Introduction: GB004 is a gut-targeted small molecule inhibitor of prolyl hydroxylases (PHDs), key enzymes involved in degradation of hypoxia inducible factor (HIF-1α), a transcription factor involved in protective cellular responses at the intersection of hypoxia and inflammation. In animal models of inflammatory bowel disease (IBD), PHD inhibition stabilizes HIF-1α to increase expression of molecules involved in supporting epithelial integrity and barrier function, promoting mucosal healing (MH). GB004 is being developed as an oral treatment for IBD. In healthy volunteers, GB004 showed a gut-targeted pharmacokinetic (PK) profile. Aims & Methods: We aimed to evaluate the safety, tolerability, and PK and explore the pharmacodynamics and clinical activity of GB004 in adults with active ulcerative colitis (UC). In this double-blind, placebo-controlled, Phase 1b study, subjects were randomized 2:1 to GB004 120 mg solution or placebo once daily for 28 days. Eligible subjects had histologic activity defined by Robarts Histopathology Index (RHI) ≥ 4 with neutrophils in the epithelium, Mayo endoscopic sub-score (MES) ≥ 1, and blood in the stool, despite treatment with 5-ASA. RHI and MES were assessed by blinded central reading. Colonic biopsies of the sigmoid and rectum were obtained at screening and Day 28. MH was defined as MES improvement (0 or 1; 0 if screening 1) and histologic remission (RHI ≤3 with lamina propria neutrophils and neutrophils in epithelium sub-scores of 0). Results: Thirty-four subjects were randomized to GB004 (n=23) or placebo (n=11). Mean age (45.4 years), total Mayo score (7.5), sigmoid MES (2.2), and sigmoid RHI (14.1) at baseline were consistent with moderately active UC. The most frequent adverse events in GB004 were nausea (22%) and dysgeusia (13%), all of which were mild in severity except for one report of moderate nausea. One GB004-treated subject discontinued due to lack of efficacy (worsening of UC unrelated to GB004). No effect on plasma erythropoietin or vascular endothelial growth factor was observed. GB004 concentrations were higher in colon tissue vs. blood on Day 28 biopsy. Preliminary microarray-based mRNA profiling of epithelial gut biopsies showed increased expression of both TJP1 and CLDN1, genes consistent with enhanced epithelial barrier function, and other trends associated with HIF-1α stabilization in GB004 vs. placebo. Initial results from myeloperoxidase staining suggest a reduction in gut epithelial neutrophil activity vs. placebo. The proportion of subjects achieving clinical activity outcomes of interest at Day 28 were higher for GB004 versus placebo: histologic remission in sigmoid or rectum (43% vs. 18%); MH in sigmoid or rectum (17% vs. 0%); clinical response (30% vs 18%); and rectal bleeding improvement (62% vs 45%), with percent decrease from baseline of 65.8% vs. 22.7%. One GB004 patient achieved clinical remission at Day 28 vs. no placebo patients. Conclusion: GB004 was well tolerated over 28 days and demonstrated a gut-targeted PK profile. GB004 showed gene expression reflective of target engagement and enhancement of barrier function in colonic biopsies. Signals of clinical activity were observed for GB004 vs placebo, including histologic endpoints. A phase 2 clinical trial in UC will be initiated in 2020 with alternative GB004 oral formulations that are currently in development. Sponsored by GB004, Inc., a wholly owned subsidiary of Gossamer Bio, Inc. Disclosure: WS, consultant fees, stock/stock options from Gossamer Bio, Inc.; BF, consultant fees from Gossamer Bio, Inc.; KR, AO, CVB, GJO, JF, RA, and BGL are employees of Gossamer Bio, Inc.; SD, AJ, and BRB have nothing relevant to disclose.