Indium-111 (111In) is a diagnostic radiometal that is important in nuclear medicine for single-photon emission computed tomography (SPECT). In order to apply this radiometal, it needs to be stably chelated and conjugated to a targeting vector that delivers it to diseased tissue. Identifying effective chelators that are capable of binding and retaining [111In]In3+ in vivo is an important research area. In this study, two 18-membered macrocyclic chelators, py-macrodipa and py2-macrodipa, were investigated for their ability to form stable coordination complexes with In3+ and to be effectively radiolabeled with [111In]In3+. The In3+ complexes of these two chelators were characterized by NMR spectroscopy, X-ray crystallography, and density functional theory calculations. These studies show that both py-macrodipa and py2-macrodipa form 8-coordinate In3+ complexes and attain an asymmetric conformation, consistent with prior studies on this ligand class with small rare earth metal ions. Spectrophotometric titrations were carried out to determine the thermodynamic stability constants (log KML) of [In(py-macrodipa)]+ and [In(py2-macrodipa)]+, which were found to be 18.96(6) and 19.53(5), respectively, where the values in parentheses are the errors of the last significant figures obtained from the standard deviation from three independent replicates. Radiolabeling studies showed that py-macrodipa and py2-macrodipa can quantitatively be radiolabeled with [111In]In3+ at 25 °C within 5 min, even at ligand concentrations as low as 1 μM. The in vitro stability of the radiolabeled complexes was investigated in human serum at 37 °C, revealing that ∼90% of [111In][In(py-macrodipa)]+ and [111In][In(py2-macrodipa)]+ remained intact after 7 days. The biodistribution of these radiolabeled complexes in mice was investigated, showing lower uptake in the kidneys, liver, and blood at the 24 h mark compared to [111In]InCl3. These results demonstrate the potential of py-macrodipa and py2-macrodipa as chelators for [111In]In3+, suggesting their value for SPECT radiopharmaceuticals.
