Abstract P1-02-11: Clinical utility of serial monitoring of circulating tumor DNA (ctDNA)in patients with neoadjuvant chemotherapy (NAC) for locally advanced breast cancer (LABC)

Abstract Introduction: Circulating tumor DNA (ctDNA) is a new biomarker which could guide further treatment. Characterization of tumor mutation profiles is required for informed choice of therapy, given that biological agents target specific pathways and effectiveness may be modulated by specific mutations. It would have clinical utility for neoadjuvant setting also. Thus, we assess the potency of ctDNA to predict tumor response to neoadjuvant chemotherapy(NAC) in locally advanced breast cancer(LABC). Methods: We performed targeted deep sequencing of 30 plasma DNAs and 10 matched germline DNAs from 10 LABC patients. Serial plasma DNAs were collected at diagnosis, after 1st NAC and curative surgery. For the target enrichment, we designed RNA baits covering a total of ~202kb regions of human genome including a total of 83 cancer-related genes. We constructed the sequencing libraries according to the optimized protocol that we recently reported and sequenced on Illumina HiSeq2500 aiming a mean sequencing depth of ~10,000. After excluding unmapped reads, PCR duplicates and off-target reads, the coverage depths for plasma DNA and germline DNA samples were 2,627x and 4,833x on average, respectively. NAC response was measured by residual cancer burden(RCB) score, calculated as a continuous index combining pathologic measurements of primary tumor and nodal metastases for prediction of distant relapse-free survival. Results: We analyzed ctDNA and primary tumor tissues from 10 patients with LABC scheduled NAC followed by operation in Samsung Medical Center. Of ten LABCs, one excluded from analysis because of angiosarcoma of breast. Five samples were triple-negative breast cancers (BCs), 2 were HER2 positive BCs and others were ER positive BCs. In tumor response, 1 patient had pathologic complete response (pCR), 1 had RCB class I, 4 and 3 patients did RCB class II and III. Of 83 genes, in analysis of ctDNA at BC diagnosis, we found 2 to 6 mutations in each samples and 3 mutations were detected averagely. Most common mutation was TP53 (6 patients), followed by PIK3CA mutation. By measuring these mutations in serial ctDNA, we found that ctDNA had disappeared after first cycle of NAC in patient with pCR. In two patients with RCB class I, ctDNA had decreased by more than 10 percent (the level of ctDNA(pg/ml): 455.9 to 30.4, 5.8 to 0.0) of primary plasma sample after first NAC. Two patients increased level of ctDNA had tumor response with RCB class III and one patient had distant tumor recurrence within 3 months after curative surgery. However, correlation between the level of ctDNA and initial stage was not observed. Patient No.Initial stageSurgical stageRCB scoreRCB classct DNA at diagnosis (pg/5ml)ctDNA after 1st NAG (pg/5ml)Tumor recurrence12A11.3331455.930.4No22B00pCR446.60.0No33B2A1.31515.80.0No42A12.132246.255.4No52B11.7972107.811.6No63B3A4.09033401.15075.5Yes73A2B3.92235088.68536.7No Conclusions: This preliminary result suggests that serial monitoring of ctDNA would be a potiential surrogate marker to predict tumor response and recurrence during NAC in LABC patients. Further results with long-term outcomes are warranted. Citation Format: Kim J-Y, Park D, Jung HH, Bae SY, Yu JH, Lee SK, Kim SW, Lee JE, Nam SJ, Ahn JS, Im Y-H, Park YH. Clinical utility of serial monitoring of circulating tumor DNA (ctDNA)in patients with neoadjuvant chemotherapy (NAC) for locally advanced breast cancer (LABC) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-02-11.