Phase I pharmacokinetic and biodistribution study with escalating doses of ²²³Ra-dichloride in men with castration-resistant metastatic prostate cancer. Eur J Nucl Med Mol Imaging. 2013;40:1384-1393. [PMID: 23653243 DOI: 10.1007/s00259-013-2427-6]

Targeted Radiopharmaceutical Therapy for Bone Metastases

Radiopharmaceutical approaches for targeting bone metastasis have traditionally focused on palliation of pain. Several agents have been clinically used over the last several decades and have proven value in pain palliation providing pain relief and improving quality of life. The role is well established across several malignancies, most commonly used in osteoblastic prostate cancer patients. These agents have primarily based on targeting and uptake in bone matrix and have mostly included beta emitting isotopes. The advent alpha emitter and FDA approval of 223Ra-dichloride has created a paradigm shift in clinical approach from application for pain palliation to treatment of bone metastasis. The approval of 223Ra-dichloride given the survival benefit in metastatic prostate cancer patients, led to predominant use of this alpha emitter in prostate cancer patients. With rapid development of radiopharmaceutical therapies and approval of other targeted agents such as 177Lu-PSMA the approach to treatment of bone metastasis has further evolved and combination treatments have increasingly been applied. Novel approaches are needed to improve and expand the use of such therapies for treatment of bone metastasis. Combination therapies with different targeting mechanisms, combining chemotherapies and cocktail of alpha and beta emitters need further exploration.

Preparation and characterization of α-zirconium phosphate as a perspective material for separation of 225Ac and 213Bi

The interest for ZrP this material is based on its physicochemical properties which makes this material a perspective candidate for applications in nuclear medicine. In this study ZrP was prepared and completely characterized using various analytical methods. Finally, the study of radiometals sorption mechanism on a surface of ZrP and the surface characterization of ZrP were done. In conclusion, ZrP appears as promising for next studies with various purposes like drug delivery system or ion-exchanger for separations of medical radionuclides such as 225Ac and 213Bi.

Targeted thorium-227 conjugates as treatment options in oncology

Targeted alpha therapy (TAT) is a promising approach for addressing unmet needs in oncology. Inherent properties make α-emitting radionuclides well suited to cancer therapy, including high linear energy transfer (LET), penetration range of 2-10 cell layers, induction of complex double-stranded DNA breaks, and immune-stimulatory effects. Several alpha radionuclides, including radium-223 (²²³Ra), actinium-225 (²²⁵Ac), and thorium-227 (²²⁷Th), have been investigated. Conjugation of tumor targeting modalities, such as antibodies and small molecules, with a chelator moiety and subsequent radiolabeling with α-emitters enables specific delivery of cytotoxic payloads to different tumor types. ²²³Ra dichloride, approved for the treatment of patients with metastatic castration-resistant prostate cancer (mCRPC) with bone-metastatic disease and no visceral metastasis, is the only approved and commercialized alpha therapy. However, ²²³Ra dichloride cannot currently be complexed to targeting moieties. In contrast to ²²³Ra, ²²⁷Th may be readily chelated, which allows radiolabeling of tumor targeting moieties to produce targeted thorium conjugates (TTCs), facilitating delivery to a broad range of tumors. TTCs have shown promise in pre-clinical studies across a range of tumor-cell expressing antigens. A clinical study in hematological malignancy targeting CD22 has demonstrated early signs of activity. Furthermore, pre-clinical studies show additive or synergistic effects when TTCs are combined with established anti-cancer therapies, for example androgen receptor inhibitors (ARI), DNA damage response inhibitors such as poly (ADP)-ribose polymerase inhibitors or ataxia telangiectasia and Rad3-related kinase inhibitors, as well as immune checkpoint inhibitors.

Comparing absorbed doses and radiation risk of the α-emitting bone-seekers [223Ra]RaCl2 and [224Ra]RaCl2

[²²³Ra]RaCl₂ and [²²⁴Ra]RaCl₂ are bone seekers, emitting high LET, and short range (< 100 μm) alpha-particles. Both radionuclides show similar decay properties; the total alpha energies are comparable (²²³Ra: ≈28 MeV, ²²⁴Ra: ≈26 MeV). [²²⁴Ra]RaCl₂ has been used from the mid-1940s until 1990 for treating different bone and joint diseases with activities of up to approximately 50 MBq [²²⁴Ra]RaCl₂. In 2013 [²²³Ra]RaCl₂ obtained marketing authorization by the FDA and by the European Union for the treatment of metastatic prostate cancer with an activity to administer of 0.055 MBq per kg body weight for six cycles. For intravenous injections in humans a model calculation using the biokinetic model of ICRP67 shows a ratio of organ absorbed dose coefficients (²²⁴Ra:²²³Ra) between 0.37 (liver) and 0.97 except for the kidneys (2.27) and blood (1.57). For the red marrow as primary organ-at-risk, the ratio is 0.57. The differences are mainly caused be the differing half-lives of the decay products of both radium isotopes. Both radionuclides show comparable DNA damage patterns in peripheral blood mononuclear cells after internal ex-vivo irradiation. Data on the long-term radiation-associated side effects are only available for treatment with [²²⁴Ra]RaCl₂. Two epidemiological studies followed two patient groups treated with [²²⁴Ra]RaCl₂ for more than 25 years. One of them was the "Spiess study", a cohort of 899 juvenile patients who received several injections of [²²⁴Ra]RaCl₂ with a mean specific activity of 0.66 MBq/kg. Another patient group of ankylosing spondylitis patients was treated with 10 repeated intravenous injections of [²²⁴Ra]RaCl₂, 1 MBq each, 1 week apart. In total 1,471 of these patients were followed-up in the "Wick study". In both studies, an increased cancer mortality by leukemia and solid cancers was observed. Similar considerations on long-term effects likely apply to [²²³Ra]RaCl₂ as well since the biokinetics are similar and the absorbed doses in the same range. However, this increased risk will most likely not be observed due to the much shorter life expectancy of prostate cancer patients treated with [²²³Ra]RaCl₂.

Dosimetry in Radiopharmaceutical Therapy

The application of radiopharmaceutical therapy for the treatment of certain diseases is well established, and the field is expanding. New therapeutic radiopharmaceuticals have been developed in recent years, and more are in the research pipeline. Concurrently, there is growing interest in the use of internal dosimetry as a means of personalizing, and potentially optimizing, such therapy for patients. Internal dosimetry is multifaceted, and the current state of the art is discussed in this continuing education article. Topics include the context of dosimetry, internal dosimetry methods, the advantages and disadvantages of incorporating dosimetry calculations in radiopharmaceutical therapy, a description of the workflow for implementing patient-specific dosimetry, and future prospects in the field.

Repair of α-particle-induced DNA damage in peripheral blood mononuclear cells after internal ex vivo irradiation with 223Ra

Purpose

As α-emitters for radiopharmaceutical therapies are administered systemically by intravenous injection, blood will be irradiated by α-particles that induce clustered DNA double-strand breaks (DSBs). Here, we investigated the induction and repair of DSB damage in peripheral blood mononuclear cells (PBMCs) as a function of the absorbed dose to the blood following internal ex vivo irradiation with [²²³Ra]RaCl₂.

Methods

Blood samples of ten volunteers were irradiated by adding [²²³Ra]RaCl₂ solution with different activity concentrations resulting in absorbed doses to the blood of 3 mGy, 25 mGy, 50 mGy and 100 mGy. PBMCs were isolated, divided in three parts and either fixed directly (d-samples) or after 4 h or 24 h culture. After immunostaining, the induced γ-H2AX α-tracks were counted. The time-dependent decrease in α-track frequency was described with a model assuming a repair rate R and a fraction of non-repairable damage Q.

Results

For 25 mGy, 50 mGy and 100 mGy, the numbers of α-tracks were significantly increased compared to baseline at all time points. Compared to the corresponding d-samples, the α-track frequency decreased significantly after 4 h and after 24 h. The repair rates R were (0.24 ± 0.05) h⁻¹ for 25 mGy, (0.16 ± 0.04) h⁻¹ for 50 mGy and (0.13 ± 0.02) h⁻¹ for 100 mGy, suggesting faster repair at lower absorbed doses, while Q-values were similar.

Conclusion

The results obtained suggest that induction and repair of the DSB damage depend on the absorbed dose to the blood. Repair rates were similar to what has been observed for irradiation with low linear energy transfer.

Dosimetry in targeted alpha therapy. A systematic review: current findings and what is needed

Objective. A systematic review of dosimetry in Targeted Alpha Therapy (TAT) has been performed, identifying the common issues. Approach. The systematic review was performed in accordance with the PRISMA guidelines, and the literature was searched using the Scopus and PubMed databases. Main results. From the systematic review, three key points should be considered when performing dosimetry in TAT. (1) Biodistribution/Biokinetics: the accuracy of the biodistribution data is a limit to accurate dosimetry in TAT. The biodistribution of alpha-emitting radionuclides throughout the body is difficult to image directly, with surrogate radionuclide imaging, blood/faecal sampling, and animal studies able to provide information. (2) Daughter radionuclides: the decay energy of the alpha-emissions is sufficient to break the bond to the targeting vector, resulting in a release of free daughter radionuclides in the body. Accounting for daughter radionuclide migration is essential. (3) Small-scale dosimetry and microdosimetry: due to the short path length and heterogeneous distribution of alpha-emitters at the target site, small-scale/microdosimetry are important to account for the non-uniform dose distribution in a target region, organ or cell and for assessing the biological effect of alpha-particle radiation. Significance. TAT is a form of cancer treatment capable of delivering a highly localised dose to the tumour environment while sparing the surrounding healthy tissue. Dosimetry is an important part of treatment planning and follow up. Being able to accurately predict the radiation dose to the target region and healthy organs could guide the optimal prescribed activity. Detailed dosimetry models accounting for the three points mentioned above will help give confidence in and guide the clinical application of alpha-emitting radionuclides in targeted cancer therapy.

Prospective testing of clinical Cerenkov luminescence imaging against standard-of-care nuclear imaging for tumour location

In oncology, the feasibility of Cerenkov luminescence imaging (CLI) has been assessed by imaging superficial lymph nodes in a few patients undergoing diagnostic ¹⁸F-fluoro-2-deoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT). However, the weak luminescence signal requires the removal of ambient light. Here we report the development of a clinical CLI fiberscope with a lightproof enclosure, and the clinical testing of the setup using five different radiotracers. In an observational prospective trial (ClinicalTrials.gov identifier NCT03484884 ) involving 96 patients with existing or suspected tumours, scheduled for routine clinical FDG PET or ¹³¹I therapy, the level of agreement of CLI with standard-of-care imaging (PET or planar single-photon emission CT) for tumour location was 'acceptable' or higher (≥3 in the 1-5 Likert scale) for 90% of the patients. CLI correlated with the concentration of radioactive activity, and captured therapeutically relevant information from patients undergoing targeted radiotherapy or receiving the alpha emitter ²²³Ra, which cannot be feasibly imaged clinically. CLI could supplement radiological scans, especially when scanner capacity is limited.

Radium-223 Dichloride in Peritoneal Dialysate Following Treatment of Metastatic Castration-resistant Prostate Cancer

ABSTRACT

Radium-223 dichloride was administrated to a patient with castration-resistant metastatic prostate cancer (mCRPC) undergoing in-home peritoneal dialysis. The peritoneal dialysate and tubing sets were collected following four treatment cycles and analyzed for removable contamination, dose rate, and radioactivity. The purpose of the study was to assess radiation safety concerns and compare data with patients having normal renal function. Sixty-two liters of dialysate in 22 samples were collected over 4 mo. The mean surface dose rate of the dialysate bags was 0.16 μSv h-1 (range 0.11-0.23 μSv h-1). The highest measured removable surface activity was 150 dpm per 100 cm2. The mean cumulative percent of administered activity was 1.7% (range 1.4-1.9%). The mean concentration of 223Ra in dialysate was 4.0 kBq L-1 (range 0.56-14.1 kBq L-1). There was no association between the measured 223Ra activity in dialysate with peritoneal dwell time or the number of exchanges following administration. The measurement of the 211Bi and 223Ra ratio in the dialysate compared with a standard showed an increase of 23% by 40 h post administration. The data presented suggest that 223Ra dialysate can be safely managed in the home without risk of radiation dose or contamination if minimal precautions are taken. Patients with normal renal function have been shown to excrete up to six times more 223Ra in urine compared to those undergoing hemodialysis or peritoneal dialysis. A potential consequence may be an increase in 223Ra activity transiting GI tract leading to intestinal effects.

Dose estimation after a mixed field exposure: Radium-223 and intensity modulated radiotherapy

INTRODUCTION

Radium-223 dichloride ([²²³Ra]RaCl₂), a radiopharmaceutical that delivers α-particles to regions of bone metastatic disease, has been proven to improve overall survival of men with metastatic castration resistant prostate cancer (mCRPC). mCRPC patients enrolled on the ADRRAD clinical trial are treated with a mixed field exposure comprising radium-223 (²²³Ra) and intensity modulated radiotherapy (IMRT). While absorbed dose estimation is an important step in the characterisation of wider systemic radiation risks in nuclear medicine, uncertainties remain for novel radiopharmaceuticals such as ²²³Ra.

METHODS

24-Colour karyotyping was used to quantify the spectrum of chromosome aberrations in peripheral blood lymphocytes of ADRRAD patients at incremental times during their treatment. Dicentric equivalent frequencies were used in standard models for estimation of absorbed blood dose. To account for the mixed field nature of the treatment, existing models were used to determine the ratio of the component radiation types. Additionally, a new approach (M-FISH_LET), based on the ratio of cells containing damage consistent with high-LET exposure (complex chromosomal exchanges) and low-LET exposure (simple exchanges), was used as a pseudo ratio for ²²³Ra:IMRT dose.

RESULTS

Total IMRT estimated doses delivered to the blood after completion of mixed radiotherapy (after 37 IMRT fractions and two [²²³Ra]RaCl₂ injections) were in the range of 1.167 ± 0.092 and 2.148 ± 0.096 Gy (dose range across all models applied). By the last treatment cycle analysed in this study (four [²²³Ra]RaCl₂ injections), the total absorbed ²²³Ra dose to the blood was estimated to be between 0.024 ± 0.027 and 0.665 ± 0.080 Gy, depending on the model used. Differences between the models were observed, with the observed dose variance coming from inter-model as opposed to inter-patient differences. The M-FISH_LET model potentially overestimates the ²²³Ra absorbed blood dose by accounting for further PBL exposure in the vicinity of metastatic sites.

CONCLUSIONS

The models presented provide initial estimations of cumulative dose received during incremental IMRT fractions and [²²³Ra]RaCl₂ injections, which will enable improved understanding of the doses received by individual patients. While the M-FISH_LET method builds on a well-established technique for external exposures, further consideration is needed to evaluate this method and its use in assessing non-targeted exposure by ²²³Ra after its localization at bone metastatic sites.

mRNA and small RNA gene expression changes in peripheral blood to detect internal Ra-223 exposure

PURPOSE

Excretion analysis is the established method for detection of incorporated alpha-emitting radionuclides, but it is laborious and time consuming. We sought a simplified method in which changes in gene expression might be measured in human peripheral blood to detect incorporated radionuclides. Such an approach could be used to quickly determine internal exposure in instances of a radiological dispersal device or a radiation accident.

MATERIALS AND METHODS

We evaluated whole blood samples from five patients with castration-resistant prostate cancer and multiple bone metastases (without visceral or nodal involvement), who underwent treatment with the alpha emitting isotope Radium-223 dichloride (Ra-223, Xofigo^®). Patients received about 4 MBq per cycle and, depending on survival and treatment tolerance, were followed for six months. We collected 24 blood samples approximately monthly corresponding to treatment cycle.

RESULTS

Firstly, we conducted whole genome screening of mRNAs (mRNA seq) and small RNAs (small RNA seq) using next generation sequencing in one patient at eight different time points during all six cycles of Ra-223-therapy. We identified 1900 mRNAs and 972 small RNAs (222 miRNAs) that were differentially up- or down-regulated during follow-up after the first treatment with Ra-223. Overall candidate RNA species inclusion criteria were a general (≥|2|-fold) change or with peaking profiles (≥|5|-fold) at specific points in time. Next we chose 72 candidate mRNAs and 101 small RNAs (comprising 29 miRNAs) for methodologic (n = 8 samples, one patient) and independent (n = 16 samples, four patients) validation by qRT-PCR. In total, 15 mRNAs (but no small RNAs) were validated by methodologic and independent testing. However, the deregulation occurred at different time points, showing a large inter-individual variability in response among patients.

CONCLUSIONS

This proof of concept provides support for the applicability of gene expression measurements to detect internalized alpha-emitting radionuclides, but further work is needed with a larger sample size. While our approach has merit for internal deposition monitoring, it was complicated by the severe clinical condition of the patients we studied.

Imaging and dosimetry for alpha-particle emitter radiopharmaceutical therapy: improving radiopharmaceutical therapy by looking into the black box

Radiopharmaceutical therapy using α-particle emitting radionuclides (αRPT) is a novel treatment modality that delivers highly potent alpha-particles to cancer cells or their environment. We review the advantages and challenges of imaging and dosimetry in implementing αRPT for cancer patients.

18F-sodium fluoride positron emission tomography (NaF-18-PET/CT) radiomic signatures to evaluate responses to alpha-particle Radium-223 dichloride therapy in osteosarcoma metastases

Patients with osteoblastic metastases from high risk osteosarcoma continue to have a poor prognosis after progression from standard-of-care multi-agent chemotherapy. In a first-in-human dose escalation trial of bone targeted Radium 223 dichloride alpha-particle therapy in 18 patients with advanced osteosarcoma only 1 patient responded based on conventional Response Evaluation Criteria in Solid Tumors (RECIST). Na¹⁸F PET response Criteria in Solid Tumors(NAFCIST), based on Sodium fluoride-18 (Na¹⁸F) positron emission tomography (PET)-CT was developed to better evaluate bone specific response. To further appreciate the spatial and temporal heterogeneity of the partial or mixed responses, a radiomics method was developed. Analyses were performed with ¹⁸F-sodium fluoride positron emission tomography imaging studies before and after alpha-particle therapy. Radioactive ¹⁸F^- -atom concentrations were measured in soft-tissues, in approximately 1000 concentration data points for ¹⁸F^- per 1 cm³ metastatic tumor. Data was analyzed from the SUV intensity values, the histogram of intensities and entropy values. Radiomics may inform intra-tumoral and inter-tumoral heterogeneity in response of bone forming osteosarcoma to alpha particle therapy. Each patient (and each tumor) represents an "N of 1" case and warrants in depth analysis individually.

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.

Graphene Quantum Dots for Molecular Radiotherapy: Radiolabeled Graphene Quantum Dots with Radium (223Ra) Showed Potent Effect Against Bone Cancer

The necessity of new drugs with special attention for the therapy of cancer is increasing each day. Despite their properties, alpha therapeutic radiopharmaceuticals, especially based on the use of radium (²²³Ra) are good choices, due to the highest and differential cytotoxicity, low adverse effects, and higher bioaccumulation on tumor sites. The use of graphene quantum dots as the carrier for ²²³Ra is a promising approach since graphene quantum dots has low toxicity, high biocompatibility, and adequate size for tumor penetration. In this study, we developed, characterized, radiolabeled with ²²³Ra, and evaluated in vitro and in vivo graphene quantum dots radiolabeled with radium (²²³Ra) for bone cancer. The results showed that ²²³Ra is incorporated into the graphene quantum dot following the Fajans-Paneth-Hahn Law. The cell viability showed a potent effect on osteosarcoma cells (MG63 and SAOS2) but a lower effect in normal fibroblast cells (hFB), corroborating the preferential targeting. Also, the results showed a more prominent effect on MG63 than SAOS2 cells, corroborating the targeting for more undifferentiated cells. The in vivo results demonstrated a renal excretion, associated with fecal excretion and accumulation in bone. The results corroborate the efficacy of ²²³RaGQDs and open new perspectives for the use of use ²²³RaGQDs, in several other diseases.

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.

Radiopharmacokinetic modelling and radiation dose assessment of 223Ra used for treatment of metastatic castration-resistant prostate cancer

PURPOSE

Ra-223 dichloride (223Ra, Xofigo®) is used for treatment of patients suffering from castration-resistant metastatic prostate cancer. The objective of this work was to apply the most recent biokinetic model for radium and its progeny to show their radiopharmacokinetic behaviour. Organ absorbed doses after intravenous injection of 223Ra were estimated and compared to clinical data and data of an earlier modelling study.

METHODS

The most recent systemic biokinetic model of 223Ra and its progeny, developed by the International Commission on Radiological Protection (ICRP), as well as the ICRP human alimentary tract model were applied for the radiopharmacokinetic modelling of Xofigo® biodistribution in patients after bolus administration. Independent kinetics were assumed for the progeny of 223Ra. The time activity curves for 223Ra were modelled and the time integrated activity coefficients, [Formula: see text] in the source regions for each progeny were determined. For estimating the organ absorbed doses, the Specific Absorbed Fractions (SAF) and dosimetric framework of ICRP were used together with the aforementioned [Formula: see text] values.

RESULTS

The distribution of 223Ra after injection showed a rapid plasma clearance and a low urinary excretion. Main elimination was via faeces. Bone retention was found to be about 30% at 4 h post-injection. Similar tendencies were observed in clinical trials of other authors. The highest absorbed dose coefficients were found for bone endosteum, liver and red marrow, followed by kidneys and colon.

CONCLUSION

The biokinetic modelling of 223Ra and its progeny may help to predict their distributions in patients after administration of Xofigo®. The organ dose coefficients of this work showed some variation to the values reported from clinical studies and an earlier compartmental modelling study. The dose to the bone endosteum was found to be lower by a factor of ca. 3 than previously estimated.

Thorium-227-Labeled Anti-CD22 Antibody (BAY 1862864) in Relapsed/Refractory CD22-Positive Non-Hodgkin Lymphoma: A First-in-Human, Phase I Study

Background: BAY 1862864 is an α-particle emitting thorium-227-labeled CD22-targeting antibody. This first-in-human dose-escalation phase I study evaluated BAY 1862864 in patients with CD22-positive relapsed/refractory B cell non-Hodgkin lymphoma (R/R-NHL). Materials and Methods: BAY 1862864 intravenous injections were administered at the starting thorium-227 radioactivity dose of 1.5 MBq (2 or 10 mg antibody), and the radioactivity dose escalated in ∼1.5 MBq increments (10 mg antibody) until the maximum tolerated dose (MTD) was reported. The primary objective was to determine the safety, tolerability, and MTD. Results: Twenty-one patients received BAY 1862864. Two dose-limiting toxicities (grade 3 febrile neutropenia and grade 4 thrombocytopenia) were reported in one patient in the 4.6 MBq (10 mg antibody) cohort. The MTD was not reached. Ten (48%) patients reported grade ≥3 treatment-emergent adverse events, with the most common being neutropenia, thrombocytopenia, and leukopenia, each occurring in 3 (14%) patients. Pharmacokinetics demonstrated the dose proportionality and stability of BAY 1862864 in the blood. The objective response rate (ORR) was 25% (5/21 patients) according to the LUGANO 2014 criteria, including 1 complete and 4 partial responses. The ORR was 11% (1/9) and 30% (3/10) in patients with relapsed high- and low-grade lymphomas, respectively. Conclusions: BAY 1862864 was safe and tolerated in patients with R/R-NHL. The Clinical Trial Registration numbers: NCT02581878 and EudraCT 2014-004140-36.

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.

Recent Advances in Radiometals for Combined Imaging and Therapy in Cancer

Nuclear medicine is defined as the use of radionuclides for diagnostic and therapeutic applications. The imaging modalities positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are based on γ-emissions of specific energies. The therapeutic technologies are based on β^- -particle-, α-particle-, and Auger electron emitters. In oncology, PET and SPECT are used to detect cancer lesions, to determine dosimetry, and to monitor therapy effectiveness. In contrast, radiotherapy is designed to irreparably damage tumor cells in order to eradicate or control the disease's progression. Radiometals are being explored for the development of diagnostic and therapeutic radiopharmaceuticals. Strategies that combine both modalities (diagnostic and therapeutic), referred to as theranostics, are promising candidates for clinical applications. This review provides an overview of the basic concepts behind therapeutic and diagnostic radiopharmaceuticals and their significance in contemporary oncology. Select radiometals that significantly impact current and upcoming cancer treatment strategies are grouped as clinically suitable theranostics pairs. The most important physical and chemical properties are discussed. Standard production methods and current radionuclide availability are provided to indicate whether a cost-efficient use in a clinical routine is feasible. Recent preclinical and clinical developments and outline perspectives for the radiometals are highlighted in each section.

TOPAS a tool to evaluate the impact of cell geometry and radionuclide on alpha particle therapy

Due to the increasing clinical application of alpha particles, accurate assessment of their dosimetry at the cellular scale should be strongly advocated. Although observations of the impact of cell and nuclear geometry have been previously reported, this effect has not been fully quantified. Additionally, alpha particle dosimetry presents several challenges and most conventional methodologies have poor resolution and are limited to average parameters across populations of cells. Meaningful dosimetry studies with alpha particles require detailed information on the geometry of the target at a subcellular scale.

METHODS

The impact of cellular geometry was evaluated for 3 different scenarios, a spherical cell with a concentric nucleus, a spherical cell with an eccentric nucleus and a model of a cell attached to a flask, consisting of a hemispherical oblate ellipsoid, all exposed to 1,700 211At radionuclide decays. We also evaluated the cross-fire effect of alpha particles as function of distance to a source cell. Finally, a nanodosimetric analysis of absorbed dose to the nucleus of a cell exposed to 1 Gy of different alpha emitting radionuclides was performed.

RESULTS

Simulated data shows the dosimetry of self-absorbed-dose strongly depends on activity localization in the source cell, but that activity localization within the source cell did not significantly affect the cross-fire absorbed dose even when cells are in direct contact with each other. Additionally, nanodosimetric analysis failed to show any significant differences in the energy deposition profile between different alpha particle emitters.

CONCLUSIONS

The collected data allows a better understanding of the dosimetry of alpha particles emitters at the sub-cellular scale. Dosimetric variations between different cellular configurations can generate complications and confounding factors for the translation of dosimetric outcomes into clinical settings, but effects of different radionuclides are generally similar.

Sodium Fluoride-18 and Radium-223 Dichloride Uptake Colocalize in Osteoblastic Mouse Xenograft Tumors

Background: Patients with osteoblastic bone metastases are candidates for radium-223 (²²³RaCl₂) therapy and may undergo sodium fluoride-18 (¹⁸F-NaF) positron emission tomography-computed tomography imaging to identify bone lesions. ¹⁸F-NaF has been shown to predict ²²³RaCl₂ uptake, but intratumor distributions of these two agents remain unclear. In this study, the authors evaluate the spatial distribution and relative uptakes of ¹⁸F-NaF and ²²³RaCl₂ in Hu09-H3 human osteosarcoma mouse xenograft tumors at macroscopic and microscopic levels to better quantify their correlation. Materials and Methods: ¹⁸F-NaF and ²²³RaCl₂ were co-injected into Hu09-H3 xenograft tumor severe combined immunodeficient mice. Tumor content was determined from in vivo biodistributions and visualized by PET, single photon emission computed tomography, and CT imaging. Intratumor distributions were visualized by quantitative autoradiography of tumor tissue sections and compared to histology of the same or adjacent sections. Results: ¹⁸F and ²²³Ra accumulated in proportional amounts in whole Hu09-H3 tumors (r² = 0.82) and in microcalcified regions within these tumors (r² = 0.87). Intratumor distributions of ¹⁸F and ²²³Ra were spatially congruent in these microcalcified regions. Conclusions: ¹⁸F-NaF and ²²³RaCl₂ uptake are strongly correlated in heterogeneously distributed microcalcified regions of Hu09-H3 xenograft tumors, and thus, tumor accumulation of ¹⁸F is predictive of ²²³Ra accumulation. Hu09-H3 xenograft tumors appear to possess certain histopathological features found in patients with metastatic bone disease and may be useful in clarifying the relationship between administered ²²³Ra dose and therapeutic effect.

A retrospective study on the potential of 99m Tc-HDP imaging before therapy for individualizing treatments with 223 Ra-Cl2 for metastatic castration resistant prostate cancer

PURPOSE

Research on dose-effect correlation is necessary to move toward an individualization of treatments of metastatic castration resistant prostate cancer (mCRPC) with ²²³ Ra-Cl₂ . We first looked for a possible correlation of ^99m Tc-HDP lesion uptake in pretreatment whole-body scans (WBSs) with lesion absorbed dose. Moreover, we looked for a possible correlation of ^99m Tc-HDP lesion uptake in pretreatment WBSs and of lesion absorbed dose with relative change in the ^99m Tc-HDP lesion uptake obtained from pre- and post-treatment WBSs in patients treated for mCRPC with six cycles of ²²³ Ra-Cl₂ .

METHODS

Eleven patients received six cycles of 55 kBq/kg of ²²³ Ra-Cl₂ separated by 4 weeks. In addition, one patient received concomitant treatment with abiraterone and two patients with enzalutamide. The ^99m Tc-HDP WBSs were acquired before the first cycle and after the sixth cycle of the treatment. For the lesions with the higher ^99m Tc-HDP uptake, the absorbed dose was calculated for the first cycle. Lesion volume was determined from ^99m Tc-HDP SPECT/CT images before the first cycle and ²²³ Ra-Cl₂ activity in the lesions was determined from ²²³ Ra-Cl₂ planar images after the first cycle. The effect of the treatment was evaluated from the relative change of the mean and the maximum counts in the lesions, both estimated from the WBSs acquired before the first cycle and after the sixth cycle.

RESULTS

The absorbed dose was calculated for 30 lesions, with values ranging between 0.4 and 3.8 Gy (mean 1.5 Gy). A significant (P < 0.05) high positive linear correlation was found between the lesion absorbed dose in the first treatment cycle and the mean and maximum counts in the lesions in the WBSs acquired before the first cycle (R = 0.75 and 0.76, respectively). The relative change of the mean and the maximum counts in the lesions in the ^99m Tc-HDP WBSs showed a significant (P < 0.05) high positive logarithmic correlation with the ^99m Tc-HDP mean and maximum counts in the lesions before the first cycle (R = 0.79 and 0.78, respectively). Lastly, a significant (P < 0.05) high positive logarithmic correlation was also found between the relative change of the mean and the maximum counts in the lesions in the ^99m Tc-HDP WBSs and the lesion absorbed dose (R = 0.86 and 0.85, respectively). For this correlation the influence of the administered activity and of the concomitant treatments was not found to be significant (P > 0.05).

CONCLUSIONS

The high correlations found for the ^99m Tc-HDP lesion uptake before the first cycle lesion with the relative change in the ^99m Tc-HDP lesion uptake after the six cycles of ²²³ Ra-Cl₂ , and with the lesion absorbed dose in the first cycle show the potential of pretreatment ^99m Tc-HDP imaging in order to personalize the performance of these treatments.

α-Particle-induced DNA damage tracks in peripheral blood mononuclear cells of [223Ra]RaCl2-treated prostate cancer patients

PURPOSE

One therapy option for prostate cancer patients with bone metastases is the use of [²²³Ra]RaCl₂. The α-emitter ²²³Ra creates DNA damage tracks along α-particle trajectories (α-tracks) in exposed cells that can be revealed by immunofluorescent staining of γ-H2AX+53BP1 DNA double-strand break markers. We investigated the time- and absorbed dose-dependency of the number of α-tracks in peripheral blood mononuclear cells (PBMCs) of patients undergoing their first therapy with [²²³Ra]RaCl₂.

METHODS

Multiple blood samples from nine prostate cancer patients were collected before and after administration of [²²³Ra]RaCl₂, up to 4 weeks after treatment. γ-H2AX- and 53BP1-positive α-tracks were microscopically quantified in isolated and immuno-stained PBMCs.

RESULTS

The absorbed doses to the blood were less than 6 mGy up to 4 h after administration and maximally 16 mGy in total. Up to 4 h after administration, the α-track frequency was significantly increased relative to baseline and correlated with the absorbed dose to the blood in the dose range < 3 mGy. In most of the late samples (24 h - 4 weeks after administration), the α-track frequency remained elevated.

CONCLUSION

The γ-H2AX+53BP1 assay is a potent method for detection of α-particle-induced DNA damages during treatment with or after accidental incorporation of radionuclides even at low absorbed doses. It may serve as a biomarker discriminating α- from β-emitters based on damage geometry.

Radiopharmaceuticals for Bone Metastases

As a single organ distributed diffusely throughout the body, bones represent both a unique challenge and unique opportunity for the treatment of symptomatic metastatic disease. While the multifocality of bone metastases often prevents effective complete treatment with focal radiotherapy, the similar pathophysiology of these diffuse sites of disease opens the door to targeted systemic therapy. The relatively rapid dose fall-off from beta- or alpha-emitting particles, if correctly and reliably targeted to osseous metastases, might reduce tumor burden and enhance pain control or improve survival. Radioisotopes have thus been studied keenly with the first generation of primarily beta-emitting radioisotopes, strontium-89 and samarium-153, which reached early FDA approval based on successful endpoints of pain control. More recently, an alpha-emitting therapy, radium-223, has demonstrated a successful endpoint of improved overall survival in patients with a burden of symptomatic, metastatic castrate-resistant prostate cancer (mCRPC) confined to the bones. With this discovery, an additional survival-improving tool beyond systemic and hormonal agents was added to the treatment arsenal for mCRPC for suitable candidates. With an improved understanding of the optimization of hormonal and systemic therapies in the context of mCRPC, there is lingering uncertainty regarding the safety and efficacy of combinatorial use of alpha and beta-emitting therapies with the current generation of systemic agents. In this narrative review, we will highlight the current understanding of the relative utility and clinical paradigms involving alpha- and beta-emitting radioisotopes. We discuss fundamental mechanisms for antineoplastic activity, initial clinical trials validating their use, the use of concurrent antiresorptive therapies to provide bone protection, and ongoing clinical trials targeted at best utilization of these agents in the broader context of mCRPC treatment.

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.

Assessment of the radiation exposure of relatives and caregivers of patients treated with Ra-223 - Results of a German multicenter study

A multicenter study was conducted to assess the radiation exposure of relatives and caregivers of patients suffering from castration resistant prostate cancer with bone metastases and treated with Ra-223 dichloride in an outpatient setting. As Ra-223 and most of its progeny emit alpha particles, especially the internal exposure of persons in the patient's vicinity had to be evaluated.

METHODS

The external radiation was measured in distances of 1 m and 2 m. Wipe-tests were taken in the patients' homes to identify significant contaminations and evaluated by liquid scintillation counting. Samples of saliva and sweat were taken and measured using gamma spectrometry.

RESULTS

The external exposure from the patients measured 10-20min post injection (p. i.) was<0.080μSv/h in median in 1 m distance (range: below decision threshold (<DT) - 0.208μSv/h). Ra-223 was found in saliva (median 20-30 min p. i.: 27 Bq/g, range: 12.6 - 110 Bq/g) and excreted with sweat (median: 0.121 Bq/cm², range: 0.020 - 0.524 Bq/cm²) in the first 24h p. i. Contaminations in restrooms and kitchens were low (median: 0.026 - 0.215 mBq/cm², depending on location; range:<DT - 2.1 mBq/cm²).

CONCLUSIONS

The potential exposure of relatives by external irradiation and incorporation of Ra-223 excreted by the patient with saliva or sweat is estimated to be well below 1 mSv. No objections are seen regarding outpatient treatment.

177Lu-EDTMP for Metastatic Bone Pain Palliation: A Systematic Review and Meta-Analysis

Purpose: Painful metastatic bone involvement is common in advanced stages of many cancers. Between available radionuclides for bone pain palliation, no consensus has been reached on lutetium ethylenediaminetetramethylene phosphonate (¹⁷⁷Lu-EDTMP) administration in this milieu. The aim of this study is to evaluate the treatment efficacy, safety profile, and toxicities of ¹⁷⁷Lu-EDTMP in patients with metastatic bone involvement, according to the published literature. Methods: A comprehensive literature search of PubMed/MEDLINE, Scopus, and Google Scholar databases was carried out to retrieve pertinent articles published until January 2019, concerning the clinical efficacy and safety of ¹⁷⁷Lu-EDTMP for bone pain palliative purposes. Results: Eight studies (172 patients) were included. This analysis revealed statistically significant effect of ¹⁷⁷Lu-EDTMP therapy on the visual analog score (4.84% (95% CI: 3.88-5.81; p < 0.001), bone palliative pain response (84%, 95% CI: 75%-90%; p < 0.001), and Karnofsky performance status (21%, 95% CI: 18%-24%; p < 0.001) overall (as well as in the high-dose and low-dose subgroups). Complete palliative pain response to treatment was observed in 32% (95% CI: 16%-53%) of patients receiving ¹⁷⁷Lu-EDTMP. Anemia was found to be the most common hematologic toxicity imposed by this therapeutic approach (grade I/II anemia in 24% (95% CI: 14%-38%; p < 0.001) and grade III/IV anemia in 19% (95% CI: 12%-28%; p < 0.001)). Conclusions: ¹⁷⁷Lu-EDTMP seems to have comparable efficacy and safety profile as that of the frequently administered radiopharmaceuticals for bone palliation. Therefore, this agent can be a good option for bone pain palliative purposes, in case of limited access to other bone palliative radiopharmaceuticals.

Microdosimetry-based determination of tumour control probability curves for treatments with 225Ac-PSMA of metastatic castration resistant prostate cancer

We performed Monte Carlo simulations in order to determine, by means of microdosimetry calculations, tumour control probability (TCP) curves for treatments with ²²⁵Ac-PSMA of metastatic castration resistant prostate cancer (mCRPC). Realistic values of cell radiosensitivity, nucleus size and lesion size were used for calculations. As the cell radiosensitivity decreased, the nucleus size decreased and the lesion size increased, the absorbed dose to reach a given TCP increased. The widest variations occurred with regard to the cell radiosensitivity. For the Monte Carlo simulations, in order to address a non-uniform PSMA expression, different ²²⁵Ac-PSMA distributions were considered. The effect of these different PSMA distributions resulted in small variations in the TCP curves (maximum variation of 5%). Absorbed doses to reach a TCP of 0.9 for a uniform ²²⁵Ac-PSMA distribution, considering a relative biological effectiveness (RBE) of 5, ranged between 35.0 Gy and 116.5 Gy. The lesion absorbed doses per administered activity reported in a study on treatments with ²²⁵Ac-PSMA of mCRPC ranged between 1.3 Gy MBq^-1 and 9.8 Gy MBq^-1 for a RBE = 5. For a 70 kg-patient to whom 100 kBq kg^-1 of ²²⁵Ac-PSMA are administered, the range of lesion absorbed doses would be between 9.1 Gy and 68.6 Gy. Thus, for a single cycle of 100 kBq kg^-1, a number of lesions would not receive an absorbed dose high enough to reach a TCP of 0.9.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: ²²⁵Ac-DOTA-Ahx-MC1RL (²²⁵Ac-Ahx) and ²²⁵Ac-DOTA-di-d-Glu-MC1RL (²²⁵Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with ²²⁵Ac-Ahx or ²²⁵Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, ²²⁵Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

Targeted Palliative Radionuclide Therapy for Metastatic Bone Pain

Bone metastasis develops in multiple malignancies with a wide range of incidence. The presence of multiple bone metastases, leading to a multitude of complications and poorer prognosis. The corresponding refractory bone pain is still a challenging issue managed through multidisciplinary approaches to enhance the quality of life. Radiopharmaceuticals are mainly used in the latest courses of the disease. Bone-pain palliation with easy-to-administer radionuclides offers advantages, including simultaneous treatment of multiple metastatic foci, the repeatability and also the combination with other therapies. Several β¯- and α-emitters as well as pharmaceuticals, from the very first [⁸⁹Sr]strontium-dichloride to recently introduced [²²³Ra]radium-dichloride, are investigated to identify an optimum agent. In addition, the combination of bone-seeking radiopharmaceuticals with chemotherapy or radiotherapy has been employed to enhance the outcome. Radiopharmaceuticals demonstrate an acceptable response rate in pain relief. Nevertheless, survival benefits have been documented in only a limited number of studies. In this review, we provide an overview of bone-seeking radiopharmaceuticals used for bone-pain palliation, their effectiveness and toxicity, as well as the results of the combination with other therapies. Bone-pain palliation with radiopharmaceuticals has been employed for eight decades. However, there are still new aspects yet to be established.

Spine and Non-spine Bone Metastases - Current Controversies and Future Direction

Bone is a common site of metastases in advanced cancers. The main symptom is pain, which increases morbidity and reduces quality of life. The treatment of bone metastases needs a multidisciplinary approach, with the main aim of relieving pain and improving quality of life. Apart from systemic anticancer therapy (hormonal therapy, chemotherapy or immunotherapy), there are several therapeutic options available to achieve palliation, including analgesics, surgery, local radiotherapy, bone-seeking radioisotopes and bone-modifying agents. Long-term use of non-steroidal analgesics and opiates is associated with significant side-effects, and tachyphylaxis. Radiotherapy is effective mainly in localised disease sites. Bone-targeting radionuclides are useful in patients with multiple metastatic lesions. Bone-modifying agents are beneficial in reducing skeletal-related events. This overview focuses on the role of surgery, including minimally invasive treatments, conventional radiotherapy in spinal and non-spinal bone metastases, bone-targeting radionuclides and bone-modifying agents in achieving palliation. We present the clinical data and their associated toxicity. Recent advances are also discussed.

Decreasing undesirable absorbed radiation to the intestine after administration of radium-223 dichloride for treatment of bone metastases

[223Ra]RaCl2 is the first alpha-particle emitting radiopharmaceutical to be used for castration-resistant prostate cancer patients with bone metastases because of its excellent therapeutic effects. [223Ra]RaCl2 is excreted via the intestine into feces, and some is absorbed from the intestine into the blood, which may be undesirable in terms of the exposure to radiation. Recently, we showed that a complex of myo-inositol-hexakisphosphate (InsP6) with zinc is a useful decorporation agent against radiostrontium. In this study, we hypothesized that Zn-InsP6 could bind to not only strontium but also to radium, and could inhibit the absorption of radium from the intestine. In in vitro binding experiments, Zn-InsP6 showed a high binding affinity for radium. In in vivo biodistribution experiments by intravenous injection of [223Ra]RaCl2 after treatment of Zn-InsP6, mice treated with Zn-InsP6 showed significantly lower bone accumulation of radioactivity (34.82 ± 1.83%Dose/g) than the mice in the non-treatment control group (40.30 ± 2.78%Dose/g) at 48 h postinjection. These results indicate that Zn-InsP6 bound radium in the intestine and inhibited the absorption of radium into the blood. Therefore, the insoluble Zn-InsP6 complex has high potential to decrease the side effects of [223Ra]RaCl2.

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.

Feasibility and limitations of quantitative SPECT for 223Ra

The aim of this paper is to investigate the feasibility and limitations of activity-concentration estimation for ²²³Ra using SPECT. Phantom measurements are performed using spheres (volumes 5.5 mL to 26.4 mL, concentrations 1.6 kBq mL^-1 to 4.5 kBq mL^-1). Furthermore, SPECT projections are simulated using the SIMIND Monte Carlo program for two geometries, one similar to the physical phantom and the other being an anthropomorphic phantom with added lesions (volumes 34 mL to 100 mL, concentrations 0.5 kBq mL^-1 to 4 kBq mL^-1). Medium-energy and high-energy collimators, 60 projections with 55 s per projection and a 20% energy window at 82 keV are employed. For the Monte Carlo simulated images, Poisson-distributed noise is added in ten noise realizations. Reconstruction is performed (OS-EM, 40 iterations, 6 subsets) employing compensation for attenuation, scatter, and collimator-detector response. The estimated concentrations in the anthropomorphic phantom are also corrected using recovery coefficients. Errors for the largest sphere in the physical phantom range from -25% to -34% for the medium-energy collimator and larger deviations for smaller spheres. Corresponding results for the high-energy collimator are -15% to -31%. The corresponding Monte Carlo simulations show standard deviations of a few percentage points. For the anthropomorphic phantom, before application of recovery coefficients the bias ranges from -16% to -46% (medium-energy collimator) and -10% to -28% (high-energy collimator), with standard deviations of 2% to 14% and 1% to 16%. After the application of recovery coefficients, the biases range from -3% to -35% (medium energy collimator) and from 0% to -18%. The errors decrease with increasing concentrations. Activity-concentration estimation of ²²³Ra with SPECT is feasible, but problems with repeatability need to be further studied.

Advances in Precision Oncology: Targeted Thorium-227 Conjugates As a New Modality in Targeted Alpha Therapy

Targeted α therapy (TAT) offers the potential for the targeted delivery of potent α-particle-emitting radionuclides that emit high linear energy transfer radiation. This leads to a densely ionizing radiation track over a short path. Localized radiation induces cytotoxic, difficult-to-repair, clustered DNA double-strand breaks (DSBs). To date, radium-223 (²²³Ra) is the only TAT approved for the treatment of patients with metastatic castration-resistant prostate cancer. Thorium-227 (²²⁷Th), the progenitor nuclide of ²²³Ra, offers promise as a wider-ranging alternative due to the availability of efficient chelators, such as octadentate 3,2-hydroxypyridinone (3,2-HOPO). The 3,2-HOPO chelator can be readily conjugated to a range of targeting moieties, enabling the generation of new targeted thorium-227 conjugates (TTCs). This review provides a comprehensive overview of the advances in the preclinical development of TTCs for hematological cancers, including CD22-positive B cell cancers and CD33-positive leukemia, as well as for solid tumors overexpressing renal cell cancer antigen CD70, membrane-anchored glycoprotein mesothelin in mesothelioma, prostate-specific membrane antigen in prostate cancer, and fibroblast growth factor receptor 2. As the mechanism of action for TTCs is linked to the formation of DSBs, the authors also report data supporting combinations of TTCs with inhibitors of the DNA damage response pathways, including those of the ataxia telangiectasia and Rad3-related protein, and poly-ADP ribose polymerase. Finally, emerging evidence suggests that TTCs induce immunogenic cell death through the release of danger-associated molecular patterns. Based on encouraging preclinical data, clinical studies have been initiated to investigate the safety and tolerability of TTCs in patients with various cancers.

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.

Radium-223 as an Approved Modality for Treatment of Bone Metastases

Radium-223 dichloride (²²³Ra) is an α-emitter radionuclide approved for treatment of osteoblastic metastases in castrate-resistant prostate cancer (mCRPC) patients. ²²³Ra increases overall survival, improves bone pain, increases the median time to the first skeletal-related event, reduces the use of external beam radiation therapy for bone pain palliation, reduces the rates of spinal cord compression, and hospitalization. ²²³Ra therapy has minimal side effects; the most common hematological side effects are anemia, thrombocytopenia and neutropenia while the nonhematological side effects that may occur are bone pain flare, nausea, fatigue, and diarrhea. Alongside ²²³Ra therapy there are currently a variety of first-line therapeutic options available to treat mCRPC patients and much debate regarding the appropriate treatment algorithm for these patients and the possible combination of therapies among the ones available. In this article, we review the rationale behind ²²³Ra therapy as well as ²²³Ra mechanisms of action, biodistribution and dosimetry, optimal timing possibilities to initiate ²²³Ra in contrast to other treatments available, the association of ²²³Ra with other therapies and the means of evaluating patients in order to properly deliver to ²²³Ra therapy. Furthermore, we will discuss ²²³Ra dose administration possibilities, patient and dose preparation and the challenges of treatment response evaluation during and after ²²³Ra.

Dosimetry, Radiobiology and Synthetic Lethality: Radiopharmaceutical Therapy (RPT) With Alpha-Particle-Emitters

As a treatment modality that is fundamentally different from other therapies against cancer, radiopharmaceutical therapy with alpha-particle emitters has drawn the attention of the therapy community and also the biopharmaceutical industry. Alpha-particles cause a preponderance of complex DNA double-strand breaks (DSBs). This provides an opportunity to either enhance cell kill by using DNA DSB repair inhibitors or identify patients who are likely to be high responders to alpha-emitter RPT. The short-range and high potency of alpha-particles requires special dosimetry considerations. These are reviewed in light of recent updates to the phantoms and associated dosimetric quantities used for dosimetry calculations. A formalism for obtaining the necessary microscale pharmacokinetic information from patient nuclear medicine imaging is presented. Alpha-emitter based radiopharmaceutical therapy is an exciting cancer therapy modality that is being revisited. Further development of imaging and dosimetric methods specific to alpha-particle emitters, coupled with standardization of the methods and rigorous evidence that dosimetry applied to alphaRPT improves patient care are needed moving forward.

225Ac-PSMA-617 for Therapy of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-targeting radio-ligand therapy with beta-emitting ¹⁷⁷Lutetium has already been investigated in several early phase dosimetry studies, demonstrated promising results in phase-2, and recently the first phase-3 trial finished recruitment. In contrast, PSMA-targeting alpha-particle therapy (TAT) has only been evaluated in few preclinical experiments, preliminary dosimetry attempts and some retrospective observational studies, yet. First clinical experience with ²²⁵Ac-PSMA-617 demonstrates promising antitumor activity with a 63%-70% PSA_>50%-response rate, 10-15 months duration of response and complete remissions in approximately ten percent of patients, some of them with enduring relapse-free survival. Nevertheless, without comparative trials there is no prove whether, applied in identical clinical situations, ²²⁵Ac-PSMA-617 is really more efficiently than ¹⁷⁷Lu-PSMA-617 or vice versa. However, there is some good rationale, that PSMA-TAT might have advantages in particular clinical indications. This includes patients with diffuse type red-marrow infiltration by reducing off-target radiation to surrounding cells; ablation of micrometastases after favorable response to other previous therapy or someday in early stage disease. Also treatment escalation of patients, either with poor response to ¹⁷⁷Lu-PSMA or harboring adverse prognostic biomarkers, appears promising. In preclinical research, alpha-radiation demonstrated stronger induction of abscopal effects than beta-radiation; favoring its usage as a combination partner with immunotherapies. So, further evaluation of PSMA-TAT is definitely warranted. Recently, de-escalated treatment protocols and application of ²²⁵Ac/¹⁷⁷Lu-PSMA "cocktail"-regimens improved the tolerability of ²²⁵Ac-PSMA-617 TAT, reducing the risk for development dry-mouth syndrome. This opens new avenues for future application in earlier stage disease.

Clinical Outcome Assessments Toolbox for Radiopharmaceuticals

For nearly 40 years, the U.S. National Cancer Institute (NCI) has funded health-related quality-of-life (HRQOL) and symptom management in oncology clinical trials as a method for including a cancer patient's experience during and after treatment. The NCI's planned scope for HRQOL, symptom and patient-reported outcomes management research is explained as it pertains to radiopharmaceutical clinical development. An effort already underway to support protocol authoring via an NCI Cancer Therapy Evaluation Program (CTEP) Centralized Protocol Writing Service (CPWS) is described as this service aids incorporation of HRQOL, symptom and patient-reported outcomes management research into sponsored protocols.

Dose-Dependent Growth Delay of Breast Cancer Xenografts in the Bone Marrow of Mice Treated with 223Ra: The Role of Bystander Effects and Their Potential for Therapy

The role of radiation-induced bystander effects in radiation therapy remains unclear. With renewed interest in therapy with α-particle emitters, and their potential for sterilizing disseminated tumor cells (DTCs), it is critical to determine the contribution of bystander effects to the overall response so they can be leveraged for maximum clinical benefit. Methods: Female Foxn1_nu athymic nude mice were administered 0, 50, or 600 kBq/kg ²²³RaCl₂ to create bystander conditions. At 24 hours after administration, MDA-MB-231 or MCF-7 human breast cancer cells expressing luciferase were injected into the tibial marrow compartment. Tumor burden was tracked weekly via bioluminescence. Results: The MDA-MB-231 xenografts were observed to have a 10-day growth delay in the 600 kBq/kg treatment group only. In contrast, MCF-7 cells had 7- and 65-day growth delays in the 50 and 600 kBq/kg groups, respectively. Histologic imaging of the tibial marrow compartment, α-camera imaging, and Monte Carlo dosimetry modeling revealed DTCs both within and beyond the range of the α-particles emitted from ²²³Ra in bone for both MCF-7 and MDA-MB-231 cells. Conclusion: Taken together, these results support the participation of ²²³Ra-induced antiproliferative/cytotoxic bystander effects in delayed growth of DTC xenografts. They indicate that the delay depends on the injected activity and therefore is dose-dependent. They suggest using ²²³RaCl₂ as an adjuvant treatment for select patients at early stages of breast cancer.

The changing role of radium-223 in metastatic castrate-resistant prostate cancer: has the EMA missed the mark with revising the label?

Radium-223 (223Ra) is a life-prolonging treatment in symptomatic men with metastatic castrate-resistant prostate cancer (mCRPC) and bone metastases, but no visceral disease, regardless of prior treatment with docetaxel. Together with four other drugs (i.e. abiraterone, cabazitaxel, docetaxel, enzalutamide), it has been available for clinical use since 2013 and has been shown to also provide benefits in quality-of-life and societal benefits. However, in 2018 the European Medicines Agency ruled to restrict the use of radium-223 to a more advanced disease setting after at least two lines of one or the other life-prolonging agent. This decision was triggered by the results of a safety interim analysis of ERA-223, a trial investigating the combination of 223Ra and abiraterone versus abiraterone alone in patients without prior chemotherapy (with the exception of adjuvant treatment) with asymptomatic bone predominant mCRPC. That safety analysis showed an early increased risk of fracture and deaths with the combination treatment. This review critically appraises the available and emerging data with 223Ra treatment in an attempt to assess the appropriateness of the revised label of radium-223.

Automated Definition of Skeletal Disease Burden in Metastatic Prostate Carcinoma: A 3D Analysis of SPECT/CT Images

To meet the current need for skeletal tumor-load estimation in castration-resistant prostate cancer (CRPC), we developed a novel approach based on adaptive bone segmentation. In this study, we compared the program output with existing estimates and with the radiological outcome. Seventy-six whole-body single-photon emission computed tomographies/x-ray computed tomography with 3,3-diphosphono-1,2-propanedicarboxylic acid from mCRPC patients were analyzed. The software identified the whole skeletal volume (S_Vol) and classified the voxels metastases (M_Vol) or normal bone (B_Vol). S_Vol was compared with the estimation of a commercial software. M_Vol was compared with manual assessment and with prostate specific antigen (PSA) levels. Counts/voxel were extracted from M_Vol and B_Vol. After six cycles of ²²³RaCl2-therapy every patient was re-evaluated as having progressive disease (PD), stable disease (SD), or a partial response (PR). S_Vol correlated with that of the commercial software (R = 0.99, p < 0.001). M_Vol correlated with the manually-counted lesions (R = 0.61, p < 0.001) and PSA (R = 0.46, p < 0.01). PD had a lower counts/voxel in M_Vol than PR/SD (715 ± 190 vs. 975 ± 215 and 1058 ± 255, p < 0.05 and p < 0.01) and B_Vol (PD 275 ± 60, PR 515 ± 188 and SD 528 ± 162 counts/voxel, p < 0.001). Segmentation-based tumor load correlated with radiological/laboratory indices. Uptake was linked with the clinical outcome, suggesting that metastases in PD patients have a lower affinity for bone-seeking radionuclides and might benefit less from bone-targeted radioisotope therapies.

Retrospective Toxicological Profiling of Radium-223 Dichloride for the Treatment of Bone Metastases in Prostate Cancer Using Adverse Event Data

Background and Objective: Radium-223 dichloride (Xofigo^®) is a calcium mimetic agent approved for the treatment of castration-resistant prostate cancer patients with symptomatic bone metastases and no known visceral metastatic disease. This targeted, α-particle-emitting therapy has demonstrated significant survival benefit accompanied by a favorable safety profile. Nevertheless, recent evidence suggests that its combined use with abiraterone and prednisone/prednisolone may be associated with increased risk of death and fractures. While the precise pathophysiologic mechanisms of these events are not yet clear, collecting evidence from more clinical trials and translational studies is necessary. The aim of our present study is to assess whether accessible sources of patient outcome data can help gain additional clinical insights to radium-223 dichloride’s safety profile. Materials and Methods: We performed a retrospective analysis of cases extracted from the FDA Adverse Event Reporting System and characterized side effect occurrence by using reporting ratios. Results: A total of ~1500 prostate cancer patients treated with radium-223 dichloride was identified, and side effects reported with the use of radium-223 dichloride alone or in combination with other therapeutic agents were extracted. Our analysis demonstrates that radium-223 dichloride may often come with hematological-related reactions, and that, when administered together with other drugs, its safety profile may differ. Conclusions: While more prospective studies are needed to fully characterize the toxicological profile of radium-223 dichloride, the present work constitutes perhaps the first effort to examine its safety when administered alone and in combination with other agents based on computational evidence from public real-world post marketing data.

Radium-223 for the treatment of bone metastases in castration-resistant prostate cancer: when and why

Radium-223 dichloride (²²³Ra) is the first, recently approved, α-particle-emitting radiopharmaceutical for the treatment of patients with bone metastases in castration-resistant prostate cancer (CRPC) and no evidence of visceral metastases. We explored MEDLINE, relevant congresses, and websites for data on ²²³Ra and prostate cancer therapies, focusing on therapeutic strategies and timing, bone metastases, and diagnostic assessment. ²²³Ra represents the only bone-targeting agent that has significantly extended patients' overall survival while reducing pain and symptomatic skeletal events. Unlike other radiopharmaceuticals, such as strontium-89 and samarium-153 EDTMP, ²²³Ra (11.4-days half-life) has shown a high biological efficiency mainly due to its short penetration range. These features potentially allow reduced bone marrow toxicity and limit undue exposure. ²²³Ra has been validated under the product name Xofigo^® by the US Food and Drug Administration and the European Medicines Agency. Patient selection, management, and treatment sequencing is recommended to be discussed in the context of a multidisciplinary environment, including oncology, urology, nuclear medicine, and radiation therapy physicians. No consensus has been achieved regarding the optimal timing and its administration as single agent or in combination with zoledronic acid or chemotherapy, so far. This review aims to provide a rationale for the use of ²²³Ra in treating metastases from CRPC, highlighting the crucial role of a multidisciplinary approach, the disputed inclusion and exclusion criteria on the basis of agencies regulations, and the value of diagnostics for therapy assessment.

223Ra-dichloride therapy of bone metastasis: optimization of SPECT images for quantification

BACKGROUND

223Ra imaging is crucial to evaluate the successfulness of the therapy of bone metastasis of castration-resistant prostate cancer (CRPC). The goals of this study were to establish a quantitative tomographic 223Ra imaging protocol with clinically achievable conditions, as well as to investigate its usefulness and limitations. We performed several experiments using the Infinia Hawkeye 4 gamma camera (GE) and physical phantoms in order to assess the optimal image acquisition and reconstruction parameters, such as the windows setting, as well as the iteration number and filter of the reconstruction algorithm. Then, based on the MIRD pamphlet 23, we used a NEMA phantom and an anthropomorphic TORSO® phantom to calibrate the gamma camera and investigate the accuracy of quantification.

RESULTS

Experiences showed that the 85 keV ± 20%, 154 keV ± 10%, and 270 keV ± 10% energy windows are the most suitable for 223Ra imaging. The study with the NEMA phantom showed that the OSEM algorithm with 2 iterations, 10 subsets, and the Butterworth filter offered the best compromise between contrast and noise. Moreover, the calibration factors for different sphere sizes (26.5 ml, 11.5 ml, and 5.6 ml) were constant for 223Ra concentrations ranging between 6.5 and 22.8 kBq/ml. The values found are 73.7 cts/s/MBq, 43.8 cts/s/MBq, and 43.4 cts/s/MBq for 26.5 ml, 11.5 ml, and 5.6 ml sphere, respectively. For concentration lower than 6.5 kBq/ml, the calibration factors exhibited greater variability pointing out the limitations of SPECT/CT imaging for quantification. By the use of a TORSO® phantom, we simulated several tumors to normal tissue ratios as close as possible to clinical conditions. Using the calibration factors obtained with the NEMA phantom, for 223Ra concentrations higher than 8 kBq/ml, we were able to quantify the activity with an error inferior to 18.8% in a 5.6 ml lesion.

CONCLUSIONS

Absolute quantitative 223Ra SPECT imaging appears feasible once the dimension of the target is determined. Further evaluation should be needed to apply the calibration factor-based quantitation to clinical 223Ra SPECT/CT imaging. This will open the possibility for patient-specific 223Ra treatment planning and therapeutic outcome prediction in patients.