Nanoparticles in Targeted Alpha Therapy

Developments in radionanotheranostic strategies for precision diagnosis and treatment of prostate cancer

BACKGROUND

Prostate Cancer (PCa) is the second most diagnosed urological cancer among men worldwide. Conventional methods used for diagnosis of PCa have several pitfalls which include lack of sensitivity and specificity. On the other hand, traditional treatment of PCa poses challenges such as long-term side effects and the development of multidrug resistance (MDR).

MAIN BODY

Hence, there is a need for novel PCa agents with the potential to lessen the burden of these adverse effects on patients. Nanotechnology has emerged as a promising approach to support both early diagnosis and effective treatment of tumours by ensuring precise delivery of the drug to the targeted site of the disease. Most cancer-related biological processes occur on the nanoscale hence application of nanotechnology has been greatly appreciated and implemented in the management and therapeutics of cancer. Nuclear medicine plays a significant role in the non-invasive diagnosis and treatment of PCa using appropriate radiopharmaceuticals. This review aims to explore the different radiolabelled nanomaterials to enhance the specific delivery of imaging and therapeutic agents to cancer cells. Thereafter, the review appraises the advantages and disadvantages of these modalities and then discusses and outlines the benefits of radiolabelled nanomaterials in targeting cancerous prostatic tumours. Moreover, the nanoradiotheranostic approaches currently developed for PCa are discussed and finally the prospects of combining radiopharmaceuticals with nanotechnology in improving PCa outcomes will be highlighted.

CONCLUSION

Nanomaterials have great potential, but safety and biocompatibility issues remain. Notwithstanding, the combination of nanomaterials with radiotherapeutics may improve patient outcomes and quality of life.

Mechanical Detection of Nuclear Decays

We report the detection of individual nuclear α decays through the mechanical recoil of the entire micron-sized particle in which the decaying nuclei are embedded. Momentum conservation ensures that such measurements are sensitive to any particles emitted in the decay, including neutral particles that may otherwise evade detection with existing techniques. Detection of the minuscule recoil of an object more than 10^{12} times more massive than the emitted particles is made possible by recently developed techniques in levitated optomechanics, which enable high-precision optical control and measurement of the mechanical motion of optically trapped particles. Observation of a change in the net charge of the particle coincident with the recoil allows decays to be identified with background levels at the micro-Becquerel level. The techniques developed here may find use in fields ranging from nuclear forensics to dark matter and neutrino physics.

Preparation of Medical 228Th-224Ra Radionuclide Generator Based on SiO2@TiO2 Microspheres

224Ra (T1/2 = 3.63 d), an α-emitting radionuclide, holds significant promise in cancer endoradiotherapy. Current 224Ra-related therapy is still scarce because of the lack of reliable radionuclide supply. The 228Th-224Ra radionuclide generator can undoubtedly introduce continuous and sustainable availability of 224Ra for advanced nuclear medicine. However, conventional metal oxides for such radionuclide generators manifest suboptimal adsorption capacities for the parent nuclide, primarily attributable to their limited surface area. In this work, core-shell SiO2@TiO2 microspheres were proposed to develop as column materials for the construction of a 228Th-224Ra generator. SiO2@TiO2 microspheres were well prepared and systematically characterized, which has also been demonstrated to have good adsorption capacity to 228Th and very weak binding affinity toward 224Ra via simulated chemical separation. Upon introducing 228Th-containing solution onto the SiO2@TiO2 functional column, a 228Th-224Ra generator with excellent retention of the parent radionuclide and ideal elution efficiency of daughter radionuclide was obtained. The prepared 228Th-224Ra generator can produce 224Ra with high purity and medical usability in good elution efficiency (98.72%) even over five cycles. To the best of our knowledge, this is the first time that the core-shell mesoporous materials have been applied in a radionuclide generator, which can offer valuable insights for materials chemistry, radiochemical separation, and biological medicine.

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.

The Different Strategies for the Radiolabeling of [211At]-Astatinated Radiopharmaceuticals

Astatine-211 (211At) has emerged as a promising radionuclide for targeted alpha therapy of cancer by virtue of its favorable nuclear properties. However, the limited in vivo stability of 211At-labeled radiopharmaceuticals remains a major challenge. This review provides a comprehensive overview of the current strategies for 211At radiolabeling, including nucleophilic and electrophilic substitution reactions, as well as the recent advances in the development of novel bifunctional coupling agents and labeling approaches to enhance the stability of 211At-labeled compounds. The preclinical and clinical applications of 211At-labeled radiopharmaceuticals, including small molecules, peptides, and antibodies, are also discussed. Looking forward, the identification of new molecular targets, the optimization of 211At production and quality control methods, and the continued evaluation of 211At-labeled radiopharmaceuticals in preclinical and clinical settings will be the key to realizing the full potential of 211At-based targeted alpha therapy. With the growing interest and investment in this field, 211At-labeled radiopharmaceuticals are poised to play an increasingly important role in future cancer treatment.

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.

In vivo Biodistribution and Clearance of Magnetic Iron Oxide Nanoparticles for Medical Applications

Magnetic iron oxide nanoparticles (magnetite and maghemite) are intensively studied due to their broad potential applications in medical and biological sciences. Their unique properties, such as nanometric size, large specific surface area, and superparamagnetism, allow them to be used in targeted drug delivery and internal radiotherapy by targeting an external magnetic field. In addition, they are successfully used in magnetic resonance imaging (MRI), hyperthermia, and radiolabelling. The appropriate design of nanoparticles allows them to be delivered to the desired tissues and organs. The desired biodistribution of nanoparticles, eg, cancerous tumors, is increased using an external magnetic field. Thus, knowledge of the biodistribution of these nanoparticles is essential for medical applications. It allows for determining whether nanoparticles are captured by the desired organs or accumulated in other tissues, which may lead to potential toxicity. This review article presents the main organs where nanoparticles accumulate. The sites of their first uptake are usually the liver, spleen, and lymph nodes, but with the appropriate design of nanoparticles, they can also be accumulated in organs such as the lungs, heart, or brain. In addition, the review describes the factors affecting the biodistribution of nanoparticles, including their size, shape, surface charge, coating molecules, and route of administration. Modern techniques for determining nanoparticle accumulation sites and concentration in isolated tissues or the body in vivo are also presented.

Targeted Alpha Therapy (TAT) with Single-Domain Antibodies (Nanobodies)

The persistent threat of cancer necessitates the development of improved and more efficient therapeutic strategies that limit damage to healthy tissues. Targeted alpha therapy (TαT), a novel form of radioimmuno-therapy (RIT), utilizes a targeting vehicle, commonly antibodies, to deliver high-energy, but short-range, alpha-emitting particles specifically to cancer cells, thereby reducing toxicity to surrounding normal tissues. Although full-length antibodies are often employed as targeting vehicles for TαT, their high molecular weight and the presence of an Fc-region lead to a long blood half-life, increased bone marrow toxicity, and accumulation in other tissues such as the kidney, liver, and spleen. The discovery of single-domain antibodies (sdAbs), or nanobodies, naturally occurring in camelids and sharks, has introduced a novel antigen-specific vehicle for molecular imaging and TαT. Given that nanobodies are the smallest naturally occurring antigen-binding fragments, they exhibit shorter relative blood half-lives, enhanced tumor uptake, and equivalent or superior binding affinity and specificity. Nanobody technology could provide a viable solution for the off-target toxicity observed with full-length antibody-based TαT. Notably, the pharmacokinetic properties of nanobodies align better with the decay characteristics of many short-lived α-emitting radionuclides. This review aims to encapsulate recent advancements in the use of nanobodies as a vehicle for TαT.

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 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.

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.

Cubosomal Lipid Formulation for Combination Cancer Treatment: Delivery of a Chemotherapeutic Agent and Complexed α-Particle Emitter 213Bi

Here, we propose tailored lipid liquid-crystalline carriers (cubosomes), which incorporate an anticancer drug (doxorubicin) and complexed short-lived α-emitter (bismuth-213), as a strategy to obtain more effective action toward the cancer cells. Cubosomes were formulated with doxorubicin (DOX) and an amphiphilic ligand (DOTAGA-OA), which forms stable complexes with ²¹³Bi radionuclide. The behavior of DOX incorporated into the carrier together with the chelating agent was investigated, and the drug liberation profile was determined. The experiments revealed that the presence of the DOTAGA-OA ligand affects the activity of DOX when they are incorporated into the same carrier. This unexpected influence was explained based on the results of release studies, which proved the contribution of electrostatics in molecular interactions between the positively charged DOX and negatively charged DOTAGA-OA in acidic and neutral solutions. A significant decrease in the viability of HeLa cancer cells was achieved using sequential cell exposure: first to the radiolabeled cubosomes containing ²¹³Bi complex and next to DOX-doped cubosomes. Therefore, the sequential procedure for the delivery of both drugs encapsulated in cubosomes is suggested for further biological and in vivo studies.

Synthesis and Characterization of Magnetic Drug Carriers Modified with Tb3+ Ions

The study aimed to synthesize and characterize the magnetic drug carrier modified with terbium (III) ions. The addition of terbium extends the possibilities of their applications for targeted anticancer radiotherapy as well as for imaging techniques using radioisotopes emitting β⁺, β⁻, α, and γ radiation. The synthesis of iron oxide nanoparticles stabilized with citrates using the co-precipitation method (IONP @ CA) was carried out during the experimental work. The obtained nanoparticles were used to synthesize a conjugate containing terbium ions and guanosine-5′-monophosphate as an analog of drugs from the thiopurine group. Conjugates and their components were characterized using Transmission Electron Microscopy, infrared spectroscopy, X-ray microanalysis, spectrofluorimetry, and thermogravimetric analysis. The hybrid was also investigated with Langmuir layers to check the interaction with analogs of biological membranes.

Nanostructures as Radionuclide Carriers in Auger Electron Therapy

The concept of nanoparticle-mediated radionuclide delivery in the cancer treatment has been widely discussed in the past decade. In particular, the use of inorganic and organic nanostructures in the development of radiopharmaceuticals enables the delivery of medically important radioisotopes for radionuclide therapy. In this review, we present the development of nanostructures for cancer therapy with Auger electron radionuclides. Following that, different types of nanoconstructs that can be used as carriers for Auger electron emitters, design principles, nanoparticle materials, and target vectors that overcame the main difficulties are described. In addition, systems in which high-Z element nanoparticles are used as radionuclide carriers, causing the emission of photoelectrons from the nanoparticle surface, are presented. Finally, future research opportunities in the field are discussed as well as issues that must be addressed before nanoparticle-based Auger electron radionuclide therapy can be transferred to clinical use.

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.

Harnessing -Emitting Radionuclides for Therapy: Radiolabeling Method Review

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

Recent Advances in Brachytherapy Using Radioactive Nanoparticles: An Alternative to Seed-Based Brachytherapy

Interstitial brachytherapy (BT) is generally used for the treatment of well-confined solid tumors. One example of this is in the treatment of prostate tumors by permanent placement of radioactive seeds within the prostate gland, where low doses of radiation are delivered for several months. However, successful implementation of this technique is hampered due to several posttreatment adverse effects or symptoms and operational and logistical complications associated with it. Recently, with the advancements in nanotechnology, radioactive nanoparticles (radio-NPs) functionalized with tumor-specific biomolecules, injected intratumorally, have been reported as an alternative to seed-based BT. Successful treatment of solid tumors using radio-NPs has been reported in several preclinical studies, on both mice and canine models. In this article, we review the recent advancements in the synthesis and use of radio-NPs as a substitute to seed-based BT. Here, we discuss the limitations of current seed-based BT and advantages of radio-NPs for BT applications. Recent progress on the types of radio-NPs, their features, synthesis methods, and delivery techniques are discussed. The last part of the review focuses on the currently used dosimetry protocols and studies on the dosimetry of nanobrachytherapy applications using radio-NPs. The current challenges and future research directions on the role of radio-NPs in BT treatments are also discussed.

A Novel Single-Step-Labeled 212Pb-CaCO3 Microparticle for Internal Alpha Therapy: Preparation, Stability, and Preclinical Data from Mice

Lead-212 is recognized as a promising radionuclide for targeted alpha therapy for tumors. Many studies of ²¹²Pb-labeling of various biomolecules through bifunctional chelators have been conducted. Another approach to exploiting the cytotoxic effect is coupling the radionuclide to a microparticle acting as a carrier vehicle, which could be used for treating disseminated cancers in body cavities. Calcium carbonate may represent a suitable material, as it is biocompatible, biodegradable, and easy to synthesize. In this work, we explored ²¹²Pb-labeling of various CaCO₃ microparticles and developed a protocol that can be straightforwardly implemented by clinicians. Vaterite microparticles stabilized by pamidronate were effective as ²¹²Pb carriers; labeling yields of ≥98% were achieved, and ²¹²Pb was strongly retained by the particles in an in vitro stability assessment. Moreover, the amounts of ²¹²Pb reaching the kidneys, liver, spleen, and skeleton of mice following intraperitoneal (i.p.) administration were very low compared to i.p. injection of unbound ²¹²Pb²⁺, indicating that CaCO₃-bound ²¹²Pb exhibited stability when administered intraperitoneally. Therapeutic efficacy was observed in a model of i.p. ovarian cancer for all the tested doses, ranging from 63 to 430 kBq per mouse. Lead-212-labeled CaCO₃ microparticles represent a promising candidate for treating intracavitary cancers.

Obstacles and Recommendations for Clinical Translation of Nanoparticle System-Based Targeted Alpha-Particle Therapy

The rationale for application of nanotechnology in targeted alpha therapy (TAT) is sound. However, the translational strategy requires attention. Formulation of TAT in nanoparticulate drug delivery systems has the potential to resolve many of the issues currently experienced. As α-particle emitters are more cytotoxic compared to beta-minus-emitting agents, the results of poor biodistribution are more dangerous. Formulation in nanotechnology is also suggested to be the ideal solution for containing the recoil daughters emitted by actinium-225, radium-223, and thorium-227. Nanoparticle-based TAT is likely to increase stability, enhance radiation dosimetry profiles, and increase therapeutic efficacy. Unfortunately, nanoparticles have their own unique barriers towards clinical translation. A major obstacle is accumulation in critical organs such as the spleen, liver, and lungs. Furthermore, inflammation, necrosis, reactive oxidative species, and apoptosis are key mechanisms through which nanoparticle-mediated toxicity takes place. It is important at this stage of the technology's readiness level that focus is shifted to clinical translation. The relative scarcity of α-particle emitters also contributes to slow-moving research in the field of TAT nanotechnology. This review describes approaches and solutions which may overcome obstacles impeding nanoparticle-based TAT and enhance clinical translation. In addition, an in-depth discussion of relevant issues and a view on technical and regulatory barriers are presented.

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.

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

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

Trastuzumab Modified Barium Ferrite Magnetic Nanoparticles Labeled with Radium-223: A New Potential Radiobioconjugate for Alpha Radioimmunotherapy

Barium ferrite nanoparticles (BaFeNPs) were investigated as vehicles for 223Ra radionuclide in targeted α-therapy. BaFe nanoparticles were labeled using a hydrothermal Ba2+ cations replacement by 223Ra with yield reaching 61.3 ± 1.8%. Radiolabeled nanoparticles were functionalized with 3-phosphonopropionic acid (CEPA) linker followed by covalent conjugation to trastuzumab (Herceptin®). Thermogravimetric analysis and radiometric method with the use of [131I]-labeled trastuzumab revealed that on average 19-21 molecules of trastuzumab are attached to the surface of one BaFe-CEPA nanoparticle. The hydrodynamic diameter of BaFe-CEPA-trastuzumab conjugate is 99.9 ± 3.0 nm in water and increases to 218.3 ± 3.7 nm in PBS buffer, and the zeta potential varies from +27.2 ± 0.7 mV in water to -8.8 ± 0.7 in PBS buffer. The [223Ra]BaFe-CEPA-trastuzumab radiobioconjugate almost quantitatively retained 223Ra (>98%) and about 96% of 211Bi and 94% of 211Pb over 30 days. The obtained radiobioconjugate exhibited high affinity, cell internalization and cytotoxicity towards the human ovarian adenocarcinoma SKOV-3 cells overexpressing HER2 receptor. Confocal studies indicated that [223Ra]BaFe-CEPA-trastuzumab was located in peri-nuclear space. High cytotoxicity of the [223Ra]BaFe-CEPA-trastuzumab bioconjugate was confirmed by radiotoxicity studies on SKOV-3 cell monolayers and 3D-spheroids. In addition, the magnetic properties of the radiobioconjugate should allow for its use in guide drug delivery driven by magnetic field gradient.