Clinical radioimmunotherapy--the role of radiobiology

MHC-I-Driven Antitumor Immunity Counterbalances Low Absorbed Doses of Radiopharmaceutical Therapy

Preclinical and clinical studies increasingly show that the immune response plays a major role in radiotherapy. Here, we investigated the role of major histocompatibility complex class I (MHC-I) molecules recognized by cytotoxic CD8+ T cells in the response to radiopharmaceutical therapy (RPT). Methods: Two murine melanoma cell lines that express low and high MHC-I levels (B16F10 and B16K1, respectively) were grafted in syngeneic or athymic and nude mice, and the response to a single injection of [225Ac]Ac-DOTA-TA99 monoclonal antibodies (9.25 or 18.5 kBq) was assessed and related to dosimetry. For clinical relevance, MHC-I expression was determined in samples from patients with well-differentiated, iodine-avid metastatic thyroid cancer and well-differentiated grade 2 mid-gut neuroendocrine tumors. Results: RPT efficacy was enhanced by T-cell presence and MHC-I expression. In mice harboring B16F10 and B16K1 melanoma tumors, RPT showed a stronger antitumor effect in C57BL/6J (immunocompetent) animals than in athymic and nude (immunodeficient) animals, suggesting a crucial role of T-cell-mediated immune responses. Moreover, the response to irradiation was the same in B16K1 MHC-Ihigh tumors with a low absorbed dose of α-RPT and in B16F10 MHC-Ilow tumors with a 4 times higher absorbed dose. These results indicate that CD8+ T cells can counterbalance low tumor irradiation. Conversely, delivering high absorbed doses leads to side effects and seems to prevent immune system activation, thereby not taking advantage of these mechanisms. Our results also indicate that MHC-I can be used as a predictive biomarker of RPT response in lesions receiving low absorbed doses and that RPT treatment regimens should be reconsidered in the function of the MHC-I expression level. Conclusion: This study shows that MHC-I expression can predict RPT immunostimulatory effects. This is relevant in metastatic disease where lesions in the same patient can receive very low or very high absorbed doses.

Assessing the Clinical Effectiveness of Radioimmunotherapy with Combined Radionuclide/Monoclonal Antibody Conjugates in Cancer Treatment: Insights from Randomised Clinical Trials

Background: Despite the development of advanced cancer therapies, achieving cancer eradication remains challenging. Radioimmunotherapy (RIT) is an innovative approach that combines radionuclides with monoclonal antibodies targeting tumour-associated antigens or those expressed by the tumour microenvironment. Over the past two decades, RIT has been extensively researched, along with two RIT products-90Y-ibritumomab tiuxetan and 131I-tositumomab. However, despite its demonstrated efficacy in non-solid tumours, RIT's clinical use remains limited, and its effectiveness in solid tumours is inconclusive. This study aimed to analyse randomised controlled trials (RCTs) to evaluate the overall clinical effectiveness of RIT across different cancer types and its impact on treatment outcomes. Methods: A systematic search of PubMed, EMBASE, Scopus, CENTRAL, and Google Scholar was conducted from January 2000 to October 2024 in accordance with PRISMA guidelines and the PICOS framework. Studies were included if they were RCTs evaluating RIT for cancer treatment and reported treatment outcomes such as overall survival (OS), progression-free survival (PFS), disease-free survival, or time to progression (TTP). Data extraction was performed using a standardised Excel form, and study quality was assessed with the Joanna Briggs Institute Critical Appraisal Tool for RCTs. A narrative synthesis of the data was complemented by meta-analyses where feasible, particularly for progression- and survival-related endpoints. Results: Out of 2241 records identified, 20 RCTs encompassing approximately 3562 patients were included. The majority of trials focused on non-solid tumours, particularly non-Hodgkin's lymphoma (NHL), while a smaller subset evaluated solid tumours such as lung, pancreatic, ovarian, and prostate cancers. Most non-solid tumour studies employed 90Y-ibritumomab tiuxetan or 131I-tositumomab, targeting the CD20 antigen, whereas limited evidence exists for RIT efficacy in solid tumours. Meta-analysis of progression-related outcomes yielded a pooled hazard ratio (HR) of 0.48 (95% CI: 0.39-0.59), indicating a 52% reduction in the risk of progression. In contrast, overall survival outcomes were more variable, with a pooled OS HR of 0.80 (95% CI: 0.60-1.07). Adverse events, predominantly haematological and nonhaematological toxicities, were common yet generally reversible. The findings suggest that RIT, especially when used as part of combination regimens, significantly improves treatment outcomes in non-solid tumours but has an inconsistent effect in solid tumour settings. Conclusions: The results underscore the clinical promise of RIT in treating non-solid tumours like NHL, where combination regimens yield superior outcomes compared to monotherapy. However, the inconclusive evidence in solid tumours highlights the need for further large-scale, well-designed RCTs to define the optimal use, dosing, and patient selection for RIT in these settings. Additionally, standardisation in outcome reporting and longer follow-up periods are essential for more accurate economic and clinical assessments. Overall, RIT represents a valuable therapeutic modality, yet its integration into cancer treatment regimens should be guided by further research aimed at mitigating toxicity and optimising combination strategies.

Clinical application of targeted α-emitter therapy in gastroenteropancreatic neuroendocrine neoplasms

Effective therapeutic strategies for advanced gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs) remain challenging, including a lack of response to therapy and post-treatment relapse. The rapid development of targeted radionuclide therapy (TRT) offers promising data for patients with somatostatin receptor (SSTR)-expressing tumors. This approach exhibits more advantages than somatostatin analog (SSA) therapy, which is primarily effective for well-differentiated and slow-growing GEP-NENs. Fortunately, some clinical studies on peptide receptor radionuclide therapy (PRRT) labeled with α-emitting radionuclides for GEP-NENs patients showed effective results for those with more advanced GEP-NENs, or those with malignant metastasis. For the improvement of clinical efficacy and the decline in the incidence of treatment-related relapse, recent progress in developing novel techniques and effective disease management strategies for optimal targeting has led to the emergence of targeted alpha therapy (TAT) in GEP-NENs patients. For instance, labeled technology and combination therapy could contribute to significantly improved long-term outcomes. However, the exact dosimetry for precision oncology, the shortage of radionuclides, and the stability of disease control are still under careful consideration. More high-quality, large-scale prospective studies are essential for obtaining valuable evidence on challenging problems and for further exploration.

225Ac/89Zr-Labeled N4MU01 Radioimmunoconjugates as Theranostics Against Nectin-4-Positive Triple-Negative Breast Cancer

Nectin-4 is an overexpressed biomarker in 60%-70% of triple-negative breast cancer (TNBC) cases and an ideal target for radiotherapy and PET imaging. In this study, theranostic radioimmunoconjugates were developed using a fully human anti-nectin-4 antibody (N4MU01). The imaging properties and therapeutic effectiveness of the radioimmunoconjugates were evaluated using TNBC models. Methods: N4MU01 was radiolabeled with 89Zr and 225Ac for imaging and radiotherapy, respectively, using TNBC xenograft and syngeneic models. Biodistribution and PET imaging of the [89Zr]Zr-deferoxamine (DFO)-N4MU01 radioimmunoconjugate was studied in mice bearing nectin-4-positive xenografts. Dosimetry and toxicity of [225Ac]Ac-Macropa-N4MU01 were studied in naïve BALB/c mice, and the therapeutic efficacy was evaluated with two doses of 13 or two doses of 18.6 kBq, administered 10 d apart in athymic BALB/c nude mice bearing either a human TNBC MDA-MB-468 xenograft or a human nectin-4-transfected 4T1 (4T1.nectin-4) syngeneic allograft. Results: The pharmacokinetic profile of the [89Zr]Zr-DFO-N4MU01 radioimmunoconjugate showed biphasic distribution with a moderate elimination half-life of 63 h. PET imaging and biodistribution of [89Zr]Zr-DFO-N4MU01 in mice bearing the MDA-MB-468 xenograft showed high tumor uptake of 13.2 ± 1.12 percent injected activity per gram at 120 h. [225Ac]Ac-Macropa-N4MU01 was effectively internalized in MDA-MB-468 and was cytotoxic to the cells with a 50% inhibition concentration of 1.2 kBq/mL. Toxicity studies revealed that 15 kBq of [225Ac]Ac-Macropa-N4MU01 was generally well tolerated, as indicated by hematologic, blood chemistry, and histopathologic analysis. Mice bearing MDA-MB-468 and 4T1.nectin-4 xenografts treated with 13 kBq of [225Ac]Ac-Macropa-N4MU01 had 100% (6/6) and 83.3% (5/6) complete tumor remissions, respectively. Conclusion: The specific tumor uptake and remarkable effectiveness against aggressive TNBC tumors are very promising and warrant the clinical development of N4MU01 radioimmunoconjugates.

Alpha Atlas: Mapping global production of α-emitting radionuclides for targeted alpha therapy

Targeted Alpha Therapy has shown great promise in cancer treatment, sparking significant interest over recent decades. However, its broad adoption has been impeded by the scarcity of alpha-emitters and the complexities related to their use. The availability of these radionuclides is often constrained by the intricate production processes and purification, as well as regulatory and logistical challenges. Moreover, the high cost and technical difficulties associated with handling and applying alpha-emitting radionuclides pose additional barriers to their clinical implementation. This Alpha Atlas provides an in-depth overview of the leading alpha-particle emitting radionuclide candidates for clinical use, focusing on their production processes and supply chains. By mapping the current facilities that produce and supply these radionuclides, this atlas aims to assist researchers, clinicians, and industries in initiating or scaling up the applications of alpha-emitters. The Alpha Atlas aspires to act as a strategic guide, facilitating collaboration and driving forward the integration of these potent therapeutic agents into cancer treatment practices.

PLK4 inhibition as a strategy to enhance non-small cell lung cancer radiosensitivity

Lung cancer is the leading cause of cancer-related mortality worldwide. Non-small cell lung cancer (NSCLC) is the most common subtype of lung cancer and comprises 85% of cases. Despite treatment advances, local control after curative-intent chemoradiation for NSCLC remains suboptimal. Polo-like kinase 4 (PLK4) is a serine-threonine kinase that plays a critical role in the regulation of centrosome duplication and cell cycle progression and is overexpressed in NSCLC, thus, making it a potential therapeutic target. CFI-400945 is an orally available PLK4 inhibitor currently undergoing clinical trial evaluation. As radiation causes cell death primarily by mitotic catastrophe, a process enhanced by alterations in centrosome amplification, we hypothesized that disruption of the mitotic machinery by inhibition of PLK4 would enhance the effects of radiation in NSCLC. PLK4 inhibition by CFI-400945 resulted in radiosensitization of NSCLC cell lines. In contrast, CFI-400945 had no effect on the radiosensitivity of normal lung fibroblasts. PLK4 inhibition did not affect cell-cycle phase distribution prior to radiation, but rather the combination of CFI-400945 and radiation resulted in increased G2/M cell cycle arrest, increased centrosome amplification, and a concomitant increase in cell death through mitotic catastrophe. Lastly, CFI-400945 treatment enhanced the radiation-induced tumor growth delay of NSCLC tumor xenografts. These data indicate that targeting PLK4 is a novel approach to enhance the radiation sensitivity of NSCLC in vitro and in vivo through potentiation of centrosome amplification and cell death through mitotic catastrophe.

Deep learning-based organ-wise dosimetry of 64Cu-DOTA-rituximab through only one scanning

This study aimed to generate a delayed 64Cu-dotatate (DOTA)-rituximab positron emission tomography (PET) image from its early-scanned image by deep learning to mitigate the inconvenience and cost of estimating absorbed radiopharmaceutical doses. We acquired PET images from six patients with malignancies at 1, 24, and 48 h post-injection (p. i.) with 8 mCi 64Cu-DOTA-rituximab to fit a time-activity curve for dosimetry. We used a paired image-to-image translation (I2I) model based on a generative adversarial network to generate delayed images from early PET images. The image similarity function between the generated image and its ground truth was determined by comparing L1 and perceptual losses. We also applied organ-wise dosimetry to acquired and generated images using OLINDA/EXM. The quality of the generated images was good, even of tumors, when using the L1 loss function as an additional loss to the adversarial loss function. The organ-wise cumulative uptake and corresponding equivalent dose were estimated. Although the absorbed dose in some organs was accurately measured, predictions for organs associated with body clearance were relatively inaccurate. These results suggested that paired I2I can be used to alleviate burdensome dosimetry for radioimmunoconjugates.

Extracellular vesicles role in radio(nuclide)therapy

Conventional radiation therapy can restore the ability of cells to undergo immunogenic cell death. Recent preclinical studies suggest that targeted radionuclide therapy, which delivers radiation to tumors at a continuous low dose rate, also stimulates the immune system and offers a promising approach for overcoming resistance to immune checkpoint inhibitors. In this context, we examined the growing body of preclinical and clinical findings showing that the immune system can be activated by the release of extracellular vesicles from irradiated cells, contributing to the antitumor immunity.

Joint Regional Uptake Quantification of Thorium-227 and Radium-223 Using a Multiple-Energy-Window Projection-Domain Quantitative SPECT Method

Thorium-227 ( )-based -particle radiopharmaceutical therapies ( -RPTs) are currently being investigated in several clinical and pre-clinical studies. After administration, decays to , another -particle-emitting isotope, which redistributes within the patient. Reliable dose quantification of both and is clinically important, and SPECT may perform this quantification as these isotopes also emit X- and -ray photons. However, reliable quantification is challenging for several reasons: the orders-of-magnitude lower activity compared to conventional SPECT, resulting in a very low number of detected counts, the presence of multiple photopeaks, substantial overlap in the emission spectra of these isotopes, and the image-degrading effects in SPECT. To address these issues, we propose a multiple-energy-window projection-domain quantification (MEW-PDQ) method that jointly estimates the regional activity uptake of both and directly using the SPECT projection data from multiple energy windows. We evaluated the method with realistic simulation studies conducted with anthropomorphic digital phantoms, including a virtual imaging trial, in the context of imaging patients with bone metastases of prostate cancer who were treated with -based -RPTs. The proposed method yielded reliable (accurate and precise) regional uptake estimates of both isotopes and outperformed state-of-the-art methods across different lesion sizes and contrasts, as well as in the virtual imaging trial. This reliable performance was also observed with moderate levels of intra-regional heterogeneous uptake as well as when there were moderate inaccuracies in the definitions of the support of various regions. Additionally, we demonstrated the effectiveness of using multiple energy windows and the variance of the estimated uptake using the proposed method approached the Cramér-Rao-lower-bound-defined theoretical limit. These results provide strong evidence in support of this method for reliable uptake quantification in -based -RPTs.

Target engagement of an anti-MT1-MMP antibody for triple-negative breast cancer PET imaging and beta therapy

PURPOSE

Triple-negative breast cancer (TNBC) is a highly aggressive subtype of breast cancer that lacks effective diagnostic and therapeutic options. Membrane type 1 matrix metalloproteinase (MT1-MMP) is an attractive biomarker for improving patient selection. This study aimed to develop a theranostic tool using a highly tumour-selective anti-MT1-MMP antibody (LEM2/15) radiolabelled with 89Zr for PET and 177Lu for therapy in a TNBC murine model.

METHODS

The LEM2/15 antibody and IgG isotype control were radiolabelled with 89Zr. PET imaging was performed in a TNBC orthotopic mouse model at 1, 2, 4, and 7 days after administration. Tissue biodistribution and pharmacokinetic parameters were analysed and Patlak linearisation was used to calculate the influx rate of irreversible uptake. The TNBC mice were treated with [177Lu]Lu-DOTA-LEM2/15 (single- or 3-dose regimen) or saline. Efficacy of [177Lu]Lu-DOTA-LEM2/15 was evaluated as tumour growth and DNA damage (γH2AX) in MDA 231-BrM2-831 tumours.

RESULTS

At 7 days post-injection, PET uptake in tumour xenografts revealed a 1.6-fold and 2.4-fold higher tumour-to-blood ratio for [89Zr]Zr-Df-LEM2/15 in the non-blocked group compared to the blocked and IgG isotype control groups, respectively. Specific uptake of LEM2/15 in TBNC tumours mediated by MT1-MMP-binding was demonstrated by the Patlak linearisation method, providing insights into the potential efficacy of LEM2/15-based treatments. A similar uptake was found for [89Zr]Zr-Df-LEM2/15 and [177Lu]Lu-DOTA-LEM2/15 in tumours 7 days post-injection (6.80 ± 1.31 vs. 5.61 ± 0.66 %ID/g). Tumour doubling time was longer in the [177Lu]Lu-DOTA-LEM2/15 3-dose regimen treated group compared to the control (50 vs. 17 days, respectively). The percentage of cells with γH2AX-foci was higher in tumours treated with [177Lu]Lu-DOTA-LEM2/15 3-dose regimen compared to tumours non-treated or treated with [177Lu]Lu-DOTA-LEM2/15 single-dose (12 % vs. 4-5 %).

CONCLUSIONS

The results showed that the 89Zr/177Lu-labelled anti-MT1-MMP mAb (LEM2/15) pair facilitated immune-PET imaging and reduced tumour growth in a preclinical TNBC xenograft model.

Targeted radionuclide therapy for head and neck squamous cell carcinoma: a review

Head and neck squamous cell carcinoma (HNSCC) is a type of head and neck cancer that is aggressive, difficult to treat, and often associated with poor prognosis. HNSCC is the sixth most common cancer worldwide, highlighting the need to develop novel treatments for this disease. The current standard of care for HNSCC usually involves a combination of surgical resection, radiation therapy, and chemotherapy. Chemotherapy is notorious for its detrimental side effects including nausea, fatigue, hair loss, and more. Radiation therapy can be a challenge due to the anatomy of the head and neck area and presence of normal tissues. In addition to the drawbacks of chemotherapy and radiation therapy, high morbidity and mortality rates for HNSCC highlight the urgent need for alternative treatment options. Immunotherapy has recently emerged as a possible treatment option for cancers including HNSCC, in which monoclonal antibodies are used to help the immune system fight disease. Combining monoclonal antibodies approved by the US Food and Drug Administration, such as cetuximab and pembrolizumab, with radiotherapy or platinum-based chemotherapy for patients with locally advanced, recurrent, or metastatic HNSCC is an accepted first-line therapy. Targeted radionuclide therapy can potentially be used in conjunction with the first-line therapy, or as an additional treatment option, to improve patient outcomes and quality of life. Epidermal growth factor receptor is a known molecular target for HNSCC; however, other targets such as human epidermal growth factor receptor 2, human epidermal growth factor receptor 3, programmed cell death protein 1, and programmed death-ligand 1 are emerging molecular targets for the diagnosis and treatment of HNSCC. To develop successful radiopharmaceuticals, it is imperative to first understand the molecular biology of the disease of interest. For cancer, this understanding often means detection and characterization of molecular targets, such as cell surface receptors, that can be used as sensitive targeting agents. The goal of this review article is to explore molecular targets for HNSCC and dissect previously conducted research in nuclear medicine and provide a possible path forward for the development of novel radiopharmaceuticals used in targeted radionuclide therapy for HNSCC, which has been underexplored to date.

Targeted Radionuclide Therapy in Glioblastoma

Despite the development of various novel therapies, glioblastoma (GBM) remains a devastating disease, with a median survival of less than 15 months. Recently, targeted radionuclide therapy has shown significant progress in treating solid tumors, with the approval of Lutathera for neuroendocrine tumors and Pluvicto for prostate cancer by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA). This achievement has shed light on the potential of targeted radionuclide therapy for other solid tumors, including GBM. This review presents the current status of targeted radionuclide therapy in GBM, highlighting the commonly used therapeutic radionuclides emitting alpha, beta particles, and Auger electrons that could induce potent molecular and cellular damage to treat GBM. We then explore a range of targeting vectors, including small molecules, peptides, and antibodies, which selectively target antigen-expressing tumor cells with minimal or no binding to healthy tissues. Considering that radiopharmaceuticals for GBM are often administered locoregionally to bypass the blood-brain barrier (BBB), we review prominent delivery methods such as convection-enhanced delivery, local implantation, and stereotactic injections. Finally, we address the challenges of this therapeutic approach for GBM and propose potential solutions.

Initial insights into the interaction of antibodies radiolabeled with Lutetium-177 and Actinium-225 with tumor microenvironment in experimental human and canine osteosarcoma

BACKGROUND

Osteosarcoma (OS) is a prevalent primary bone cancer affecting both humans and canines. This study describes initial insights into the interaction of the human monoclonal antibody IF3 to an insulin-like growth factor 2 receptor (IGF2R) radiolabeled with either alpha-emitting Actinium-225 (225Ac) or beta-emitting Lutetium-177 (177Lu) radionuclides with the OS cells and tumor microenvironment (TME) in experimental human and canine OS.

BASIC PROCEDURES

SCID mice bearing canine Gracie or human OS-33 OS tumors were treated with 177Lu- or 225Ac-labeled IF3 antibody, sacrificed at 24, 72 or 168 h post-treatment and their tumors were analyzed by immunohistochemistry (IHC) for the presence of OS cells, various elements of TME as well as for the double DNA strand breaks with γH2AX and caspase 3 assays.

MAIN FINDINGS

IHC revealed a reduction in IGF2R-positive OS cells and OS stem cell populations post therapy with 225Ac- and 177Lu-labeled IF3 antibody. Notably, radiolabeled IF3 antibody effectively diminished pro-tumorigenic M2 macrophages, highlighting its therapeutic promise. The study also unveiled varied responses of natural killer (NK) cells and M1 macrophages, shedding light on the intricate TME interplay. Time-dependent increase in γ-H2AX staining in canine Gracie and human OS-33 tumors treated with [177Lu]Lu-IF3 and [225Ac]Ac-IF3 was observed at 24 and 72 h post-RIT.

PRINCIPAL CONCLUSIONS

These findings suggest that radiolabeled antibodies offer a hopeful avenue for personalized OS treatment, emphasizing the importance of understanding their impact on the TME and potential synergies with immunotherapy.

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

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

Immunological effects of radiopharmaceutical therapy

Radiation therapy (RT) is a pillar of cancer therapy used by more than half of all cancer patients. Clinically, RT is mostly delivered as external beam radiation therapy (EBRT). However, the scope of EBRT is limited in the metastatic setting, where all sites of disease need to be irradiated. Such a limitation is attributed to radiation-induced toxicities, for example on bone marrow and hematologic toxicities, resulting from a large EBRT field. Radiopharmaceutical therapy (RPT) has emerged as an alternative to EBRT for the irradiation of all sites of metastatic disease. While RPT can reduce tumor burden, it can also impact the immune system and anti-tumor immunity. Understanding these effects is crucial for predicting and managing treatment-related hematological toxicities and optimizing their integration with other therapeutic modalities, such as immunotherapies. Here, we review the immunomodulatory effects of α- and β-particle emitter-based RPT on various immune cell lines, such as CD8+ and CD4+ T cells, natural killer (NK) cells, and regulatory T (Treg) cells. We briefly discuss Auger electron-emitter (AEE)-based RPT, and finally, we highlight the combination of RPT with immune checkpoint inhibitors, which may offer potential therapeutic synergies for patients with metastatic cancers.

Ultrasmall Gold Nanoparticles Radiolabeled with Iodine-125 as Potential New Radiopharmaceutical

The relatively high linear energy transfer of Auger electrons, which can cause clustered DNA damage and hence efficient cell death, makes Auger emitters excellent candidates for attacking metastasized tumors. Moreover, gammas or positrons are usually emitted along with the Auger electrons, providing the possibility of theragnostic applications. Despite the promising properties of Auger electrons, only a few radiopharmaceuticals employing Auger emitters have been developed so far. This is most likely explained by the short ranges of these electrons, requiring the delivery of the Auger emitters to crucial cell parts such as the cell nucleus. In this work, we combined the Auger emitter 125I and ultrasmall gold nanoparticles to prepare a novel radiopharmaceutical. The 125I labeled gold nanoparticles were shown to accumulate at the cell nucleus, leading to a high tumor-killing efficiency in both 2D and 3D tumor cell models. The results from this work indicate that ultrasmall nanoparticles, which passively accumulate at the cell nucleus, have the potential to be applied in targeted radionuclide therapy. Even better tumor-killing efficiency can be expected if tumor-targeting moieties are conjugated to the nanoparticles.

Revolutionizing cancer care strategies: immunotherapy, gene therapy, and molecular targeted therapy

Despite the availability of technological advances in traditional anti-cancer therapies, there is a need for more precise and targeted cancer treatment strategies. The wide-ranging shortfalls of conventional anticancer therapies such as systematic toxicity, compromised life quality, and limited to severe side effects are major areas of concern of conventional cancer treatment approaches. Owing to the expansion of knowledge and technological advancements in the field of cancer biology, more innovative and safe anti-cancerous approaches such as immune therapy, gene therapy and targeted therapy are rapidly evolving with the aim to address the limitations of conventional therapies. The concept of immunotherapy began with the capability of coley toxins to stimulate toll-like receptors of immune cells to provoke an immune response against cancers. With an in-depth understating of the molecular mechanisms of carcinogenesis and their relationship to disease prognosis, molecular targeted therapy approaches, that inhibit or stimulate specific cancer-promoting or cancer-inhibitory molecules respectively, have offered promising outcomes. In this review, we evaluate the achievement and challenges of these technically advanced therapies with the aim of presenting the overall progress and perspective of each approach.

Targeted Radium Alpha Therapy in the Era of Nanomedicine: In Vivo Results

Targeted alpha-particle therapy using radionuclides with alpha emission is a rapidly developing area in modern cancer treatment. To selectively deliver alpha-emitting isotopes to tumors, targeting vectors, including monoclonal antibodies, peptides, small molecule inhibitors, or other biomolecules, are attached to them, which ensures specific binding to tumor-related antigens and cell surface receptors. Although earlier studies have already demonstrated the anti-tumor potential of alpha-emitting radium (Ra) isotopes-Radium-223 and Radium-224 (223/224Ra)-in the treatment of skeletal metastases, their inability to complex with target-specific moieties hindered application beyond bone targeting. To exploit the therapeutic gains of Ra across a wider spectrum of cancers, nanoparticles have recently been embraced as carriers to ensure the linkage of 223/224Ra to target-affine vectors. Exemplified by prior findings, Ra was successfully bound to several nano/microparticles, including lanthanum phosphate, nanozeolites, barium sulfate, hydroxyapatite, calcium carbonate, gypsum, celestine, or liposomes. Despite the lengthened tumor retention and the related improvement in the radiotherapeutic effect of 223/224Ra coupled to nanoparticles, the in vivo assessment of the radiolabeled nanoprobes is a prerequisite prior to clinical usage. For this purpose, experimental xenotransplant models of different cancers provide a well-suited scenario. Herein, we summarize the latest achievements with 223/224Ra-doped nanoparticles and related advances in targeted alpha radiotherapy.

Development and Characterization of Novel FAP-Targeted Theranostic Pairs: A Bench-to-Bedside Study

Fibroblast activation protein (FAP) is among the most popular targets in nuclear medicine imaging and cancer theranostics. Several small-molecule moieties (FAPI-04, FAPI-46, etc.) are used for developing FAP-targeted theranostic agents. Nonetheless, the circulation time of FAP inhibitors is relatively short, resulting in rapid clearance via kidneys, low tumor uptake, and associated unsatisfactory treatment efficacy. To address the existing drawbacks, we engineered 3 peptides named FD1, FD2, and FD3 with different circulation times through solid-phase peptide synthesis. All the 3 reported peptides bind to human and murine FAP with single-digit nanomolar affinity measured by surface plasmon resonance. The diagnostic and therapeutic potential of the agents labeled with 68Ga and 177Lu was assessed in several tumor models exhibiting different levels of FAP expression. While radiolabeled FD1 was rapidly excreted from kidneys, radiolabeled FD2/FD3 have significantly prolonged circulation, increased tumor uptake, and decreased kidney accumulation. Our findings indicated that [68Ga]Ga-DOTA-FD1 positron emission tomography (PET) effectively detected FAP dynamics, whereas [177Lu]Lu-DOTA-FD2 and [177Lu]Lu-DOTA-FD3 exhibited remarkable therapeutic efficacy in FAP-overexpressing tumor models, including pancreatic cancer cell models characterized by abundant stroma. Moreover, a pilot translational investigation demonstrated that [68Ga]Ga-DOTA-FD1 had the capability to identify both primary and metastatic tumors with precision and distinction. In summary, we developed [68Ga]Ga-DOTA-FD1 for same-day PET imaging of FAP dynamics and [177Lu]Lu-DOTA-FD2 and [177Lu]Lu-DOTA-FD3 for effective radioligand therapy of FAP-overexpressing tumors.

Radiopharmaceuticals as combinatorial partners for immune checkpoint inhibitors

Immune checkpoint inhibitors (ICIs) have revolutionized the treatment of multiple cancer types. However, only a fraction of patients with cancer responds to ICIs employed as stand-alone therapeutics, calling for the development of safe and effective combinatorial regimens to extend the benefits of ICIs to a larger patient population. In addition to exhibiting a good safety and efficacy profile, targeted radionuclide therapy (TRT) with radiopharmaceuticals that specifically accumulate in the tumor microenvironment has been associated with promising immunostimulatory effects that (at least in preclinical cancer models) provide a robust platform for the development of TRT/ICI combinations. We discuss preclinical and clinical findings suggesting that TRT stands out as a promising partner for the development of safe and efficient combinatorial regimens involving ICIs.

Coded aperture and Compton imaging for the development of 225 Ac-based radiopharmaceuticals

BACKGROUND

Targeted alpha-particle therapy (TAT) has great promise as a cancer treatment. Arguably the most promising TAT radionuclide that has been proposed is 225 Ac. The development of 225 Ac-based radiopharmaceuticals has been hampered due to the lack of effective means to study the daughter redistribution of these agents in small animals at the preclinical stage.

PURPOSE

The ability to directly image the daughters, namely 221 Fr and 213 Bi, via their gamma-ray emissions would be a boon for preclinical studies. That said, conventional medical imaging modalities, including single photon emission computed tomography (SPECT) based on nonmultiplexed collimation, cannot be employed due to sensitivity limitations.

METHODS

As an alternative, we propose the use of both coded aperture and Compton imaging with the former modality suited to the 218-keV gamma-ray emission of 221 Fr and the latter suited to the 440-keV gamma-ray emission of 213 Bi.

RESULTS

This work includes coded aperture images of 221 Fr and Compton images of 213 Bi in tumor-bearing mice injected with 225 Ac-based radiopharmaceuticals.

CONCLUSIONS

These results are the first demonstration of visualizing and quantifying the 225 Ac daughters in small animals through the application of coded aperture and Compton imaging.

Rationale for Prostate-Specific-Membrane-Antigen-Targeted Radionuclide Theranostic Applied to Metastatic Clear Cell Renal Carcinoma

Prostate-specific membrane antigen (PSMA), whose high expression has been demonstrated in metastatic aggressive prostate adenocarcinoma, is also highly expressed in the neovessels of various solid tumors, including clear cell renal cell carcinoma (ccRCC). In the VISION phase III clinical trial, PSMA-targeted radioligand therapy (PRLT) with lutetium 177 demonstrated a 4-month overall survival OS benefit compared to the best standard of care in heavily pretreated metastatic prostate cancer. Despite the improvement in the management of metastatic clear cell renal cell carcinoma (mccRCC) with antiangiogenic tyrosine kinase inhibitor (TKI) and immunotherapy, there is still a need for new treatments for patients who progress despite these drugs. In this study, we discuss the rationale of PRLT applied to the treavtment of mccRCC.

Updates on radiotherapy-immunotherapy combinations: Proceedings of 6th annual ImmunoRad conference

Focal radiation therapy (RT) has attracted considerable attention as a combinatorial partner for immunotherapy (IT), largely reflecting a well-defined, predictable safety profile and at least some potential for immunostimulation. However, only a few RT-IT combinations have been tested successfully in patients with cancer, highlighting the urgent need for an improved understanding of the interaction between RT and IT in both preclinical and clinical scenarios. Every year since 2016, ImmunoRad gathers experts working at the interface between RT and IT to provide a forum for education and discussion, with the ultimate goal of fostering progress in the field at both preclinical and clinical levels. Here, we summarize the key concepts and findings presented at the Sixth Annual ImmunoRad conference.

Radioimmunotherapy of Non-Hodgkin B-cell Lymphoma: An update

Systemic radioimmunotherapy (RIT) is arguably the most effective and least toxic anticancer treatment for non-Hodgkin lymphoma (NHL). In treatment-naïve patients with indolent NHL, the efficacy of a single injection of RIT compares with that of multiple cycles of combination chemotherapy. However, 20 years following the approval of the first CD20-targeting radioimmunoconjugates ⁹⁰Y-Ibritumomab-tiuxetan (Zevalin) and ¹³¹I-tositumomab (Bexxar), the number of patients referred for RIT in western countries has dramatically decreased. Notwithstanding this, the development of RIT has continued. Therapeutic targets other than CD20 have been identified, new vector molecules have been produced allowing for faster delivery of RIT to the target, and innovative radionuclides with favorable physical characteristics such as alpha emitters have been more widely available. In this article, we reviewed the current status of RIT in NHL, with particular focus on recent clinical and preclinical developments.

In Vivo evaluation of newly synthesized 213Bi-conjugated alpha-melanocyte stimulating hormone (α-MSH) peptide analogues in melanocortin-1 receptor (MC1-R) positive experimental melanoma model

Given the rising pervasiveness of melanocortin-1 receptor (MC1-R) positive melanoma malignum (MM) and pertinent metastases, radiolabelled receptor-affine alpha-melanocyte stimulating hormone-analogue (α-MSH analogue) imaging probes would be of crucial importance in timely tumor diagnostic assessment. Herein we aimed at investigating the biodistribution and the MM targeting potential of newly synthesized ²¹³Bi-conjugated MC1-R specific peptide-based radioligands with the establishment of MC1-R overexpressing MM preclinical model. DOTA-conjugated NAP, -HOLD, -FOLD, -and MARSamide were labelled with ²¹³Bi. Ex vivo biodistribution studies were conducted post-administration of 3.81 ± 0.32 MBq [²¹³Bi]Bi-DOTA conjugated deriva-tives into twenty B16-F10 tumor-bearing C57BL/6 J and healthy mice. Organ Level Internal Dose Assessment (OLINDA) and IDAC-Dose were used to calculate translational data-based absorbed radiation dose in human organs. Moderate or low %ID/g uptake of [²¹³Bi]Bi-DOTA conjugated NAP, -HOLD, -and MARSamide and significantly increased [²¹³Bi]Bi-DOTA-FOLDamide accumulation was observed in the thoracic and abdominal organs (p ≤ 0.01). High [²¹³Bi]Bi-DOTA-NAP (%ID/g:3.76 ± 0.96), -and FOLDamide (%ID/g:3.28 ± 0.95) tumor tracer activity confirmed their MC1-R-affinity. The bladder wall received the highest radiation absorbed dose followed by the kidneys (bladder wall: 1.95·10^-2 and 8.97·10^-2 mSv/MBq; kidneys: 7.47·10^-3 vs. 5.88·10^-2 mSv/MBq measured by IDAC and OLINDA; respectively) indicating the suitability of the NAPamide derivative for clinical use. These novel [²¹³Bi]Bi-DOTA-linked peptide probes displaying meaningful MC1-R affinity could be promising molecular probes in MM imaging.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

Immunomodulatory effects of targeted radionuclide therapy

It is now clear that conventional radiation therapy can reinstate cell death immunogenicity. Recent preclinical data indicate that targeted radionuclide therapy that irradiate tumors at continuous low dose rate also can elicit immunostimulatory effects and represents a promising strategy to circumvent immune checkpoint inhibitor resistance. In this perspective, we discuss the accumulating preclinical and clinical data suggesting that activation of the immune system through the cGAS-STING axis and the release of extracellular vesicles by irradiated cells, participate to this antitumor immunity. This should need to be considered for adapting clinical practices to state of the art of the radiobiology and to increase targeted radionuclide therapy effectiveness.

Selective Chelation of the Exotic Meitner-Auger Emitter Mercury-197 m/g with Sulfur-Rich Macrocyclic Ligands: Towards the Future of Theranostic Radiopharmaceuticals

Mercury-197 m/g are a promising pair of radioactive isomers for incorporation into a theranostic as they can be used as a diagnostic agent using SPECT imaging and a therapeutic via Meitner-Auger electron emissions. However, the current absence of ligands able to stably coordinate ^197m/g Hg to a tumour-targeting vector precludes their use in vivo. To address this, we report herein a series of sulfur-rich chelators capable of incorporating ^197m/g Hg into a radiopharmaceutical. 1,4,7,10-Tetrathia-13-azacyclopentadecane (NS₄ ) and its derivatives, (2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetic acid (NS₄ -CA) and N-benzyl-2-(1,4,7,10-tetrathia-13-azacyclopentadecan-13-yl)acetamide (NS₄ -BA), were designed, synthesized and analyzed for their ability to coordinate Hg²⁺ through a combination of theoretical (DFT) and experimental coordination chemistry studies (NMR and mass spectrometry) as well as ^197m/g Hg radiolabeling studies and in vitro stability assays. The development of stable ligands for ^197m/g Hg reported herein is extremely impactful as it would enable their use for in vivo imaging and therapy, leading to personalized treatments for cancer.

Determination of distribution coefficients of mercury and gold on selected extraction chromatographic resins - towards an improved separation method of mercury-197 from proton-irradiated gold targets

Radioisotope mercury-197g (^197gHg, half-life: 64.14 h) along with its metastable isomer (^197mHg, half-life: 23.8 h) are potential candidates for targeted Meitner-Auger electron therapy due to their suitable decay properties. Their production can be achieved via proton irradiation of a natural gold target, but the number of studies surrounding their separation from an irradiated gold target is limited. This study focuses on the determination of distribution coefficients (K_d) of gold (III) and mercury (II) on seven extraction chromatographic resins. Mercury K_d were measured by means of radiotracers and Inductively Coupled Plasma Mass Spectrometry (ICP_MS); values obtained from the two methods were generally in good agreement. These results can provide insight on Hg and Au chemistry and aid in the design of improved separation system(s).

NADPH Selective Depletion Nanomedicine-Mediated Radio-Immunometabolism Regulation for Strengthening Anti-PDL1 Therapy against TNBC

Anti-PD(L)1 immunotherapy recently arises as an effective treatment against triple-negative breast cancer (TNBC) but is only applicable to a small portion of TNBC patients due to the low PD-L1 expression and the immunosuppressive tumor microenvironment (TME). To address these challenges, a multifunctional "drug-like" copolymer that possesses the auto-changeable upper critical solution temperature and the capacity of scavenging reduced nicotinamide adenine dinucleotide phosphate (NADPH) inside tumor cells is synthesized and employed to develop a hypoxia-targeted and BMS202 (small molecule antagonist of PD-1/PD-L1 interactions)-loaded nanomedicine (BMS202@HZP NPs), combining the anti-PD-L1 therapy and the low-dose radiotherapy (LDRT) against TNBC. In addition to the controlled release of BMS202 in the hypoxic TNBC, BMS202@HZP NPs benefit the LDRT by upregulating the pentose phosphate pathway (PPP, the primary cellular source for NADPH) of TME whereas scavenging the NADPH inside tumor cells. As a result, the BMS202@HZP NPs-mediated LDRT upregulate the PD-L1 expression of tumor to promote anti-PD-L1 therapy response while reprogramming the immunometabolism of TME to alleviate its immunosuppression. This innovative nanomedicine-mediated radio-immunometabolism regulation provides a promising strategy to reinforce the anti-PD-L1 therapy against TNBC.

A Projection-Domain Low-Count Quantitative SPECT Method for α-Particle-Emitting Radiopharmaceutical Therapy

Single-photon emission-computed tomography (SPECT) provides a mechanism to estimate regional isotope uptake in lesions and at-risk organs after administration of α-particle-emitting radiopharmaceutical therapies (α-RPTs). However, this estimation task is challenging due to the complex emission spectra, the very low number of detected counts (~20 times lower than in conventional SPECT), the impact of stray-radiation-related noise at these low counts, and the multiple image-degrading processes in SPECT. The conventional reconstruction-based quantification methods are observed to be erroneous for α-RPT SPECT. To address these challenges, we developed a low-count quantitative SPECT (LC-QSPECT) method that directly estimates the regional activity uptake from the projection data (obviating the reconstruction step), compensates for stray-radiation-related noise, and accounts for the radioisotope and SPECT physics, including the isotope spectra, scatter, attenuation, and collimator-detector response, using a Monte Carlo-based approach. The method was validated in the context of 3-D SPECT with 223Ra, a commonly used radionuclide for α-RPT. Validation was performed using both realistic simulation studies, including a virtual clinical trial, and synthetic and 3-D-printed anthropomorphic physical-phantom studies. Across all studies, the LC-QSPECT method yielded reliable regional-uptake estimates and outperformed the conventional ordered subset expectation-maximization (OSEM)-based reconstruction and geometric transfer matrix (GTM)-based post-reconstruction partial-volume compensation methods. Furthermore, the method yielded reliable uptake across different lesion sizes, contrasts, and different levels of intralesion heterogeneity. Additionally, the variance of the estimated uptake approached the Cramér-Rao bound-defined theoretical limit. In conclusion, the proposed LC-QSPECT method demonstrated the ability to perform reliable quantification for α-RPT SPECT.

Development of radiopharmaceuticals for targeted alpha therapy: Where do we stand?

Targeted alpha therapy is an oncological treatment, where cytotoxic doses of alpha radiation are locally delivered to tumor cells, while the surrounding healthy tissue is minimally affected. This therapeutic strategy relies on radiopharmaceuticals made of medically relevant radionuclides chelated by ligands, and conjugated to targeting vectors, which promote the drug accumulation in tumor sites. This review discusses the state-of-the-art in the development of radiopharmaceuticals for targeted alpha therapy, breaking down their key structural components, such as radioisotope, targeting vector, and delivery formulation, and analyzing their pros and cons. Moreover, we discuss current drawbacks that are holding back targeted alpha therapy in the clinic, and identify ongoing strategies in field to overcome those issues, including radioisotope encapsulation in nanoformulations to prevent the release of the daughters. Lastly, we critically discuss potential opportunities the field holds, which may contribute to targeted alpha therapy becoming a gold standard treatment in oncology in the future.

Biologicals as theranostic vehicles in paediatric oncology

Biologicals, such as antibodies or antibody-fragments e.g. nanobodies, have changed the landscape of cancer therapy and can be used in combination with traditional cancer treatments. They have been demonstrated to be excellent vehicles for molecular imaging. Several biologicals for nuclear imaging of adult cancer may be used in combination with (nuclear) therapy. Though it's great potential, molecular imaging using biologicals is rarely applied in paediatric oncology. This paper describes the current status of biologicals as radiopharmaceuticals for childhood cancer. Furthermore, the importance and potential for developing additional biological theranostics as opportunity to image and treat childhood cancer is discussed.

Radiopharmaceutical Treatments for Cancer Therapy, Radionuclides Characteristics, Applications, and Challenges

Advances in the field of molecular biology have had an impact on biomedical applications, which provide greater hope for both imaging and therapeutics. Work has been intensified on the development of radionuclides and their application in radiopharmaceuticals (RP_S) which will certainly influence and expand therapeutic approaches in the future treatment of patients. Alpha or beta particles and Auger electrons are used for therapy purposes, and each has advantages and disadvantages. The radionuclides labeled drug delivery system will deliver the particles to the specific targeting cell. Different radioligands can be chosen to uniquely target molecular receptors or intracellular components, making them suitable for personal patient-tailored therapy in modern cancer therapy management. Advances in nanotechnology have enabled nanoparticle drug delivery systems that can allow for specific multivalent attachment of targeted molecules of antibodies, peptides, or ligands to the surface of nanoparticles for therapy and imaging purposes. This review presents fundamental radionuclide properties with particular reference to tumor biology and receptor characteristic of radiopharmaceutical targeted therapy development.

Application of Cleavable Linkers to Improve Therapeutic Index of Radioligand Therapies

Radioligand therapy (RLT) is an emergent drug class for cancer treatment. The dose administered to cancer patients is constrained by the radiation exposure to normal tissues to maintain an appropriate therapeutic index. When a radiopharmaceutical or its radiometabolite is retained in the kidneys, radiation dose deposition in the kidneys can become a dose-limiting factor. A good exemplar is [¹⁷⁷Lu]Lu-DOTATATE, where patients receive a co-infusion of basic amino acids for nephroprotection. Besides peptides, there are other classes of targeting vectors like antibody fragments, antibody mimetics, peptidomimetics, and small molecules that clear through the renal pathway. In this review, we will review established and emerging strategies that can be used to mitigate radiation-induced nephrotoxicity, with a focus on the development and incorporation of cleavable linkers for radiopharmaceutical designs. Finally, we offer our perspectives on cleavable linkers for RLT, highlighting future areas of research that will help advance the technology.

Evaluation of the Photophysical Properties of Two Scintillators: Crystalline Para-terphenyl and Plastic-Embedded 2,5-Diphenyloxazole Dye (EJ-276) at Room and Cryogenic Temperatures

The photophysical characterization of two dyes used as scintillators, crystalline para-terphenyl and EJ-276, a plastic heavily doped with 2,5-diphenyloxazole (DPO), was investigated with steady-state absorption, time-resolved emission, and transient absorption at room and cryogenic temperatures. Application of time-gated emission spectroscopy allowed for the measurement of phosphorescence spectra and their temporal dynamics. The photophysical properties of plastic-embedded DPO are not substantially altered compared to those previously determined for this dye in solvents. Notably, the amount of delayed fluorescence is always greater than that of phosphorescence. However, our study of crystalline para-terphenyl suggests that a second phase called β (perhaps comprising more planar molecules) functions as a triplet trap and decreases the amount of delayed fluorescence relative to phosphorescence. While the "main form" of para-terphenyl dominates absorption, the emissive properties (fluorescence, phosphorescence, and delayed fluorescence) are dominated by the β-phase. Studies of the para-terphenyl crystal performed with femtosecond time-resolved transient absorption demonstrate that excitation from the main form of the para-terphenyl crystal is promptly transferred to the β-phase with a time constant of roughly 300 ps. This work provides insight into the photophysical properties of two scintillators utilized to differentiate γ-ray- and neutron-induced signals.

Core-shell structured gold nanoparticles as carrier for 166Dy/166Ho in vivo generator

Background

Radionuclide therapy (RNT) has become a very important treatment modality for cancer nowadays. Comparing with other cancer treatment options, sufficient efficacy could be achieved in RNT with lower toxicity. β⁻ emitters are frequently used in RNT due to the long tissue penetration depth of the β⁻ particles. The dysprosium-166/holmium-166 (¹⁶⁶Dy/¹⁶⁶Ho) in vivo generator shows great potential for treating large malignancies due to the long half-life time of the mother nuclide ¹⁶⁶Dy and the emission of high energy β⁻ from the daughter nuclide ¹⁶⁶Ho. However, the internal conversion occurring after β⁻ decay from ¹⁶⁶Dy to ¹⁶⁶Ho could cause the release of about 72% of ¹⁶⁶Ho when ¹⁶⁶Dy is bound to conventional chelators. The aim of this study is to develop a nanoparticle based carrier for ¹⁶⁶Dy/¹⁶⁶Ho in vivo generator such that the loss of the daughter nuclide ¹⁶⁶Ho induced by internal conversion is prevented. To achieve this goal, we radiolabelled platinum-gold bimetallic nanoparticles (PtAuNPs) and core–shell structured gold nanoparticles (AuNPs) with ¹⁶⁶Dy and studied the retention of both ¹⁶⁶Dy and ¹⁶⁶Ho under various conditions.

Results

The ¹⁶⁶Dy was co-reduced with gold and platinum precursor to form the ¹⁶⁶DyAu@AuNPs and ¹⁶⁶DyPtAuNPs. The ¹⁶⁶Dy radiolabelling efficiency was determined to be 60% and 70% for the two types of nanoparticles respectively. The retention of ¹⁶⁶Dy and ¹⁶⁶Ho were tested in MiliQ water or 2.5 mM DTPA for a period of 72 h. In both cases, more than 90% of both ¹⁶⁶Dy and ¹⁶⁶Ho was retained. The results show that the incorporation of ¹⁶⁶Dy in AuNPs can prevent the escape of ¹⁶⁶Ho released due to internal conversion.

Conclusion

We developed a chelator-free radiolabelling method for ¹⁶⁶Dy with good radiolabelling efficiency and very high stability and retention of the daughter nuclide ¹⁶⁶Ho. The results from this study indicate that to avoid the loss of the daughter radionuclides by internal conversion, carriers composed of electron-rich materials should be used.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00170-3.

ASTRO's Framework for Radiopharmaceutical Therapy Curriculum Development for Trainees

In 2017, the American Society for Radiation Oncology (ASTRO) board of directors prioritized radiopharmaceutical therapy (RPT) as a leading area for new therapeutic development, and the ASTRO RPT workgroup was created. Herein, the workgroup has developed a framework for RPT curriculum development upon which education leaders can build to integrate this modality into radiation oncology resident education. Through this effort, the workgroup aims to provide a guide to ensure robust training in an emerging therapeutic area within the context of existing radiation oncology training in radiation biology, medical physics, and clinical radiation oncology. The framework first determines the core RPT knowledge required to select patients, prescribe, safely administer, and manage related adverse events. Then, it defines the most important topics for preparing residents for clinical RPT planning and delivery. This framework is designed as a tool to supplement the current training that exists for radiation oncology residents. The final document was approved by the ASTRO board of directors in the fall of 2021.

H2ampa─Versatile Chelator for [203Pb]Pb2+, [213Bi]Bi3+, and [225Ac]Ac3

A new decadentate chelator, H₂ampa, was designed to be a potential radiopharmaceutical chelator component. The chelator involves both amide and picolinate functional groups on a large non-macrocyclic, ether-bridged backbone. With its large scaffold, H₂ampa was paired with [^nat/203Pb]Pb²⁺, [^nat/213Bi]Bi³⁺, and ^natLa³⁺/[²²⁵Ac]Ac³⁺ ions. Nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry were used to study the non-radioactive metal complexes. A single crystal of [Bi(ampa)](NO₃) was obtained; its asymmetric, 10-coordinate complex structure was revealed by X-ray diffraction. Optimal conformations of the metal complexes were assessed by density functional theory studies to provide further structural information. Solution studies providing thermodynamic insights into metal complex formation revealed H₂ampa coordinated Bi³⁺, Pb²⁺, and La³⁺ ions to obtain pM values of 26, 14.8, and 15.1, respectively. Preliminary concentration-dependent radiolabeling experiments were carried out between H₂ampa and three different radiometals to evaluate their compatibility for radiopharmaceutical applications. The chelator radiolabeled [²⁰³Pb]Pb²⁺, [²¹³Bi]Bi³⁺, and [²²⁵Ac]Ac³⁺ in short reaction times (7-30 min), at dilute concentrations, and under mild conditions. Thus, H₂ampa was proven to be a versatile chelator able to well coordinate a small range of radiometals frequently considered to be alpha therapeutic candidates.

Radiolabeled Nanocarriers as Theranostics-Advancement from Peptides to Nanocarriers

Endogenous targeted radiotherapy is emerging as an integral modality to treat a variety of cancer entities. Nevertheless, despite the positive clinical outcome of the treatment using radiolabeled peptides, small molecules, antibodies, and nanobodies, a high degree of hepatotoxicity and nephrotoxicity still persist. This limits the amount of dose that can be injected. In an attempt to mitigate these side effects, the use of nanocarriers such as nanoparticles (NPs), dendrimers, micelles, liposomes, and nanogels (NGs) is currently being explored. Nanocarriers can prolong circulation time and tumor retention, maximize radiation dosage, and offer multifunctionality for different targeting strategies. In this review, the authors first provide a summary of radiation therapy and imaging and discuss the new radiotracers that are used preclinically and clinically. They then highlight and identify the advantages of radio-nanomedicine and its potential in overcoming the limitations of endogenous radiotherapy. Finally, the review points to the ongoing efforts to maximize the use of radio-nanomedicine for efficient clinical translation.

Emerging applications of nanobodies in cancer therapy

Cancer is a heterogeneous disease, requiring treatment tailored to the unique phenotype of the patient's tumor. Monoclonal antibodies (mAbs) and variants thereof have enabled targeted therapies to selectively target cancer cells. Cancer cell-specific mAbs have been used for image-guided surgery and targeted delivery of radionuclides or toxic agents, improving classical treatment strategies. Cancer cell-specific mAbs can further inhibit tumor cell growth or can stimulate immune-mediated destruction of cancer cells, a feature that has also been achieved through mAb-mediated manipulation of immune cells and pathways. Drawbacks of mAbs and their variants, together with the discovery of camelid heavy chain-only antibodies and the many advantageous features of their variable domains, referred to as V_HHs, single domain antibodies or nanobodies (Nbs), resulted in the exploration of Nbs as an alternative targeting moiety. We therefore review the state-of-the-art as well as novel exploitation strategies of Nbs for targeted cancer therapy.

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.

Radiolabeled Antibodies for Cancer Imaging and Therapy

Radioimmunoconjugates consist of a monoclonal antibody (mAb) linked to a radionuclide. Radioimmunoconjugates as theranostics tools have been in development with success, particularly in hematological malignancies, leading to approval by the US Food and Drug Administration (FDA) for the treatment of non-Hodgkin's lymphoma. Radioimmunotherapy (RIT) allows for reduced toxicity compared to conventional radiation therapy and enhances the efficacy of mAbs. In addition, using radiolabeled mAbs with imaging methods provides critical information on the pharmacokinetics and pharmacodynamics of therapeutic agents with direct relevance to the optimization of the dose and dosing schedule, real-time antigen quantitation, antigen heterogeneity, and dynamic antigen changes. All of these parameters are critical in predicting treatment responses and identifying patients who are most likely to benefit from treatment. Historically, RITs have been less effective in solid tumors; however, several strategies are being investigated to improve their therapeutic index, including targeting patients with minimal disease burden; using pre-targeting strategies, newer radionuclides, and improved labeling techniques; and using combined modalities and locoregional application. This review provides an overview of the radiolabeled intact antibodies currently in clinical use and those in development.

Harnessing -Emitting Radionuclides for Therapy: Radiolabeling Method Review

Targeted α-therapy (TAT) is an emerging powerful tool treating late-stage cancers for which therapeutic options are limited. At the core of TAT are targeted radiopharmaceuticals, where isotopes are paired with targeting vectors to enable tissue- or cell-specific delivery of α-emitters. DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and DTPA (diethylenetriamine pentaacetic acid) are commonly used to chelate metallic radionuclides but have limitations. Significant efforts are underway to develop effective stable chelators for α-emitters and are at various stages of development and community adoption. Isotopes such as 149Tb, 212/213Bi, 212Pb (for 212Bi), 225Ac, and 226/227Th have found suitable chelators, although further studies, especially in vivo studies, are required. For others, including 223Ra, 230U, and, arguably 211At, the ideal chemistry remains elusive. This review summarizes the methods reported to date for the incorporation of 149Tb, 211At, 212/213Bi, 212Pb (for 212Bi), 223Ra, 225Ac, 226/227Th, and 230U into radiopharmaceuticals, with a focus on new discoveries and remaining challenges.