Production of GMP-grade radioactive holmium loaded poly(L-lactic acid) microspheres for clinical application

Transarterial Radioembolization Planning and Treatment with Microspheres Containing Holmium-166: Determination of Renal and Intestinal Radionuclide Elimination, Effective Half-Life, and Regulatory Aspects

After transarterial radioembolization (TARE) with microspheres loaded with holmium-166, radioactivity is excreted from the body. The aim of this study was to evaluate radioactive renal and intestinal excretions after TARE planning and treatment procedures with holmium-166-loaded microspheres and to correlate the findings with the intratherapeutic effective half-life. Urinary and intestinal excretions of patients who underwent TARE procedures were collected during postinterventional intervals of 24 h (TARE planning) and 48 h (TARE treatment). Whole-body effective half-life measurements were performed. Calibrations of the ¹⁶⁶Ho measuring system showed evidence of long-living nuclides. For excretion determination, 22 TARE planning procedures and 29 TARE treatment procedures were evaluated. Mean/maximum total excretion proportions of the injected ¹⁶⁶Ho were 0.0038%/0.0096% for TARE planning procedures and 0.0061%/0.0184% for TARE treatment procedures. The mean renal fractions of all measured excretions were 97.1% and 98.1%, respectively. Weak correlations were apparent between the injected and excreted activities (R² planning/treatment: 0.11/0.32). Mean effective ¹⁶⁶Ho half-lives of 24.03 h (planning) and 25.62 h (treatment) confirmed low excretions. Radioactive waste disposal regulations of selected jurisdictions can be met but must be reviewed before implementing this method into clinical practice. Inherent long-living nuclide impurities should be considered.

Intratumoral injection of holmium-166 microspheres as neoadjuvant therapy of soft tissue sarcomas in dogs

INTRODUCTION

Minimally invasive microbrachytherapy is in development to treat solid tumors by intratumoral injection of (radioactive) holmium-166 (¹⁶⁶Ho) microspheres (MS). A high local dose can be administered with minimal damage to surrounding tissue because of the short soft tissue penetration depth of ¹⁶⁶Ho beta radiation. We aimed to prospectively evaluate the safety and efficacy of ¹⁶⁶Ho microbrachytherapy in client-owned canine patients with soft tissue sarcomas (STS).

METHODS

We included seven dogs with STS not suitable for local excision due to tumor size and/or location. ¹⁶⁶HoMS were suspended in a carrier fluid and multiple needle-injections were performed in predetermined tumor segments to maximize tumor coverage. Tumor response was evaluated using 3D caliper and CT measurements. Follow-up further included monitoring for potential side effects and registration of subsequent treatments and survival, until at least two years after treatment.

RESULTS

Delivered radioactive doses ranged from 70 to 969 Gy resulting in a mean tumor volume reduction of 49.0 ± 21.3% after 33 ± 25 days. Treatment-related side effects consisted of local necrosis (n = 1) and ulceration of the skin covering the tumor (n = 1), which resolved with basic wound care, and surgical excision of residual tumor, respectively. Residual tumor was surgically resected in six patients after 22-93 days. After a mean follow-up of 1,005 days, four patients were alive, two patients were euthanized because of unrelated causes, and one patient was euthanized because of disease progression after the owner(s) declined subsequent surgical treatment.

CONCLUSION

¹⁶⁶Ho microbrachytherapy was a safe and effective neoadjuvant treatment option for canine patients with STS.

Case Report: Radioactive Holmium-166 Microspheres for the Intratumoral Treatment of a Canine Pituitary Tumor

Introduction: In this case study, a client-owned dog with a large pituitary tumor was experimentally treated by intratumoral injection of radioactive holmium-166 microspheres (166HoMS), named 166Ho microbrachytherapy. To our knowledge, this is the first intracranial intratumoral treatment through needle injection of radioactive microspheres. Materials and Methods: A 10-year-old Jack Russell Terrier was referred to the Clinic for Companion Animal Health (Faculty of Veterinary Medicine, Utrecht University, The Netherlands) with behavioral changes, restlessness, stiff gait, and compulsive circling. MRI and CT showed a pituitary tumor with basisphenoid bone invasion and marked mass effect. The tumor measured 8.8 cm3 with a pituitary height-to-brain area (P/B) ratio of 1.86 cm-1 [pituitary height (cm) ×10/brain area (cm2)]. To reduce tumor volume and neurological signs, 166HoMS were administered in the tumor center by transsphenoidal CT-guided needle injections. Results: Two manual CT-guided injections were performed containing 0.6 ml of 166HoMS suspension in total. A total of 1097 MBq was delivered, resulting in a calculated average tumor dose of 1866 Gy. At 138 days after treatment, the tumor volume measured 5.3 cm3 with a P/B ratio of 1.41 cm-1, revealing a total tumor volume reduction of 40%. Debulking surgery was performed five months after 166HoMS treatment due to recurrent neurological signs. The patient was euthanized two weeks later at request of the owners. Histopathological analysis indicated a pituitary adenoma at time of treatment, with more malignant characteristics during debulking surgery. Conclusion: The 40% tumor volume reduction without evident severe periprocedural side effects demonstrated the feasibility of intracranial intratumoral 166HoMS treatment in this single dog.

Microspheres Used in Liver Radioembolization: From Conception to Clinical Effects

Inert microspheres, labeled with several radionuclides, have been developed during the last two decades for the intra-arterial treatment of liver tumors, generally called Selective Intrahepatic radiotherapy (SIRT). The aim is to embolize microspheres into the hepatic capillaries, accessible through the hepatic artery, to deliver high levels of local radiation to primary (such as hepatocarcinoma, HCC) or secondary (metastases from several primary cancers, e.g., colorectal, melanoma, neuro-endocrine tumors) liver tumors. Several types of microspheres were designed as medical devices, using different vehicles (glass, resin, poly-lactic acid) and labeled with different radionuclides, 90Y and 166Ho. The relationship between the microspheres' properties and the internal dosimetry parameters have been well studied over the last decade. This includes data derived from the clinics, but also computational data with various millimetric dosimetry and radiobiology models. The main purpose of this paper is to define the characteristics of these radiolabeled microspheres and explain their association with the microsphere distribution in the tissues and with the clinical efficacy and toxicity. This review focuses on avenues to follow in the future to optimize such particle therapy and benefit to patients.

Holmium-Containing Bioactive Glasses Dispersed in Poloxamer 407 Hydrogel as a Theragenerative Composite for Bone Cancer Treatment

Holmium-containing bioactive glasses can be applied in bone cancer treatment because the holmium content can be neutron activated, having suitable properties for brachytherapy applications, while the bioactive glass matrix can regenerate the bone alterations induced by the tumor. To facilitate the application of these glasses in clinical practice, we proposed a composite based on Poloxamer 407 thermoresponsive hydrogel, with suitable properties for applications as injectable systems. Therefore, in this work, we evaluated the influence of holmium-containing glass particles on the properties of Poloxamer 407 hydrogel (20 w/w.%), including self-assembly ability and biological properties. 58S bioactive glasses (58SiO₂-33CaO-9P₂O₅) containing different Ho₂O₃ amounts (1.25, 2.5, 3.75, and 5 wt.%) were incorporated into the hydrogel. The formulations were characterized by scanning electron microscopy, differential scanning calorimetry, rheological tests, and [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] MTT cell viability against pre-osteoblastic and osteosarcoma cells. The results evidenced that neither the glass particles dispersed in the hydrogel nor the holmium content in the glasses significantly influenced the hydrogel self-assembly ability (T_mic ~13.8 °C and T_gel ~20 °C). Although, the glass particles considerably diminished the hydrogel viscosity in one order of magnitude at body temperature (37 °C). The cytotoxicity results evidenced that the formulations selectively favored pre-osteoblastic cell proliferation and osteosarcoma cell death. In conclusion, the formulation containing glass with the highest fraction of holmium content (5 wt.%) had the best biological results outcomes aiming its application as theragenerative materials for bone cancer treatment.

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.

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.

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.

Poly(3-hydroxi-butyrate-co-3-hydroxy-valerate) (PHB-HV) microparticles loaded with holmium acetylacetonate as potential contrast agents for magnetic resonance images

Introduction

Biodegradable polymers that contain radioactive isotopes such as Holmium 166 have potential applications as beta particle emitters in tumor tissues. Also, Ho(III) is paramagnetic, which makes it suitable as a contrast agent for magnetic resonance (MR) images.

Methods

Holmium acetylacetonate (Ho(acac)₃) loaded poly(3-hydroxy-butyrate-co-3-hydroxy-valerate) microspheres, with 5% or 8% of 3-hydroxy-valerate (HV), were prepared by emulsification/evaporation process within 20–53 μm size. Microspheres characterization was done using scanning electron microscopy, energy-dispersive X-ray, and infrared spectroscopies. The release of holmium(III) in sodium phosphate buffer (pH 7.4) was followed for 9 days with inductively coupled plasma. Finally, T₂ and T₂* magnetic resonance images (MRI) were acquired and compared with the MRI of the inclusion complex of holmium acetylacetonate in some β-cyclodextrins.

Results

Holmium acetylacetonate loading, evaluated by thermogravimetry, was up to 20 times higher for copolymer with 5% of HV. It was shown that microspheres loaded with Ho(acac)₃ exhibited an accumulation of Ho(III) on their surfaces but were stable over time, as no expressive release of holmium(III) was detected in 9-day exposition to sodium phosphate buffer. Holmium acetylacetonate in both microspheres or inclusion complexes was very efficient in obtaining T₂ and T₂* weighted images in magnetic resonance, thus, might be used as contrast agents.

Conclusion

This is the first description of the use of inclusion complexes of holmium acetylacetonate in biodegradable polymers as contrast agents. New investigations are underway to evaluate the resistance of PHB-HV polymer microparticles to nuclear activation to assess their potential for use as radiopharmaceuticals for the treatment of liver cancer.

Theranostics in Interventional Oncology: Versatile Carriers for Diagnosis and Targeted Image-Guided Minimally Invasive Procedures

We are continuously progressing in our understanding of cancer and other diseases and learned how they can be heterogeneous among patients. Therefore, there is an increasing need for accurate characterization of diseases at the molecular level. In parallel, medical imaging and image-guided therapies are rapidly developing fields with new interventions and procedures entering constantly in clinical practice. Theranostics, a relatively new branch of medicine, refers to procedures combining diagnosis and treatment, often based on patient and disease-specific features or molecular markers. Interventional oncology which is at the convergence point of diagnosis and treatment employs several methods related to theranostics to provide minimally invasive procedures tailored to the patient characteristics. The aim is to develop more personalized procedures able to identify cancer cells, selectively reach and treat them, and to assess drug delivery and uptake in real-time in order to perform adjustments in the treatment being delivered based on obtained procedure feedback and ultimately predict response. Here, we review several interventional oncology procedures referring to the field of theranostics, and describe innovative methods that are under development as well as future directions in the field.

Blood and urine analyses after radioembolization of liver malignancies with [166Ho]Ho-acetylacetonate-poly(l-lactic acid) microspheres

BACKGROUND

[¹⁶⁶Ho]Ho-acetylacetonate-poly(L-lactic acid) microspheres were used in radioembolization of liver malignancies by intra-arterial administration. The primary aim of this study was to assess the stability and biodistribution of these microspheres.

MATERIALS AND METHODS

Peripheral blood and urine samples were obtained from two clinical studies. Patient and in vitro experiment samples were analyzed using inductively coupled plasma mass spectrometry (ICP-MS), gamma-ray spectroscopy, light microscopy, Coulter particle counting, and high performance liquid chromatography (HPLC).

RESULTS

The median percentage holmium compared to the total amount injected into the hepatic artery was 0.19% (range 0.08-2.8%) and 0.32% (range 0.03-1.8%) in the 1 h blood plasma and 24 h urine, respectively. Both the blood plasma and urine were correlated with the neutron irradiation exposure required for [¹⁶⁶Ho]Ho-AcAc-PLLA microsphere production (ρ = 0.616, p = 0.002). After a temporary interruption of the phase 2 clinical study, the resuspension medium was replaced to precipitate [¹⁶⁶Ho]Ho³⁺ pre-administration using phosphate. The in vitro near-maximum neutron irradiation experiments showed significant [¹⁶⁶Ho]Ho-AcAc-PLLA microsphere damage.

CONCLUSION

The amount of holmium in the peripheral blood and urine samples after [¹⁶⁶Ho]Ho-AcAc-PLLA microsphere intrahepatic infusion was low. A further decrease was observed after reformulation of the resuspension solution but minimization of production damage is necessary.

Transarterial Radioembolization (TARE) Agents beyond 90Y-Microspheres

Liver malignancies, either primary tumours (mainly hepatocellular carcinoma and cholangiocarcinoma) or secondary hepatic metastases, are a major cause of death, with an increasing incidence. Among them, hepatocellular carcinoma (HCC) presents with a dark prognosis because of underlying liver diseases and an often late diagnosis. A curative surgical treatment can therefore only be proposed in 20 to 30% of the patients. However, new treatment options for intermediate to advanced stages, such as internal radionuclide therapy, seem particularly attractive. Transarterial radioembolization (TARE), which consists in the use of intra-arterial injection of a radiolabelled embolising agent, has led to very promising results. TARE with 90Y-loaded microspheres is now becoming an established procedure to treat liver tumours, with two commercially available products (namely, SIR-Sphere® and TheraSphere®). However, this technology remains expensive and is thus not available everywhere. The aim of this review is to describe TARE alternative technologies currently developed and investigated in clinical trials, with special emphasis on HCC.

Additional hepatic 166Ho-radioembolization in patients with neuroendocrine tumours treated with 177Lu-DOTATATE; a single center, interventional, non-randomized, non-comparative, open label, phase II study (HEPAR PLUS trial)

Background

Neuroendocrine tumours (NET) consist of a heterogeneous group of neoplasms with various organs of origin. At diagnosis 21% of the patients with a Grade 1 NET and 30% with a Grade 2 NET have distant metastases. Treatment with peptide receptor radionuclide therapy (PRRT) shows a high objective response rate and long median survival after treatment. However, complete remission is almost never achieved. The liver is the most commonly affected organ in metastatic disease and is the most incriminating factor for patient survival. Additional treatment of liver disease after PRRT may improve outcome in NET patients. Radioembolization is an established therapy for liver metastasis. To investigate this hypothesis, a phase 2 study was initiated to assess effectiveness and toxicity of holmium-166 radioembolization (¹⁶⁶Ho-RE) after PRRT with lutetium-177 (¹⁷⁷Lu)-DOTATATE.

Methods

The HEPAR PLUS trial (“HolmiumEmbolizationParticles forArterialRadiotherapyPlus¹⁷⁷Lu-DOTATATE inSalvage NET patients”) is a single centre, interventional, non-randomized, non-comparative, open label study. In this phase 2 study 30–48 patients with > 3 measurable liver metastases according to RECIST 1.1 will receive additional ¹⁶⁶Ho-RE within 20 weeks after the 4th and last cycle of PRRT with 7.4 GBq ¹⁷⁷Lu-DOTATATE. Primary objectives are to assess tumour response, complete and partial response according to RECIST 1.1, and toxicity, based on CTCAE v4.03, 3 months after ¹⁶⁶Ho-RE. Secondary endpoints include biochemical response, quality of life, biodistribution and dosimetry.

Discussion

This is the first prospective study to combine PRRT with ¹⁷⁷Lu-DOTATATE and additional ¹⁶⁶Ho-RE in metastatic NET. A radiation boost on intrahepatic disease using ¹⁶⁶Ho-RE may lead to an improved response rate without significant additional side-effects.

Trial registration

Clinicaltrials.gov NCT02067988, 13 February 2014. Protocol version: 6, 30 november 2016.

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.

Intratumoral injection of radioactive holmium-166 microspheres in recurrent head and neck squamous cell carcinoma: preliminary results of first use

BACKGROUND

Limited treatment options exist for patients with locoregional recurrences of head and neck squamous cell carcinoma (HNSCC). In the palliative setting, a single session, minimally invasive, and relatively safe therapy is desirable. This case series illustrates the feasibility of a direct intratumoral injection of radioactive holmium-166 microspheres (HoMS) in patients as a palliative treatment for recurrent HNSCC.

PATIENTS AND METHODS

In this retrospective analysis, patients with already reirradiated irresectable recurrent HNSCC, for whom palliative chemotherapy was unsuccessful or impossible, were offered microbrachytherapy with HoMS. The intratumoral injection was administered manually under ultrasound guidance. Parameters scored were technical feasibility (i.e. administration, leakage, and distribution), clinical response (response evaluation criteria in solid tumors 1.1), and complications (Common Terminology Criteria for Adverse Events 4.3).

RESULTS

From 2015 to 2017, three patients were treated. None of the patients experienced adverse events; however, therapeutic effects were minimal. Technical difficulties, including precipitating of microspheres and high intratumoral pressure, resulted in suboptimal distribution of the microspheres.

CONCLUSION

Intratumoral injections with HoMS are minimally invasive and relatively safe in palliation of HNSCC patients. Careful patient selection and improved administration techniques are required to provide a more effective treatment. Further investigation of this novel treatment modality should be carried out because of the absence of side effects and lack of other treatment options.

Efficacy of Radioembolization with 166Ho-Microspheres in Salvage Patients with Liver Metastases: A Phase 2 Study

Radioembolization of liver malignancies with ¹⁶⁶Ho-microspheres has been shown to be safe in a phase 1 dose-escalation study. The purpose of this study was to investigate the efficacy of ¹⁶⁶Ho radioembolization. Methods: In this prospective single-arm study, 56 patients were enrolled, all with liver metastases refractory to systemic therapy and ineligible for surgical resection. The primary outcome was a response by 2 target lesions on triphasic liver CT scans 3 mo after therapy, as assessed using RECIST, version 1.1. Secondary outcomes included overall tumor response, time to imaging progression, overall survival, toxicity, quality of life, and quantification of the microspheres on SPECT and MRI. Results: Between May 2012 and March 2015, 38 eligible patients were treated, one of whom was not evaluable. In 27 (73%) of 37 patients, the target lesions showed complete response, partial response, or stable disease (disease control) at 3 mo (95% confidence interval [CI], 57%-85%). The median overall survival was 14.5 mo (95% CI, 8.6-22.8 mo). For colorectal cancer patients (n = 23), the median overall survival was 13.4 mo (95% CI, 8.2-15.7 mo). Grade 3 or 4 toxic events after treatment (according to the Common Terminology Criteria for Adverse Events, version 4.03) included abdominal pain (in 18% of patients), nausea (8%), ascites (3%), fatigue (3%), gastric stenosis (3%), hepatic failure (3%), liver abscesses (3%), paroxysmal atrial tachycardia (3%), thoracic pain (3%), upper gastrointestinal hemorrhage (3%), and vomiting (3%). On SPECT, ¹⁶⁶Ho could be quantified with high accuracy and precision, with a mean overestimation of 9.3% ± 7.1% in the liver. Conclusion: Radioembolization with ¹⁶⁶Ho-microspheres induced a tumor response with an acceptable toxicity profile in salvage patients with liver metastases.

Intratumoral injection of radioactive holmium ( 166 Ho) microspheres for treatment of oral squamous cell carcinoma in cats

BACKGROUND & AIMS

A "microbrachytherapy" was developed as treatment option for inoperable tumours by direct intratumoral injection of radioactive holmium-166 ( ¹⁶⁶ Ho) microspheres (MS). ¹⁶⁶ Ho emits β-radiation which potentially enables a high, ablative, radioactive-absorbed dose on the tumour tissue while sparing surrounding tissues.

MATERIALS & METHODS

Safety and efficacy of ¹⁶⁶ Ho microbrachytherapy were evaluated in a prospective cohort study of 13 cats with inoperable oral squamous cell carcinoma without evidence of distant metastasis.

RESULTS

Local response rate was 55%, including complete response or partial response (downstaging) enabling subsequent marginal resection. Median survival time was 113 days overall, and 296 days for patients with local response. Side effects were minimal. Tumour volume was a significant predictor of response.

DISCUSSION

Response rate may be further improved by optimizing the intratumoral spatial distribution of ¹⁶⁶ Ho MS.

CONCLUSION

¹⁶⁶ Ho microbrachytherapy has potential as a minimally invasive, single procedure radio-ablation treatment of unresectable tumours with minimal morbidity.

A review of 3D image-based dosimetry, technical considerations and emerging perspectives in 90Y microsphere therapy

Yttrium-90 radioembolization (⁹⁰Y-RE) is a well-established therapy for the treatment of hepatocellular carcinoma (HCC) and also of metastatic liver deposits from other malignancies. Nuclear Medicine and Cath Lab diagnostic imaging takes a pivotal role in the success of the treatment, and in order to fully exploit the efficacy of the technique and provide reliable quantitative dosimetry that are related to clinical endpoints in the era of personalized medicine, technical challenges in imaging need to be overcome. In this paper, the extensive literature of current ⁹⁰Y-RE techniques and challenges facing it in terms of quantification and dosimetry are reviewed, with a focus on the current generation of 3D dosimetry techniques. Finally, new emerging techniques are reviewed which seek to overcome these challenges, such as high-resolution imaging, novel surgical procedures and the use of other radiopharmaceuticals for therapy and pre-therapeutic planning.

The elements of life and medicines

Which elements are essential for human life? Here we make an element-by-element journey through the periodic table and attempt to assess whether elements are essential or not, and if they are, whether there is a relevant code for them in the human genome. There are many difficulties such as the human biochemistry of several so-called essential elements is not well understood, and it is not clear how we should classify elements that are involved in the destruction of invading microorganisms, or elements which are essential for microorganisms with which we live in symbiosis. In general, genes do not code for the elements themselves, but for specific chemical species, i.e. for the element, its oxidation state, type and number of coordinated ligands, and the coordination geometry. Today, the biological periodic table is in a position somewhat similar to Mendeleev's chemical periodic table of 1869: there are gaps and we need to do more research to fill them. The periodic table also offers potential for novel therapeutic and diagnostic agents, based on not only essential elements, but also non-essential elements, and on radionuclides. Although the potential for inorganic chemistry in medicine was realized more than 2000 years ago, this area of research is still in its infancy. Future advances in the design of inorganic drugs require more knowledge of their mechanism of action, including target sites and metabolism. Temporal speciation of elements in their biological environments at the atomic level is a major challenge, for which new methods are urgently needed.

(⁹⁹m)Tc-MAA overestimates the absorbed dose to the lungs in radioembolization: a quantitative evaluation in patients treated with ¹⁶⁶Ho-microspheres

PURPOSE

Radiation pneumonitis is a rare but serious complication of radioembolic therapy of liver tumours. Estimation of the mean absorbed dose to the lungs based on pretreatment diagnostic (99m)Tc-macroaggregated albumin ((99m)Tc-MAA) imaging should prevent this, with administered activities adjusted accordingly. The accuracy of (99m)Tc-MAA-based lung absorbed dose estimates was evaluated and compared to absorbed dose estimates based on pretreatment diagnostic (166)Ho-microsphere imaging and to the actual lung absorbed doses after (166)Ho radioembolization.

METHODS

This prospective clinical study included 14 patients with chemorefractory, unresectable liver metastases treated with (166)Ho radioembolization. (99m)Tc-MAA-based and (166)Ho-microsphere-based estimation of lung absorbed doses was performed on pretreatment diagnostic planar scintigraphic and SPECT/CT images. The clinical analysis was preceded by an anthropomorphic torso phantom study with simulated lung shunt fractions of 0 to 30 % to determine the accuracy of the image-based lung absorbed dose estimates after (166)Ho radioembolization.

RESULTS

In the phantom study, (166)Ho SPECT/CT-based lung absorbed dose estimates were more accurate (absolute error range 0.1 to -4.4 Gy) than (166)Ho planar scintigraphy-based lung absorbed dose estimates (absolute error range 9.5 to 12.1 Gy). Clinically, the actual median lung absorbed dose was 0.02 Gy (range 0.0 to 0.7 Gy) based on posttreatment (166)Ho-microsphere SPECT/CT imaging. Lung absorbed doses estimated on the basis of pretreatment diagnostic (166)Ho-microsphere SPECT/CT imaging (median 0.02 Gy, range 0.0 to 0.4 Gy) were significantly better predictors of the actual lung absorbed doses than doses estimated on the basis of (166)Ho-microsphere planar scintigraphy (median 10.4 Gy, range 4.0 to 17.3 Gy; p < 0.001), (99m)Tc-MAA SPECT/CT imaging (median 2.5 Gy, range 1.2 to 12.3 Gy; p < 0.001), and (99m)Tc-MAA planar scintigraphy (median 5.5 Gy, range 2.3 to 18.2 Gy; p < 0.001).

CONCLUSION

In clinical practice, lung absorbed doses are significantly overestimated by pretreatment diagnostic (99m)Tc-MAA imaging. Pretreatment diagnostic (166)Ho-microsphere SPECT/CT imaging accurately predicts lung absorbed doses after (166)Ho radioembolization.

In vivo dosimetry based on SPECT and MR imaging of 166Ho-microspheres for treatment of liver malignancies

(166)Ho-poly(l-lactic acid) microspheres allow for quantitative imaging with MR imaging or SPECT for microsphere biodistribution assessment after radioembolization. The purpose of this study was to evaluate SPECT- and MR imaging-based dosimetry in the first patients treated with (166)Ho radioembolization.

METHODS

Fifteen patients with unresectable, chemorefractory liver metastases of any origin were enrolled in this phase 1 study and were treated with (166)Ho radioembolization according to a dose escalation protocol (20-80 Gy). The contours of all liver segments and all discernible tumors were manually delineated on T2-weighted posttreatment MR images and registered to the posttreatment SPECT images (n = 9) or SPECT/CT images (n = 6) and MR imaging-based R2* maps (n = 14). Dosimetry was based on SPECT (n = 15) and MR imaging (n = 9) for all volumes of interest, tumor-to-nontumor (T/N) activity concentration ratios were calculated, and correlation and agreement of MR imaging- and SPECT-based measurements were evaluated.

RESULTS

The median overall T/N ratio was 1.4 based on SPECT (range, 0.9-2.8) and 1.4 based on MR imaging (range, 1.1-3.1). In 6 of 15 patients (40%), all tumors had received an activity concentration equal to or higher than the normal liver (T/N ratio ≥ 1). Analysis of SPECT and MR imaging measurements for dose to liver segments yielded a high correlation (R(2) = 0.91) and a moderate agreement (mean bias, 3.7 Gy; 95% limits of agreement, -11.2 to 18.7).

CONCLUSION

With the use of (166)Ho-microspheres, in vivo dosimetry is feasible on the basis of both SPECT and MR imaging, which enables personalized treatment by selective targeting of inadequately treated tumors.

99mTc-macroaggregated albumin poorly predicts the intrahepatic distribution of 90Y resin microspheres in hepatic radioembolization

In hepatic (90)Y radioembolization, pretreatment (99m)Tc-macroaggregated albumin ((99m)Tc-MAA) nuclear imaging is used for lung shunt analysis, evaluation of extrahepatic deposition, and sometimes for treatment planning, using a partition model. A high level of agreement between pretreatment (99m)Tc-MAA distribution and final (90)Y-microsphere distribution is assumed. The aim of this study was to investigate the value of pretreatment (99m)Tc-MAA SPECT to predict intrahepatic posttreatment (90)Y-microsphere distribution.

METHODS

Volumes of interest (VOIs) were delineated on pretreatment contrast-enhanced CT or MR images according to Couinaud liver segmentation. All VOIs were registered to the (99m)Tc-MAA SPECT and (90)Y SPECT images. The (99m)Tc-MAA SPECT and (90)Y SPECT activity counts were normalized to the total administered activity of (90)Y. For each VOI, this practice resulted in a predictive amount of (90)Y (MBq/cm(3)) based on (99m)Tc-MAA SPECT in comparison with an actual amount of (90)Y based on (90)Y SPECT. Bland-Altman analysis was used to investigate the agreement of the activity distribution between (99m)Tc-MAA SPECT and (90)Y SPECT.

RESULTS

A total of 39 procedures (225 VOIs) in 31 patients were included for analysis. The overall mean difference between pretreatment and posttreatment distribution of activity concentration for all segments was -0.022 MBq/cm(3) with 95% limits of agreement of -0.581 to 0.537 MBq/cm(3) (-28.9 to 26.7 Gy absorbed dose). A difference of >10%, >20%, and >30% of the mean activity per milliliter was found in, respectively, 153 (68%), 97 (43%), and 72 (32%) of the 225 segments. In every (99m)Tc-MAA procedure, at least 1 segment showed an under- or overestimation of >10%. The position of the catheter tip during administrations, as well as the tumor load of the liver segments, significantly influenced the disagreement.

CONCLUSION

In current clinical practice, (99m)Tc-MAA distribution does not accurately predict final (90)Y activity distribution. Awareness of the importance of catheter positioning and adherence to specific recommendations may lead to optimization of individualized treatment planning based on pretreatment imaging.

The necessity of nuclear reactors for targeted radionuclide therapies

Nuclear medicine has been contributing towards personalized therapies. Nuclear reactors are required for the working horses of both diagnosis and treatment, i.e., Tc-99m and I-131. In fact, reactors will remain necessary to fulfill the demand for a variety of radionuclides and are essential in the expanding field of targeted radionuclide therapies for cancer. However, the main reactors involved in the global supply are ageing and expected to shut down before 2025. Therefore, the fields of (nuclear) medicine, nuclear industry and politics share a global responsibility, faced with the task to secure future access to suitable nuclear reactors. At the same time, alternative production routes should be industrialized. For this, a coordinating entity should be put into place.

In vivo distribution of (131)I and (125)I dual-labeled gelatin microspheres after implantation into rabbit liver

OBJECTIVE

The purpose of this study was to explore the radionuclide distribution and metabolism of (131)I and (125)I dual-labeled gelatin microspheres ((131)I-(125)I-GMSs) implanted in rabbit liver.

METHODS

The simultaneous radiolabeling of (131)I and (125)I into GMSs was performed by a chloramine-T method to prepare biodegradable dual-labeled radionuclide microspheres. The microspheres were injected into rabbit liver. Radionuclide distribution and metabolism in vivo were examined using single photon emission computed tomography (SPECT) and by blood and urine radioactivity counting.

RESULTS

(131)I and (125)I were labeled into the biodegradable GMSs in accordance with the mixture ratio of batch feeding. After (131)I-(125)I-GMSs had been implanted in rabbit liver, small amounts of (131)I and (125)I were released into the blood along with the degradation of microspheres and excreted via the urine within 24 days. The radionuclides in the rabbit liver injection site could be detected by SPECT until day 48. The microspheres could be observed by histological methods on day 32. No signs of thyroid damage were observed throughout the entire experimental period.

CONCLUSION

(131)I-(125)I-GMS can be retained long term in the injection site. Due to the advantages of combining two radionuclides, (131)I-(125)I-GMS may be a safe and effective choice for cancer brachytherapy.

Preparation and quality control of 166Ho-DTPA-antiCD20 for radioimmunotherapy

In this work, anti-CD20 was successively labeled with beta-particle emitting radionuclide, Ho-166, for ultimate radioimmunotherapy applications. Ho-166 chloride was obtained by thermal neutron flux (1×1013 n cm−2 s−1) of natural Ho2(NO3)3 sample, dissolved in acidic media. 166Ho-holmium chloride (185 MBq) was added to the conjugated antibody after ccDTPA residulation at room temperature. Radiochemical purity of 95% (ITLC) and 98% (HPLC) were obtained for final radioimmunoconjugate (specific activity =3–3.5 GBq/mg). The final isotonic 166Ho-rituximab complex was checked by gel electrophoresis for protein integrity retention. Biodistribution studies of Ho-166 chloride and radioimmunoconjugate were performed in wild-type rats to determine the biodistribution. The accumulation of the radiolabeled antibody in lungs, liver and spleen demonstrates a similar pattern to the other radiolabeled anti-CD20 immunoconjugates.

Holmium-166 radioembolization for the treatment of patients with liver metastases: design of the phase I HEPAR trial

BACKGROUND

Intra-arterial radioembolization with yttrium-90 microspheres ( 90Y-RE) is an increasingly used therapy for patients with unresectable liver malignancies. Over the last decade, radioactive holmium-166 poly(L-lactic acid) microspheres ( 166Ho-PLLA-MS) have been developed as a possible alternative to 90Y-RE. Next to high-energy beta-radiation, 166Ho also emits gamma-radiation, which allows for imaging by gamma scintigraphy. In addition, Ho is a highly paramagnetic element and can therefore be visualized by MRI. These imaging modalities are useful for assessment of the biodistribution, and allow dosimetry through quantitative analysis of the scintigraphic and MR images. Previous studies have demonstrated the safety of 166Ho-PLLA-MS radioembolization ( 166Ho-RE) in animals. The aim of this phase I trial is to assess the safety and toxicity profile of 166Ho-RE in patients with liver metastases.

METHODS

The HEPAR study (Holmium Embolization Particles for Arterial Radiotherapy) is a non-randomized, open label, safety study. We aim to include 15 to 24 patients with liver metastases of any origin, who have chemotherapy-refractory disease and who are not amenable to surgical resection. Prior to treatment, in addition to the standard technetium-99m labelled macroaggregated albumin ( 99mTc-MAA) dose, a low radioactive safety dose of 60-mg 166Ho-PLLA-MS will be administered. Patients are treated in 4 cohorts of 3-6 patients, according to a standard dose escalation protocol (20 Gy, 40 Gy, 60 Gy, and 80 Gy, respectively). The primary objective will be to establish the maximum tolerated radiation dose of 166Ho-PLLA-MS. Secondary objectives are to assess tumour response, biodistribution, performance status, quality of life, and to compare the 166Ho-PLLA-MS safety dose and the 99mTc-MAA dose distributions with respect to the ability to accurately predict microsphere distribution.

DISCUSSION

This will be the first clinical study on 166Ho-RE. Based on preclinical studies, it is expected that 166Ho-RE has a safety and toxicity profile comparable to that of 90Y-RE. The biochemical and radionuclide characteristics of 166Ho-PLLA-MS that enable accurate dosimetry calculations and biodistribution assessment may however improve the overall safety of the procedure.

The studies on the aromaticity of fullerenes and their holmium endohedral compounds

Density functional theory BLYP/DNP was employed to optimize a series of fullerenes and their holmium endohedral compounds, including C(20), Ho@C(20), Ho(3+)@C(20), C(60), Ho@C(60), Ho(3+)@C(60),C(70), Ho@C(70), Ho(3+)@C(70) C(78), Ho@C(78), Ho(3+)@C(78), C(82),Ho@C(82) and Ho(3+)@C(82). DFT semi core pseudospot approximation was taken into consideration in the calculations of the element holmium because of its particular electronic structure. Fullerenes and their holmium endohedral compounds' aromaticity were studied in terms of structural criteria, energetic criteria, and reactivity criteria. The results indicate that the aromaticity of fullerenes was reduced when a holmium atom was introduced into the carbon cage, and the endohedral fullerenes' reactive activity enhance; but the aromaticity of the carbon cage increased when a Ho(3+) cation was encapsulated into a fullerene. Calculations of aromaticity and stability indicate that two paths can lead to the similar aim of preparing holmium endohedral fullerenes; that is, they can form from either a holmium atom or a holmium cation (Ho(3+)) reacting with fullerenes, respectively, and the latter is more favorable.

Holmium-166 poly(L-lactic acid) microsphere radioembolisation of the liver: technical aspects studied in a large animal model

Objective

To assess the accuracy of a scout dose of holmium-166 poly(L-lactic acid) microspheres (¹⁶⁶Ho-PLLA-MS) in predicting the distribution of a treatment dose of ¹⁶⁶Ho-PLLA-MS, using single photon emission tomography (SPECT).

Methods

A scout dose (60 mg) was injected into the hepatic artery of five pigs and SPECT acquired. Subsequently, a ‘treatment dose’ was administered (540 mg) and SPECT, computed tomography (CT) and magnetic resonance imaging (MRI) of the total dose performed. The two SPECT images of each animal were compared. To validate quantitative SPECT an ex vivo liver was instilled with ¹⁶⁶Ho-PLLA-MS and SPECT acquired. The liver was cut into slices and planar images were acquired, which were registered to the SPECT image.

Results

Qualitatively, the scout dose and total dose images were similar, except in one animal because of catheter displacement. Quantitative analysis, feasible in two animals, tended to confirm this similarity (r² = 0.34); in the other animal the relation was significantly better (r² = 0.66). The relation between the SPECT and planar images acquired from the ex vivo liver was strong (r² = 0.90).

Conclusion

In the porcine model a scout dose of ¹⁶⁶Ho-PLLA-MS can accurately predict the biodistribution of a treatment dose. Quantitative ¹⁶⁶Ho SPECT was validated for clinical application.

Neutron activation of holmium poly(L-lactic acid) microspheres for hepatic arterial radio-embolization: a validation study

Poly(L-lactic acid) microspheres loaded with holmium-166 acetylacetonate (¹⁶⁶Ho-PLLA-MS) are a novel microdevice for intra-arterial radioembolization in patients with unresectable liver malignancies. The neutron activation in a nuclear reactor, in particular the gamma heating, damages the ¹⁶⁶Ho-PLLA-MS. The degree of damage is dependent on the irradiation characteristics and irradiation time in a particular reactor facility. The aim of this study was to standardize and objectively validate the activation procedure in a particular reactor. The methods included light- and scanning electron microscopy (SEM), particle size analysis, differential scanning calorimetry, viscometry, thermal neutron flux measurements and energy deposition calculations. Seven hours-neutron irradiation results in sufficient specific activity of the ¹⁶⁶Ho-PLLA-MS while structural integrity is preserved. Neutron flux measurements and energy deposition calculations are required in the screening of other nuclear reactors. For the evaluation of microsphere quality, light microscopy, SEM and particle size analysis are appropriate techniques.

Unilateral intracarotid injection of holmium microspheres to induce bilateral MRI-validated cerebral embolization in rats

BACKGROUND

Cerebral embolization models have been hindered by the fact that delivery is predominantly one-sided and cannot be quantified easily. We have developed a model for bilateral cerebral micro-embolization. By using holmium microspheres, it is possible to quantify intracerebral delivery using MRI.

METHODS

To validate the quantification of holmium microspheres a phantom study was performed in which concentration of microspheres in solution was compared with the number of holmium-induced artifacts on MRI. After that identical microspheres were administered by unilateral injection in the carotid artery, while the opposite carotid artery was clamped. On post-injection MRI scans, intracerebral delivery and right/left distribution of the microspheres was determined.

RESULTS

In the phantom study it was shown that quantification by MRI is possible and that MRI artifacts represent single microspheres. In the rat brain, about one-third of the injected dose was consistently located on the contralateral side. The administration was reproducible regarding distribution and number of microspheres.

CONCLUSIONS

The use of holmium microspheres enables quantification of delivered dose as single microspheres induce artifacts on MRI. By clamping the contralateral carotid artery, one-third of the dose is diverted to the contralateral hemisphere.

Clinical effects of transcatheter hepatic arterial embolization with holmium-166 poly(L-lactic acid) microspheres in healthy pigs

Purpose

The aim of this study is to evaluate the toxicity of holmium-166 poly(l-lactic acid) microspheres administered into the hepatic artery in pigs.

Methods

Healthy pigs (20–30 kg) were injected into the hepatic artery with holmium-165-loaded microspheres (¹⁶⁵HoMS; n = 5) or with holmium-166-loaded microspheres (¹⁶⁶HoMS; n = 13). The microspheres’ biodistribution was assessed by single-photon emission computed tomography and/or MRI. The animals were monitored clinically, biochemically, and (¹⁶⁶HoMS group only) hematologically over a period of 1 month (¹⁶⁵HoMS group) or over 1 or 2 months (¹⁶⁶HoMS group). Finally, a pathological examination was undertaken.

Results

After microsphere administration, some animals exhibited a slightly diminished level of consciousness and a dip in appetite, both of which were transient. Four lethal adverse events occurred in the ¹⁶⁶HoMS group due either to incorrect administration or comorbidity: inadvertent delivery of microspheres into the gastric wall (n = 2), preexisting gastric ulceration (n = 1), and endocarditis (n = 1). AST levels were transitorily elevated post-¹⁶⁶HoMS administration. In the other blood parameters, no abnormalities were observed. Nuclear scans were acquired from all animals from the ¹⁶⁶HoMS group, and MRI scans were performed if available. In pigs from the ¹⁶⁶HoMS group, atrophy of one or more liver lobes was frequently observed. The actual radioactivity distribution was assessed through ex vivo ^166mHo measurements.

Conclusion

It can be concluded that the toxicity profile of HoMS is low. In pigs, hepatic arterial embolization with ¹⁶⁶HoMS in amounts corresponding with liver-absorbed doses of over 100 Gy, if correctly administered, is not associated with clinically relevant side effects. This result offers a good perspective for upcoming patient trials.

Holmium-loaded poly(L-lactic acid) microspheres: in vitro degradation study

The clinical application of holmium-loaded poly(L-lactic acid) (PLLA) microspheres for the radionuclide treatment of liver malignancies requires in depth understanding of the degradation characteristics of the microspheres. To this end, an in-vitro degradation study was conducted. PLLA-microspheres with and without HoAcAc loading, and before and after neutron or gamma irradiation, were incubated in a phosphate buffer at 37 degrees C for 12 months. In contrast with the other microsphere formulations, only the neutron-irradiated Ho-PLLA-MS disintegrated. At the end of the experiment (52 weeks) highly crystalline fragments, as evidenced from Differential Scanning Calorimetry, were present. Infrared spectroscopy showed that these fragments consisted of holmium lactate. In conclusion, this study demonstrates that the degradation of neutron-irradiated Ho-PLLA-MS was substantially accelerated by the HoAcAc incorporation and subsequent neutron irradiation. The degradation of these microspheres in aqueous solution resulted in the formation of insoluble holmium lactate microcrystals without release of Ho3+.