1. Departments of Medicinal Chemistry, Cellular Biochemistry, and Pharmacology, Glaxo Wellcome, 5 Moore Drive, Research Triangle Park, North Carolina 27709, Center for Digestive Diseases, Mayo Clinic, Rochester, Minnesota 55905, and Department of Medicinal Chemistry, Glaxo Wellcome Medicines Research Center, Gunnels Wood Road, Stevenage, Herts, SG1 2NY U.K.