Preparation of highly purified 68Ga solutions via ion exchange in hydrochloric acid–ethanol mixtures

Preclinical evaluation of 68Ga-labeled folic acid conjugates for visualization of inflammatory foci

INTRODUCTION

Folate receptors (FR) have been considered a convenient target for different radiopharmaceuticals in recent years. Multifarious 68Ga-labeled folate conjugates have been proposed as promising agents for the PET imaging of FR-overexpressing malignant neoplasms. In addition, radiolabeled folate-based conjugates can be effective for imaging non-tumor pathological foci characterized by a pronounced cluster of activated macrophages. We previously reported that a conjugate of folic acid with the NODAGA-chelator, labeled with gallium-68 and containing a (His-Glu)2-tag ([68Ga]Ga-NODAGA-[Lys-(HE)2]-folic acid), is suitable for imaging tumor lesions characterized by an increased density of FR. Introduction of the (His-Glu)2-tag into the structure of the folate radioconjugate significantly reduced its accumulation in non-target tissues (e.g., kidneys), leaving the accumulation in tumors at least at the same level, and even increasing it. The present study assessed the suitability of the developed molecule (in comparison with the unmodified analog) for imaging foci of non-oncological etiology characterized by a pronounced macrophage response.

METHODS

Systemic juvenile idiopathic arthritis (JIA), reproduced in Wistar rats, was used as the pathology model. Acute inflammatory processes of soft tissues of septic and aseptic etiologies were selected as differential models.

RESULTS

The results obtained in this study showed a significantly elevated level of accumulation in the JIA focus compared to healthy rat paws and accumulation in the foci of differential models of the inflammatory process, which confirms the macrophage-mediated pathway of accumulation of the studied compounds. Simultaneously, the ratio of accumulation in pathology to accumulation in comparable healthy tissues in all studied pathologies was significantly high.

CONCLUSION

The data obtained allowed us to conclude the diagnostic potential of new radiolabeled folate-based conjugate with (His-Glu)2-tag for pharmacokinetic property optimization in the radionuclide diagnosis of rheumatoid and other diseases characterized by a pronounced macrophage immune response. The mathematical compartment model quantitatively confirmed that the additional (His-Glu)2 fragment introduced in the molecule acts in favor of potential radiopharmaceutical use to visualize inflammatory processes by positron emission tomography.

Evaluation of chelating agents based on pyridine-azacrown compounds H4PATA, PATAM, and H4PATPA for 68Ga and 177Lu

In this article, we present the synthesis and characterization of three macrocyclic chelators, H4PATA, PATAM, and H4PATPA, based on a pyridine-azacrown compound. Their complexation with 68Ga and 177Lu has been thoroughly investigated using MALDI TOF MS, 1H NMR spectroscopy, radiolabeling studies, and experiments in vitro with fetal bovine serum and a 1000-fold molar excess of H4EDTA. Our studies have shown that the chelators H4PATA and H4PATPA form complexes at room temperature with both radionuclides (RCY > 80 % and > 90 % for complexes with H4PATA and H4PATPA after 30 min, respectively). The chelator PATAM requires high temperature (95 °C) for complexation. In vitro stability assays in fetal bovine serum as well as H4EDTA-challenge revealed that transchelation occurs for all complexes with 68Ga. However, complexes of the ligands H4PATA and PATAM with 177Lu were found stable. Thus, taking into account the radiolabeling at room temperature and in vitro stability of the complex [177Lu]Lu·PATA, our investigations revealed the chelator H4PATA is a candidate for radiopharmaceutical use with 177Lu.

Improvement of End-of-Synthesis Radiochemical Purity of 177Lu-DOTA-PSMA-Ligands with Alternative Synthesis Approaches: Conversion Upswing and Side-Products Minimization

BACKGROUND

Radiochemical purity is a key criterion for the quality of radiopharmaceuticals used in clinical practice. The joint improvement of analytical methods capable of identifying related radiochemical impurities and determining the actual radiochemical purity, as well as the improvement of synthesis methods to minimize the formation of possible radiochemical impurities, is integral to the implementation of high-tech nuclear medicine procedures. PSMA-targeted radionuclide therapy with lutetium-177 has emerged as an effective treatment option for prostate cancer, and [177Lu]Lu-PSMA-617 and [177Lu]Lu-PSMAI&T have achieved global recognition as viable radiopharmaceuticals. Recently, it was shown that specific radiochemical impurities can form during the synthesis of [177Lu]Lu-PSMA-617 because of a spontaneous, thermally mediated condensation of the Glu-C(O)-Lys fragment, resulting in the formation of three different cyclic forms (with no affinity for PSMA). During this study, we identified another impurity, a product of detachment of the Glu-CO fragment from PSMA-617, caused by heating. The total content of all four thermally mediated degradation products may reach 9-11% during classical incubation for 30 min at 95 °C, reducing the radiochemical purity to an unacceptable level (albeit with high levels of radiochemical conversion). It is reasonable to assume that the formation of similar impurities is characteristic of all PSMA-specific vectors that contain Glu-C(O)-Lys pharmacophores. Because the formation of these impurities directly depends on the temperature and incubation time, to reduce their content in the reaction mixture at the end of the synthesis, it is necessary to select conditions to achieve a high level of radiochemical conversion for the minimum possible time and/or at the minimum sufficient temperature.

METHODS

In this study, using [177Lu]Lu-PSMA-617 as an example, we evaluated the efficiency of alternative methods of synthesis with microwave heating and co-solvent (ethanol) addition to ensure radiochemical yield and radiochemical purity in the shortest possible time and at the minimum necessary and sufficient synthesis temperature.

RESULTS

Both approaches achieved a significant reduction in the impurities content, while achieving satisfactory synthesis yields in a short time. In addition to improving the synthesis parameters and radiochemical purity, the use of microwave heating and the addition of ethanol reduces the negative influence of other auxiliaries on labeling kinetics. Notably, the addition of ethanol under certain conditions allowed [177Lu]Lu-PSMA-617 to be synthesized at room temperature for only 10 min. This makes it possible to achieve exceptionally high real radiochemical purity of the preparations, determined only by the quality of the original precursor. The approaches considered in this study can be successfully applied to improve the synthesis process and quality parameters of the finished product, both for known radiopharmaceuticals and for those under development.

Synthesis of new acyclic chelators H4aPyta and H6aPyha and their complexes with Cu2+, Ga3+, Y3+, and Bi3

In this article, we present the synthesis and characterization of new acyclic pyridine-containing polyaminocarboxylate ligands H4aPyta and H6aPyha, which differ in structural rigidity and the number of chelating groups. Their abilities to form complexes with Cu2+, Ga3+, Y3+, and Bi3+ cations, as well as the stability of the complexes, were evaluated by potentiometric titration method, radiolabeling with the corresponding radionuclides, in vitro studies, mass spectrometry, and HPLC. The structures of the resulting complexes were determined using NMR spectroscopy and DFT calculations. The results obtained made it possible to evaluate the influence of the structural features of the complexes on their stability. The developed chelators H4aPyta and H6aPyha were proved to be promising for further research in the field of radiopharmaceuticals.

Cyclotron Production of Gallium-68 Radiopharmaceuticals Using the 68Zn(p,n)68Ga Reaction and Their Regulatory Aspects

Designing and implementing various radionuclide production methods guarantees a sustainable supply, which is important for medical use. The use of medical cyclotrons for radiometal production can increase the availability of gallium-68 (⁶⁸Ga) radiopharmaceuticals. Although generators have greatly influenced the demand for ⁶⁸Ga radiopharmaceuticals, the use of medical cyclotrons is currently being explored. The resulting ⁶⁸Ga production is several times higher than obtained from a generator. Moreover, the use of solid targets yields end of purification and end of synthesis (EOS) of up to 194 GBq and 72 GBq, respectively. Furthermore, experiments employing liquid targets have provided promising results, with an EOS of 3 GBq for [⁶⁸Ga]Ga-PSMA-11. However, some processes can be further optimized, specifically purification, to achieve high ⁶⁸Ga recovery and apparent molar activity. In the future, ⁶⁸Ga will probably remain one of the most in-demand radionuclides; however, careful consideration is needed regarding how to reduce the production costs. Thus, this review aimed to discuss the production of ⁶⁸Ga radiopharmaceuticals using Advanced Cyclotron Systems, Inc. (ACSI, Richmond, BC, Canada) Richmond, Canada and GE Healthcare, Wisconsin, USA cyclotrons, its related factors, and regulatory concerns.

Separation of Scandium from Hydrochloric Acid-Ethanol Leachate of Bauxite Residue by a Supported Ionic Liquid Phase

Solvometallurgy is a new branch of extractive metallurgy in which green organic solvents are used instead of aqueous solutions to improve selectivity in separation processes. In the present study, nonaqueous leaching of a Greek bauxite residue (BR) was performed and scandium was separated from other elements in the leachate by column chromatography. At first, the selectivity of sorbents for scandium(III) over iron(III) was tested in batch mode using various organic solvents. The following three sorbents were tested: (1) a carboxylic acid-functionalized supported ionic liquid phase (SILP), (2) silica (SiO2), and (3) silica functionalized with ethylenediaminetetraacetic acid (SiO2-TMS-EDTA). The best separation of scandium and iron was achieved from ethanolic solution by the SILP. The BR was then leached with 0.7 mol L-1 HCl in ethanol or in water. The leaching efficiency of scandium with both lixiviants was similar. However, much less sodium was leached, and silica remained in solution when leaching was performed with the ethanolic lixiviant. By using ethanol as opposed to water, the serious drawback of silica gel formation that is taking place in the aqueous leachate of BR was circumvented. The sorption preference of the SILP for metal ions in the ethanolic leachate was partly reversed compared to the aqueous leachate. Iron was separated from other metals of the ethanolic BR leachate by a simple elution with ethanol. The formation of the anionic tetrachloroferrate(III) complex, [FeCl4]-, enabled the selective elution. This complex was not observed in the aqueous leachate of BR. Scandium was separated from the vast majority of other components of the BR by elution with 0.1 mol L-1 H3PO4.

Pharmacokinetic Properties of 68Ga-labelled Folic Acid Conjugates: Improvement Using HEHE Tag

The folate receptor (FR) is a promising cell membrane-associated target for molecular imaging and radionuclide therapy of cancer (FR-α) and potentially also inflammatory diseases (FR-β) through use of folic acid-based radioconjugate. FR is often overexpressed by cells of epithelial tumors, including tumors of ovary, cervix, endometrium, lungs, kidneys, etc. In healthy tissues, FR can be found in small numbers by the epithelial cells, mainly in the kidneys. Extremely high undesired accumulation of the folate radioconjugates in the renal tissue is a main drawback of FR-targeting concept. In the course of this work, we aimed to reduce the undesirable accumulation of folate radioconjugates in the kidneys by introducing a histidine/glutamic acid tag into their structure. Two folic acid based compounds were synthesized: NODAGA-1,4-butanediamine-folic acid (FA-I, as control) and NODAGA-[Lys-(HE)₂]-folic acid (FA-II) which contains a (His-Glu)₂ fragment. In vitro studies with FR (+) cells (KB and others) showed that both compounds have specificity for FR. Introduction of (HE)₂-tag does not affect FR binding ability of the conjugates. In vivo biodistribution studies with normal laboratory animals, as well as with KB tumor bearing animals, were carried out. The results showed that introduction of the (HE)₂ tag into the structure of folate radioconjugates can significantly reduce the accumulation of these compounds in non-target tissues and important organs (the accumulation in the kidneys is reduced 2-4 times), leaving the accumulation in tumor at least at the same level, and even increasing it.