In vitro and in vivo rationale for the triple combination of panobinostat (LBH589) and dexamethasone with either bortezomib or lenalidomide in multiple myeloma

Background Combinations of drug treatments based on bortezomib or lenalidomide plus steroids have resulted in very high response rates in multiple myeloma. However, most patients still relapse, indicating the need for novel combination partners to increase duration of response or to treat relapsed disease. We explored the antimyeloma activity of triple combinations of these well-established schemes with panobinostat, a novel deacetylase inhibitor with a multi-targeted profile. Design and Methods The activity of these combinations was explored in vitro in cell lines by using MTT and annexin V, ex vivo by flow cytometry, and in vivo using two different murine models of human myeloma: one bearing a subcutaneous plasmacytoma and another with a disseminated myeloma. Moreover, gene expression profiling and immunohistochemical studies were performed. Results The addition of panobinostat (LBH589) to dexamethasone and either bortezomib or lenalidomide resulted in clear potentiation in multiple myeloma cell lines, freshly isolated plasma cells, and murine models of multiple myeloma. The quantification of the potency of these combinations by using the Chou-Talalay method showed synergistic combination indices for all of them. This effect derived from the deregulation of a cluster of genes that was completely different from the sum of genes affected by the single agents (895 and 1323 genes exclusively deregulated by panobinostat and dexamethasone plus bortezomib or lenalidomide, respectively). Functional experiments, such as annexin V staining, cell cycle analysis, and immunohistochemical studies also supported this potentiation. Anti-myeloma efficacy was confirmed in an extramedullary plasmacytoma model and a disseminated luciferized model, in which panobinostat also provided a marked benefit in bone disease. Conclusions The potent activity, together with the exclusive mechanistic profile, provides the rationale for the clinical evaluation of these drug combinations in multiple myeloma.