Luminescent Alendronic Acid-Conjugated Micellar Nanostructures for Potential Application in the Bone-Targeted Delivery of Cholecalciferol

Vitamin D, an essential micronutrient crucial for skeletal integrity and various non-skeletal physiological functions, exhibits limited bioavailability and stability in vivo. This study is focused on the development of polyethylene glycol (PEG)-grafted phospholipid micellar nanostructures co-encapsulating vitamin D3 and conjugated with alendronic acid, aimed at active bone targeting. Furthermore, these nanostructures are rendered optically traceable in the UV-visible region of the electromagnetic spectrum via the simultaneous encapsulation of vitamin D3 with carbon dots, a newly emerging class of fluorescents, biocompatible nanoparticles characterized by their resistance to photobleaching and environmental friendliness, which hold promise for future in vitro bioimaging studies. A systematic investigation is conducted to optimize experimental parameters for the preparation of micellar nanostructures with an average hydrodynamic diameter below 200 nm, ensuring colloidal stability in physiological media while preserving the optical luminescent properties of the encapsulated carbon dots. Comprehensive chemical-physical characterization of these micellar nanostructures is performed employing optical and morphological techniques. Furthermore, their binding affinity for the principal inorganic constituent of bone tissue is assessed through a binding assay with hydroxyapatite nanoparticles, indicating significant potential for active bone-targeting. These formulated nanostructures hold promise for novel therapeutic interventions to address skeletal-related complications in cancer affected patients in the future.

High-Spatial-Resolution Benchtop X-ray Fluorescence Imaging through Bragg-Diffraction-Based Focusing with Bent Mosaic Graphite Crystals: A Simulation Study

X-ray fluorescence imaging (XFI) can localize diagnostic or theranostic entities utilizing nanoparticle (NP)-based probes at high resolution in vivo, in vitro, and ex vivo. However, small-animal benchtop XFI systems demonstrating high spatial resolution (variable from sub-millimeter to millimeter range) in vivo are still limited to lighter elements (i.e., atomic number Z≤45). This study investigates the feasibility of focusing hard X-rays from solid-target tubes using ellipsoidal lens systems composed of mosaic graphite crystals with the aim of enabling high-resolution in vivo XFI applications with mid-Z (42≤Z≤64) elements. Monte Carlo simulations are performed to characterize the proposed focusing-optics concept and provide quantitative predictions of the XFI sensitivity, in silico tumor-bearing mice models loaded with palladium (Pd) and barium (Ba) NPs. Based on simulation results, the minimum detectable total mass of PdNPs per scan position is expected to be on the order of a few hundred nanograms under in vivo conform conditions. PdNP masses as low as 150 ng to 50 ng could be detectable with a resolution of 600 μm when imaging abdominal tumor lesions across a range of low-dose (0.8 μGy) to high-dose (8 μGy) exposure scenarios. The proposed focusing-optics concept presents a potential step toward realizing XFI with conventional X-ray tubes for high-resolution applications involving interesting NP formulations.

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.

Equilibrium Thermodynamics of Macropa Complexes with Selected Metal Isotopes of Radiopharmaceutical Interest

To pursue the design of in vivo stable chelating systems for radiometals, a concise and straightforward method toolbox was developed combining NMR, isothermal titration calorimetry (ITC), and europium time-resolved laser-induced fluorescence spectroscopy (Eu-TRLFS). For this purpose, the macropa chelator was chosen, and Lu3+, La3+, Pb2+, Ra2+, and Ba2+ were chosen as radiopharmaceutically relevant metal ions. They differ in charge (2+ and 3+) and coordination properties (main group vs lanthanides). 1H NMR was used to determine four pKa values (±0.15; carboxylate functions, 2.40 and 3.13; amino functions, 6.80 and 7.73). Eu-TRLFS was used to validate the exclusive existence of the 1:1 Mn+/ligand complex in the chosen pH range at tracer level concentrations. ITC measurements were accomplished to determine the resulting stability constants of the desired complexes, with log K values ranging from 18.5 for the Pb-mcp complex to 7.3 for the Lu-mcp complex. Density-functional-theory-calculated structures nicely mirror the complexes' order of stabilities by bonding features. Radiolabeling with macropa using ligand concentrations from 10-3 to 10-6 M was accomplished by pointing out the complex formation and stability (212Pb > 133La > 131Ba ≈ 224Ra > 177Lu) by means of normal-phase thin-layer chromatography analyses.

The Curies' element: state of the art and perspectives on the use of radium in nuclear medicine

BACKGROUND

The alpha-emitter radium-223 (223Ra) is presently used in nuclear medicine for the palliative treatment of bone metastases from castration-resistant prostate cancer. This application arises from its advantageous decay properties and its intrinsic ability to accumulate in regions of high bone turnover when injected as a simple chloride salt. The commercial availability of [223Ra]RaCl2 as a registered drug (Xofigo®) is a further additional asset.

MAIN BODY

The prospect of extending the utility of 223Ra to targeted α-therapy of non-osseous cancers has garnered significant interest. Different methods, such as the use of bifunctional chelators and nanoparticles, have been explored to incorporate 223Ra in proper carriers designed to precisely target tumor sites. Nevertheless, the search for a suitable scaffold remains an ongoing challenge, impeding the diffusion of 223Ra-based radiopharmaceuticals.

CONCLUSION

This review offers a comprehensive overview of the current role of radium radioisotopes in nuclear medicine, with a specific focus on 223Ra. It also critically examines the endeavors conducted so far to develop constructs capable of incorporating 223Ra into cancer-targeting drugs. Particular emphasis is given to the chemical aspects aimed at providing molecular scaffolds for the bifunctional chelator approach.

Radiolabeled nanomaterials for biomedical applications: radiopharmacy in the era of nanotechnology

BACKGROUND

Recent advances in nanotechnology have offered new hope for cancer detection, prevention, and treatment. Nanomedicine, a term for the application of nanotechnology in medical and health fields, uses nanoparticles for several applications such as imaging, diagnostic, targeted cancer therapy, drug and gene delivery, tissue engineering, and theranostics.

RESULTS

Here, we overview the current state-of-the-art of radiolabeled nanoparticles for molecular imaging and radionuclide therapy. Nanostructured radiopharmaceuticals of technetium-99m, copper-64, lutetium-177, and radium-223 are discussed within the scope of this review article.

CONCLUSION

Nanoradiopharmaceuticals may lead to better development of theranostics inspired by ingenious delivery and imaging systems. Cancer nano-theranostics have the potential to lead the way to more specific and individualized cancer treatment.

The aqueous chemistry of radium

Available literature data on the aqueous chemistry of radium are compiled. There are limited available experimental data and a significant portion of the data has been estimated using electrostatic techniques, typically based on the corresponding data of barium. The available data are compared with the corresponding data of barium (and strontium) and a methodology for estimating additional radium thermochemical data is described.

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.

Calcium Carbonate Microparticles as Carriers of 224Ra: Impact of Specific Activity in Mice with Intraperitoneal Ovarian Cancer

BACKGROUND

Patients with advanced-stage ovarian cancer face a poor prognosis because of recurrent peritoneal cavity metastases following surgery and chemotherapy. Alpha-emitters may enable the efficient treatment of such disseminated diseases because of their short range and highly energetic radiation. Radium-224 is a candidate α-emitter due to its convenient 3.6-day half-life, with more than 90% of the decay energy originating from α-particles. However, its inherent skeletal accumulation must be overcome to facilitate intraperitoneal delivery of the radiation dose. Therefore, ²²⁴Ra-labeled CaCO₃ microparticles have been developed.

OBJECTIVE

The antitumor effect of CaCO₃ microparticles as a carrier for ²²⁴Ra was investigated, with an emphasis on the ratio of activity to mass dose of CaCO₃, that is, specific activity.

METHODS

Nude athymic mice were inoculated intraperitoneally with human ovarian cancer cells (ES-2) and treated with a single intraperitoneal injection of ²²⁴Ra-labeled CaCO₃ microparticles with varying combinations of mass and activity dose, or cationic ²²⁴Ra in solution. Survival and ascites volume at sacrifice were evaluated.

RESULTS

Significant therapeutic effect was achieved for all tested specific activities ranging from 0.4 to 4.6 kBq/mg. Although treatment with a mean activity dose of 1305 kBq/kg of cationic ²²⁴Ra prolonged the survival compared with the control, equivalent median survival could be achieved with ²²⁴Ra-labeled microparticles with a mean dose of only 420 kBq/kg. The best outcome was achieved with the highest specific activities (2.6 and 4.6 kBq/mg).

CONCLUSION

Radium-224-labeled CaCO₃ microparticles present a promising therapy against cancer dissemination in body cavities.

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 impact of barium isotopes in radiopharmacy and nuclear medicine - From past to presence

With the exception of beryllium, divalent cations of every alkaline earth metal are characterized by their calcimimetic behavior. Thus, in vivo biodistribution of these cations mostly occurs in form of a massive accumulation in bone tissues, consisting of hydroxyapatite to a major extent. Apart from the lightest elements beryllium and magnesium, animal studies and human studies regarding the overall in vivo behavior were carried out by using radioisotopes of the elements calcium, strontium, barium and radium. To date, only strontium with its radioisotopes and radium gained importance for applications in nuclear medicine, mainly for pain-reducing and palliative treatment of bone metastases. In contrast, barium radioisotopes can be ascertained as useful imaging agents and possible diagnostic analogues for theranostic approaches. This review focuses on the characteristic and chemical behavior of barium compounds, possible radioactive barium isotopes for future applications in nuclear medicine and radiopharmacy as well as recent results regarding barium-131 as diagnostic match for radium isotopes used in targeted alpha therapy.

Ra-224 activity, half-life, and 241 keV gamma ray absolute emission intensity: A NIST-NPL bilateral comparison

The national metrology institutes for the United Kingdom (UK) and the United States of America (USA) have compared activity standards for 224Ra, an α-particle emitter of interest as the basis for therapeutic radiopharmaceuticals. Solutions of 224RaCl2 were assayed by absolute methods, including digital coincidence counting and triple-to-double coincidence ratio liquid scintillation counting. Ionization chamber and high-purity germanium (HPGe) γ-ray spectrometry calibrations were compared; further, a solution was shipped between laboratories for a direct comparison by HPGe spectrometry. New determinations of the absolute emission intensity for the 241 keV γ ray (Iγ = 4.011(16) per 100 disintegrations of 224Ra) and of the 224Ra half-life (T1/2 = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.

Radiochemical separation of 224Ra from 232U and 228Th sources for 224Ra/212Pb/212Bi generator

The application of diagnostic and therapeutic radionuclides in nuclear medicine has grown significantly and has translated into the increased interest in radionuclide generators and their development. ²²⁴Ra and its shorter-lived daughters, ²¹²Pb and ²¹²Bi, are very interesting radionuclides from Targeted Alpha Therapy point of view for treatment of small cancers or metastatic forms. The purpose of the present work was to develop a simple generator for rapid elution of carrier-free ²²⁴Ra from ²³²U or ²²⁸Th sources by radiochemical separation based on extraction chromatography with the utilization of a home-made material. The bis(2-ethylhexyl) hydrogen phosphate (HDEHP) extractant was immobilized on polytetrafluroethylene (PTFE) grains and its ability to selectively adsorb ²³²U and ²²⁸Th, with simultaneous high elution recovery of ²²⁴Ra, was checked over few years. The ²²⁴Ra was quantitatively eluted with small volume (3-5 mL) of 0.1 M HNO₃ with low breakthrough (<0.005%) and was used for further milking of ²¹²Bi and ²¹²Pb from DOWEX 50WX12 by 0.75 M and 2.0 M HCl, respectively. The elaborated here methods allowed high recovery of ²²⁴Ra, ²¹²Pb and ²¹²Bi radionuclides and their application in radiolabeling of various biomolecules.

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.

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.

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.

Recent Insights in Barium-131 as a Diagnostic Match for Radium-223: Cyclotron Production, Separation, Radiolabeling, and Imaging

Barium-131 is a single photon emission computed tomography (SPECT)-compatible radionuclide for nuclear medicine and a promising diagnostic match for radium-223/-224. Herein, we report on the sufficient production route 133Cs(p,3n)131Ba by using 27.5 MeV proton beams. An average of 190 MBq barium-131 per irradiation was obtained. The SR Resin-based purification process led to barium-131 in high radiochemical purity. An isotopic impurity of 0.01% barium-133 was detectable. For the first time, radiolabeling of the ligand macropa with barium-131 was performed. Radiolabeling methods under mild conditions and reaction controls based on TLC systems were successfully applied. Small animal SPECT/ computed tomography (CT) measurements and biodistribution studies were performed using [131Ba]Ba(NO3)2 as reference and 131Ba-labeled macropa in healthy mice for the first time. Biodistribution studies revealed the expected rapid bone uptake of [131Ba]Ba2+, whereas 131Ba-labeled macropa showed a fast clearance from the blood, thereby showing a significantly (p < 0.001) lower accumulation in the bone. We conclude that barium-131 is a promising SPECT radionuclide and delivers appropriate imaging qualities in small animals. Furthermore, the relative stability of the 131Ba-labeled macropa complex in vivo forms the basis for the development of sufficient new chelators, especially for radium isotopes. Thereby, barium-131 will attain its goal as a diagnostic match to the alpha emitters radium-223 and radium-224.

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.

Sub-10 nm Radiolabeled Barium Sulfate Nanoparticles as Carriers for Theranostic Applications and Targeted Alpha Therapy

The treatment of cancer patients with α-particle-emitting therapeutics continues to gain in importance and relevance. The range of radiopharmaceutically relevant α-emitters is limited to a few radionuclides, as stable chelators or carrier systems for safe transport of the radioactive cargo are often lacking. Encapsulation of α-emitters into solid inorganic systems can help to diversify the portfolio of candidate radionuclides, provided, that these nanomaterials effectively retain both the parent and the recoil daughters. We therefore focus on designing stable and defined nanocarrier-based systems for various clinically relevant radionuclides, including the promising α-emitting radionuclide 224Ra. Hence, sub-10 nm barium sulfate nanocontainers were prepared and different radiometals like 89Zr, 111In, 131Ba, 177Lu or 224Ra were incorporated. Our system shows stabilities of >90 % regarding the radiometal release from the BaSO4 matrix. Furthermore, we confirm the presence of surface-exposed amine functionalities as well as the formation of a biomolecular corona.

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.

A 224Ra-labeled polyoxopalladate as a putative radiopharmaceutical

Despite their attractive properties, internal targeted alpha therapies using 223/224Ra are limited to bone-seeking applications. As there is no suitable chelator available, the search for new carriers to stably bind Ra2+ and to connect it to biological target molecules is necessary. Polyoxopalladates represent a class of compounds where Ra2+ can be easily introduced into the Pd-POM core during a facile one-pot preparation. Due to the formation of a protein corona, the connection to other targeting (bio)macromolecules is possible.