Background: Endometrial cancer is the most common gynecologic cancer and is primarily treated with chemotherapy. Many endometrial cancers though express hormone receptors and may respond to hormonal therapy, a potentially attractive non-chemotherapy alternative in such a population of women. The aim of OATH is to investigate the potential of dual hormonal blockade through the combination of antiprogesterone and anti-estrogen therapy via the administration of onapristone extended release (ONA) plus anastrozole (ANA), respectively. OATH is an open-label, multicenter, investigator-initiated, non-randomized, phase 2 study to evaluate the safety and efficacy of ONA + ANA in patients (pts) with endometrial cancer (EC). ClinicalTrials.gov Identifier: NCT04719273. Methods: Pts received ONA 50 mg twice daily plus ANA 1 mg once daily and were treated until progressive disease (PD) or unacceptable toxicity. Co-primary endpoints were 4-month progression-free survival (PFS) rate and overall response rate (ORR). A sample size of 25 pts will achieve 80% power to detect an improvement in 4-month PFS estimates from 25% (historical) to 52% and an improvement in response rate from 15% (historical) to 37%, using two-sided exact test at 5% significance level. Secondary endpoints include PFS, disease control rate (DCR), and safety. Results: As of January 20, 2023, 14 pts were enrolled. Median age was 67 years (44-80). ECOG performance status was 0 (36%), and 1 (64%). Number of prior chemotherapy regimens was 0 (1 pt,7%), 1 (9 pts, 64%) and 2 (4 pts, 29%). Seven (50%) pts received radiotherapy and 3 (21%) pts received prior checkpoint inhibitor therapy. Median treatment duration was 4.2 (1.0-19.9) months, and 8 pts remain on treatment. Adverse events were mainly grade 1 and 2. The most common treatment-related adverse events (AEs) and abnormal laboratory values were hot flashes (36%), alanine transferase (ALT) increased (29%), aspartate aminotransferase (AST) increased (29%), nausea (29%), and diarrhea (21%). No treatment-related serious AE, deaths on treatment, or treatment-related drug discontinuations were reported. The 4-month PFS Kaplan-Meier rate was 63.5%. Among 13 pts who had at least one tumor assessment after baseline, the best overall response was: 1 complete response, 1 partial response (response rate: 15.4%), 9 (69%) stable disease, and 2 (15.4%) progressions. Updated results will be presented. Conclusions: Preliminary results of dual hormonal blockade using ONA and ANA in heavily pretreated pts with EC suggest that this combination ONA + ANA has promising activity and is well tolerated. Clinical trial information: NCT04719273.