Combined Effects of As4S4 and Imatinib on Chronic Myeloid Leukemia Cells and BCR-ABL Oncoprotein.

Imatinib is a tailored drug for chronic myeloid leukemia (CML), which has very good effects on patients at chronic phase (CP), but not on those at accelerated phase or blast phase. In addition, even among patients at CP, Imatinib seems unable to eradicate the malignant progenitors and a significant portion of patients develops drug resistance after long time use. Arsenic compounds were known as ancient remedies for CML with certain efficacy. The aim of this study was to investigate the potential benefit of combination therapy with Imatinib and arsenic sulfide (As₄S₄) on BCR-ABL+ K562 cells and fresh CD34⁺ hematopoietic progenitor cells isolated from CML patients and non-leukemic donors. Analysis of cell proliferation and clonogenic ability showed that As₄S₄ and Imatinib exerted synergistic effects on both K562 cells and fresh CML cells. The effective concentrations on fresh CML cells were pharmacokinetically available in vivo but had much less inhibitory effect on CD34⁺ cells from the non-leukemia donors. The synergistic effect of Imatinib/As4S4 combination in terms of anti-proliferation might be connected with their distinct but complementary roles in interfering with the cell cycle progression. Our data showed that Imatinib induced G1 arrest of K562 cells, while As4S4 induced G2/M arrest. In addition, Imatinib induces significant down-regulation of phosphorylated Rb and CDK1, which is in agreement with the G1/S but not G2/M arrest under this drug. However, As4S4 shows no obvious effect on these proteins in spite of a visible effect on G2/M block. Using a number of parameters such as morphology, Annexin V/PI, mitochondrial transmembrane potential, caspase3 activity and Fas/Fas-L, the synergistic effects were revealed on induction of cell apoptosis, largely through mitochondrial pathway. What’s more, the two drugs also exhibited synergistic effect in targeting BCR-ABL protein. While As₄S₄ triggered its degradation and Imatinib inhibited its tyrosine kinase activity, combined use of the two led to lower protein/enzymatic activity levels of BCR-ABL. In conclusion, our study suggests As₄S₄ and Imatinib have synergistic effects in inhibiting proliferation, inducing apoptosis of cells and reduction the tyrosine kinase activity of BCR-ABL. Our in vitro data thus strongly suggest a potential clinical application of Imatinib/As₄S₄ combination on CML.