CHAPTER 4. Hydrophilic Ir(iii) Complexes for In vitro and In vivo Oxygen Imaging

Abstract

Ir(iii) complexes are efficiently taken up into cells and emit oxygen-dependent phosphorescence, which can be exploited in in vivo oxygen measurements. Here, we newly synthesized the hydrophilic Ir(iii) complexes BTP-PEGn (n = 12, 24 and 48) and PPY-PEG24, which bear polyethylene glycol (PEG) groups of different chain lengths on the btp ligand of Ir(btp)3 (btp = benzothienylpyridine) and 24 units long on the ppy ligand of Ir(ppy)3 (ppy = phenylpyridine). BTP-PEGn and PPY-PEG24 were soluble in water and emitted red and green phosphorescence, respectively, that was efficiently quenched by oxygen in water. The photophysical properties of these complexes were determined by steady-state and time-resolved phosphorescence measurements. Introduction of a PEG chain onto the ligand significantly inhibited the interaction of the chromophores with human serum albumin in H2O. Cell permeation experiments using HT-29 cells showed that the cellular uptake efficiencies of these complexes are very low. Intravascular staining was achieved by administering BTP-PEG48 to mice via the tail vein. Lifetime images of the renal cortex were obtained by fluorescence and phosphorescence lifetime imaging microscopy (FLIM/PLIM). The PLIM images obtained using BTP-PEG48 clearly visualized the peritubular capillaries and urinary space, and the oxygen levels of these spaces were evaluated from the lifetime values.