Photothermal effect by 808-nm laser irradiation of melanin: a proof-of-concept study of photothermal therapy using B16-F10 melanotic melanoma growing in BALB/c mice

Abstract

The photothermal effect is undergoing great interest due to advances in new photosensitizing materials and better-suited light sources, but studies are frequently hampered by the need to employ exogenous photothermal agents and expensive irradiation devices. Here we present a simple strategy based on direct NIR irradiation of the melanin pigment with a commercial 808-nm laser pointer. Proof-of-concept studies showed efficient photothermal effects on melanin in vitro and in vivo. After NIR irradiation, BALB/c mice bearing B16-F10 melanotic melanoma tumors revealed severe histopathological damage and massive necrosis in melanin-containing tumor tissue, while surrounding healthy tissues showed no damage. Therefore, the feasibility of this approach may allow implementing direct procedures for photothermal therapy of pigmented tumors.

Fig. 1

Scheme illustrating the photothermal effect. The energy of NIR photons is absorbed by electrons in the valence band (VB) of photothermal polymers or nanoparticles, pumping them to the conduction band (CB). Electronic excitation leaves positive charges (holes) in the VB, and after a very short time, electrons and holes recombine due to their mutual electrostatic attraction, converting a great part of the absorbed energy in lattice vibrations (wiggly arrows) and thermal energy.

Fig. 2

(a) Chemical structure of eumelanin, according to recent overviews [41,46], showing 4 units of the linear 4,7’-linked indole-5,6-quinone (and 5,6-dihydroxy)-2-carboxylic acid polymer, with atom numbering. (b) Comparative absorption spectra of sepia ink (A) and China ink (B), both diluted 1:2000 (v/v) with distilled water, showing the position of the 808-nm laser excitation (dashed line). The broad-band absorption curves decrease monotonically going from UV to NIR, a typical feature of these black polymeric chromophores.

Fig. 3

(a) Ignition time (seconds) of paper stripes impregnated with sepia ink at different concentrations (circles and squares). Papers were air dried, and then irradiated with 808-nm laser for different times. The higher the ink content, the faster the paper ignition. Note that even low ink concentration lead to ignition after 5 s, proof of its very high photothermal efficiency. (b) Temperature increase (in °C) of water (squares) and 50% sepia ink in water (circles) during 808-nm irradiation for different times (min). The temperature increases significantly for the sepia ink solution (58 °C at 4 min), while the temperature remains constant for water alone (26 °C at 4 min).

Fig. 4

Histological H&E images of the melanotic melanoma B16-F10 growing in BALB/c mice, (a) without irradiation (control tumor), and (b) after irradiation with the 808-nm laser for 10 min and observed 24 h later. (a) Small and large black granules (melanosomes) appear within tumor cells, but sometimes they are located in the extracellular space (white arrows). The irradiated tumor section (b) shows extensive damaged and necrotic areas with rounded or disrupted tumor cells (white arrows), big brown-black melanophages (encircled), as well as pycnotic cell nuclei (black arrows), and cytoplasm fragmentation. Scale bars: 30 µm.