Background: There are currently no approved maintenance therapies for metastatic and recurrent endometrial cancer, which carries a dismal long-term prognosis. The PARP inhibitor drug class has demonstrated significant clinical benefit as a maintenance therapy for patients with germline/somatic BRCA mutations or homologous recombination deficiency (HRD) in ovarian cancers. However, BRCA mutations are not common in endometrial cancer. Loss of function of the tumor suppressor gene PTEN has been demonstrated in greater than 80% of endometrioid endometrial cancers ¹. PTEN loss of function is well known to lead to activation of the PI3K-AKT-mTOR pathway but has also been shown to lead to defects in homologous recombination (HR) DNA repair of double strand breaks ². Specifically, PTEN silencing leads to decreased RAD51 foci formation. Based on this mechanism, in vitro data on endometrial cancer cell lines has demonstrated sensitivity to PARP inhibitors. Notably, sensitivity of PARP inhibition in PTEN loss of function is independent of microsatellite instability (MSI) ³. Methods: This is a multi-center phase II, placebo controlled, double-blinded study of the use of rucaparib as maintenance therapy in patients with metastatic or recurrent endometrial cancer. A maximum of 138 patients will be randomized 1:1 to receive rucaparib at starting dose of 600mg BID dosing vs. placebo. Standard dose reductions of known toxicities will be followed based on FDA approved indications of rucaparib. Patients will remain on therapy until progression of disease or toxicity induced discontinuation. Eligible patients will have Stage III/IV or recurrent endometrial cancer and must have received one or two prior lines of chemotherapy. All histologic subtypes, including carcinosarcoma will be included. Treatment arms will be stratified on histology, number of lines of prior therapy, and complete vs partial response to prior line of therapy. The primary endpoint is progression free survival (PFS). Secondary endpoints include overall survival (OS), overall response rate (ORR), and toxicity. To date, 79 patients have been enrolled. 1. Mutter GL, Lin MC, Fitzgerald JT, et al. Altered PTEN expression as a diagnostic marker for the earliest endometrial precancers. J Natl Cancer Inst. 2000;92(11):924-930. 2. Shen WH, Balajee AS, Wang J, et al. Essential role for nuclear PTEN in maintaining chromosomal integrity. Cell. 2007;128(1):157-170. 3. Dedes KJ, Wetterskog D, Mendes-Pereira AM, et al. PTEN deficiency in endometrioid endometrial adenocarcinomas predicts sensitivity to PARP inhibitors. Sci Transl Med. 2010;2(53):53ra75. Clinical trial information: NCT03617679.