Increased survival of normal cells during laser photodynamic therapy: implications for ex vivo autologous bone marrow...

Laser light-induced, dye-mediated photolysis of leukemic cells was tested in an in vitro model for its efficacy in eliminating occult tumor cells for ex vivo autologous bone marrow purging. Merocyanine 540 (MC540) was mixed with acute promyelocytic leukemia (HL-60) cells in the presence of human albumin. This cell-dye mixture was irradiated with 514 nm argon laser light. Results show that in the presence of 0.1%, 0.25% and 0.5% albumin, laser light doses of 62.4 J/cm2, 93.6 J/cm2 and 109.2 J/cm2, respectively, were required for a 5 log reduction in the survival of leukemic cells. Under identical conditions, 80% to 84% of the normal bone marrow cells and 41% of the granulocyte-macrophage colony forming cells survived. The number of surviving stromal cells was reduced (1+) compared to the untreated control (4+). Mixing of irradiated bone marrow cells with equal number of HL-60 cells did not interfere with the killing of HL-60 cells treated with MC540 and laser light. The non-specific cytotoxicity of laser light alone was less than 6% for normal bone marrow cells. These results suggest that the concentration of human albumin plays an important role in laser light-induced phototoxicity. This laser light-induced selective photolysis of leukemic cells can be used in ex vivo purging of tumor cell-contaminated bone marrow grafts to achieve very high survival rates of normal bone marrow cells and granulocyte-macrophage colony forming cells.