Clinical activity of abiraterone acetate in patients with castration-resistant prostate cancer who received prior ketoconazole therapy.

Abstract

99 Background: Studies have shown activity of Abiraterone acetate (AA) in patients (pts) with castration-resistant prostate cancer (CRPC) who have received prior ketoconazole. Prostate-specific antigen (PSA) response to AA in relation to previous PSA response to ketoconazole was investigated. Methods: A retrospective analysis was conducted to determine the clinical activity of AA in men with CRPC who have received prior ketoconazole therapy at our institution. Time to PSA progression (PSA TTP) was defined by PCWG2 criteria, a PSA reduction of 50% or more was considered as PSA response. Results: Thirty four pts were identified. Nineteen pts (56%) had previous PSA responses on ketoconazole, with a median PSA TTP of 11 months (95% confidence interval [CI] 6.8-19.9). Subsequently, 11 of 34 (33%) of pts achieved a PSA response on AA, with a median PSA TTP of 6 months (95% CI 4.9-9.5). Among the 19 pts having a PSA response on ketoconazole, only four (21%) pts subsequently had PSA response to AA. Two of these pts had transient PSA response with PSA TTP less than 3 months on kKetoconazole, one patient discontinued Ketoconazole due to side effects, one patient had intermittent non-castrate testosterone levels. In contrast, 7 of 15 (46.7%) pts without prior PSA response to ketoconazole subsequently achieved PSA response on AA (p=0.11). Of note, PSA reduction of less than 50% on AA was observed in 9 of 34 pts (26%), which was associated with a longer median PSA TTP compared to pts who had PSA-progressive disease (5.9 months [95% CI 3.5-7.3] vs.1.5 months [95% CI 1.0-3.5], p=0.028). Five of these nine patients had a prior PSA response to ketoconazole but required drug discontinuation for reasons other than disease progression. Conclusions: PSA response to prior ketoconazole therapy is associated with lower PSA response rate to subsequent AA. The observation suggests that there is a biologically distinct subset of patients who are ketoconazole-resistant but abiraterone-sensitive, the underlying mechanism needs to be further investigated.