Treatment with mTOR Inhibitor, Everolimus (RAD001) Overcomes Resistance to Imatinib In Ph-Leukemia Quiescent Cells.

Abstract 1579

Recent studies suggest that leukemia stem cells (LSCs) are responsible for relapse of leukemia and eradication of LSCs should be necessary for the cure. In order to examine mechanisms of drug resistance in Ph⁺ leukemia to imatinib (IM) due to stem cell properties and to seek strategies to overcome the resistance, we've previously established in vivo-murine and ex vivo-culture models using murine hematopoietic pluripotent progenitors transduced with BCR-ABL (Minami, et al., PNAS, 2008). Furthermore, Ph⁺ leukemia (including T315I-, F311I-mutated CML-BC, or Y253H-mutated Ph-ALL) patient cells were serially xenotransplanted into immunodeficient NOD/SCID/IL2rγ^null (NOG) mice. Engrafted bone marrow and spleen cells were almost identical to the original leukemia cells as to phenotypes including karyotypes and distribution of primitive populations. Spleen cells from leukemic NOG mice were treated ex vivo with IM and growth factors, and cell viablility (PI/AnnexinV-staining) was compared between treated and non-treated cells. After treatment with IM, significantly more residual cells were observed in the CD34⁺/38^- population compared to the CD34⁺/38⁺ or CD34^-/38⁺ populations. Phosphorylation of BCR-ABL and CrkL was completely inhibited in all populations with IM treatment. Regarding cell cycle states, a higher percentage of quiescent slow-cycling (Hoechst 33342^low/Pyronin Y^low) cells was observed in the CD34⁺/38^- population relative to the other populations. Recently, aberrant activation of mTOR signaling has also been reported to be involved in LSCs. Leukemic spleen cells in longer co-culturing with stromal cells were treated with IM and mTOR inhibitor, everolimus (Eve, RAD001). While slow-cycling CD34^high+ cells were insensitive to IM, combination treatment with IM and Eve induced significant cell death also in the quiescent population. In vivo-treatment with Eve decreased tumor cells in leukemic NOD mice inoculated with the leukemic NOG cells under the sub-lethally irradiated condition. Treatment with Eve also reduced long-term colony formation of Ph⁺ leukemia patient samples including Y253H-mutated Ph-ALL with less toxicity to normal cord blood cells. These results imply that treatment with Eve is promising for overcoming the resistance to IM due to quiescent property in Ph⁺ LSCs.