Potentiation of phototherapy cytotoxicity with light scattering media

Abstract

Lung cancer cells are susceptible to photodynamic therapy (PDT) using 630 nm light and dihematoporphyrin ether (DHE). A light scattering media, intralipid (IL), was compared to balanced salt solution (PBS) for PDT of A549 human lung cancer cells. Differences in cellular DHE content after IL or PBS exposure were determined. Cells were incubated in 25 μg/ml DHE for 2 hr and then incubated in various concentrations of IL or PBS at room temperature for 2.5 to 10.0 min. Significant amounts of DHE were lost from IL-incubated cells compared to cells incubated in PBS. After 5 min in 1% IL, cellular DHE content was 0.32 ± 0.04 μg DHE/10 6 cells compared to 0.56 ± 0.11 μg DHE/10 6 cells in PBS-incubated cells ( P < 0.05). Despite this, superior PDT cytotoxicity was noted when cells were treated in IL with energy densities ⩾ 105 mJ/cm 2. At an energy density of 210 mJ/cm 2, the survival fraction (SF) of cells treated in 1% IL was 0.004 ± 0.001 compared to 0.071 ± 0.022 in PBS-treated cells ( P < 0.05). SF was dependent upon the IL concentration with the greatest cell killing noted with 1% IL. An apparent loss of cellular DHE (“DHE washout”) was confirmed by demonstration of a higher SF of cells incubated in IL, rinsed, and subsequently PDT-treated in PBS with 157.5 mJ/cm 2 (SF = 0.85 ± 0.11) compared to cells incubated and treated in PBS (SF = 0.50 ± 0.03, P < 0.05). Separation of the cells from 1% IL via a transparent, impermeable barrier resulted in the greatest PDT effect (SF = 0.037 ± 0.02, P < 0.05). These data suggest that the light-dispersing characteristics of IL may result in greater PDT efficacy despite DHE loss into the lipid media during treatment. These factors may be important in maximizing the efficiency and safety of PDT.