Exploration of commercial cyclen-based chelators for mercury-197 m/g incorporation into theranostic radiopharmaceuticals

Abstract

A comprehensive investigation of the Hg2+ coordination chemistry and 197m/gHg radiolabeling capabilities of cyclen-based commercial chelators, namely, DOTA and DOTAM (aka TCMC), along with their bifunctional counterparts, p-SCN-Bn-DOTA and p-SCN-Bn-TCMC, was conducted to assess the suitability of these frameworks as bifunctional chelators for the 197m/gHg2+ theranostic pair. Radiolabeling studies revealed that TCMC and DOTA exhibited low radiochemical yields (0%–6%), even when subjected to harsh conditions (80°C) and high ligand concentrations (10–4 M). In contrast, p-SCN-Bn-TCMC and p-SCN-Bn-DOTA demonstrated significantly higher 197m/gHg radiochemical yields (100% ± 0.0% and 70.9% ± 1.1%, respectively) under the same conditions. The [197 m/gHg]Hg-p-SCN-Bn-TCMC complex was kinetically inert when challenged against human serum and glutathione. To understand the differences in labeling between the commercial chelators and their bifunctional counterparts, non-radioactive natHg2+ complexes were assessed using NMR spectroscopy and DFT calculations. The NMR spectra of Hg-TCMC and Hg-p-SCN-Bn-TCMC suggested binding of the Hg2+ ion through the cyclen backbone framework. DFT studies indicated that binding of the Hg2+ ion within the backbone forms a thermodynamically stable product. However, competition can form between isothiocyanate binding and binding through the macrocycle, which was experimentally observed. The isothiocyanate bound coordination product was dominant at the radiochemical scale as, in comparison, the macrocycle bound product was seen at the NMR scale, agreeing with the DFT result. Furthermore, a bioconjugate of TCMC (TCMC-PSMA) targeting prostate-specific membrane antigen was synthesized and radiolabeled, resulting in an apparent molar activity of 0.089 MBq/nmol. However, the complex demonstrated significant degradation over 24 h when exposed to human serum and glutathione. Subsequently, cell binding assays were conducted, revealing a Ki value ranging from 19.0 to 19.6 nM. This research provides crucial insight into the effectiveness of current commercial chelators in the context of 197m/gHg2+ radiolabeling. It underscores the necessity for the development of specific and customized chelators to these unique “soft” radiometals to advance 197m/gHg2+ radiopharmaceuticals.

Capturing Mercury-197m/g for Auger Electron Therapy and Cancer Theranostic with Sulfur-Containing Cyclen-Based Macrocycles

“…The results obtained for DOTA are in agreement with the outcomes reported in a recent publication where the inability of this chelator of incorporating [ 197m/g Hg]Hg 2+ was already described. 10 This finding underscores the important role of the soft donor atoms on the pendant arms to trigger the formation of [ 197m/g Hg]Hg-complexes, albeit rather high ligand concen-trations have been used to achieve quantitative yields. Nonetheless, it cannot be excluded that the presence of contaminating metals originating from the in-house production of [ 197m/g Hg]Hg 2+ , such as gold derived from a target material, may contribute to this effect (which seems to be extremely marked for DO3S among the S-containing macrocycles) by competing with the studied chelators.…”

“…10,26 The mixture was then applied onto SDS-PAGE gels and electrophoresed at room temperature and 150 V until the dye front traversed the resolving gel (60 min). 10,26 Following electrophoresis, the gel underwent scanning using the radio-TLC scanner previously mentioned.…”

“…25,26 Commercially available chelators, specifically 1,4,7,10-tetrazacyclododecane-1,4,7,10-tetracetic acid (DOTA), 1,4,7,10-tetrakis-(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (TCMC), and their bifunctional counterparts, namely, S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA) and S-2-(4-isothiocyanatobenzyl)-1,4,7,10tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane (p-SCN-Bn-TCMC), have also been recently explored (Figure 1C). 10 While DOTA and TCMC were not able to incorporate 197m/g Hg, the introduction of the p-SCN-Bn moiety increased the radiochemical yields due to Hg 2+ -isothiocyanate-favorable interactions. However, only p-SCN-Bn-TCMC demonstrated quantitative labeling ability, though the stability of its 197m/g Hg complex in human serum remained moderate.…”

“…Thus, the development of enhanced chelating platforms is still an imperative need to fully unlock the theranostic potential of 197m/g Hg. 10 To address this pressing demand, this research embarks on a comprehensive theoretical and experimental coordination chemistry exploration, delving into the thermodynamic, kinetic, and structural properties of a series of Hg 2+ complexes formed with different sulfur-containing macrocycles. 27 This class of chelators, comprising 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), and 1,7-bis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane-4,10, diacetic acid (DO2A2S), are based on the 1,4,7,10-tetrazacyclododecane (cyclen) ring and varies in the nature of the pendant arms which modulate their overall chemical softness (Figure 1D).…”

“…10,26 The migration patterns of free [ 197m/g Hg]Hg 2+ incubated in serum, [ 197m/g Hg]Hg 2+ complex incubated in PBS, and [ 197m/g Hg]-Hg 2+ complex incubated in serum are illustrated in FigureS31.…”