Potent antileukemia activity of the combination of the heat shock protein 90 inhibitor NVP-AUY922 and the histone deacetylase inhibitor LBH589 (panobinostat) against human AML and CML cells

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NVP-AUY922 (Novartis Pharmaceuticals) is a novel 4,5-diarylsoxazole ATP-binding site heat shock protein 90 (hsp90) inhibitor, which has been shown to inhibit the chaperone function of hsp90 and deplete the levels of hsp90 client proteins (Brough PA et al., J Med Chem. 2007). Treatment with AUY922 has been shown to exert potent in vitro anti-tumor activity, as well as in vivo tumor retention and growth inhibitory effects. Present studies demonstrate that AUY922 dose-dependently induced accumulation of human acute myeloid leukemia MV4-11 and Bcr-Abl-expressing K562 cells in G1 and G2/M phases of the cell cycle, with concomitant decline in the percentage of cells in the S phase of the cell cycle. In U937 and MV4-11 cells AUY922 dose-dependently (20 to 50 nM) induced apoptosis (40-60% of cells). This was associated with depletion of FLT-3, p-STAT5, AKT, and CDK4 levels, with concomitant induction of hsp70 levels in MV4-11 cells. In K562 cells, treatment with 50-100 nM of AUY922 depleted Bcr-Abl, p-STAT, p-CrkL and AKT levels and induced apoptosis (30-50% of cells). Our previous studies had shown that, by inhibiting histone deacetylase (HDAC) 6, the pan-HDAC inhibitor panobinostat (LBH589, Novartis Pharmaceuticals) induces acetylation and inhibition of hsp90 chaperone function, resulting in depletion of unmutated or mutant forms of Bcr-Abl, as well as of c-Raf and AKT in CML cells. In the present studies we demonstrate that co-treatment with AUY922 (10 or 20 nM) and panobinostat (5 nM) caused more depletion of FLT-3, p-STAT5 and AKT and synergistically induced apoptosis of MV4-11 cells. Similarly, co-treatment with AUY922 (20 to 100 nM) and panobinostat (50 nM) caused more depletion of Bcr-Abl, p-STAT5, p-CrkL and AKT and synergistically induced apoptosis of K562 cells (by isobologram analyses). Importantly, co-treatment with AUY922 and panobinostat also caused more depletion of the ectopically expressed unmutated Bcr-Abl, mutant Bcr-AblE255K and Bcr-AblT315K, p-STAT5, p-CrkL, p-AKT, c-Raf, as well as synergistically induced apoptosis of BaF3 cells with the unmutated or the mutant forms of Bcr-Abl. Finally co-treatment with AUY922 and panobinostat caused more loss of cell viability than either agent alone in four primary AML samples and five imatinib-refractory, primary CML samples. These in vitro studies show that the combination of AUY922 and panobinostat exert synergistic antileukemia activity against human AML and CML cells. The findings also create a strong rationale to test the in vivo efficacy of the combination of AUY922 and panobinostat.