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PDT-induced in vitro bystander effect

The mechanisms of Photodynamic therapy (PDT) include singlet oxygen and reactive oxygen species (ROS) production that damage tumor cells and vasculature. The resulting effect is a balance between photo-oxidations via primary or secondary ROS and scavenging activity. Sensitizers distribute in the extra-cellular space before and during cell sensitization, suggesting that PDT could act directly on cell structures and on extra-cellular compartments, including sera. In this paper we endeavored to determine whether the application of PDT to culture media could have an effect on cell survival. Culture media (RPMI supplemented with Fetal Calf Serum (FCS)) was incubated with Rose Bengal (RB) and irradiated before being added to cells for various times of contact, as a replacement for untreated media. Treatedmedia reduced cell survival by up to 40% after 30 min of contact for 10 μg/mL of RB and 20 J/cm2. This effect was RB or light dose-dependent. The survival reduction observed when adding treated-media was more pronounced when cell doubling time was shorter. Analysis of ROS or peroxide production in treated-media revealed a long-lasting oxidizing activity. Our findings support the hypothesis of a ROS or peroxide-mediated, PDT-induced, delayed cell toxicity