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Treatment patterns and outcomes among patients with microsatellite stable (MSS) advanced endometrial cancer in the United States: Endometrial Cancer Health Outcomes (ECHO) retrospective chart review Study.

5581 Background: Traditional platinum-based systemic chemotherapy continue to be the SOC for aEC in the first line. Phase 2 clinical trials of chemotherapy (GOG 129 series) and some targeted therapies (229 series) for second line advanced endometrial cancer (aEC) have proved disappointing. Recently the treatment landscape for aEC patients has significantly changed with newer targeted therapies focusing on the microsatellite instability (MSI) status of endometrial tumors. The objective of the ECHO study was to describe real-world treatment patterns and outcomes in non-MSI-high or DNA mismatch repair proficient (pMMR) aEC patients in clinical practice in the United States (US) prior to 2019. Methods: The ECHO study is a multicenter, retrospective chart review study in women diagnosed with aEC in the US. Data were obtained from medical records of adult women (≥18 years) diagnosed with advanced or inoperable aEC (stages III or IV) with known MSI status, who had received at least one prior systemic therapy and progressed between July 1, 2016 – June 30, 2019. De-identified patient data extracted by treating oncologists included patient demographics, clinical and treatment characteristics, and clinical outcomes. Kaplan-Meier analyses were performed to estimate real-world progression-free survival (rwPFS) and overall survival (OS). Results: A total of 124 non-MSI-high or pMMR aEC patients who had progression following first line therapy were included in this interim analysis. Average age was 63 years, 62.9% White/Caucasian, 16.9% Hispanic/Latino, and 86% had ECOG ≤1. Metastases were observed in 70% of patients at diagnosis, with the most common metastatic sites being lung (47.6%), liver (32.3%), and distant lymph nodes (29%). As 2nd line therapy, 69% of patients received mono or combination chemotherapy (primarily with doxorubicin), 13% hormonal therapy, and 18% targeted therapy ± chemotherapy. Median duration of 2nd line therapy was 4 months. The majority (86.3%) discontinued 2nd line therapy, with disease progression the most common reason (66.4%). A quarter (26.6%) of patients initiated an additional line of therapy. Median rwPFS from initiation of 2nd line therapy was 5 months (95% confidence interval [CI]: 4-9). Median OS from initiation of 2nd line therapy was 12 months (95%CI: 9-18). Estimated OS rates from initiation of 2nd line therapy at 6, 12, and 24 months were 66%, 47%, and 30%, respectively. Conclusions: In this retrospective, chart review study, patients with non-MSI-high/pMMR aEC in the US who failed at least one systemic therapy had poor prognosis on subsequent therapies. There continues to be a significant unmet need in this group of women. Novel therapies are needed that delay progression and/or improve overall survival and further research is indicated to explore this.