PROJECT SUMMARY/ABSTRACT Endometriosis is estimated to affect 10% of reproductive age women. It results in considerable morbidity with chronic and debilitating pain, which substantially affect the quality of life of women and their families. Because it is an estrogen-dependent disorder, hormonal therapies are available for the medical treatment of endometriosis. However, these hormonal treatments along with laparoscopic surgery are often of limited efficacy with high recurrence rates, frequent side effects, additional costs, and potential morbidity. Thus, a critical need exists to develop new and effective therapies for endometriosis targeting biologically important mechanisms that underlie pathophysiology of this disease. Endometriosis is known as a chronic inflammatory disease. Aberrant inflammatory dysfunction contributes to development and progression of the disease. We have recently determined a small molecule, niclosamide (Niclo) that could serve as a potential new effective, non-hormonal, fertility-sparing option for the treatment of endometriosis. We have demonstrated that Niclo reduces growth and progression of endometriosis-like lesions (ELL) via inflammatory signaling using a mouse model of endometriosis. Our studies show that large peritoneal MΦ (LPM) are increased in the peritoneal fluid (PF) of ELL mice and invaded into the ELL. Elevated LPM populations in the PF are reduced by Niclo. Niclo also inhibits aberrant inflammation established in the PF, ELL, pelvic organs (uterus and vagina) and dorsal root ganglion (DRG), as well as MΦ infiltration, vascularization and innervation in the ELL. Therefore, we hypothesize that understanding the complex chronic inflammatory mechanisms associated with endometriosis is crucial to develop a new therapeutic strategy and provides the rationale for targeting immune dysfunction. The objective of this application is to test this hypothesis by examining: 1) how loss of LPM impacts pathophysiology of endometriosis, 2) how ELL induction alters the functionally heterogenic population of ELL and peritoneal exudate cells, and how their inflammatory dysfunction is inhibited by Niclo and 3) how inhibitory interactions from Niclo correlate with pain-related symptomology. Niclo is an efficacious Food and Drug Administration-approved drug for the treatment of helminthosis in humans that has been used for decades. Thus, drug re-purposing of Niclo could result in a rapidly-distributable, fertility-sparing and effective non- hormonal therapy that has fewer side effects than current treatments.