Background: 5-FU is a key anti-cancer agent used across a broad range of tumors. The anti-cancer metabolite of 5-FU, fluorodeoxyuridine-monophosphate (FUDR-MP), binds and inhibits thymidylate synthase (TS), disrupting DNA synthesis and repair. 5-FU is often dosed with leucovorin (LV) to enhance the binding of FUDR-MP to TS. NUC-3373 is a targeted inhibitor of TS designed to bypass 5-FU resistance mechanisms associated with transport, activation and breakdown and avoid the generation of toxic metabolites such as FUTP and FBAL. NUC-3373 has a longer plasma t_1/2 (~10 hours) than 5-FU (8-14 minutes), generating substantially higher intracellular levels of FUDR-MP and lower levels of the toxic metabolites FUTP and FBAL. Part 1 interim data from the NuTide:302 study demonstrated NUC-3373’s favorable PK and safety profile was unaffected by LV. Therefore, all subsequent patients in NuTide:302 are receiving NUC-3373 + LV. Here we present the next data series from NuTide:302. Methods: NuTide:302 is a 3-part, Phase Ib study in patients with advanced CRC who have relapsed after ≥2 prior lines of fluoropyrimidine- containing therapies. In Part 1, patients are receiving NUC-3373 with or without LV. In Part 2, NUC-3373 +LV is being administered in dose-escalation cohorts with either oxaliplatin (NUFOX) or irinotecan (NUFIRI). In Part 3, the NUFOX and NUFIRI regimens selected from Part 2 will be combined with biologics targeting VEGF and EGFR pathways. Results:36 patients have been treated in Part 1: 21 received 1500 mg/m² NUC-3373 ± LV q2w; 11 received 1500 mg/m²NUC-3373 + LV q1w; and 4 received 2500 mg/m² NUC-3373 + LV q1w. Clinical activity has been observed including tumor shrinkages and stabilization of disease for up to 5 months following rapid progression (≤2 months) on prior lines of therapy. One fluoropyrimidine-refractory patient demonstrated a 28% reduction in target lesions and achieved a stable disease of 5 months after rapid progression on CAPOX (2 months) and FOLFIRI (1.5 months). Safety data for all patients treated with NUC-3373 ± LV in Part 1 of NuTide:302 is shown below. Updated data on the clinical activity and safety of NUC-3373 will be presented. Clinical trial information: NCT03428958Conclusions:NUC-3373 ± LV has shown clinical activity in heavily pre-treated CRC patients, including tumor shrinkage in a fluoropyrimidine-refractory patient. The safety profile of NUC-3373 ± LV is very encouraging: no neutropenia or hand-foot syndrome of any grade and no diarrhea or mucositis above Grade 2. NUC-3373 +LV is currently being dose escalated further in Part 1 and dosed with either oxaliplatin (NUFOX) or irinotecan (NUFIRI) in Part 2 of NuTide:302.