Characterization of ESR1 mutations in endometrial and ovarian cancers.

5598 Background: Low-grade ovarian (OC) and endometrial (EC) cancers frequently express estrogen receptor (ERα, encoded by ESR1), and are considered hormonally responsive tumors. The use of endocrine therapy in the advanced and recurrent disease setting is common. In breast cancer, ESR1 mutations ( ESR1mt) convey resistance to endocrine therapy. Mechanisms for endocrine failure in gynecologic cancers are not well understood. In this study, we aim to evaluate the prevalence of ESR1mt, and associated characteristics, in EC and OC. Methods: 17,666 EC and 21,879 OC samples were analyzed by NGS of DNA (NextSeq, 592 genes and NovaSeq, WES) and RNA (NovaSeq, WTS) (Caris Life Sciences, Phx, AZ). Tumor mutational burden (TMB) totaled all somatic mutations (mt) per tumor (TMB-H: > 10 mt/MB). Statistical significance determined using chi-square and Mann-Whitney U test and adjusted for multiple comparisons (q<0.05). Real-world overall survival (rwOS) and time on treat (ToT) obtained from insurance claims data and calculated from first treatment to last contact or last of treatment, respectively. Results: L536, Y537, and D538 were the most common ESR1 hotspot mutations. ESR1mt (EC: 2.38%, OC: 0.22%) were more prevalent in endometrioid EC tumors (4.2%) compared to other EC histologies (1.7%)(p<0.05). Compared to high-grade serous (HGS) OC tumors (0.07%), ESR1mtmore prevalent in in low-grade serous (1.08%) and endometrioid (1.48%) (p<0.05). ESR1mt was associated with higher ESR1 RNA expression (EC: 1.19-fc, OC: 1-.28-fc), ER+ (EC: 94.1% vs 63%, OC: 80.9% vs 49.3%), and PR+ (EC: 90.8% vs 46.7%, OC: 36% vs 24.5%) (q<0.05). ESR1mt were associated with decreased TP53 mt (EC: 10.7% vs 50.9%, OC: 16.7% vs 78.3%), but increased PI3K pathway alterations (EC: PTEN mt: 70.4% vs 40%, PIK3R1 mt: 34.1% vs 19.6%, PIK3CA mt: 47.9% vs 36.6%, AKT1 mt: 11% vs 2.8%; OC: PTEN mt: 36.2% vs 4.%, PIK3CA mt: 32.7% vs 8.97%, AKT1 mt: 8.33% vs 0.62%), CTNNB1 mt (EC: 52.5% vs 13.9%; OC: 39.6% vs 3.09%), ARID1A mt (EC: 56% vs 32.2%; OC:28.6% vs 8.04%) and TGFBR1 mt (EC: 55.6% vs 9.16%) (q<0.05). ESR1mt was associated with TMB high (30.7% vs 21.6%) and high median IFN score (EC: -0.42 vs -0.35, OC: -0.48 vs -0.27) (q<0.05). ESR1mt enriched in patients with prior AI treatment (EC: 6.38% vs 2.65%, q=0.02; OC: 1.26% vs 0.18%), especially in EOC (14.7% vs 3.37%) (q<0.05). Conclusions: ESR1mt were more common in EC and were enriched in the endometrioid subtype, and in general associated with increased molecular alterations but a more cold immune microenvironment. Hotspot mutations known to confer endocrine resistance were most common and enrichment was observed in cases previously exposed to AIs, suggesting this may be a mechanism of resistance to endocrine therapy for some gynecologic malignancies.