The effects of protoporphyrin IX-induced photodynamic therapy with and without iron chelation on human squamous carcinoma cells cultured under normoxic, hypoxic and hyperoxic conditions

Abstract

Photodynamic therapy requires the combined interaction of a photosensitiser, light and oxygen to ablate target tissue. In this study we examined the effect of iron chelation and oxygen environment manipulation on the accumulation of the clinically useful photosensitiser protoporphyrin IX (PpIX) within human squamous epithelial carcinoma cells and the subsequent ablation of these cells on irradiation. Cells were incubated at concentrations of 5%, 20% or 40% oxygen for 24h prior to and for 3h following the administration of the PpIX precursors aminolevulinic acid (ALA), methyl aminolevulinate (MAL) or hexylaminolevulinate (HAL) with or without the iron chelator 1,2-diethyl-3-hydroxypyridin-4-one hydrochloride (CP94). PpIX accumulation was monitored using a fluorescence plate reader, cells were irradiated with 37 J/cm(2) red light and cell viability measured using the neutral red uptake assay. Manipulation of the oxygen environment and/or co-administration of CP94 with PpIX precursors resulted in significant changes in both PpIX accumulation and photobleaching. Incubation with 5% or 40% oxygen produced the greatest levels of PpIX and photobleaching in cells incubated with ALA/MAL. Incorporation of CP94 also resulted in significant decreases in cell viability following administration of ALA/MAL/HAL, with oxygen concentration predominantly having a significant effect in cells incubated with HAL. Experimentation with human squamous epithelial carcinoma cells has indicated that the iron chelator CP94 significantly increased PpIX accumulation induced by each PpIX congener investigated (ALA/MAL/HAL) at all oxygen concentrations employed (5%/20%/40%) resulting in increased levels of photobleaching and reduced cell viability on irradiation. Further detailed investigation of the complex relationship of PDT cytotoxicity at various oxygen concentrations is required. It is therefore concluded that iron chelation with CP94 is a simple protocol modification with which it may be much easier to enhance clinical PDT efficacy than the complex and less well understood process of oxygen manipulation.