Global comparison of targeted alpha vs targeted beta therapy for cancer: In vitro, in vivo and clinical trials

On-chip fabrication of calcium carbonate nanoparticles loaded with various compounds using microfluidic approach

Engineered calcium carbonate (CaCO3) particles are extensively used as drug delivery systems due to their availability, biological compatibility, biodegradability, and cost-effective production. The synthesis procedure of CaCO3 particles, however, suffers from poor reproducibility. Furthermore, reducing the size of CaCO3 particles to <100 nm requires the use of additives in the reaction, which increases the total reaction time. Here we propose on-chip synthesis and loading of nanoscaled CaCO3 particles using microfluidics. After the development and fabrication of a microfluidic device, we optimized the synthesis of CaCO3 NPs by varying different parameters such as flow rates in the microfluidic channels, concentration of reagents, and the reaction time. To prove the versatility of the used synthesis route, we performed single and double loading of CaCO3 NPs with various compounds (Doxorubicin, Cy5 or FITC conjugated with BSA, and DNA) using the same microfluidic device. Further, the on-chip loaded CaCO3 NPs were used as carriers to transfer compounds to model cells. We have developed a microfluidic synthesis method that opens up a new pathway for easy on-chip fabrication of functional nanoparticles for clinical use.

Affinity of PET-MRI Tracers for Hypoxic Cells in Breast Cancer: A Systematic Review

Tumour hypoxia is a known microenvironmental culprit for treatment resistance, tumour recurrence and promotion of metastatic spread. Despite the long-known existence of this factor within the tumour milieu, hypoxia is still one of the greatest challenges in cancer management. The transition from invasive and less reliable detection methods to more accurate and non-invasive ways to identify and quantify hypoxia was a long process that eventually led to the promising results showed by functional imaging techniques. Hybrid imaging, such as PET-CT, has the great advantage of combining the structural or anatomical image (offered by CT) with the functional or metabolic one (offered by PET). However, in the context of hypoxia, it is only the PET image taken after appropriate radiotracer administration that would supply hypoxia-specific information. To overcome this limitation, the development of the latest hybrid imaging systems, such as PET-MRI, enables a synergistic approach towards hypoxia imaging, with both methods having the potential to provide functional information on the tumour microenvironment. This study is designed as a systematic review of the literature on the newest developments of PET-MRI for the imaging of hypoxic cells in breast cancer. The analysis includes the affinity of various PET-MRI tracers for hypoxia in this patient group as well as the correlations between PET-specific and MRI-specific parameters, to offer a broader view on the potential for the widespread clinical implementation of this hybrid imaging technique.

Targeted Alpha Therapy for Glioblastoma: Review on In Vitro, In Vivo and Clinical Trials

Glioblastoma (GB), a prevalent and highly malignant primary brain tumour with a very high mortality rate due to its resistance to conventional therapies and invasive nature, resulting in 5-year survival rates of only 4-17%. Despite recent advancements in cancer management, the survival rates for GB patients have not significantly improved over the last 10-20 years. Consequently, there exists a critical unmet need for innovative therapies. One promising approach for GB is Targeted Alpha Therapy (TAT), which aims to selectively deliver potentially therapeutic radiation doses to malignant cells and the tumour microenvironment while minimising radiation exposure to surrounding normal tissue with or without conventional external beam radiation. This approach has shown promise in both pre-clinical and clinical settings. A review was conducted following PRISMA 2020 guidelines across Medline, SCOPUS, and Embase, identifying 34 relevant studies out of 526 initially found. In pre-clinical studies, TAT demonstrated high binding specificity to targeted GB cells, with affinity rates between 60.0% and 84.2%, and minimal binding to non-targeted cells (4.0-5.6%). This specificity significantly enhanced cytotoxic effects and improved biodistribution when delivered intratumorally. Mice treated with TAT showed markedly higher median survival rates compared to control groups. In clinical trials, TAT applied to recurrent GB (rGB) displayed varying success rates in extending overall survival (OS) and progression-free survival. Particularly effective when integrated into treatment regimens for both newly diagnosed and recurrent cases, TAT increased the median OS by 16.1% in newly diagnosed GB and by 36.4% in rGB, compared to current standard therapies. Furthermore, it was generally well tolerated with minimal adverse effects. These findings underscore the potential of TAT as a viable therapeutic option in the management of GB.

Recent advances and impending challenges for the radiopharmaceutical sciences in oncology

This paper is the first of a Series on theranostics that summarises the current landscape of the radiopharmaceutical sciences as they pertain to oncology. In this Series paper, we describe exciting developments in radiochemistry and the production of radionuclides, the development and translation of theranostics, and the application of artificial intelligence to our field. These developments are catalysing growth in the use of radiopharmaceuticals to the benefit of patients worldwide. We also highlight some of the key issues to be addressed in the coming years to realise the full potential of radiopharmaceuticals to treat cancer.

Synthesis of new acyclic chelators H4aPyta and H6aPyha and their complexes with Cu2+, Ga3+, Y3+, and Bi3

In this article, we present the synthesis and characterization of new acyclic pyridine-containing polyaminocarboxylate ligands H4aPyta and H6aPyha, which differ in structural rigidity and the number of chelating groups. Their abilities to form complexes with Cu2+, Ga3+, Y3+, and Bi3+ cations, as well as the stability of the complexes, were evaluated by potentiometric titration method, radiolabeling with the corresponding radionuclides, in vitro studies, mass spectrometry, and HPLC. The structures of the resulting complexes were determined using NMR spectroscopy and DFT calculations. The results obtained made it possible to evaluate the influence of the structural features of the complexes on their stability. The developed chelators H4aPyta and H6aPyha were proved to be promising for further research in the field of radiopharmaceuticals.

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.

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Development of 225Ac Production from Low Isotopic Dilution 229Th

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

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

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

Development of [225Ac]Ac-DOTA-C595 as radioimmunotherapy of pancreatic cancer: in vitro evaluation, dosimetric assessment and detector calibration

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy which may benefit from radioimmunotherapy. Previously, [177Lu]Lu-DOTA-C595 has been developed as a beta-emitting radioimmunoconjugate to target cancer-specific mucin 1 epitopes (MUC1-CE) overexpressed on PDAC. However, the therapeutic effect may be enhanced by using an alpha-emitting radionuclide such as Actinium-225 (Ac-225). The short range and high linear energy transfer of alpha particles provides dense cellular damage and can overcome typical barriers related to PDAC treatment such as hypoxia. Despite the added cytotoxicity of alpha-emitters, their clinical implementation can be complicated by their complex decay chains, recoil energy and short-range impeding radiation detection. In this study, we developed and evaluated [225Ac]Ac-DOTA-C595 as an alpha-emitting radioimmunotherapy against PDAC using a series of in vitro experiments and conducted a preliminary dosimetric assessment and cross-calibration of detectors for the clinical implementation of Ac-225.

RESULTS

Cell binding and internalisation of [225Ac]Ac-DOTA-C595 was rapid and greatest in cells with strong MUC1-CE expression. Over 99% of PDAC cells had positive yH2AX expression within 1 h of [225Ac]Ac-DOTA-C595 exposure, suggesting a high level of DNA damage. Clonogenic assays further illustrated the cytotoxicity of [225Ac]Ac-DOTA-C595 in a concentration-dependent manner. At low concentrations of [225Ac]Ac-DOTA-C595, cells with strong MUC1-CE expression had lower cell survival than cells with weak MUC1-CE expression, yet survival was similar between cell lines at high concentrations irrespective of MUC1-CE expression. A dosimetric assessment was performed to estimate the dose-rate of 1 kBq of [225Ac]Ac-DOTA-C595 with consideration to alpha particles. Total absorption of 1 kBq of Ac-225 was estimated to provide a dose rate of 17.5 mGy/h, confirmed via both detector measurements and calculations.

CONCLUSION

[225Ac]Ac-DOTA-C595 was shown to target and induce a therapeutic effect in MUC1-CE expressing PDAC cells.

Immunotherapy for Acute Myeloid Leukemia: Current Trends, Challenges, and Strategies

BACKGROUND

In the past decade, there have been significant breakthroughs in immunotherapies for B-cell lymphoid malignancies and multiple myeloma, but progress has been much less for acute myeloid leukemia (AML). Nevertheless, challenge begets innovation and several therapeutic strategies are under investigation.

SUMMARY

In this review, we review the state of the art in AML immunotherapy including CD33- and CD123-targeted agents, immune checkpoint inhibition, and adoptive cell therapy strategies. We also share conceptual frameworks for approaching the growing catalog of investigational AML immunotherapies and propose future directions for the field.

KEY MESSAGES

Immunotherapies for AML face significant challenges but novel strategies are in development.

Site-Specifically Conjugated Single-Domain Antibody Successfully Identifies Glypican-3-Expressing Liver Cancer by Immuno-PET

Primary liver cancer is the third leading cause of cancer-related deaths, and its incidence and mortality are increasing worldwide. Hepatocellular carcinoma (HCC) accounts for 80% of primary liver cancer cases. Glypican-3 (GPC3) is a heparan sulfate proteoglycan that histopathologically defines HCC and represents an attractive tumor-selective marker for radiopharmaceutical imaging and therapy for this disease. Single-domain antibodies are a promising scaffold for imaging because of their favorable pharmacokinetic properties, good tumor penetration, and renal clearance. Although conventional lysine-directed bioconjugation can be used to yield conjugates for radiolabeling full-length antibodies, this stochastic approach risks negatively affecting target binding of the smaller single-domain antibodies. To address this challenge, site-specific approaches have been explored. Here, we used conventional and sortase-based site-specific conjugation methods to engineer GPC3-specific human single-domain antibody (HN3) PET probes. Methods: Bifunctional deferoxamine (DFO) isothiocyanate was used to synthesize native HN3 (nHN3)-DFO. Site-specifically modified HN3 (ssHN3)-DFO was engineered using sortase-mediated conjugation of triglycine-DFO chelator and HN3 containing an LPETG C-terminal tag. Both conjugates were radiolabeled with 89Zr, and their binding affinity in vitro and target engagement of GPC3-positive (GPC3+) tumors in vivo were determined. Results: Both 89Zr-ssHN3 and 89Zr-nHN3 displayed nanomolar affinity for GPC3 in vitro. Biodistribution and PET/CT image analysis in mice bearing isogenic A431 and A431-GPC3+ xenografts, as well as in HepG2 liver cancer xenografts, showed that both conjugates specifically identify GPC3+ tumors. 89Zr-ssHN3 exhibited more favorable biodistribution and pharmacokinetic properties, including higher tumor uptake and lower liver accumulation. Comparative PET/CT studies on mice imaged with both 18F-FDG and 89Zr-ssHN3 showed more consistent tumor accumulation for the single-domain antibody conjugate, further establishing its potential for PET imaging. Conclusion: 89Zr-ssHN3 showed clear advantages in tumor uptake and tumor-to-liver signal ratio over the conventionally modified 89Zr-nHN3 in xenograft models. Our results establish the potential of HN3-based single-domain antibody probes for GPC3-directed PET imaging of liver cancers.

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.

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of ¹⁷⁷Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of ²²⁵Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of ²²⁵Ac, its physical and radiochemical properties, as well as the place of ²²⁵Ac-DOTATOC and ²²⁵Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.

Advances in antibody-based therapy in oncology

Monoclonal antibodies are a growing class of targeted cancer therapeutics, characterized by exquisite specificity, long serum half-life, high affinity and immune effector functions. In this review, we outline key advances in the field with a particular focus on recent and emerging classes of engineered antibody therapeutic candidates, discuss molecular structure and mechanisms of action and provide updates on clinical development and practice.

Dual targeting with 224Ra/212Pb-conjugates for targeted alpha therapy of disseminated cancers: A conceptual approach

Metastases are the primary cause of death among cancer patients and efficacious new treatments are sorely needed. Targeted alpha-emitting radiopharmaceuticals that are highly cytotoxic may fulfill this critical need. The focus of this paper is to describe and explore a novel technology that may improve the therapeutic effect of targeted alpha therapy by combining two radionuclides from the same decay chain in the same solution. We hypothesize that the dual targeting solution containing bone-seeking 224Ra and cell-directed complexes of progeny 212Pb is a promising approach to treat metastatic cancers with bone and soft tissue lesions as well as skeletal metastases of mixed lytic/osteoblastic nature. A novel liquid 224Ra/212Pb-generator for rapid preparation of a dual targeting solution is described. Cancer cell targeting monoclonal antibodies, their fragments, synthetic proteins or peptides can all be radiolabeled with 212Pb in the 224Ra-solution in transient equilibrium with daughter nuclides. Thus, 224Ra targets stromal elements in sclerotic bone metastases and 212Pb-chelated-conjugate targets tumor cells of metastatic prostate cancer or osteosarcoma. The dual targeting solution may also be explored to treat metastatic breast cancer or multiple myeloma after manipulation of bone metastases to a more osteoblastic phenotype by the use of bisphosphonates, denosumab, bortezomib or hormone therapy prior to treatment. This may improve targeting of bone-seeking 224Ra and render an augmented radiation dose deposited within metastases. Our preliminary preclinical studies provide conceptual evidence that the dual 224Ra-solution with bone or tumor-targeted delivery of 212Pb has potential to inhibit cancer metastases without significant toxicity. In some settings, the use of a booster dose of purified 212Pb-conjugate alone could be required to elevate the effect of this tumor cell directed component, if needed, e.g., in a fractionated treatment regimen, where the dual targeting solution will act as maintenance treatment.

The inhibiting effect of alpha-based TARE on embolized vessels and neovascularization

Transarterial embolization (TAE) is a personalized technology that offers precise delivery of chemotherapeutic drugs or selective internal radiation therapy for hepatocellular carcinoma (HCC). Beta-emitting radionuclide embolisms for TAE (β-based TARE) are commonly used in the clinic via inducing biochemical lethality on tumor cells, while alpha-emitting radionuclides-based embolisms for TAE (α-based TARE) are still under study. The feeding artery plays a key role in tumor growth, metastasis, and recurrence. In this research, the auricular central arteries (ACAs) of rabbits were embolized with silk fibroin-based microspheres (SFMs) or SFMs integrated with α (Ra-223) or β (I-131) radionuclides to investigate the influence on vessels. TARE-induced tissue necrosis and the following neovascularization were measured by pathological analysis and 68Ga-DOTA-RGD PET/CT. The results showed that, compared to I-131, Ra-223 enhanced the growth inhibition of human hepatoma cells Huh-7 and induced more DNA double-strand breaks in vascular smooth muscle cells. Unlike β-based TARE, which mainly led to extensive necrosis of surrounding tissues, α-based TARE induced irreversible necrosis of a limited area adjacent to the embolized vessels. RGD PET revealed the inhibition on neovascularization in α-based TARE (SUVmax = 0.053 ± 0.004) when compared with normal group (SUVmax = 0.099 ± 0.036), the SFMs-lipiodol group (SUVmax = 0.240 ± 0.040), and β-based TARE (SUVmax = 0.141 ± 0.026), owing to the avoidance of the embolism-induced neovascularization. In conclusion, α-based TARE provided a promising strategy for HCC treatments via destroying the embolized vessels and inhibiting neovascularization.

Challenges and opportunities in developing Actinium-225 radiopharmaceuticals

Actinium-225 (225Ac) has emerged as a promising therapeutic radioisotope for targeted alpha therapy. It emits net four alpha particles during its decay to stable daughter bismuth-209, rightly called an in-vivo nano-generator. Compared to the worldwide demand of 225Ac, the amount produced via depleted thorium-229 sources is minimal, making it an expensive radionuclide. However, many research groups are working on optimizing the parameters for the production of 225Ac via different routes, including cyclotrons, reactors and high-energy linear accelerators. The present review article focuses on the various aspects associated with the development of 225Ac radiopharmaceuticals. It includes the challenges and opportunities associated with the production methods, labeling chemistry, in-vivo kinetics and dosimetry of 225Ac radiopharmaceuticals. A brief description is also given about the 225Ac radiopharmaceuticals at preclinical stages, clinical trials and used routinely.

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.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

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.

Nanomicelles of Radium Dichloride [223Ra]RaCl2 Co-Loaded with Radioactive Gold [198Au]Au Nanoparticles for Targeted Alpha-Beta Radionuclide Therapy of Osteosarcoma

Alpha and beta particulate radiation are used for non-treated neoplasia, due to their ability to reach and remain in tumor sites. Radium-223 (²²³Ra), an alpha emitter, promotes localized cytotoxic effects, while radioactive gold (¹⁹⁸Au), beta-type energy, reduces radiation in the surrounding tissues. Nanotechnology, including several radioactive nanoparticles, can be safely and effectively used in cancer treatment. In this context, this study aims to analyze the antitumoral effects of [²²³Ra]Ra nanomicelles co-loaded with radioactive gold nanoparticles ([¹⁹⁸Au]AuNPs). For this, we synthesize and characterize nanomicelles, as well as analyze some parameters, such as particle size, radioactivity emission, dynamic light scattering, and microscopic atomic force. [²²³Ra]Ra nanomicelles co-loaded with [¹⁹⁸Au]AuNPs, with simultaneous alpha and beta emission, showed no instability, a mean particle size of 296 nm, and a PDI of 0.201 (±0.096). Furthermore, nanomicelles were tested in an in vitro cytotoxicity assay. We observed a significant increase in tumor cell death using combined alpha and beta therapy in the same formulation, compared with these components used alone. Together, these results show, for the first time, an efficient association between alpha and beta therapies, which could become a promising tool in the control of tumor progression.

Why bother with alpha particles?

The approval of 223RaCl2 for cancer therapy in 2013 has heralded a resurgence of interest in the development of α-particle emitting radiopharmaceuticals. In the last decade, over a dozen α-emitting radiopharmaceuticals have entered clinical trials, spawned by strong preclinical studies. In this article, we explore the potential role of α-particle therapy in cancer treatment. We begin by providing a background for the basic principles of therapy with α-emitters, and we explore recent breakthroughs in therapy with α-emitting radionuclides, including conjugates with small molecules and antibodies. Finally, we discuss some outstanding challenges to the clinical adoption of α-therapies and potential strategies to address them.

Neopentyl Glycol as a Scaffold to Provide Radiohalogenated Theranostic Pairs of High In Vivo Stability

The high in vivo stability of 2,2-dihydroxymethyl-3-[¹⁸F]fluoropropyl-2-nitroimidazole ([¹⁸F]DiFA) prompted us to evaluate neopentyl as a scaffold to prepare a radiotheranostic system with radioiodine and astatine. Three DiFA analogues with one, two, or without a hydroxyl group were synthesized. While all ¹²⁵I-labeled compounds remained stable against nucleophilic substitution, only a ¹²⁵I-labeled neopentyl glycol was stable against cytochrome P450 (CYP)-mediated metabolism and showed high stability against in vivo deiodination. ²¹¹At-labeled neopentyl glycol also remained stable against both nucleophilic substitution and CYP-mediated metabolism. ²¹¹At-labeled neopentyl glycol showed the biodistribution profiles similar to those of its radioiodinated counterpart in contrast to the ¹²⁵I/²¹¹At-labeled benzoate pair. The urine analyses confirmed that ²¹¹At-labeled neopentyl glycol was excreted in the urine as a glucuronide conjugate with the absence of free [²¹¹At]At^-. These findings indicate that neopentyl glycol would constitute a promising scaffold to prepare a radiotheranostic system with radioiodine and ²¹¹At.

Medicinal chemistry and biomedical applications of bismuth-based compounds and nanoparticles

Bismuth as a relatively non-toxic and inexpensive metal with exceptional properties has numerous biomedical applications. Bismuth-based compounds are used extensively as medicines for the treatment of gastrointestinal disorders including dyspepsia, gastric ulcers and H. pylori infections. Recently, its medicinal application was further extended to potential treatments of viral infection, multidrug resistant microbial infections, cancer and also imaging, drug delivery and biosensing. In this review we have highlighted the unique chemistry and biological chemistry of bismuth-209 as a prelude to sections covering the unique antibacterial activity of bismuth including a description of research undertaken to date to elucidate key molecular mechanisms of action against H. pylori, the development of novel compounds to treat infection from microbes beyond H. pylori and the significant role bismuth compounds can play as resistance breakers. Furthermore we have provided an account of the potential therapeutic application of bismuth-213 in targeted alpha therapy as well as a summary of the biomedical applications of bismuth-based nanoparticles and composites. Ultimately this review aims to provide the state of the art, highlight the untapped biomedical potential of bismuth and encourage original contributions to this exciting and important field.

Pancreatic Ductal Adenocarcinoma: The Dawn of the Era of Nuclear Medicine?

Pancreatic ductal adenocarcinoma (PDAC), accounting for 90-95% of all pancreatic tumors, is a highly devastating disease associated with poor prognosis. The lack of accurate diagnostic tests and failure of conventional therapies contribute to this pejorative issue. Over the last decade, the advent of theranostics in nuclear medicine has opened great opportunities for the diagnosis and treatment of several solid tumors. Several radiotracers dedicated to PDAC imaging or internal vectorized radiotherapy have been developed and some of them are currently under clinical consideration. The functional information provided by Positron Emission Tomography (PET) or Single Photon Emission Computed Tomography (SPECT) could indeed provide an additive diagnostic value and thus help in the selection of patients for targeted therapies. Moreover, the therapeutic potential of β-- and α-emitter-radiolabeled agents could also overcome the resistance to conventional therapies. This review summarizes the current knowledge concerning the recent developments in the nuclear medicine field for the management of PDAC patients.

In situ Generated ²¹²Pb-PSMA Ligand in a ²²⁴Ra-Solution for Dual Targeting of Prostate Cancer Sclerotic Stroma and PSMA-positive Cells

Background: New treatments combating bone and extraskeletal metastases are needed for patients with metastatic castration-resistant prostate cancer. The majority of metastases overexpress prostate-specific membrane antigen (PSMA), making it an ideal candidate for targeted radionuclide therapy.

Objective: The aim of this study was to test a novel liquid 224Ra/212Pb-generator for the rapid preparation of a dual-alpha targeting solution. Here, PSMA-targeting ligands are labelled with 212Pb in the 224Ra-solution in transient equilibrium with daughter nuclides. Thus, natural bone-seeking 224Ra targeting sclerotic bone metastases and 212Pb-chelated PSMA ligands targeting PSMA-expressing tumour cells are obtained.

Methods: Two PSMA-targeting ligands, the p-SCN-Bn-TCMC-PSMA ligand (NG001), specifically developed for chelating 212Pb, and the most clinically used DOTA-based PSMA-617 were labelled with 212Pb. Radiolabelling and targeting potential were investigated in situ, in vitro (PSMA-positive C4-2 human prostate cancer cells) and in vivo (athymic mice bearing C4-2 xenografts).

Results: NG001 was rapidly labelled with 212Pb (radiochemical purity >94% at concentrations of ≥15 µg/ml) using the liquid 224Ra/212Pb-generator. The high radiochemical purity and stability of [212Pb]Pb-NG001 were demonstrated over 48 hours in the presence of ascorbic acid and albumin. Similar binding abilities of the 212Pb-labelled ligands were observed in C4-2 cells. The PSMA ligands displayed comparable tumour uptake after 2 hours, but NG001 showed a 3.5-fold lower kidney uptake than PSMA-617. Radium-224 was not chelated and, hence, showed high uptake in bones.

Conclusion: A fast method for the labelling of PSMA ligands with 212Pb in the 224Ra/212Pb-solution was developed. Thus, further in vivo studies with dual tumour targeting by alpha-particles are warranted.

Combination Therapy, a Promising Approach to Enhance the Efficacy of Radionuclide and Targeted Radionuclide Therapy of Prostate and Breast Cancer

In recent years, radionuclide therapy (RT) and targeted radionuclide therapy (TRT) have gained great interest in cancer treatment. This is due to promising results obtained in both preclinical and clinical studies. However, a complete response is achieved in only a small percentage of patients that receive RT or TRT. As a consequence, there have been several strategies to improve RT and TRT outcomes including the combination of these treatments with other well-established anti-cancer therapies, for example, chemotherapy. Combinations of RT and TRT with other therapies with distinct mechanisms of action represent a promising strategy. As for prostate cancer and breast cancer, the two most prevalent cancer types worldwide, several combination-based therapies have been evaluated. In this review, we will provide an overview of the RT and TRT agents currently used or being investigated in combination with hormone therapy, chemotherapy, immunotherapy, and external beam radiation therapy for the treatment of prostate cancer and breast cancer.

Combined Radionuclide Therapy and Immunotherapy for Treatment of Triple Negative Breast Cancer

Triple negative breast cancer (TNBC) is an aggressive subtype of the disease with poor clinical outcomes and limited therapeutic options. Immune checkpoint blockade (CP) has surged to the forefront of cancer therapies with widespread clinical success in a variety of cancer types. However, the percentage of TNBC patients that benefit from CP as a monotherapy is low, and clinical trials have shown the need for combined therapeutic modalities. Specifically, there has been interest in combining CP therapy with radiation therapy where clinical studies primarily with external beam have suggested their therapeutic synergy, contributing to the development of anti-tumor immunity. Here, we have developed a therapeutic platform combining radionuclide therapy (RT) and immunotherapy utilizing a radiolabeled biomolecule and CP in an E0771 murine TNBC tumor model. Survival studies show that while neither monotherapy is able to improve therapeutic outcomes, the combination of RT + CP extended overall survival. Histologic analysis showed that RT + CP increased necrotic tissue within the tumor and decreased levels of F4/80+ macrophages. Flow cytometry analysis of the peripheral blood also showed that RT + CP suppressed macrophages and myeloid-derived suppressive cells, both of which actively contribute to immune escape and tumor relapse.

Evaluation of the PSMA-Binding Ligand 212Pb-NG001 in Multicellular Tumour Spheroid and Mouse Models of Prostate Cancer

Radioligand therapy targeting the prostate-specific membrane antigen (PSMA) is rapidly evolving as a promising treatment for metastatic castration-resistant prostate cancer. The PSMA-targeting ligand p-SCN-Bn-TCMC-PSMA (NG001) labelled with ²¹²Pb efficiently targets PSMA-positive cells in vitro and in vivo. The aim of this preclinical study was to evaluate the therapeutic potential of ²¹²Pb-NG001 in multicellular tumour spheroid and mouse models of prostate cancer. The cytotoxic effect of ²¹²Pb-NG001 was tested in human prostate C4-2 spheroids. Biodistribution at various time points and therapeutic effects of different activities of the radioligand were investigated in male athymic nude mice bearing C4-2 tumours, while long-term toxicity was studied in immunocompetent BALB/c mice. The radioligand induced a selective cytotoxic effect in spheroids at activity concentrations of 3–10 kBq/mL. In mice, the radioligand accumulated rapidly in tumours and was retained over 24 h, while it rapidly cleared from nontargeted tissues. Treatment with 0.25, 0.30 or 0.40 MBq of ²¹²Pb-NG001 significantly inhibited tumour growth and improved median survival with therapeutic indexes of 1.5, 2.3 and 2.7, respectively. In BALB/c mice, no signs of long-term radiation toxicity were observed at activities of 0.05 and 0.33 MBq. The obtained results warrant clinical studies to evaluate the biodistribution, therapeutic efficacy and toxicity of ²¹²Pb-NG001.

CD38-Targeted Theranostics of Lymphoma with 89Zr/177Lu-Labeled Daratumumab

Lymphoma is a heterogeneous disease with varying clinical manifestations and outcomes. Many subtypes of lymphoma, such as Burkitt's lymphoma and diffuse large B cell lymphoma, are highly aggressive with dismal prognosis even after conventional chemotherapy and radiotherapy. As such, exploring specific biomarkers for lymphoma is of high clinical significance. Herein, a potential marker, CD38, is investigated for differentiating lymphoma. A CD38-targeting monoclonal antibody (mAb, daratumumab) is then radiolabeled with Zr-89 and Lu-177 for theranostic applications. As the diagnostic component, the Zr-89-labeled mAb is highly specific in delineating CD38-positive lymphoma via positron emission tomography (PET) imaging, while the Lu-177-labeled mAb serves well as the therapeutic component to suppress tumor growth after a one-time administration. These results strongly suggest that CD38 is a lymphoma-specific marker and prove that 89Zr/177Lu-labeled daratumumab facilitates immunoPET imaging and radioimmunotherapy of lymphoma in preclinical models. Further clinical evaluation and translation of this CD38-targeted theranostics may be of significant help in lymphoma patient stratification and management.

Prognostic and Theranostic Applications of Positron Emission Tomography for a Personalized Approach to Metastatic Castration-Resistant Prostate Cancer

Metastatic castration-resistant prostate cancer (mCRPC) represents a condition of progressive disease in spite of androgen deprivation therapy (ADT), with a broad spectrum of manifestations ranging from no symptoms to severe debilitation due to bone or visceral metastatization. The management of mCRPC has been profoundly modified by introducing novel therapeutic tools such as antiandrogen drugs (i.e., abiraterone acetate and enzalutamide), immunotherapy through sipuleucel-T, and targeted alpha therapy (TAT). This variety of approaches calls for unmet need of biomarkers suitable for patients’ pre-treatment selection and prognostic stratification. In this scenario, imaging with positron emission computed tomography (PET/CT) presents great and still unexplored potential to detect specific molecular and metabolic signatures, some of whom, such as the prostate specific membrane antigen (PSMA), can also be exploited as therapeutic targets, thus combining diagnosis and therapy in the so-called “theranostic” approach. In this review, we performed a web-based and desktop literature research to investigate the prognostic and theranostic potential of several PET imaging probes, such as ¹⁸F-FDG, ¹⁸F-choline and ⁶⁸Ga-PSMA-11, also covering the emerging tracers still in a pre-clinical phase (e.g., PARP-inhibitors’ analogs and the radioligands binding to gastrin releasing peptide receptors/GRPR), highlighting their potential for defining personalized care pathways in mCRPC.

Anti-Tumor Efficacy of PD-L1 Targeted Alpha-Particle Therapy in a Human Melanoma Xenograft Model

Simple Summary

In recent years, the development of immune checkpoint inhibitors, such as anti-PD‑1 and anti-PD-L1, proved to prolong melanoma patient survival and are now used in routine clinical practice. PD-L1 also represents a potent biomarker for in vivo molecular imaging using radiolabeled anti-PD-L1 mAbs and positron emission tomography and is currently in development to select patients and assess response to treatment. The aim of our study was to investigate in a preclinical model of human melanoma if PD-L1 could also be a good target for treatment using targeted alpha-particle therapy. Our results show that targeting PD-L1 with bismuth-213, an alpha particle emitter, was associated with efficient anti-tumor response, significant tumor growth delay, and improved survival. This demonstrates that anti-PD-L1 antibodies could be used as theranostics in molecular imaging but also in targeted alpha-particle therapy to treat the tumor and its stroma.

Abstract

PD-L1 (programmed death-ligand 1, B7-H1, CD274), the ligand for PD-1 inhibitory receptor, is expressed on various tumors, and its expression is correlated with a poor prognosis in melanoma. Anti-PD-L1 mAbs have been developed along with anti-CTLA-4 and anti-PD-1 antibodies for immune checkpoint inhibitor (ICI) therapy, and anti-PD-1 mAbs are now used as first line treatment in melanoma. However, many patients do not respond to ICI therapies, and therefore new treatment alternatives should be developed. Because of its expression on the tumor cells and on immunosuppressive cells within the tumor microenvironment, PD-L1 represents an interesting target for targeted alpha-particle therapy (TAT). We developed a TAT approach in a human melanoma xenograft model that stably expresses PD-L1 using a ²¹³Bi-anti-human-PD-L1 mAb. Unlike treatment with unlabeled anti-human-PD-L1 mAb, TAT targeting PD-L1 significantly delayed melanoma tumor growth and improved animal survival. A slight decrease in platelets was observed, but no toxicity on red blood cells, bone marrow, liver or kidney was induced. Anti-tumor efficacy was associated with specific tumor targeting since no therapeutic effect was observed in animals bearing PD-L1 negative melanoma tumors. This study demonstrates that anti-PD-L1 antibodies may be used efficiently for TAT treatment in melanoma.

Glypican-3 targeted delivery of 89Zr and 90Y as a theranostic radionuclide platform for hepatocellular carcinoma

Glypican-3 (GPC3) is a tumor associated antigen expressed by hepatocellular carcinoma (HCC) cells. This preclinical study evaluated the efficacy of a theranostic platform using a GPC3-targeting antibody αGPC3 conjugated to zirconium-89 (⁸⁹Zr) and yttrium-90 (⁹⁰Y) to identify, treat, and assess treatment response in a murine model of HCC. A murine orthotopic xenograft model of HCC was generated. Animals were injected with ⁸⁹Zr-labeled αGPC3 and imaged with a small-animal positron emission/computerized tomography (PET/CT) imaging system (immuno-PET) before and 30 days after radioimmunotherapy (RIT) with ⁹⁰Y-labeled αGPC3. Serum alpha fetoprotein (AFP), a marker of tumor burden, was measured. Gross tumor volume (GTV) and SUV_max by immuno-PET was measured using fixed intensity threshold and manual segmentation methods. Immuno-PET GTV measurements reliably quantified tumor burden prior to RIT, strongly correlating with serum AFP (R² = 0.90). Serum AFP was significantly lower 30 days after RIT in ⁹⁰Y-αGPC3 treated animals compared to those untreated (p = 0.01) or treated with non-radiolabeled αGPC3 (p = 0.02). Immuno-PET GTV measurements strongly correlated with tumor burden after RIT (R² = 0.87), and GTV of animals treated with ⁹⁰Y-αGPC3 was lower than in animals who did not receive treatment or were treated with non-radiolabeled αGPC3, although this only trended toward statistical significance. A theranostic platform utilizing GPC3 targeted ⁸⁹Zr and ⁹⁰Y effectively imaged, treated, and assessed response after radioimmunotherapy in a GPC3-expressing HCC xenograft model.

Translational Research in FLASH Radiotherapy-From Radiobiological Mechanisms to In Vivo Results

FLASH radiotherapy, or the administration of ultra-high dose rate radiotherapy, is a new radiation delivery method that aims to widen the therapeutic window in radiotherapy. Thus far, most in vitro and in vivo results show a real potential of FLASH to offer superior normal tissue sparing compared to conventionally delivered radiation. While there are several postulations behind the differential behaviour among normal and cancer cells under FLASH, the full spectra of radiobiological mechanisms are yet to be clarified. Currently the number of devices delivering FLASH dose rate is few and is mainly limited to experimental and modified linear accelerators. Nevertheless, FLASH research is increasing with new developments in all the main areas: radiobiology, technology and clinical research. This paper presents the current status of FLASH radiotherapy with the aforementioned aspects in mind, but also to highlight the existing challenges and future prospects to overcome them.

Effect of microdistribution of alpha and beta-emitters in targeted radionuclide therapies on delivered absorbed dose in a GATE model of bone marrow

Acute hematologic toxicity is a frequent adverse effect of beta-emitter targeted radionuclide therapies (TRTs). Alpha emitters have the potential of delivering high linear energy transfer (LET) radiation to the tumor attributed to its shorter range. Antibody-based TRTs have increased blood-pool half-lives, and therefore increased marrow toxicity, which is a particular concern with alpha emitters. Accurate 3D absorbed dose calculations focusing on the interface region of blood vessels and bone can elucidate energy deposition patterns. Firstly, a cylindrical geometry model with a central blood vessel embedded in the trabecular tissue was modeled. Monte Carlo simulations in GATE were performed considering beta (177Lu, 90Y) and alpha emitters (211At, 225Ac) as sources restricted to the blood pool. Subsequently, the radioactive sources were added in the trabecular bone compartment in order to model bone marrow metastases infiltration (BMMI). Radial profiles, dose-volume histograms and voxel relative differences were used to evaluate the absorbed dose results. We demonstrated that alpha emitters have a higher localized energy deposition compared to beta emitters. In the cylindrical geometry model, when the sources are confined to the blood pool, the dose to the trabecular bone is greater for beta emitting radionuclides, as alpha emitters deposit the majority of their energy within 70 μm of the vessel wall. In the BMMI model, alpha emitters have a lower dose to untargeted trabecular bone. Our results suggest that when alpha emitters are restricted to the blood pool, as when labeled to antibodies, hematologic toxicities may be lower than expected due to differences in the microdistribution of delivered absorbed dose.

The Role of Theranostics in Prostate Cancer

Theranostics in men with metastatic castration-resistant prostate cancer (mCRPC) has been developed to target bone and the tumor itself. Currently, bone-directed targeted alpha therapy with radium-223 (²²³Ra) is the only theranostic agent proven to prolong survival in men with mCRPC who have symptomatic bone metastases and no known visceral metastases. The clinical utility and therapeutic success of ²²³Ra has encouraged the development of other tumor-targeting theranostic agents in mCRPC, primarily targeting prostate-specific membrane antigen (PSMA) with radioligand therapy (RLT). There is increasing evidence of promising response rates and a low toxicity profile with ¹⁷⁷Lu-labeled PSMA RLT in patients with mCRPC. A phase III randomized study of ¹⁷⁷Lu-labeled PSMA RLT has completed accrual and is awaiting results as to whether the drug improves radiographic progression-free survival and overall survival in men with mCRPC receiving standard of care treatments. Additional early clinical trials are investigating the role of tumor-directed targeted alpha therapy with radiotracers such as ²²⁵Ac. In this article, we review the current status of theranostics in prostate cancer, discussing the challenges and opportunities of combination therapies with more conventional agents such as androgen receptor inhibitors, cytotoxic chemotherapy, and immunotherapy.

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.

The Expression Profile and Textural Characteristics of C595-Reactive MUC1 in Pancreatic Ductal Adenocarcinoma for Targeted Radionuclide Therapy

Simple Summary

Pancreatic ductal adenocarcinoma (PDAC) is a cancer of low survival needing novel treatment approaches such as targeted therapies. If a target is overexpressed on PDAC cells but has minimal expression on normal cells, it is considered a good candidate for targeted therapy. Identifying targets with this expression pattern can help to optimise targeted therapies to be therapeutically effective without compromising on tolerability. The aim of this study was to assess the expression of the MUC1 receptor using the C595 antibody. We performed a series of cell line and tissue studies to identify if the expression of the MUC1 receptor changes between different pancreatic pathologies, including PDAC and normal pancreatic tissue. We found that the MUC1 receptor is both overexpressed and more uniformly expressed in PDAC compared to the other tissue types assessed. This indicates that the MUC1 receptor is a feasible target for targeted therapies of PDAC.

Abstract

Improvements in the prognosis of pancreatic ductal adenocarcinoma (PDAC) rely on the development of effective treatments to target advanced disease. Mucin 1 (MUC1) is a transmembrane glycoprotein which is involved in the metastatic progression of PDAC and is a receptor-of-interest for targeted radionuclide therapy. The aim of this study was to determine the feasibility of MUC1-based targeted radionuclide therapy for PDAC, by evaluating the expression profile of MUC1 in different pancreatic cells and tissues using the C595 antibody. MUC1 expression was evaluated in four PDAC cell lines (PANC-1, BxPC-3, CAPAN-1 and AsPC-1) using flow cytometry and immunocytochemistry. Immunohistochemistry was performed on primary and metastatic PDAC, pancreatitis, pancreatic intra-epithelial neoplasia and normal pancreatic tissue samples to identify potential changes in C595-reactive MUC1 expression across different disease groups. C595-reactive MUC1 expression was found to varying degrees in the cell lines (11.5–93.1%). A pixel analysis of the immunohistochemical staining demonstrated highest MUC1 expression in primary PDAC tissue (mean pixel value of 205.4), followed by other pancreatic cancer types (204.9), pancreatic intra-epithelial neoplasia (203.8), metastatic PDAC (201.5), chronic pancreatitis (198.1) and normal pancreatic tissue (191.4). The increased expression in malignant tissues and reduced expression in benign tissues indicate that C595-reactive MUC1 is a potential target for targeted radionuclide therapy of PDAC.

An octadentate bis(semicarbazone) macrocycle: a potential chelator for lead and bismuth radiopharmaceuticals

A variant of 1,4,7,10-tetraazacyclododecane (cyclen) bearing two semicarbazone pendant groups has been prepared. The octadentate ligand forms complexes with Bi3+ and Pb2+. X-ray crystallography showed that the neutral ligand provides an eight-coordinate environment for both metal ions and intermolecular hydrogen bond interactions have influenced the coordination environments of both complexes in the solid state. NMR spectroscopy revealed a fluxional environment for both complexes. The ligand was radiolabeled with the α-emitting radioactive isotope 213Bi3+, which is used in systemic targeted radiotherapy. The resulting complex was stable in serum for at least 90 min (two decay half-lives). The Pb2+ complex has reasonably fast kinetics of formation (t1/2 = 20 min) at 25 °C and pH 7.4. The Bi3+ and Pb2+ complexes show kinetic stability in 1.2 M HCl (half-lives of 214 min and 47 min, respectively). This is the first description of a macrocycle bearing semicarbazone pendant groups and its utility in coordinating main group metals, specifically those with radiotherapeutic potential.

Preclinical evaluation of new α-radionuclide therapy targeting LAT1: 2-[211At]astato-α-methyl-L-phenylalanine in tumor-bearing model

INTRODUCTION

Targeted α-radionuclide therapy has attracted attention as a promising therapy for refractory cancers. However, the application is limited to certain types of cancer. Since L-type amino acid transporter 1 (LAT1) is highly expressed in various human cancers, we prepared an LAT1-selective α-radionuclide-labeled amino acid analog, 2-[²¹¹At]astato-α-methyl-L-phenylalanine (2-[²¹¹At]AAMP), and evaluated its potential as a therapeutic agent.

METHODS

2-[²¹¹At]AAMP was prepared from the stannyl precursor. Stability of 2-[²¹¹At]AAMP was evaluated both in vitro and in vivo. In vitro studies using an LAT1-expressing human ovarian cancer cell line, SKOV3, were performed to evaluate cellular uptake and cytotoxicity of 2-[²¹¹At]AAMP. Biodistribution and therapeutic studies in SKOV3-bearing mice were performed after intravenous injection of 2-[²¹¹At]AAMP.

RESULTS

2-[²¹¹At]AAMP was stable in murine plasma in vitro and excreted intact into urine. Cellular uptake of 2-[²¹¹At]AAMP was inhibited by treatment with an LAT1-selective inhibitor. After 24 h incubation, 2-[²¹¹At]AAMP suppressed clonogenic growth at 10 kBq/ml, and induced cell death and DNA double-strand breaks at 25 kBq/ml. When injected into mice, 2-[²¹¹At]AAMP exhibited peak accumulation in the tumor at 30 min postinjection, and radioactivity levels in the tumor were retained up to 60 min. The majority of the radioactivity was rapidly eliminated from the body into urine in an intact form immediately after injection. 2-[²¹¹At]AAMP significantly improved the survival of mice (P < 0.05) without serious side effects.

CONCLUSION

2-[²¹¹At]AAMP showed α-radiation-dependent cellular growth inhibition after it was taken up via LAT1. In addition, 2-[²¹¹At]AAMP had a beneficial effect on survival in vivo. These findings suggest that 2-[²¹¹At]AAMP would be useful for the treatment of LAT1-positive cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This is the first report of an LAT1-targeting radiopharmaceutical for α-radionuclide therapy; this agent would be applicable for the treatment of various types of cancer.