Serial circulating tumor DNA (ctDNA) sequencing to monitor response and define acquired resistance to letrozole/abemaciclib in endometrial cancer (EC).

5592 Background: In a phase 2 study, letrozole/abemaciclib demonstrated an objective response rate (ORR) of 30% and median progression free survival (PFS) of 9.1 months in recurrent ER positive EC. While tissue-based tumor profiling revealed several mechanistically relevant candidate baseline genomic predictors of response ( CTNNB1, KRAS,and CDKN2A mutations) and absence of response ( TP53mutations), circulating tumor DNA (ctDNA) is a less invasive alternative to monitor therapeutic efficacy and define acquired resistance. Methods: Serial plasma specimens were obtained at baseline, cycle 2 day 1 (C2D1), C3D1, C8D1, time of objective response, and at the time of progression from participants who consented for this optional collection. Samples were analyzed using the Guardant Infinity assay, which includes genotyping of > 750 genes and methylation-based tumor fraction (TF). Treatment response was assessed using the Guardant methylation-informed Molecular Response algorithm. Patients with 50% or more decrease in ctDNA levels or with absolute low ctDNA levels were defined as achieving Molecular Response (MR). Comparison of survival curves was performed using the logrank test. Results: 99/102(97%) samples from 28 patients were successfully analyzed. Three samples were excluded from analysis for not meeting sequencing quality control thresholds. Detection of ctDNA at baseline, measured via methylation-based TF, was associated with worse median PFS, (ctDNA detected (n = 18): 3.5 months vs ctDNA undetected (n = 8): 16.5 months, p = 0.006, HR = 12.5) and overall survival (OS), (ctDNA detected: 11.6 months vs ctDNA undetected: not yet reached, p = 0.008, HR = 4.7e7). Patients who achieved MR after the first or second cycle of letrozole/abemaciclib therapy exhibited better median PFS (MR(n = 13): 12.8 vs not-MR(n = 10): 3.5 months, p < 0.001, HR = 0.1) and were more likely to demonstrate clinical benefit (objective response and/or progression free survival for ≥6 months) from this regimen (MR: 84.6% vs not-MR: 10.0%, p < 0.001, odds ratio (OR) = 49.5). ctDNA analysis of post-progression specimens identified several acquired genomic alterations associated with resistance to combined aromatase/CDK4&6 inhibitor therapy, such as ESR1 mutations/amplification, RB1 mutations, ERBB2 amplification/mutations and CCNE1 amplification. Of note, 2 of the 3 patients with mismatch repair deficient (dMMR) ECs acquired ESR1 mutations at the time of progression; both patients had exhibited durable objective responses to therapy. Conclusions: Baseline and on-treatment ctDNA dynamics may provide an early indication of long-term outcomes and benefit from letrozole/abemaciclib in EC. ctDNA at the time of progression may identify genomic alterations of acquired resistance (such as ESR1, RB1, CCNE1,and ERBB2 alterations) that may guide selection of subsequent therapy.