Abstract 2854: Targeting constitutive NF-kB activation through Bruton's tyrosine kinase (Btk) and the proteasome in mantle cell lymphoma
Authors: Zhishuo Ou, Archito T. Tamayo, Liang Zhang, John Lee, Kejie Zhang, Changping Li, Jacqueline Wang, James Pham, Luhong Sun, Christopher J. Kirk, Richard J. Ford, Lan V. Pham
Journal: Cancer Research (2012)

Abstract Mantle cell lymphoma (MCL) is an aggressive B-cell non-Hodgkin lymphoma (NHL) with poor prognoses; novel agents are needed for its therapy. Bruton's tyrosine kinase (Btk) has been identified as an essential kinase for B-cell survival and it is activated through the B-cell receptor (BCR) pathway. Btk has recently emerged as a promising target in MCL, as demonstrated by recent clinical trials on the Btk inhibitor PCI-32765 (Pharmacyclics, Sunnyvale, CA), suggesting that elucidating critical signaling pathways emanating through Btk will hold an important key to deciphering the pathogenesis of MCL that can lead to the development of more effective targeted therapies. In this study, we showed that Btk is constitutively phosphorylated in most MCL cell lines except Rec1 and DB SP53 and is variable among primary cells from patients. We demonstrated that knockdown of Btk by siRNA diminished Btk expression, reduced constitutive NF-kB activation by luciferase assays, leading to the cell growth inhibition and induction of apoptosis in MCL cell lines. MCL cells treated with the Btk inhibitor PCI-32765 effectively inhibits Btk activity, leading to reduced MCL cell growth with IC50 values range from 2-6 uM in Mino, Jeko1, Z138 and JMP1. Interestingly, the IC50 value in DB SP53, which lacks both phosphorylated Btk and membrane immunoglobulins, is 35 uM, suggesting that BCR signaling is not active in these cell lines. PCI-32765 also induced caspase-dependent apoptosis in a dose- and time-dependent manner in representative MCL cell lines and in patient primary cells. We further demonstrated that PCI-32765 down-regulates NF-kB activity through both, the canonical and alternative NF-kB pathways. Since previous studies have indicated synergy between proteasome inhibitors and tyrosine kinase inhibitors, we evaluated whether there is synergism between PCI-32765 and the next generation proteasome inhibitor Carfilzomib (Onyx Pharmaceuticals, South San Francisco, CA). Our data suggest potential synergy between Carfilzomib and PCI-32765 in represented MCL cell lines in terms of inhibiting cell growth and induction of apoptosis. Both compounds also synergize to inhibit NF-kB activation in MCL cells. In summary, our data suggest that Btk is a key survival kinase in MCL and strategic targeting of growth/survival Btk-mediated NF-kB pathways with novel therapeutic agents such as PCI-32765 should provide a novel therapy regimen for MCL patients. Combining PCI-32765 with the proteasome inhibitor Carfilzomib can synergize its effect in MCL and may be a useful therapeutic strategy, particularly for patients with relapsed/refractory MCL. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2854. doi:1538-7445.AM2012-2854