SL-801, a Novel, Reversible Inhibitor of Exportin-1 (XPO1) / Chromosome Region Maintenance-1 (CRM1) with Broad and Potent Anti-Cancer Activity
Authors: Chen Janice , Brooks Christopher L. , McDonald Peter , Schwartz Jonathan D. , Schneider Rebecca S. , Sakakibara Keiichi , Saito Naoya , Sato Takuji , Kawabe Takumi , Rowinsky Eric K. 
Published: October 2019

Abstract
XPO1/CRM1, the principal nuclear export protein in eukaryotic cells, is required for the nuclear-cytoplasmic transport of both proteins and RNAs. Overexpression of XPO1 is reported in many cancers, causing dysregulated protein localization, aberrant cell proliferation, and resistance to apoptosis, and is associated with aggressive characteristics and poor patient outcome. Recent work has revealed XPO1 to be a clinically relevant target, and nuclear export inhibitors have emerged as a new class of anti-cancer agents with clinical activity in multiple hematologic and solid malignancies. SL-801 is a novel small molecule that binds covalently to Cys528 of XPO1, blocking the ability of XPO1 to interact with substrate cargos (e.g., p53, FOXO, p21, p27, and others). In contrast to the prototypical XPO1 inhibitor leptomycin B, which binds irreversibly to XPO1 and caused significant toxicities in Phase 1 trials, SL-801 binding to XPO1 is reversible, a characteristic that may be exploited to maximize its therapeutic index. Exposure to SL-801 results in potent inhibition of XPO1-dependent nuclear export, cell cycle arrest, and induction of apoptosis in a time- and dose-dependent manner. Here, the anti-tumor activity of SL-801 was investigated against a panel of 240 cell lines representing a broad range of solid and hematologic malignancies and confirmed in several SCID xenograft models.
The OncoPanelTM high content screening platform was used to evaluate the cytotoxicity of SL-801 against 205 solid tumor and 35 liquid tumor cell lines. SL-801 demonstrated potent activity, with 50% growth inhibitory (GI50) values ≤ 10 nM in 51/240 (21.3%) cell lines and GI50 values ≤ 100 nM in 230/240 (95.8%) cell lines. SL-801 sensitivity was independent of cell proliferation rate or XPO1 expression levels. While SL-801 was broadly cytotoxic, cell lines of hematopoietic origin exhibited greater sensitivity. GI50s in hematologic cancers ranged from 3-93 nM in leukemias, 1-103 nM in lymphomas, and 3-11 nM in multiple myelomas. SL-801 also inhibited solid tumor growth, with GI50s ≤ 10 nM in several breast, brain, cervical, ovarian, gastric, kidney, liver, lung, melanoma, prostate, and sarcoma lines. In addition, a 5-fold increase in active caspase-3 staining was observed at SL-801 concentrations ≤ 100 nM in 117/240 (48.8%) cell lines, consistent with induction of apoptosis. To understand tumor sensitivity to SL-801, results of the cell line cytotoxicity screen were analyzed against publicly available genomic datasets. This analysis revealed that SL-801 was cytotoxic towards cell lines regardless of mutation status of key oncogenes (e.g., KRAS) and tumor suppressor genes (e.g., TP53).
The in vitro cytotoxicity of SL-801 against tumor cell lines was further validated in several xenograft models in SCID mice. In the RPMI-8226 multiple myeloma xenograft model, tumor growth was significantly inhibited at oral SL-801 doses of 31.25 mg/kg administered daily for five days for two weeks. In the ARH-77 human multiple myeloma xenograft model, overall survival was significantly prolonged by daily oral administration of 125 mg/kg SL-801 for ten days. This dose and regimen were well tolerated, and 90% of SL-801-treated mice survived > 150 days, whereas median survival was 39.5 days in the vehicle-treated group (p < 0.001). Significant tumor growth inhibition was also observed in the NCI-H226 non-small cell lung cancer and 22RV1 prostate cancer xenograft models.
These data demonstrate that SL-801 is a promising clinical candidate that inhibits a novel, clinically validated target and support its clinical development in a broad range of oncologic indications. The reversible binding of SL-801 to XPO1 may offer the potential to develop dosing schedules to enable recovery in normal tissues, thus broadening the therapeutic index of this class of agents. IND-enabling work is underway to support entry into clinical trials, and a Phase I trial design will be discussed.

Disclosures
Chen: Stemline Therapeutics, Inc.: Employment, Equity Ownership. Brooks:Stemline Therapeutics, Inc.: Employment, Equity Ownership, Patents & Royalties. McDonald:Stemline Therapeutics, Inc.: Employment, Equity Ownership. Schwartz:Stemline Therapeutics, Inc.: Employment, Equity Ownership. Schneider:Stemline Therapeutics, Inc.: Employment. Sakakibara:CanBas Co., Ltd.: Employment, Equity Ownership. Saito:CanBas Co., Ltd.: Employment, Equity Ownership. Sato:CanBas Co., Ltd.: Employment, Equity Ownership, Patents & Royalties. Kawabe:CanBas Co., Ltd.: Employment, Equity Ownership, Patents & Royalties. Rowinsky:Stemline Therapeutics: Employment, Equity Ownership.