Single Particle and PET-based Platform for Identifying Optimal Plasmonic Nano-Heaters for Photothermal Cancer Therapy

Abstract

Plasmonic nanoparticle-based photothermal cancer therapy is a promising new tool to inflict localized and irreversible damage to tumor tissue by hyperthermia, without harming surrounding healthy tissue. We developed a single particle and positron emission tomography (PET)-based platform to quantitatively correlate the heat generation of plasmonic nanoparticles with their potential as cancer killing agents. In vitro, the heat generation and absorption cross-section of single irradiated nanoparticles were quantified using a temperature sensitive lipid-based assay and compared to their theoretically predicted photo-absorption. In vivo, the heat generation of irradiated nanoparticles was evaluated in human tumor xenografts in mice using 2-deoxy-2-[F-18]fluoro-D-glucose ((18)F-FDG) PET imaging. To validate the use of this platform, we quantified the photothermal efficiency of near infrared resonant silica-gold nanoshells (AuNSs) and benchmarked this against the heating of colloidal spherical, solid gold nanoparticles (AuNPs). As expected, both in vitro and in vivo the heat generation of the resonant AuNSs performed superior compared to the non-resonant AuNPs. Furthermore, the results showed that PET imaging could be reliably used to monitor early treatment response of photothermal treatment. This multidisciplinary approach provides a much needed platform to benchmark the emerging plethora of novel plasmonic nanoparticles for their potential for photothermal cancer therapy.

Figure 1. Characteristics of the plasmonic nanoparticles.

(a) Sketch of the investigated plasmonic nanoparticles. (b) Absorption spectra in water for the three investigated nanoparticle types show that AuNPs with diameters of 80 nm and 150 nm exhibit resonances in the visible region (400–700 nm), whereas the AuNSs (diameter 150 nm) have their resonance peak in the near infrared region (700–1100 nm).

Figure 2. Measuring single particle heat generation using a 2D phospholipid bilayer assay.

(a) The temperature profile of an irradiated AuNS (diameter 150 nm) at the laser intensity P = 2.7 × 10⁶ W/cm². The temperature decays with increasing distance from the particle center. (b) The biological assay used to map out the temperature profile. The assay consists of a 2D phospholipid bilayer with a well-characterized phase transition and a fluorescent molecule that senses the phase and preferentially partitions into the fluid phase. The surface temperature of the nanoparticle can be extracted by determining one point on the temperature profile, ΔT, in this assay corresponding to half the diameter of the melted footprint, D_m. (c) Optically heating a nanoparticle embedded in the bilayer with a tightly focused laser beam induces local melting. Here is shown representative melted footprints for all three nanoparticles at the same laser intensity, P = 4.3 × 10⁶ W/cm². The fluorescent images are recorded by acquiring an emission bandwidth of 496 nm–587 nm and the nanoparticles are imaged by recording 594 nm laser light reflected from the nanoparticle. Scale bars are 2 μm. (d) The surface temperature as a function of laser intensity for all three nanoparticles derived from the size of the melted footprints and using equation (1). n = 12 for all datasets and error bars represent one standard deviation (SD).

Figure 3. Thermographic assessment of the heat generation in vitro.

(a) Illustration of the assay using thermographic imaging to assess the laser mediated bulk heating. (b,c) Plots of the maximum absolute temperature in aqueous bulk solutions as a function of laser intensity measured in (b) 5 × 10¹⁰ nanoparticle/mL solutions or (c) 5 × 10⁸ nanoparticle/mL solutions of AuNSs (blue circle), 80 nm AuNPs (red diamonds), 150 nm AuNPs (yellow squares), and saline (grey stars). The laser intensity was 0.58 W/cm².

Figure 4. PET evaluation of the treatment response.

(a) Images showing the coronal planes of the ¹⁸F-FDG PET scan of mice from each nanoparticle group (columns). The rows show the baseline PET scan, the scan immediately after the laser irradiation (Day 0), and the scan two days post-treatment (Day 2). White arrows mark the tumor location where the nanoparticles were administered and subsequently irradiated with the 807 nm laser (laser intensity of 0.58 W/cm²). In the mice from the AuNS and 150 nm AuNP groups there is a tumor subvolume with decreased uptake of ¹⁸F-FDG (marked by red arrows) both at Day 0 and Day 2. (b,c) at different timepoints. If the treatment is effective this volume will increase after laser irradiation. Each individual data set has been normalized to the baseline value. The mean relative changes in the tumor volume with low ¹⁸F-FDG uptake in the four treatment groups are shown in (b) and for individual animals in (c) (red: 80 nm AuNP, yellow: 150 nm AuNP, blue: 150 nm AuNS, and grey: saline). n = 6, each group. (d) Plots of the temperature evolution measured using thermographic imaging at the skin surface of the tumors of mice receiving AuNSs (blue circle, n = 3), 80 nm AuNP (red diamonds, n = 3), 150 nm AuNP (yellow squares, n = 3), and saline (grey stars, n = 7). The laser intensity was 0.58 W/cm². Error bars represent one SD. * denotes p value < 0.05, ** denotes p value < 0.01, and **** denotes p value < 0.0001.