Therapeutic Radionuclides: Production, Physical Characteristics, and Applications

Towards Clinical Development of Scandium Radioisotope Complexes for Use in Nuclear Medicine: Encouraging Prospects with the Chelator 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic Acid (DOTA) and Its Analogues

Scandium (Sc) isotopes have recently attracted significant attention in the search for new radionuclides with potential uses in personalized medicine, especially in the treatment of specific cancer patient categories. In particular, Sc-43 and Sc-44, as positron emitters with a satisfactory half-life (3.9 and 4.0 h, respectively), are ideal for cancer diagnosis via Positron Emission Tomography (PET). On the other hand, Sc-47, as an emitter of beta particles and low gamma radiation, may be used as a therapeutic radionuclide, which also allows Single-Photon Emission Computed Tomography (SPECT) imaging. As these scandium isotopes follow the same biological pathway and chemical reactivity, they appear to fit perfectly into the "theranostic pair" concept. A step-by-step description, initiating from the moment of scandium isotope production and leading up to their preclinical and clinical trial applications, is presented. Recent developments related to the nuclear reactions selected and employed to produce the radionuclides Sc-43, Sc-44, and Sc-47, the chemical processing of these isotopes and the main target recovery methods are also included. Furthermore, the radiolabeling of the leading chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), and its structural analogues with scandium is also discussed and the advantages and disadvantages of scandium complexation are evaluated. Finally, a review of the preclinical studies and clinical trials involving scandium, as well as future challenges for its clinical uses and applications, are presented.

Radiopharmaceutical transport in solid tumors via a 3-dimensional image-based spatiotemporal model

Lutetium-177 prostate-specific membrane antigen (177Lu-PSMA)-targeted radiopharmaceutical therapy is a clinically approved treatment for patients with metastatic castration-resistant prostate cancer (mCRPC). Even though common practice reluctantly follows "one size fits all" approach, medical community believes there is significant room for deeper understanding and personalization of radiopharmaceutical therapies. To pursue this aim, we present a 3-dimensional spatiotemporal radiopharmaceutical delivery model based on clinical imaging data to simulate pharmacokinetic of 177Lu-PSMA within the prostate tumors. The model includes interstitial flow, radiopharmaceutical transport in tissues, receptor cycles, association/dissociation with ligands, synthesis of PSMA receptors, receptor recycling, internalization of radiopharmaceuticals, and degradation of receptors and drugs. The model was studied for a range of values for injection amount (100-1000 nmol), receptor density (10-500 nmol•l-1), and recycling rate of receptors (10-4 to 10-1 min-1). Furthermore, injection type, different convection-diffusion-reaction mechanisms, characteristic time scales, and length scales are discussed. The study found that increasing receptor density, ligand amount, and labeled ligands improved radiopharmaceutical uptake in the tumor. A high receptor recycling rate (0.1 min-1) increased radiopharmaceutical concentration by promoting repeated binding to tumor cell receptors. Continuous infusion results in higher radiopharmaceutical concentrations within tumors compared to bolus administration. These insights are crucial for advancing targeted therapy for prostate cancer by understanding the mechanism of radiopharmaceutical distribution in tumors. Furthermore, measures of characteristic length and advection time scale were computed. The presented spatiotemporal tumor transport model can analyze different physiological parameters affecting 177Lu-PSMA delivery.

Development of purification of no-carrier-added 47Sc of theranostic interest: selective separation study from the natTi(n,p) process

Scandium-47 is one of the most promising medical radioisotopes, and its production and trace separation make it an attractive candidate for theranostic application. In this study, the production of no-carrier-added (NCA) 47Sc through the natTi(n,p) reaction and subsequent purification using liquid–liquid extraction was done for the theranostic application. The comparative separation of NCA 47Sc after the dissolution of an activated Ti target using Di-2-Ethylhexyl Phosphoric Acid (HDEHP) in kerosene was evaluated. The extraction process was optimized in terms of the concentration of extractant, extraction time, pH, and reaction temperature to achieve the maximum possible separation. HDEHP is efficient and promising for rapid extraction and separation of NCA 47Sc from Ti ions at low acidity (pH 0.85) with high extraction percent (>99%), contaminated with 22.3% of Ti ions after 5 min of extraction time. Different stripping reagents were used to separate loaded 47Sc and Ti ions. Firstly, 5 M HCl was enough for stripping the loaded Ti ions. Then the loaded 47Sc was separated with a purity of 100% using 0.05 M NaOH. The obtained results find the HDEHP a promising extractant for efficient separation of 47Sc from irradiated Ti target for preparing the 47Sc radiopharmaceuticals for theranostics applications.

Copper Coordination Chemistry of Sulfur Pendant Cyclen Derivatives: An Attempt to Hinder the Reductive-Induced Demetalation in 64/67Cu Radiopharmaceuticals

The Cu²⁺ complexes formed by a series of cyclen derivatives bearing sulfur pendant arms, 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO3S), 1,4,7-tris[2-(methylsulfanyl)ethyl]-10-acetamido-1,4,7,10-tetraazacyclododecane (DO3SAm), and 1,7-bis[2-(methylsulfanyl)ethyl]-4,10-diacetic acid-1,4,7,10-tetraazacyclododecane (DO2A2S), were studied in aqueous solution at 25 °C from thermodynamic and structural points of view to evaluate their potential as chelators for copper radioisotopes. UV-vis spectrophotometric out-of-cell titrations under strongly acidic conditions, direct in-cell UV-vis titrations, potentiometric measurements at pH >4, and spectrophotometric Ag⁺-Cu²⁺ competition experiments were performed to evaluate the stoichiometry and stability constants of the Cu²⁺ complexes. A highly stable 1:1 metal-to-ligand complex (CuL) was found in solution at all pH values for all chelators, and for DO2A2S, protonated species were also detected under acidic conditions. The structures of the Cu²⁺ complexes in aqueous solution were investigated by UV-vis and electron paramagnetic resonance (EPR), and the results were supported by relativistic density functional theory (DFT) calculations. Isomers were detected that differed from their coordination modes. Crystals of [Cu(DO4S)(NO₃)]·NO₃ and [Cu(DO2A2S)] suitable for X-ray diffraction were obtained. Cyclic voltammetry (CV) experiments highlighted the remarkable stability of the copper complexes with reference to dissociation upon reduction from Cu²⁺ to Cu⁺ on the CV time scale. The Cu⁺ complexes were generated in situ by electrolysis and examined by NMR spectroscopy. DFT calculations gave further structural insights. These results demonstrate that the investigated sulfur-containing chelators are promising candidates for application in copper-based radiopharmaceuticals. In this connection, the high stability of both Cu²⁺ and Cu⁺ complexes can represent a key parameter for avoiding in vivo demetalation after bioinduced reduction to Cu⁺, often observed for other well-known chelators that can stabilize only Cu²⁺.

Production scheme for diagnostic-therapeutic radioisotopes by accelerator neutrons

Interest has been growing in the development of medical radioisotopes used for noninvasive nuclear medicine imaging of disease and cancer therapy. Especially the development of an alternative production scheme of ⁹⁹Mo, the mother radioisotope of ^99mTc used for imaging, is required, because the current supply chain of the reactor product ⁹⁹Mo is fragile worldwide. We have proposed a new production scheme of ⁹⁹Mo as well as therapeutic radioisotopes, such as ⁶⁴Cu and ⁶⁷Cu, using accelerator neutrons provided by the ^natC(d,n) reaction. Based on this scheme we have obtained high-quality ^99mTc, ⁶⁴Cu, and ⁶⁷Cu suitable for clinical use by developing both production and separation methods of the radioisotopes. We proposed a new facility to constantly and reliably produce a wide variety of high-quality, carrier-free radioisotopes, including ⁹⁹Mo, with accelerator neutrons. We report on the development of the proposed scheme and future prospects of the facility toward the domestic production of medical radioisotopes.

An assessment of bone-seeking radionuclides for palliation of metastatic bone pain in a vertebral model

OBJECTIVE

Bone-seeking radiopharmaceuticals have the main role in the treatment of painful bone metastases. The aim of this study was to dosimetrically compare radiopharmaceuticals in use for bone pain palliation therapy and bone scan.

METHODS

The MCNPX code was used to simulate the radiation transport in a vertebral phantom. Absorbed fractions were calculated for monoenergetic electrons, photons and alpha particles. S values were obtained for radionuclides ³²P, ³³P, ⁸⁹Sr, ⁹⁰Y, ^99mTc, ^117mSn, ¹⁵³Sm, ¹⁶⁶Ho, ¹⁶⁹Er, ¹⁷⁷Lu, ¹⁸⁶Re, ¹⁸⁸Re, ²²³Ra, ²²⁴Ra and their progenies for target regions including the active marrow and the bone endosteum.

RESULTS

The results demonstrated the dependence of dosimetric parameters on the source or target size, particle energy and location of the source. The electron emitters including ³³P, ^117mSn, ¹⁶⁹Er and ¹⁷⁷Lu and ²²³Ra as an α-emitter gave the lower absorbed dose to the active marrow. These radionuclides gave the highest values of the Relative Advantage Factor (RAF).

CONCLUSIONS

According to the results, ³³P, ^117mSn, ¹⁶⁹Er, ¹⁷⁷Lu and ²²³Ra have fewer side effects on the active marrow than other investigated radionuclides. Therefore, these radionuclides may be a better choice for use in palliative radiotherapy.

Development of separation technology for the removal of radium-223 from targeted thorium conjugate formulations. Part II: purification of targeted thorium conjugates on cation exchange columns

Tumor targeting pharmaceuticals will play a crucial role in future pharma pipelines. The targeted thorium conjugate (TTC) therapeutic platform could provide real benefit to patients, whereby targeting moieties like monoclonal antibodies are radiolabelled with the alpha-emitting radionuclide thorium-227 (²²⁷Th, t_1/2 = 18.7 days). A potential problem could be the accumulation of the long-lived daughter nuclide radium-223 (²²³Ra, t_1/2 = 11.4 days) in the drug product during manufacturing and distribution. Therefore, the level of ²²³Ra must be standardized before administration to the patient. The focus in this study has been the removal of ²²³Ra, as the other progenies will have a very limited stay in the formulation. In this study, the purification of TTCs labeled with decayed ²²⁷Th has been explored. Columns packed with a strong cation exchange resin have been used to sequester ²²³Ra. The separation of TTC from ²²³Ra has been evaluated as influenced by both formulation and process parameters with a design of experiments (DOE) study; including citrate or acetate buffer, pH, buffer concentration, presence or absence of pABA + EDTA, resin amount and sodium chloride concentration. The aim was to achieve a separation with high sorption of ²²³Ra and accompanying low TTC sorption. The results were analyzed by multivariate analysis. Four regression models of TTC and ²²³Ra sorption from citrate and acetate buffered formulations were developed. The predictive accuracy of sorption in the four statistical models was given by standard deviations and confidence intervals. The TTC sorption in citrate and acetate buffered formulations was affected by the identical variables and the variation in TTC sorption was comparable for the two models. However, the DOE variables had a significantly stronger impact on the ²²³Ra sorption in citrate buffered formulations than the ²²³Ra sorption in acetate buffer. An optimal separation with a TTC sorption below 25% and ²²³Ra sorption above 90% can be achieved in both citrate and acetate buffered formulations. Stability studies of radiochemical purity (RCP) indicated that the measured ²²⁷Th values may be partly due to free ²²⁷Th and not TTC, but the results indicate that TTC stability may be controlled by optimizing formulation parameters. Hence, the sorption data and the regression models presented must be reviewed and further explored with regard to what is known about the stability of the TTC in the different buffered formulations.