Differences in heat sensitivity between normal and acute myeloid leukemic stem cells: Feasibility of hyperthermic purging of leukemic cells from autologous stem cell grafts

Objectives. In autologous stem cell transplantation contamination of the graft with malignant cells is frequently noticed and necessitates the use of in vivo or in vitro purging modalities. The hematopoietic recovery after transplantation depends on the number of stem and progenitor cells in the transplant. Therefore, in the present study the effects of hyperthermic treatment on the human normal and acute myeloid leukemic (AML) stem cell compartment were investigated. Methods. Normal bone marrow and AML blasts were heat treated up to 120 minutes at 43°C. The surviving fractions of the different stem cell subsets were determined using in vitro methylcellulose and cobblestone area-forming cell (CAFC) clonogenic assays, as well as the in vivo NOD/SCID repopulating assay. The leukemic nature of the colonies from AML cells was confirmed by RT-PCR analysis. In order to increase the therapeutic index of the hyperthermic purging modality, the heat treatment was preceded by a 3-hour incubation at 37°C with the ether lipid ET-18-OCH 3 (25 μg/mL). Results. It could be demonstrated that normal progenitor cells are far more resistant to hyperthermia than leukemic progenitor cells (56%±7% vs 9.9%±2.6% survival after 60 minutes at 43°C, respectively). Furthermore, normal hematopoietic stem cells appear to be extremely resistant to the heat treatment (94%±9% survival after 60 minutes at 43°C). In contrast, in the leukemic stem cell compartment no significant differences in heat sensitivity between the stem cells and progenitor subsets could be observed (12.3%±2.9% vs 9.9%±2.6% survival after 60 minutes at 43°C, respectively). The combined treatment resulted in a survival for normal progenitor and stem cells of 32%±6% and 85%±15% after 60 minutes at 43°C, respectively. Under these conditions the number of leukemic stem cells was reduced to 1%±0.3%. After 120 minutes at 43°C, no AML-colonies could be detected anymore. Conclusions. Our data demonstrate that leukemic stem cells have an increased hyperthermic sensitivity compared to their normal counterparts and that this difference can be further increased in combination with ET-18-OCH 3. These striking differences in heat sensitivity warrant the use of hyperthermia as a clinically applicable purging modality in autologous stem cell transplantation.