Salarius Pharmaceuticals, Inc. announced interim clinical trial results from the company's Phase1/2 trial of its novel oral, reversible, targeted LSD1 inhibitor, seclidemstat, as a treatment for Ewing sarcoma and FET-rearranged sarcomas. These interim results appear to indicate that first- and second-relapse Ewing sarcoma patients treated with seclidemstat in combination with topotecan and cyclophosphamide who achieve disease control may have an increased timeto tumor progression (TTP) compared with treatment of topotecan and cyclophosphamide alone, per the Phase 3 rEECur study described below. The Ewing sarcoma and FET-rearranged sarcoma trial is currently on a partial clinical hold as described below.

As of October 31, 2022, 13 first- and second-relapse Ewing sarcoma patients were evaluable for confirmed disease control rate assessment. Of these 13 patients, five patients (38%) achieved confirmed disease control with no tumor progression observed while treated with seclidemstat in combination with topotecan and cyclophosphamide. Treatment duration for the 13 patients ranged from 0.7 months to 13.8 months.

Five first-relapse Ewing sarcoma patients were treated and three (60%) achieved confirmed disease control, includingone complete response, one partial response and one stable disease at 12.8 months and continuing. First-relapse patientshad a median TTP of 7.4 months and treatment duration ranged from 1.4 months to 13.8 months. At ASCO 2022, Euro Ewing Consortium presented rEECur(2) Phase 3 study results in relapsed/refractory Ewing sarcomapatients that showed median event-free survival of 3.5 months in the topotecan/cyclophosphamide arm (n=73) compared with 5.7 months in the high-dose ifosfamide arm (n=73)(2) .

The rEECur data includes approximately 80% primary refractoror first-relapse. Ewing sarcoma patients after relapse have 5-year overall survival of about 13% and 10-year overall survival of about 9%(3) . As previously reported, on October 18, 2022, enrollment of new patients in the Salarius-sponsored seclidemstat sarcoma clinical trial and the MD Anderson investigator-initiated hematologic clinical trial was voluntarily paused due to a suspected unexpected serious adverse reaction (SUSAR) observed in the sarcoma trial; patients currently enrolled in both studies are able to continue treatment after consulting with their physician.

The U.S. Food and Drug Administration(FDA) subsequently agreed with Salarius' approach and placed the sarcoma trial on partial clinical hold; Salarius is working with the FDA to further analyze the available data with the goal of understanding how best to proceed and restart enrollment. Additional interim trial results include the following: Among the 13 Ewing sarcoma patients treated with seclidemstat, to poteca and cyclophosphamide . There was a 38% confirmed disease control rate and 1.6 months median TTP which ranged from 0.7 months to 13.8 months.

One second relapse patient with confirmed stable disease withdrew from the study with no observed progression at 3.5 months due to a non-study related adverse even. One second relapse patient with a confirmed partial response withdrew from the study with no observed progression at 3.1 months. Among the five first-relapse Ewing sarcoma patients treated, three (60%) achieved confirmed disease control.

One patient achieved a complete response and withdrew from the study at 7.4 months. One patient achieved a partial response with 78% target lesion reduction; this patient subsequently elected radiation treatment as consolidation and withdrew from the study at 13.8 months. One patient achieved stable disease and continues on treatment after 12.8 months.

Single-agent activity has not been observed in the FET-rearranged sarcoma cohort of the trial. More than 85 patients have been treated in either the dose-escalation or the dose-expansion portions of the trial with seclidemstat as a standalone therapy or in combination with standard-of-care chemotherapy Conference call and webcast Salarius plans to hold an investor call in mid-December to discuss these interim resesults and other clinical and preclinical data following presentation at the American Society of Hematology annual meeting by MD Anderson Cancer Center and Salarius.