High denticity oxinate-linear-backbone chelating ligand for diagnostic radiometal ions [111In]In3+ and [89Zr]Zr4+

Advances in nuclear medicine depend on chelating ligands that form highly stable and kinetically inert complexes with relevant radiometal ions for use in diagnosis or therapy. A new potentially decadentate ligand, H(5)decaox, was synthesised to incorporate two 8-hydroxyquinoline moieties on either end of a diethylenetriamine backbone decorated with three carboxylic acids, one at each N atom of the backbone. Metal complexation was assessed using nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HR-MS) with In3+, Zr4+ and La3+. Solution thermodynamic studies provided the stepwise protonation constants and metal formation constants, indicating a high affinity for both In3+ and Zr4+ (pIn = 32.3 and pZr = 34.7), and density functional theory (DFT) calculations provided insight into the coordination environments with either metal ion. Concentration dependent radiolabeling experiments with [In-111]InCl3 and [Zr-89]ZrCl4 showed promise as quantitative radiolabeling (>95%) occurred at micromolar concentrations, under mild, near-physiological conditions of pH 7 and room temperature for 30 minutes. Serum stability of both radiometal complexes was investigated and the [In-111]In(decaox) complex remained 91% intact after 24 hours while the [Zr-89]Zr(decaox) complex was 86% intact over the same time, comparable to other chelating ligands previously assessed with the same methods. The high radiolabeling yields, limited serum protein transchelation and structural insight of the [Zr-89]Zr(decaox) complex suggest a promising fit between the oxinate-containing ligand and the Zr4+ ion, setting the stage for further investigations with a functionalised version of the chelator for its potential in PET imaging.