Phase II study of SM-88 in Ewing's and other sarcomas

e23505 Background: Sarcomas are rare heterogeneous malignancies. Once recurrent, cure is uncommon. SM-88 (racemetyrosine) is an amino acid analogue with no known cross resistance to typical sarcoma regimens. Based on previous anecdotal experience in Ewing’s (EWS) we initiated a Phase 2 trial (HopES) in EWS and other sarcomas (Ss) after >1 prior systemic therapy. We now report preliminary data after having met prespecified continuation criteria. Methods: Open label prospective trial in 2 separate cohorts (EWS and Ss) of oral SM-88 used with MPS conditioning agents (SM-88 920 mg, methoxsalen 10 mg, phenytoin 50 mg, sirolimus 0.5 mg) all daily until progression. Results: As of Feb 5 2021, 10 pts with incurable sarcomas were enrolled; 4 had high risk but stable EWS. Average age 43.9 yrs (13–77); 70% white; 20% female. Median number of prior regimens 4 (1–9); 70% received prior RT; 50% prior surgery. Median time from initial diagnosis 39.5 months with 50% T2 (40% unknown), 30% M1 (30% unknown). Prespecified futility stopping was exceeded (i.e., >1 of first 5 subjects/cohort) upon achieving clinical benefit in each. Stable disease was achieved in 75.0% (6/8 with available data). Time on treatment (TTx) exceeded last known TTx in 80% (95% CI 44.4–97.5). Median SM-88 TTx was 4.9 vs 2.9 mo for prior TTx (logrank HR 0.53; p=0.12). One EWS subject had unresectable disease that became resectable, was completely resected, and remained disease-free for ≥ 6 months. Prior to SM-88, longest TTx was 12 mo (on IT*) and shortest TTx 1 mo (on IEV*) vs SM-88 TTx of 11.9 mo. An angiosarcoma subject had a 21% reduction in the sum of all target lesions and exceeded all prior TTx (including 8 mo on Ap/N* with 12+ mo duration of treatment of SM-88). There were no serious drug-related AEs. ECOG performance remained stable for all. Conclusions: SM-88 has exceeded pre-specified futility in both cohorts (EWS maintenance and Ss salvage). HopES continues to enroll toward the planned total of 12 subjects to more precisely define its benefit in this ultra-orphan, extremely recalcitrant disease. This trial now confirms the previously reported clinical utility of oral SM-88 in EWS and other high-risk sarcomas. Based on durable response (>6mo), SD and prolonged TTx, SM-88 warrants additional investigation in this setting. Clinical trial information: NCT03778996. [Table: see text]