Introduction We investigated whether circulating cell-free (cf)DNA sequencing analysis is associated with response to immune checkpoint inhibition (ICI) in patients with endometrial cancer (EC) enrolled on a phase II trial (NCT03241745).

Methods Patients with MSI-H/dMMR/hypermutated EC and ≥1 prior cytotoxic lines received nivolumab until progression/toxicity. Pretreatment EC and matched blood-derived DNA underwent whole-exome sequencing; cfDNA at baseline, 2 weeks, and then every 6 weeks underwent high-depth sequencing using MSK-ACCESS (129 genes). cfDNA allele fraction changes were correlated with RECIST responses and AquariusNet-generated tumor volume.

Results At baseline, median circulating tumor (ct)DNA fraction in cfDNA was 13.1% (range, 0.0-86.0%), and high-depth sequencing captured somatic mutations (median, 29.5; range: 2-75). Liquid biopsy MSI status matched tumor MSI status defined by whole-exome sequencing. Median ctDNA fraction percent difference from week 0 to week 8 was -51.6% (IQR: -6.2%, -85.1%) in 5 patients with partial response/stable disease versus +17.7% (IQR: -6.6%, +356.6%) in 5 patients with progressive disease (P=0.08). ctDNA fraction changes correlated with tumor volume changes (R²=0.684, P<0.001). ctDNA predicted ICI response by week 2 in all patients but 1, who had a stable ctDNA fraction but increasing allele frequency of B2M alterations, associated with ICI resistance. Among 2 patients with long-term nivolumab response, ctDNA fraction remained low up to week 140.

Conclusion/Implications In patients with advanced MSI-H/dMMR/hypermutated EC, changes in ctDNA are detected as early as 2 weeks after ICI initiation, can anticipate radiologic progression/response, and can monitor durable responders. Future studies may use ctDNA to assess mechanisms of ICI resistance and offer adaptive therapy intervention.