In a healthy gut, intestinal epithelial cells, with their tight junctions, allow controlled passage of gluten and other fragments. When integrity of this system is compromised, as in celiac disease (CD), an inappropriate immune response to environmental antigens (i.e. gluten) develops. This is called hyper-intestinal permeability or 'leaky gut'. AT-1001 is a protein derived from a Gram-negative bacteria called Vibrio cholera. AT-1001 inhibits leaky gut and appears to have an impact on autoimmunity, making it a potential candidate for the treatment of CD. This double-blind, randomised placebo controlled study aims to determine the safety and tolerability of 12 mg doses of AT-1001 in CD subjects challenged with gluten. Intestinal permeability (IP) (measured urinary lactulose-to-mannitol) is used as a measure of drug efficacy. Male and female in-patients (n=20) aged 18-59y with diagnosed CD, on gluten-free diets for 6 months+ were, on days 1 and 3, treated with 12mg AT-1001 or placebo, followed by a sham gluten challenge, followed by the intestinal permeability measure. On day 2 the sham gluten was replaced by gluten. Puddings (containing sham or gluten) were served to all participants by kitchen staff in singe-blind fashion. For day 2 to day 1 IP change, there was a 70% increase in IP in the placebo group (P = 0.041) and no increase in the drug group, confirming the effects of gluten exposure on IP and the protective effects of AT-1001. Adverse events were mild (n=49) or moderate (n=3). Both groups experienced diarrhoea (an expected symptom in CD patients after exposure to gluten) but the AT-1001 treated volunteers reported less diarrhoea than placebo (P=0.017) suggesting a protective effect for AT-1001. This data demonstrate that 12 mg AT-1001 was generally safe, well tolerated and effectively mitigated gluten-induced GI adverse effects in coeliac patients when compared to placebo. Interferon (IFN)-γ (a marker of immune activity after acute dietary gluten) increased on day 3 in both the placebo group and AT-1001 group, but less so in the AT-1001 group though the difference was not statistically significant (likely due to small sample size). AT-1001 is well tolerated and appears to reduce intestinal barrier dysfunction, proinflammatory cytokine production, and gastrointestinal symptoms in coeliacs after gluten exposure. Larger studies are required to further elucidate the effects of AT-1001.