The HDAC Inhibitor LBH589 Enhances the Antimyeloma Effects of the IGF-1RTK Inhibitor Picropodophyllin

We have previously shown the use of the insulin-like growth factor type 1 receptor tyrosine kinase (IGF-1RTK) inhibitor picropodophyllin (PPP) as an attractive strategy to combat multiple myeloma (MM) in vitro and in vivo. After a combinatorial drug screening, the histone deacetylase inhibitor LBH589 was shown to act in synergy with PPP reducing survival of MM cells. In this study, we tried to elucidate the molecular mechanisms underlying this combinatorial effect. The in vitro anti-MM effects of PPP and LBH589 alone and in combination were evaluated by studying apoptosis, cell cycle distribution, and downstream transcriptome using both human MM cell lines and cells from the murine 5T3MM model. In vivo the effect on survival of 5T33MM-inoculated mice was evaluated. In the human MM cell line RPMI8226, treatment with PPP and LBH589 in combination resulted in a five-fold increase of apoptosis, and an additive effect on the cleavage of the active forms of caspase-8 was observed as compared with the single drug treatments. Cell cycle analysis revealed an accumulation of cells in the G(2)-M phase and subsequent downregulation of cell cycle regulating proteins. These data were also confirmed in the 5T33MM cells in vitro. Also, the transcriptome was analyzed by Affymetrix arrays showing gene expression alterations mainly in categories of genes regulating apoptosis and cell adhesion. Combined treatment in vivo resulted in a significantly prolonged survival of 5T33MM-inoculated mice. The results indicate an improved MM treatment opportunity in using a combination of PPP and LBH589.