Abstract 1341: Inhibiting the nuclear exporter XPO1 and the antiapoptotic factor BCL2 is synergistic in XPO1 and SF3B1 mutant hematologic malignancies
Authors: Totiger Tulasigeri M. , Chaudhry Sana , Afaghani Jumana , Taylor Justin 
Published: July 2021

Abstract
Mutations in the nuclear exporter XPO1 and the spliceosomal protein SF3B1 are bona fide cancer drivers and occur across a spectrum of hematologic cancers, specifically in chronic lymphocytic leukemia (CLL), myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). Selinexor is a potent, XPO1 inhibitor that was recently approved in diffuse large B-cell lymphoma and is being tested in MDS and AML. The BCL2 inhibitor, venetoclax, is approved for use in CLL and has also shown to be effective in myeloid malignancies. XPO1 inhibition exerts its antineoplastic effects by blocking key tumorigenic pathways, such as NFκB, and decreasing the anti-apoptotic protein MCL1. We therefore hypothesized that combining selinexor and venetoclax would have potential synergy and could be used to target hematologic malignancies bearing XPO1 and SF3B1 mutations.
We first determined whether XPO1 and SF3B1 mutant cell lines showed differential response to either selinexor or venetoclax monotherapy. Five lymphoma cell lines; 3 diffuse large B-cell lymphoma (SU-DHL-6, SU-DHL-16, FARAGE) and 2 classical Hodgkin lymphoma (L428 and SUP-HD1), were used based on the presence or absence of XPO1 mutations. SUDHL-16 and SUP-HD1 are heterozygous for the XPO1 E571K hotspot mutation while SUDHL-6, FARAGE and SUP-HD1 are wildtype. These cell lines were used to assess sensitivity to Selinexor or Venetoclax using the CellTiter Glo assay. The XPO1 mutant cells showed increased sensitivity to selinexor (IC50 = 16-35nM vs 41-231nM). Additionally, the XPO1 mutant cell lines showed increased sensitivity to single-agent venetoclax (XPO1 mutant IC50 = 2-13nM vs 5-2853nM). SF3B1 K666N mutant K562 and Nalm-6 cells also showed increased sensitivity to selinexor compared to wildtype cells (IC50 =116-190nM vs 212-1405nM), as well as to venetoclax as compared to wildtype (IC50 =149nM-1.28 μM vs 1.26-1.4μM).
Next, we tested the synergy of the combination of these drugs in the XPO1 and SF3B1 mutant cell lines. Increasing concentrations of the individual drugs were applied to each individual cell line in a 6x6 matrix and the Bliss Independence model was used to calculate synergy of the combination. As hypothesized, the combination of Selinexor and venetoclax showed synergy in both the XPO1 and SF3B1 mutant cell lines (synergy scores 4.5-16.5). Western blots of treated wildtype versus mutant cells showed decreased levels of p65 and MCL1 with selinexor (but not venetoclax) that was enhanced with combination therapy, specifically in mutant cell lines.
In conclusion, inhibiting the nuclear exporter XPO1 and the anti-apoptotic factor BCL2 is synergistic in both XPO1 and SF3B1 mutant cells. This combination is highly promising as an all oral regimen for hematologic malignancies bearing these mutations. Further, preclinical testing in in vivo using XPO1 and SF3B1 mutant and wildtype mice is ongoing.
Citation Format: Tulasigeri M. Totiger, Sana Chaudhry, Jumana Afaghani, Justin Taylor. Inhibiting the nuclear exporter XPO1 and the antiapoptotic factor BCL2 is synergistic in XPO1 and SF3B1 mutant hematologic malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1341.