Aim: To elucidate the mechanism of the potentiating influence of heating associated with photoirradiation on the antitumor efficiency of photodynamic therapy (PDT) with hematoporphyrin derivative (HPD). Methods: The study was carried out on Ehrlich ascites carcinoma (EAC) cells, which were loaded with HPD in a serum-free medium and then irradiated with red light (lambda(max) = 630 nm) at various temperatures. Cytotoxicity was estimated by the trypan blue exclusion assay. Results: Our data,support the view that in PDT the hyperthermia (around 44 degreesC) produced by irradiation can enhance synergistically the HPD-photoinduced tumor eradication; it was found that raising the irradiation temperature from 30 to 44 degreesC caused a substantial (similar to 1.5 fold) increase in the rate of HPD-photosensitized inactivation of EAC cells, while hyperthermia (44 degreesC) itself showed little toxic effects towards the cells. Conclusion: Studies indicated that the potentiating effect of heating on the antitumor efficiency of HPD-PDT could be largely explained by the stimulation of reactive oxygen species formation such as H2O2, superoxide and hydroxyl radicals. It was also found that photosensitization of EAC cells by HPD caused a strong fall in the activity of catalase (CAT) and glutathione (GSH) peroxidase, and that heating sensitized the H2O2-detoxifying enzymes to HPD-photoinduced inactivation. Under HPD-PDT, these events could result in loss of protection against accumulating H2O2; we revealed that cell-bound CAT and the GSH redox cycle play an important role in the protection of EAC cells against HPD-PDT. Moreover, our findings suggest that during PDT with HPD, an increase in the temperature of tumors could enhance the efficiency of this therapy via the stimulation of a chlorin-type photoproduct formation.
