5015 Background: Assessment of radiographic response by RECIST in the majority of mCRPC pts is limited by the lack of measurable disease. Changes in CTC counts (CTCs) enumerated using Veridex CellSearch from unfavorable at baseline (BL [≥ 5 cells/7.5 mL]) to favorable (≤ 4) are prognostic for survival, and the test is FDA cleared as an aid in the monitoring of metastatic PC. The CTC cutpoint of ≥ 5 excludes many pts from response assessment. Examining CTCs alone and in combination with other biomarkers as a potential surrogate for clinical benefit was a secondary objective of COU-AA-301, a phase 3 trial of abiraterone acetate + prednisone vs prednisone alone in mCRPC. Methods: Pts from both treatment (tmt) groups with BL CTC > 0 were combined to assess CTC = 0 as a response criterion. Association between CTC response, defined as BL CTC > 0 and post-BL CTC = 0, and clinical outcomes was assessed. CTCs were determined at BL and 4, 8, and 12 wks. Pts with BL CTC > 0 and missing post-tmt CTCs were considered nonresponders. Radiographic response was first assessed at Wk 12. Overall survival (OS) was estimated using the Kaplan-Meier method. Results: Among739 pts with BL CTC > 0, 141 had measurable disease. At Wk 12, 19% (141/739) of pts were CTC responders and 81% (598/739) were CTC nonresponders. Among CTC responders, 74% (104/141) had stable disease or better by RECIST; 26% (37/141) were either not evaluable or had disease progression by RECIST. Median OS was 23.8 and 10.0 mos for CTC responders (n = 141) and nonresponders (n = 598), respectively. Among pts with liver and/or lung metastases, 86% (24/28) of CTC responders at Wk 12 had stable disease or better by RECIST; 14% (4/28) had disease progression by RECIST. Median OS was 19.9 and 7.1 mos for CTC responders (n = 28) and nonresponders (n = 127), respectively. Similar results were observed in Wk 8 CTC responders. Conclusions: For mCRPC pts with BL CTC > 0, CTC response on tmt (CTC = 0) is associated with longer survival and could be considered a response criterion. Additional analysis is required to fully characterize the relationship between CTC = 0 and objective response by RECIST in pts with measurable disease. Clinical trial information: NCT00638690.