Photothermal therapy is promising cancer therapy and is especially studied for brain tumors. It is based on near-infrared laser irradiation of metallic nanoparticles, which absorb the light and dissipate it as heat. The extreme heating of nanoparticles causes hyperthermia (+41-44 ⁰C) in tumors and leads to localized irreversible damage of the cancerous tissue.
Cancer cells are by nature more susceptible to thermal effects than healthy cells, which further enhances the effect. Gold nanoshells are currently the only metallic nanoparticles approved by the FDA for clinical use.
Motivation for the study
Most focus has been on using gold for photothermal therapy and remarkably little attention has been on alternative materials that might possess equally or even better photothermal efficiency. In this study, the potential of platinum nanoparticles for photothermal cancer therapy was explored. More knowledge is also needed to clarify unintentional heating of healthy brain tissue, so temperature changes upon NIR laser irradiation were measured in different porcine cerebral tissues.
Photothermal therapy efficiency
Human ovarian cancer cells were plated on Petri dishes and incubated with platinum nanoparticles of varying sizes between 30 and 70 nm. After 24-hour incubation, irradiation was performed using a Modulight ML6600 laser at 808 nm together with an illumination kit (MLAKIT) tailored for Petri dish illumination. A small area of the dish was illuminated with the laser for five minutes with 45 W/cm2 irradiance. Calcein cell viability staining was performed 24 hours after the laser treatment and cells were examined with a fluorescence microscope to determine how the treatment had affected the cells.
Results showed that cancer cells that were not incubated with platinum nanoparticles were not affected by the laser treatment. Also the cells treated with the smallest 30 nm nanoparticles remained not affected, suggesting that 30 nm diameter is too small to generate efficient heating effect. However, with 50 nm and 70 nm nanoparticles, a clear circular area with no living cells was observed where the laser spot hit which suggests the treatment to be very efficient. No difference was observed between 50 and 70 nm nanoparticles
Image from the original publication.
Laser-induced heating in brain tissue
Laser-induced heating in brain tissue was examined on three different porcine cerebral tissue extracts. Tissue extracts in cuvettes were irradiated with ML6600 laser system at 808 nm wavelength together with Modulight illumination kit (MLAKIT) to generate a uniform 1 cm spot over the sample. Different power levels and irradiation times were used and their effect on temperature increase was studied by performing continuous heat gradient measurements with IR camera.
Heat gradient measurements showed that the brain tissue temperature increased significantly during 30 min irradiation, but returned to room temperature within 30 minutes. Different cerebral tissues behaved differently upon irradiation as the brain stem heated significantly less than cerebellum and cerebrum. This is expected to be related to increased light scattering so that less light can be absorbed into brain stem. In all tissue types, the temperature increase was seen to be linearly proportional to the laser power used.
Image from the original publication. Reproduced with permission from the contact author of the publication.
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