Phosphorescent Light–Emitting Iridium Complexes Serve as a Hypoxia-Sensing Probe for Tumor Imaging in Living Animals

Iridium complex is a promising organic light-emitting diode material for next generation video displays that emits phosphorescence quenched by oxygen. We used this oxygen-quenching feature for imaging tumor hypoxia. Red light-emitting Ir(btp)(2)(acac) (BTP) presented hypoxia-dependent light emission in culture cell lines, whose intensity was in parallel with hypoxia-inducible factor-1alpha images. BTP was further applied to imaging five nude mouse transplanted with tumors. All tumors presented a bright BTP-emitting image even 5 minutes after injection. The minimal image recognition size was approximately 2 mm in diameter. By morphologic examination and phosphorescence lifetime measurement, BTP appeared to localize to the tumor cells. Because BTP is easily modifiable, we synthesized BTP analogues with a longer excitation/emission wavelength. One of them, BTPHSA, depicted clear imaging from tumors transplanted 6 to 7 mm deep from the skin surface. We suggest that iridium complex materials have a vast potential for imaging hypoxic lesions such as tumor tissues.