Circulating tumor cell (CTC) number as a response endpoint in metastatic castration resistant (mCRPC) compared with PSA across five randomized phase 3 trials.

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Abstract
5007 Background: Radiographic progression and overall survival (OS) are the traditional clinical benefit measures for mCRPC trials. Reliable indicators of response that occur early are a critical unmet need in practice and clinical research. We explored a week 13 CTC and prostate-specific antigen (PSA) endpoint relative to baseline in 5 prospective randomized phase 3 registration trials that enrolled 5912 pts. OS was the primary endpoint. Methods: CTC number (CellSearch) and PSA values in patients who survived at least 13 weeks were evaluated as response endpoints in COU-AA-301, AFFIRM, ELM-PC-5, ELM-PC-4 and COMET-1. Pts with missing values at week 13 were considered non-responders. The endpoints considered are shown in Table 1. Results: The discriminatory strength of the response endpoints with respect to OS was estimated using the weighted c-index. To summarize discrimination results for each measure, the mean and the standard deviation based on the weighted c-indices from the 5 studies were computed. Conclusions: CTC0 and CTC conversion endpoints had the highest discriminatory power for OS relative to the % decline in CTC or PSA endpoints. The percent of pts eligible and evaluable for the CTC0 endpoint was significantly higher than the conversion endpoint, 75% vs. 51%, respectively. These two absolute measures of CTC can be considered meaningful response indicators in mCRPC clinical trials. [Table: see text] [Table: see text]