Development of novel GI-centric prostaglandin E₂ receptor type 4 (EP4) agonist prodrugs as treatment for ulcerative colitis and other intestinal inflammatory diseases

Prostaglandin E₂ receptor type 4 (EP4) agonists have been shown to be effective in treating experimental ulcerative colitis (UC) in animals and in human clinical trials, but their development has been impeded by unacceptable systemic side effects. In this study, a series of methylene phosphate prodrugs of a highly potent and selective prostaglandin EP4 receptor agonist were designed to target and remain localized in the gastrointestinal (GI) tract after either oral or rectal instillation. The prodrugs were designed to be converted to liberate active EP4 agonist by intestinal alkaline phosphate (IAP), a ubiquitous enzyme found at the luminal of the intestinal wall thus exposing the colon epithelial barrier while reducing systemic exposure to the active agonist. The prodrugs were shown to hydrolyze in plasma and after contact with GI tissue slices from ileum and colon. When optimized prodrugs were dosed orally, systemic peak exposure to the active agonist was not reduced, presumably due to IAP activity in the duodenum and small intestine. However, when dosed rectally, the prodrugs gave much reduced levels of EP4 agonist in the blood. An optimized prodrug was shown to be retained in the colon, when compared with free agonist after rectal administration in healthy mice and to be efficacious in a model of UC (the DSS mouse model). Plasma exposure to the active agonist was also much reduced in the mouse model of UC after 4 days of rectal dosing but after 7 days, one DSS mouse showed elevated systemic levels of the free agonist in the blood. The concept of efficacy and intestinal retention of an EP4 agonist-methylene phosphate prodrug was proven for rectal instillation but in DSS treated mice, severe disease appears to compromise the epithelia barrier sufficiently to allow some absorption of the prodrug to occur. Thus, further optimization of these prodrugs is required before a candidate can be selected for development for treating severe ulcerative colitis.