Abstract 2646: The liquid biopsy network in advanced breast cancer (ABC): Benchmarking the prognostic role of circulating tumor DNA (ctDNA) to the goal standard circulating tumor cells (CTCs) enumeration
Authors: Gerratana Lorenzo , Davis Andrew A. , Rossi Giovanna , Strickland Kimberly , Mu Zhaomei , Flaum Lisa , Gradishar William J. , Santa-Maria Cesar , Yang Hushan , Cristofanilli Massimo 
Published: August 2018

Abstract
Background: Liquid biopsy technologies, including CTCs and ctDNA, may provide prognostic information for advanced breast cancer (ABC) patients. We evaluated quantification and molecular characterization of ctDNA-based testing for prognostic stratification and predictive analysis.
Methods: This retrospective study analyzed a cohort of 91 patients (pts) with ABC, which was longitudinally characterized for CTCs and ctDNA. CTCs were isolated through the CellSearch immunomagnetic kit (Menarini Silicon Biosystems, PA), and ctDNA was analyzed using the Guardant360 NGS-based assay (Guardant Health, CA). A receiver operating characteristic (ROC) analysis was performed to identify the best ctDNA percentage (%ctDNA) cut-off capable to separate pts with ≥ 5 CTC/7.5 ml of blood. Survival analyses for both CTC and %ctDNA were investigated by log-rank test. Associations between %ctDNA stratification and detectable ctDNA mutations were explored through Fisher's exact test, and multiplicity was corrected through Bonferroni-Holm method (Q value).
Results: 40 pts (44%) were diagnosed with luminal-like ABC, 25 pts, (27%) with HER2-positive ABC, and 26 pts (29%) with triple negative ABC. 84 pts (92%) had stage IV disease and 26 pts (31%) were de novo metastatic. 22 pts (35%) had a CTC count ≥ 5, while among pts with CTC &lt; 5, 21 (34%) had no detectable CTCs. The median %ctDNA was 1.65 (IQR: 0.3 - 9.8), and 11 pts had no detectable ctDNA. A cut-off of 5.7 was selected (AUC: 0.7533) by ROC analysis. A significant association for both PFS and OS was observed for the selected threshold (log-rank test P=0.024 and P=0.0015, respectively). Pts with a %ctDNA &lt; 5.7 had a median PFS and OS of 6.2 ms (IQR: 2.1 - 12.1) and 21.7 ms (IQR: 12.82 - not reached), respectively. For pts with a %ctDNA ≥ 5.7, median PFS and OS were 3 ms (IQR: 2.1 - 7) and 7.2 ms (IQR: 7.2 - not reached), respectively. The prognostic impact of CTCs was confirmed for PFS and OS (P=0.024 and P&lt;0.0001, respectively). Notably, the 5.7 %ctDNA cut-off retained its significance also in pts without detectable CTCs (P=0.0209), and subgroup analysis highlighted a significantly different prognostic impact across breast cancer subtypes (P for interaction = 0.029). Interestingly, pts with ≥ 5.7 %ctDNA had a higher frequency of ctDNA detected MET and PIK3CA mutations (P=0.0007 and P=0.003 respectively; Q=0.0126 and Q=0.051, respectively) and higher number of detectable mutations (P&lt;0.0001).
Conclusions: The CTCs-derived %ctDNA cut-off is capable of prognostic stratification in ABC. Interestingly, ctDNA can stratify pts also without detectable CTCs, providing valuable information on prognosis, mutational tumor burden and disease monitoring and characterization. Further prospective studies are needed to validate these findings.
Citation Format: Lorenzo Gerratana, Andrew A. Davis, Giovanna Rossi, Kimberly Strickland, Zhaomei Mu, Lisa Flaum, William J. Gradishar, Cesar Santa-Maria, Hushan Yang, Massimo Cristofanilli. The liquid biopsy network in advanced breast cancer (ABC): Benchmarking the prognostic role of circulating tumor DNA (ctDNA) to the goal standard circulating tumor cells (CTCs) enumeration [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2646.