Indocyanine green (ICG) is known to produce singlet oxygen under IR irradiation, which oxidizes intracellular components to create a photodynamic killing effect. However, its killing efficiency is relatively low. Some research suggested that ICG molecules dissociate into two aldehydes upon its reaction with singlet oxygen, leading to irreversible photobleaching. This alternative reaction path for singlet oxygen may contribute to the low photodynamic killing effect of ICG. This work investigates the role of <i>in situ</i> oxygen in ICG photobleaching; the effect of albumin on stabilizing ICG was also examined. Dissolved oxygen (DO) was measured before and after irradiation with a diode laser (803 nm) under constant temperature to avoid temperature effects. Photobleaching was monitored by UV-visible spectrophotometry. Photobleaching was observed in cells after IR irradiation. DO decreased with IR irradiation in a separate experiment. Photobleaching was more efficient in ICG monomers than in aggregates. Bovine serum albumin (BSA) was found to have the effect of stabilizing ICG monomers by reducing the presence of small aggregates that form at higher concentrations. Consequently, the addition of 1% BSA to ICG aqueous solution induced a larger decrease of DO. On the other hand, photobleaching was not observed in oxygen-free aqueous solution; externally photobleached ICG had minimal cell killing effect. Taken together, oxygen is required for photobleaching to occur; proteins in ICG solution not only have the effect of stabilizing ICG monomers, but also compete with ICG to react with the singlet oxygen, effectively preventing ICG from oxidation and allowing a longer photodynamic effect before photobleaching. Due to restrictions in oxygen diffusion across protein-imbedded membranes, intracellular oxygen supply may not replenish quickly if oxygen consumption increases rapidly within the cell. Acute changes in intracellular oxygen tension may occur too rapidly to allow signaling pathways to adjust metabolism and should be investigated further.

Role of dissolved oxygen and albumin in indocyanine green photobleaching

2019 Fall National Meeting

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