Abstract 3969: CTCs and CTC clusters in breast cancer patient-derived xenograft models
Authors: Sahay Debashish , Ramirez Arturo B. , Bhat Raksha R. , Dobrolecki Lacey E. , Nardone Agostina , Lewis Michael T. , Osborne C. Kent , Rimawi Mothaffar , Stilwell Jackie L. , Kaldjian Eric P. , Schiff Rachel , Trivedi Meghana V. 
Published: July 2016

Abstract
Breast cancer (BC) patient-derived xenograft (PDX) models represent a continuous and reproducible source of circulating tumor cells (CTCs). Using various BC PDX models, we describe the utility of CTCs and CTC clusters in detecting tumor-specific mutations and our preliminary results in understanding their predictive value for treatment response and long-term outcomes. CTCs were detected in 300-450 μl of blood of PDX-bearing mice using the RareCyte technology adapted for small blood volume. CTCs were isolated without cell surface marker-based enrichment and identified them as DAPI+, human Cytokeratin (CK)+, and mouse CD45-. Collective expression of cell surface markers (EpCAM, EGFR, and HER2) was assessed using a cocktail of target-specific antibodies in CTCs and primary PDX tumors. Individual CTCs and tumor cells from primary and metastatic tumors were isolated using CytePicker® for single cell analysis of tumor-specific mutations. Single CTCs (1-41 per mouse) and CTC clusters (1-2 per mouse) were detected in the blood of one ER+/PR+/HER2- (BCM-4888) and two triple-negative BC (TNBC, BCM-4272 and BCM-3887) PDX models. The PIK3CA T1035A mutation found in primary tumors of BCM-4888 was also detected in isolated CTCs and PDX primary and metastatic tumor cells. As a proof-of-principle experiment, we have evaluated numbers of single CTCs and CTC clusters at baseline and after treatment with 4 weekly cycles of vehicle (N = 5-6) or chemotherapy regimens (N = 2-3) [docetaxel or carboplatin or their combination] in TNBC PDX models BCM-4272 and BCM-3887. Preliminary analysis from these studies suggests dynamic and differential effects of chemotherapy regimens on single CTCs and CTC clusters, potentially reflecting the genetic characteristics of tumors and their unique response to the selected chemotherapy agents. Ongoing experiments in additional mice and PDX models will determine the predictive role of CTCs and CTC clusters in treatment response and long-term outcomes such as recurrence-free survival. In conclusion, we have demonstrated that RareCyte technology detects CTCs from small volumes of blood without the use of cell surface marker-based enrichment method. Furthermore, CTCs and CTC clusters can be used to assess the presence of tumor-specific mutations. Ongoing studies will fully reveal the potential of CTCs and CTC clusters as surrogate markers of treatment response and outcomes within PDX models.
Citation Format: Debashish Sahay, Arturo B. Ramirez, Raksha R. Bhat, Lacey E. Dobrolecki, Agostina Nardone, Michael T. Lewis, C. Kent Osborne, Mothaffar Rimawi, Jackie L. Stilwell, Eric P. Kaldjian, Rachel Schiff, Meghana V. Trivedi. CTCs and CTC clusters in breast cancer patient-derived xenograft models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3969.