Exploring Cyclic Aminopolycarboxylate Ligands for Sb(III) Complexation: PCTA and Its Derivatives as a Promising Solution

Towards the Stable Chelation of Radioantimony(V) for Targeted Auger Theranostics

Antimony-119 (119Sb) is one of the most attractive Auger-electron emitters identified to date, but it remains practically unexplored for targeted radiotherapy because no chelators have been identified to stably bind this metalloid in vivo. In a departure from current studies focused on chelator development for Sb(III), we explore the chelation chemistry of Sb(V) using the tris-catecholate ligand TREN-CAM. Through a combination of radiolabeling, spectroscopic, solid-state, and computational studies, the radiochemistry and structural chemistry of TREN-CAM with 1XX/natSb(V) were established. The resulting [1XXSb]Sb-TREN-CAM complex remained intact for several days in human serum, signifying high stability under biological conditions. Finally, the first in vivo single photon emission computed tomography and positron emission tomography imaging studies were carried out using 117Sb, the diagnostic analogue of 119Sb. These studies revealed marked differences in the uptake and distribution of activity in mice administered unchelated [117Sb]Sb(OH)6 - versus [117Sb]Sb-TREN-CAM, suggesting that 117Sb is largely retained by TREN-CAM over the time course of the study. Collectively, these findings demonstrate the most physiologically stable complex of no-carrier-added 1XXSb yet reported, offering new promise for the clinical implementation of radioantimony in nuclear medicine. Our results also establish the feasibility of 117Sb as an elementally matched partner to 119Sb for theranostic applications.

Chemistry of Antimony in Radiopharmaceutical Development: Unlocking the Theranostic Potential of Sb Isotopes

Antimony-119 (119Sb) holds promise for radiopharmaceutical therapy (RPT), emitting short-range Auger and conversion electrons that can deliver cytotoxic radiation on a cellular level. While it has high promise theoretically, experimental validation is necessary for 119Sb in vivo applications. Current 119Sb production and separation methods face robustness and compatibility challenges in radiopharmaceutical synthesis. Limited progress in chelator development hampers targeted experiments with 119Sb. This review compiles literature on the toxicological, biodistribution and redox properties of Sb, along with existing Sb complexes, evaluating their suitability for radiopharmaceuticals. Sb(III) is suggested as the preferred oxidation state for radiopharmaceutical elaboration due to its stability in vivo and lack of skeletal uptake. While Sb complexes with both hard and soft donor atoms can be achieved, Sb thiol complexes offer enhanced stability and compatibility with the desired Sb(III) oxidation state. For 119Sb to find application in RPT, scientists need to make discoveries and advancements in the areas of isotope production, and radiometal chelation. This review aims to guide future research towards harnessing the therapeutic potential of 119Sb in RPT.

Facile transmetallation of [SbIII(DOTA)]- renders it unsuitable for medical applications

The antimony(iii) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) has been prepared and its exceptionally low stability observed. The Sb(iii) ion in Na[Sb(DOTA)]·4H2O shows an approximately square antiprismatic coordination geometry that is close to superimposable to the Bi(iii) geometry in [Bi(DOTA)]- in two phases containing this anion, Na[Bi(DOTA)]·4H2O, [H3O][Bi(DOTA)]·H2O for which structures are also described. Interestingly, DOTA itself in [(H6DOTA)]Cl2·4H2O·DMSO shows the same orientation of the N4O4 metal binding cavity reflecting the limited flexibility of DOTA in an octadentate coordination mode. In 8-coordinate complexes it can however accommodate M(iii) ions with r ion spanning a relatively wide range from 87 pm (Sc(iii)) to 117 pm (Bi(iii)). The larger Bi3+ ion appears to be the best metal-ligand size match since [Bi(DOTA)]- is associated with greater complex stability. In the solution state, [Sb(DOTA)]- is extremely susceptible to transmetallation by trivalent ions (Sc(iii), Y(iii), Bi(iii)) and, significantly, even by biologically important divalent metal ions (Mg(ii), Ca(ii), Zn(ii)). In all cases just one equivalent is enough to displace most of the Sb(iii). [Sb(DOTA)]- is resistant to hydrolysis; however, since biologically more abundant metal ions easily substitute the antimony, DOTA complexes will not be suitable for deployment for the delivery of the, so far unexploited, theranostic isotope pair 119Sb and 117Sb.