The multi-kinase inhibtor Sorafenib interacts synergistically with the HDAC inhibitor Vorinostat to induce apoptosis in CML cells in association with downregulation of MCL-1 and p21^CIP1

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Interactions between the multi-kinase inhibitor Sorafenib (Bay 43-9006; Bayer Pharmaceuticals) and the histone deacetylase inhibitor (HDACI) Vorinostat (Merck Pharmaceuticals) were examined in CML cells. Pretreatment of K562 or LAMA84 for 24 hr with a subtoxic concentration of Vorinostat (1.5-2.0 μM) followed by a marginally toxic concentration of Sorafenib (7.5 μM; 24 hr) resulted in a highly synergistic induction of apoptosis, and loss of clonogenic survival. Simultaneous exposure also resulted in greater than additive antileukemic effects, but these were slightly less pronounced as those observed with sequential treatment. This regimen potently induced apoptosis in imatinib mesylate-resistant cells including those expressing the T315I Bcr/Abl mutation or amplified Lyn, as well as in primary CD34+ CML patient-derived samples. In contrast, synergism was not observed in normal bone marrow mononuclear cells. Sequential but not individual treatment with Sorafenib/Vorinostat resulted in pronounced mitochondrial dysfunction (i.e. cytochrome c, Smac and AIF release), caspase 8 activation, PARP cleavage, downregulation of Mcl-1, and attenuation of p21^CIP1 induction. Enforced expression of Mcl-1 substantially circumvented Sorafenib/Vorinostat lethality while ecotopic expression of p21^CIP1 partially but significantly protected cells from this regimen. Downregulation of Mcl-1 was entirely caspase-independent whereas attenuation of p21^CIP1 induction was partially caspase-dependent. Furthermore, RT-PCR analysis revealed that Mcl-1 downregulation was largely transcription-independent, whereas p21^CIP1 down-regulation displayed a transcriptional component. Together, these findings indicate that combined exposure to Vorinostat and Sorafenib potently induces mitochondrial injury and apoptosis in CML cells, including those resistant to imatinib mesylate, through a process that involves downregulation of Mcl-1 and p21^CIP1. They also suggest that a strategy combining Vorinostat and Sorafenib warrants further attention in CML, particularly in disease resistant to imatinib mesylate therapy.