In this study, we demonstrated that aminopolycarboxylate chelators with five and six carboxylic groups, Ca-DTPA and Ca-TTHA, respectively, induced whole-body clearance of free 225Ac with a significant reduction in the liver as the critical organ. 225Ac was excreted in the urine and faeces after chelation. The liver showed the highest retention of free 225Ac with an estimated human absorbed dose of 4.76 SvRBE5/MBq. Therefore, Ca-DTPA and Ca-TTHA administration may be a treatment option for unexpected radiation exposure caused by free 225Ac.
Aminopolycarboxylate chelators, such as DTPA, EDTA, GEDTA, and TTHA, showed higher reductions in 225Ac retention in the liver than those with D-penicillamine, dimercaprol, and DO3A. These results reflect a difference in the number of ligating atoms and the number of coordinating groups. The Ac3+ ion is classified as a ‘hard’ Lewis acid according to the hard and soft acids and bases (HSAB) theory [33], as it behaves similar to the La3+ ion, carrying a large charge and low polarisability. Thus, the Ac3+ ion prefers hard bases or non-polarisable and negatively charged Lewis bases such as carboxylates [34] and thus shows a high affinity for aminopolycarboxylates, especially those that have a higher number of carboxylic groups, such as DTPA and TTHA. D-penicillamine and dimercaprol are sulphur-coordinating soft Lewis bases that are effective decorporation chelators for soft metal ions, such as Pb2+, Hg2+, and As3+.
The aminopolycarboxylates used in this study behave as multidentate chelators for the Ac3+ ion. The thermodynamic stability increases with the number of coordinating atoms [20]. Indeed, our results are consistent with the stability constants for La3+ complexes [35]. The low decorporation ability of DO3A can be explained in a similar manner. The low ability of CyDTA may occur because of kinetic issues, as considerable time is required to complete the coordination to the Ac3+ ion, thereby explaining why this chelator showed lower reduction.
Ca-TTHA and the sodium salt of TTHA exhibited similar effects on the 225Ac distribution pattern in mice. Previous studies using EDTA have reported that its calcium salt, rather than the sodium salt, is preferred as a chelator drug because Na-EDTA chelates Ca in the body and may cause hypocalcaemic tetany [36]. Therefore, Ca-TTHA should be selected for the development of TTHA as a chelator drug.
In this study, we administered a series of chelators 1 h following 225Ac injection. The chelator administration time was selected based on the distribution data indicating that the majority of the free 225Ac was distributed in the liver. We found that Ca-DTPA and Ca-TTHA reduced free 225Ac retention in the liver as well as in other organs. These results show that Ca-DTPA and Ca-TTHA are effective in reducing radiation exposure from free 225Ac. Ca-TTHA showed a higher tendency to reduce 225Ac retention compared to that by Ca-DTPA in the organs. These data support the development of Ca-TTHA for the removal of free 225Ac. To produce 225Ac for medical use, 229Th generators are currently used, while accelerator synthesis of 225Ac is under investigation to increase the supply [37]. In the accelerator synthesis, 227Ac (half-life = 21.8 y), which cannot be chemically separated, is included as an unavoidable by-product [37]. The method proposed by the present study may thus also be useful to reduce the 227Ac, which will be retained in the liver substantially longer than 225Ac in 225Ac TAT. Aminopolycarboxylates are known to form stable chelates with a wide range of metal ions. DTPA and TTHA can be expected to capture the daughter nuclide 213Bi (and to a lesser extent, 209Pb2+) de-chelated by the recoil during the decay process of 225Ac, as these two chelators form highly stable complexes with Bi3+ [35].
This study had several limitations. First, the free 225Ac was evaluated in non-tumour-bearing mice. Further preclinical and clinical studies with specific target 225Ac-labeled drugs are necessary to determine the timing of administration not to avoid tumour uptake of 225Ac in tumour-bearing mice. The 225Ac internalisation by tumour cells following drug delivery is important in drug design in the development of agents for 225Ac TAT, because internalisation causes short-lived daughter radionuclides generated by 225Ac to be trapped in the cells [38, 39]. Therefore, the chelator administration timing would be appropriate after tumour delivery and internalisation, because chelators with negatively charged carboxylic groups do not penetrate tumour cell membranes [40]. Second, this study used a fixed administration dose and the same administration route for aminopolycarboxylate chelators (150 mg/kg, i.p.) for comparisons. Optimisation of the administration dose, route, and safety tests are necessary in future studies.
In conclusion, we found that aminopolycarboxylate chelators with five and six carboxyl groups, i.e. Ca-DTPA and Ca-TTHA, were useful for whole-body clearance of free 225Ac, with marked reduction in the liver. Our findings provide a novel strategy to remove accumulated free 225Ac released from 225Ac-labeled drugs and encourage the future development of 225Ac TAT.