Melphalan and XPO1 Inhibitor Combination Therapy for the Treatment of Multiple Myeloma

Introduction

High-dose melphalan chemotherapy with autologous stem cell transplant remains the standard of care for the treatment of multiple myeloma. However, patients eventually develop drug resistance and die from progressive disease despite the introduction of therapies using proteosome inhibitors (PIs) and immunomodulatory drugs (IMIDs). The incurable nature of multiple myeloma clearly demonstrates the need for novel agents and treatments. Here, our aim was to investigate whether the use of XPO1 (exportin 1, CRM1) inhibitors (XPO1i) could sensitize de novo and acquired drug-resistant multiple myeloma cells both in vitro and ex vivo to the alkylating agent melphalan.

Materials and Methods

Human multiple myeloma cell lines NCI-H929, RPMI-8226, U266 and PBMC controls were treated in vitro with the XPO1i KOS-2464 and the orally available Selective Inhibitor of Nuclear Export (SINE) selinexor (KPT-330) or) +/- melphalan. Multiple myeloma cells were grown at high-density conditions (>3-5x10⁶ cells/mL). High-density multiple myeloma cells have been shown to possess de novo drug resistance. Sensitivity of the XPO1i/melphalan-treated NCI-H929 cells was measured by cell viability assay (CellTiter-Blue). Apoptosis in XPO1i/melphalan-treated NCI-H929, RPMI-8226, and U266 cells was assayed using flow cytometry (activated caspase 3). Proximity ligation assays were performed to assess XPO1-p53 binding in the presence of an XPO1i. Western blots of XPO1i-treated myeloma cells were performed for nuclear and total p53. Drug-resistant U266 (PSR) and 8226 (8226/B25) myeloma cell lines were developed by incremental exposure to bortezomib. PSR cells are able to grow in 15 nM bortezomib and the 8226/B25 in 25 nM. These resistant myeloma cells were treated in vitro with XPO1i +/- melphalan. Sensitivity to therapy was measured by apoptosis and cell viability assay. Multiple myeloma cells isolated from patients with newly diagnosed, relapsed, or refractory disease were treated with XPO1i +/- melphalan and CD138+/light chain+ myeloma cells and assayed for apoptosis.

Results

Multiple myeloma cell (NCI-H929) viability was decreased synergistically by XPO1i when used in combination with melphalan, as shown by the calculated combinatorial index (CI) values. We examined sequencing of the drugs and found that concurrent treatment with melphalan (10 µM) and selinexor (300 nM) for 48 hours produced the best results (CI value 0.370, n=6). Sequential treatment (selinexor for 24 hours followed by melphalan for an additional 24 hours) or the reverse sequence had slightly less synergy, with CI values of 0.491 (n=9) and 0.565 (n=3), respectively. Normal PBMC control cells were unaffected by XPO1i/melphalan treatment as shown by viability and apoptotic assays. Proximity ligation assay demonstrated that XPO1i blocks XPO1/p53 binding. Western blot showed that the XPO1i treatment of myeloma cells increased nuclear and total p53. Drug-resistant 8226/B25 myeloma cells but not PSR cells were found to be resistant to melphalan when compared to parental cell lines. Both resistant myeloma cell lines were sensitized by XPO1i to melphalan as shown by apoptosis assay (3- to 10-fold). CD138+/light chain+ myeloma cells derived from newly diagnosed, relapsed, and refractory myeloma patients were also sensitized by XPO1 inhibitors to melphalan as demonstrated by apoptotic assays (e.g. activated caspase 3).

Conclusions

XPO1i synergistically improved the response of de novo and acquired drug-resistant myeloma cells to melphalan in vitro and ex vivo. It is possible that this synergy may be due to an increase of nuclear p53 by XPO1i and the reported activation of p53 by melphalan. Future studies include in vitro experiments using drug-resistant human U266 myeloma cells in NOD-SCID-gamma mice and clinical trials using melphalan in combination with the SINE selinexor. Combination therapies using selinexor and melphalan may significantly improve the treatment of myeloma.