e15178 Background: Enumeration of circulating tumor cells (CTCs) using the CellSearch platform has prognostic value in patients with metastatic castration resistant prostate cancer. However, the prognostic value of CTC enumeration in high-risk, localized prostate cancer (HRLPC) remains undefined. Methods: Patients with HRLPC (defined by ≥1 of the following criteria: ≥ cT3a disease, Gleason score 8-10, or PSA > 20 ng/mL) who have chosen prostatectomy for their definitive management were prospectively identified. Patients were consented to receive 4 sequential 30 mL blood draws, each collected in 3 separate 10 mL EDTA tubes. The first 2 blood draws were conducted 2 weeks prior and immediately prior to surgery, while the second 2 blood draws were conducted 4-6 weeks and 3 months following surgery. Within 4 hrs of blood collection, the white blood cell (WBC) fraction was pooled and Ficoll purified. The WBC fraction was transferred to a CellSave tube, and CTCs were enumerated using the CellSearch system. Expression of CD133 and E-cadherin was characterized using the CellSearch system in patients with detectable CTCs. Results: Within 3 monthsof study initiation in Nov 2011, 19 of a planned 37 patients have been accrued. Median age in the cohort was 65 (range, 51-74), and the number of patients with Gleason score 8-10, ≥ cT3a disease, or PSA > 20 ng/mL was 16, 4, and 2, respectively. The majority of patients (17/19, or 89%) had only one high-risk feature. Mean baseline PSA for the cohort was 11.4 (range, 3.4-37). CTCs were detectable in 67% of patients prior to surgery, 27% of patients at 1 month following surgery and 67% of patients at 3 months following surgery. Amongst those patients with detectable CTCs, the median count was 3 (range, 1-7). Further, in these patients, CD133 and E-cadherin were detected in 44% and 46% of specimens assessed, respectively. Full details of these analyses will be provided at the time of the meeting. Conclusions: Using a modified methodology, CTC enumeration using the CellSearch platform is feasible in patients with HRLPC. Interestingly, markers of epithelial-mesenchymal transition and stem cell lineage are detectable in a proportion of patients with localized disease.