isotope, as well as its existing and possible production methods.

225Ac-iPSMA-RGD for Alpha-Therapy Dual Targeting of Stromal/Tumor Cell PSMA and Integrins

Prostate-specific membrane antigens (PSMAs) are frequently overexpressed in both tumor stromal endothelial cells and malignant cells (stromal/tumor cells) of various cancers. The RGD (Arg-Gly-Asp) peptide sequence can specifically detect integrins involved in tumor angiogenesis. This study aimed to preclinically evaluate the cytotoxicity, biokinetics, dosimetry, and therapeutic efficacy of 225Ac-iPSMA-RGD to determine its potential as an improved radiopharmaceutical for alpha therapy compared with the 225Ac-iPSMA and 225Ac-RGD monomers. HEHA-HYNIC-iPSMA-RGD (iPSMA-RGD) was synthesized and characterized by FT-IR, UV-vis, and UPLC mass spectroscopy. The cytotoxicity of 225Ac-iPSMA-RGD was assessed in HCT116 colorectal cancer cells. Biodistribution, biokinetics, and therapeutic efficacy were evaluated in nude mice with induced HCT116 tumors. In vitro results showed increased DNA double-strand breaks through ROS generation, cell apoptosis, and death in HCT116 cells treated with 225Ac-iPSMA-RGD. The results also demonstrated in vivo cytotoxicity in cancer cells after treatment with 225Ac-iPSMA-RGD and biokinetic and dosimetric properties suitable for alpha therapy, delivering ablative radiation doses up to 237 Gy/3.7 kBq to HCT116 tumors in mice. Given the phenotype of HCT116 cancer cells, the results of this study warrant further dosimetric and clinical studies to determine the potential of 225Ac-iPSMA-RGD in the treatment of colorectal cancer.

Theranostic Imaging Surrogates for Targeted Alpha Therapy: Progress in Production, Purification, and Applications

This article highlights recent developments of SPECT and PET diagnostic imaging surrogates for targeted alpha particle therapy (TAT) radiopharmaceuticals. It outlines the rationale for using imaging surrogates to improve diagnostic-scan accuracy and facilitate research, and the properties an imaging-surrogate candidate should possess. It evaluates the strengths and limitations of each potential imaging surrogate. Thirteen surrogates for TAT are explored: 133La, 132La, 134Ce/134La, and 226Ac for 225Ac TAT; 203Pb for 212Pb TAT; 131Ba for 223Ra and 224Ra TAT; 123I, 124I, 131I and 209At for 211At TAT; 134Ce/134La for 227Th TAT; and 155Tb and 152Tb for 149Tb TAT.

Precision strikes: PSMA-targeted radionuclide therapy in prostate cancer - a narrative review

Introduction

Radio-ligand targeted therapy is a new and promising concept of treatment Castration resistant prostate cancer (CRPC). Only a few radio-pharmaceutics were approved for usage in treating prostate cancer, among the multiple others tested. We aimed to review and summarize the literature on the therapeutic isotopes specific for PSMA.

Methods

We performed a scoping literature review of PubMed from January 1996 to December 2022.

Results

98 publications were selected for inclusion in this review. The studies contained in publications allowed to summarize the data on pharmacokinetics, therapeutic effects, side effects and the medical use of 225Ac and 177Lu radionuclides. The review also presents new research directions for specific PSMA radionuclides.

Conclusion

Radioligand targeted therapy is a new and promising concept where Lu-177-PSMA-617 have promising outcomes in treatment according to standard of care.

Early Normal Tissue Effects and Bone Marrow Relative Biological Effectiveness for an Actinium 225-Labeled HER2/neu-Targeting Antibody

PURPOSE

In this study we determined the dose-independent relative biological effectiveness (RBE2) of bone marrow for an anti-HER2/neu antibody labeled with the alpha-particle emitter actinium 225 (225Ac). Hematologic toxicity is often a consequence of radiopharmaceutical therapy (RPT) administration, and dosimetric guidance to the bone marrow is required to limit toxicity.

METHODS AND MATERIALS

Female neu/N transgenic mice (MMTV-neu) were intravenously injected with 0 to 16.65 kBq of the alpha-particle emitter labeled antibody, 225Ac-DOTA-7.16.4, and euthanized at 1 to 9 days after treatment. Complete blood counts were performed. Femurs and tibias were collected, and bone marrow was isolated from 1 femur and tibia and counted for radioactivity. Contralateral intact femurs were fixed, decalcified, and assessed by histology. Marrow cellularity was the biologic endpoint selected for RBE2 determination. For the reference radiation, both femurs of the mice were photon irradiated with 0 to 5 Gy using a small animal radiation research platform.

RESULTS

Response as measured by cellularity for the alpha-particle emitter RPT (αRPT) RPT and the external beam radiation therapy were linear and linear quadratic, respectively, as a function of absorbed dose. The resulting dose-independent RBE2 for bone marrow was 6.

CONCLUSIONS

As αRPT gains prominence, preclinical studies evaluating RBE in vivo will be important in relating to human experience with beta-particle emitter RPT. Such normal tissue RBE evaluations will help mitigate unexpected toxicity in αRPT.

Production, isolation, and shipment of clinically relevant quantities of astatine-211: A simple and efficient approach to increasing supply

The alpha emitter astatine-211 (211At) is a promising candidate for cancer treatment based on Targeted Alpha (α) Therapy (TAT). A small number of facilities, distributed across the United States, are capable of accelerating α-particle beams to produce 211At. However, challenges remain regarding strategic methods for shipping 211At in a form adaptable to advanced radiochemistry reactions and other uses of the radioisotope.

PURPOSE

Our method allows shipment of 211At in various quantities in a form convenient for further radiochemistry.

PROCEDURES

For this study, a 3-octanone impregnated Amberchrom CG300M resin bed in a column cartridge was used to separate 211At from the bismuth matrix on site at the production accelerator (Texas A&M) in preparation for shipping. Aliquots of 6 M HNO3 containing up to ≈2.22 GBq of 211At from the dissolved target were successfully loaded and retained on columns. Exempt packages (<370 MBq) were shipped to a destination radiochemistry facility, University of Texas MD Anderson Cancer Center, in the form of a convenient air-dried column. Type A packages have been shipped overnight to University of Alabama at Birmingham.

MAIN FINDINGS

Air-dried column hold times of various lengths did not inhibit simple and efficient recovery of 211At. Solution eluted from the column was sufficiently high in specific activity to successfully radiolabel a model compound, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1), with 211At. The method to prepare and ship 211At described in this manuscript has also been used to ship larger quantities of 211At a greater distance to University of Alabama at Birmingham.

PRINCIPAL CONCLUSIONS

The successful proof of this method paves the way for the distribution of 211At from Texas A&M University to research institutions and clinical oncology centers in Texas and elsewhere. Use of this simple method at other facilities has the potential increase the overall availability of 211At for preclinical and clinical studies.

Actinium-225 Targeted Agents: Where Are We Now?

α-particle targeted radionuclide therapy has shown promise for optimal cancer management, an exciting new era for brachytherapy. Alpha-emitting nuclides can have significant advantages over gamma- and beta-emitters due to their high linear energy transfer (LET). While their limited path length results in more specific tumor 0kill with less damage to surrounding normal tissues, their high LET can produce substantially more lethal double strand DNA breaks per radiation track than beta particles. Over the last decade, the physical and chemical attributes of Actinium-225 (225Ac) including its half-life, decay schemes, path length, and straightforward chelation ability has peaked interest for brachytherapy agent development. However, this has been met with challenges including source availability, accurate modeling for standardized dosimetry for brachytherapy treatment planning, and laboratory space allocation in the hospital setting for on-demand radiopharmaceuticals production. Current evidence suggests that a simple empirical approach based on 225Ac administered radioactivity may lead to inconsistent outcomes and toxicity. In this review article, we highlight the recent advances in 225Ac source production, dosimetry modeling, and current clinical studies.

Tandem Isotope Therapy with 225Ac- and 177Lu-PSMA-617 in a Murine Model of Prostate Cancer

Radionuclide therapy targeting prostate-specific membrane antigen (PSMA) is a promising option for metastatic castration-resistant prostate cancer. Clinical experience using 177Lu or 225Ac has demonstrated encouraging treatment responses; however, responses are not durable. Dual-isotope combinations, or "tandem" approaches, may improve tolerability while retaining a high tumor dose. In this study, we directly compared α- versus β-particle treatment, as well as a combination thereof, at different stages of disease in a murine model of disseminated prostate cancer. Methods: First, to determine comparable injected activities from 177Lu- and 225Ac-PSMA-617, ex vivo biodistribution studies were performed at 5 time points after treatment of C4-2 subcutaneous tumor-bearing NSG mice. To establish a more representative model of metastatic prostate cancer, NSG mice were inoculated with luciferase-expressing C4-2 cells in the left ventricle, leading to disseminated visceral and bone lesions. At either 3 or 5 wk after inoculation, the mice were treated with equivalent tumor dose-depositing activities of 177Lu- or 225Ac-PSMA-617 alone or in combination (35 MBq of 177Lu, 40 kBq of 225Ac, or 17 MBq of 177Lu + 20 kBq 225Ac; 10/group). Disease burden was assessed by weekly bioluminescence imaging. Treatment efficacy was evaluated using whole-body tumor burden and overall survival. Results: The ex vivo biodistribution studies revealed that 35 MBq of 177Lu and 40 kBq of 225Ac yield equivalent absorbed tumor doses in a subcutaneous C4-2 model. The disease burden of mice treated at 3 wk after inoculation (microscopic disease) with 177Lu was not significantly different from that of untreated mice. However, 225Ac-PSMA-617 both as a single agent and in combination with 177Lu (17 MBq of 177Lu + 20 kBq of 225Ac) were associated with significant whole-body tumor growth retardation and survival benefit (overall survival, 8.3 wk for nontreatment, 9.4 wk for 177Lu, 15.3 wk for 225Ac alone, and 14.1 wk for tandem therapy). When treated at 5 wk after inoculation (macroscopic disease), all treatment groups showed retarded tumor growth and improved survival, with no significant differences between 225Ac alone and administration of half the 225Ac activity in tandem with 177Lu (overall survival, 7.9 wk for nontreatment, 10.3 wk for 177Lu, 14.6 wk for 225Ac alone, and 13.2 wk for tandem therapy). Conclusion: Treatment of a disseminated model of prostate cancer with simultaneous 225Ac- and 177Lu-PSMA-617 results in significantly decreased tumor growth compared with 177Lu, which was ineffective as a single agent against microscopic lesions. Mice treated later in the disease progression and bearing macroscopic, millimeter-sized lesions experienced significant tumor growth retardation and survival benefit in both monoisotopic and tandem regimens of 177Lu and 225Ac. Although the greatest benefits were observed with the single agent 225Ac, the tandem arm experienced no significant difference in disease burden or survival benefit, suggesting that the reduced activity of 225Ac was adequately compensated in the tandem arm. The superior therapeutic efficacy of 225Ac in this model suggests a preference for α-emitters alone, or possibly in combination, in the microscopic disease setting.

Development of 225Ac Production from Low Isotopic Dilution 229Th

The promise of 225Ac targeted alpha therapies has been on the horizon for the last two decades. TerraPower Isotopes are uniquely suited to produce clinically relevant quantities of 225Ac through the decay of 229Th. Herein, a rapid processing scheme to isolate radionuclidic and radioisotopically pure 225Ac in good yield (98%) produced from 229Th that contains significant quantities of 228Th activity is described. The characterization of each step of the process is presented along with the detailed characterization of the resulting 225Ac isotopic starting material that will support the cancer research and development efforts.

Initial Findings on the Use of [225Ac]Ac-DOTATATE Therapy as a Theranostic Application in Patients with Neuroendocrine Tumors

Objectives

This study aimed to evaluate the stability, safety, and efficacy of alpha-targeted therapy with [225Ac]Ac-DOTATATE in patients with grade 1/2 metastatic neuroendocrine tumors (NETs).

Methods

This retrospective cohort included patients (n=11) with metastatic NETs from different primary sites (bronchial, pancreatic, nonpancreatic gastroenteropancreatic NETs, paraganglioma, and unknown primary site) treated with [225Ac]Ac-DOTATATE with a mean activity of 8.2±0.6 MBq (range: 7.5-10.0 MBq) at our institution between November 2019 and March 2022. The in vivo and in vitro stability of [225Ac]Ac-DOTATATE was calculated. The safety profile was evaluated according to the CTCAE-v5.0. Treatment efficacy was evaluated according to [68Ga] Ga-DOTATATE positron emission tomography/computed tomography (PET/CT) images and the RECIST 1.1 criteria.

Results

Patients had 73% (n=8) lymph node metastases, 91% (n=10) liver metastases, 36% (n=4) lung metastases, and 73% (n=8) bone metastases. All but one patient was refractory to treatment with [177Lu]Lu-DOTATATE. [225Ac]Ac-DOTATATE was stable for at least 5 h in vitro (in saline) and 3 h in vivo (urine and blood samples). Grade 2 renal toxicity and grade 2 hematotoxicity were observed in one patient. No grade 3-4 toxicities were reported. According to post-treatment [68Ga]Ga-DOTATATE PET/CT (n=9), 11% (n=1) had progressive disease, 44.4% (n=4) had stable disease, and 44.4% (n=4) had a partial response. The disease control rate was 89% (n=8). The median progression-free survival estimated according to Kaplan-Meier analysis was 12 months.

Conclusion

The preliminary results of this study suggest that [225Ac]Ac-DOTATATE is stable, safe, and effective for treating advanced and [177Lu] Lu-DOTATATE-refractory NETs. However, prospective studies are needed to determine the impact of treatment on overall survival and to uncover potential side effects.

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.

Emerging Role of Nuclear Medicine in Prostate Cancer: Current State and Future Perspectives

Prostate cancer is the most frequent epithelial neoplasia after skin cancer in men starting from 50 years and prostate-specific antigen (PSA) dosage can be used as an early screening tool. Prostate cancer imaging includes several radiological modalities, ranging from ultrasonography, computed tomography (CT), and magnetic resonance to nuclear medicine hybrid techniques such as single-photon emission computed tomography (SPECT)/CT and positron emission tomography (PET)/CT. Innovation in radiopharmaceutical compounds has introduced specific tracers with diagnostic and therapeutic indications, opening the horizons to targeted and very effective clinical care for patients with prostate cancer. The aim of the present review is to illustrate the current knowledge and future perspectives of nuclear medicine, including stand-alone diagnostic techniques and theragnostic approaches, in the clinical management of patients with prostate cancer from initial staging to advanced disease.

[225Ac]Ac- and [111In]In-DOTA-trastuzumab theranostic pair: cellular dosimetry and cytotoxicity in vitro and tumour and normal tissue uptake in vivo in NRG mice with HER2-positive human breast cancer xenografts

BACKGROUND

Trastuzumab (Herceptin) has improved the outcome for patients with HER2-positive breast cancer (BC) but brain metastases (BM) remain a challenge due to poor uptake of trastuzumab into the brain. Radioimmunotherapy (RIT) with trastuzumab labeled with α-particle emitting, 225Ac may overcome this challenge by increasing the cytotoxic potency on HER2-positive BC cells. Our first aim was to synthesize and characterize [111In]In-DOTA-trastuzumab and [225Ac]Ac-DOTA-trastuzumab as a theranostic pair for imaging and RIT of HER2-positive BC, respectively. A second aim was to estimate the cellular dosimetry of [225Ac]Ac-DOTA-trastuzumab and determine its cytotoxicity in vitro on HER2-positive BC cells. A third aim was to study the tumour and normal tissue uptake of [225Ac]Ac-DOTA-trastuzumab using [111In]In-DOTA-trastuzumab as a radiotracer in vivo in NRG mice with s.c. 164/8-1B/H2N.luc+ human BC tumours that metastasize to the brain.

RESULTS

Trastuzumab was conjugated to 12.7 ± 1.2 DOTA chelators and labeled with 111In or 225Ac. [111In]In-DOTA-trastuzumab exhibited high affinity specific binding to HER2-positive SK-BR-3 human BC cells (KD = 1.2 ± 0.3 × 10-8 mol/L). Treatment with [225Ac]Ac-DOTA-trastuzumab decreased the surviving fraction (SF) of SK-BR-3 cells dependent on the specific activity (SA) with SF < 0.001 at SA = 0.74 kBq/µg. No surviving colonies were noted at SA = 1.10 kBq/µg or 1.665 kBq/µg. Multiple DNA double-strand breaks (DSBs) were detected in SK-BR-3 cells exposed to [225Ac]Ac-DOTA-trastuzumab by γ-H2AX immunofluorescence microscopy. The time-integrated activity of [111In]In-DOTA-trastuzumab in SK-BR-3 cells was measured and used to estimate the absorbed doses from [225Ac]Ac-DOTA-trastuzumab by Monte Carlo N-Particle simulation for correlation with the SF. The dose required to decrease the SF of SK-BR-3 cells to 0.10 (D10) was 1.10 Gy. Based on the D10 reported for γ-irradiation of SK-BR-3 cells, we estimate that the relative biological effectiveness of the α-particles emitted by 225Ac is 4.4. Biodistribution studies in NRG mice with s.c. 164/8-1B/H2N.luc+ human BC tumours at 48 h post-coinjection of [111In]In-DOTA-trastuzumab and [225Ac]Ac-DOTA-trastuzumab revealed HER2-specific tumour uptake (10.6 ± 0.6% ID/g) but spleen uptake was high (28.9 ± 7.4% ID/g). Tumours were well-visualized by SPECT/CT imaging using [111In]In-DOTA-trastuzumab.

CONCLUSION

We conclude that [225Ac]Ac-DOTA-trastuzumab exhibited potent and HER2-specific cytotoxicity on SK-BR-3 cells in vitro and HER2-specific uptake in s.c. 164/8-1B/H2N.luc+ human BC tumours in NRG mice, and these tumours were imaged by SPECT/CT with [111In]In-DOTA-trastuzumab. These results are promising for combining [111In]In-DOTA-trastuzumab and [225Ac]Ac-DOTA-trastuzumab as a theranostic pair for imaging and RIT of HER2-positive BC.

Evaluation of the therapeutic efficacy of 213Bi-labelled DOTA-conjugated alpha-melanocyte stimulating hormone peptide analogues in melanocortin-1 receptor positive preclinical melanoma model

Melanocortin-1 receptor (MC1-R) targeting alpha-melanocyte stimulating hormone-analogue (α-MSH) biomolecules labelled with α-emitting radiometal seem to be valuable in the targeted radionuclide therapy of MC1-R positive melanoma malignum (MM). Herein is reported the anti-tumor in vivo therapeutic evaluation of MC1-R-affine [213Bi]Bi-DOTA-NAPamide and HOLDamide treatment in MC1-R positive B16-F10 melanoma tumor-bearing C57BL/6J mice. On the 6th, 8th and 10th days post tumor cell inoculation; the treated groups of mice were intravenously injected with approximately 5 MBq of both amide derivatives. Beyond body weight and tumor volume assessment, [68Ga]Ga-DOTA-HOLDamide and NAPamide-based PET/MRI scans, and ex vivo biodistribution studies were executed 30,- and 90 min postinjection. In the PET/MRI imaging studies the B16-F10 tumors were clearly visualized with both 68Ga-labelled tracers, however, significantly lower tumor-to-muscle (T/M) ratios were observed by using [68Ga]Ga-DOTA-HOLDamide. After alpha-radiotherapy treatment the tumor size of the control group was larger relative to both treated cohorts, while the smallest tumor volumes were observed in the NAPamide-treated subclass on the 10th day. Relatively higher [213Bi]Bi-DOTA-NAPamide accumulation in the B16-F10 tumors (%ID/g: 2.71 ± 0.15) with discrete background activity led to excellent T/M ratios, particularly 90 min postinjection. Overall, the therapeutic application of receptor selective [213Bi]Bi-DOTA-NAPamide seems to be feasible in MC1-R positive MM management.

Preclinical Development in Radiopharmaceutical Therapy for Prostate Cancer

Prostate cancer is a leading cause of cancer death in men worldwide. Among the various treatment options, radiopharmaceutical therapy has shown notable success in metastatic, castration-resistant disease. Radiopharmaceutical therapy is a systemic approach that delivers cytotoxic radiation doses precisely to the malignant tumors and/or tumor microenvironment. Therapeutic radiopharmaceuticals are composed of a therapeutic radionuclide and a high-affinity, tumor-targeting carrier molecule. Therapeutic radionuclides used in preclinical prostate cancer studies are primarily α-, β--, or Auger-electron-emitting radiometals or radiohalogens. Monoclonal antibodies, antibody-derived fragments, peptides, and small molecules are frequently used as tumor-targeting molecules. Over the years, several important membrane-associated proteases and receptors have been identified, validated, and subsequently used for preclinical radiotherapeutic development for prostate cancer. Prostate-specific membrane antigen (PSMA) is the most well-studied prostate cancer-associated protease in preclinical literature. PSMA-targeting radiotherapeutic agents are being investigated using high-affinity antibody- and small-molecule-based agents for safety and efficacy. Early generations of such agents were developed simply by replacing radionuclides of the imaging agents with therapeutic ones. Later, extensive structure-activity relationship studies were conducted to address the safety and efficacy issues obtained from initial patient data. Recent regulatory approval of the 177Lu-labeled low-molecular-weight agent, 177Lu-PSMA-617, is a significant accomplishment. Current preclinical experiments are focused on the structural modification of 177Lu-PSMA-617 and relevant investigational agents to increase tumor targeting and reduce off-target binding and toxicity in healthy organs. While lutetium-177 (177Lu) remains the most widely used radionuclide, radiolabeled analogs with iodine-131 (128I), yttrium-90 (89Y), copper-67 (67Cu), and terbium-161 (161Tb) have been evaluated as potential alternatives in recent years. In addition, agents carrying the α-particle-emitting radiohalogen, astatine-211 (211At), or radiometals, actinium-225 (225Ac), lead-212 (212Pb), radium-223 (223Ra), and thorium-227 (227Th), have been increasingly investigated in preclinical research. Besides PSMA-based radiotherapeutics, other prominent prostate cancer-related proteases, for example, human kallikrein peptidases (HK2 and HK3), have been explored using monoclonal-antibody-(mAb)-based targeting platforms. Several promising mAbs targeting receptors overexpressed on the different stages of prostate cancer have also been developed for radiopharmaceutical therapy, for example, Delta-like ligand 3 (DLL-3), CD46, and CUB domain-containing protein 1 (CDCP1). Progress is also being made using peptide-based targeting platforms for the gastrin-releasing peptide receptor (GRPR), a well-established membrane-associated receptor expressed in localized and metastatic prostate cancers. Furthermore, mechanism-driven combination therapies appear to be a burgeoning area in the context of preclinical prostate cancer radiotherapeutics. Here, we review the current developments related to the preclinical radiopharmaceutical therapy of prostate cancer. These are summarized in two major topics: (1) therapeutic radionuclides and (2) tumor-targeting approaches using monoclonal antibodies, small molecules, and peptides.

Methods for Radiolabeling Nanoparticles (Part 3): Therapeutic Use

Following previously published systematic reviews on the diagnostic use of nanoparticles (NPs), in this manuscript, we report published methods for radiolabeling nanoparticles with therapeutic alpha-emitting, beta-emitting, or Auger's electron-emitting isotopes. After analyzing 234 papers, we found that different methods were used with the same isotope and the same type of nanoparticle. The most common type of nanoparticles used are the PLGA and PAMAM nanoparticles, and the most commonly used therapeutic isotope is 177Lu. Regarding labeling methods, the direct encapsulation of the isotope resulted in the most reliable and reproducible technique. Radiolabeled nanoparticles show promising results in metastatic breast and lung cancer, although this field of research needs more clinical studies, mainly on the comparison of nanoparticles with chemotherapy.

A GATE simulation study for dosimetry in cancer cell and micrometastasis from the 225Ac decay chain

BACKGROUND

Radiopharmaceutical therapy (RPT) with alpha-emitting radionuclides has shown great promise in treating metastatic cancers. The successive emission of four alpha particles in the ²²⁵Ac decay chain leads to highly targeted and effective cancer cell death. Quantifying cellular dosimetry for ²²⁵Ac RPT is essential for predicting cell survival and therapeutic success. However, the leading assumption that all ²²⁵Ac progeny remain localized at their target sites likely overestimates the absorbed dose to cancer cells. To address limitations in existing semi-analytic approaches, this work evaluates S-values for ²²⁵Ac's progeny radionuclides with GATE Monte Carlo simulations.

METHODS

The cellular geometries considered were an individual cell (10 µm diameter with a nucleus of 8 µm diameter) and a cluster of cells (micrometastasis) with radionuclides localized in four subcellular regions: cell membrane, cytoplasm, nucleus, or whole cell. The absorbed dose to the cell nucleus was scored, and self- and cross-dose S-values were derived. We also evaluated the total absorbed dose with various degrees of radiopharmaceutical internalization and retention of the progeny radionuclides ²²¹Fr (t_1/2 = 4.80 m) and ²¹³Bi (t_1/2 = 45.6 m).

RESULTS

For the cumulative ²²⁵Ac decay chain, our self- and cross-dose nuclear S-values were both in good agreement with S-values published by MIRDcell, with per cent differences ranging from - 2.7 to - 8.7% for the various radionuclide source locations. Source location had greater effects on self-dose S-values than the intercellular cross-dose S-values. Cumulative ²²⁵Ac decay chain self-dose S-values increased from 0.167 to 0.364 GyBq^-1 s^-1 with radionuclide internalization from the cell surface into the cell. When progeny migration from the target site was modelled, the cumulative self-dose S-values to the cell nucleus decreased by up to 71% and 21% for ²²¹Fr and ²¹³Bi retention, respectively.

CONCLUSIONS

Our GATE Monte Carlo simulations resulted in cellular S-values in agreement with existing MIRD S-values for the alpha-emitting radionuclides in the ²²⁵Ac decay chain. To obtain accurate absorbed dose estimates in ²²⁵Ac studies, accurate understanding of daughter migration is critical for optimized injected activities. Future work will investigate other novel preclinical alpha-emitting radionuclides to evaluate therapeutic potency and explore realistic cellular geometries corresponding to targeted cancer cell lines.

Diagnosis, Management and Theragnostic Approach of Gastro-Entero-Pancreatic Neuroendocrine Neoplasms

Gastro-entero-pancreatic neuroendocrine neoplasms (GEP-NENs) constitute an ideal target for radiolabeled somatostatin analogs. The theragnostic approach is able to combine diagnosis and therapy by the identification of a molecular target that can be diagnosed and treated with the same radiolabeled compound. During the last years, advances in functional imaging with the introduction of somatostatin analogs and peptide receptor radionuclide therapy, have improved the diagnosis and treatment of GEP-NENs. Moreover, PET/CT imaging with ¹⁸F-FDG represents a complementary tool for prognostic evaluation of patients with GEP-NENs. In the field of personalized medicine, the theragnostic approach has emerged as a promising tool in diagnosis and management of patients with GEP-NENs. The aim of this review is to summarize the current evidence on diagnosis and management of patients with GEP-NENs, focusing on the theragnostic approach.

Evaluation of 134Ce/134La as a PET Imaging Theranostic Pair for 225Ac α-Radiotherapeutics

225Ac-targeted α-radiotherapy is a promising approach to treating malignancies, including prostate cancer. However, α-emitting isotopes are difficult to image because of low administered activities and a low fraction of suitable γ-emissions. The in vivo generator 134Ce/134La has been proposed as a potential PET imaging surrogate for the therapeutic nuclides 225Ac and 227Th. In this report, we detail efficient radiolabeling methods using the 225Ac-chelators DOTA and MACROPA. These methods were applied to radiolabeling of prostate cancer imaging agents, including PSMA-617 and MACROPA-PEG4-YS5, for evaluation of their in vivo pharmacokinetic characteristics and comparison to the corresponding 225Ac analogs. Methods: Radiolabeling was performed by mixing DOTA/MACROPA chelates with 134Ce/134La in NH4OAc, pH 8.0, at room temperature, and radiochemical yields were monitored by radio-thin-layer chromatography. In vivo biodistributions of 134Ce-DOTA/MACROPA.NH2 complexes were assayed through dynamic small-animal PET/CT imaging and ex vivo biodistribution studies over 1 h in healthy C57BL/6 mice, compared with free 134CeCl3 In vivo, preclinical imaging of 134Ce-PSMA-617 and 134Ce-MACROPA-PEG4-YS5 was performed on 22Rv1 tumor-bearing male nu/nu-mice. Ex vivo biodistribution was performed for 134Ce/225Ac-MACROPA-PEG4-YS5 conjugates. Results: 134Ce-MACROPA.NH2 demonstrated near-quantitative labeling with 1:1 ligand-to-metal ratios at room temperature, whereas a 10:1 ligand-to-metal ratio and elevated temperatures were required for DOTA. Rapid urinary excretion and low liver and bone uptake were seen for 134Ce/225Ac-DOTA/MACROPA. NH2 conjugates in comparison to free 134CeCl3 confirmed high in vivo stability. An interesting observation during the radiolabeling of tumor-targeting vectors PSMA-617 and MACROPA-PEG4-YS5-that the daughter 134La was expelled from the chelate after the decay of parent 134Ce-was confirmed through radio-thin-layer chromatography and reverse-phase high-performance liquid chromatography. Both conjugates, 134Ce-PSMA-617 and 134Ce-MACROPA-PEG4-YS5, displayed tumor uptake in 22Rv1 tumor-bearing mice. The ex vivo biodistribution of 134Ce-MACROPA.NH2, 134Ce-DOTA and 134Ce-MACROPA-PEG4-YS5 corroborated well with the respective 225Ac-conjugates. Conclusion: These results demonstrate the PET imaging potential for 134Ce/134La-labeled small-molecule and antibody agents. The similar 225Ac and 134Ce/134La-chemical and pharmacokinetic characteristics suggest that the 134Ce/134La pair may act as a PET imaging surrogate for 225Ac-based radioligand therapies.

Development of Novel 111In/225Ac-Labeled Agent Targeting PSMA for Highly Efficient Cancer Radiotheranostics

Prostate-specific membrane antigen (PSMA) is a promising target for metastatic castration-resistant prostate cancer. We previously reported the effectiveness of PSMA-DA1 as a PSMA-targeting radiotheranostic agent containing an albumin binder moiety. To further enhance tumor uptake, we newly designed PSMA-NAT-DA1 (PNT-DA1) by the introduction of a lipophilic linker into PSMA-DA1. The PSMA affinity of [¹¹¹In]In-PNT-DA1 was increased (K_d = 8.20 nM) compared with that of [¹¹¹In]In-PSMA-DA1 (K_d = 89.4 nM). [¹¹¹In]In-PNT-DA1 showed markedly high tumor accumulation (131.6% injected dose/g at 48 h post-injection), and [¹¹¹In]In-PNT-DA1 enabled the visualization of the tumor clearly at 24 h post-injection with SPECT/CT imaging. The administration of [²²⁵Ac]Ac-PNT-DA1 (2.5 kBq) led to shrinkage of the tumor without marked toxicity, and the antitumor effects of [²²⁵Ac]Ac-PNT-DA1 were superior to those of [²²⁵Ac]Ac-PSMA-DA1 and [²²⁵Ac]Ac-PSMA-617, which is the current gold standard for PSMA-targeting ²²⁵Ac-endoradiotherapy. These results suggest that the combination of [¹¹¹In]In-PNT-DA1 and [²²⁵Ac]Ac-PNT-DA1 comprises a promising method of PSMA-targeting radiotheranostics.

Next generation radiotheranostics promoting precision medicine

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

Diagnostic value of two-time point [68Ga]Ga-PSMA-11 PET/CT in the primary staging of untreated prostate cancer

The emerging PET tracer [⁶⁸Ga]Ga-PSMA-11 has been established for staging in prostate cancer (PCa). Aim was to determine the value of early static imaging in two-phase PET/CT. 100 men with newly diagnosed histopathologically confirmed untreated PCa who underwent [⁶⁸Ga]Ga-PSMA-11 PET/CT from January 2017 to October 2019 were included. The two-phase imaging protocol consisted of an early static scan of the pelvis (6 min p.i.) and a late total-body scan (60 min p.i). Associations of semi-quantitative parameters derived via volumes of interest (VOI) with Gleason grade group and PSA were investigated. In 94/100 patients (94%) the primary tumor was detected in both phases. In 29/100 patients (29%) metastases were detected at a median PSA level of 32.2 ng/ml (0.41-503 ng/ml). In 71/100 patients (71%) without metastasis a median PSA level of 10.1 ng/ml (0.57-103 ng/ml) was observed (p = < 0.001). Primary tumors demonstrated a median standard uptake value maximum (SUVmax) of 8.2 (3.1-45.3) in early phase versus 12.2 (3.1-73.4) in late phase and a median standard uptake value mean (SUVmean) of 4.2 (1.6-24.1) in early phase versus 5.8 (1.6-39.9) in late phase, significantly increasing over time (p = < 0.001). Higher SUVmax and SUVmean were associated with higher Gleason grade group (p = 0.004 and p = 0.003, respectively) and higher PSA levels (p = < 0.001). In 13/100 patients the semi-quantitative parameters including SUVmax were declining in the late phase compared to early phase. Two-phase [⁶⁸Ga]Ga-PSMA-11 PET/CT demonstrates a high detection rate for primary tumor of untreated PCa of 94% and improves diagnostic accuracy. Higher PSA levels and Gleason grade group are associated with higher semi-quantitative parameters in the primary tumor. Early imaging provides additional information in a small sub-group with declining semi-quantitative parameters in the late phase.

Scandium-44: Diagnostic Feasibility in Tumor-Related Angiogenesis

Angiogenesis-related cell-surface molecules, including integrins, aminopeptidase N, vascular endothelial growth factor, and gastrin-releasing peptide receptor (GRPR), play a crucial role in tumour formation. Radiolabelled imaging probes targeting angiogenic biomarkers serve as valuable vectors in tumour identification. Nowadays, there is a growing interest in novel radionuclides other than gallium-68 (⁶⁸Ga) or copper-64 (⁶⁴Cu) to establish selective radiotracers for the imaging of tumour-associated neo-angiogenesis. Given its ideal decay characteristics (E_β⁺_average: 632 KeV) and a half-life (T_1/2 = 3.97 h) that is well matched to the pharmacokinetic profile of small molecules targeting angiogenesis, scandium-44 (⁴⁴Sc) has gained meaningful attention as a promising radiometal for positron emission tomography (PET) imaging. More recently, intensive research has been centered around the investigation of ⁴⁴Sc-labelled angiogenesis-directed radiopharmaceuticals. Previous studies dealt with the evaluation of ⁴⁴Sc-appended a_vb₃ integrin-affine Arg-Gly-Asp (RGD) tripeptides, GRPR-selective aminobenzoyl-bombesin analogue (AMBA), and hypoxia-associated nitroimidazole derivatives in the identification of various cancers using experimental tumour models. Given the tumour-related hypoxia- and angiogenesis-targeting capability of these PET probes, ⁴⁴Sc seems to be a strong competitor of the currently used positron emitters in radiotracer development. In this review, we summarize the preliminary preclinical achievements with ⁴⁴Sc-labelled angiogenesis-specific molecular probes.