The HDAC inhibitor LBH589 enhances the anti-myeloma effect of the IGF-1 RTK inhibitor picropodophyllin (PPP)

Our previous studies have shown that inhibition of the IGF-1R pathway by the IGF-1RTK inhibitor picropodophyllin (PPP) is an effective strategy against multiple myeloma (MM) in vitro and in vivo. Here we performed a combinatorial drug screen (HTS) to select the most efficient combination with PPP. The HDAC inhibitor LBH589 was shown to act in synergy with PPP to reduce survival in MM cells. We analyzed the combinatorial effects on apoptosis, cell cycle distribution and the impact on downstream gene and protein expression in human and mouse MM models in vitro. In the human MM cell line RPMI 8226 treatment either drug alone induced a 3-fold increase of apoptotic and late apoptotic/necrotic cells, as compared to controls, while the combination caused a 5-fold increase. With both drugs we observed an additive effect on the cleavage of the active forms of caspase-8 compared to the single drugs. Also combination resulted in an accumulation of cells in the G2/M phase, and subsequent down-regulation of cell cycle regulated proteins cyclin B1, -E and -D2. These data were also confirmed in the mouse 5T33MM cells in vitro. Confirming the potential of this drug combination, gene expression arrays were performed showing regulated genes mainly in the categories of apoptosis and cell adhesion. Combined treatment in vivo resulted in a significant prolonged survival of 5T33MM inoculated mice when compared to the control and to single drug treatment. In conclusion, the results indicate an improved MM treatment opportunity in using a combination of PPP and LBH589.