Functionalized TiO2 nanoparticles labelled with 225Ac for targeted alpha radionuclide therapy

In vitro and in vivo study of 221Fr and 213Bi progeny release from the 225Ac-labelled TiO2 nanoparticles

BACKGROUND

Targeted alpha therapy (TAT) is an effective option for cancer treatment. To maximize its efficacy and minimize side effects, carriers must deliver radionuclides to target tissues. Most of the nuclides used in TAT decay via the alpha cascade, producing several radioactive daughter nuclei with sufficient energy to escape from the original carrier. Therefore, studying these daughter atoms is crucial in the search for new carriers. Nanoparticles have potential as carriers due to their structure, which can prevent the escape of daughter atoms and reduce radiation exposure to non-target tissues. This work focuses on determining the released activity of 221Fr and 213Bi resulting from the decay of 225Ac labelled TiO2 nanoparticles.

RESULTS

Labelling of TiO2 nanoparticles has shown high sorption rates of 225Ac and its progeny, 221Fr and 213Bi, with over 92 % of activities sorbed on the nanoparticle surface for all measured radionuclides. However, in the quasi-dynamic in vitro system, the released activity of 221Fr and 213Bi is strongly dependent on the nanoparticles concentration, ranging from 15 % for a concentration of 1 mg/mL to approximately 50 % for a nanoparticle concentration of 10 μg/mL in saline solution. The released activities of 213Bi were lower, with a maximum value of around 20 % for concentrations of 0.05, 0.025, and 0.01 mg/mL. The leakage of 225Ac and its progeny was tested in various biological matrices. Minimal released activity was measured in saline at around 10 % after 48 h, while the maximum activity was measured in blood serum and plasma at 20 %. The amount of 225Ac released into the media was minimal (<3 %). The in vitro results were confirmed in a healthy mouse model. The difference in %ID/g was clearly visible immediately after dissection and again after 6 h when 213Bi reached equilibrium with 225Ac.

CONCLUSION

The study verified the potential release of 225Ac progeny from the labelled TiO2 nanoparticles. Experiments were performed to determine the dependence of released activity on nanoparticle concentration and the biological environment. The results demonstrated the high stability of the prepared 225Ac@TiO2 NPs and the potential release of progeny over time. In vivo studies confirmed our hypothesis. The data obtained suggest that the daughter atoms can escape from the original carrier and follow their own biological pathways in the organism.

Nanoradiopharmaceuticals: An Attractive Concept in Oncotherapy

Radiopharmaceuticals are of significant importance in the fields of tumor imaging and therapy. In recent decades, the increasing role of nanotechnology has led to the attractive concept of nanoradiopharmaceuticals. Consequently, it is imperative to provide a concise summary of the necessary guidelines to facilitate the translation of nanoradiopharmaceuticals. In this work, we have presented the contents of radiolabeling strategies and some applications of nanoradiopharmaceuticals. Such a framework can assist researchers in identifying more pertinent insights or making more informed decisions in the study of nanoradiopharmaceuticals.

Recent Advances in Metal Oxide and Phosphate Nanomaterials Radiolabeling with Medicinal Nuclides

The utilization of nanomaterials in biomedical applications has surged in recent years; yet, the transition from research to practical implementation remains a great challenge. However, a promising area of research has emerged with the integration of nanomaterials with diagnostic and therapeutic radionuclides. In this Review, we elucidate the motivations behind selecting metal oxide- and phosphate-based nanomaterials in conjunction with these radionuclides, while addressing its issues and limitations. Various metal oxide- and phosphate-based nanoparticles, exhibiting low toxicity and high tolerability, have been proposed for diverse biomedical applications, ranging from bone substitutes to drug delivery systems and controlled release vectors for pharmaceuticals, including radionuclides for nuclear medicine imaging and therapy. Moreover, the potential synergistic effects of multimodal combinational therapies, integrating chemotherapeutics, immunomodulators, or hyperthermia, underscore the versatility of these nanoconstructs. Our comprehensive exploration includes the underlying principles of radiolabeling strategies, the pivotal attributes of nanomaterial platforms, and their applications. Through this perspective, we present the potential of nanotechnology-enabled nuclear medicine. Furthermore, we discuss the potential systemic and local applications of these nanoconstructs, considering their in vitro and in vivo characteristics, as well as their physicochemical properties.

Nanoscale Radiotheranostics for Cancer Treatment: From Bench to Bedside

In recent years, the application of radionuclides-containing nanomaterials in cancer treatment has garnered widespread attention. The diversity of nanomaterials allows researchers to selectively combine them with appropriate radionuclides for biomedical purposes, addressing challenges faced by peptides, small molecules, or antibodies used for radionuclide labeling. However, with advantages come challenges, and nanoradionuclides still encounter significant issues during clinical translation. This review summarized the recent progress of nanosized radionuclides for cancer treatment or diagnosis. The discussion began with representative radionuclides and the methods of incorporating them into nanomaterial structures. Subsequently, new combinations of nanomaterials and radionuclides, along with their applications, were introduced to demonstrate their future trends. The benefits of nanoradionuclides included optimized pharmacokinetic properties, enhanced disease-targeting efficacy, and synergistic application with other treatment techniques. Besides, the basic rule of this section was to summarize how these nanoradionuclides can truly impact the diagnosis and therapy of various cancer types. In the last part, the focus was devoted to the nanoradionuclides currently applicable in clinics and how to address the existing issues and problems based on our knowledge.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

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

BACKGROUND

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

MAIN BODY

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

CONCLUSION

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Recent Innovations and Nano-Delivery of Actinium-225: A Narrative Review

The actinium-225 (²²⁵Ac) radioisotope exhibits highly attractive nuclear properties for application in radionuclide therapy. However, the ²²⁵Ac radionuclide presents multiple daughter nuclides in its decay chain, which can escape the targeted site, circulate in plasma, and cause toxicity in areas such as kidneys and renal tissues. Several ameliorative strategies have been devised to circumvent this issue, including nano-delivery. Alpha-emitting radionuclides and nanotechnology applications in nuclear medicine have culminated in major advancements that offer promising therapeutic possibilities for treating several cancers. Accordingly, the importance of nanomaterials in retaining the ²²⁵Ac daughters from recoiling into unintended organs has been established. This review expounds on the advancements of targeted radionuclide therapy (TRT) as an alternative anticancer treatment. It discusses the recent developments in the preclinical and clinical investigations on ²²⁵Ac as a prospective anticancer agent. Moreover, the rationale for using nanomaterials in improving the therapeutic efficacy of α-particles in targeted alpha therapy (TAT) with an emphasis on ²²⁵Ac is discussed. Quality control measures in the preparation of ²²⁵Ac-conjugates are also highlighted.

In vitro studies of 223Ra- and 225Ac-labelled α-zirconium phosphate as potential carrier for alpha targeted therapy

In this study suitability of α-ZrP nanoparticles as a 223Ra and 225Ac carriers for TAT was investigated. The yields of radiolabelling were higher than 98% in both cases. Subsequently, in vitro stability studies were carried out in various biological matrices during 48 h period. Measurements of released radioactivity showed the highest stability in saline. Released activity of 223Ra, 225Ac and their daughter radionuclides was around 0.5%. On the other hand, the lowest stability was shown in plasma and serum. Released activity for 223Ra, 225Ac and their progeny atoms was from 15 to 32%.

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.

Pr(H3BNMe2BH3)3 and Pr(thd)3 as Volatile Carriers for Actinium-225. Deposition of Actinium-Doped Praseodymium Boride Thin Films for Potential Use in Brachytherapy

Here we show that the praseodymium N,N-dimethylaminodiboranate complex Pr(H₃BNMe₂BH₃)₃ and the 2,2,6,6-tetramethylheptane-3,5-dionate complex Pr(thd)₃ can serve as volatile carriers for ²²⁵Ac. The actinium coordination complexes Ac(H₃BNMe₂BH₃)₃ and Ac(thd)₃ are the likely species subliming with the carrier material. A sample of ²²⁵Ac-doped Pr(H₃BNMe₂BH₃)₃ was used to deposit amorphous ²²⁵Ac-doped praseodymium boride films on glass and Si(100) at 300 °C. The α emission spectra of the refractory films are well-resolved, suggesting that they could be used as radioactive implants for brachytherapy and related treatments.

Impact of metallic coating on the retention of 225Ac and its daugthers within core-shell nanocarriers

Currently, alpha-emitting radionuclide 225Ac is one of the most promising isotopes in alpha therapy due to its high linear energy transfer during four sequential alpha decays. However, the main obstacle preventing the full introduction of 225Ac into clinical practice is the lack of stable retention of radionuclides, leading to free circulation of toxic isotopes in the body. In this work, the surface of silica nanoparticles (SiO2 NPs) has been modified with metallic shells composed of titanium dioxide (TiO2) and gold (Au) nanostructures to improve the retention of 225Ac and its decay products within the developed nanocarriers. In vitro and in vivo studies in healthy mice show that the metallic surface coating of SiO2 NPs promotes an enhanced sequestering of radionuclides (225Ac and its daughter isotopes) compared to non-modified SiO2 NPs for a prolonged period of time. Histological analysis reveals that for the period of 3-10 d after the injections, the developed nanocarriers have no significant toxic effects in mice. At the same time, almost no accumulation of leaked radionuclides can be detected in non-target organs (e.g., in the kidneys). In contrast, non-modified carriers (SiO2 NPs) demonstrate the release of free radionuclides, which are distributed over the whole animal body with the consequent morphological changes in the lung, liver and kidney tissues. These results highlight the potential of the developed nanocarriers to be utilized as radionuclide delivery systems and offer an insight into design rules for the fabrication of new nanotherapeutic agents.

Radiolabeled Gold Nanoseeds Decorated with Substance P Peptides: Synthesis, Characterization and In Vitro Evaluation in Glioblastoma Cellular Models

Despite some progress, the overall survival of patients with glioblastoma (GBM) remains extremely poor. In this context, there is a pressing need to develop innovative therapy strategies for GBM, namely those based on nanomedicine approaches. Towards this goal, we have focused on nanoparticles (AuNP-SP and AuNP-SPTyr8) with a small gold core (ca. 4 nm), carrying DOTA chelators and substance P (SP) peptides. These new SP-containing AuNPs were characterized by a variety of analytical techniques, including TEM and DLS measurements and UV-vis and CD spectroscopy, which proved their high in vitro stability and poor tendency to interact with plasma proteins. Their labeling with diagnostic and therapeutic radionuclides was efficiently performed by DOTA complexation with the trivalent radiometals ⁶⁷Ga and ¹⁷⁷Lu or by electrophilic radioiodination with ¹²⁵I of the tyrosyl residue in AuNP-SPTyr8. Cellular studies of the resulting radiolabeled AuNPs in NKR1-positive GBM cells (U87, T98G and U373) have shown that the presence of the SP peptides has a crucial and positive impact on their internalization by the tumor cells. Consistently, ¹⁷⁷Lu-AuNP-SPTyr8 showed more pronounced radiobiological effects in U373 cells when compared with the non-targeted congener ¹⁷⁷Lu-AuNP-TDOTA, as assessed by cell viability and clonogenic assays and corroborated by Monte Carlo microdosimetry simulations.

Nano-hydroxyapatite as a delivery system: overview and advancements

Nano-hydroxyapatite is being investigated as vital components of implants and dental and tissue engineering devices. It is found as a bone replacement due to its non-toxicity and cytocompatibility with dental tissues and bone. The reality that nanocrystalline hydroxyapatite can be made of porous granules and scaffolds. Additionally, it has a massive loading potential indicating its use as a transporter for drugs or a regulated drug release mechanism in pharmaceutical research. This review aims to present existing nano-hydroxyapatite research developments as a drug carrier employed in bone tissue disorders locally and deliver poorly soluble drugs with reduced bioavailability. We have discussed the nano-hydroxyapatite role in the delivery of drugs (i.e. anti-resorptive, anti-cancer, and antibiotics), proteins, genetic material, and radionuclides.

Titanium and Iron Oxide Nanoparticles for Cancer Therapy: Surface Chemistry and Biological Implications

Currently, cancer is among the most challenging diseases due to its ability to continuously evolve into a more complex muldimentional system, in addition to its high capability to spread to other organs and tissues. In this context, the relevance of nanobiomaterials (NBMs) for the development of new more effective and less harmful treatments is increasing. NBMs provide the possibility of combining several functionalities on a single system, expectedly in a synergic way, to better perform the treatment and cure. However, the control of properties such as colloidal stability, circulation time, pharmacokinetics, and biodistribution, assuring the concentration in specific target tissues and organs, while keeping all desired properties, tends to be dependent on subtle changes in surface chemistry. Hence, the behavior of such materials in different media/environments is of uttermost relevance and concern since it can compromise their efficiency and safety on application. Given the bright perspectives, many efforts have been focused on the development of nanomaterials fulfilling the requirements for real application. These include robust and reproducible preparation methods to avoid aggregation while preserving the interaction properties. The possible impact of nanomaterials in different forms of diagnosis and therapy has been demonstrated in the past few years, given the perspectives on how revolutionary they can be in medicine and health. Considering the high biocompatibility and suitability, this review is focused on titanium dioxide– and iron oxide–based nanoagents highlighting the current trends and main advancements in the research for cancer therapies. The effects of phenomena, such as aggregation and agglomeration, the formation of the corona layer, and how they can compromise relevant properties of nanomaterials and their potential applicability, are also addressed. In short, this review summarizes the current understanding and perspectives on such smart nanobiomaterials for diagnostics, treatment, and theranostics of diseases.

Nanoradiopharmaceuticals Based on Alpha Emitters: Recent Developments for Medical Applications

The application of nanotechnology in nuclear medicine offers attractive therapeutic opportunities for the treatment of various diseases, including cancer. Indeed, nanoparticles-conjugated targeted alpha-particle therapy (TAT) would be ideal for localized cell killing due to high linear energy transfer and short ranges of alpha emitters. New approaches in radiolabeling are necessary because chemical radiolabeling techniques are rendered sub-optimal due to the presence of recoil energy generated by alpha decay, which causes chemical bonds to break. This review attempts to cover, in a concise fashion, various aspects of physics, radiobiology, and production of alpha emitters, as well as highlight the main problems they present, with possible new approaches to mitigate those problems. Special emphasis is placed on the strategies proposed for managing recoil energy. We will also provide an account of the recent studies in vitro and in vivo preclinical investigations of α-particle therapy delivered by various nanosystems from different materials, including inorganic nanoparticles, liposomes, and polymersomes, and some carbon-based systems are also summarized.

The Peptide Functionalized Inorganic Nanoparticles for Cancer-Related Bioanalytical and Biomedical Applications

In order to improve their bioapplications, inorganic nanoparticles (NPs) are usually functionalized with specific biomolecules. Peptides with short amino acid sequences have attracted great attention in the NP functionalization since they are easy to be synthesized on a large scale by the automatic synthesizer and can integrate various functionalities including specific biorecognition and therapeutic function into one sequence. Conjugation of peptides with NPs can generate novel theranostic/drug delivery nanosystems with active tumor targeting ability and efficient nanosensing platforms for sensitive detection of various analytes, such as heavy metallic ions and biomarkers. Massive studies demonstrate that applications of the peptide-NP bioconjugates can help to achieve the precise diagnosis and therapy of diseases. In particular, the peptide-NP bioconjugates show tremendous potential for development of effective anti-tumor nanomedicines. This review provides an overview of the effects of properties of peptide functionalized NPs on precise diagnostics and therapy of cancers through summarizing the recent publications on the applications of peptide-NP bioconjugates for biomarkers (antigens and enzymes) and carcinogens (e.g., heavy metallic ions) detection, drug delivery, and imaging-guided therapy. The current challenges and future prospects of the subject are also discussed.

Radiolabeling strategies and pharmacokinetic studies for metal based nanotheranostics

Radiolabeled metal-based nanoparticles (MNPs) have drawn considerable attention in the fields of nuclear medicine and molecular imaging, drug delivery, and radiation therapy, given the fact that they can be potentially used as diagnostic imaging and/or therapeutic agents, or even as theranostic combinations. Here, we present a systematic review on recent advances in the design and synthesis of MNPs with major focuses on their radiolabeling strategies and the determinants of their in vivo pharmacokinetics, and together how their intended applications would be impacted. For clarification, we categorize all reported radiolabeling strategies for MNPs into indirect and direct approaches. While indirect labeling simply refers to the use of bifunctional chelators or prosthetic groups conjugated to MNPs for post-synthesis labeling with radionuclides, we found that many practical direct labeling methodologies have been developed to incorporate radionuclides into the MNP core without using extra reagents, including chemisorption, radiochemical doping, hadronic bombardment, encapsulation, and isotope or cation exchange. From the perspective of practical use, a few relevant examples are presented and discussed in terms of their pros and cons. We further reviewed the determinants of in vivo pharmacokinetic parameters of MNPs, including factors influencing their in vivo absorption, distribution, metabolism, and elimination, and discussed the challenges and opportunities in the development of radiolabeled MNPs for in vivo biomedical applications. Taken together, we believe the cumulative advancement summarized in this review would provide a general guidance in the field for design and synthesis of radiolabeled MNPs towards practical realization of their much desired theranostic capabilities. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Diagnostic Nanodevices Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Tailoring the Radionuclide Encapsulation and Surface Chemistry of La(223Ra)VO4 Nanoparticles for Targeted Alpha Therapy

The development of targeted alpha therapy (TAT) as a viable cancer treatment requires innovative solutions to challenges associated with radionuclide retention to enhance local tumor cytotoxicity and to minimize off-target effects. Nanoparticles (NPs) with high encapsulation and high retention of radionuclides have shown potential in overcoming these issues. This article shows the influence of pH on the structure of lanthanum vanadate (LaVO4) NPs and its impact on the radiochemical yield of 223Ra and subsequent retention of its decay daughters, 211Pb and 211Bi. An acidic pH (4.9) results in a high fraction of La(223Ra)VO4 NPs with tetragonal structure (44.6–66.1%) and a 223Ra radiochemical yield <40%. Adjusting the pH to 11 yields >80% of La(223Ra)VO4 NPs with monoclinic structure and increases the 223Ra radiochemical yield >85%. The leakage of decay daughters from La(223Ra)VO4 NPs (pH 11) was <5% and <0.5% when exposed to deionized water and phosphate-buffered saline, respectively. Altering the surface chemistry of La(223Ra)VO4 NPs with carboxylate and phosphate compounds resulted in a threefold decrease in hydrodynamic diameter and a 223Ra radiochemical yield between 74.7% and 99.6%. These results show the importance of tailoring the synthesis parameters and surface chemistry of LaVO4 NPs to obtain high encapsulation and retention of radionuclides.

Alpha emitting nuclides for targeted therapy

Targeted alpha therapy (TAT) is an area of research with rapidly increasing importance as the emitted alpha particle has a significant effect on inducing cytotoxic effects on tumor cells while mitigating dose to normal tissues. Two significant isotopes of interest within the area of TAT are thorium-227 and actinium-225 due to their nuclear characteristics. Both isotopes have physical half-lives suitable for coordination with larger biomolecules, and additionally actinium-225 has potential to serve as an in vivo generator. In this review, the authors will discuss the production, purification, labeling reactions, and biological studies of actinium-225 and thorium-227 complexes and clinical studies.

Radiolabeling of Theranostic Nanosystems

In the recent years, progress in nanotechnology has significantly contributed to the development of novel pharmaceutical formulations to overcome the drawbacks of conventional treatments and improve the therapeutic outcome in many diseases, especially cancer. Nanoparticle vectors have demonstrated the potential to concomitantly deliver diagnostic and therapeutic payloads to diseased tissue. Due to their special physical and chemical properties, the characteristics and function of nanoparticles are tunable based on biological molecular targets and specific desired features (e.g., surface chemistry and diagnostic radioisotope labeling). Within the past decade, several theranostic nanoparticles have been developed as a multifunctional nanosystems which combine the diagnostic and therapeutic functionalities into a single drug delivery platform. Theranostic nanosystems can provide useful information on a real-time systemic distribution of the developed nanosystem and simultaneously transport the therapeutic payload. In general, the diagnostic functionality of theranostic nanoparticles can be achieved through labeling gamma-emitted radioactive isotopes on the surface of nanoparticles which facilitates noninvasive detection using nuclear molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT), meanwhile, the therapeutic effect arises from the potent drug released from the nanoparticle. Moreover, some radioisotopes can concurrently emit both gamma radiation and high-energy particles (e.g., alpha, beta, and Auger electrons), prompting the use either alone for radiotheranostics or synergistically with chemotherapy. This chapter provides an overview of the fundamentals of radiochemistry and relevant radiolabeling strategies for theranostic nanosystem development as well as the methods for the preclinical evaluation of radiolabeled nanoparticles. Furthermore, preclinical case studies of recently developed theranostic nanosystems will be highlighted.

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.

Encapsulation and retention of 225Ac, 223Ra, 227Th, and decay daughters in zircon-type gadolinium vanadate nanoparticles

Unwanted targeting of healthy organs caused by the relocation of radionuclides from the target site has been one of the limiting factors in the widespread application of targeted alpha therapy in patient regimens. GdVO4 nanoparticles (NPs) were developed as platforms to encapsulate α-emitting radionuclides 223Ra, 225Ac, and 227Th, and retain their decay daughters at the target site. Polycrystalline GdVO4 NPs with different morphologies and a zircon-type tetragonal crystal structure were obtained by precipitation of GdCl3 and Na3VO4 in aqueous media at room temperature. The ability of GdVO4 crystals to host multivalent ions was initially assessed using La, Cs, Bi, Ba, and Pb as surrogates of the radionuclides under investigation. A decrease in Ba encapsulation was obtained after increasing the concentration of surrogate ions, whereas the encapsulation of La cations in GdVO4 NPs was quantitative (∼100%). Retention of radionuclides was assessed in vitro by dialyzing the radioactive GdVO4 NPs against deionized water. While 227Th was quantitatively encapsulated (100%), a partial encapsulation of 223Ra (∼75%) and 225Ac (>60%) was observed in GdVO4 NPs. The maximum leakage of 221Fr (1st decay daughter of 225Ac) was 55.4 ± 3.6%, whereas for 223Ra (1st decay daughter of 227Th) the maximum leakage was 73.0 ± 4.0%. These results show the potential of GdVO4 NPs as platforms of α-emitting radionuclides for their application in targeted alpha therapy.

Hydroxyapatite and Titanium Dioxide Nanoparticles: Radiolabelling and In Vitro Stability of Prospective Theranostic Nanocarriers for 223Ra and 99mTc

Hydroxyapatite and titanium dioxide are widely used materials in a broad spectrum of branches. Due to their appropriate properties such as a large specific surface area, radiation stability or relatively low toxicity, they could be potentially used as nanocarriers for medicinal radionuclides for diagnostics and therapy. Two radiolabelling strategies of both nanomaterials were carried out by 99mTc for diagnostic purposes and by 223Ra for therapeutic purposes. The first one was the radionuclide sorption on ready-made nanoparticles and the second one was direct radionuclide incorporation into the structure of the nanoparticles. Achieved labelling yields were higher than 94% in all cases. Afterwards, in vitro stability tests were carried out in several solutions: physiological saline, bovine blood plasma, bovine blood serum, 1% and 5% human albumin solutions. In vitro stability studies were performed as short-term (59 h for 223Ra and 31 h for 99mTc) and long-term experiments (five half-lives of 223Ra, approx. 55 days). Both radiolabelled nanoparticles with 99mTc have shown similar released activities (about 20%) in all solutions. The best results were obtained for 223Ra radiolabelled titanium dioxide nanoparticles, where overall released activities were under 6% for 59 h study in all matrices and under 3% for 55 days in a long-term perspective.

Nanoparticles in Targeted Alpha Therapy

Recent advances in the field of nanotechnology application in nuclear medicine offer the promise of better therapeutic options. In recent years, increasing efforts have been made on developing nanoconstructs that can be used as carriers for immobilising alpha (α)-emitters in targeted drug delivery. In this publication, we provide a comprehensive overview of available information on functional nanomaterials for targeted alpha therapy. The first section describes why nanoconstructs are used for the synthesis of α-emitting radiopharmaceuticals. Next, we present the synthesis and summarise the recent studies demonstrating therapeutic applications of α-emitting labelled radiobioconjugates in targeted therapy. Finally, future prospects and the emerging possibility of therapeutic application of radiolabelled nanomaterials are discussed.

Quantitative encapsulation and retention of 227Th and decay daughters in core-shell lanthanum phosphate nanoparticles

Targeted alpha therapy (TAT) offers great promise for treating recalcitrant tumors and micrometastatic cancers. One drawback of TAT is the potential damage to normal tissues and organs due to the relocation of decay daughters from the treatment site. The present study evaluates La(227Th)PO4 core (C) and core +2 shells (C2S) nanoparticles (NPs) as a delivery platform of 227Th to minimize systemic distribution of decay daughters, 223Ra and 211Pb. In vitro retention of decay daughters within La(227Th)PO4 C NPs was influenced by the concentration of reagents used during synthesis, in which the leakage of 223Ra was between 0.4 ± 0.2% and 20.3 ± 1.1% in deionized water. Deposition of two nonradioactive LaPO4 shells onto La(227Th)PO4 C NPs increased the retention of decay daughters to >99.75%. The toxicity of the nonradioactive LaPO4 C and C2S NP delivery platforms was examined in a mammalian breast cancer cell line, BT-474. No significant decrease in cell viability was observed for a monolayer of BT-474 cells for NP concentrations below 233.9 μg mL-1, however cell viability decreased below 60% when BT-474 spheroids were incubated with either LaPO4 C or C2S NPs at concentrations exceeding 29.2 μg mL-1. La(227Th)PO4 C2S NPs exhibit a high encapsulation and in vitro retention of radionuclides with limited contribution to cellular cytotoxicity for TAT applications.

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

We report the structural properties of ultra-small ThO2 and UO2 nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO2 and UO2 NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO2 and UO2 materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th-O and U-O bond distances despite retaining average bond lengths that are characteristic of bulk ThO2 and UO2. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

Trastuzumab Conjugated Superparamagnetic Iron Oxide Nanoparticles Labeled with 225Ac as a Perspective Tool for Combined α-Radioimmunotherapy and Magnetic Hyperthermia of HER2-Positive Breast Cancer

It has been proven and confirmed in numerous repeated tests, that the use of a combination of several therapeutic methods gives much better treatment results than in the case of separate therapies. Particularly promising is the combination of ionizing radiation and magnetic hyperthermia in one drug. To achieve this objective, magnetite nanoparticles have been modified in their core with α emitter ²²⁵Ac, in an amount affecting only slightly their magnetic properties. By 3-phosphonopropionic acid (CEPA) linker nanoparticles were conjugated covalently with trastuzumab (Herceptin^®), a monoclonal antibody that recognizes ovarian and breast cancer cells overexpressing the HER2 receptors. The synthesized bioconjugates were characterized by transmission electron microscopy (TEM), Dynamic Light Scattering (DLS) measurement, thermogravimetric analysis (TGA) and application of ¹³¹I-labeled trastuzumab for quantification of the bound biomolecule. The obtained results show that one ²²⁵Ac@Fe₃O₄-CEPA-trastuzumab bioconjugate contains an average of 8–11 molecules of trastuzumab. The labeled nanoparticles almost quantitatively retain ²²⁵Ac (>98%) in phosphate-buffered saline (PBS) and physiological salt, and more than 90% of ²²¹Fr and ²¹³Bi over 10 days. In human serum after 10 days, the fraction of ²²⁵Ac released from ²²⁵Ac@Fe₃O₄ was still less than 2%, but the retention of ²²¹Fr and ²¹³Bi decreased to 70%. The synthesized ²²⁵Ac@Fe₃O₄-CEPA-trastuzumab bioconjugates have shown a high cytotoxic effect toward SKOV-3 ovarian cancer cells expressing HER2 receptor in-vitro. The in-vivo studies indicate that this bioconjugate exhibits properties suitable for the treatment of cancer cells by intratumoral or post-resection injection. The intravenous injection of the ²²⁵Ac@Fe₃O₄-CEPA-trastuzumab radiobioconjugate is excluded due to its high accumulation in the liver, lungs and spleen. Additionally, the high value of a specific absorption rate (SAR) allows its use in a new very perspective combination of α radionuclide therapy with magnetic hyperthermia.

A Proof-of-Concept Study on the Therapeutic Potential of Au Nanoparticles Radiolabeled with the Alpha-Emitter Actinium-225

Actinium-225 (²²⁵Ac) is receiving increased attention for its application in targeted radionuclide therapy, due to the short range of its emitted alpha particles in conjunction with their high linear energy transfer, which lead to the eradication of tumor cells while sparing neighboring healthy tissue. The objective of our study was the evaluation of a gold nanoparticle radiolabeled with ²²⁵Ac as an injectable radiopharmaceutical form of brachytherapy for local radiation treatment of cancer. Au@TADOTAGA was radiolabeled with ²²⁵Ac at pH 5.6 (30 min at 70 °C), and in vitro stability was evaluated. In vitro cytotoxicity was assessed in U-87 MG cancer cells, and in vivo biodistribution was performed by intravenous and intratumoral administration of [²²⁵Ac]²²⁵Ac-Au@TADOTAGA in U-87 MG tumor-bearing mice. A preliminary study to assess therapeutic efficacy of the intratumorally-injected radio-nanomedicine was performed over a period of 22 days, while the necrotic effect on tumors was evaluated by a histopathology study. We have shown that [²²⁵Ac]²²⁵Ac-Au@TADOTAGA resulted in the retardation of tumor growth after its intratumoral injection in U87MG tumor-bearing mice, even though very low activities were injected per mouse. This gold nanoparticle radiopharmaceutical could be applied as an unconventional brachytherapy in injectable form for local radiation treatment of cancer.

Study of 223Ra uptake mechanism on hydroxyapatite and titanium dioxide nanoparticles as a function of pH

The mechanism of 223Ra uptake on hydroxyapatite and titanium dioxide nanoparticles was studied as a function of pH. Both materials are widely used in food industry and medicine. They offer properties suitable for labelling with medicinal radionuclides, particularly for targeted radionuclide therapy. The selected isotope, 223Ra, is an alpha emitter widely used in targeted alpha particle therapy due to high-dose delivery in very small tissue volume, nevertheless the results are applicable for any radium isotope including 226Ra. The study was performed in the pH range 4.5 to 12 for hydroxyapatite nanoparticles and 2 to 12 for titanium dioxide nanoparticles in Britton-Robinson buffer solution. Both nanomaterials at pH 6 and higher showed that over 95% of the radium has been sorbed. According to the applied chemical equilibrium model, the most important species playing a role in sorption on the edge-sites were RaCO3, RaPO4 -, RaHPO4 and Ra(Ac-)2, and Ra2+ and RaH2PO4 + on layer-sites. All experiments were conducted under free air conditions and no negative impact of CO2 was found. The surface complexation model was found suitable for describing radium uptake by the studied hydroxyapatite and titanium dioxide nanomaterials.

Surface protolytic property characterization of hydroxyapatite and titanium dioxide nanoparticles

We provide characterization data of hydroxyapatite (nHAp) and titanium dioxide (nTiO2) nanoparticles as potential materials for ion sorption, e.g. in targeted therapy, barrier materials for waste repositories or photovoltaics. The study is focused on the determination of the values of protonation and ion exchange constants and site densities (∑SOH, ∑X; [mol kg-1]) of nTiO2 and nHAp for further Ra kinetics and sorption experiments. These data are very important for further investigation of the materials, which can be used e.g. as drug delivery systems or in engineered barriers of deep geological repositories. The characterization was based on the evaluation of the dependence of titrating agent consumption on pH. Titration results were evaluated on the basis of several model combinations, however the combination of the Chemical Equilibrium Model (CEM) and Ion Exchange Model (IExM) fits best to the experimental titration curves. However, the differences between the two sorbents were relatively large. Due to stability in a broad pH range and available surface sites, nTiO2 seems to have a wide application range. The applicability of nHAp is not so wide because of its dissolution under pH 5. Both sorbents are virtually able to sorb cationic species on deprotonated edge and layer sites with different capacities, which can be important for sorption and decontaminating applications.

Review of Therapeutic Applications of Radiolabeled Functional Nanomaterials

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

Actinium-225 for Targeted α Therapy: Coordination Chemistry and Current Chelation Approaches

The α-emitting radionuclide actinium-225 possesses nuclear properties that are highly promising for use in targeted α therapy (TAT), a therapeutic strategy that employs α particle emissions to destroy tumors. A key factor, however, that may hinder the clinical use of actinium-225 is the poor understanding of its coordination chemistry, which creates challenges for the development of suitable chelation strategies for this ion. In this article, we provide an overview of the known chemistry of actinium and a summary of the chelating agents that have been explored for use in actinium-225-based TAT. This overview provides a starting point for researchers in the field of TAT to gain an understanding of this valuable therapeutic radionuclide.

Multifunctional GdVO4:Eu core–shell nanoparticles containing 225Ac for targeted alpha therapy and molecular imaging

Development of actinium-225 doped Gd_0.8Eu_0.2VO₄ core–shell nanoparticles as multifunctional platforms for multimodal molecular imaging and targeted radionuclide therapy.