Background. Cisplatin/gemcitabine is standard 1st-line treatment for patients with advanced biliary tract cancer (ABC). NUC-1031 (phosphoramidate transformation of gemcitabine) is designed to enhance efficacy by maximising intra-tumoural active metabolites. Methods. Patients with untreated ABC, ECOG-PS 0-1 received NUC-1031 (625 or 725mg/m 2) and cisplatin (25mg/m 2) on days 1 and 8, every 21 days. Objectives: safety and maximum tolerated dose (primary); and ORR, pharmacokinetics, PFS and OS (secondary). Results. Twenty-one patients (median age 61 years, n=13 male; 17 cholangiocarcinoma, 2 ampullary and 2 gallbladder cancer) received NUC-1031 625mg/m 2 (n=8 and expansion n=7; median 6 cycles) or 725mg/m 2 (n=6; median 7.5 cycles). Treatment was well tolerated; most common treatment-emergent grade 3-4 adverse-events occurring in >1 patient with 625mg/m 2 NUC-1031 were increased GGT: 40%, ALT: 20%, bilirubin: 13%, neutropenia: 27%, decreased WCC: 20%, thrombocytopenia: 13%, nausea: 13%, diarrhea: 13%, fatigue: 13%, and thrombus: 20% and with 725mg/m 2 , increased GGT: 67% and fatigue: 33%. NUC-1031 725mg/m 2 was selected as the recommended dose with cisplatin in ABC. ORR: 33% (1 CR, 6 PRs), DCR 76%, median PFS 7.2 months (95%-CI 4.3-10.1), median OS 9.6 months (95%-CI 6.7-13.1). The median plasma AUC 0-24 and C max estimates were highest for NUC-1031 (218-324 μg•h/mL and 309-889 μg/mL, respectively) and lowest for dFdC (0.47-1.56 μg•h/ mL and 0.284-0.522 μg/mL, respectively). Conclusions. This is the first study reporting on the combination of NUC-1031 with cisplatin in ABC and demonstrated a favourable safety profile; 725mg/m 2 NUC-1031 in combination with cisplatin is undergoing phase III trial evaluation in ABC.