4059 Background: BOLD-100 is a first-in-class ruthenium-based anticancer agent in Phase 2 clinical development for the treatment of advanced gastrointestinal (GI) cancers in combination with FOLFOX. BOLD-100 demonstrated synergy in established preclinical models in combination with various anticancer therapies, particularly in treatment resistant cell lines. Methods: This is a prospective, Phase 2 study. Advanced gastric cancer (GC) patients received BOLD-100 (625 mg/m2) with FOLFOX on day 1 of each 14-day cycle until progressive disease or unacceptable toxicity. The primary objective was to evaluate progression free survival (PFS), overall survival (OS), overall response rate (ORR), and disease control rate (DCR) of BOLD-100+FOLFOX. Bayesian modelling was used to continually reassess these endpoints, the posterior probability of superiority to an historical landmark for each endpoint. Results: As of 31 Dec 2023, 21 pts with advanced gastric cancer, median age 61 years [range 35, 84] were treated. All patients had stage IV disease and ECOG ≤ 1. Patients had a median of 4 prior systemic therapies [0, 7], 1 with no prior therapy, 2 had 2 prior therapies, 5 with 3 prior therapies, and 13 patients with 4 or more prior therapies. 20/21 patients received prior platinum with 18/21 receiving prior FOLFOX/CAPOX. While on study, pts received a median of 6 cycles BOLD-100 + FOLFOX [range 1-27]. Median PFS and OS was 4.3 [95% credible interval (CI) 2.8, 7.1] months and 7.9 [CI 4.8, 15] months, respectively. ORR was 11% [CI 2, 31] and DCR was 72% [49, 89] in the 18 evaluable patients. Two pts achieved a partial response, 4 pts had target tumor reductions, and 11 pts had stable disease. Treatment was well tolerated. 19 pts had ≥1 treatment-related adverse events (AEs), most commonly neutrophil count decreased (n = 7, 33%), nausea (n = 6, 29%), and peripheral sensory neuropathy (n = 4, 19%). Most AEs were grade (G) 1-2. 7 patients (33%) had G3/4 neutrophil count decreased. Conclusions: BOLD-100 plus FOLFOX is an active, well-tolerated treatment regimen in the heavily pre-treated advanced GC. The reported mPFS, mOS, ORR and DCR data in this analysis shows promising clinical activity. The combination of BOLD-100 with FOLFOX is worthy of further study. Clinical trial information: NCT04421820 .