Serial Circulating Tumor DNA Sequencing to Monitor Response and Define Acquired Resistance to Letrozole/Abemaciclib in Endometrial Cancer.

Objective: In a phase II study, letrozole/abemaciclib demonstrated an objective response rate of 30% and a median progression-free survival (PFS) of 9.1 months in recurrent estrogen receptor-positive endometrial cancer (EC). While tissue-based tumor profiling revealed several mechanistically relevant candidate baseline genomic predictors of response, 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, C2D1, C3D1, C8D1, the time of objective response, and the time of progression. Samples were analyzed using the Guardant Reveal assay to assess methylation-based tumor fraction (TF), with the Guardant360 assay providing genotyping of >700 genes in samples with detectable ctDNA. Treatment response was assessed using a measure of the relative change in TF pre- versus on-treatment.

Results: A total of 99 of 102 (97%) samples from 28 patients were successfully analyzed. Patients with above median baseline TF exhibited worse median PFS (2.0 months v 16.5 months, P < .005, hazard ratio [HR], 24.1) and worse overall survival (OS) (10.7 months v not yet reached, P < .005, HR, 14.8). Patients with molecular response (MR) after the first or second cycle of letrozole/abemaciclib therapy had significantly better median PFS and OS regardless of the cutoff used for definition of MR. ctDNA analysis of postprogression specimens identified several acquired genomic alterations associated with resistance to letrozole/abemaciclib therapy in more than half of the patients, including PI3K pathway, receptor tyrosine kinase (FGFR1,2 and ERBB2 alterations), cell cycle pathway (RB1 and CCNE1 alterations), and ESR1 and MAPK pathway alterations. Two of the three patients with mismatch repair-deficient ECs acquired ESR1 mutations at the time of progression.

Conclusions: Baseline and on-treatment ctDNA dynamics may provide an early indication of benefit from letrozole/abemaciclib in EC. ctDNA at the time of progression may identify resistance alterations that may inform subsequent therapy.