Abstract 1691: Synergistic anti-tumor activity of lenalidomide with the BET bromodomain inhibitor CPI203 in bortezomib-resistant mantle cell lymphoma
Authors: Moros Alexandra , Rodriguez Vanina , Saborit-Villarroya Ifigenia , Montraveta Arnau , Balsas Patricia , Sandy Peter , Martinez Antonio , Normant Emmanuel , Perez-Galan Patricia , Campo Elias , Colomer Dolors , Roue Gael 
Published: May 2015

Abstract
Mantle cell lymphoma (MCL) is an aggressive B lymphoid neoplasm genetically characterized by the t(11;14)(q13;q32) leading to the overexpression of cyclin D1. As a consequence of its poor responses to conventional chemotherapy and relatively short patient survival, new therapeutic strategies are required. Despite the promising introduction of the proteasome inhibitor bortezomib in the treatment of MCL, not all the patients respond and relapse frequently occurs. To unravel the factors involved in the acquisition of bortezomib resistance in vivo, immunodeficient mice were engrafted with a set of MCL cell lines with different levels of sensitivity to the drug, followed by gene expression profiling of the tumors and functional validation of the identified gene signatures. We observed an increased tumorigenicity of bortezomib-resistant MCL cells in vivo, that was associated with plasmacytic differentiation features, like IRF4 and Blimp-1 upregulation. As the immunomoduladory drug lenalidomide has been shown to modulate IRF4 expression in various B-cell malignancies, we assessed its activity in in vitro and in vivo settings by means of flow cytometry, western blot, antibody array, real-time PCR, immunofluorescence, in vivo imaging, and immunohistochemistry. In vitro, lenalidomide as single agent was found to exert antitumor activity in 4/11 MCL cell lines, corresponding to those cells with either primary or acquired resistance to bortezomib. Lenalidomide-treated cells showed decreased IRF4 expression, increased cytosolic amounts of p27 and caspase-dependent apoptosis. Accordingly, mice bearing bortezomib-resistant tumors and treated for 3 weeks with a lenalidomide regimen of 10-50 mg/kg/day, showed a 30 to 45% reduction in tumor burden when compared to vehicle-treated mice (p=0.04), with several hallmarks of lenalidomide activity, like downregulation of IRF4 and its target gene MYC, decreased mitotic index, p27 cytosolic accumulation and caspase-3 processing. Importantly, the inhibition of tumor growth induced by the combination of lenalidomide with bortezomib (0.15 mg/kg, twice a week) was 37% and 66% greater than that for lenalidomide alone and vehicle arms, respectively (p=0.02). Moreover, repression of MYC in bortezomib-resistant cells by gene knockdown or treatment with CPI203, a BET bromodomain inhibitor, synergistically induced cell death when combined with lenalidomide therapy. Accordingly, co-treatment of mice with lenalidomide plus CPI203 synergistically reduced MYC and IRF4 expression and tumor burden, and induced caspase processing. Together, these results suggest that exacerbated IRF4/MYC signaling is associated to bortezomib resistance in MCL in vivo and warrant clinical evaluation of lenalidomide plus BET inhibitor combination in MCL cases refractory to proteasome inhibition.
Citation Format: Alexandra Moros, Vanina Rodriguez, Ifigenia Saborit-Villarroya, Arnau Montraveta, Patricia Balsas, Peter Sandy, Antonio Martinez, Emmanuel Normant, Patricia Perez-Galan, Elias Campo, Dolors Colomer, Gael Roue. Synergistic anti-tumor activity of lenalidomide with the BET bromodomain inhibitor CPI203 in bortezomib-resistant mantle cell lymphoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1691. doi:10.1158/1538-7445.AM2014-1691