PLK1 inhibitors synergistically potentiate HDAC inhibitor lethality in imatinib mesylate-sensitive or -resistant BCR/ABL+ leukemia cells in vitro and in vivo

These findings suggest that concomitant PLK1 and HDAC inhibition is active against imatinib mesylate-sensitive or refractory CML and ALL cells both in vitro and in vivo and that this strategy warrants further evaluation in the setting of BCR/ABL(+) leukemias.

To determine whether Polo-like kinase 1 (PLK1) inhibitors (e.g., BI2536) and histone deacetylase (HDAC) inhibitors (e.g., vorinostat) interact synergistically in the BCR/ABL(+) leukemia cells sensitive or resistant to imatinib mesylate (IM) in vitro and in vivo. Experimental design: K562 and LAMA84 cells sensitive or resistant to imatinib mesylate and primary CML cells were exposed to BI2536 and vorinostat. Effects on cell viability and signaling pathways were determined using flow cytometry, Western blotting, and gene transfection. K562 and BV173/E255K animal models were used to test in vivo efficacy.

Cotreatment with BI2536 and vorinostat synergistically induced cell death in parental or imatinib mesylate-resistant BCR/ABL(+) cells and primary CD34(+) bone marrow cells but was minimally toxic to normal cells. BI2536/vorinostat cotreatment triggered pronounced mitochondrial dysfunction, inhibition of p-BCR/ABL, caspase activation, PARP cleavage, reactive oxygen species (ROS) generation, and DNA damage (manifest by increased expression of γH2A.X, p-ATM, p-ATR), events attenuated by the antioxidant TBAP. PLK1 short hairpin RNA (shRNA) knockdown significantly increased HDACI lethality, whereas HDAC1-3 shRNA knockdown reciprocally increased BI2536-induced apoptosis. Genetic interruption of the DNA damage linker H1.2 partially but significantly reduced PLK1/HDAC inhibitor-mediated cell death, suggesting a functional role for DNA damage in lethality. Finally, BI2536/vorinostat cotreatment dramatically reduced tumor growth in both subcutaneous and systemic BCR/ABL(+) leukemia xenograft models and significantly enhanced animal survival. Conclusions: These findings suggest that concomitant PLK1 and HDAC inhibition is active against imatinib mesylate-sensitive or refractory CML and ALL cells both in vitro and in vivo and that this strategy warrants further evaluation in the setting of BCR/ABL(+) leukemias.