PSA AS SURROGATE MARKER FOR SURVIVAL

J Clin Oncol. 2013 Oct 7. [Epub ahead of print]

Prostate-Specific Antigen Changes As Surrogate for Overall Survival in Men With Metastatic Castration-Resistant Prostate Cancer Treated With Second-Line Chemotherapy.

Halabi S, Armstrong AJ, Sartor O, de Bono J, Kaplan E, Lin CY, Solomon NC, Small EJ.

Source

Susan Halabi, Andrew J. Armstrong, Ellen Kaplan, Chen-Yen Lin, and Nicole C. Solomon, Duke University, Durham, NC; Oliver Sartor, Tulane University, New Orleans, LA; Johann de Bono, Royal Marsden Hospital, Sutton, United Kingdom; and Eric J. Small, University of California at San Francisco, San Francisco, CA.

Abstract

PURPOSE:

Prostate-specific antigen (PSA) kinetics, and more specifically a ≥ 30% decline in PSA within 3 months after initiation of first-line chemotherapy with docetaxel, are associated with improvement in overall survival (OS) in men with metastatic castration-resistant prostate cancer (mCRPC). The objective of this analysis was to evaluate post-treatment PSA kinetics as surrogates for OS in patients receiving second-line chemotherapy.

PATIENTS AND METHODS:

Data from a phase III trial of patients with mCRPC randomly assigned to cabazitaxel plus prednisone (C + P) or mitoxantrone plus prednisone were used. PSA decline (≥ 30% and ≥ 50%), velocity, and rise within the first 3 months of treatment were evaluated as surrogates for OS. The Prentice criteria, proportion of treatment explained (PTE), and meta-analytic approaches were used as measures of surrogacy.

RESULTS:

The observed hazard ratio (HR) for death for patients treated with C + P was 0.66 (95% CI, 0.55 to 0.79; P < .001). Furthermore, a ≥ 30% decline in PSA was a statistically significant predictor of OS (HR for death, 0.52; 95% CI, 0.43 to 0.64; P < .001). Adjusting for treatment effect, the HR for a ≥ 30% PSA decline was 0.50 (95% CI, 0.40 to 0.62; P < .001), but treatment remained statistically significant, thus failing the third Prentice criterion. The PTE for a ≥ 30% decline in PSA was 0.34 (95% CI, 0.11 to 0.56), indicating a lack of surrogacy for OS. The values of R² were < 1, suggesting that PSA decline was not surrogate for OS.

CONCLUSION:

Surrogacy for any PSA-based end point could not be demonstrated in this analysis. Thus, the benefits of cabazitaxel in mediating a survival benefit are not fully captured by early PSA changes.