Biocompatible ring-deformed indium phthalocyanine label for near-infrared photoacoustic imaging

Abstract Photoacoustic imaging of small animals plays an increasingly important role in preclinical studies and is poised to become a valuable diagnostic imaging tool for a large diversity of human diseases. Exogenous contrast agents absorbing in the near-infrared (NIR) window where tissues are most transparent (650 – 950 nm) may help to increase the resolution and sensitivity of photoacoustic imaging, and may also offer specificity to targeted tissues. We show that a ring-deformed indium phthalocyanine (InPc) with a strong absorption band at ∼840 nm is a promising photoacoustic probe, while presents adequate photostability for imaging, very fast and efficient radiationless processes, and negligible cytotoxicity or phototoxicity in vitro up to concentrations of 30 µM. The sensitivity of its photoacoustic detection was demonstrated by following the time-dependence of InPc uptake in a monolayer of 4T1-luc2 cells in vitro. In vivo imaging using a commercial photoacoustic tomography equipment confirmed the ability to detect InPc in 4T1-luc2 tumors orthotopically inoculated in the mammary gland female BALB/c mice (8-12 weeks old) within a few minutes of its intravenous administration. InPc remained in the tumor for hours and proved to be biocompatible.