Treatment with Bortezomib Overcomes Resistance to Imatinib in Ph-Leukemia Quiescent Cells.

Abstract 1426

Poster Board I-449

Recent studies suggest that leukemia stem cells (LSCs) are responsible for relapse of leukemia following conventional or targeted agents and that eradication of LSCs might be necessary to cure the disease. In order to examine mechanisms of drug resistance in LSCs and to seek strategies to overcome the resistance, we used Ph-positive acute lymphoblastic leukemia patient cells 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. Recently several publications have suggested that proteasome inhibitors can induce selective cell death in LSCs. Spleen cells derived from leukemic NOG mice were treated ex vivo with imatinib and the proteasome inhibitor, bortezomib and cell viablility (PI-/Annexin-V-) was compared between treated and non-treated cells. After treatment with imatinib, significantly more residual cells were observed in the CD34+CD38- population compared to the CD34+CD38+ or CD34-CD38+ populations. With nM level of bortezomib, substantial cell death was induced in all populations with up-regulation of phospho-p53 (Ser15). Phosphorylation of BCR-ABL and CrkL was completely inhibited in all populations with imatinib treatment, but not with bortezomib treatment. Regarding cell cycle states, a higher percentage of Hoechst-33342^low/Pyronin-Y^low cells was observed in the CD34+CD38- population relative to the other populations, suggesting more cells in the G₀ state among the CD34+CD38- population. In co-culturing with S17 stromal cells, quiescent (Hoechst-33342^low/Pyronin-Y^low) CD34⁺ cells were insensitive to imatinib, while substantial cell death including CD34⁺ population was induced with nM level of bortezomib. We are also investigating more detailed biomarkers in the cell death and effects of these drugs both on the primitive leukemia cells and normal hematopoietic cells using the in vivo leukemic NOG mice systems. These results imply that resistance to imatinib in Ph-positive leukemia quiescent cells is independent of BCR-ABL phosphorylation and that treatment with bortezomib can overcome the resistance of Ph-positive LSCs.