TPS544 Background: The UK ABC-02 study established cisplatin and gemcitabine as the reference regimen for first-line treatment of patients (pts) with advanced biliary tract cancers (BTCs) (median overall survival (OS): 11.7 months). No clinical studies since ABC-02 have reported an extension in OS, and therefore effective new agents/combinations are required. NUC-1031 was designed to improve on gemcitabine’s relatively poor efficacy by overcoming its associated key cancer resistance mechanisms, through cellular uptake independent of nucleoside transporters, activation independent of deoxycytidine kinase and protection from cytidine deaminase inactivation, resulting in over 200x the intracellular levels of the anti-cancer metabolite, dFdCTP, greater stability and reduction in the generation of toxic metabolites. NUC-1031 showed activity as monotherapy in a phase I/II study in 7 pts with BTC, refractory to all standard treatments (Blagden et al ASCO 2015; abstract 2514). Methods: ABC-08 is a multi-centre phase Ib study of NUC-1031 combined with cisplatin in pts with non-resectable or recurrent/metastatic cholangiocarcinoma, gallbladder or ampullary carcinoma, aged ≥18 years with an ECOG performance status of 0-1, who have received no prior systemic therapy. The starting dose for NUC-1031 is 625 mg/m² administered IV on days 1 and 8 in combination with cisplatin (standard dose of 25 mg/m²) (21 day schedule). The dose will be escalated sequentially in cohorts of 3-6 pts using an accelerated titration procedure (725mg/m², 825mg/m², 925mg/m²). Treatment will continue until intolerable toxicity/progressive disease. The primary endpoints are safety and RP2D. Secondary endpoints are progression-free survival, OS, response rate and pharmacokinetic endpoints, including assessments of multiple plasma and intracellular analytes: NUC-1031, cisplatin, dFdC, dFdCMP, dFdCDP, dFdCTP and dFdU, and will be correlated with safety profile and clinical activity. Planned accrual is 15-24 pts over 2 years. Cohort 1 has been completed without dose-limiting toxicities. Enrolment to cohort 2 is on-going. Clinical trial information: NCT02351765.