Surface layer-preserving photodynamic therapy (SPPDT) in a subcutaneous mouse model of lung cancer

Background and Objectives Photodynamic therapy (PDT) may be a less invasive treatment for lung cancer. Our newly developed surface layer-preserving PDT (SPPDT) technique enables us to irradiate deep tumor while preserving the overlying tissue. The aim of this basic study was to verify that the SPPDT technique might be applied to lung cancer. Study Design/Materials and Methods PDT with talaporfin sodium was performed using a pulsed laser with different pulse dose rates (PDRs, 2.5-20.0 mJ/cm²/pulse) in a mouse model of subcutaneous tumor. To mimic the tracheal wall structure and a thoracic tumor in the tracheobronchus, we also made a mouse model in which a piece of swine cartilage was placed between the dermis and the tumor, and PDT was carried out 2 weeks after implantation. In both experiments, the tissue samples were collected 48 hours after PDT and evaluated microscopically. Results SPPDT using a high-PDR laser damaged the underlying tissue but left the superficial tissue intact in the mouse subcutaneous tumor model. In SPPDT, a higher PDR produced a thicker layer of intact superficial tissue than a lower PDR, while a lower PDR produced a deeper layer of damaged tissue than a higher PDR. SPPDT was also able to preserve the superficial tissue and to damage the tumor tissue beneath the cartilage implant. Conclusion SPPDT was able to damage tumor beneath the superficial normal tissue layer, which included tracheal cartilage in the mouse model. The thickness control of SPPDT was provided by controlling laser pulse intensity. SPPDT is a new technology, whose future potential is unknown. The initial clinical application of this technology could be endoscopic treatment (e.g., palliative therapy of thoracic malignancies via bronchoscopy).