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) can be achieved and also constitutes a favorable therapeutic window in multiple myeloma (MM). As no complete remission using in vivo models of MM could be obtained, a combinatorial drug screen (HTS) was performed to select the most performant combination with PPP. The HDAC inhibitor LBH589 was shown to act in synergy with PPP on survival of MM cells. The contribution from both drugs and the combination were further monitored for apoptosis, cell cycle distribution, and the impact on downstream gene and protein expression in human and mouse MM models in vitro. In the RPMI 8226 human MM cell line, simultaneous treatment with both compounds for 48h caused a 5-fold increase of apoptotic and late apoptotic/necrotic cells as compared to controls, while treatment with either compound alone only induced a 3-fold increase. After 24h cleavage of apoptotic proteins caspase -9, -8 and -3 could be found in RPMI 8226 cells treated with both drugs individually, but in the combination we observed an additive effect on the cleavage of the active forms of caspase 8 as compared to single drug treatments. The combination of LBH589 and PPP could be monitored as an accumulation of cells in the G2/M phase, and subsequent down-regulation of cell cycle regulated proteins. The effect of both compounds on the expression of cyclin B1, -E and -D2 was additive, as demonstrated by western blot. These data were also confirmed in the mouse 5T33MM cells in vitro. Gene expression analysis and validations of the RPMI 8226 cells reveal that the drug combination has better effects than the single drug alone. Combined treatment in vivo resulted in a significant prolonged survival of 5T33MM inoculated mice when compared to the control group and to treatment with the drugs alone. In conclusion, the results indicate an improved MM treatment opportunity in using a combination of PPP and LBH589.

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