Radiation Stability of Sorbents in Medical 225Ac/213Bi Generators

A review of recent advancements in Actinium-225 labeled compounds and biomolecules for therapeutic purposes

In nuclear medicine, cancers that cannot be cured or can only be treated partially by traditional techniques like surgery or chemotherapy are killed by ionizing radiation as a form of therapeutic treatment. Actinium-225 is an alpha-emitting radionuclide that is highly encouraging as a therapeutic approach and more promising for targeted alpha therapy (TAT). Actinium-225 is the best candidate for tumor cells treatment and has physical characteristics such as high (LET) linear energy transfer (150 keV per μm), half-life (t1/2  = 9.92d), and short ranges (400-100 μm) which prevent the damage of normal healthy tissues. The introduction of various new radiopharmaceuticals and radioisotopes has significantly assisted the advancement of nuclear medicine. Ac-225 radiopharmaceuticals continuously demonstrate their potential as targeted alpha therapeutics. 225 Ac-labeled radiopharmaceuticals have confirmed their importance in medical and clinical areas by introducing [225 Ac]Ac-PSMA-617, [225 Ac]Ac-DOTATOC, [225 Ac]Ac-DOTA-substance-P, reported significantly improved response in patients with prostate cancer, neuroendocrine, and glioma, respectively. The development of these radiopharmaceuticals required a suitable buffer, incubation time, optimal pH, and reaction temperature. There is a growing need to standardize quality control (QC) testing techniques such as radiochemical purity (RCP). This review aims to summarize the development of the Ac-225 labeled compounds and biomolecules. The current state of their reported resulting clinical applications is also summarized as well.

Gamma radiation effects on AG MP-50 cation exchange resin and sulfonated activated carbon for bismuth-213 separation

Medical 225Ac/213Bi radionuclide generators are designed to provide a local supply of the short-lived 213Bi for cancer treatment. However, radiation-induced damage to the sorbents commonly used in such radionuclide generators remains a major concern. In this study, the effects of gamma radiation on AG MP-50 cation exchange resin and sulfonated activated carbon (SAC) were studied by analyzing the changes in the morphological characteristics, functional groups, and the La3+/Bi3+ sorption performance, with La3+ being a suitable non-radioactive substitute for Ac3+. The surface sulfonic acid groups of AG MP-50 resin suffered from severe radiation-induced degradation, while the particle morphology was changed markedly after being exposed to absorbed doses of approximately 11 MGy. As a result, the sorption performance of irradiated AG MP-50 for La3+ and Bi3+ was significantly decreased with increasing absorbed doses. In contrast, no apparent changes in acquired morphological characteristics were observed for pristine and irradiated SAC based on SEM and XRD characterization. The surface oxygen content (e.g., O-C[double bond, length as m-dash]O) of irradiated SAC increased for an absorbed dose of 11 MGy due to free radical-induced oxidation. The sorption performance of pristine and irradiated SAC materials for La3+ and Bi3+ remained generally the same at pH values of 1 and 2. Furthermore, the applicability of AG MP-50 and SAC in the 225Ac/213Bi generators was illustrated in terms of their radiolytic stability. This study provides further evidence for the practical implementation of both AG MP-50 and SAC in 225Ac/213Bi radionuclide generators.

DTPA(DOTA)-Nimotuzumab Radiolabeling with Generator-produced Thorium for Radioimmunotherapy of EGFR-overexpressing Carcinomas

INTRODUCTION

The feasibility of preparing the "in-house" generators and the Th- DTPA(DOTA)-Nimotuzumab radioimmunoconjugate was evaluated. 226Th is perspective for TAT, however, due to short half-life it is preferable to apply this radionuclide for readily available epithelial malignancies. Nimotuzumab being specific for EGFR expressing cells as a targeting moiety is considered to be suitable for thorium delivery.

METHODS

TEVA extraction chromatographic resin and anion exchange resin AG 1x8 were used as sorbents for 226Th generator. In order to determine features of labeling by Th4+ we applied 234Th as a longer-lived analog of short-lived 226Th and the immunoconjugates DTPA(DOTA)-Nimotuzumab were used for radiolabeling.

RESULTS

The generator on the base of TEVA resin has shown higher volume activity of the product compared to the AG 1x8. The 226Th volume concentration was up to 80%/mL. The radiolabeling of BFCA by thorium radioisotopes reached 95% at the MR(Th:p-SCN-Bn-DTPA) = 1:100 and 86% for MR(Th:p-SCN-Bn-DOTA) = 1:5000 at 90°C. The procedure of Nimotuzumab labeling with Th4+ for radiotherapy of EGFR-overexpressing carcinomas was established. The overall labeling yield in both radioimmunoconjugates - DTPA and DOTA functionalized - was in the range of 45-50%. The immunoconjugate Nimotuzumab-p-SCN-Bn-DTPA was obtained with a molar ratio 1:25 (Nimotuzumab: BFCA), within 1 hour of conjugation at 25°C and labelled via postconjugation approach. Whereas Nimotuzumab-p-SCN-Bn-DOTA was obtained at the same conditions, but radiolabeled by the method of pre-conjugation.

CONCLUSION

Thorium-234 incorporation into both radioimmunoconjugates reached 45-50%. It has been shown that Th-DTPA-Nimotuzumab radioimmunoconjugate specifically bound with EGFR overexpressing epidermoid carcinoma A431 cells.

Rapid Elution of 226Th from a Two-Column 230U/226Th Generator with Diluted and Buffer Solutions

A radionuclide generator of the short-lived alpha emitter ²²⁶Th was proposed. An original scheme consisting of two in-series chromatographic columns was developed for rapidly producing a neutral citric-buffered eluate of high purity ²²⁶Th. The first column filled with TEVA resin retained the parent ²³⁰U, while ²²⁶Th was eluted with 7 M HCl solution to be immediately adsorbed on the second column containing DGA resin or UTEVA resin. Having substituted the strongly acidic medium of second column with neutral salt solution, ²²⁶Th was desorbed with diluted citric buffer solution. One cycle of generator milking took 5-7 min and produced >90% of ²²⁶Th in 1.5 mL of eluate (pH 4.5-5.0) appropriate for direct use in radiopharmaceutical synthesis. The ²³⁰U impurity in ²²⁶Th eluate was less than 0.01%. The proposed two-column ²³⁰U/²²⁶Th generator was tested over 2 months including a second loading of ²³⁰U additionally accumulated from ²³⁰Pa.

A Radionuclide Generator of High-Purity Bi-213 for Instant Labeling

A new two-column ²²⁵Ac/²¹³Bi generator was developed specifically for using ²²⁵Ac containing an impurity of long lived ²²⁷Ac. The parent ²²⁵Ac was retained on the first Actinide Resin column, while ²¹³Bi was accumulated on the second column filled with AG MP-50 resin via continuous elution and decay of intermediate ²²¹Fr. The ²¹³Bi accumulation was realized in circulation mode which allowed a compact generator design. It was demonstrated that ²¹³Bi could be quickly and effectively extracted from AG MP-50 in form of complexes with various chelating agents including DTPA and DOTA. The performance of the generator presented and a conventional single-column generator on the base of AG MP-50 was tested and both generators were loaded with ²²⁵Ac containing ²²⁷Ac impurity. The ²¹³Bi generation efficiencies were comparable and greater than 70%, whereas the developed generator provided a deeper degree of purification of ²¹³Bi from Ac isotopes and decay products of ²²⁷Ac.