Cancer photothermal therapy in the near-infrared region by using single-walled carbon nanotubes

Abstract

Single-walled carbon nanotubes (SWNTs) have a high optical absorbance in the near-infrared (NIR) region. In this special optical window, biological systems are known to be highly transparent. The optical properties of SWNTs provide an opportunity for selective photothermal therapy for cancer treatment. Specifically, CoMoCAT nanotubes with a uniform size (about 0.81 nm) and a narrow absorption peak at 980 nm are ideal candidates for such a novel approach. Here, CoMoCAT SWNTs are conjugated to folate, which can bind specifically to the surface of the folate receptor tumor markers. Folate-SWNT (FA-SWNT) targeted tumor cells were irradiated by a 980-nm laser. In our in vitro and in vivo experiments, FA-SWNT effectively enhanced the photothermal destruction on tumor cells and noticeably spared the photothermal destruction for nontargeted normal cells. Thus, SWNTs, combined with suitable tumor markers, can be used as novel nanomaterials for selective photothermal therapy for cancer treatment.

Fig. 1

Absorption spectra of CoMoCAT^® single-walled carbon nanotubes solubilized in PEG. (a) UV-vis-NIR absorption spectra of the SWNTs suspension and water. (b) Absorbance of SWNT suspension of different concentrations at 980 nm (optical path=0.5 cm). The solid line is a linear fit. (c) Temperature measurements of an ex vitro control experiment using a SWNT solution (3.5 μg/mL) and water during irradiation by a 980-nm laser at 1 W/cm² for 2 min at room temperature. The results demonstrated the enhanced laser energy absorption by the SWNTs. (d) Cytotoxicity of SWNTs. Cells were incubated in the SWNT solution of different concentrations (1.75 μg/mL to 7 μg/mL) for 12 h, then washed and incubated in complete medium for 72 h before assessing cell viability. Cells not incubated with SWNTs were used as control. Bars (means+SD, n=4).

Fig. 2

Selective targeting and killing of cancer cells. (a) Chemical structure of PL-PEG-FA and PL-PEG-FITC synthesized by conjugating PL-PEG-NH₂ with FA or FITC, respectively, for solubilizing individual SWNTs. (b) Upper panel: Confocal images of EMT6 cells of FR+ or FR− after incubation in a solution of SWNTs with two cargoes (PL-PEG-FA and PL-PEG-FITC). The strong green FITC fluorescence inside the FR+ cells confirms the SWNT uptake by FA and FITC cargoes. The small amount of green fluorescence inside the FR− cells, confirmed the lack of SWNT uptake by FA and FITC cargoes (×40). (Inset) High-magnification images show the details of fluorescence in the cells. Lower panel: Images of dead FR+ cells with rounded cell morphology and unharmed FR− cells after irradiation (980-nm laser radiation at 0.5 W/cm² for 2 min). (Inset) High-magnification images show the details of the dead cells (FR+) and the live cells (FR−). (Color online only).

Fig. 3

Viability of tumor cells under different treatments. (a) Cells tumor cells treated with laser only or with laser-FA-SWNT The treated cells were incubated in complete medium for 12 h before assessing cell viability. Cells without treatment were used as control. Bars (means+SD, n=4). (b) Optical images of tumor cells under different treatments as indicated.

Fig. 4

Laser treatment induced cell death in vivo. (a) Temperature on the surface of a mouse tumor during irradiation by the 980-nm laser at 1 W/cm² for 5 min with and without FA-SWNT (1 mg/kg). This result clearly demonstrated the selective thermal effect caused by SWNT absorption of 980-nm light in the tumor. (b) Fluorescent emissions of FITC from a mouse tumor sample and normal tissue around the tumor, 6 h after injection, observed with a fluorescence stereo microscope. The normal tissue surrounding the tumor injected with SWNT-FITC shows more intense fluorescent emission than that surrounding the tumor injected with FA-SWNT-FITC The control sample was injected with PBS These results indicated that FA-SWNT has higher tumor selectivity than SWNT. (c) Images of tumor tissue upper panel and normal tissue (lower panel) sections after different treatments. TUNEL staining analysis of tissue sections from a mouse was performed 3 h after treatment. Samples were selected from the tumor and the normal tissue within the laser beam but 0.5 cm away from the tumor. This result clearly demonstrated that FA-SWNT enhanced the thermal damages of the tumor while contributing limited damage to the normal tissue around the tumors. (d) Cell death rates of the tumor section, after different treatments (100 cells/group ×5). (e) Cell death rates of the normal section after different treatments (100 cells/group ×5).