Introduction: Ewing sarcoma is an aggressive pediatric and young adult bone tumor dependent almost exclusively on the EWS/ETS fusion protein family for transcriptional activity. EWS/FLI is the most common fusion resulting in the repression of vital tumor suppressor genes by the activity of lysine-specific histone demethylase 1 (LSD1). Seclidemstat (SP-2577) is a first in class, orally bioavailable, small molecule with reversible and noncompetitive selective inhibition of LSD1 at low nanomolar concentrations (IC50: 25-50 nM). Seclidemstat has demonstrated disruption of global transcriptional function of EWS/ETS fusions and impairs multiple EWS/ETS-associated oncogenic phenotypes in xenograft models of Ewing sarcoma. This Phase I study aims primarily to assess the safety and tolerability of seclidemstat and to secondarily characterize the pharmacokinetics (PK), food effects, and preliminary anti-tumor activity via RECIST 1.1. Exploratory objectives include using circulating tumor cells, cell-free DNA, and hemoglobin F as pharmacodynamic markers of response. Methods: This multi-center phase I study (NCT03600649) is open-label and non-randomized and utilizes an accelerated dose escalation phase followed by a conventional 3+3 design to determine the maximum tolerated dose (MTD). Initially, one subject per dose cohort will be recruited in the accelerated dose escalation phase until the first instance of grade 2 or greater drug related toxicity or dose limiting toxicity (DLT). Further cohorts will be recruited in cohorts of three subjects in the 3+3 dose escalation phase. Once a declared suitable dose and schedule for further investigation has been identified, 14 additional patients will be enrolled in the dose expansion part of the study for a total of 20 patients treated at the MTD. The starting dose of seclidemstat is 75 mg PO BID with seven dose levels planned by modified Fibonacci schema. Food effects on PK are planned on day 1 and 2 of cycles 1 and 2. Patients at least 12 years of age will be included with histologically confirmed relapsed or refractory Ewing sarcoma with adequate performance status and organ function. Archival tumor tissue is required during screening. Tumor biopsies and measurable disease are required in dose expansion only. This trial is currently open for requirement at five locations in the United States. Dose level 2 (150 mg PO BID) began enrollment in November 2018 with no drug related grade 2 or greater adverse events or DLTs observed in dose level 1. Citation Format: Damon Reed, Steven G. DuBois, Richard Gorlick, Leo Mascarenhas, Douglas Harrison, Jonathan Metts, Stephen L. Lessnick, Brian D. Crompton, David M. Loeb, Rose Hernandez, Jeff Larson, David D. Stenehjem. A Phase I dose escalation and expansion study of seclidemstat (SP-2577) a first-in class reversible LSD1 inhibitor for patients with relapsed or refractory Ewing sarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT109.

Abstract Osteosarcoma (OS) is the most common bone cancer in children and adolescents, and patients with metastatic disease still have extremely poor prognosis and no effective targeted therapy. It has been reported that the activation of Wnt/β-catenin pathway is closely associated with OS development and metastatic progression. Tegavivint (BC2059), a novel small molecule inhibitor of the Wnt/β-catenin pathway, has been reported to be active against multiple types of cancer cells in vitro, and anti-tumor efficacy has been published in animal models of acute myeloid leukemia and multiple myeloma. In this study, we investigated the antitumor activity of Tegavivint against metastatic OS both in vitro and in vivo using established human OS cell lines (143B, SaOS-2, LM7) and patient-derived xenograft (PDX) models. In vitro, Tegavivint effectively inhibited tumor cell survival in a dose-dependent manner in all established OS cell lines and OS PDX-derived cells. Subsequent in vivo studies using an orthotopic model of LM7 cells engrafted in the tibia of NSG mice demonstrated complete regression of the primary tumors in all treated mice as well as a significant reduction in lung metastasis by the treatment with Tegavivint. We further examined the activity of Tegavivint using a pair of PDX models derived from an inherently chemo-resistant OS patient at different disease stages: PDX63 was derived from the pre-treatment biopsy of the primary tumor, and PDX84 was derived from the relapsed metastatic lung lesion of the same patient after chemotherapy. The growth of subcutaneously engrafted PDX63 tumors was significantly suppressed by the treatment with Tegavivint alone and Tegavivint enhanced the antitumor activity of doxorubicin. Analysis of mRNA expression by qPCR in the tumor tissue demonstrated significant downregulation of critical genes by Tegavivint, including c-Myc and ALDH1, which is a potential marker for cancer stem cells. Furthermore, in an innovative lung metastasis model using PDX84 tumor-dissociated cells intravenously (IV) injected in NSG mice, we demonstrated that Tegavivint treatment markedly reduced the number of lung metastases. Finally, PDX84-derived cells were sorted into ALDH1-high and ALDH1-low populations. ALDH1-high cells showed higher expression of β-catenin and higher sensitivity to Tegavivint than the ALDH1-low cells in vitro and subsequent IV injection of ALDH1-high PDX84 or ALDH1-low PDX84 cells demonstrated significantly more lung lesions in the ALDH1-high group than the ALDH1-low group. The number of ALDH1-high derived lung metastases was significantly suppressed by Tegavivint. Taken together, our preclinical findings demonstrate that Tegavivint has promising therapeutic potential for primary and metastatic OS through the blockade of Wnt signaling /ALDH1 axis. Citation Format: Motonari Nomura, Nino C. Rainusso, Ruolan Han, Jeff Larson, Ryan L. Shuck, Lyazat Kurenbekova, Jason T. Yustein. Tegavivint suppresses progression and metastasis of osteosarcoma via blockade of Wnt signaling/ALDH1 axis: Preclinical study of a novel Wnt/β-catenin pathway inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3186.