818P - Monitoring patient-specific mutation in ctDNA and CTC for tumour response evaluation after neoadjuvant chemotherapy in locally advanced gastric cancer

Abstract

BackgroundWe conduct a trial using neoadjuvant chemotherapy (nCRT) and verify the value of ctDNA and CTC as biomarkers for tumor response in the nCRT treatment of locally advance gastric adenocarcinoma. MethodsTwenty milliliters of plasma were collected at 3 points: before nCRT; after 2 cycles of nCRT; and after surgery. Firefly ctDNA NGS assays were used to track ctDNA mutations previously characterized in paired tumor tissue by massively parallel sequencing. Using patient-specific mutation derived from exome sequencing of tumor tissues samples, bespoke amplicon-based sequencing panel were synthesized and used for ctDNA profiling. CTCs in 7ml peripheral blood were separated by a negative enrichment method and identified by FISH using two frequently proliferation chromosome probes, CEP8 and CEP17. Meanwhile, the HER2 expression in CTCs was determined by FISH and immunofluorescence. CEP8+ and/or CEP17+, DAPI+, CD45- cell were justified as CTCs, HER2 FISH+ or HER2 IF+, DAPI+, CD45- cells were justified as HER2 positive CTCs. ResultsIn comparison to basal line value of pre-chemotherapy blood, the ctDNA loading were decreased in post-chemotherapy specimens of 8 patients with TRG1 by histopathological grading or partial and complete response by CT scan. However, the amount of ctDNA in 25 patients diagnosed as TRG 2 or 3 showed minor changes during the neoadjuvant therapy. 22 have integral FISH data (CTCs range from 0-29, average: 4.9, 2.7 and 4.0, positive rate: 82%,73% and 73%), and 12 have integral HER2 data (CTCS range from 0-9, average: 2.9, 2.2 and 2.9, positive rate: 25%, 50% and 33%). The dynamic variations were coincidence to the clinical evaluation. For HER2, we found three positive model in CTCs, single FISH or IF positive or double positive. Double positive HER2-CTCs are found in some patients. By negative enrichment and FISH, CTCs can be detected at a low cutoff of 2 cells in 73%∼82% patients. ConclusionsThe ctDNA and CTC alteration during neoadjuvant therapy are consistent with histopathological grading and response assessmentand and can be surrogate markers to prediction efficacy of nCRT for GC. Clinical trial identificationNCT03425058 11/02/2018. Legal entity responsible for the studyThe author. FundingBeijing Municipal Administration of Hospital Clinical Medicine Development of Special Funding Support (ZYLX201701). DisclosureThe author has declared no conflicts of interest.