Hematoporphyrin derivatives potentiate the radiosensitizing effects of 2-deoxy-D-glucose in cancer cells

PURPOSE: Two deoxy-D-glucose (2-DG), an inhibitor of glucose transport and glycolysis, has been shown to differentially inhibit the repair of radiation damage in cancer cells by reducing the flow of metabolic energy. Since hematoporphyrin derivatives (Hpd) inhibit certain enzymes of the respiratory metabolism, resulting in an increase in the glucose usage and glycolysis, Hpd could possibly enhance the energy-linked radiosensitizing effects of 2-DG in cancer cells. The purpose of the present work was to verify this suggestion. METHODS AND MATERIALS: Two human tumor cell lines (cerebral glioma, BMG-1 and squamous cell carcinoma, 4197) and a murine tumor cell line (Ehrlich ascites tumor [EAT], F-15) in vitro were investigated. A commercially available preparation of Hpd, Photosan-3 (PS-3) was used in the present studies. Cells incubated with 0-10 microg/ml PS-3 for 0-24 h before irradiation were exposed to 2.5 Gy of Co-60 gamma rays and maintained under liquid holding conditions for 1-4 h to facilitate repair. 2-DG (0-5 mM) added at the time of irradiation was present during the liquid holding. Radiation-induced cytogenetic damage (micronuclei formation) and cell death (macrocolony assay) were analyzed as parameters of radiation response. Effects of these radiosensitizers on glucose usage and glycolysis were also studied by measuring the glucose consumption and lactate production using enzymatic assays. RESULTS: The glucose consumption and lactate production of BMG-1 cells (0.83 and 1.43 pmole/cell/h) were twofold higher than in the 4197 cells (0.38 and 0.63 pmole/cell/h). Presence of PS-3 (10 microg/ml) enhanced the rate of glycolysis (glucose consumption and lactate production) in these cells by 35% to 65%, which was reduced by 20% to 40% in the presence of 5 mM 2-DG. In exponentially growing BMG-1 and EAT cells, presence of 2-DG (5 mM; equimolar with glucose) for 4 hours after irradiation increased the radiation-induced micronuclei formation and cell death by nearly 40%, whereas no significant effects could be observed in 4197 cells. In EAT cells, radiation was also observed to induce apoptotic death, which was significantly increased in the presence of the combination (PS-3 + 2-DG). The combination (PS-3 + 2-DG) enhanced the radiation damage in all three cell systems by 60-100%. Furthermore, the radiosensitizing effects of the combination (PS-3 + 2-DG) were higher at pH 6.7 as compared to pH 7. 4. In the plateau phase, presence of 2-DG alone did not significantly influence the radiation response of either BMG-1 or of 4197 cells, whereas in combination with PS-3, 2-DG enhanced the radiation damage in both these cell lines by 40% to 50%. Furthermore, in BMG-1 cells, the effects of 2-DG were observed to be reversible to a very great extent, while that of the combination were mostly irreversible. CONCLUSION: The hematoporphyrin derivative PS-3 enhances the radiosensitizing effects of 2-DG in cancer cells, possibly by further reducing the energy supply leading to an irreversible inhibition of DNA repair, and increased cytogenetic damage and cell death. Since both these compounds have been used in clinical practice, further studies to investigate their use in improving radiotherapy of tumors are warranted.