264 Background: The novel drug EDO-S7.1 (CAP7.1) is converted to active etoposide by CES allowing administration of higher doses, reducing resistance, and permitting treatment of advanced tumors. Methods: The primary objective was to compare disease control rate (DCR) in 22 pts with unresectable BTC randomized 1:1 to 3-week cycles of EDO-S7.1 (200 or 150mg/m2; iv) given on days (d) 1–5, or best supportive care (BSC) until progression (assessed every 4 weeks). Secondary objectives were progression-free survival (PFS), time to treatment failure (TTF), overall survival (OS) and safety. BSC pts could crossover to EDO-S7.1 upon progression. Results: DCR favored EDO-S7.1 (55.6% [CI 21.2, 86.3] vs BSC (20.0% [2.5, 55.6]; treatment difference –12.80, 72.39). More EDO-S7.1 treated pts achieved sustainable stable disease (SD) or partial response (PR) vs BSC. Progressive disease occurred in 40% EDO-S7.1 vs 70% BSC pts. Three pts (30%) who progressed on BSC achieved SD following crossover to EDO-S7.1, and one pt (10%) PR (total: 40.0% [12.2, 73.8]) vs two pts with SD following BSC (20.0% [2.5, 55.6]; treatment difference –0.17, 0.57; p=0.0786). Median PFS was 103d with EDO-S7.1 vs 39d with BSC (p=0.006), and median TTF 92d vs 39d, respectively (p=0.006). Median OS was 227d with EDO-S7.1 vs 162d with BSC (p=0.088), and estimated 1-year OS 44% vs 11.3%, respectively. EDO-S7.1 had significant efficacy in pts with metastatic disease (p=0.029) or without prior surgery (p=0.032). Seven pts were eligible for exploratory analysis of tumor CES via immunohistochemistry. 4/7 pts had CES+ tumors and longer median PFS (195d [163, 233]) and OS (302d [214, 838]) vs pts with CES– tumors (46d [46, 83] and 83d [43, 319], respectively). The most common drug-related adverse events (G3–4) (%) were leukopenia 65 (26.1), neutropenia 78.3 (56.5), thrombocytopenia 56.5 (17.4), anemia 52.2 (17.4), alopecia 34.4 (0), fatigue 26.1 (0), and abdominal pain 30.4 (0). Conclusions: EDO-S7.1 demonstrated efficacy in pts with advanced BTC and may provide a new biomarker-guided therapeutic option with stratification by intra-tumor CES assessment. These findings will be explored in a larger patient cohort. Clinical trial information: NCT02094560.