XPO1 Inhibitor Selinexor Overcomes Ibrutinib Resistance in Mantle Cell Lymphoma Via Nuclear Retention of IκB

Background: B-cell receptor signaling (BCR) plays a central role in mantle cell lymphoma (MCL) and inhibition of BCR signaling through the BTK inhibitor, ibrutinib, has generated remarkable responses in patients. However, approximately one-third of the patients do not respond well to the drug, and disease relapse on ibrutinib is nearly universal. Alternative therapeutic strategies aimed to prevent and overcome ibrutinib resistance are needed. Selinexor is a first-in-class selective inhibitor of nuclear export. It binds and inhibits exportin XPO-1, which mediates the nuclear export of proteins and mRNAs. Inactivation of XPO-1 increases the retention of tumor suppressors in the nuclei to enhance suppression of tumor cell growth. Inactivation of exportin also decreases the the expression of oncogenes of which translation is eIF4E-dependent. Since many of the oncogenes and tumor suppressor genes in MCL also shuttle between nuclei and cytoplasm, we speculate that selinexor would be active in MCL. As selinexor acts on a cellular process rather than a particular molecular target, we thus postulate selinexor may have the potential to act broadly in both ibrutinib-sensitive and -resistant mantle lymphoma cells. In this study, we compared and contrasted the intrinsic cellular response of MCL cells to ibrutinib versus selinexor. We aim to identify molecular mechanisms underlying differential cellular response to these two drugs in order to obtain a deeper understanding of what underlies tumor cells' susceptibility to drug intervention.

Methods: We first determined the sensitivity of six MCL cell lines to both ibrutinib and selinexor in a cell-based MTT assay, which measures cellular metabolic activity, and then tested the effects of the drugs on cell growth. Next, we examined the effects of both inhibitors on apoptosis and proliferation to further delineate the differential sensitivity of MCL cells to ibrutinib and selinexor. We performed RNA-seq analyses to identify signaling pathways that underlie differential cellular response to these two drugs. Lastly, we validated the major findings in both cell lines and primary MCL tumors using a variety of assays including Flowsight analysis.

Results: We found that selinexor had a broader anti-tumor activity in MCL than ibrutinib. MCL cell lines resistant to ibrutinib were sensitive to selinexor. We showed that selinexor induced apoptosis in addition to causing cell cycle arrest. Further, inhibition of NF-kB gene expression signature, as opposed to BCR signature, was a common feature that underlies the response of MCL cell lines to both selinexor and ibrutinib while unchanged NF-kB signature was associated exclusively with ibrutinib resistance. Using Flowsight, we showed that selinexor increased nuclear retention of IκB that was accompanied by the reduction of NF-kB DNA binding activity. In primary MCL tumor cells, we further demonstrated that the number of cells with IκB nuclear retention was linearly correlated with the degree of cellular apoptosis.

Conclusions: Together, our results show that inhibition of NF-κB transcription via IκB nuclear retention is an important action of selinexor in MCL tumor cells. Selinexor is effective MCL cells with intrinsic resistance to ibrutinib and has the potential to prevent and overcome ibrutinib resistance. Our study warrants further clinical investigation of this compound in MCL.