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

Abstract

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-propyl_pyr-FITC and H₂dedpa-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 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-propyl_pyr-NH₂, was quantitatively radiolabeled with ⁶⁷Ga (RCY>99%) under reaction conditions commensurate with unfunctionalized H₂dedpa (10min at room temperature) at ligand concentrations as low as 10^-5M. The resultant ⁶⁷Ga-complex withstood transchelation to the in vivo metal-binding competitor apo-transferrin (2h at 37°C, 93% intact), signifying that [Ga(dedpa-propyl_pyr-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.

Keywords: Bifunctional chelates; Fluorescence imaging; Gallium-68; Nitroimidazole; Positron-emission tomography; Spheroids.