Abstract 5472: Selinexor, a selective inhibitor of nuclear export (SINE), acts through NF-κB deactivation and combines with proteasome inhibitors to synergistically induce tumor cell death

Background: The primary nuclear export protein, Exportin 1 (XPO1/CRM1) is overexpressed in most cancers, which frequently correlates with poor prognosis. SINE compounds are a family of small-molecule bioavailable drugs that bind covalently to XPO1 leading to nuclear retention of major tumor suppressor proteins such as p53 and IκB, resulting in selective tumor cell death. Selinexor is the most advanced SINE with >500 hematological and solid tumor cancer patients treated to date in Phase I/II clinical trials. Here we investigated the role of NF-κB transcriptional deactivation in SINE induced anti tumor activity by nuclear retention of IκB. We report the results of combining selinexor or related SINE compounds with proteasome inhibitors, for which inhibition of NF-κB has also been shown to be an important mechanism for cancer cell killing. Methods: Whole protein cell lysates from solid and hematological cancer cell lines treated with selinexor with or without proteasome inhibitors were analyzed by immunoblots. IκB localization was evaluated by immunofluorescence. Cytotoxic effects of SINE compounds were evaluated in the presence or absence of proteasome inhibitors. NF-κB transcriptional activity was analyzed using an ELISA assay. H929 multiple myeloma xenografts in NOD-SCID mice were treated with selinexor alone or in combination with carfilzomib to determine effects on tumor growth. Results: NF-κB transcriptional activity was upregulated in SINE resistant sarcoma cell lines HT1080R (IC50 = 2.4 μM versus HT1080 parental IC50 = 0.03 μM) and ASPS-KY (IC50>10μM). In those resistant lines, combination of the proteasome inhibitors bortezomib or carfilzomib with SINE compound showed dramatic synergistic cytotoxic potency of more than 10 folds. Also, the combination of bortezomib with selinexor led to synergistic nuclear retention of IκB. Furthermore, IκB silencing by siRNA led to reduced selinexor cytotoxic potency, increasing IC50 from 58 nM to 830 nM (9-fold) in IM-9 multiple myeloma cells and from 27 nM to 1.9 μM (66-fold) in U2OS osteosarcoma cells. Finally, the combination of selinexor with carfilzomib was synergistic in reducing tumor growth in the H929 multiple myeloma xenograft model. Conclusion: Inhibition of NF-κB transcriptional activity through forced nuclear retention of IκB appears to be an important mechanism in the selective tumor cell cytotoxicity of selinexor and related SINE compounds. Furthermore, the combination of selinexor and proteasome inhibitors, which are also known to act at least in part through inhibition of NF-κB, leads to synergistic activity in vitro and in vivo, suggesting that such combinations may provide clinically more effective than the single agents. A Phase 1 trial to study the safety and efficacy of selinexor in combination with carfilzomib in multiple myeloma is ongoing (NCT02199665). Citation Format: Yosef Landesman, Trinayan Kashyap, Marsha Crochiere, Boris Klebanov, Sharon Friedlander, William Senapedis, Robert Carlson, Michael Kauffman, Sharon Shacham. Selinexor, a selective inhibitor of nuclear export (SINE), acts through NF-κB deactivation and combines with proteasome inhibitors to synergistically induce tumor cell death. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5472. doi:10.1158/1538-7445.AM2015-5472