Interaction of photodynamic therapy and hyperthermia: tumor response and cell survival studies after treatment of mice in vivo.

The interaction of photodynamic therapy (PDT) and hyperthermia was studied in the radiation-induced-fibrosarcoma experimental mouse tumor system by tumor regrowth experiments as well as in vivo to in vitro cloning assays. In vivo, PDT (Photofrin II, 10 mg/kg i.p.), followed 24 h later by light (135 J/cm2, 630 nm) and/or heat (44 degrees C, 30 min) caused severe vascular damage (congestion of tumor vessels and hemorrhage) and subsequent disappearance of palpable tumor mass. While heat-treated tumors always started to regrow within 2 days of treatment, regrowth if it occurred was delayed to 4-5 days after PDT and 6-7 days following combined treatments. Only PDT followed by heat cured a considerable number of animals (45%), while PDT alone and heat followed by PDT cured less than 10% of animals, and heat alone caused no tumor cures. The various treatments differed in their immediate as well as their delayed effects on tumor clonogenicity when observed over a 24-h period. Tumors treated with PDT showed no immediate changes in clonogenicity, but progressive delayed cell death occurred if tumors remained in situ. Heat alone led to an immediate reduction in the number of clonogenic tumor cells, followed by some additional cell death for 4 h and subsequent recovery of clonogenicity. PDT followed by heat caused markedly potentiated immediate reduction in cell survival which may be the result of direct interaction of heat and PDT damage affecting the tumor cells. Some tumors rapidly progressed to total eradication, whereas others showed delayed survival values similar to those for tumor having received PDT only. In the reverse sequence, heat before PDT, the tumor cell survival kinetics resembled those following heat treatment alone. The comparative lack of effectiveness of this treatment regimen can be explained by the severe tumor hemorrhage caused by the initial heat treatment which reduces the transmission of light essential for the subsequent PDT treatment. This study shows that despite pronounced similarities in the microscopic and macroscopic appearance shortly after treatment by PDT or hyperthermia, these two modalities lead to tumor destruction by different mechanisms. Furthermore the combination of these two modalities in the proper sequence leads to potentiated cytocidal effects on the tumor cells in vivo.