Simultaneous Targeting of SRC and BCR-ABL Kinases by BMS-354825 Cures Ph+ Acute Lymphoblastic Leukemia in Mice.

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The ABL kinase inhibitor imatinib mesylate is the preferred treatment for Philadelphia chromosome-positive (Ph+) chronic myeloid leukemia (CML) in chronic phase, but is less effective in CML blast crisis or Ph+ B-cell acute lymphoblastic leukemia (B-ALL). We have shown that SRC kinases are required for BCR-ABL-induced B-ALL but not CML (Hu et al., Nat. Genet. , 36:453, 2004), suggesting that SRC kinases may also serve as therapeutic targets in addition to BCR-ABL for B-ALL. To test this idea, we compared the activities of Abl-selective imatinib with BMS-354825, a potent dual inhibitor of SRC and ABL kinases, using the BCR-ABL mutant P210T315I resistant to inhibition by both drugs (Shah et al, Science , 16:399, 2004). We treated P210T315I-expressing pre-B cells with BMS-354825 in vitro. While BMS-354825 did not reduce phosphorylation of P210T315I, it inhibited phosphorylation of SRC kinases at concentrations as low as 25 nM, confirming that BMS-354825 is a potent SRC inhibitor in cells. Moreover, BMS-354825 inhibited proliferation and induced apoptosis of the cells, suggesting that inhibition of SRC kinases alone may suffice in inhibiting the growth of BCR-ABL inhibitor-insensitive leukemic cells. To test whether SRC inhibition was sufficient to exert a therapeutic effect on P210T315I-induced B-ALL, donor bone marrow cells from BALB/c mice were transduced with P210T315I-IRES-GFP retrovirus, followed by transplantation into BALB/c recipients. 8 days after the transplantation, BMS-354825 was given orally twice a day at a dose of 30 mg/kg for 6 days, followed by switching to 10 mg/kg for 3 days and then back to 30mg/kg. Imatinib was given orally twice a day at a dose of 100mg/kg. At day 17 post BMT, the mean peripheral blood (PB) GFP+/B220+ cell count (/ml) was: 1.8x107 for placebo; 0.7x107 for BMS-354825; and 3.0x107 for imatinib. So far (30 days post BMT), 50% (3/6) of placebo-treated and 69% (9/13) of imatinib-treated mice died, whereas all BMS-354825-treated mice are alive. These results suggested that SRC kinases could serve as therapeutic targets for B-ALL although targeting SRC alone may not cure the disease. Similarly, targeting BCR-ABL alone with imatinib could not cure B-ALL. To test dual-target therapy of B-ALL with BMS-354825, we treated mice with B-ALL induced by wild type BCR-ABL. Compared to placebo- and imatinib-treated mice at day 17 post BMT, GFP+/B220+ cells in BMS-354825-treated mice were no longer detectable while PB GFP+/B220+ cells for placebo- and imatinib-treated groups were 3.6x107and 2.4x107/ml, respectively. Although this therapeutic experiment is still ongoing, complete eradication of leukemic cells in BMS-354825-treated mice demonstrates that blockade of both SRC kinases and BCR-ABL cured B-ALL. To understand how SRC kinases function in B-leukemic cells, we looked for SRC downstream signaling molecules using mice deficient for SRC kinases LYN, HCK or FGR. The activation of AKT and STAT5 but not ERK and JNK was down-regulated in leukemic cells lacking LYN, HCK and FGR. The lack of HCK or FGR but not LYN was responsible for their down-regulation. Together, these results indicate that SRC kinases are valuable therapeutic targets for Ph+ B-ALL, and that simultaneous inhibition of SRC and BCR-ABL kinases and their downstream pathways may be beneficial to Ph+ B-ALL patients.