Selinexor, a Selective Inhibitor of Nuclear Export (SINE) Compound, Shows Synergistic Anti-Tumor Activity in Combination with Dexamethasone Characterized By Specific Pattern of Gene Expression in Multiple Myeloma (MM)

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.