Background: Biliary tract cancer (BTC) is an aggressive disease with a poor prognosis. Gemcitabine + cisplatin (GemCis) is the accepted global standard of care (SoC), however key cancer resistance mechanisms associated with the transport, activation and breakdown of gemcitabine are known to limit its clinical activity across a range of tumor types, including BTC. NUC-1031 is a phosphoramidate transformation of gemcitabine designed to overcome these key resistance mechanisms and generate much higher levels of the active anti-cancer metabolite, dFdCTP, in cells. Promising efficacy has been observed with single-agent NUC-1031 in a phase I study in advanced solid tumors and in the phase Ib ABC-08 study of NUC-1031 + cisplatin for first-line treatment of advanced BTC. Of 21 patients enrolled in 2 dose cohorts (NUC-1031 625 mg/m² or 725 mg/m² + cisplatin 25 mg/m² on Days 1 and 8 of 21-day cycle), 16 were considered to be efficacy evaluable. In this population, 1 patient had a CR and 6 patients had PRs, resulting in an ORR of 44% (7/16). This compares favorably to the 26% ORR reported for the SoC regimen. In addition, 6 patients had SD, resulting in a DCR of 81% (13/16). The combination was well tolerated with no unexpected AEs or DLTs. The recommended dose of NUC-1031 with cisplatin was 725 mg/m². The tolerability profile, together with encouraging efficacy led to initiation of a global registrational study. Methods: NuTide:121 is a phase III, open-label, randomized study of NUC-1031 + cisplatin vs GemCis for first-line treatment of advanced BTC. Patients ≥18 years with histologically- or cytologically-confirmed BTC (including cholangiocarcinoma, gallbladder, or ampullary cancer), who have had no prior systemic chemotherapy for locally advanced/metastatic disease, are eligible. A total of 828 patients are being randomized (1:1) to either 725 mg/m² NUC-1031 or 1000 mg/m² gemcitabine, both with 25 mg/m² cisplatin, administered on Days 1 and 8 of 21-day cycles. Primary endpoints are OS and ORR. Secondary endpoints include PFS, safety, PK and patient-reported quality of life. In addition to the final analysis, three interim analyses are planned. The study has passed an initial safety analysis, with no protocol changes required. NuTide:121 is being conducted at approximately 130 sites across North America, Europe and Asia Pacific. Clinical trial information: NCT04163900