Imatinib, dasatinib or nilotinib resistance may emerge through point mutations in Bcr-Abl, Bcr-Abl gene amplification and/or an increase in Bcr-Abl protein levels. To investigate alternative treatments for these particular cases, we have indeed developed two different cell lines derived from K562 and LAMA84 cell lines, which are completely resistant to 1��M imatinib. While the levels of Bcr-Abl and P-Bcr-Abl in LAMA84-R are much AB1010 higher than in LAMA84-S cells, the levels of Bcr-Abl and P-Bcr-Abl in K562-R compared with K562-S are much closer to each other. Thus, the increased expression of Bcr-Abl is probably at least in part responsible for the LAMA-R resistance to imatinib, dasatinib and nilotinib, while possible mutations may be responsible for the K562-R resistance. Additionally, we have used the Baf3 Bcr-Abl T315I cell line, derived from Baf3, which is also resistant to 1��M imatinib and at least partially resistant to dasatinib and nilotinib treatments. In addition to its effect on imatinib-sensitive cell lines, the bortezomib/paclitaxel regimen was able to induce caspase cleavage, a measure of caspase activation, in K562-R cells and significant downregulation of the total levels and phosphorylation of Bcr-Abl in all tested TKIs-resistant cell lines. Thus, such combination may be a good strategy to treat resistant cases due to either an increase in Bcr-Abl expression or Bcr-Abl mutations that abrogate imatinib, dasatinib or nilotinib inhibitory effects. Notably, in addition to the bortezomib/paclitaxel regimen, our results demonstrate that bortezomib, in combination with other mitotic inhibitors that act by inducing mitotic arrest through various mechanisms, inhibits Bcr-Abl and results in caspase 3 activation. It has previously been established that inhibition of Bcr-Abl or knock-down of Bcr-Abl induces caspase activation and apoptosis. Thus, our results indicate that Bcr-Abl down-modulation contributes, at least in part, to caspase activation and induction of cell death. Both docetaxel and vincristine are FDA-approved for the treatment of several malignancies, alone or in combination. Interestingly, a recent study concluded that BI 2536 has growth inhibitory effects on Bcr-Abl-positive cells that are not amplified by bortezomib after 16h of co-treatment. In contrast, we are showing here that the combined treatment of bortezomib 9nM with BI 2536 8nM for 60h is significantly more effective in inducing caspase activation, PARP cleavage and cell death compared with single treatments, in both K562 and K562-R cells. The longer time needed for bortezomib to amplify the effects of BI 2536 might be explained by the involvement of transcriptional mechanisms in bortezomib/BI 2536-induced cell death, although further experiments are needed to clarify this aspect. Recently, two other drugs were approved by FDA for the treatment of patients with CML whose tumors are resistant to or who cannot tolerate Imatinib, Dasatinib or Nilotinib therapies: bosulif and synribo. Bosutinib, approved on September 4, 2012, is a TKI inhibitor efficient against many Bcr-Abl mutations, except T315I. Omacetaxine mepesuccinate, approved on October 26, 2012, is a non-TKI drug intended to be used when leukemia progresses after therapy with at least two TKIs. While the drug can be used for the treatment of CML patients 
with T315I mutation, it shows significant hematologic toxicity in clinical trials: thrombocytopenia, neutropenia, and anemia. While these two new approved drugs offer an option for many patients with imatinib, dasatinib and nilotinibresistant CML, novel better strategies have to be developed. In contrast with bosutinib, our combined treatment with bortezomib and mitotic inhibitors is able to target Bcr-Abl with T315I mutation. Moreover, lower concentrations of each drug can be used in synergistic combinations, which may reduce toxicity. However, the toxicity of our MLN4924 regimens remains to be established.