11-Chloro-1,1'-di-n-propyl-3,3,3',3'-tetramethyl-10,12-trimethyleneindatricarbocyanine iodide (IR-780) is a near infrared (NIR) photosensitizer for cancer treatment. Under NIR irradiation, IR-780 efficiently generates singlet oxygen or other reactive oxygen species (ROS) in lesion position that ultimately cause cell apoptosis and necrosis. However, biomedical application of IR-780 was limited by its poor water solubility. In this work, we designed a water-soluble IR-780 polymer (Poly-IR) for mitochondria-targeted photodynamic therapy via condensation polymerization. The results of dynamic light scattering (DLS) and transmission electron microscope (TEM) displayed that Poly-IR was self-assembled into nanoparticles in water. And ROS detection experiments demonstrated that Poly-IR quickly and efficiently generated ROS under NIR irradiation in solution and cells. The cellular distribution of the Poly-IR was monitored by confocal laser scanning microscopy (CLSM). Colocalization experiments with mitochondrial stain revealed a high degree of colocalization between Poly-IR and mitochondria, which illustrated that Poly-IR selectively accumulated in mitochondria. Furthermore, we explored the photodamages of Poly-IR to mitochondria through monitoring the change of mitochondrial membrane potential that was stained by JC-1 probe. In the dark, red fluorescence emerged with Poly-IR treated A549 cells. Under NIR irradiation, the red fluorescence was disappeared and green fluorescence was generated in Poly-IR treated cells, which confirmed the photodamage of Poly-IR to mitochondria. The cytotoxicity of Poly-IR was measured by MTT assay (MTT=3-(4,5-dimethylthiazol- 2-yl)-2,5-diphenyltetrazolium bromide). The IC₅₀ values of Poly-IR for A549 cells and MCF-7 cells were 9.13 and 10.98 μg/mL respectively in the dark. At the same time, the IC₅₀ values of Poly-IR for A549 cells and MCF-7 cells were 4.57 and 0.22 μg/mL respectively under NIR irradiation. The cytotoxicity of Poly-IR for MCF-7 cells treated with NIR exposure was significantly increased 50 times compared to without irradiation. Live/dead cell staining experiments also verified that Poly-IR had more phototoxicity. Meanwhile, cytotoxicity on tumor cells was also detected by flow cytometry apoptosis assay according to the typical Annexin V-fluorescein isothiocyanate (FITC) and propidium iodide (PI) double staining principle. These results suggested that Poly-IR promoted tumor cells apoptosis under near-IR irradiation, which expanded ideas in mi-tochondria-targeted photodynamic therapy in cancer treatment.

Key words: photodynamic therapy, reactive oxygen species (ROS), IR-780, mitochondria-target, nanoparticles