Photodynamic therapy (PDT) uses the strong cytotoxicity of singlet oxygen and hyperthermia produced by irradiating excitation light on a photosensitizer. The phototoxic effects of indocyanine green (ICG) and near-infrared light (NIR) have been studied in different types of cancer cells. Plasma proteins bind strongly to ICG, followed by rapid clearance by the liver, resulting in no tumor-selective accumulation after systemic administration. Kimura et al. have proposed using a novel nanoparticle labeled with ICG (ICG-lactosome) that has tumor selective accumulation owing to enhanced permeability and retention (EPR) effect. In this study, we investigated the efficacy of PDT using ICG-lactosome and NIR for a bone metastatic mouse model of breast cancer.

Cells from the human breast cancer cell line, MDA-MB-231 were injected into the right tibia of 26 anesthetized BALB/C nu/nu mice at a concentration. The mice were then randomly divided into three groups: the PDT group (n = 9), the laser (laser irradiation only) group (n = 9), and the control group (n = 8). PDT was performed thrice (7, 21, 35 days after cell inoculation) following ICG-lactosome administration via the tail vein 24 hours before irradiation. The mice were percutaneously irradiated with an 810-nm medical diode laser for 10 min. In the laser group, mice were irradiated following saline administration 24 hours before irradiation. Radiographic analysis was performed for 49 days after cell inoculation. The area of osteolytic lesion was quantified. The right hind legs of 3 mice were amputated 24 hours after the third treatment. Histological analysis was performed using hematoxylin-eosin staining and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining of sagittal sections. The data was analyzed using Tukey-Kramer post-hoc test. P-value of <0.05 was considered significant.

X-ray on day 49 of the three groups are considered. The area of osteolytic lesion in the PDT group (7.9 ± 1.2 mm²: mean ± SD) was significantly smaller than that of the control (11.4 ± 1.4 mm²) and laser (11.9 ± 1.2 mm²) groups. In histological findings, we observed many TUNEL-positive cells in the metastatic tissue 24 hours after PDT. In the control and laser groups, TUNEL-positive cells were occasionally observed.

We have previously reported the effect of ICG-lactosome-enhanced PDT on the cytotoxicity of human breast cancer cells in vitroand on the delay of paralysis in a rat spinal metastasis model. In this study, we demonstrated the inhibitory effect of ICG-lactosome-enhanced PDT on bone destruction caused by human breast cancer cells in vivo. This PDT induced apoptosis and necrosis in the tumor cells. Intralesional resection is often performed for spinal metastases in an emergency. The residual tumor may regrow and cause neurological deficits. We believe that ICG-lactosome-enhanced PDT can decrease the rate of local recurrence through reduction of the residual tumor. PDT with ICG-lactosome and NIR had an inhibitory effect on the growth of bone metastasis of a human breast cancer.

Summary

Photodynamic therapy with ICG lactosome and near-infrared light has phototoxic effects on human breast cancer cells. With the same total energy, phototoxic effects depend on output of irradiation light rather than irradiation time.