Significant and sustained antitumor activity in post-docetaxel, castration-resistant prostate cancer with the CYP17 inhibitor abiraterone acetate.

PURPOSE: The principal objective of this trial was to evaluate the antitumor activity of abiraterone acetate, an oral, specific, irreversible inhibitor of CYP17 in docetaxel-treated patients with castration-resistant prostate cancer (CRPC). PATIENTS AND METHODS: In this multicenter, two-stage, phase II study, abiraterone acetate 1,000 mg was administered once daily continuously. The primary end point was achievement of a prostate-specific antigen (PSA) decline of > or = 50% in at least seven of 35 patients. Per an attained phase II design, more than 35 patients could be enrolled if the primary end point was met. Secondary objectives included: PSA declines of > or = 30% and > or = 90%; rate of RECIST (Response Evaluation Criteria in Solid Tumors) responses and duration on study; time to PSA progression; safety and tolerability; and circulating tumor cell (CTC) enumeration. RESULTS: Docetaxel-treated patients with CRPC (N = 47) were enrolled. PSA declines of > or = 30%, > or = 50% and > or = 90% were seen in 68% (32 of 47), 51% (24 of 47), and 15% (seven of 47) of patients, respectively. Partial responses (by RECIST) were reported in eight (27%) of 30 patients with measurable disease. Median time to PSA progression was 169 days (95% CI, 113 to 281 days). The median number of weeks on study was 24, and 12 (25.5%) of 47 patients remained on study > or = 48 weeks. CTCs were enumerated in 34 patients; 27 (79%) of 34 patients had at least five CTCs at baseline. Eleven (41%) of 27 patients had a decline from at least five to less than 5 CTCs, and 18 (67%) of 27 had a > or = 30% decline in CTCs after starting treatment with abiraterone acetate. Abiraterone acetate was well tolerated. CONCLUSION: Abiraterone acetate has significant antitumor activity in post-docetaxel patients with CRPC. Randomized, phase III trials of abiraterone acetate are underway to define the future role of this agent.