Background: Efficient cancer therapy is sought not only for primary tumor treatment but also for the prevention of metastatic cancer growth. Immunotherapy has been shown to prevent cancer metastasis by inducing antigen-specific immune responses. Indocyanine green (ICG) has a peak spectral absorption at about 800 nm, which makes it a photothermal reagent for direct treatment of solid tumors by photothermal therapy (PTT). Since PTT alone cannot fully induce antigen-specific immune response for prevention of cancer metastasis, the combination of PTT and immunotherapy has been developed as a new strategy of cancer treatment.
Methods: Thermal responsive liposomes (TRL) were synthesized by incorporating ICG into the lipid bilayer and encapsulating the water-soluble immune stimulatory molecule polyinosinic:polycytidylic acid (poly I:C) in the hydrophilic core. The poly I:C- and ICG-containing TRLs (piTRLs) were analyzed according to size, and their photothermal effect was evaluated following laser irradiation at 808 nm. Moreover, the temperature-dependent release of poly I:C was also measured. For cancer therapy, CT-26 (carcinoma) and B16 (melanoma) cells were subcutaneously inoculated to build the 1st transplanted tumor in BALB/c and C57BL/6 mice, respectively. These mice received a 2nd transplantation with the same cancer cells by intravenous inoculation, for evaluation of the anti-metastatic effects of the liposomes after PTT.
Results: Near-infrared (NIR) laser irradiation increased the temperature of piTRLs and effectively released poly I:C from the liposomes. The increased temperature induced a photothermal effect, which promoted cancer cell apoptosis and dissolution of the 1st transplanted tumor. Moreover, the released poly I:C from the piTRL induced activation of dendritic cells (DCs) in tumor draining lymph node (tdLN). Cancer cell apoptosis and DC-activation-mediated cancer antigen-specific immune responses further prevented growth of lung metastatic cancer developed following intravenous transplantation of cancer cells.
Conclusion: These results demonstrated the potential usage of a piTRL with laser irradiation for immuno-photothermal therapy against various types of cancer and their metastases.
Keywords: Anti-metastasis; Immunotherapy; Indocyanine green; Liposome; Photothermal therapy; Polyinosinic:polycytidylic acid.