Secretory diarrhea is a leading cause of death worldwide among children under the age of five. Its pathology involves cystic fibrosis transmembrane conductance regulator CFTR, which provides a primary pathway for Cl- secretion. Targeting CFTR to prevent/reduce losses of electrolytes and water is a therapeutic strategy. Lysophosphatidic acid (LPA), a naturally occurring phospholipid mediator acts through G-protein-coupled receptors (GPCRs). Our research group has demonstrated that the LPA2 GPCR inhibits CFTR-dependent Cl- transport in intestinal epithelial cells. We have synthesized Rx100, a potent and metabolically stabilized agonist for the LPA2 GPCR. We have demonstrated that Rx100 significantly inhibited cholera toxin (CTX)-induced CFTR-mediated secretory diarrhea in mice. We propose to develop Rx100 into a highly effective anti-diarrheal agent. Aim #1. Optimize the efficacy of Rx100 in inhibiting (CTX)-induced diarrhea 1.1. Determine the oral bioavailability of Rx100 in mice. 1.2. Determine and optimize the anti-diarrheal effect of Rx100 by oral delivery. 1.3. Determine and optimize the anti-diarrheal effect of Rx100 by parenteral delivery Aim #2. Confirm the efficacy of Rx100 in inhibiting CTX-induced diarrhea Aim #3. Evaluate the anti-diarrheal effect of Rx100 in inhibiting Escherichia coli toxin and Citrobacter rodentium infection-induced diarrhea in mice. Methods: (1) Oral Rx100 bioavailability under 1.1 will be determined using tandem mass spectrometry detection (LC/MS/MS). (2) For Rx100's efficacy studies under 1.2 & 1.3, we will use an open-loop mouse model of CTX-induced diarrhea. (3) We will confirm Rx100's efficacy under aim #2 using a closed-loop mouse model of CTX-induced diarrhea. (4) We will also test Rx100's efficacy in a closed-loop model of Escherichia coli heat-stable toxin-induced intestinal fluid secretion under aim #3. We will also test Rx100's efficacy using a mouse model of C. rodentium infection under aim #3. Relevance to Public Health: Rx100 can be developed into a highly effective anti-diarrheal drug for the benefit of human beings.