Abstract 4434: Selinexor and melphalan combination therapy for the treatment of multiple myeloma
Authors: Turner Joel G. , Dawson Jana L. , Cubitt Christopher , Nishihori Taiga , Anasetti Claudio , Alsina Melissa , Baz Rachid C. , Kauffman Michael , Shacham Sharon , Dalton William S. , Sullivan Daniel M. 
Published: August 2015

Abstract
Introduction
High-dose melphalan (MEL) followed by autologous stem cell transplant remains the standard of care for the treatment of multiple myeloma (MM). However, patients eventually develop drug resistance and die from progressive disease despite newer therapies using proteasome inhibitors and immunomodulatory drugs. The incurable nature of MM demonstrates the need for novel treatments. Our aim was to investigate whether MEL therapy could be improved by the addition of the XPO1 inhibitor selinexor (SEL) in drug-resistant and parental MM cells both in vitro and ex vivo.
Materials and Methods
SEL/MEL-treated human MM cells were assayed for cell viability (CT-Blue) and apoptosis by flow cytometry (activated caspase 3). Proximity ligation assays (PLD) were used to assess if XPO1-p53 binding was inhibited by SEL. Western blots of SEL-treated MM cells were performed for nuclear and total p53. MEL-resistant U266 (LR6) and 8226 (LR5) MM cell lines were developed by incremental exposure to MEL. MEL resistant and parental MM cells were treated in vitro with SEL +/- MEL and assayed for apoptosis and cell viability. Cells isolated from patients with newly diagnosed or relapsed MM, were treated with SEL +/- MEL and assayed for apoptosis.
Results
MM cell viability was decreased synergistically by SEL when used in combination with MEL, as shown by combination index (CI) values. Drug sequencing assays showed that concurrent treatment with MEL (10 μM) and SEL (300 nM) for 48 hours produced synergistic results in human H929 MM cells (CI value 0.079, n = 3). Sequential treatment, SEL for 24 hours followed by MEL for an additional 24 hours or the reverse sequence, also demonstrated synergy with CI = 0.208 (n = 3) and 0.142 (n = 3), respectively. Normal PBMCs (control) were unaffected by SEL/MEL treatment as shown by viability and apoptotic assays. PLD demonstrated that SEL blocks XPO1/p53 binding. Western blot data showed that the SEL treatment of MM cells increased nuclear and total p53. Drug-resistant LR5 and LR6 MM cells were found to be resistant to MEL when compared to parental cell lines. Both resistant MM cell lines were sensitized by SEL to MEL as shown by apoptosis assay (20-fold). CD138+/light chain+ MM cells derived from newly diagnosed and relapsed MM patients were also sensitized (5 to 10-fold) by SEL to MEL as demonstrated by apoptosis assay.
Conclusions
SEL synergistically improved the response of drug-resistant and parental MM cells to MEL in vitro and ex vivo. It is possible that this synergy may be due to an increase of nuclear p53 by SEL and the reported activation of p53 by MEL. Ongoing studies include in vitro experiments to investigate whether this drug combination reverses MEL resistance by the Fanconi Anemia/BRCA pathway, in vivo treatment of MM in NSG mice with SEL/MEL and a clinical trial using high-dose MEL in combination with SEL. Combination therapies using SEL and MEL may significantly improve the treatment outcomes of MM.
Citation Format: Joel G. Turner, Jana L. Dawson, Christopher Cubitt, Taiga Nishihori, Claudio Anasetti, Melissa Alsina, Rachid C. Baz, Michael Kauffman, Sharon Shacham, William S. Dalton, Daniel M. Sullivan. Selinexor and melphalan combination therapy for the treatment of multiple myeloma. [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 4434. doi:10.1158/1538-7445.AM2015-4434