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Gold nanorods, which have a strong surface plasmon band at the near-infrared region, absorb light energy which is then converted to heat. Since near-infrared light can penetrate deeply into tissue, gold nanorods are expected to be useful as photosensitizers for photothermal therapy. In this study, the length of the poly(ethylene glycol) (PEG) chain was optimized in order to stabilize the gold nanorods in the blood circulation after intravenous injection. PEG(5000)- and PEG(10000)-modified gold nanorods showed higher stability in the blood circulation compared with PEG(2000)- and PEG(20000)-modified gold nanorods. As a demonstration of photothermal tissue damage, PEG(5000)-modified gold nanorods were injected into the muscle in the hind limbs of a mouse, and then irradiated with near-infrared pulsed laser light. Significant tissue damage was observed only in the presence of gold nanorods and laser irradiation. We next injected the gold nanorods directly into subcutaneous tumors in mice, and then irradiated the tumor with near-infrared pulsed laser light. Significant suppression of tumor growth was observed. In the case of the intravenous injection of gold nanorods, the suppression of tumor growth was weaker than for the case of direct injection, indicating that the targeted delivery of gold nanorods to the tumor tissue is an important key to improve the therapeutic effect.

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