Purpose: To improve treatment efficacy and tumor cell selectivity of delta-aminolevulinic acid (ALA)-based photodynamic therapy (PDT) via pretreatment of cells and tumors with methotrexate to enhance intracellular photosensitizer levels.
Experimental design: Skin carcinoma cells, in vitro and in vivo, served as the model system. Cultured human SCC13 and HEK1 cells, normal keratinocytes, and in vivo skin tumor models were preconditioned with methotrexate for 72 h and then incubated with ALA for 4 h. Changes in protoporphyrin IX (PpIX) levels and cell survival after light exposure were assessed.
Results: Methotrexate preconditioning of monolayer cultures preferentially increased intracellular PpIX levels 2- to 4-fold in carcinoma cells versus normal keratinocytes. Photodynamic killing was synergistically enhanced by the combined therapy compared with PDT alone. Methotrexate enhancement of PpIX levels was achieved over a broad methotrexate concentration range (0.0003-1.0 mg/L; 0.6 nmol/L-2 mmol/L). PpIX enhancement correlated with changes in protein expression of key porphyrin pathway enzymes, approximately 4-fold increase in coproporphyrinogen oxidase and stable or slightly decreased expression of ferrochelatase. Differentiation markers (E-cadherin, involucrin, and filaggrin) were also selectively induced by methotrexate in carcinoma cells. In vivo relevance was established by showing that methotrexate preconditioning enhances PpIX accumulation in three models: (a) organotypic cultures of immortalized keratinocytes, (b) chemically induced skin tumors in mice; and (c) human A431 squamous cell tumors implanted subcutaneously in mice.
Conclusion: Combination therapy using short-term exposure to low-dose methotrexate followed by ALA-PDT should be further investigated as a new combination modality to enhance efficacy and selectivity of PDT for epithelial carcinomas.