Background: Unselected pts with EC who progressed on prior chemotherapy have a poor prognosis with limited treatment options. SG is a novel antibody-drug conjugate that targets Trop-2, a cell surface glycoprotein highly expressed in many epithelial tumors. It is conjugated to deliver SN-38, the active metabolite of irinotecan, via a proprietary hydrolyzable linker. Preclinical studies show SG has activity against chemotherapy-resistant EC and significant bystander effect against EC with heterogenous Trop-2 expression (Perrone E. Mol Oncol. 2019). Methods: The phase I/II basket study (NCT01631552) evaluated pts unselected for Trop-2 with advanced solid tumors who received intravenous SG (days 1 and 8 of 21-day cycles), until progression or unacceptable toxicity. CT/MRI scans were obtained at 8-week intervals for response assessment by RECIST 1.1. We report results for mEC pts who progressed after ≥1 prior systemic therapy and were treated with SG 10 mg/kg. Endpoints include safety, objective response rate (ORR), clinical benefit rate (CBR), duration of response (DOR), progression-free survival (PFS), and overall survival (OS). Results: 18 mEC pts (all women; 17 white and 1 black; median age 69 years [range, 41–76]) had a median 3.5 (range 2–6) prior lines of therapy. All pts received prior treatment with platinum therapies. At a median follow-up of 12.7 months, the ORR (95% CI) was 22.2% (6.4–47.6), with 4 partial responses. CBR (95% CI) was 44.4% (21.5–69.2), with 8 of 18 pts having either an objective response or stable disease ≥6 months. The DOR of responders ranged from 9.1 to 26.6 months, with 2 of 4 responders having a duration of ≥18 months. Median PFS (95% CI) was 3.2 months (1.9–9.4), and median OS (95% CI) was 11.9 months (4.7–not calculable). Key grade ≥3 TRAEs in the overall basket study safety population (n=495) included neutropenia (28%), neutrophil count decrease (14%), anemia (10%), diarrhea (8%), fatigue (6%), and febrile neutropenia (5%). A similar safety profile was seen in the mEC cohort. Conclusions: Median OS in unselected pts with mEC who progressed on prior platinum therapy is ~10 months with an ORR of ~10%. SG monotherapy showed clinical activity in pts with relapsed/refractory mEC, consistent with previous preclinical findings, and support further clinical investigation (NCT04251416). The phase II TROPiCS-03 (NCT03964727) study in pts with metastatic solid tumors selected based on elevated Trop-2 expression by a validated IHC assay will also provide further insights. Clinical trial information: NCT01631552