A Radium-223 microgenerator from cyclotron-produced trace Actinium-227

Actinium-225 photonuclear production in nuclear reactors using a mixed radium-226 and gadolinium-157 target

BACKGROUND

Actinium-225 is one of the most promising radionuclides for targeted alpha therapy. Its limited availability significantly restricts clinical trials and potential applications of 225Ac-based radiopharmaceuticals.

METHODS

In this work, we examine the possibility of 225Ac production from the thermal neutron flux of a nuclear reactor. For this purpose, a target consisting of 1.4 mg of 226Ra(NO3)2 (T1/2 = 1600 years) and 115.5 mg of 90 % enriched, stable 157Gd2O3 was irradiated for 48 h in the Breazeale Nuclear Reactor with an average neutron flux of 1.7·1013 cm-2·s-1. Gadolinium-157 has one of the highest thermal neutron capture cross sections of 0.25 Mb, and its neutron capture results in emission of high-energy, prompt γ-photons. Emitted γ-photons interact with 226Ra to produce 225Ra according to the 226Ra(γ, n)225Ra reaction. Gadolinium debulking and separation of undesirable, co-produced 227Ac from 225Ra was achieved in one step by using 60 g of branched DGA resin. After 225Ac ingrowth from 225Ra (T1/2 = 14.8 d), 225Ac was extracted from the 226Ra and 225Ra fraction using 5 g of bDGA resin and then eluted using 5 mM HNO3.

RESULTS

Measured activity of 225Ac showed that 6(1) kBq or 0.16(3) μCi (1σ) of 225Ra was produced at the end of bombardment from 0.9 mg of 226Ra.

CONCLUSION

The developed 225Ac separation is a waste-free process which can be used to obtain pure 225Ac in a nuclear reactor.

ISIT-QA: In Silico Imaging Trial to Evaluate a Low-Count Quantitative SPECT Method Across Multiple Scanner-Collimator Configurations for 223Ra-Based Radiopharmaceutical Therapies

Personalized dose-based treatment planning requires accurate and reproducible noninvasive measurements to ensure safety and effectiveness. Dose estimation using SPECT is possible but challenging for alpha (α)-particle-emitting radiopharmaceutical therapy (α-RPT) because of complex γ-emission spectra, extremely low counts, and various image-degrading artifacts across a plethora of scanner-collimator configurations. Through the incorporation of physics-based considerations and skipping of the potentially lossy voxel-based reconstruction step, a recently developed projection-domain low-count quantitative SPECT (LC-QSPECT) method has the potential to provide reproducible, accurate, and precise activity concentration and dose measures across multiple scanners, as is typically the case in multicenter settings. To assess this potential, we conducted an in silico imaging trial to evaluate the LC-QSPECT method for a 223Ra-based α-RPT, with the trial recapitulating patient and imaging system variabilities. Methods: A virtual imaging trial titled In Silico Imaging Trial for Quantitation Accuracy (ISIT-QA) was designed with the objectives of evaluating the performance of the LC-QSPECT method across multiple scanner-collimator configurations and comparing performance with a conventional reconstruction-based quantification method. In this trial, we simulated 280 realistic virtual patients with bone-metastatic castration-resistant prostate cancer treated with 223Ra-based α-RPT. The trial was conducted with 9 simulated SPECT scanner-collimator configurations. The primary objective of this trial was to evaluate the reproducibility of dose estimates across multiple scanner-collimator configurations using LC-QSPECT by calculating the intraclass correlation coefficient. Additionally, we compared the reproducibility and evaluated the accuracy of both considered quantification methods across multiple scanner-collimator configurations. Finally, the repeatability of the methods was evaluated in a test-retest study. Results: In this trial, data from 268 223RaCl2 treated virtual prostate cancer patients, with a total of 2,903 lesions, were used to evaluate LC-QSPECT. LC-QSPECT provided dose estimates with good reproducibility across the 9 scanner-collimator configurations (intraclass correlation coefficient > 0.75) and high accuracy (ensemble average values of recovery coefficients ranged from 1.00 to 1.02). Compared with conventional reconstruction-based quantification, LC-QSPECT yielded significantly improved reproducibility across scanner-collimator configurations, accuracy, and test-retest repeatability ([Formula: see text] Conclusion: LC-QSPECT provides reproducible, accurate, and repeatable dose estimations in 223Ra-based α-RPT as evaluated in ISIT-QA. These findings provide a strong impetus for multicenter clinical evaluations of LC-QSPECT in dose quantification for α-RPTs.

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

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

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.

Radiochemical Quality Control Methods for Radium-223 and Thorium-227 Radiotherapies

Background: The majority of radiopharmaceuticals for use in disease detection and targeted treatment undergo a single radioactive transition (decay) to reach a stable ground state. Complex emitters, which produce a series of daughter radionuclides, are emerging as novel radiopharmaceuticals. The need for validation of chemical and radiopurity with such agents using common quality control instrumentation is an area of active investigation. Here, we demonstrate novel methods to characterize 227Th and 223Ra. Materials and Methods: A radio-TLC scanner and a γ-counter, two common and widely accessible technologies, as well as a solid-state α-particle spectral imaging camera were evaluated for their ability to characterize and distinguish 227Th and 223Ra. We verified these results through purity evaluation of a novel 227Th-labeled protein construct. Results: The γ-counter and α-camera distinguished 227Th from 223Ra, enabling rapid and quantitative determination of radionuclidic purity. The radio-TLC showed limited ability to describe purity, although use under α-particle-specific settings enhanced resolution. All three methods were able to distinguish a pure from impure 227Th-labeled protein. Conclusions: The presented quality control evaluation for 227Th and 223Ra on three different instruments can be applied to both research and clinical settings as new alpha particle therapies are developed.

Radiopharmaceutical Quality Control Considerations for Accelerator-Produced Actinium Therapies

Background: Alpha-particle-emitting radiotherapies are of great interest for the treatment of disseminated cancer. Actinium-225 (²²⁵Ac) produces four α-particles through its decay and is among the most attractive radionuclides for use in targeted radiotherapy applications. However, supply issues for this isotope have limited availability and increased cost for research and translation. Efforts have focused on accelerator-based methods that produce ²²⁵Ac in addition to long-lived ²²⁷Ac. Objective: The authors investigated the impact of ²²⁵Ac/²²⁷Ac material in the radiolabeling and radiopharmaceutical quality control evaluation of a DOTA chelate-conjugated peptide under good manufacturing practices. The authors use an automated module under identical conditions with either generator or accelerator-produced actinium radiolabeling. Methods: The authors have performed characterization of the radiolabeled products, including thin-layer chromatography, high-pressure liquid chromatography, gamma counting, and high-energy resolution gamma spectroscopy. Results: Peptide was radiolabeled and assessed at >95% radiochemical purity with high yields for generator produced ²²⁵Ac. The radiolabeling results produced material with subtle but detectable differences when using ²²⁵Ac/²²⁷Ac. Gamma spectroscopy was able to identify peptide initially labeled with ²²⁷Th, and at 100 d for quantification of ²²⁵Ac-bearing peptide. Conclusion: Peptides produced using ²²⁵Ac/²²⁷Ac material may be suitable for translation, but raise new issues that include processing times, logistics, and contaminant detection.

Radiation Safety Considerations and Clinical Advantages of Alpha-Emitting Therapy Radionuclides

Alpha-emitting radionuclides provide an effective means of delivering large radiation doses to targeted treatment locations. Radium-223 dichloride (²²³RaCl₂) is FDA approved for treatment of metastatic castration-resistant prostate cancer (mCRPC) and Actinium-225 (²²⁵Ac-Lintuzumab) radiolabeled antibodies have been shown to be beneficial for patients with acute myeloid leukemia. In recent years, there is an increasing use of alpha emitters in theranostic agents with both small and large molecule constructs. The proper precautionary means for their use and surveying documentation of these isotopes in a clinical setting are an essential accompaniment to these treatments. Methods: Patient treatment data collected over a three-year period, as well as regulatory requirements and safety practices, are described. Commonly used radiation instrumentation was evaluated for their ability to identify potential radioactive material spills and contamination events during a clinical administration of ²²⁵Ac. These instruments were placed at 0.32 cm from a 1.0 cm ²²⁵Ac disk source for measurement purposes. Radiation background values, efficiencies, and minimal detectable activities were measured and calculated for each type of detector. Results: The median external measured patient dose rate from ²²³RaCl₂ patients (n = 611) was 2.5 µSv hr-1 on contact and 0.2 µSv hr-1 at 1 meter immediately after administration. Similarly, ²²⁵Ac-Lintuzumab (n = 19) patients had median external dose rates of 2.0 µSv hr-1 on contact and 0.3 µSv hr-1 at 1 meter. For the measurement of ²²⁵Ac samples, a liquid scintillation counter was found to have the highest overall efficiency (97%), while a zinc sulfide (ZnS) alpha probe offered the lowest minimal detectable activity at 3 counts per minute. Conclusion: In this study, we report data from 630 patients who were undergoing treatment with alpha-emitting isotopes ²²³Ra and ²²⁵Ac. While alpha emitters have ability to deliver higher internal radiation dose to the tissues exposed as compared with other unsealed radionuclides, they typically present minimal external dose rate concerns. Additionally, alpha radiation can be efficiently detected with appropriate radiation instrumentation, such as a liquid scintillation counter or ZnS probe, that should be prioritized when surveying for spills of alpha-emitters.

Production and Supply of α-Particle-Emitting Radionuclides for Targeted α-Therapy

Encouraging results from targeted α-therapy have received significant attention from academia and industry. However, the limited availability of suitable radionuclides has hampered widespread translation and application. In the present review, we discuss the most promising candidates for clinical application and the state of the art of their production and supply. In this review, along with 2 forthcoming reviews on chelation and clinical application of α-emitting radionuclides, The Journal of Nuclear Medicine will provide a comprehensive assessment of the field.

A new way to ensure selective zirconium ion adsorption

This work studies the adsorption of zirconium ions by mesoporous titanium dioxide with surface arsenate groups. Experimental maximal adsorption values of zirconium ions were found to be 109.6 mg/g in neutral medium. This process depends on the interaction time, the equilibrium concentration of zirconium ions, and the acidity of the solution. Adsorption kinetics fit well into the kinetic model based on the pseudo-second-order equation (R 2 = 0.9984). Equilibrium adsorption of zirconium ions is well described by Langmuir’s adsorption theory (R 2 = 0.9856 and χ 2 = 1.307). Although zirconium ions are less actively adsorbed from a neutral medium than strontium or yttrium ions, in the 2% nitric acid only zirconium is adsorbed out of the mixture of zirconium, strontium, and yttrium. The results obtained by inductively coupled plasma mass spectrometry have shown that the investigated adsorbent selectively adsorbs zirconium ions from their mixture with strontium and yttrium in the range of solution acidity pH = 0–1. The average percentage of maximum extraction of zirconium ions is 94.3 ± 2.4%, and the highest percent of zirconium ions taken up from the mixture with strontium and yttrium is ∼98.4%. Investigated titanium dioxide selectively separate 90Zr from 90Sr with the presence of 1000-fold excess of stable 88Sr in radioactive liquid β − source. This fact is extremely valuable for the age dating of 90Sr-containing device in nuclear forensics or the determination of 90Sr in low activity background samples.

Capturing an elusive but critical element: Natural protein enables actinium chemistry

Actinium-based therapies could revolutionize cancer medicine but remain tantalizing due to the difficulties in studying and limited knowledge of Ac chemistry. Current efforts focus on small synthetic chelators, limiting radioisotope complexation and purification efficiencies. Here, we demonstrate a straightforward strategy to purify medically relevant radiometals, actinium(III) and yttrium(III), and probe their chemistry, using the recently discovered protein, lanmodulin. The stoichiometry, solution behavior, and formation constant of the ²²⁸Ac³⁺-lanmodulin complex and its ⁹⁰Y³⁺/^natY³⁺/^natLa³⁺ analogs were experimentally determined, representing the first actinium-protein and strongest actinide(III)-protein complex (sub-picomolar K_d) to be characterized. Lanmodulin’s unparalleled properties enable the facile purification recovery of radiometals, even in the presence of >10⁺¹⁰ equivalents of competing ions and at ultratrace levels: down to 2 femtograms ⁹⁰Y³⁺ and 40 attograms ²²⁸Ac³⁺. The lanmodulin-based approach charts a new course to study elusive isotopes and develop versatile chelating platforms for medical radiometals, both for high-value separations and potential in vivo applications.

Practical considerations for quantitative clinical SPECT/CT imaging of alpha particle emitting radioisotopes

Rationale: Alpha particle emitting radiopharmaceuticals are generating considerable interest for the treatment of disseminated metastatic disease. Molecular imaging of the distribution of these agents is critical to safely and effectively maximize the clinical potential of this emerging drug class. The present studies aim to investigate the feasibility and limitations of quantitative SPECT for 223Ra, 225Ac and 227Th. Methods: Three state-of-the-art SPECT/CT systems were investigated: the GE Discovery NM/CT 670, the GE Optima NM/CT 640, and the Siemens Symbia T6. A series of phantoms, including the NEMA IEC Body phantom, were used to compare and calibrate each camera. Additionally, anthropomorphic physical tumor and vertebrae phantoms were developed and imaged to evaluate the quantitative imaging protocol. Results: This work describes and validates a methodology to calibrate each clinical system. The efficiency of each gamma camera was analyzed and compared. Using the calibration factors obtained with the NEMA phantom, we were able to quantify the activity in 3D-printed tissue phantoms with an error of 2.1%, 3.5% and 11.8% for 223Ra, 225Ac, and 227Th, respectively. Conclusion: The present study validates that quantitative SPECT/CT imaging of 223Ra, 225Ac, and 227Th is achievable but that careful considerations for camera configuration are required. These results will aid in future implementation of SPECT-based patient studies and will help to identify the limiting factors for accurate image-based quantification with alpha particle emitting radionuclides.

Recent Achievements about Targeted Alpha Therapy-Based Targeting Vectors and Chelating Agents

One of the most rapidly growing options in the management of cancer therapy is Targeted Alpha Therapy (TAT) through which lethal α-emitting radionuclides conjugated to tumor-targeting vectors selectively deliver high amount of radiation to cancer cells.²²⁵Ac, ²¹²Bi, ²¹¹At, ²¹³Bi, and ²²³Ra have been investigated by plenty of clinical trials and preclinical researches for the treatment of smaller tumor burdens, micro-metastatic disease, and post-surgery residual disease. In order to send maximum radiation to tumor cells while minimizing toxicity in normal cells, a high affinity of targeting vectors to cancer tissue is essential. Besides that, the stable and specific complex between chelating agent and α-emitters was found as a crucial parameter. The present review was planned to highlight recent achievements about TAT-based targeting vectors and chelating agents and provide further insight for future researches.

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

Improved Radium-223 Therapy with Combination Epithelial Sodium Channel Blockade

Background: Radium-223 dichloride ([223Ra]RaCl2) is the first approved alpha particle-emitting therapy and is indicated for treatment of bone metastatic castrate resistant prostate cancer. Approximately half of the dose is absorbed into the gastrointestinal (GI) tract within minutes of administration, limiting disease-site uptake and contributing to toxicity. Here, we investigate the role of enteric ion channels and their modulation for improved therapeutic efficacy and reduced side effects. Methods: Utilizing primary human duodenal organoids (enteroids) as in vitro models of the functional GI epithelium, we found that Amiloride (ENaC blocker) and NS-1619 (K+ channel activator) presented significant effects in 223Ra membranal transport. The radioactive drug distribution was evaluated for lead combinations in vivo, and in osteosarcoma and prostate cancer models. Results: Amiloride shifted 223Ra uptake in vivo from the gut, to nearly double the uptake at sites of bone remodeling. Bone tumor growth inhibition with the combination as measured by bioluminescent and X-ray imaging was significantly greater than single agents alone, and the combination resulted in no weight loss. Conclusion: This combination of approved agents may be readily implemented as a clinical approach to improve outcomes of bone metastatic cancer patients with the benefit of ameliorated tolerability.

Towards the stable chelation of radium for biomedical applications with an 18-membered macrocyclic ligand

Targeted alpha therapy is an emerging strategy for the treatment of disseminated cancer. [223Ra]RaCl2 is the only clinically approved alpha particle-emitting drug, and it is used to treat castrate-resistant prostate cancer bone metastases, to which [223Ra]Ra2+ localizes. To specifically direct [223Ra]Ra2+ to non-osseous disease sites, chelation and conjugation to a cancer-targeting moiety is necessary. Although previous efforts to stably chelate [223Ra]Ra2+ for this purpose have had limited success, here we report a biologically stable radiocomplex with the 18-membered macrocyclic chelator macropa. Quantitative labeling of macropa with [223Ra]Ra2+ was accomplished within 5 min at room temperature with a radiolabeling efficiency of >95%, representing a significant advancement over conventional chelators such as DOTA and EDTA, which were unable to completely complex [223Ra]Ra2+ under these conditions. [223Ra][Ra(macropa)] was highly stable in human serum and exhibited dramatically reduced bone and spleen uptake in mice in comparison to bone-targeted [223Ra]RaCl2, signifying that [223Ra][Ra(macropa)] remains intact in vivo. Upon conjugation of macropa to a single amino acid β-alanine as well as to the prostate-specific membrane antigen-targeting peptide DUPA, both constructs retained high affinity for 223Ra, complexing >95% of Ra2+ in solution. Furthermore, [223Ra][Ra(macropa-β-alanine)] was rapidly cleared from mice and showed low 223Ra bone absorption, indicating that this conjugate is stable under biological conditions. Unexpectedly, this stability was lost upon conjugation of macropa to DUPA, which suggests a role of targeting vectors in complex stability in vivo for this system. Nonetheless, our successful demonstration of efficient radiolabeling of the β-alanine conjugate with 223Ra and its subsequent stability in vivo establishes for the first time the possibility of delivering [223Ra]Ra2+ to metastases outside of the bone using functionalized chelators, marking a significant expansion of the therapeutic utility of this radiometal in the clinic.

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

Targeted Alpha Therapy: Current Clinical Applications

α-Emitting radionuclides have been approved for cancer treatment since 2013, with increasing degrees of success. Despite this clinical utility, little is known regarding the mechanisms of action of α particles in this setting, and accurate assessments of the dosimetry underpinning their effectiveness are lacking. However, targeted alpha therapy (TAT) is gaining more attention as new targets, synthetic chemistry approaches, and α particle emitters are identified, constructed, developed, and realized. From a radiobiological perspective, α particles are more effective at killing cells compared to low linear energy transfer radiation. Also, from these direct effects, it is now evident from preclinical and clinical data that α emitters are capable of both producing effects in nonirradiated bystander cells and stimulating the immune system, extending the biological effects of TAT beyond the range of α particles. The short range of α particles makes them a potent tool to irradiate single-cell lesions or treat solid tumors by minimizing unwanted irradiation of normal tissue surrounding the cancer cells, assuming a high specificity of the radiopharmaceutical and good stability of its chemical bonds. Clinical approval of ²²³RaCl₂ in 2013 was a major milestone in the widespread application of TAT as a safe and effective strategy for cancer treatment. In addition, ²²⁵Ac-prostate specific membrane antigen treatment benefit in metastatic castrate-resistant prostate cancer patients, refractory to standard therapies, is another game-changing piece in the short history of TAT clinical application. Clinical applications of TAT are growing with different radionuclides and combination therapies, and in different clinical settings. Despite the remarkable advances in TAT dosimetry and imaging, it has not yet been used to its full potential. Labeled ²²⁷Th and ²²⁵Ac appear to be promising candidates and could represent the next generation of agents able to extend patient survival in several clinical scenarios.

Preclinical Single Photon Emission Computed Tomography of Alpha Particle-Emitting Radium-223

Objective: Dose optimization and pharmacokinetic evaluation of α-particle emitting radium-223 dichloride (²²³RaCl₂) by planar γ-camera or single photon emission computed tomography (SPECT) imaging are hampered by the low photon abundance and injected activities. In this study, we demonstrate SPECT of ²²³Ra using phantoms and small animal in vivo models. Methods: Line phantoms and mice bearing ²²³Ra were imaged using a dedicated small animal SPECT by detecting the low-energy photon emissions from ²²³Ra. Localization of the therapeutic agent was verified by whole-body and whole-limb autoradiography and its radiobiological effect confirmed by immunofluorescence. Results: A state-of-the-art commercial small animal SPECT system equipped with a highly sensitive collimator enables collection of sufficient counts for three-dimensional reconstruction at reasonable administered activities and acquisition times. Line sources of ²²³Ra in both air and in a water scattering phantom gave a line spread function with a full-width-at-half-maximum of 1.45 mm. Early and late-phase imaging of the pharmacokinetics of the radiopharmaceutical were captured. Uptake at sites of active bone remodeling was correlated with DNA damage from the α particle emissions. Conclusions: This work demonstrates the capability to noninvasively define the distribution of ²²³RaCl₂, a recently approved α-particle-emitting radionuclide. This approach allows quantitative assessment of ²²³Ra distribution and may assist radiation-dose optimization strategies to improve therapeutic response and ultimately to enable personalized treatment planning.

Previous, Current, and Future Pharmacotherapy and Diagnosis of Prostate Cancer-A Comprehensive Review

Prostate cancer (PCa) is one of the most common cancers in men that usually develops slowly. Since diagnostic methods improved in the last decade and are highly precise, more cancers are diagnosed at an early stage. Active surveillance or watchful waiting are appealing approaches for men diagnosed with low-risk prostate cancer, and they are an antidote to the overtreatment problem and unnecessary biopsies. However, treatment depends on individual circumstances of a patient. Older hormonal therapies based on first generation antiandrogens and steroids were widely used in metastatic castration-resistant prostate cancer (mCRPC) patients prior to the implementation of docetaxel. Nowadays, accordingly to randomized clinical trials, abiraterone, enzalutamide, apalutamide. and docetaxel became first line agents administrated in the treatment of mCRPC. Furthermore, radium-223 is an optional therapy for bone-only metastasis patients. Sipuleucel-T demonstrated an overall survival benefit. However, other novel immunotherapeutics showed limitations in monotherapy. Possible combinations of new vaccines or immune checkpoint blockers with enzalutamide, abiraterone, radium-223, or docetaxel are the subject of ongoing rivalry regarding optimal therapy of prostate cancer.

Review of Therapeutic Applications of Radiolabeled Functional Nanomaterials

In the last two decades, various nanomaterials have attracted increasing attention in medical science owing to their unique physical and chemical characteristics. Incorporating radionuclides into conventionally used nanomaterials can confer useful additional properties compared to the original material. Therefore, various radionuclides have been used to synthesize functional nanomaterials for biomedical applications. In particular, several α- or β-emitter-labeled organic and inorganic nanoparticles have been extensively investigated for efficient and targeted cancer treatment. This article reviews recent progress in cancer therapy using radiolabeled nanomaterials including inorganic, polymeric, and carbon-based materials and liposomes. We first provide an overview of radiolabeling methods for preparing anticancer agents that have been investigated recently in preclinical studies. Next, we discuss the therapeutic applications and effectiveness of α- or β-emitter-incorporated nanomaterials in animal models and the emerging possibilities of these nanomaterials in cancer therapy.

The impact of age on radium-223 distribution and an evaluation of molecular imaging surrogates

INTRODUCTION

Radium-223 dichloride is the first alpha-particle emitting therapeutic agent approved by FDA and EMA for bone metastatic castration-resistant prostate cancer. We studied its age-dependent biodistribution in mice, and compared it with [99mTc]Tc-MDP and [18F]NaF aiming to identify a potential imaging surrogate to predict [223Ra]RaCl2 whole-body localization.

METHODS

Male C57Bl/6 mice dosed with [223Ra]RaCl2 were sacrificed at different time points to explore [223Ra]RaCl2 whole-body distribution. In another experiment, mice at different ages were dosed with [223Ra]RaCl2 to evaluate the aging impact. Finally, [99mTc]Tc-MDP and [18F]NaF were administered to mice, and we compared their biodistributions with [223Ra]RaCl2. Detailed micro-localization of each tracer was visualized using autoradiography and histochemical staining.

RESULTS

[223Ra]RaCl2 uptake in bone was rapid and stable. We observed persistent localization at bone epiphyses, as well as the red pulp of the spleen, while its uptake in most soft tissues cleared within 24 h. [223Ra]RaCl2 distribution in soft tissues is similar in all age groups tested, while bone activity significantly decreased with aging. Although the diagnostic tracers cleared much faster from soft tissues than the therapeutic radionuclide, [99mTc]Tc-MDP and [18F]NaF both co-localized with [223Ra]RaCl2 in the skeletal compartment.

CONCLUSIONS

Radium-223 localization to the bone is dependent on age-varying factors, which implies that radium-223 dosimetry should take patient age into account. [99mTc]Tc-MDP shows a different biodistribution from [223Ra]RaCl2, both in soft tissues and in bone. [18F]NaF presents a high similarity with [223Ra]RaCl2 in skeletal uptake, which validates the potential of [18F]NaF as an imaging surrogate to predict radium-223 radiotherapeutic distribution in bone.

Targeted and Nontargeted α-Particle Therapies

α-Particle irradiation of cancerous tissue is increasingly recognized as a potent therapeutic option. We briefly review the physics, radiobiology, and dosimetry of α-particle emitters, as well as the distinguishing features that make them unique for radiopharmaceutical therapy. We also review the emerging clinical role of α-particle therapy in managing cancer and recent studies on in vitro and preclinical α-particle therapy delivered by antibodies, other small molecules, and nanometer-sized particles. In addition to their unique radiopharmaceutical characteristics, the increased availability and improved radiochemistry of α-particle radionuclides have contributed to the growing recent interest in α-particle radiotherapy. Targeted therapy strategies have presented novel possibilities for the use of α-particles in the treatment of cancer. Clinical experience has already demonstrated the safe and effective use of α-particle emitters as potent tumor-selective drugs for the treatment of leukemia and metastatic disease.

Feed-forward alpha particle radiotherapy ablates androgen receptor-addicted prostate cancer

Human kallikrein peptidase 2 (hK2) is a prostate specific enzyme whose expression is governed by the androgen receptor (AR). AR is the central oncogenic driver of prostate cancer (PCa) and is also a key regulator of DNA repair in cancer. We report an innovative therapeutic strategy that exploits the hormone-DNA repair circuit to enable molecularly-specific alpha particle irradiation of PCa. Alpha-particle irradiation of PCa is prompted by molecularly specific-targeting and internalization of the humanized monoclonal antibody hu11B6 targeting hK2 and further accelerated by inherent DNA-repair that up-regulate hK2 (KLK2) expression in vivo. hu11B6 demonstrates exquisite targeting specificity for KLK2. A single administration of actinium-225 labeled hu11B6 eradicates disease and significantly prolongs survival in animal models. DNA damage arising from alpha particle irradiation induces AR and subsequently KLK2, generating a unique feed-forward mechanism, which increases binding of hu11B6. Imaging data in nonhuman primates support the possibility of utilizing hu11B6 in man.

Simultaneous Separation of Actinium and Radium Isotopes from a Proton Irradiated Thorium Matrix

A new method has been developed for the isolation of ^223,224,225Ra, in high yield and purity, from a proton irradiated ²³²Th matrix. Herein we report an all-aqueous process using multiple solid-supported adsorption steps including a citrate chelation method developed to remove >99.9% of the barium contaminants by activity from the final radium product. A procedure involving the use of three columns in succession was developed, and the separation of ^223,224,225Ra from the thorium matrix was obtained with an overall recovery yield of 91 ± 3%, average radiochemical purity of 99.9%, and production yields that correspond to physical yields based on previously measured excitation functions.