Objectives: ER-positive ECs are characterized by PI3K and receptor tyrosine kinase (RTK)/RAS/b-catenin pathway alterations in approximately 90% and 80% of cases, respectively. Extensive crosstalk between ER, PI3K, and RTK/RAS/b-catenin pathways leads to both ligand-dependent and independent ER transcriptional activity as well as upregulation of cyclin D1 which, in complex with cyclin-dependent kinases 4 and 6 (CDK4 and CDK6), is a critical regulator of cell cycle progression and a key mediator of resistance to hormonal therapy. Given that retinoblastoma (RB) loss is rare in ER-positive EC, we hypothesized that the combination of the aromatase inhibitor letrozole and CDK4/6 inhibitor abemaciclib would demonstrate promising activity and acceptable toxicity in this setting. Methods: This was an investigator-initiated, single-arm, phase II study of letrozole/abemaciclib in recurrent ER-positive EC (defined as ≥1% of tumor cell nuclei being immunoreactive by immunohistochemistry [IHC]). Eligibility criteria included measurable disease, no limit on prior therapies, and all EC histologies (in the 2nd stage, only endometrioid and carcinosarcomas with endometrioid epithelial components were allowed). Any prior hormonal therapy was allowed. Co-primary endpoints were objective response rate (ORR) by RECIST 1.1 and progression-free survival (PFS) rate at six months. Abemaciclib 150 mg PO twice daily and letrozole 2.5 mg daily were administered until progression or unacceptable toxicity. A two-stage design was employed whereby 16 pts were enrolled in the 1st stage; if there were ≥2 ORRs or ≥2 pts without disease progression or death at six months, accrual would continue to the 2nd stage with an enrollment of 19 additional pts. Overall, if there were ≥4 ORRs or ≥8 pts without disease progression or death at six months, letrozole/abemaciclib would be considered worthy of further study. Results: As of May 21, 2021, 28 pts-initiated therapy. Median follow-up was 10.2 months, median number of prior lines was 3 (range: 1-8) and 13 (46.4%) pts had prior hormonal therapy. Seven pts exhibited ORR (seven PRs, five confirmed as of the cut-off date, ORR 25% [95% CI: 10.7-44.9]), all in pts with endometrioid adenocarcinoma, including four pts with prior hormonal therapy. Responses were observed regardless of grade, mismatch repair and progesterone receptor (PR) status. Twelve pts (42.9%) exhibited stable disease as best response. PFS at six months was 42.9% (95% CI: 21%-63.1%), and median PFS was 5.3 months (95% CI: 2-16.5). Most common G3+ treatment-related toxicities were neutropenia (n=5, 17.6%) and anemia (n=3, 10.7%). Sixteen (57.1%) pts had at least one dose reduction of abemaciclib; two (7.1%) pts discontinued therapy because of toxicity. IHC and targeted next-generation sequencing will be reported at the meeting. Conclusions: Letrozole/abemaciclib met the predetermined criteria to be considered worthy of further evaluation in this population of recurrent ER-positive EC. Objectives: ER-positive ECs are characterized by PI3K and receptor tyrosine kinase (RTK)/RAS/b-catenin pathway alterations in approximately 90% and 80% of cases, respectively. Extensive crosstalk between ER, PI3K, and RTK/RAS/b-catenin pathways leads to both ligand-dependent and independent ER transcriptional activity as well as upregulation of cyclin D1 which, in complex with cyclin-dependent kinases 4 and 6 (CDK4 and CDK6), is a critical regulator of cell cycle progression and a key mediator of resistance to hormonal therapy. Given that retinoblastoma (RB) loss is rare in ER-positive EC, we hypothesized that the combination of the aromatase inhibitor letrozole and CDK4/6 inhibitor abemaciclib would demonstrate promising activity and acceptable toxicity in this setting. Methods: This was an investigator-initiated, single-arm, phase II study of letrozole/abemaciclib in recurrent ER-positive EC (defined as ≥1% of tumor cell nuclei being immunoreactive by immunohistochemistry [IHC]). Eligibility criteria included measurable disease, no limit on prior therapies, and all EC histologies (in the 2nd stage, only endometrioid and carcinosarcomas with endometrioid epithelial components were allowed). Any prior hormonal therapy was allowed. Co-primary endpoints were objective response rate (ORR) by RECIST 1.1 and progression-free survival (PFS) rate at six months. Abemaciclib 150 mg PO twice daily and letrozole 2.5 mg daily were administered until progression or unacceptable toxicity. A two-stage design was employed whereby 16 pts were enrolled in the 1st stage; if there were ≥2 ORRs or ≥2 pts without disease progression or death at six months, accrual would continue to the 2nd stage with an enrollment of 19 additional pts. Overall, if there were ≥4 ORRs or ≥8 pts without disease progression or death at six months, letrozole/abemaciclib would be considered worthy of further study. Results: As of May 21, 2021, 28 pts-initiated therapy. Median follow-up was 10.2 months, median number of prior lines was 3 (range: 1-8) and 13 (46.4%) pts had prior hormonal therapy. Seven pts exhibited ORR (seven PRs, five confirmed as of the cut-off date, ORR 25% [95% CI: 10.7-44.9]), all in pts with endometrioid adenocarcinoma, including four pts with prior hormonal therapy. Responses were observed regardless of grade, mismatch repair and progesterone receptor (PR) status. Twelve pts (42.9%) exhibited stable disease as best response. PFS at six months was 42.9% (95% CI: 21%-63.1%), and median PFS was 5.3 months (95% CI: 2-16.5). Most common G3+ treatment-related toxicities were neutropenia (n=5, 17.6%) and anemia (n=3, 10.7%). Sixteen (57.1%) pts had at least one dose reduction of abemaciclib; two (7.1%) pts discontinued therapy because of toxicity. IHC and targeted next-generation sequencing will be reported at the meeting. Conclusions: Letrozole/abemaciclib met the predetermined criteria to be considered worthy of further evaluation in this population of recurrent ER-positive EC.