This study compares apoptosis induction in squamous cell carcinoma keratinocytes following exposure to 5-aminolevulinic acid combined with either blue light (417 ± 5 nm) or red light (635 ± 5 nm) photodynamic therapy, with apoptotic activity quantified via levels of Annexin V expression. Our results demonstrate that blue light induces superior levels of apoptosis in squamous cell carcinoma keratinocytes compared to red light at the parameters tested. Both wavelengths demonstrated additional dose- and temperature-dependent effects across two squamous cell carcinoma cell lines. These conclusions have significant implications for the clinical selection of a light source in combination 5-aminolevulinic acid photodynamic therapy in the treatment of actinic keratosis and early squamous cell carcinoma lesions.

5-Aminolevulinic (5-ALA) acid photodynamic therapy (PDT) is a treatment for actinic keratosis and has been studied as a treatment for non-invasive cutaneous squamous cell carcinoma (cSCC). PDT induces apoptosis and necrosis in actinic keratosis and cSCC. 5-ALA blue light PDT may modulate gene expression and pathways in surviving cells. In this study, differential gene expression and pathway analysis of cSCC and human dermal fibroblasts were compared before and after 5-ALA blue light PDT using RNA-Sequencing. No genes were differentially expressed after correcting for multiple testing (FDR <0.05). As a result, transcription factor, gene enrichment, and pathway analysis were performed with genes identified before multiple testing (p<0.05). Pathways associated with proliferation and carcinogenesis were downregulated. These findings using 5-ALA blue light PDT are similar to previously published studies using methyl-aminolevulinic and red light protocols, indicating that surviving residual cells may undergo changes consistent with a less aggressive cancerous phenotype.