Abstract
The BCR-ABL inhibitor imatinib mesylate is the current approved treatment for Philadelphia chromosome-positive (Ph+) chronic myeloid leukemia (CML). While this agent is effective in the chronic phase of CML, it is less effective in advanced disease (acelerated phase or blast crisis), and resistance to imatinib is an issue at all stages of disease, particularly advanced. Resistance is mediated primarily by BCR-ABL mutations, although other mechanisms have also been implicated. Another key issue with imatinib therapy is that molecular remission in imatinib-treated CML patients is difficult to achieve, leaving patients at risk of relapse. We have previously observed that imatinib significantly prolongs survival of CML mice, but is not curative (Hu et al, Nature Genetics36[5]:453–461, 2004). We hypothesize that this can be attributed to the inability of imatinib to completely kill CML stem cells. We identified that BCR-ABL-expressing Lin-c-KIT+Sca-1+ bone marrow cells are CML stem cells in mice. We tested whether BMS-214662 (which has been shown to have an inhibitory effect on growth of non-proliferating cancer cells) (Lee et al, Proceedings of the AACR42:260s, 2001) reduces leukemia stem cell populations in CML mice. Donor bone marrow cells from C57BL/6 mice were transduced with P210BCR-ABL-IRES-GFP retrovirus, followed by transplantation into lethally irradiated C57BL/6 recipient mice. Eight days after transplantation, BMS-214662 was given orally once a day at a dose of 300 mg/kg for 7 days. Bone marrow cells from the treated CML mice were then analyzed by FACS for CML stem cells (GFP+Lin-c-Kit+Sca-1+). CML mice treated with placebo, dasatinib (a novel, oral, multi-targeted kinase inhibitor that targets BCR-ABL and SRC family kinases) 10 mg/kg, twice daily (BID), BMS-214662, or dasatinib 10 mg/kg BID in combination with BMS-214662. Numbers of leukemia stem cells per bone were significantly lower in mice treated with BMS-214662 alone, dasatinib alone, or both BMS-214662 and dasatinib, compared with placebo-treated mice. Among different treatments, the combination of BMS-214662 and dasatinib had the strongest inhibitory effect on CML stem cells. Inhibition of the leukemia stem cells by dasatinib could be due to its inhibitory effect on BCR-ABL or SRC kinases, whereas BMS-214662 must function through other mechanisms. BMS-214662 is also a farnesyl transferase inhibitor (FTI), which reduces Ras activation. However, our control experiment showed that other FTIs did not inhibit proliferation of non-proliferating cancer cells (data not shown). This suggests that BMS-214662 inhibits CML stem cells through unknown mechanisms. In summary, BMS-214662 is a potent inhibitor of CML stem cells, and combinatorial use of BMS-214662 and dasatinib may provide more durable responses, and potentially a curative therapy for CML patients. Given the proven activity of dasatinib against a spectrum of imatinib-resistant BCR-ABL mutations (O’Hare, et al. Cancer Res65[11]:4500–5, 2005; Shah et al, Science, 305:399, 2004), and the apparent activity of dasatinib against stem cells in vivo shown here, this combination could potentially suppress the emergence of resistance, further adding to the durability of response.
Human chronic myeloid leukemia stem cells are insensitive to imatinib despite inhibition of BCR-ABL activity
