e14567 Background: Although the prognostic relevance of circulating tumor cells (CTCs) is well established in metastatic colorectal cancer (mCRC), their clinical use is currently constrained by the phenotypic definition of such cells that is largely limited to epithelial markers. Our objective in this study is to characterize the prognostic significance of a larger group of CTCs including those with characteristics of epithelial-mesenchymal transition (EMT) or cancer stem cells (CSCs) in a cohort of pre-treated mCRC patients (pts). Methods: Peripheral blood was collected in AdnaCollect preservation tubes from pts for immunomagnetic tumor cell enrichment using the AdnaTest ColonCancer Select kit and multiplex qRT-PCR analyses of tumor-associated epithelial (CEA, EPCAM, EGFR), cancer stem cell (ALDH1) and mesenchymal (TWIST1, AKT2, PIK3CA) transcripts. Results: In the initial phase, 41 previously treated mCRC pts (median prior therapies 3) were enrolled over a 4 month period and were followed prospectively. In this cohort, there was a low rate of CEA, EPCAM, and EGFR expression in the CTCs (12%, 2%, and 0%, respectively), consistent with loss of epithelial markers. Conversely, ALDH1, an established CSC marker was expressed in 30% of pts, with only one pt’s CTCs expressing both ALDH1 and CEA expression. ALDH1 expression was associated with very poor prognosis (HR 24.6, p=0.037) and 57% 60-day mortality, compared to 0% 60-day mortality for pts with no detectable ALDH1 expression. The mesenchymal markers of TWIST1, AKT2, and PIK3CA were strongly correlated with ALDH1 expression (Spearman r=0.55, 0.59, 0.63, respectively, p < 0.01 for each). Conclusions: The majority of CTCs detected in refractory mCRC pts have lost epithelial markers. Acquisition of markers associated with EMT or CSCs appear to be associated with poor prognosis, and may provide important insights into better prognostication and also biology of refractory colorectal cancer. Further enrolment into this study is ongoing and updated results will be presented.