Non-invasive cell-free DNA monitoring for predicting the response to neoadjuvant immunotherapy in locally advanced esophageal squamous cell carcinoma.

e16040 Background: The combined modality therapy including neoadjuvant chemoradiotherapy has been selected as the standard care for locally advanced Esophageal squamous cell carcinoma (ESCC). However, the conclusive evidences to support the effectiveness of neoadjuvant immunotherapy for ESCC are rarely discussed. This study aims to investigate the molecular features of cell-free DNA (cfDNA) and the impact of their dynamic changes on the response to neoadjuvant immunotherapy in locally advanced ESCC patients. Methods: Twenty-four eligible locally advanced ESCC patients with received neoadjuvant immunotherapy were enrolled in this study. 7 patients achieved pathological complete regression (pCR) and the other 17 patients had non-pCR. The baseline, second and third blood samples (before neoadjuvant immunotherapy, at one month and two months after neoadjuvant immunotherapy, respectively) were collected from Cancer Institute and Hospital, Chinese Academy of Medical Sciences. Both cfDNA fragments and genomic DNA were extracted for enrichment of a customized panel covering exon regions of 599 genes. The dynamic analysis of gene alterations, which allows the monitoring of the response to neoadjuvant immunotherapy in locally advanced ESCC patients, were conducted based on multiple blood sampling strategy. Results: The most frequently mutated genes among all cases were TP53, KMT2D, CREBBP and KMT2B. MSI scores of both second and third blood samples were remarkablely higher than those in baseline blood samples (p = 0.002, 0.04). On the contrary, the TMB values and mean variant allele frequency (VAF) of second and third blood samples were significantly decreased compared with the baseline blood (TMB, p = 0.04, 0.03; mean VAF, p = 0.01, 0.02). Moreover, a decreasing trend of mean VAF was observed at the time-series blood samples from ESCC patients with pCR, while the mean VAF of ESCC patients with non-pCR declined in the blood samples from the second time point then increased in the blood samples from the thrid time point. Conclusions: The detection of dynamic changes of mean VAF could distinguish the pCR and non-pCR ESCC patients with neoadjuvant immunotherapy. These findings warrant further expanded prospective cohorts to validate.