Resistance of oral cancer cells to 5-ALA-mediated photodynamic therapy

Photodynamic therapy (PDT) has been indicated for oral squamous cell carcinoma (OSCC) at early stages. Chemo and radio-resistance are frequently observed in OSCC, but it is unknown whether this tumor can develop resistance to PDT. It was investigated the process of acquiring resistance to multiple cycles of PDT by using OSCC cells. We also analyzed the expression of anti-apoptotic proteins and those related to Akt/mTOR pathway. Sub-lethal doses of PDT were applied, consisting of a constant concentration of 5-aminolevulinic acid (5-ALA) (1 mM, 4-h incubation) and increasing irradiation dose with LED (from 5.86 to 10.54 J/cm² ). Cell viability, migration capacity, intracellular expression of protoporphyrin IX (PpIX), mitochondrial density, and pro-survival proteins were investigated in PDT-resistant cells. Six OSCC cell generations resistant to PDT were isolated. The resistant cells exhibited a survival phenotype characterized by a reduction in the expression of endogenous PpIX, increase in mitochondrial density, increase in migration capacity, and up-regulation of p-NFκB, p-survivin, iNOS, p-Akt Ser⁴⁷³ , cyclin D1, p-mTOR Ser²⁴⁸¹ , and p-mTOR Ser²⁴⁴⁸ . OSCC cells are able to survive doses of 5-ALA-PDT by reducing PpIX synthesis and activating signaling pathways related to the inhibition of apoptosis and improvement of cell proliferation. Further studies are necessary to confirm whether this PDT-resistance phenotype can be clinically present, mainly in OSCC showing clinical recurrences after exposure to different PDT protocols.