Intratumoral treatment with radioactive beta-emitting microparticles: a systematic review

Labelling of micro- and nanoplastics for environmental studies: state-of-the-art and future challenges

Studying microplastics and nanoplastics (MNP) in environmental matrices is extremely challenging, and recent developments in labelling techniques may hold much promise to further our knowledge in this field. Here, we reviewed MNP labelling techniques and applications to provide the first systematic and in-depth insight into MNP labelling. We classified all labelling techniques for MNP into four main types (fluorescent, metal, stable isotope and radioisotope) and discussed per type the synthesis methods, detection methods, influencing factors, and the current and future applications and challenges. Direct labelling of environmental MNP with fluorescent dyes and metals enables simple visualisation and selective detection of MNP to improve detection efficiency. However, it is still an open question how to avoid co-labelling of non-plastic (i.e. non-target, matrix) materials. Labelling of MNP that are intentionally added in the environment may allow semi-automatic detection of MNP particles with high accuracy and sensitivity during studies on e.g. transport and degradation. The detection limit of labelled MNP largely depends on particle size and the type of matrix. Fluorescent labelling allows efficient detection of microplastics, whereas metal labelling is preferred for nanoplastics research due to a potentially higher sensitivity. A major challenge for fluorescent and metal labelling is to develop techniques that do not alter the inherent MNP properties or only do so minimally, in particular the surface properties. Stable and radioactive isotope labelling (13C and 14C, but also 15N, 2H) of the polymer itself allows to preserve inherent MNP properties, but have been largely ignored. Overall, labelling of MNP holds great promise for advancing our fundamental understanding of the behaviour of plastics, notably the smallest fractions, in the environment.

Effective therapy with Bismuth-212 labeled macroaggregated albumin in orthotopic mouse breast tumor models

Intravascularly administered radiation therapy using beta (β-)-emitting radioisotopes has relied on either intravenously injected radiolabeled peptides that target cancer or radiolabeled microspheres that are trapped in the tumor following intra-arterial delivery. More recently, targeted intravenous radiopeptide therapies have explored the use of alpha (α)-particle emitting radioisotopes, but microspheres radiolabeled with α-particle emitters have not yet been studied. Here, FDA-approved macroaggregated albumin (MAA) particles were radiolabeled with Bismuth-212 (Bi-212-MAA) and evaluated using clonogenic and survival assays in vitro and using immune-competent mouse models of breast cancer. The in vivo biodistribution of Bi-212-MAA was investigated in Balb/c and C57BL/6 mice with 4T1 and EO771 orthotopic breast tumors, respectively. The same orthotopic breast cancer models were used to evaluate the treatment efficacy of Bi-212-MAA. Our results showed that macroaggregated albumin can be stably radiolabeled with Bi-212 and that Bi-212-MAA can deliver significant radiation therapy to reduce the growth and clonogenic potential of 4T1 and EO771 cells in vitro. Additionally, Bi-212-MAA treatment upregulated γH2AX and cleaved Caspase-3 expression in 4T1 cells. Biodistribution analyses showed 87-93% of the Bi-212-MAA remained in 4T1 and EO771 tumors 2 and 4 h after injection. Following single-tumor treatments with Bi-212-MAA there was a significant reduction in the growth of both 4T1 and EO771 breast tumors over the 18-day monitoring period. Overall, these findings showed that Bi-212-MAA was stably radiolabeled and inhibited breast cancer growth. Bi-212-MAA is an exciting platform to study α-particle therapy and will be easily translatable to larger animal models and human clinical trials.

Intra-Arterial Delivery of Radiopharmaceuticals in Oncology: Current Trends and the Future of Alpha-Particle Therapeutics

A paradigm shift is underway in cancer diagnosis and therapy using radioactivity-based agents called radiopharmaceuticals. In the new strategy, diagnostic imaging measures the tumor uptake of radioactive agent “X” in a patient’s specific cancer, and if uptake metrics are realized, the patient can be selected for therapy with radioactive agent “Y”. The X and Y represent different radioisotopes that are optimized for each application. X–Y pairs are known as radiotheranostics, with the currently approved route of therapy being intravenous administration. The field is now evaluating the potential of intra-arterial dosing of radiotheranostics. In this manner, a higher initial concentration can be achieved at the cancer site, which could potentially enhance tumor-to-background targeting and lead to improved imaging and therapy. Numerous clinical trials are underway to evaluate these new therapeutic approaches that can be performed via interventional radiology. Of further interest is changing the therapeutic radioisotope that provides radiation therapy by β- emission to radioisotopes that also decay by α-particle emissions. Alpha (α)-particle emissions provide high energy transfer to the tumors and have distinct advantages. This review discusses the current landscape of intra-arterially delivered radiopharmaceuticals and the future of α-particle therapy with short-lived radioisotopes.

Current and upcoming radionuclide therapies in the direction of precision oncology: A narrative review

As new molecular tracers are identified to target specific receptors, tissue, and tumor types, opportunities arise for the development of both diagnostic tracers and their therapeutic counterparts, termed "theranostics." While diagnostic tracers utilize positron emitters or gamma-emitting radionuclides, their theranostic counterparts are typically bound to beta and alpha emitters, which can deliver specific and localized radiation to targets with minimal collateral damage to uninvolved surrounding structures. This is an exciting time in molecular imaging and therapy and a step towards personalized and precise medicine in which patients who were either without treatment options or not candidates for other therapies now have expanded options, with tangible data showing improved outcomes. This manuscript explores the current state of theranostics, providing background, treatment specifics, and toxicities, and discusses future potential trends.

Estimation of the Internal Dose Imparted by 18F-Fluorodeoxyglucose to Tissues by Using Fricke Dosimetry in a Phantom and Positron Emission Tomography

Purpose

Assessment of the radiation dose delivered to a tumor and different organs is a major issue when using radiolabelled compounds for diagnostic imaging or endoradiotherapy. The present article reports on a study to correlate the mean 18F-fluorodeoxyglucose (18F-FDG) activity in different tissues measured in a mouse model by positron emission tomography (PET) imaging, with the dose assessed in vitro by Fricke dosimetry.

Methods

The dose-response relationship of the Fricke dosimeter and PET data was determined at different times after adding 18F-FDG (0-80 MBq) to a Fricke solution (1 mM ferrous ammonium sulfate in 0.4 M sulfuric acid). The total dose was assessed at 24 h (~13 half-lives of 18F-FDG). The number of coincident events produced in 3 mL of Fricke solution or 3 mL of deionized water that contained 60 MBq of 18F-FDG was measured using the Triumph/LabPET8TM preclinical PET/CT scanner. The total activity concentration measured by PET was correlated with the calculated dose from the Fricke dosimeter, at any exposure activity of 18F-FDG.

Results

The radiation dose measured with the Fricke dosimeter increased rapidly during the first 4 h after adding 18F-FDG and then gradually reached a plateau. Presence of non-radioactive-FDG did not alter the Fricke dosimetry. The characteristic responses of the dosimeter and PET imaging clearly exhibit linearity with injected activity of 18F-FDG. The dose (Gy) to time-integrated activity (MBq.h) relationship was measured, yielding a conversion factor of 0.064 ± 0.06 Gy/MBq.h in the present mouse model. This correlation provides an efficient alternative method to measure, three-dimensionally, the total and regional dose absorbed from 18F-radiotracers.

Conclusions

The Fricke dosimeter can be used to calibrate a PET scanner, thus enabling the determination of dose from the measured radioactivity emitted by 18F-FDG in tissues. The method should be applicable to radiotracers with other positron-emitting radionuclides.

Hybrid theranostic microbubbles for ultrasound/photoacoustic imaging guided starvation/low-temperature photothermal/hypoxia-activated synergistic cancer therapy

Constructing a theranostic agent for high-contrast multimodality imaging-guided synergistic therapy with long-term tumor retention and minimum systemic side effects still remains a major challenge. Herein, a hybrid microbubble-based theranostic platform was developed for dual-modality ultrasound (US) and enhanced photoacoustic (PA) imaging-guided synergistic tumor therapy by combining starvation therapy, low-temperature photothermal therapy (PTT), and hypoxia-activated therapy, based on polydopamine (PDA) doped poly(vinyl alcohol) microbubbles loaded with glucose oxidase (GOx) (PDA-PVAMBs@GOx) and hypoxia-activated prodrug (HAP) tirapazamine (TPZ). For dual-modality US/enhanced PA imaging, PDA-PVAMBs provided 6.5-fold amplified PA signals relative to freely dispersed PDA nanoparticles (PDA NPs). For synergistic cancer therapy, oxygen (O₂) carried by PDA-PVAMBs@GOx was first released to promote starvation therapy by loaded GOx. Then, moderate near-infrared (NIR) laser irradiation triggered PTT and improved enzymatic activity of GOx with its optimal activity around 47 °C. Subsequently, GOx-mediated tumor starvation depleted O₂ and exacerbated the hypoxia environment, thereby activating the toxicity of TPZ in the tumor site. Through dual-modality US/PA imaging monitoring, PDA-PVAMBs@GOx with long-term retention (∼7 days) combined with PTT and TPZ significantly inhibited the growth of solid tumors with minimum systemic side effects, which might be a powerful tool for effective tumor treatment.

Evaluation of Long-Term Outcomes and Toxicity After Stereotactic Phosphorus-32-Based Intracavitary Brachytherapy in Patients With Cystic Craniopharyngioma

PURPOSE

Interstitial brachytherapy based on phosphorus-32 (P-32) has an established role as a minimally invasive treatment modality for patients with cystic craniopharyngioma. However, reporting on long-term outcomes with toxicity profiles for large cohorts is lacking in the literature. The purpose of this study is therefore to evaluate the long-term visual, endocrinal, and neurocognitive functions in what is the largest patient series having received this treatment to date.

METHODS AND MATERIALS

We retrospectively evaluated 90 patients with cystic craniopharyngiomas who were treated with stereotactic intracavitary brachytherapy between 1998 and 2010. Colloidal activity of injected radioisotope P-32 was based on an even distribution within the tumor. After treatment, patients were followed-up for a minimum of 5 years and over a mean of 121 months (60-192 months) to assess radiographic and clinical responses.

RESULTS

The 90 patients included in our study cohort underwent a total of 108 stereotactic surgical procedures for 129 craniopharyngioma-related cysts. Of the included tumors, 65 (72.2%) were associated with a single cyst, 15 (16.7%) were associated with 2 cysts, and 10 (11.1%) tumors had developed septations with 3 to 4 cysts. Stereotactic cyst puncture and content aspiration were used to drain a mean cyst fluid volume of 21.4 mL (1.0-55.0 mL). Each cyst was then instilled for interstitial brachytherapy with colloidal P-32 solution. Based on radiographic follow-up assessments, 56 cysts (43.4%) showed resolution and/or nonrecurrence, which was classified as a complete response to treatment; 47 cysts (36.4%) showed a partial response; and 5 cysts (3.9%) displayed a stable appearance. Treatment resulted in immediate and clinically significant vision improvement in 54 of 63 (86%) symptomatic patients, and this improvement was maintained. Progression-free survival rates at 5 and 10 years were 95.5% and 84.4%, respectively.

CONCLUSIONS

P-32-based interstitial brachytherapy can play an effective role in managing patients with cystic craniopharyngiomas. It can be considered a valid alternative to surgery in select patients with a favorable toxicity profile and long-term clinical outcomes.

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

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

Injectable biomaterials for treatment of glioblastoma

Despite ongoing advancements in the field of medicine, glioblastoma multiforme (GBM) is presently incurable, making this advanced brain tumor the deadliest tumor type in the central nervous system. The primary treatment strategies for GBM (i.e. surgical resection, radiation therapy, chemotherapy, and newly incorporated targeted therapies) fail to overcome the challenging characteristics of highly aggressive GBM tumors and are presently given with the goal of increasing the quality of life for patients. With the aim of creating effective treatment solutions, research has shifted toward utilizing injectable biomaterial adjuncts to minimize invasiveness of treatment, provide spatiotemporal control of therapeutic delivery, and engage with cells through material-cell interfaces. This review aims to summarize the limitations of the current standard of care for GBM, discuss how these limitations can be addressed by local employment of injectable biomaterial systems, and highlight developments in the field of biomaterials for these applications.

Emerging technologies for local cancer treatment

The fundamental limitations of systemic therapeutic administration have prompted the development of local drug delivery platforms as a solution to increase effectiveness and reduce side effects. By confining therapeutics to the site of disease, local delivery technologies can enhance therapeutic index. This review highlights recent advances and opportunities in local drug delivery strategies for cancer treatment in addition to challenges that need to be addressed to facilitate clinical translation. The benefits of local cancer treatment combined with technological advancements and increased understanding of the tumor microenvironment, present a prime breakthrough opportunity for safer and more effective therapies.

Characterization of holmium(iii)-acetylacetonate complexes derived from therapeutic microspheres by infrared ion spectroscopy

Microspheres containing radioactive ¹⁶⁶holmium-acetylacetonate are employed in emerging radionuclide therapies for the treatment of malignancies. At the molecular level, details on the coordination geometries of the Ho complexes are however elusive. Infrared ion spectroscopy (IRIS) was used to characterize several ¹⁶⁵Ho-acetylacetonate complexes derived from non-radioactive microspheres. The coordination geometry of four distinct ionic complexes were fully assigned by comparison of their measured IR spectra with spectra calculated at the density functional theory (DFT) level. The coordination of each acetylacetonate ligand is dependent on the presence of other ligands, revealing an asymmetric chelation motif in some of the complexes. A fifth, previously unknown constituent of the microspheres was identified as a coordination complex containing an acetic acid ligand. These results pave the way for IRIS-based identification of microsphere constituents upon neutron activation of the metal center.

Human intratumoral therapy: Linking drug properties and tumor transport of drugs in clinical trials

Cancer therapies aim to kill tumor cells directly or engage the immune system to fight malignancy. Checkpoint inhibitors, oncolytic viruses, cell-based immunotherapies, cytokines, and adjuvants have been applied to prompt the immune system to recognize and attack cancer cells. However, systemic exposure of cancer therapies can induce unwanted adverse events. Intratumoral administration of potent therapies utilizes small amounts of drugs, in an effort to minimize systemic exposure and off-target toxicities. Here, we discuss the properties of the tumor microenvironment and transport considerations for intratumoral drug delivery. Specifically, we consider various tumor tissue factors and physicochemical factors that can affect tumor retention after intratumoral injection. We also review approved and clinical-stage intratumoral therapies and consider how the molecular and biophysical properties (e.g. size and charge) of these therapies influences intratumoral transport (e.g. tumor retention and cellular uptake). Finally, we offer a critical review and highlight several emerging approaches to promote tumor retention and limit systemic exposure of potent intratumoral therapies.

Feasibility of CT quantification of intratumoural 166Ho-microspheres

Background

Microspheres loaded with radioactive ¹⁶⁶Ho (¹⁶⁶Ho-MS) are novel particles for radioembolisation and intratumoural treatment. Because of the limited penetration of β radiation, quantitative imaging of microsphere distribution is crucial for optimal intratumoural treatment. Computed tomography (CT) may provide high-resolution and fast imaging of the distribution of these microspheres, with lower costs and widespread availability in comparison with current standard single-photon emission tomography (SPECT) and magnetic resonance imaging. This phantom study investigated the feasibility of CT quantification of ¹⁶⁶Ho-MS.

Methods

CT quantification was performed on a phantom with various concentrations of HoCl and Ho-MS to investigate the CT sensitivity and calibrate the CT recovery. ¹⁶⁶Ho-MS were injected into ex vivo tissues, in VX-2 cancer-bearing rabbits, and in patients with head-neck cancer, to demonstrate sensitivity and clinical visibility. The amount of Ho-MS was determined by CT scanning, using a density-based threshold method and compared with a validated ¹⁶⁶Ho SPECT quantification method.

Results

In the phantom, a near perfect linearity (least squares R² > 0.99) between HU values and concentration of ¹⁶⁶Ho was found. Ex vivo tissue experiments showed an excellent correlation (r = 0.99, p < 0.01) between the dose calibrator, SPECT, and CT imaging. CT recovery was on average 86.4% ex vivo, 76.0% in rabbits, and 99.1% in humans.

Conclusion

This study showed that CT-based quantification of Ho microspheres is feasible and is a high-resolution alternative to SPECT-based determination of their local distribution.

Fluorescent hollow mesoporous carbon spheres for drug loading and tumor treatment through 980-nm laser and microwave co-irradiation

Hollow mesoporous particles for drug delivery and cancer therapy have attracted significant attention over recent decades. Here, we develop a simple and highly efficient strategy for preparing fluorescent hollow mesoporous carbon spheres (HMCSs). Compared with typical carbon materials such as fullerene C60, carbon nanotubes, reduced graphene oxide, and carbon nanohorns; HMCSs showed fewer effects on cell cycle distribution and lower toxicity to cells. Ten different drugs were incorporated into the HMCSs, and the maximum loading efficiency reached 42.79 ± 2.7%. Importantly, microwaves were found to improve the photothermal effect generated by HMCSs when combined with 980-nm laser irradiation. The cell killing and tumor growth inhibition efficiencies of HMCSs and drug-loaded HMCSs under co-irradiation with laser and microwaves were significantly improved compared with those under laser irradiation alone. After local administration HMCSs were only distributed in tissue at the injection site. HMCSs showed almost no toxicity in mice after local injection and could be completely removed from the injection site.

Biocompatible chitosan-carbon nanocage hybrids for sustained drug release and highly efficient laser and microwave co-irradiation induced cancer therapy

Graphitic carbon nanocages (GCNCs) are unique graphene-based nanomaterials that can be used for cancer photothermal therapy (PTT). However, low toxicity GCNC-based organic/inorganic hybrid biomaterials for microwave irradiation assisted PTT have not yet been reported. In the present study, chitosan (CS)-coated GCNCs (CS-GCNCs) loaded with 5-fluorouracil (5Fu) were used for cancer therapy when activated by 808-nm laser and microwave co-irradiation. The cytotoxicity of GCNCs was significantly reduced after coating with CS. For example, fewer cell-cycle defects were caused by CS-GCNCs in comparison with non-coated GCNCs. The release rate of 5Fu from CS-GCNCs was significantly slower than that of 5Fu from GCNCs, providing sustained release. The release rate could be accelerated by 808-nm laser and microwave co-irradiation. The 5Fu in CS-GCNCs retained high cancer cell killing bioactivity by enhancing the caspase-3 expression level. The cancer cell killing and tumor inhibition efficiencies of the 5Fu-loaded nanomaterials increased significantly under 808-nm laser and microwave co-irradiation. The strong cell killing and tumor ablation activities were due to the synergy of the enhanced GCNC thermal effect caused by laser and microwave co-irradiation and the chemotherapy of 5Fu. Our research opens a door for the development of drug-loaded GCNC-based nano-biomaterials for chemo-photothermal synergistic therapy with the assistance of microwave irradiation. STATEMENT OF SIGNIFICANCE: Graphitic carbon nanocages (GCNCs) are graphene-based nanomaterials that can be used for both drug loading and cancer photothermal therapy (PTT). Herein, we showed that chitosan (CS)-GCNCs hybrid biomaterials had very low cytotoxicity, high ability for loading drug, and exhibited sustained drug release. In particular, although low-power microwaves alone are unable to trigger cancer cell damage by GCNCs, the cell killing and mouse tumor inhibition efficiencies were significantly improved by near-infrared (NIR) laser and microwave co-irradiation compared with laser-triggered PTT alone. This combined use of laser and microwave co-irradiation promises essential therapeutic modality and opens a new avenue for PTT.

Preparation and characterization of inorganic radioactive holmium-166 microspheres for internal radionuclide therapy

Microspheres with high specific activities of radionuclides are very interesting for internal radiotherapy treatments. This work focuses on the formulation and characterization of inorganic microspheres with a high content of holmium and therefore a high specific radioactivity of holmium-166. Two novel formulations of inorganic microspheres were obtained by dispersing solid holmium acetylacetonate microspheres (Ho₂(AcAc)₃-ms) in NaH₂PO₄ or NaOH solutions followed by 2 h incubation at room temperature. By exchange of acetylacetonate with phosphate or hydroxyl ions, holmium phosphate microspheres (HoPO₄-ms) and holmium hydroxide microspheres (Ho(OH)₃-ms) were formed respectively. The inorganic microspheres had a significantly smaller diameter (28.5 ± 4.4 μm (HoPO₄-ms) and 25.1 ± 3.5 μm (Ho(OH)₃-ms)) than those of Ho₂(AcAc)₃-ms (32.6 ± 5.2 μm). The weight percentage of holmium-165 in the microspheres increased significantly from 47% (Ho₂(AcAc)₃-ms) to 55% (HoPO₄-ms) and 73% (Ho(OH)₃-ms). After preparation of both HoPO₄-ms and Ho(OH)₃-ms, the stable holmium-165 isotope was partly converted by neutron activation into radioactive holmium-166 to yield radioactive microspheres. High specific activities were achieved ranging from 21.7 to 59.9 MBq/mg (¹⁶⁶HoPO₄-ms) and from 28.8 to 79.9 MBq/mg (¹⁶⁶Ho(OH)₃-ms) depending on the neutron activation time. The structure of both microspheres was preserved up to neutron activations of 6 h in a thermal neutron flux of 4.72 × 10¹⁶ n m^-2 s^-1. After activation, both microspheres revealed excellent stability in administration fluids (saline and phosphate buffer) having less than 0.05% of holmium released after 72 h incubation. Finally, the hemocompatibility of these inorganic microspheres was evaluated and it was shown that the microspheres did cause neither hemolysis nor depletion or inhibition of the coagulation factors of the intrinsic blood coagulation pathway meaning that the microspheres have a good hemocompatibility. Overall, this work shows that radioactive inorganic microspheres with high specific activities of holmium-166 can be prepared which potentially can be used for internal radionuclide therapy.

Cachexia induced by Yoshida ascites hepatoma in Wistar rats is not associated with inflammatory response in the spleen or brain

Recent data indicate that peripheral, as well as hypothalamic pro-inflammatory cytokines play an important role in the development of cancer cachexia. However, there are only a few studies simultaneously investigating the expression of inflammatory molecules in both the periphery and hypothalamic structures in animal models of cancer cachexia. Therefore, using the Yoshida ascites hepatoma rat's model of cancer cachexia we investigated the gene expression of inflammatory markers in the spleen along with the paraventricular and arcuate nuclei, two hypothalamic structures that are involved in regulating energy balance. In addition, we investigated the effect of intracerebroventricular administration of PS-1145 dihydrochloride (an Ikβ inhibitor) on the expression of selected inflammatory molecules in these hypothalamic nuclei and spleen. We observed significantly reduced food intake in tumor-bearing rats. Moreover, we found significantly decreased expression of IL-6 in the spleen as well as decreased NF-κB in the paraventricular nucleus of rats with Yoshida ascites hepatoma. Similarly, expression of TNF-α, IL-1β, NF-κB, and COX-2 in the arcuate nucleus was significantly reduced in tumor-bearing rats. Administration of PS-1145 dihydrochloride reduced only the gene expression of COX-2 in the hypothalamus. Based on our findings, we suggest that the growing Yoshida ascites hepatoma decreased food intake by mechanical compression of the gut and therefore this model is not suitable for investigation of the inflammation-related mechanisms of cancer cachexia development.

The various therapeutic applications of the medical isotope holmium-166: a narrative review

Over the years, a broad spectrum of applications of the radionuclide holmium-166 as a medical isotope has been established. The isotope holmium-166 is attractive as it emits high-energy beta radiation which can be used for a therapeutic effect and gamma radiation which can be used for nuclear imaging purposes. Furthermore, holmium-165 can be visualized by MRI because of its paramagnetic properties and by CT because of its high density. Since holmium-165 has a natural abundance of 100%, the only by-product is metastable holmium-166 and no costly chemical purification steps are necessary for production of nuclear reactor derived holmium-166. Several compounds labelled with holmium-166 are now used in patients, such Ho¹⁶⁶-labelled microspheres for liver malignancies, Ho¹⁶⁶-labelled chitosan for hepatocellular carcinoma (HCC) and [¹⁶⁶Ho]Ho DOTMP for bone metastases. The outcomes in patients are very promising, making this isotope more and more interesting for applications in interventional oncology. Both drugs as well as medical devices labelled with radioactive holmium are used for internal radiotherapy. One of the treatment possibilities is direct intratumoural treatment, in which the radioactive compound is injected with a needle directly into the tumour. Numerous other applications have been developed, like patches for treatment of skin cancer and holmium labelled antibodies and peptides. The second major application that is currently clinically applied is selective internal radiation therapy (SIRT, also called radioembolization), a novel treatment option for liver malignancies. This review discusses medical drugs and medical devices based on the therapeutic radionuclide holmium-166.

Radioactive holmium phosphate microspheres for cancer treatment

The aim of this study was the development of radioactive holmium phosphate microspheres (HoPO₄-MS) with a high holmium content and that are stable in human serum for selective internal radiation therapy (SIRT) of liver cancer. To this end, holmium acetylacetonate microspheres (HoAcAc-MS) were prepared (34.2 ± 1.0 µm in diameter, holmium content of 46.2 ± 0.8 and density of 1.7 g/cm³) via an emulsification and solvent evaporation method. The concentration of HoAcAc in the organic solvent, the temperature of emulsification and the stirring speed were varied for the preparation of the HoAcAc-MS to obtain microspheres with different diameters ranging from 11 to 35 µm. Subsequently, the AcAc ligands of the HoAcAc-MS were replaced by phosphate ions by simply incubating neutron irradiated HoAcAc-MS in a phosphate buffer solution (0.116 M, pH 4.2) to yield radioactive HoPO₄-MS. The obtained microspheres were analyzed using different techniques such as SEM-EDS, ICP-OES and HPLC. The prepared HoPO₄-MS (29.5 ± 1.2 µm in diameter and a density of 3.1 g/cm³) present an even higher holmium content (52 wt%) than the HoAcAc-MS precursor (46 wt%). Finally, the stability of the HoPO₄-MS was tested by incubation in human serum at 37 °C which showed no visible changes of the microspheres morphology and only 0.1% of holmium release was observed during the 2 weeks period of incubation. In conclusion, this study shows that stable radioactive HoPO₄-MS can be prepared with suitable properties to be used for cancer therapy.