Photobiomodulation pre-treatment enhances ALA-induced PpIX accumulation and photodynamic therapy response in oral squamous cell carcinoma cells in cell culture

INTRODUCTION: Photodynamic therapy (PDT) and photobiomodulation (PBM) are light-based modalities with differing applications but which share overlapping spectral and irradiance requirements. PDT, which uses an exogenous photosensitizer, can be used to impart a selective cytotoxic response in target malignant tissues, while PBM is typically used to stimulate biological responses that are beneficial in healthy tissues. The inherent compatibility in light delivery requirements suggests the potential for exploring interactions between these modalities which could enhance tumor response to PDT. OBJECTIVE: We sought to evaluate the capability of photobiomodulation (PBM) pretreatment to enhance aminolaevulinic acid (ALA)-induced protoporhyrin IX (PpIX) photosensitization, and subsequent PDT response in oral squamous cell carcinoma (OSCC) cells. MATERIAL AND METHODS: OSCC cell cultures were initially treated with 635 or 830 nm irradiation at various fluences to examine fluence and wavelength dependence of PBM effects. Fluence-dependent changes in mitochondrial dehydrogenase activity as an indicator of mitochondrial metabolic function were assessed by the MTS assay. PBM treated cultures were also lysed to assess PpIX accumulation via fluorescence analysis. Cytotoxic response in cultures treated with PDT with or without PBM priming/pre-treatment at varying fluences were assessed by image analysis following staining with calcein acetoxymethyl ester and ethidium bromide. RESULTS: Photobiomodulation (PBM) using red (635 nm) or infrared (830 nm) light sources produced dose-dependent increases in mitochondrial dehydrogenase activity (as measured by the MTS assay) with minimal evidence of phototoxicity in oral squamous cell carcinoma (OSCC) cells. The accumulation of PpIX resulting from ALA incubation increased significantly in OSCC cells that were pre-treated with PBM relative to controls. The combination of PBM and PDT showed significantly increased cytotoxic response compared to PDT alone. CONCLUSIONS: PBM enhances ALA-induced PpIX accumulation, and subsequent PDT response in OSCC cells. This enhancement effect can be achieved using the same light source as for PpIX photoactivation suggesting potential as a clinically practical pre-treatment to enhance efficacy of ALA-PDT.