Abstract
889
Objectives: The successful application of radiometals for diagnosis and therapy in nuclear medicine requires chelators that efficiently and stably complex the radiometal at low chelator concentration, preferably under mild reaction conditions. We aim to develop new chelators for the radiometals 89Zr and 213Bi. 89Zr (β+, t1/2 78 h), has utility in PET imaging of antibodies, which require 2 - 5 days post-injection to localize at target tissue prior to imaging. The hexadentate tris(hydroxamate), desferrioxamine-B can coordinate 89Zr4+, but there is some evidence of in vivo instability. The radiotherapeutic α-emitting metal, 213Bi (t1/2 45 min) has demonstrated clinical efficacy, but the currently used chelator, DOTA, requires heating at high temperatures to incorporate 213Bi. Based on our previous experience of hexadentate tris(hydroxypyridinone) chelators that demonstrate high affinity for oxophilic metal ions, we aimed to develop an octadentate tetrakis(hydroxypyridinone) ligand containing four 3,4-hydroxypyridinone groups, and evaluate its ability to coordinate 89Zr4+ and 213Bi3+ under mild reaction conditions.
Methods: Synthesis involving consecutive coupling reactions to form amide bonds between 3,4-hydroxypyridinone building blocks provided tetrakis(3,4-hydroxypyridinone) (TkHP) (Figure 1). Generator-produced solutions of 213BiI4-/213BiI52- were reacted with TkHP (10 mM - 1 μM TkHP, 1 - 5 MBq 213Bi, 0.4 M tris buffer, 50 μL total volume) for 5 min at ambient temperature. Radiochemical yield (RCY) was determined with ITLC, and the reaction products were analysed by reverse phase HPLC. 213Bi-TkHP was incubated in serum at 37 C and aliquots were analyzed with ITLC to evaluate the stability of the complex in serum over 2 h. For 89Zr studies, solutions of [89Zr(oxalate)4]4- were reacted with TkHP (1 mM - 100 mM TkHP, 0.5 MBq 89Zr, 0.2 M ammonium acetate solution, 20 μL total volume) for 10 min at ambient temperature. RCY was also determined by ITLC, and reaction products analysed by reverse phase HPLC.
Results: After 5 min reaction at ambient temperature, RCY of 213Bi-TkHP was > 95 % at 1 mMTkHP concentration and > 80% at 0.1 mM TkHP concentration. However, in serum 213Bi3+ dissociated from TkHP over 2 h. Preliminary radiolabelling studies with 89Zr4+ demonstrated that RCY of 89Zr-TkHP = 95 % at TkHP concentration of 0.1 mM.
Conclusion: A novel octadentate ligand containing four 3,4-hydroxypyridinones, TkHP, was synthesised. TkHP contains an apical primary amine allowing functionalization for attachment to biomolecules. 213Bi3+ radiolabelling experiments demonstrated that 213Bi-TkHP forms rapidly at ambient temperature, but that the resulting complex is not stable in serum. Preliminary experiments indicated that TkHP also complexes 89Zr4+ rapidly under mild conditions. Figure 1. Structure of the tetrakis(3,4-hydroxypyridinone) ligand TkHP