Merocyanine 540-sensitized photoinactivation of leukemia cells: effects of dose fractionation.

The differential sensitivity to merocyanine 540 (MC540)-sensitized photoirradiation of leukemia cells, selected solid tumor cells, and normal pluripotent hematopoietic stem cells has been successfully exploited for the extracorporeal purging of simulated autologous remission bone marrow grafts. In this communication, we compare the effects of fractionated vs continuous irradiation upon the MC540-sensitized photoinactivation of L1210 and K562 leukemia cells. Exposure to MC540 (15 micrograms/mL) and fractionated doses of white light inactivated fewer in vitro clonogenic cells than exposure to an equivalent dose of continuous irradiation, provided the irradiation doses were small (8.1-16.2 kJ/m2) and spaced 1-2 h apart. The dye-sensitized photoinactivation of leukemia cells was enhanced when cells were stored at 4 degrees C instead of 37 degrees C between irradiation periods, most likely in part because the cells were unable to repair sublethal photodynamic damages at the lower temperature. These data suggest that cells can recover from sublethal damage inflicted by the plasma membrane-active photosensitizer, MC540.