Triptolide Induces Cell Death Independent of Cellular Responses to Imatinib in Blast Crisis CML Cells and Sensitizes Imatinib.

Imatinib, a Bcr-Abl tyrosine kinase inhibitor has revolutionized the treatment of patients with CML. However, resistance develops due to Bcr-Abl gene mutations and various other mechanisms. Although second generation Bcr-Abl inhibitors can overcome most of the mutation driven resistance, they cannot overcome other resistance mechanisms. Furthermore, Imatinib has limited effectiveness in patients with blast crisis (BC) CML. We have previously shown that triptolide, an anti-cancer agent isolated from a Chinese herb, potently induces apoptosis in AML cells in part by decreasing the levels of XIAP and Mcl-1, two potent antiapoptotic proteins. Here we investigated its effect on Philadelphia chromosome positive (Ph+) cells and found that at low nM concentrations, triptolide induced significant cell death in K562 (IC50=113.4±3.9 nM) and KBM5 (IC50=30.0±2.1 nM) cells, two cell lines derived from BC CML patients, as well as in ALL-1 cells (IC50=113.8±1.4 nM), a cell line derived from Ph+ ALL. Interestingly, KBM5-STI571 cells, an Imatinib resistant KBM5 subline bearing the T315I mutation which is resistant to most available Bcr-Abl tyrosine kinase inhibitors, were as sensitive as KBM5 cells to triptolide. Likewise, triptolide killed Ba/F3 cells harboring BCR-ABL mutants (E255K and T315I) with similar efficacy as Ba/F3 cells carrying the wild type BCR-ABL gene. We then treated 8 samples from 7 CML patients with blasts ranging from 10–91% with triptolide (up to 100 nM) in vitro. Triptolide induced cell death in all samples tested. Importantly, 6/7 samples were from patients resistant/relapsed after Imatinib. Three were also nonresponsive to Nilotinib and one to neither Nilotinib nor Dasatinib. Next we tried to elucidate the possible apoptosis regulators involved in triptolide-induced cell death. Triptolide decreased antiapoptotic XIAP, Mcl-1 and Bcr-Abl protein levels in K562 cells and in blast cells from CML patients. Based on this observation, we treated CML cells with both triptolide and Imatinib. The combination synergistically induced cell death in K562 cells (CI=0.50±0.14). In KBM5 cells, Imatinib antagonized rather than enhanced triptolide when administrated simultaneously: Triptolide alone induced cell death with IC50=24.3±2.8 nM at 48 hours, while in combination with 1 μM Imatinib, the IC50 increased to 82.9±4.1 nM. This is probably due to the fact that Imatinib primarily blocks KBM5 cells in G0/G1 and that resting cells were less sensitive to triptolide. We therefore pretreated KBM5 cells with triptolide for 24 hrs followed by 1 μM Imatinib for 24 hrs. This sequential treatment was more effective to induce cell death in KBM5 cells (IC50=15.4±0.6 nM). Triptolide did not sensitize Imatinib resistant KBM5-STI571 cells. Conclusion: Results suggest that triptolide potently induces cell death in BC CML cells and that the cell death induced by triptolide is independent of response to Imatinib or other second generation Bcr-Abl kinase inhibitors. Triptolide could be of potential benefit to CML patients in blast crisis and CML patients failing Bcr-Abl tyrosine kinase inhibitors.