LAUSR

5579 Background: Hormone receptor (HR) positivity has been reported as a good prognostic indicator in endometrial cancer. This study investigated estrogen receptor (ER) and progesterone receptor (PR) positivity as indicators of platinum response and improved survival in uterine serous carcinoma (USC), and determined differences in molecular profiles between these tumors and hormone receptor negative tumors. Methods: Tumor profiling was done with immunohistochemistry (IHC), next generation sequencing (NGS), and whole transcriptome sequencing (WTS). PD-L1 expression was determined by IHC using SP-142 (cut-off >1%). Microsatellite instability (MSI) status was evaluated with IHC and NGS, and tumor mutational burden (TMB) by totaling somatic mutations per tumor (high if > 10 mutations/ MB). Immune-cell fraction was determined with QuantiSeq. We used insurance claims data to calculate Kaplan-Meier estimates for overall survival (OS). Statistical significance was determined with chi-square and Wilcoxon rank sum test and adjusted for multiple comparisons. Results: 2806 USC tumors were available for molecular profiling with 717 HR+/966 HR-, 1559 ER+/1059 ER- and 805 PR+/1809 PR-. Median OS for HR+ patients was longer than patients who were HR- regardless of treatment (1152 v 797 days; hazard ratio 0.68, 95% CI 0.58-0.80, p < 0.01). This OS benefit of HR positivity remained for patients receiving platinum therapy (1134 v 889 days; hazard ratio 0.75, 95% CI 0.58-0.98, p < 0.01). HR+ status trended towards improved OS in those treated with hormone therapy (407 vs 771 days, hazard ratio 0.78, 95% CI 0.59-1.02, p = 0.07). In addition to increased androgen receptor expression (54.2 vs 6.2%, p = < 0.001), PTEN mutations were more common in the HR+ group (10.3 vs 5.1%, p = 0.016). This resulted in in more frequent alterations of the PI3K pathway when compared to HR- tumors 64.5 vs 52.2%, p = < 0.001). PD-L1, TMB, and MSI status were similar between the two cohorts. Checkpoint inhibitor gene expression was notable for higher IDO expression in the HR+ group, but lower CD80, CD86, HAVCR2, IFNG, and PDC1 expression. The immune micro-environment was notable for less B-cells and M1 macrophages, but more M2 macrophages. Breaking the ER and PR positive cohort down to individual expression of each gene resulted in similar OS and molecular findings. Conclusions: HR positivity is associated with improved survival in allcomers with USC and those treated with platinum therapy, and there was a trend to improved OS with hormone therapy. More data is needed to determine if HR status is a prognostic marker for IO treatment response. This cohort had a distinct molecular profile compared to HR- tumors.