IntroductionAn array of chronic inflammatory diseases, including Crohn disease, psoriasis, rheumatoid arthritis, and multiple sclerosis, cause widespread and severe health problems. These and other immune diseases can be considered to have a predominantly T helper cell type 1 (Th1) bias. Although injectable biologic treatments such as antibodies and soluble protein that block TNF-␣ have provided significant clinical benefit, there is still a high unmet medical need, particularly for a safe targeted drug that can be taken orally.Interleukin-12 (IL-12) is a heterodimeric cytokine (p70) composed of 2 independently regulated protein subunits, p35 and p40. IL-12 plays a central role in the immune response by bridging innate resistance and antigen-specific adaptive immunity. 1 It is produced from phagocytic cells and antigen-presenting cells, in particular macrophages and dendritic cells, upon stimulation with bacteria, bacterial products such as lipopolysaccharide (LPS), and intracellular parasites. 2,3 The well-documented biologic functions of IL-12 are the induction of interferon-␥ (IFN-␥) expression from T and natural killer (NK) cells 4,5 and the differentiation of naive T cells toward a Th1-cell type. 6,7 IL-12 also appears to be critical in the expansion and maintenance of the Th1 phenotype. [8][9][10] IL-23, a more recently discovered member of the IL-12 family, also promotes Th1 response, but has distinct functions from IL-12. IL-23 is required for the generation of effector memory T cells. 11 IL-23 is also needed for the generation of IL-17-producing T cells, which play a significant role in the inflammatory response. 12 Through these activities, IL-23 plays a critical role in chronic inflammatory diseases.Although the inflammatory effector function of Th1 is essential for the clearance of intracellular pathogens, the excessive production of proinflammatory cytokines leads to serious tissue damage typical of organ-specific autoimmunity, 13,14 including type 1 diabetes, 15 multiple sclerosis, 16,17 rheumatoid arthritis, 18,19 inflammatory bowel disease, 20 psoriasis, 21 and sepsis. 22 IL-12/IL-23 has been validated as an attractive clinical target by a number of studies. Mice lacking the gene encoding the p40 subunit shared by IL-12 and IL-23 or lacking the IL-23-specific subunit p19 have proven the integral role of IL-12/IL-23 to the pathogenesis of these disorders, implying that blockade of the cytokines could be effective in the treatment of a variety of autoimmune diseases. 21,[23][24][25][26][27][28] Monoclonal antibodies to the IL-12/23 p40 subunit have been shown to be effective in human clinical trials in patients with Crohn disease and psoriasis. 29,30 While antibodies against IL-12/IL-23 could provide significant medical benefit, a small-molecule IL-12/IL-23 inhibitor that could be administered orally would be highly desirable. Although some compounds weakly or nonselectively inhibit IL-12, there are no potent and selective small-molecule IL-12 inhibitors. To find an effective inhibitor against the...