Reactor production and electrochemical purification of (169)Er: a potential step forward for its utilization in in vivo therapeutic applications

Laser isotope enrichment of 168Er

This present study investigates a three-step laser isotope separation method for the enrichment of 168Er isotope using 631.052 nm - 586.912 nm - 566.003 nm three-step photoionization scheme. The lineshape contours observed in three-step photoionization process have been investigated in detail. This study shows that enrichment of 168Er isotope can be achieved with a relatively simple experimental configuration. With the derived system configuration, it has been shown that it is possible to produce 18 g/day of 90% enriched 168Er. Using the enriched 168Er isotope obtained from the laser isotope separation process, irradiation in low, medium, and high flux reactors can produce 180, 1800, and 18,000 doses per day (each with an activity of 7.4 GBq) respectively. After 24 h of irradiation and chemical separation, the radioisotopic purity of the medical isotope reaches to > 99% making it suitable for the medical applications. This is the first ever study on the laser isotope separation of 168Er isotope.

Emerging role of electrochemistry in radiochemical separation of medically important radiometals: state of the art

Electrochemical separation technology has brought a renaissance in the field of nuclear medicine towards obtaining clinical-grade radiometals for preparation of a wide variety of radiopharmaceuticals. This article is a comprehensive summary of the electrochemical processes developed for the separation of radiometals that could be used for diagnostic or therapeutic applications in nuclear medicine. For using electrochemistry as a tool for the separation of radiometals, intricate knowledge is essential to understand the basic parameters of electrochemical separation processes which include applied potential, selection of electrolyte, choice of the electrode, the temperature of the electrolyte, pH of the electrolyte and time of electrolysis. The advantages of the electrochemical separation approach over the other conventional methodologies such as solvent extraction, column chromatography, sublimation, etc., have also been discussed. The latest research and development from our laboratory on electrochemical methodologies developed for separation of 90Y from 90Sr, 188Re from 188W, 99mTc from 99Mo, 47Sc from 46Ca, 45Ca from 46Sc,153Sm from 154Eu, 169Er from 169Yb, 177Lu from Yb and 132/135La from Ba have been described. In all the cases, the final product is obtained either in a 'no-carrier-added' (NCA) form or free from inextricable impurities and thus found suitable for formulation of radiopharmaceuticals.

Cutting edge rare earth radiometals: prospects for cancer theranostics

Background

With recent advances in novel approaches to cancer therapy and imaging, the application of theranostic techniques in personalised medicine has emerged as a very promising avenue of research inquiry in recent years. Interest has been directed towards the theranostic potential of Rare Earth radiometals due to their closely related chemical properties which allow for their facile and interchangeable incorporation into identical bifunctional chelators or targeting biomolecules for use in a diverse range of cancer imaging and therapeutic applications without additional modification, i.e. a “one-size-fits-all” approach. This review will focus on recent progress and innovations in the area of Rare Earth radionuclides for theranostic applications by providing a detailed snapshot of their current state of production by means of nuclear reactions, subsequent promising theranostic capabilities in the clinic, as well as a discussion of factors that have impacted upon their progress through the theranostic drug development pipeline.

Main body

In light of this interest, a great deal of research has also been focussed towards certain under-utilised Rare Earth radionuclides with diverse and favourable decay characteristics which span the broad spectrum of most cancer imaging and therapeutic applications, with potential nuclides suitable for α-therapy (¹⁴⁹Tb), β⁻-therapy (⁴⁷Sc, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁶⁹Er, ¹⁴⁹Pm, ¹⁴³Pr, ¹⁷⁰Tm), Auger electron (AE) therapy (¹⁶¹Tb, ¹³⁵La, ¹⁶⁵Er), positron emission tomography (⁴³Sc, ⁴⁴Sc, ¹⁴⁹Tb, ¹⁵²Tb, ¹³²La, ¹³³La), and single photon emission computed tomography (⁴⁷Sc, ¹⁵⁵Tb, ¹⁵²Tb, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁴⁹Pm, ¹⁷⁰Tm). For a number of the aforementioned radionuclides, their progression from ‘bench to bedside’ has been hamstrung by lack of availability due to production and purification methods requiring further optimisation.

Conclusions

In order to exploit the potential of these radionuclides, reliable and economical production and purification methods that provide the desired radionuclides in high yield and purity are required. With more reactors around the world being decommissioned in future, solutions to radionuclide production issues will likely be found in a greater focus on linear accelerator and cyclotron infrastructure and production methods, as well as mass separation methods. Recent progress towards the optimisation of these and other radionuclide production and purification methods has increased the feasibility of utilising Rare Earth radiometals in both preclinical and clinical settings, thereby placing them at the forefront of radiometals research for cancer theranostics.

[Evaluation of radionuclide impurities in several radiopharmaceuticals]

PURPOSE

Revisions to German radiation protection laws have resulted in updated limit values, which could affect the unrestricted release of waste produced by nuclear medicine therapy. In addition, signs of long-lived concomitant nuclides in ¹⁵³Sm and ²²³Ra radiopharmaceuticals have been seen in the past. Therefore, the goal of this article was to analyze the radionuclidic purity of selected radiopharmaceuticals.

METHOD

48 samples from 12 different radiopharmaceuticals were examined. A high purity germanium semiconductor detector (HPGe detector) was used for the qualitative and quantitative evaluation of concomitant nuclides.

RESULTS

Various europium isotopes were identified in ⁹⁰Y-citrate, ¹⁵³Sm-Quadramet, ¹⁶⁶Ho-QuiremSpheres, and ¹⁶⁹Er-erbium citrate, with the greatest amount being found in ¹⁵³Sm (7.0 ppm (¹⁵²Eu), 8.4 ppm (¹⁵⁴Eu), and 2.1 ppm (¹⁵⁵Eu)). ¹⁶⁹Yb was the most significant impurity in ¹⁶⁹Er (513 ppm). In the case of ¹⁷⁷Lu radiopharmaceuticals, there was a significant difference in the ^177mLu content (0.8 ppm vs. 0.0024 ppm) between two different manufacturers. No concomitant nuclides could be found within the detection limits in the case of ⁹⁰Y spheres, ²²³Ra, and ²²⁵Ac.

CONCLUSION

The limit values for unrestricted release are exceeded manyfold in the case of the identified concomitant nuclides. As a result, alternative release procedures (extension of the decay time, specific release, release in the individual case) or transfer to collection facilities must be considered. Technical methods for reducing or preventing impurities could also be a possible solution. Consequences for patient radiation exposure were able to be ruled out.

Production of Mass-Separated Erbium-169 Towards the First Preclinical in vitro Investigations

The β--particle-emitting erbium-169 is a potential radionuclide toward therapy of metastasized cancer diseases. It can be produced in nuclear research reactors, irradiating isotopically-enriched 168Er2O3. This path, however, is not suitable for receptor-targeted radionuclide therapy, where high specific molar activities are required. In this study, an electromagnetic isotope separation technique was applied after neutron irradiation to boost the specific activity by separating 169Er from 168Er targets. The separation efficiency increased up to 0.5% using resonant laser ionization. A subsequent chemical purification process was developed as well as activity standardization of the radionuclidically pure 169Er. The quality of the 169Er product permitted radiolabeling and pre-clinical studies. A preliminary in vitro experiment was accomplished, using a 169Er-PSMA-617, to show the potential of 169Er to reduce tumor cell viability.

Ce-141-labeled DOTMP: A theranostic option in management of pain due to skeletal metastases

Owing to its favorable radioactive decay characteristics (T_1/2  = 32.51 d, E_β [max] = 434.6 keV [70.5%] and 580.0 keV [29.5%], E_γ  = 145.4 keV [48.5%]), ¹⁴¹ Ce could be envisaged as a theranostic radionuclide for use in nuclear medicine. The present article reports synthesis and evaluation of ¹⁴¹ Ce complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylenephosphonic acid (DOTMP) as a potent theranostic agent targeting metastatic skeletal lesions. Ce-141 was produced with 314 ± 29 MBq/mg (n = 6) specific activity and >99.9% radionuclidic purity (n = 6). Around 185 MBq dose of [¹⁴¹ Ce]Ce-DOTMP was synthesized with 98.6 ± 0.5% (n = 4) radiochemical yield under optimized conditions of reaction, and the preparation showed adequately high in vitro stability. Biodistribution studies in normal Wistar rats demonstrated significant skeletal localization and retention of injected activity (2.73 ± 0.28% and 2.63 ± 0.22% of injected activity per gram in femur at 3 hours and 14 days post-injection, respectively) with rapid clearance from non-target organs. The results of biodistribution studies were corroborated by serial scintigraphic imaging studies. These results demonstrate the potential utility of ¹⁴¹ Ce-DOTMP as a theranostic molecule for personalized patient care of cancer patients suffering from painful metastatic skeletal lesions.

Palliative care of bone pain due to skeletal metastases: Exploring newer avenues using neutron activated (45)Ca

INTRODUCTION

With an objective to develop a cost-effective radiochemical formulation for palliation of pain due to skeletal metastases, we have demonstrated a viable method for large-scale production of (45)Ca (t½=163 days, Eβmax=0.3MeV) using moderate flux research reactor, its purification from radionuclidic impurities adopting electrochemical approach and preclinical evaluation of (45)CaCl2.

METHODS

Irradiation parameters were optimized by theoretical calculations for production of (45)Ca with highest possible specific activity along with minimum radionuclidic impurity burden. Based on this, the radioisotope was produced in reactor by irradiation of isotopically enriched (98% in (44)Ca) CaO target at a thermal neutron flux of ~1 × 10(14) n.cm(-2).s(-1) for 4 months. Scandium-46 impurity co-produced along with (45)Ca was efficiently removed adopting an electrochemical separation approach. The bone specificity of (45)CaCl2 was established by in vitro studies involving its uptake in hydroxyapatite (HA) particles and also evaluating its biodistribution pattern over a period of 2 weeks after in vivo administration in Wistar rats.

RESULTS

Thermal neutron irradiation of 100mg of enriched (98% in (44)Ca) CaO target followed by radiochemical processing and electrochemical purification procedure yielded ~37 GBq of (45)Ca with a specific activity of ~370 MBq/mg and radionuclidic purity>99.99%. The reliability and reproducibility of this approach were amply demonstrated by process demonstration in several batches. In vitro studies indicated significant uptake of (45)CaCl2 (up to 65%) in HA particles. In vivo biodistribution studies in Wistar rats showed specific skeletal accumulation (40-46%ID) with good retention over a period of 2 weeks.

CONCLUSIONS

To the best of our knowledge, this is the first study on utilization of (45)CaCl2 in the context of nuclear medicine. The results obtained in this study hold promise and warrant further investigations for future translation of (45)CaCl2 to the clinics, thereby potentially enabling a cost-effective approach for metastatic bone pain palliation especially in developing countries.

An overview of radioisotope separation technologies for development of 188W/188Re radionuclide generators providing 188Re to meet future research and clinical demands

Separation technologies for ¹⁸⁸W/¹⁸⁸Re radionuclide generators.