Photothermal and immunological reactions against metastatic tumors using laser photosensitizer immunoadjuvant

Abstract

Photothermal tissue interaction is the most common phenomenon when laser energy is deposited in tissue. Because of the sensitivity of cancer cells to temperature increase, photothermal reaction can be an effective mechanism of direct cancer destruction using lasers. Tumor-specific immune response is crucial in achieving systemic and long-term cures for cancers, particularly for metastatic cancers. Can photothermal interaction induce sufficient immunological reaction when the local destruction of tumor cells occurs? To achieve selective photothermal destruction, indocyanine green as a photosensitizer was directly injected into rat mammary tumors, followed by irradiation of 805 nm laser light. Although extensive photothermal tumor killing was achieved and tumor growth was slowed down immediately following the treatment, photothermal reaction alone was shown not sufficient in controlling the treated primary tumors and their metastases. When an immunoadjuvant was used with the indocyanine green, however, the same laser treatment not only could eventually eradicate the treated primary tumors but also eradicate the untreated metastases at remote sites. The tumor eradication went through a growth-regression process over a period of six to nine weeks post-treatment, indicating an induced immune response. The Western Blot analysis using the serum from a laser-immunotherapy cured rat showed that the tumor-specific antibody induced by the treatment had a long- lasting effect. Our experimental data indicated that photothermal interaction alone was not sufficient to slow and eventually reverse tumor growth. However, it can reduce the tumor burden and at the same time release tumor antigens to be recognized by the host immune system. Therefore, in conjunction with specific immunological stimulation using in situ immunoadjuvants, the selective thermal injury to tumors plays an important and a direct role in this laser immunotherapy.