Novel “bi-modal” H 2 dedpa derivatives for radio- and fluorescence imaging

A novel pyridyl functionalized analog of the promising hexadentate 68Ga3 + chelate H2dedpa (N4O2, 1,2-[[6-carboxy-pyridin-2-yl]-methylamine]ethane) was successfully synthesized and characterized. This new bifunctional chelate (BFC) was used to prepare the first proof-of-principle bi-modal H2dedpa derivative for fluorescence and nuclear imaging. Two bi-modal H2dedpa derivatives were prepared: H2dedpa-propylpyr-FITC and H2dedpa-propylpyr-FITC-(N,N′-propyl-2-NI) (FITC = fluorescein, pyr = pyridyl functionalized, NI = nitroimidazole). The ligands possess the strong gallium-coordinating atoms contained within dedpa2 − that are ideal for radiolabeling with 68Ga3 + for positron-emission tomography (PET) imaging, and two fluorophores for optical imaging. In addition, one analog contains two NI moieties for specific entrapment of the tracer in hypoxic cells. These new bi-modal analogs were compared to the native unfunctionalized H2dedpa scaffold to determine the extent to which the addition of pyridyl functionalization would affect metal coordination, and complex stability. The non-radioactive gallium complexes were tested in a 3D tumor spheroid model. The novel pyridyl bis-functionalized H2dedpa ligand, H2dedpa-propylpyr-NH2, was quantitatively radiolabeled with 67Ga (RCY > 99%) under reaction conditions commensurate with unfunctionalized H2dedpa (10 min at room temperature) at ligand concentrations as low as 10− 5 M. The resultant 67Ga-complex withstood transchelation to the in vivo metal-binding competitor apo-transferrin (2 h at 37 °C, 93% intact), signifying that [Ga(dedpa-propylpyr-NH2)]+ is a kinetically inert complex suitable for in vivo use, but exhibited slightly reduced stability compared to the native [67Ga(dedpa)] scaffold (> 99% intact). Finally, bi-model fluorescent Ga-dedpa compounds were successfully imaged in a 3D tumor spheroid model. The Ga-dedpa-FITC-NI derivative was specifically localized in the central hypoxic core of the spheroid.