In-Vivo Phosphorescence Imaging Of Cancer Using Iridium Complexes

Iridium complexes exhibit highly-efficient phosphorescence that is quenched notably by ambient molecular oxygen. We utilized the oxygen-sensitive properties of the phosphorescence to visualize tumors in vivo because the levels of oxygen in tumors may be significantly below those of normal tissue. We used (btp)(2)Ir(acac) (BTP) as an oxygen probe because the phosphorescence of BTP appears in the red to near-infrared region and has a relatively long lifetime (5.8 mu 3s) and high quantum yield (0.32) in solution. The oxygen-quenching rate constants were determined to be 5.7 x 10(4) and 1.2 x 10(4) mmHg(-1)s(-1) in n-hexane and in lipid bilayer of DMPC in Tris-HCl buffer solution, respectively, at 35 degrees C. We took the phosphorescence image of HeLa cells that had been incubated under 5% and 20% O-2 conditions. In 20% O-2 culture condition, HeLa cells did not exhibit notable phosphorescence, while in 5% O-2 culture condition, they emitted bright red phosphorescence due to BTP. Then we tested this probe to image tumors transplanted in nude mice. After 5 minutes of the BTP injection, tumor moieties began to emit red phosphorescence and after one hour each tumor was visualized very clearly by the BTP phosphorescence that could be seen only in a low oxygen state.