Novel "bi-modal" H2dedpa derivatives for radio- and fluorescence imaging.

Abstract A novel pyridyl functionalized analog of the promising hexadentate 68 Ga 3 + chelate H 2 dedpa (N 4 O 2 , 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 H 2 dedpa derivative for fluorescence and nuclear imaging. Two bi-modal H 2 dedpa derivatives were prepared: H 2 dedpa-propyl pyr -FITC and H 2 dedpa-propyl pyr -FITC-( N , N ′-propyl-2-NI) (FITC = fluorescein, pyr = pyridyl functionalized, NI = nitroimidazole). The ligands possess the strong gallium-coordinating atoms contained within dedpa 2 − that are ideal for radiolabeling with 68 Ga 3 + 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 H 2 dedpa 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 H 2 dedpa ligand, H 2 dedpa-propyl pyr -NH 2 , was quantitatively radiolabeled with 67 Ga (RCY > 99%) under reaction conditions commensurate with unfunctionalized H 2 dedpa (10 min at room temperature) at ligand concentrations as low as 10 − 5  M. The resultant 67 Ga-complex withstood transchelation to the in vivo metal-binding competitor apo -transferrin (2 h at 37 °C, 93% intact), signifying that [Ga(dedpa-propyl pyr -NH 2 )] + is a kinetically inert complex suitable for in vivo use, but exhibited slightly reduced stability compared to the native [ 67 Ga(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.