Combination of PDT and HT increases free radical induced tumor cell death in vivo

From recent experiments we have strong evidence that low energy radiation (photo dynamic therapy ‐ PDT) and hyperthermia (HT) exert their therapeutic effects via formation of free radicals. We carried out a couple of anmial experiments (rats) to elucidate whether the effect of either HT or PDT and the combination of both. During hyperthermia of a DS carcinosarcoma superoxide anions are formed due to specific oxygenation conditions of the tumor tissue. PDT alone produces severe damage of endothelial cells (mitrosylation) with the result of impaired oxygenation and blood flow in the tumor. To optimize this approach both treatment methods were combined. HT induces formation of free radicals with subsequent protein‐ and lipid‐peroxidation and apoptosis, an event which is potentiated if oxygen is delivered via the mask (induction of ischemia/reperfusion). In comparison to HT alone, an increase in the number of cells undergoing apoptosis was seen with ALA‐PDT and ALA‐PDT+HT (TUNEL‐assay, DNA‐fragmentation). Increased caspase‐3 and ‐8 activities were found after HT, ALA‐PDT, or ALA‐PDT+HT. Immunohistological detection of protein nitration was used to localize reactive nitrogen‐related damage. Increased peroxynitrite formation was most prominent after ALA‐PDT+HT preferably in endothelial cells. The formation of cytotoxic peroxynitrate however, strongly depends on the presence of superoxide anions. Taken together the formation of reactive oxygen species with subsequent oxidation/nitration of proteins and induction of apoptotic signaling pathways is a major event during HT and PDT. The combination of HT and PDT is a new and promising approach which uses synergistic effects on ROS formation within the tumor tissue.