Hyperthermia potentiates the activity of immunotoxin conjugates against common acute lymphoblastic leukaemia cells in...

We studied the in-vitro cytotoxic effect of hyperthermia at 42 degrees C, both alone and in combination with ricin-linked immunotoxins, reactive with the common acute lymphoblastic leukaemia cell lines Reh and KM-3. Assessment of cytotoxicity was by incorporation of 3H-leucine and limiting dilutions analysis. The effect of immunotoxins alone and in combination with hyperthermia on normal human marrow progenitor cells was assessed by conventional colony forming units-granulocyte macrophage (CFU-GM) assay. We found that incubation of either of the cell lines with a mixture of the two immunotoxins, RPH-7-ricin and PHM-6-ricin, at 42 degrees C for one hour (h) potentiated the cytotoxic activity of the immunotoxins at 37 degrees C. At a concentration of 10(-8) mol/L, a 2.2-log kill was seen with KM-3 leukaemic cells at 37 degrees C and a 3.3-log kill at 42 degrees C, an increase of approximately 10 fold in cytotoxic activity. Survival of CFU-GM following treatment at 42 degrees C for one h with a similar concentration of immunotoxins was 26.2% (+/- 13.7%) (equivalent to 0.6 log kill) and 76.0% (+/- 1.83%) (0.1 log kill) when normal marrow was incubated with immunotoxins at 37 degrees C. This suggests relative sparing of normal marrow cells compared with the leukaemic cells tested as indicated by the 2.1-log kill difference (approximately 100 fold) between normal and leukaemic cells at 37 degrees C and the 2.7-log kill (approximately 500-fold) difference at 42 degrees C. We conclude that hyperthermia may have a role in addition to immunotoxins in the purging of marrow ex vivo to remove leukaemic cells.