Identification of a predictive factor of metastatic castration-resistant prostate cancer patients’ response to alternative antiandrogen therapy with flutamide.

321 Background: Recently, new androgen pathway inhibitors, abiraterone and enzalutamide, are demonstrated to improve overall survival for metastatic castration-resistant prostate cancer (mCRPC). In Japan, alternative antiandrogen (AA) as second-line hormonal therapy for mCRPC that relapses after initial hormone therapy have been commonly used before new androgen pathway inhibitors. In this study, we attempted to identify the predictive factors for efficacy of AA as second-line hormone therapy. Methods: We identified consecutive 65 mCRPC patients treated with AA as second-line hormonal therapy. All patients were treated with maximum androgen blockade (MAB) initially and evaluated antiandrogen withdrawal syndrome after relapse. We analyzed the correlations between progression-free survival (PFS) of AA and clinicopathological characteristics, including patients’ age, initial PSA levels, PSA levels at flutamide induction, Gleason scores, T stage, N stage, extent of disease (EOD) classifications on bone scan, and the previous duration of prostate cancer sensitivity to MAB. Results: The median duration of prostate cancer sensitivity to MAB was 11.3 months (range: 1.5-53.0 months). In multivariate analysis, four significant risk factors for poor PFS were identified; initial PSA levels ( > 263 ng/mL vs ≤ 263; HR 0.53, p = 0.038), N stage (1 vs 0; HR 3.00, p = 0.001), EOD classifications (3-4 vs 1-2; HR 2.50, p = 0.007), and the previous duration of prostate cancer sensitivity to MAB ( < 12 months vs ≥ 12; HR 2.16, p = 0.026). We stratified the patients into two cohorts with low risk (0-2 risk factor present) and high risk (3-4 risk factors present). We found a significant difference in PFS among risk groups (median PFS 7.3 months vs 1.5, p < 0.000). Conclusions: Initial PSA, N stage, EOD classifications on bone scan, and the previous duration of prostate cancer sensitivity to MAB were the significant predictive factors for efficacy of AA as second-line hormone therapy in patients with mCRPC. These findings might support that decision-making of when to start the new AR pathway inhibitors.