Co-production of 155Tb and 152Tb irradiating 155Gd / 151Eu tandem target with a medium energy α-particle beam

INTRODUCTION

Four terbium isotopes 149,152,155,161Tb emitting various types of radiation can be used for both diagnostics and therapy. 152Tb emits positrons and is ideal for PET. 155Tb is considered a promising Auger emitter and a diagnostic pair for other terbium therapeutic isotopes. Several methods for the production of 155Tb using charged particle accelerators have been proposed, but they all have significant limitations. The restricted availability of this isotope hinders its medical applications. We have proposed a new method for production of 155Tb, irradiating enriched 155Gd by alpha particles. The possibility of simultaneous production of two isotopes of terbium, 152,155Tb, was also studied for more efficient cyclotron beam use.

METHODS

Irradiation of 155Gd enriched targets and 155Gd / 151Eu tandem target with alpha-particles with an energy of 54 MeV was carried out at the U-150 cyclotron at the NRC "Kurchatov Institute". The cross sections of nuclear reactions on enr-155Gd were measured by the stack foil technique, detecting the gamma-radiation of the activation products. The separation of rare earth elements was performed by extraction chromatography with the LN Resin. 155Tb was produced via 155Dy decay.

RESULTS

The cross sections for the 155,156Tb and 155,157Dy production were measured by the irradiation of a gadolinium target enriched with the 155Gd isotope with alpha-particles in an energy range of 54 → 33 MeV. The yield of 155Dy on a thick target at 54 MeV was 130 MBq/μAh, which makes it possible to obtain 1 GBq of 155Tb in 11 hour-irradiation with 20 μA beam current. The possibility of simultaneous production of 152,155Tb by irradiation of 155Gd and 151Eu tandem target with medium-energy alpha-particles is implemented. Optimal irradiation energy ranges of alpha -particles as 54 → 42 MeV for 155Tb and 42 → 34 MeV for 152Tb were suggested. Product activity and radionuclidic purity were calculated.

Experimental assessment of nuclear cross sections for the production of Tb radioisotopes with a medical cyclotron

155Tb is one of the most interesting radionuclides for theranostic applications. It is suitable for SPECT imaging and it can be used as a true diagnostic partner of the therapeutic 149Tb and 161Tb. Its production by proton irradiation using enriched 155Gd and 156Gd oxide targets is currently being investigated and represents a promising solution. To achieve the level of radionuclidic purity required in the clinical setting, the co-production of Tb impurities has to be minimized. For this purpose, an accurate knowledge of the cross sections of the nuclear reactions involved is of paramount importance. In this paper, we report on the assessment of cross sections of the reactions 154Gd(p,xn)153,154,154m1,154m2Tb, 155Gd(p,xn)154,154m1,154m2,155Tb, 156Gd(p,xn)155,156Tb and 157Gd(p,2n)156Tb derived with a specific data analysis procedure developed by our group. This method allows to disentangle the nuclear contributions from the production cross section by inverting linear systems of equations and it requires the measurement of the cross sections from as many materials as the reactions involved in the production of the radionuclide under study. For this purpose, the experimental data previously measured by our group at the Bern medical cyclotron by irradiating natural Gd2O3, enriched 155Gd2O3 and enriched 156Gd2O3 targets were used. For some of these nuclear reactions, cross sections were assessed for the first time. On the basis of our findings, production yield and purity can be calculated for any kind of isotopic composition of the enriched material.

Photonuclear production of medical radioisotopes 161Tb and 155Tb

The production possibility of ¹⁶¹Tb and ¹⁵⁵Tb by irradiating of natural dysprosium with gamma rays obtained by decelerating an electron beam with an energy of 55 MeV has been demonstrated experimentally. The yield of ¹⁶¹Tb was 14.4 × 10³ Bq × μA^-1 × h^-1 × cm² × g_Dy2O3^-1. Simultaneously, upon irradiation, ¹⁵⁵Dy is formed with the yield of 25 × 10³ Bq × μA^-1 × h^-1 × cm² × g_Dy2O3^-1, which leads to the formation of 1.6 × 10³ Bq × μA^-1 × h^-1 × cm² × g_Dy2O3^-1 of ¹⁵⁵Tb. It has been shown that the isolation of terbium radioisotopes from tens of mg of dysprosium target can be achieved by extraction chromatography, and final separation yield was 39%. The impurity of ¹⁶⁰Tb is 7.3% of the ¹⁶¹Tb activity at EOB.

The role of chemistry in accelerator-based production and separation of radionuclides as basis for radiolabelled compounds for medical applications

Radiochemical separations used in large scale routine production of diagnostic and therapeutic radionuclides at a particle accelerator for patient care are briefly outlined. The role of chemistry at various stages of development of a production route of a novel radionuclide, namely nuclear data measurement, high-current targetry, chemical processing and quality control of the product, is discussed in detail. Special attention is paid to production of non-standard positron emitters (e.g. 44gSc, 64Cu, 68Ga, etc.) at a cyclotron and novel therapeutic radionuclides (e.g. 67Cu, 225Ac, etc.) at an accelerator. Some typical examples of radiochemical methods involved are presented.

New method for production of 155Tb via 155Dy by irradiation of natGd by medium energy alpha particles

INTRODUCTION

¹⁵⁵Tb (T_1/2 = 5.32 d) is considered both as a promising Auger electron emitter and as a diagnostic pair for other therapeutic terbium radionuclides. Despite several methods for its production proposed, it remains scarcely available. Most of the methods using low-energy protons and deuterons beams result in a high content of radionuclidic impurities. High purity ¹⁵⁵Tb can be obtained using high-energy proton beams combined with online mass separation of products, but the method remains inaccessible to most potential consumers. We have proposed an indirect method for the production of ¹⁵⁵Tb via formation of ¹⁵⁵Dy (T_1/2 = 9.9 h), which can be implemented using medium energy alpha particles beam.

METHODS

Gadolinium oxide targets of natural isotopic composition were irradiated by 60 MeV alpha particles beam on a U-150 cyclotron of the National Research Center "Kurchatov Institute". The cross sections of nuclear reactions were measured by the stack foil technique, detecting the gamma radiation of the activation products. Gd, Tb, and Dy were separated by extraction chromatography using the LN Resin sorbent in nitric media. The isolated dysprosium fraction was stored for a day, and the formed ¹⁵⁵Tb was isolated by the same method.

RESULTS

The cross sections for the formation of ¹⁵⁹Gd, ^153-156Tb, and ^155,157Dy under irradiation by alpha particles of a gadolinium target of natural isotopic composition in the energy range 20-60 MeV have been measured. The ¹⁵⁵Dy yield on a thick target at 60 MeV was 35 MBq/μAh, which makes it possible to obtain 1 GBq ¹⁵⁵Tb as a result of 12-hour irradiation with a beam current of 50 μA. Extraction chromatography on LN Resin sorbent in nitric enabled quick and efficient separation of Gd, Tb, and Dy. The radiochemical yield of Dy was 95%, for Tb > 95%. The main radionuclidic impurity is ¹⁵³Tb (T_1/2 = 2.34 d; <5.4% of ¹⁵⁵Tb activity).

CONCLUSIONS

The developed method allows the production of therapeutic amounts of ¹⁵⁵Tb with acceptable radionuclidic purity without the need for isotopically enriched materials. The amount of ¹⁵⁵Tb is sufficient for its use in Auger therapy, as well as for preclinical studies of the suitability of SPECT preparations in laboratory animals. Nevertheless, to obtain higher activities, a longer irradiation time and a higher projectile current are proposed. The ¹⁵³Tb radionuclide present in the final preparation has a shorter half-life than the target radionuclide, and its hard γ-lines have a probability of emission of less than 1%, from which it can be concluded that the negative effect will not be significant. However, a product of this purity and type of contamination requires additional testing for toxicity in living organisms. The final sample also includes a certain amount of ¹⁵⁷Tb (T_1/2 = 71 a, the only γ-line 54.5 keV Iγ = 0.0084%), which will complicate the labeling conditions. Thus, more research is needed in the labeling area. It should be noted that the use of gadolinium enriched in the ¹⁵⁵Gd or ¹⁵⁶Gd nuclide as a target will help not only reduce the amount of impurities but also increase the yield of ¹⁵⁵Tb.

A heavy-ion production channel of 149Tb via 63Cu bombardment of 89Y

The radionuclide ¹⁴⁹Tb (t_1/2 = 4.1 h) is a potential theranostic isotope which can simultaneously be used for targeted-alpha-particle therapy and positron-emission tomography. Feasibility experiments were performed to test a near-symmetric heavy-ion reaction of ⁶³Cu bombardment on monoisotopic ⁸⁹Y. The indirect reaction was studied to avoid isomer production. Offline gamma spectroscopy was used to quantify thick-target physical yields and experimental results show modest agreement to the fusion-evaporation code PACE4. A near-symmetric fission yield was also observed.

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

Cross section measurements of 151Eu(3He,5n) reaction: new opportunities for medical alpha emitter 149Tb production

Method for production of alpha emitter ¹⁴⁹Tb by irradiation of ¹⁵¹Eu with 70 MeV ³He nuclei is proposed. For the first time, the cross sections for the formation of isotopes ^{149,150,151,152}Tb were measured experimentally using a stack foil technique in the ³He particles energy range 70 → 12 MeV. The thick target yield of ¹⁴⁹Tb is 39 MBq/μAh, or 230 MBq/μA ¹⁴⁹Tb at saturation. The optimal energy range from the point of view of radioisotopic purity is 70 → 40 MeV. At these conditions about 150 MBq/μA ¹⁴⁹Tb can be produced in 8 hours irradiation, which is sufficient for therapeutic applications. The main impurities are ¹⁵⁰Tb (~100% in activity) and ¹⁵¹Tb (~30% in activity). The proposed method surpasses its counterparts by the high content of the target isotope in the natural mixture and the simplicity of the radiochemical separation of ¹⁴⁹Tb from the bulk target material.

Chemical Purification of Terbium-155 from Pseudo-Isobaric Impurities in a Mass Separated Source Produced at CERN

Four terbium radioisotopes (^{149, 152, 155, 161}Tb) constitute a potential theranostic quartet for cancer treatment but require any derived radiopharmaceutical to be essentially free of impurities. Terbium-155 prepared by proton irradiation and on-line mass separation at the CERN-ISOLDE and CERN-MEDICIS facilities contains radioactive ¹³⁹Ce¹⁶O and also zinc or gold, depending on the catcher foil used. A method using ion-exchange and extraction chromatography resins in two column separation steps has been developed to isolate ¹⁵⁵Tb with a chemical yield of ≥95% and radionuclidic purity ≥99.9%. Conversion of terbium into a form suitable for chelation to targeting molecules in diagnostic nuclear medicine is presented. The resulting ¹⁵⁵Tb preparations are suitable for the determination of absolute activity, SPECT phantom imaging studies and pre-clinical trials.

Studies on purification of 89Sr from irradiated yttria target by multi-column extraction chromatography using DtBuCH18-C-6/XAD-7 resin

89Sr is being produced using yttria target via the nuclear reaction 89Y(n,p)89Sr in Fast Breeder Test Reactor (FBTR), Kalpakkam. The isotope 89Sr is a pure beta emitter with a half-life of 50.53 days which is useful mainly for bone pain palliation in patients with bone metastases. The existing method for processing the irradiated yttria target to obtain the pure 89Sr source involves separation of the bulk yttrium target by solvent extraction using TBP-HNO3 followed by purification of 89Sr source by cation exchange chromatography technique using Dowex resin. The study described here involves the selective extraction and purification of 89Sr by multi-column extraction chromatography technique using the Sr-specific crown ether, DtBuCH18C6 (CE) coated onto an XAD-7 resin matrix for superior separation and increased yield compared to single column technique. The 89Sr source thus purified from the irradiated yttria target is free from other radionuclidic impurities produced during the target irradiation i.e. 88Y, 65Zn, 139,141Ce, 154Eu and 160Tb.