Acitretin enhances the cytotoxic effect of 5-aminolevulinic acid photodynamic therapy on squamous cell carcinoma cells

•It is the first report of using acitretin as an enhancer to promote cytotoxic effect of ALA-PDT on SCC cells in vitro.•This study provides a theoretical basis for the combined treatment of acitretin and ALA-PDT in cSCC, as well as a new idea for the treatment of refractory skin tumors of epidermal origin. The efficacy of 5-aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) on invasive cutaneous squamous cell carcinoma (cSCC) remains to be improved due to the limited penetration of this treatment. Previous study showed that acitretin and ALA-PDT had synergistic effect on cSCC, but whether acitretin can enhance the cytotoxic effect of ALA-PDT on cSCC is unclear. To investigate whether acitretin can enhance the cytotoxic effect of ALA-PDT on SCL-1 cells, as well as the possible mechanism involved. Inverted microscopy, trypan blue exclusion assay, and flow cytometry were used to studied the morphology, viability and apoptosis of SCL-1 cells treated with acitretin, ALA-PDT and acitretin followed by ALA-PDT treatment, respectively. Confocal microscopy was applied to detect the ROS formation of SCL-1 cells treated with acitretin of four different concentrations. The ROS formation of SCL-I cells treated with acitretin of four different concentrations followed by ALA-PDT treatment was detected using confocal microscopy and flow cytometry. SCL-1 cells exhibited a significant morphological alteration when treated with acitretin followed by ALA-PDT. The combination of acitretin and ALA-PDT induced a higher cell death rate and apoptosis than that with acitretin or ALA-PDT treatment alone. ROS could be induced when incubated with acitretin at a concentration of 6.4 × 10−4mg /mL or above. However, a higher level of ROS formation was observed when SCL-1 cells were treated with acitretin followed by ALA-PDT than that with ALA-PDT or acitretin alone. Acitretin can enhance the cytotoxic effect of ALA-PDT on SCL-1 cells, possibly via the ROS pathway.