Background: CRC is characterized by hyperactivation of the Wnt pathway where DKK1 plays a critical regulatory role. DKN-01 is an IgG4 monoclonal antibody that potently neutralizes DKK1. DKN-01 has immunomodulatory activity and stimulates a pro-inflammatory tumor microenvironment. Elevated DKK1 expression has been shown to correlate with fluorouracil (5FU) resistance in CRC tumors. 5FU-based combination therapies have demonstrated promising clinical activity in combination with DKN-01 in gastric cancer. In pre-clinical models of 5FU-resistant CRC, DKN-01 showed potent anti-tumor effects. Methods: DeFianCe (NCT05480306) is a Phase 2 randomized, open-label, two-part, multicenter study to evaluate efficacy and safety of DKN-01 plus FOLFIRI/FOLFOX and bevacizumab versus standard of care (SOC) [FOLFIRI/FOLFOX and bevacizumab] as 2L treatment of advanced MSS CRC patients (pts). The primary endpoint of the single arm Part A (DKN-01 + SOC) was safety and tolerability with secondary endpoints including overall response rate (ORR), progression-free survival (PFS) and overall survival. Retrospective analysis of tumoral DKK1 mRNA expression was measured centrally by in situ hybridization. Results: Thirty-three pts enrolled in Part A between Sept 2022-April 2023. As of 21 Aug 2023, median age 56 years (35, 84); 20 males. 29 pts had tumors with evaluable baseline DKK1 expression; 55% were DKK1-expressing (≥1% tumor cells). All patients had received prior 5FU-based therapies, 30 pts (91%) with oxaliplatin in combination and 17 pts (52%) had prior bevacizumab. 24 pts (73%) had RAS mutations (22 KRAS, 2 NRAS) and 23 pts (70%) had liver metastasis. 30 pts (91%) received DKN-01 + bevacizumab + FOLFIRI. 17 pts (52%) had ≥grade 3 treatment related adverse events (TRAE) with neutrophil count decreased, anemia and fatigue representing the most common. No pt discontinued DKN-01 due to a TRAE. Of the 27 response evaluable pts the ORR was 30% and the DCR was 93%: 8 PR, 17 SD and 2 PD. The median PFS has not been reached. Conclusions: DKN-01 in combination with SOC was well tolerated with promising preliminary activity (ORR 30% and DCR 93%) in the single arm Part A. PFS has not been reached and survival follow up is ongoing. Part B randomized expansion phase has begun and is enrolling an additional 130 pts. Tumoral DKK1 expression and correlation with clinical outcomes will be evaluated as an exploratory efficacy endpoint. Clinical trial information: NCT05480306.