Background: Although 5-fluorouracil-based chemotherapies (5-FU, capecitabine, and floxuridine) remain the cornerstone of combination therapies for colorectal cancer (CRC), their clinical utility is limited by key cancer resistance mechanisms associated with breakdown, transport, and activation. These agents require intracellular conversion to the active metabolite fluorodeoxyuridine-monophosphate (FUDR-MP) before they can exert their core anti-cancer activity through inhibition of the enzyme thymidylate synthase (TS). NUC-3373 is a phosphoramidate transformation of FUDR-MP designed to bypass the key resistance mechanisms associated with 5-FU. Results from the first-in-human study of NUC-3373 (NuTide:301) in patients with advanced solid tumours demonstrated a favourable PK/PD profile for NUC-3373, with a longer plasma t_1/2 (9.7 hours) than 5-FU (8-14 minutes) and much higher levels of the active anti-cancer metabolite, FUDR-MP (Ghazaly et alESMO, 2017). TS is efficiently inhibited and sequestered into TS-ternary complexes (TS-T), depleting the pool of dTMP within 2-4 hours. Methods: NuTide:302 is a two-part, Phase Ib study in patients with CRC who have relapsed after ≥ 2 prior lines of 5-FU-containing therapies. The primary objective is to identify a recommended NUC-3373 dose when administered every 2 weeks in combination with standard agents used in CRC treatment. Secondary objectives include safety, PK/PD, and anti‐tumour activity. In Part 1, approximately 12 patients will be administered NUC-3373 with leucovorin (LV) to determine if LV is beneficial in augmenting the formation of TS-T. If so, it will be administered in Part 2. In Part 2, the following combination agents will be administered with NUC-3373 (±LV): oxaliplatin; oxaliplatin + bevacizumab; oxaliplatin + panitumumab; irinotecan; and irinotecan + cetuximab. Up to 62 patients will be enrolled in cohorts of 3-6, in a modified 3+3 design. Enrollment to Part 1 initiated in September 2018. Clinical trial information: NCT03428958