A comparative study of IR-820 and indocyanine green (ICG)

Objectives Indocyanine green (ICG) is a clinically used near-infrared (NIR) fluorescence imaging agent. Major disadvantages of ICG include limited stability in aqueous solution and short plasma residence time. Our study aim was to explore potential imaging applications of IR-820, another NIR dye, by comparing it to ICG in vivo and in vitro.

Methods (1) Spectrofluorometer measurements of aqueous solutions of ICG and IR820 over 4 days under different light and temperature conditions (light, dark, foil, room temperature, 4 °C and 42 °C); (2) Fluorescent microscope images of human ovarian cancer cells (SKOV-3) and human umbilical endothelial cells (HUVEC) exposed to IR820 or ICG; (3) Dye distribution in vivo after an equimolar rat tail vein injection of either dye, imaged with a CCD camera.

Results The in vitro fluorescent yield ratio for IR-820 vs. ICG is 1:10. Unlike ICG, the peak emission wavelength of IR820 is not concentration-dependent. The degradation of both dyes in aqueous solution follows pseudo-first order kinetics, and the half-time for IR820 is approximately double that of ICG under all experimental conditions. In cellular studies, we observed evenly distributed cytoplasmic and perinuclear localization for both dyes. For in vivo rat experiments, both dyes accumulated primarily in the liver, with some lung uptake for IR820. The image intensity of IR820 in the abdomen was initially lower than for ICG, but in 24-hour images the IR-820 intensity became higher.

Conclusions IR-820 is a better alternative than ICG when stability of the dye solution over time is a concern, or when a consistent peak emission wavelength is desired. The increased stability of IR-820 that we observed in vitro translates into increased stability in vivo as well. In practical applications, the enhanced stability of IR-820 should result in longer plasma resident time compared to ICG, and thus allow for longer image collection times.

Research Support A.F.F. supported by NIH/NIGMS R25 GM06134