Combination of 5-Aminolevulinic Acid and Indocyanine Green in Photodynamic Therapy for Squamous Cell Carcinoma: An In Vivo Study

Photodynamic therapy (PDT) is a promising minimally invasive treatment for non-melanoma skin cancer (NMSC). Still, its clinical efficacy is limited by light attenuation and drug distribution within tumor tissue, which restricts photosensitizer activation in deeper tumor regions. PDT with 5-aminolevulinic acid (ALA) is widely used as a precursor to induce accumulation of protoporphyrin IX (PpIX), followed by red light irradiation; it is effective for superficial lesions but often fails to achieve complete tumor control in thicker tumors, contributing to long-term lesion recurrence. To address this limitation, we investigated a two-photosensitizer, two-wavelength-PDT approach that combines ALA with indocyanine green (ICG), a near-infrared (NIR)-responsive photosensitizer that can be activated at greater tissue depths. This also promotes different cellular death targets, since ALA-mediated PDT mainly induces direct tumor cell killing through apoptosis and necrosis, whereas ICG-mediated PDT may contribute to treatment effects. In this study, a preclinical model of cutaneous squamous cell carcinoma (SCC) was used, with animals assigned to control, single-photosensitizer PDT (ALA, ICG), and combined photosensitizer PDT treatment groups. The effect of photosensitizer administration and light irradiation sequence on therapeutic efficacy was also investigated, and tumor progression and survival outcomes were monitored over time. The results indicated that the combined ALA+ICG-PDT approach produced the most sustained suppression of tumor growth and significantly prolonged animal survival compared with single-photosensitizer PDT. Importantly, these findings demonstrated a strong dependence on the sequence in which the photosensitizer and light are applied. These findings indicate that integrating photosensitizers activated at complementary wavelengths may improve treatment coverage across the tumor and partially overcome depth-related limitations associated with PDT, representing a promising strategy to enhance PDT efficacy for NMSC and other solid tumors.