Abstract B147: Preclinical evaluation of XPO1 inhibition with topoisomerase-I (topo-I) inhibition in colorectal cancer (CRC) cell lines and patient-derived xenograft (PDX) models
Authors: Lentz Robert W , Dominguez Adrian , Bagby Stacey , Binns Cameron , Leal Alexis , Kim Sunnie , Davis Sarah Lindsey , Lieu Christopher H , Messersmith Wells A , Pitts Todd M 
Published: December 2023

Abstract
Background: Resistance to topo-I inhibitors is of significant importance in treating patients with metastatic CRC (mCRC). Selinexor and eltanexor are selective inhibitors of nuclear export, by binding to exportin 1 (XPO1, the primary nucleic export receptor, and is overexpressed in CRC). Eltanexor is an investigational XPO1 inhibitor, with similar mechanism and potency but improved tolerability in animal models. XPO1 inhibition results in nuclear localization of tumor suppressor proteins, oncoprotein mRNAs, and topoisomerases, and decreases DNA damage repair (DDR) response. Irinotecan is a topo-I inhibitor which induces DNA damage and is commonly used in mCRC. We hypothesized that XPO1 inhibition may improve response to topo-I inhibitor in mCRC by impairing the DDR response.
Methods: The synergism of XPO1 inhibition (selinexor or eltanexor) with SN38 was assessed in CRC cell lines by CellTiter-Glo 2.0 and quantitated using Bliss Additivity Model using Synergy Finder 2.0. Each condition was plated either concurrent (both drugs for 72 hours) or sequential (SN38 for 24 hours then removed, followed by XPO1 inhibitor for 48 hours). Nude mice were implanted with one microsatellite stable CRC PDX model in the flanks followed by treatment with vehicle, XPO1 inhibitor, irinotecan (15 mg/kg intraperitoneal weekly), or combination. Tumor growth was measured by specific growth rate (SGR), within an IACUC-approved protocol. Tumor tissue was collected at day 7 for molecular analysis. Tissues were lysed and protein was run on 4%-12% SDS-PAGE Gel. p21 Waf/Cip and P-Histone H2A.X were used to determine DNA damage by western blot. P-Histone H2AX (pH2AX) was evaluated by immunofluorescence using primary conjugated P-Histone H2A.X antibody and DAPI for nuclear counter stain. Slides were imaged using Olympus iX83 microscope and analysis was performed using Olympus CellSens software.
Results: In CRC cell lines, concurrent combination eltanexor and SN38 did not synergistically decrease viability but increased pH2AX compared to SN38 alone (~30-fold). However, sequential SN38 followed by eltanexor synergistically decreased viability at multiple dose levels; evaluation of pH2AX is ongoing. In PDX model CRC254, eltanexor (10 mg/kg and reduced to 7.5 mg/kg on day 8 to improve tolerability, orally 5 days per week) with irinotecan decreased SGR compared to irinotecan alone (SGR=0.007 vs 0.026, respectively, p=0.0003) and compared to eltanexor alone (SGR=0.029, p &lt;0.0001). Combination treatment increased pH2AX and increased p21 expression compared to either single-agent treatment. Similar SGR results were observed with selinexor (5 mg/kg orally twice weekly).
Conclusions: XPO1 inhibition and topo-I inhibition decreased CRC tumor growth in PDX models, presumably by increasing DNA double strand breaks. Cell line data suggests sequential dosing of topo-I inhibition followed by XPO1 inhibition may be effective. This work supports continued development of this combination in mCRC.
Citation Format: Robert W Lentz, Adrian Dominguez, Stacey Bagby, Cameron Binns, Alexis Leal, Sunnie Kim, Sarah Lindsey Davis, Christopher H Lieu, Wells A Messersmith, Todd M Pitts. Preclinical evaluation of XPO1 inhibition with topoisomerase-I (topo-I) inhibition in colorectal cancer (CRC) cell lines and patient-derived xenograft (PDX) models [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B147.