ALBERT

Description: Introduction: Exportin 1 (XPO1) is a well characterized and essential nucleo-cytoplasmic transport protein in the karyopherin family, and is responsible for the nuclear export of over 200 cargo proteins, including the major tumor suppressor proteins (TSPs) p53, p21, FOXO and the translation regulator elF4E. XPO1 is overexpressed in numerous cancer types including solid and hematological malignancies, often correlating with poor prognosis. Recently, a novel class of Selective Inhibitors of Nuclear Export (SINE) compounds, selinexor (KPT-330) and the second generation KPT-8602, have been developed for the treatment of advanced cancers. We have previously shown that selinexor has marked activity in AML and DLBCL pre-clinical models. The BCL-2 family of anti-apoptotic proteins are deregulated and linked to maintenance and survival in AML and DLBCL. For its translation, the mRNA for BCL-2 is transported from the nucleus to the cytoplasm by forming a complex with XPO1 cargo, elF4E. Other important mRNAs exported from the nucleus via this mechanism include BCL6 and MYC. We hypothesize that SINE compounds inhibit XPO1/elF4E-mediated nuclear-cytoplasmic transport by covalently binding to the XPO1 cargo binding site and that in the absence of protein translation, BCL-2, BCL6 and MYC levels rapidly decline. Venetoclax (VEN; ABT-199) is a potent, selective inhibitor of BCL-2. In vitro, AML cells acquire resistance to VEN over time, often due to up-regulation of another BCL-2 family anti-apoptotic protein, MCL-1. MCL-1 is regulated by the anti-apoptotic transcription factor and XPO1 cargo NF-kB. We have previously shown that SINE compounds significantly decreased MCL-1 levels, presumably via inactivation of NF-kB. The goal of this study was to test whether SINE compounds will synergize with VEN via BCL-2 modulation and whether the combination would diminish MCL-1 mediated resistance to BCL-2 inhibition in DLBCL and AML models, respectively. Methods: BH3 profiling was performed in a sample of cell lines using a cytochrome c release assay to identify anti-apoptotic dependencies. The effects of SINE compounds and VEN as single agents or in combination on cell viability were performed in AML (K-562, MOLM-13, MV-4-11, and U-937) and DLBCL cell lines (SU-DHL-6, DoHH-2 and Toledo). Whole cell protein lysates were extracted 24 hours after treatment for immunoblot analysis. The activity of SINE compounds (5 mg/kg) and VEN (25 mg/kg) as single agents, or in combination were measured in AML (MV-4-11) and DLBCL (DoHH-2 and Toledo) xenografts in NSGS and nude mice, respectively. Tumor growth and survival were measured throughout these animal studies. Tumor tissue was collected at the end of treatment for flow cytometric analysis, western blotting and immunohistochemistry (IHC). Results: By employing BH3 profiling, we identified AML cell lines that were dependent (MV-4-11 and MOLM-13) and not dependent (U-937 and K-562) on MCL-1. Dose response analysis demonstrated that each of the AML cell lines was sensitive to the SINE compounds, while VEN only reduced viability in the MV-4-11 and MOLM-13 cells. Additionally, there was enhanced growth inhibition when the SINE compounds were combined with VEN in the MCL-1 dependent cells. SINE compound treatment synergistically decreased c-MYC protein levels in all 4 AML cell lines with the combination treatment (Figure 1), whereas PARP cleavage was only enhanced with the combination in the MV-4-11 and MOLM-13 cells. Likewise, MCL-1 is reduced in the presence of SINE compound or SINE compound-VEN combinations. In DLBCL xenograft studies (DoHH-2 and Toledo), combination of selinexor with VEN was synergistic for tumor reduction and increased animal survival when compared to either single agent alone. By IHC we observed a concomitant reduction in BCL-2 and BCL-6 and an increase in cleaved caspase 3 in DLBCL tumors after combination treatment. Conclusions: SINE compound-VEN combinations show enhanced antitumor effect, with reduction of oncogenic activity. SINE compounds reduce MCL-1 in VEN-resistant cells. As MCL-1 driven anti-apoptotic machinery is responsible for resistance to inhibition of BCL-2 in DLBCL and AML, SINE compound regulation of MCL-1 may lead to rescue of VEN resistance. SINE compounds and VEN are excellent candidate partners for combination therapies in AML and DLBCL. Disclosures Friedlander: Karyopharm Therapeutics: Employment. Chang:Karyopharm Therapeutics: Employment, Equity Ownership. Kashyap:Karyopharm Therapeutics: Employment, Equity Ownership. Argueta:Karyopharm Therapeutics: Employment, Equity Ownership. Klebanov:Karyopharm Therapeutics: Employment, Equity Ownership. Senapedis:Karyopharm Therapeutics: Employment, Equity Ownership. Baloglu:Karyopharm Therapeutics: Employment, Equity Ownership. Lee:Karyopharm Therapeutics: Employment, Equity Ownership. Shacham:Karyopharm Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Savona:TG Therapeutics: Research Funding; Amgen Inc.: Membership on an entity's Board of Directors or advisory committees; Takeda: Research Funding; Sunesis: Research Funding; Incyte: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees; Ariad: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees.

Description: Background and Purpose: Although novel approaches to the treatment of acute myeloid leukemia (AML) are urgently needed, the heterogeneity of AML and the paucity of known actionable targets indicate that there is an important need for broadly active treatment approaches that are not limited to specific genetic subtypes. Selinexor, an oral selective inhibitor of nuclear export, inhibits XPO1 and causes differentiation and apoptosis of a variety of AML subtypes while sparing normal hematopoiesis. We therefore undertook a pilot study to determine the safety and to explore the efficacy of selinexor in combination with fludarabine and cytarabine in pediatric patients with relapsed leukemia. Patients and Methods: Twelve children and adolescents with relapsed or refractory AML (n=10) or mixed phenotype leukemia (n=2) have been enrolled on the study to date. Four patients previously received only chemotherapy, whereas eight had undergone at least one prior stem cell transplant. High-risk features included t(6;9), t(6;12), t(4;11), -7 (2 cases), and megakaryoblastic leukemia. Selinexor, initially at 30 mg/m2/dose, was given orally on days 1, 3, 8, 10, 22, and 24 and escalated according to a rolling-6 design. Fludarabine (30 mg/m2) and cytarabine (2 g/m2) were administered on days 15-19. Results: Among 11 patients who completed at least one cycle and were evaluable for toxicity and response, 3 were treated at dose level 1 (30 mg/m2), 3 at dose level 2 (40 mg/m2), 4 at dose level 3 (55 mg/m2), and 1 at dose level 4 (70 mg/m2). No dose-limiting toxicities were observed at any dose level. The most common Grade 3 non-hematologic toxicity related to selinexor was hyponatremia, which was observed in 10 patients and easily corrected in all cases. As expected, the combination of selinexor plus fludarabine and cytarabine resulted in Grade 3 neutropenia and thrombocytopenia in all patients. Mean pharmacokinetic parameters indicate that plasma exposure is generally dose proportional across selinexor doses, with no apparent accumulation. Plasma exposure in pediatric patients is similar to adult patients treated in phase I trials. Inhibition of XPO1 was assessed by qRT-PCR of XPO1, which is upregulated at the RNA level in response to XPO1 protein inactivation (PDn). Six of the first seven patients enrolled on the trial demonstrated at least 2-fold induction of XPO1 that persisted for at least 48 hours (see figure), indicating prolonged inhibition of the protein by selinexor. The overall response rate in this group of heavily pretreated, relapsed, and refractory patients was 55%. Five patients achieved complete remission (4 with complete count recovery) and 1 had a partial response. Eight of the 11 patients underwent subsequent stem cell transplantation. A trend toward stronger and persistent XPO1 inhibition (measured by PDn) was observed among patients who achieved CR compared to those who did not. Conclusion: Selinexor, given in combination with fludarabine and cytarabine, is tolerable in pediatric patients with relapsed leukemia. Selinexor pharmacokinetic parameters are generally dose proportional and are similar to those seen in adult patients. Most patients demonstrate XPO1 target inhibition. Response rates are encouraging and will be further explored in the Phase II portion of this trial. Figure 1. Figure 1. Disclosures Kaufman: Karyopharm Therapeutics Inc: Employment. Klebanov:Karyopharm Therapeutics Inc: Employment. Ellis:Karyopharm Therapeutics Inc: Employment. Landesman:Karyopharm Therapeutics: Employment. Youssoufian:Karyopharm Therapeutics Inc: Employment. Rashal:Karyopharm Therapeutics Inc: Employment. Shacham:Karyopharm: Employment, Equity Ownership.

Description: Background: SINE compounds are a family of small-molecule drugs that inhibit XPO1 mediated nuclear export, resulting in nuclear retention of major tumor suppressor proteins (TSPs) such as p53, FOXO, pRB and IκB and subsequently in specific cancer cell death. Selinexor is a clinical stage SINE compound currently in human phase I/II clinical trials in patients with solid and hematological malignancies. Glucocorticoid Receptor (GR) is an XPO1 cargo and dexamethasone (Dex) acts as a GR agonist and inhibits NFκB activity. The combination of selinexor with Dex (Sel-Dex) shows enhanced anti-tumor potency in vitro and in vivo. The current study aimed at deciphering the molecular changes that contribute to the synergism of Sel-Dex. Methods: Total RNA and whole protein cell lysates from MM cell lines treated with selinexor with or without dexamethasone were analyzed by quantitative PCR and by immunoblots. Localization of GR was evaluated by immunofluorescence microscopy. GR and NFκB transcriptional activity was analyzed using ELISA assays (Affimetrix and Thermo Scientific). RNA from naïve and drug treated cells was analyzed by deep sequencing (by Asuragen). Results: Dexamethasone, but not selinexor, induced phosphorylation of GR. Selinexor blocked nuclear export of phosphorylated GR, enhancing GR transcriptional activation. Deep sequencing revealed a set of genes, whose level of expression was synergistically modified by Sel-Dex. Those genes belong to several pathways including GPCR signaling, cell metabolism and ERK, TGF-β and PI3-AKT signaling. Among the genes that were synergistically up regulated by the combination were genes from the Early Growth Response family (EGR1, EGR3, EGR4). EGR1 is a tumor suppressor protein which down-regulates survivin and triggers Caspase signaling and cell death. Interestingly, EGR1 also mediates the cytotoxic effects of bortezomib and lenalidomide in multiple myeloma cells. The Glucocorticoid-Induced Leucine Zipper (GILZ) gene was also up regulated by Sel-Dex combination. It has been shown by others that silencing GILZ expression decreased the therapeutic effects of dexamethasone in multiple myeloma. Conclusions: Selinexor increased the nuclear retention of dexamethasone-activated-GR. The increased GR transcriptional activity induces expression of genes across different pathways leading to inhibition of cell proliferation and increase cancer cell death. We are currently testing the function of several of these genes in the context of Sel-Dex combination. The current finding supports the the previously reported anti cancer activity of Sel-Dex combination in patients with heavily pretreated relapsed/refractory multiple myeloma. Disclosures Kashyap: Pharma: Employment. Klebanov:Karyopharm Therapeutics Inc: Employment. Lee:Karyopharm Therapeutics Inc: Employment. Landesman:Karyopharm Therapeutics: Employment.

Description: Selinexor is a novel, first-in-class, selective inhibitor of nuclear export compound, which blocks exportin 1 (XPO1) function, leads to nuclear accumulation of tumor suppressor proteins, and induces cancer cell death. A phase 1 dose-escalation study was initiated to examine the safety and efficacy of selinexor in patients with advanced hematological malignancies. Ninety-five patients with relapsed or refractory acute myeloid leukemia (AML) were enrolled between January 2013 and June 2014 to receive 4, 8, or 10 doses of selinexor in a 21- or 28-day cycle. The most frequently reported adverse events (AEs) in patients with AML were grade 1 or 2 constitutional and gastrointestinal toxicities, which were generally manageable with supportive care. The only nonhematological grade 3/4 AE, occurring in 〉5% of the patient population, was fatigue (14%). There were no reported dose-limiting toxicities or evidence of cumulative toxicity. The recommended phase 2 dose was established at 60 mg (∼35 mg/m2) given twice weekly in a 4-week cycle based on the totality of safety and efficacy data. Overall, 14% of the 81 evaluable patients achieved an objective response (OR) and 31% percent showed ≥50% decrease in bone marrow blasts from baseline. Patients achieving an OR had a significant improvement in median progression-free survival (PFS) (5.1 vs 1.3 months; P = .008; hazard ratio [HR], 3.1) and overall survival (9.7 vs 2.7 months; P = .01; HR, 3.1) compared with nonresponders. These findings suggest that selinexor is safe as a monotherapy in patients with relapsed or refractory AML and have informed subsequent phase 2 clinical development. This trial was registered at www.clinicaltrials.gov as #NCT01607892.

Description: Selinexor (KPT-330) is a first in class nuclear transport inhibitor of exportin-1(XPO1) currently in advanced clinical trials to treat patients with solid and hematological malignancies. To determine how selinexor might impact anti-tumor immunity, we analyzed immune homeostasis in mice treated with high selinexor doses (15 mg/kg, three times a week: M, W, F) and found disruptions in T cell development, a progressive loss of CD8 T cells and increases in inflammatory monocytes. Antibody production in response to immunization was mostly normal. Precursor populations in bone marrow and thymus were unaffected by high doses of selinexor, suggesting that normal immune homeostasis could recover. We found that high dose of selinexor given once per week preserved nearly normal immune functioning, whereas a lower dose given 3 times per week (7.5 mg/kg, M, W, F) was not able to restore immune homeostasis. Both naïve and effector CD8 T cells cultured in vitro showed impaired activation in the presence of selinexor. These experiments suggest that XPO1 function is required for T cell development and function. We then determined the minimum concentration of selinexor required to block T cell activation, and showed that T cell inhibitory effects of selinexor occur at levels above 100nM, corresponding to the first 24 hours post-oral dosing of 10 mg/kg. In a model of implantable melanoma, we used selinexor treatment at the clinically relevant dosing regimen of 10 mg/kg with a 5-day drug holiday (M, W selinexor treatment). After two weeks of treatment, tumors were harvested and tumor infiltrating leukocyte (TIL) populations were analyzed. This treatment led to intratumoral IFNg+, granzyme B+ cytotoxic CD8 T cells that were comparable to vehicle treated mice. Overall, selinexor treatment leads to transient inhibition of T cell activation but the clinically relevant once and twice weekly dosing schedules that incorporate sufficient drug holidays allow for normal CD8 T cell functioning and development of anti-tumor immunity. These results provide additional support to the recommended selinexor phase 2 dosing regimen, as was determined recently (Razak et al. 2016). Disclosures Klebanov: Karyopharm Therapeutics: Employment, Equity Ownership. Kashyap:Karyopharm Therapeutics: Employment, Equity Ownership. Shacham:Karyopharm Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Landesman:Karyopharm Therapeutics: Employment, Equity Ownership. Dougan:Karyopharm Therapeutics: Consultancy. Dougan:Karyopharm Therapeutics: Consultancy.

Description: Background: XPO1 (exportin-1/CRM1) mediates nuclear export of proteins containing leucine-rich amino-acid consensus sequences. XPO1 cargo proteins include many of the major tumor suppressor proteins (p53, IkB, pRB, FOXOs) and their export leads to the inactivation of cell cycle checkpoints. Overexpression of XPO1 has been reported to correlate with poor cancer prognosis. The Selective Inhibitor of Nuclear Export (SINE) compound, selinexor, binds covalently to the cargo pocket on XPO1, inhibits nuclear export which leads to cell cycle arrest and specific cancer cell death. Selinexor is currently in advanced clinical trials for patients with solid and hematological malignancies including patients with relapsed/refractory Diffuse Large B-Cell Lymphoma (DLBCL) (NCT02227251). Using preclinical models, we recently demonstrated that proteasome inhibitors (PI) can re-sensitize multiple myeloma that acquired resistance to selinexor. Here, we aimed to find if treatment with selinexor and bortezomib is beneficial for the treatment of DLBCL. Methods: DLBCLcell lines were treated with selinexor in combination with bortezomib. Cell viability was examined using standard viability assays after 72 hours of treatment. Whole cell protein lysates were evaluated by immunoblotting. NF-κB transcriptional activity was analyzed using an ELISA assay. WSU-DLCL2 cells were grown as sub-cutaneous tumors in ICR SCID mice. Tumor bearing mice were divided into 4 groups and were administered either vehicle, sub-maximum tolerated doses of selinexor (10 mg/kg p.o. twice a week, M, Th), bortezomib (1 mg/kg i.v. twice a week, M, TH) and the combination of selinexor (10 mg/kg p.o. twice a week) plus bortezomib (1 mg/kg i.v. twice a week). Results: The combination treatment of selinexor with bortezomib synergistically killed DLBCL cells compared to the single agents alone. Co-treatment with bortezomib enhanced selinexor mediated nuclear retention of IκB-α. Selinexor plus bortezomib treatment decreased NF-κB transcriptional activity. Finally, the combination of selinexor with bortezomib showed superior anti-tumor efficacy in the combination group compared to single agent treatments in WSU-DLCL2 xenograft model. Conclusions: Based on our results, inhibition of NF-κB transcriptional activity through forced nuclear retention of IκB appears to be an important mechanism underlying the synergistic effects of selinexor plus bortezomib in many different cell lines including DLBCL. The superior efficacy of selinexor plus bortezomib combination both in vitro and in vivo when compared to single agents along provides a rational for conducting clinical trials with these combinations in DLBCL patients. Disclosures Kashyap: Karyopharm Therapeutics: Employment, Equity Ownership. Klebanov:Karyopharm Therapeutics: Employment, Equity Ownership. Senapedis:Karyopharm Therapeutics: Employment, Equity Ownership. Shacham:Karyopharm Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Kauffman:Karyopharm Therapeutics Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Landesman:Karyopharm Therapeutics: Employment, Equity Ownership.

Description: Introduction: SINE are a family of small molecules that selectively inhibit nuclear export by forming a slowly reversible covalent bond with Cysteine 528 (Cys528) in the cargo binding pocket of Exportin 1 (XPO1/CRM1). SINE binding to XPO1 leads to forced nuclear retention and activation of major tumor suppressor proteins (TSPs) such as p53, FOXO, pRB and IkB, resulting in selective death of cancer cells. Selinexor is an orally bioavailable SINE compound currently in human phase I and II clinical trials for advanced hematological and solid cancers. Oral selinexor demonstrates maximal pharmacokinetic exposure at 1-2 hours in humans with associated increases in pharmacodynamic markers of XPO1 inhibition in 2-4 hours that last for up to 48 hours. The goal of this study was to develop a binding assay that would enable quantification of XPO1 occupancy in PBMCs from patients following oral administration of selinexor. Methods: To measure the binding of SINE to XPO1, biotinylated leptomycin B (LMB) was utilized. Biotinylated LMB binds covalently and irreversibly to Cys528 in the cargo-binding site of free XPO1 with activity confirmed to be similar to that of unmodified LMB in cytotoxicity assays. To measure SINE binding to XPO1 in vitro, cancer cell lines and PBMCs from normal human donors were treated with SINE compounds prior to treatment with biotinylated LMB. Any XPO1 that did not bind SINE instead binds to biotinylated LMB and can be quantified. In in vivo studies, mice were treated with selinexor, followed by collection of PBMCs for treatment with biotinylated LMB. After incubation with biotinylated LMB, cells were harvested, lysed, and protein lysates were subjected to pull-down experiments with streptavidin-conjugated beads followed by immunoanalysis of XPO1. Results: To evaluate selinexor-XPO1 binding kinetics in vitro, MM.1S, AML2, AML3, and HEL cells were treated with 0 - 10 µM of SINE compounds and unbound XPO1 was pulled down from cell lysates treated with biotinylated LMB. Immunoanalysis showed that 50% XPO1 occupancy with selinexor was achieved at 0.07 µM in MM.1S, 0.1 µM in AML2, 0.03 µM in AML3, and 0.12 µM in HEL cells. Selinexor-XPO1 occupancy experiments using human PBMCs isolated from donor whole blood showed 50% XPO1 occupancy at 0.05 µM. In mice, 50% XPO1 occupancy in PMBCs was achieved after 4 hours treatment with 1.2 mg/kg (3.6 mg/m2) selinexor, while 90% XPO1 occupancy was achieved at 8.1 mg/kg (24.3 mg/m2). Mice treated with a single dose of selinexor from 1.5 to 10 mg/kg for 4-96 hours revealed sustained, dose dependent XPO1 occupancy in PBMCs for up to 72 hours. Conclusions: We have developed a sensitive and robust assay to measure selinexor binding to XPO1 that can be used to evaluate drug exposure following treatment with oral selinexor in preclinical and clinical studies. Studies are ongoing to determine whether there is a correlation between XPO1 occupancy (pharmacodynamics measurement) with disease response in patients with solid and hematological malignancies. Disclosures Crochiere: Karyopharm: Employment. Klebanov:Karyopharm Therpeutics: Employment. Baloglu:Karyopharm: Employment. Kalid:Karyopharm Therapeutics: Employment. Kashyap:Karyopharm Therapeutics: Employment. Senapedis:Karyopharm: Employment. del Alamo:Karyopharm: Employment. Rashal:Karyopharm Therapeutics: Employment. Tamir:Karyopharm: Employment. McCauley:Karyopharm Therapeutics: Employment, Equity Ownership. Carlson:Karyopharm Therapeutics: Employment. Savona:Karyopharm: Consultancy, Equity Ownership; Gilead: Consultancy; Incyte: Consultancy; Celgene: Consultancy. Kauffman:Karyopharm Therapeutics, Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties. Shacham:Karyopharm Therapeutics, Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties. Landesman:Karyopharm Therapeutics: Employment.

Description: Background: Selinexor is a Selective Inhibitor of Nuclear Export (SINE) compound that binds to and inhibits XPO1 mediated nuclear export, resulting in nuclear accumulation of tumor suppressor proteins (TSPs) including p53, pRB, and IκB-α. Selinexor has therapeutic benefit both pre-clinically and clinically (NCT01607892, NCT02336815) in multiple myeloma (MM). It has been previously demonstrated that the extent of NF-κB transcriptional inhibition is one of the critical mechanisms contributing to the efficacy and/or resistance to selinexor in cells. However, the mechanism leading to NF-κB inhibition after selinexor treatment is not fully understood. We hypothesized that the level of the cellular inhibitor of NF-κB, IκB-α, and its compartmental localization play an important role in NF-κB transactivation and response to selinexor. In this study, we investigate the effect of selinexor treatment on MM cells with low expression of IκB-α and high NF-kB activity in order to understand the mechanism of NF-κB inhibition by selinexor. Methods: IM9 and RPMI-8226cells were treated with selinexor in the presence or absence of 20 ng/mL tumor necrosis factor α (TNFα; inflammatory mimic) and whole protein lysates were analyzed by immunoblotting. Cytotoxic effects of selinexor were evaluated using standard viability assays. IκB-α knockdown was performed using transfection of specific siRNA duplexes. NF-κB transcriptional activity was analyzed using an ELISA assay. Results: Selinexor induces nuclear localization of IκB-α in MM cells. RNAi of IκB-α in MM cells reduced the cytotoxic effects of selinexor by 10-fold. In addition, knockdown of IκB-α reduces the synergy of the selinexor plus proteasome inhibitor (bortezomib or carfilzomib) combination. This data suggests that in MM, IκB-α plays a major role in cellular sensitivity to selinexor potentially through NF-κB activity. Selinexor inhibited NF-kB transcriptional activity in IM9 and RPMI-8226 cells with IC50 of 1079 nM and 591 nM respectively. Although the difference in NF-κB activity IC50 is only 2-fold between the two MM cell lines (MTT IC50s are ~100 nM), IM9 cells have a 100-fold higher basal NF-kB activity when compared to RPMI-8226 cells. Under TNFα stimulation NF-kB activity was induced by 1.5- and 35-fold in IM9 and RPMI-8226, respectively. We observed that selinexor treatment caused a dose dependent inhibition of IκB kinase (IKK)-mediated phosphorylation of serine 32/36 on IκB-α and serine 536 on the NF-κB p65 subunit (RelA) upon TNFα stimulation in both cell lines. In RPMI-8226 cells, selinexor reduced TNFα-induced IκB-α phosphorylation in a dose dependent manner and protected IκB-α from degradation. In IM9 cells that have high basal NF-κB activity, TNFα did not induce NF-kB activity or cause IκB-α degradation. However, selinexor treatment inhibited NF-kB activity below its basal level (70% reduction) which resulted in dose dependent reduction in the level of IκB-α protein perhaps through inhibition of NF-κB transcriptional control of IκB-α mRNA expression. Conclusions: IκB-α plays a major role in the cellular cytotoxicity of selinexor in cancer cells. Multiple myeloma cells lose sensitivity to selinexor treatment upon IκB-α silencing, which in turn reduces the cytotoxicity of selinexor. TNFα stimulation induces the phosphorylation of NF-κB p65 subunit and IκB-α through increased IKK activity resulting in IκB-α degradation and NF-κB activation. In RPMI-8226 cells, selinexor treatment blocked TNFα-induced degradation of IκB-α. However, in IM9 cells TNFα alone did not have any significant effect on IκB-α which might be due to the high basal NF-kB activity. Interestingly, IκB-α is also a transcriptional target of NF-kB. In IM9 cells, selinexor treatment reduces NF-kB activity below the high basal level in a dose dependent manner resulting in near complete inhibition of NF-kB-controlled IκB-α mRNA transcription and a loss of the IκB-α protein. Ultimately, selinexor treatment inhibits cell viability and NF-kB transcriptional activity regardless of basal NF-κB activity in MM cells. Because of this IκB-α/NF-kB transcriptional mechanism, selinexor treatment can inhibit both chronic (unresponsive to TNFα) and acute (TNFα-simulated) inflammatory signaling which makes selinexor an applicable therapy to cancer cells with a variety of aberrant signaling pathways. Disclosures Kashyap: Karyopharm Therapeutics: Employment, Equity Ownership. Klebanov:Karyopharm Therapeutics: Employment, Equity Ownership. Argueta:Karyopharm Therapeutics: Employment, Equity Ownership. Lee:Karyopharm Therapeutics: Employment, Equity Ownership. Shacham:Karyopharm Therapeutics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Kauffman:Karyopharm Therapeutics Inc: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Landesman:Karyopharm Therapeutics Inc: Employment, Other: stockholder. Senapedis:Karyopharm Therapeutics: Employment, Equity Ownership.