Background: Lysine-specific demethylase 1 (LSD1) is an epigenetic enzyme that is aberrantly expressed in many solid tumors. High levels of LSD1 expression are often correlated with poor patient prognosis due to LSD1’s role in cancer cell proliferation, metastasis, and chemoresistance. Seclidemstat is a novel, selective, reversible and oral LSD1 inhibitor capable of inhibiting both LSD1’s catalytic and scaffolding functions. We report preliminary efficacy in AST from an ongoing phase 1 trial. Methods: SALA-003-AC19 (NCT03895684) is a phase 1 trial of single agent SP-2577 in pts with AST. All pts had progressive disease (PD) at time of study entry. Pts received oral SP-2577 twice a day under fasting condition, in 28-day cycles (C). The primary objective is safety and tolerability. Secondary objectives are to determine maximum-tolerated dose, preliminary efficacy, pharmacokinetics, and pharmacodynamics. Results: As of December 30, 2020, 19 pts with AST (10 sarcoma, 2 prostate, 2 ovarian, 2 pancreatic, 1 renal, 1 cervical, 1 breast) were enrolled. Pts received escalating doses of SP-2577 from 150 to 600 mg BID and the dose escalation is ongoing. The median age was 63 years (range, 21–79). 42% were male, and pts had received a median of 4 (range, 1–8) prior systemic therapies. The most common (>5%) grade 3 treatment-related adverse events were GI related including diarrhea (5.3%) and abdominal pain (5.3%). No grade 4 events were reported and there were no treatment-related deaths. Safety data will be presented after completion of phase 1. Three pts had at least one dose reduction. Among the 13 pts who were evaluable for response at end of C2, 7 pts (54%) had best response of stable disease (SD) with median time to progression (TTP) of 4.3 months (range, 2.1–11.5). Four of the 7 pts had genetic abnormalities that may demonstrate increased sensitization to SP-2577 according to preclinical studies. Characteristics of 7 pts with SD at C2 and beyond are shown in the table. Conclusions: Seclidemstat has shown activity among advanced sarcoma pts with a manageable safety profile. The dose escalation is ongoing and preliminary clinical data supports further exploration in FET-translocated sarcoma as single agent and in combination therapy. Clinical trial information: NCT03895684

¹Underwent wedge resection of target lung lesion after C2 and other residual lung lesions did not progress until 11.5 months. ²No PD, discontinued SP-2577 due to adverse events.