An excellent response with a complete remission, maintained for more than 4 years since the patient’s initial diagnosis and beginning of the treatment was observed. Based on these results, more than five different clinical trials have been initiated, using bortezomib alone or in combination with other drugs for the treatment of CML and/or Ph+ALL. Thus, bortezomib is a promising treatment in Bcr-Abl-positive leukemias. An interesting study suggested that bortezomib in combination with the cyclin-dependent kinase inhibitor flavopiridol synergizes to induce apoptosis in CML cells. Flavopiridol causes an inhibition of the cell cycle in G1 or G2, based on the inhibition of CDK. Other studies have shown that INCB28060 side effects leukemic cells are particularly sensitive when survival pathway inhibitors are combined with mitotic inhibitors. Moreover, combination of bortezomib with mitotic inhibitors are currently in clinical trials for the treatment of non-small-cell lung carcinoma and other solid tumors. Thus, we hypothesized that a strategy based on the combined treatment with bortezomib and mitotic inhibitors for the treatment of Bcr-Abl-positive leukemias may be promising. Especially important might be to determine the effectiveness of this strategy in TKIs-resistant Bcr-Abl-positive cases. Paclitaxel, a mitotic inhibitor drug acting by stabilization of microtubules, is FDA approved for the treatment of lung, ovarian, breast cancers and advanced forms of Kaposi’s sarcoma. Paclitaxel is now in clinical trials for the treatment of CML. However, to our knowledge, there are no clinical trials or published studies employing the combined bortezomib and paclitaxel regimen for the treatment of Bcr-Ablpositive CML. Such a combination, if synergistic in inducing apoptosis in Bcr-Abl-positive cells, would significantly decrease the dose of each compound necessary to achieve a therapeutic effect. Here we demonstrate that bortezomib, in combination with the mitotic inhibitor paclitaxel, efficiently kill TKIs-resistant and sensitive Bcr-Abl-positive leukemic cells. In addition, bortezomib in combination with either paclitaxel or BI 2536, another mitotic inhibitor that inhibits PLK1, induces a marked downregulation of total and phosphorylated Bcr-Abl protein levels, thus downregulating the critical Bcr-Abl downstream signaling pathways and activating caspases. Similarly, bortezomib, in combination with other mitotic inhibitors, is able to decrease Bcr-Abl activity and increase caspase activation. Taken together, our findings unravel a novel promising treatment for TKIs-resistant and sensitive CML cases, as well as other Bcr-Abl positive leukemias. The effects of 9nM bortezomib, 6nM paclitaxel or both drugs in combination were evaluated in both parental K562 and TKIs-resistant K562-R cells after 48h of treatment. Similar to K562, K562-R cells are synergistically killed by bortezomib/paclitaxel regimen, as shown by a CI lower than 1. Notably, combined treatment with bortezomib and paclitaxel strongly decreases phosphorylation of Bcr-Abl, in both K562 and K562-R cell lines. The effects of 4nM bortezomib, 5nM paclitaxel or both drugs in combination for 48h were also evaluated in both parental LAMA84 and TKIs-resistant LAMA84-R cells. Interestingly, the LAMA84-R cells show a significant increase in total levels and phosphorylation of Bcr-Abl oncoprotein when compared with parental LAMA84 cells. This suggests that these cells adapted to resist imatinib, dasatinib and nilotinib treatments through the upregulation of Bcr-Abl levels and activity. Combined treatment with bortezomib and paclitaxel was able to downregulate total levels and phosphorylation of Bcr-Abl in LAMA84-R cell lines. The T315I point mutation in Bcr-Abl is known to confer resistance to imatinib, dasatinib, nilotinib, and bosutinib. It is XL880 849217-64-7 therefore important to test whether the combined treatment of bortezomib and paclitaxel is also active on cells expressing Bcr-Abl with the T315I mutation.