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Combined treatment of ionizing radiation and photosensitization by 5-aminolevulinic acid-induced protoporphyrin IX.

The response of human colon adenocarcinoma cells of the line WiDr to the combined treatment of ionizing radiation and photosensitization by 5-aminolevulinic acid-induced protoporphyrin IX was assessed by a colony-forming assay. A dose of X rays inactivating approximately 50% of the cells was used. Seventy to 85% of the cells accumulated in S and G2 + M phase 12-24 h after such a treatment as measured by flow cytometry, while the distribution of cells in the phases of the cell cycle approached that of untreated cells 48 h after X-ray treatment. Cellular photosensitization was developed by endogenous synthesis of protoporphyrin IX (PPIX) from the precursor 5-aminolevulinic acid (5-ALA). This was performed by treating the cells with 1 mM 5-ALA for 4 h in a serum-free medium. The endogenous synthesis of PPIX increased with time after the cells had been subcultured, i.e. the ability of the cells to synthesize PPIX increased 1.5-2-fold within 48 h of incubation. This was not due to effects of trypsin on the cells. Photochemotherapy with 5-ALA was given 0-48 h after X rays. The combined cytotoxic effect was analyzed by an isobologram after correction of the survival curves for microcolony formation and differences in intracellular concentration of PPIX. The results indicate that 5-ALA PCT given 0-4 h after X rays acts slightly antagonistically while 5-ALA PCT given 12-48 h after X rays acts slightly synergistically.