PLK1 Inhibitors Synergistically Potentiate HDAC Inhibitor Lethality in Imatinib Mesylate–Sensitive or –Resistant BCR/ABL+ Leukemia Cells In Vitro and In Vivo

Abstract

Purpose To determine whether PLK1 inhibitors (e.g. BI2536) and HDAC inhibitor (e.g. vorinostat) interact synergistically in CML cells sensitive or resistant to imatinib mesylate (IM) in vitro and in vivo. Experimental Design K562 and LAMA84 cells sensitive or resistant to IM 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. Results Co-treatment with BI2536 and vorinostat synergistically induced cell death in parental or IM-resistant BCR/ABL+ cells and and primary CD34+ bone marrow cells but was minimally toxic to normal cells. BI2536/vorinostat co-treatment triggered pronounced mitochondrial dysfunction, inhibition of p-BCR/ABL, caspase activation, PARP cleavage, ROS generation, and DNA damage (manifest by increased expression of γH2A.X, p-ATM, p-ATR), events attenuated by the anti-oxidant TBAP. PLK1 shRNA knockdown significantly increased HDACI lethality, whereas or HDAC 1–3 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 co-treatment 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 IM-sensitive or refractory CML cells both in vitro and in vivo, and that this strategy warrants further evaluation in the setting of BCR/ABL+ leukemias.

BI 6727 and GSK461364 suppress growth and radiosensitize osteosarcoma cells, but show limited cytotoxic effects when combined with conventional treatments

“…Previous reports with BI 2536 have shown that inhibition of PLK1 in vitro also boosts the antineoplastic effects of other chemotherapeutics, such as temozolomide (TMZ) [30], MTX and CDDP [31], DXR and cyclophosphamide [32], docetaxel [33], and histone deacetylase inhibitors [34], among others. It is well recognized that treatment efficacy (irrespective of the tumor type) is often achieved by combining agents with diverse mechanisms of action and different spectra of toxicity.…”

Histone deacetylase inhibitor (HDACI) mechanisms of action: Emerging insights

“…S. Liu, Song, & Liu, 2010). Co-treatment of imatinib-sensitive and –resistant Bcr-Abl + cells with the PLK-1 inhibitor BI2536 and vorinostat synergistically induced cell death with minimal toxicity towards normal cells (Dasmahapatra, Patel, Nguyen, Attkisson, & Grant, 2013). The combination triggered pronounced mitochondrial dysfunction, inhibition of phospho-BCR/ABL, caspase activation, PARP (poly ADP ribose polymerase) cleavage, ROS generation and DNA damage, and was dramatically effective in a xenograft model of Bcr-Abl + leukemia (Dasmahapatra et al, 2013).…”

“…Co-treatment of imatinib-sensitive and –resistant Bcr-Abl + cells with the PLK-1 inhibitor BI2536 and vorinostat synergistically induced cell death with minimal toxicity towards normal cells (Dasmahapatra, Patel, Nguyen, Attkisson, & Grant, 2013). The combination triggered pronounced mitochondrial dysfunction, inhibition of phospho-BCR/ABL, caspase activation, PARP (poly ADP ribose polymerase) cleavage, ROS generation and DNA damage, and was dramatically effective in a xenograft model of Bcr-Abl + leukemia (Dasmahapatra et al, 2013). Although Bcr-Abl signals downstream to PLK-1 (Gleixner et al, 2010), enhanced Bcr-Abl pathway inhibition was not felt to be the sole mechanism for lethality of the BI2536/vorinostat regimen; rather, potentiation of DNA damage and disabling of the DDR appeared to predominate (Dasmahapatra et al, 2013).…”

“…The combination triggered pronounced mitochondrial dysfunction, inhibition of phospho-BCR/ABL, caspase activation, PARP (poly ADP ribose polymerase) cleavage, ROS generation and DNA damage, and was dramatically effective in a xenograft model of Bcr-Abl + leukemia (Dasmahapatra et al, 2013). Although Bcr-Abl signals downstream to PLK-1 (Gleixner et al, 2010), enhanced Bcr-Abl pathway inhibition was not felt to be the sole mechanism for lethality of the BI2536/vorinostat regimen; rather, potentiation of DNA damage and disabling of the DDR appeared to predominate (Dasmahapatra et al, 2013). Potentiation by vorinostat of BI2536-mediated inhibition of PLK-1 phosphorylation and enhanced ROS generation were noted, and abrogation of G0/G1 arrest with striking accumulation of cells in G2/M phase was observed (Dasmahapatra et al, 2013).…”

Alternative approaches to eradicating the malignant clone in chronic myeloid leukemia: tyrosine-kinase inhibitor combinations and beyond

“…The pan-HDACIs suberoylanilide hydroxamic acid 96 and LAQ824 97 down-regulated BCR-ABL1 levels and induced apoptosis in CML-mBC cells, whereas HDACIs potentiated the activity of ABL1 TKIs 98 and inhibitors of polo-like kinase 1 99 against resistant CML cells in vitro and in vivo. More relevant to targeting CML stem cells, a recent study showed that LBH589 (panobinostat) induced apoptosis in quiescent CML progenitors that are resistant to elimination by imatinib and reduced NSG engraftment by these cells.…”