Abstract 4146: Selinexor synergizes with DNA damaging agents through down-regulation of key DNA damage response genes
Authors: Kashyap Trinayan , Argueta Christian , Klebanov Boris , Gonzalez Oscar , Baloglu Erkan , Landesman Yosef , Lee Margaret , Gardner Humphrey , Shacham Sharon , Senapedis William 
Published: July 2017

Abstract
Background: Selinexor is a first-in-class, orally bioavailable SINE (Selective Inhibitor of Nuclear Export) compound currently in phase 1/2 clinical trials of patients with solid and hematological malignancies. SINE compounds bind exportin 1 (XPO1/CRM1) and inhibit nuclear export, resulting in nuclear retention of major tumor suppressor proteins (TSPs). We previously found through Reverse Phase Protein Array analysis that selinexor treatment alters the level of key DNA damage response (DDR) proteins. We also observed enhanced anti-tumor activity when combining selinexor with standard DNA damaging agents (DDA) in pre-clinical models and clinical studies. We hypothesize that a reduction in DDR protein expression by selinexor drives the synergy observed in combination with DDAs.
Methods: Solid and hematological cancer cell lysates treated with selinexor +/- DDAs (cisplatin, doxorubicin, docetaxel and gemcitabine) were analyzed by immunoblotting and qPCR. Comet assay and γH2A.X staining were analyzed by immunofluorescence to evaluate DNA damage. Mice bearing MDA-MB-231 (triple negative breast cancer) tumors were treated with sub-toxic doses of selinexor (2.5 mg/kg), cisplatin (4 mg/kg) or docetaxel (4 mg/kg) alone or in combination. Tumor growth and mouse weight were followed for 25 days.
Results: Selinexor reduced the expression of DDR mRNA and proteins involved in single-stranded (SSB) and double-stranded break (DSB) repair as early as 2 hours post treatment. We observed a dose-dependent correlation between the degree of DDR protein reduction and selinexor cytotoxicity in vitro. Selinexor prevented DDR activation following exposure to DDAs and led to synergistic cell death. Treatment of cells with selinexor plus DDA showed enhanced activity in comet assays and an increase in γH2A.X staining when compared to treatment with the single agents. Exposure to selinexor 6 - 24 hours after DDA treatment inhibited the DDR mechanism and was more synergistic than the opposite order (Sel → DDA) or simultaneous treatment. In a MDA-MB-231 xenograft, treatment with selinexor, docetaxel (SSB) or cisplatin (DSB) alone resulted in 66.7%, 51.5% or 26.6% TGI, respectively when compared to vehicle control whereas sequential treatment of docetaxel or cisplatin followed by selinexor resulted in 93.9% TGI or 9.6% tumor regression, respectively. No significant weight loss was seen in any of the groups by the end of the study. IHC staining and immunoblot analysis of tumor tissue and sections showed a reduction of DDR proteins.
Conclusion: Selinexor synergizes with DDAs by increasing both SSB and DSB through inhibition of relevant DDR proteins. This data supports clinical development of selinexor in combination with standard chemotherapeutic agents in patients with solid and hematological malignancies. The results also suggest that sequential treatment of patients with DDAs followed by selinexor might be more beneficial.
Citation Format: Trinayan Kashyap, Christian Argueta, Boris Klebanov, Oscar Gonzalez, Erkan Baloglu, Yosef Landesman, Margaret Lee, Humphrey Gardner, Sharon Shacham, William Senapedis. Selinexor synergizes with DNA damaging agents through down-regulation of key DNA damage response genes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4146. doi:10.1158/1538-7445.AM2017-4146