Leukocyte trafficking is a therapeutic target in IBD. The integrins alpha(4)beta(7) and alpha(4)beta(1) regulate leukocyte migration into tissues and lymphoid organs. Current strategies rely on biologics, such as mAb, to inhibit leukocyte recruitment. Here we show the in vivo therapeutic effects of a small molecule alpha 4-integrin antagonist (GSK223618A) in a leukocyte-trafficking model and a murine model of colitis. Leukocytes isolated from MLNs of transgenic beta-actinluc(+) mice were injected i.v. into recipients with DSS-induced colitis. Recipient mice were orally gavaged with vehicle or an alpha(4)-integrin antagonist 1 h pre-adoptive transfer, followed by bioluminescence whole body and ex vivo organ imaging 4 h post-transfer. To confirm its therapeutic effect, the alpha(4)-integrin antagonist was given orally twice daily for 6 days to mice with DSS-induced colitis, starting on Day 3. Clinical, macroscopic, and histological signs of inflammation were assessed and gene-expression profiles analyzed. Using bioluminescence imaging, we tracked and quantified leukocyte migration to the inflamed gut and demonstrated its inhibition by a small molecule alpha(4)-integrin antagonist. Additionally, the therapeutic effect of the antagonist was confirmed in DSS-induced colitis in terms of clinical, macroscopic, and histological signs of inflammation. Gene expression analysis suggested enhancement of tissue healing in compound-treated animals. Inhibition of leukocyte trafficking using small molecule integrin antagonists is a promising alternative to large molecule biologics. Furthermore, in vivo bioluminescence imaging is a valuable strategy for preclinical evaluation of potential therapeutics that target leukocyte trafficking in inflammatory diseases. J. Leukoc. Biol. 88: 1271-1278; 2010.Leukocyte trafficking is a therapeutic target in IBD. The integrins alpha(4)beta(7) and alpha(4)beta(1) regulate leukocyte migration into tissues and lymphoid organs. Current strategies rely on biologics, such as mAb, to inhibit leukocyte recruitment. Here we show the in vivo therapeutic effects of a small molecule alpha 4-integrin antagonist (GSK223618A) in a leukocyte-trafficking model and a murine model of colitis. Leukocytes isolated from MLNs of transgenic beta-actinluc(+) mice were injected i.v. into recipients with DSS-induced colitis. Recipient mice were orally gavaged with vehicle or an alpha(4)-integrin antagonist 1 h pre-adoptive transfer, followed by bioluminescence whole body and ex vivo organ imaging 4 h post-transfer. To confirm its therapeutic effect, the alpha(4)-integrin antagonist was given orally twice daily for 6 days to mice with DSS-induced colitis, starting on Day 3. Clinical, macroscopic, and histological signs of inflammation were assessed and gene-expression profiles analyzed. Using bioluminescence imaging, we tracked and quantified leukocyte migration to the inflamed gut and demonstrated its inhibition by a small molecule alpha(4)-integrin antagonist. Additionally, the therapeutic effect of the antagonist was confirmed in DSS-induced colitis in terms of clinical, macroscopic, and histological signs of inflammation. Gene expression analysis suggested enhancement of tissue healing in compound-treated animals. Inhibition of leukocyte trafficking using small molecule integrin antagonists is a promising alternative to large molecule biologics. Furthermore, in vivo bioluminescence imaging is a valuable strategy for preclinical evaluation of potential therapeutics that target leukocyte trafficking in inflammatory diseases. J. Leukoc. Biol. 88: 1271-1278; 2010.