CAST: A retrospective analysis of cabazitaxel and abiraterone acetate sequential treatment in patients with metastatic castrate-resistant prostate cancer previously treated with docetaxel.

Cabazitaxel and abiraterone have both received approval for treating metastatic castrate-resistant prostate cancer (mCRPC) patients after first-line docetaxel therapy. In the cabazitaxel and abiraterone sequential treatment (CAST) study, the clinical outcome of docetaxel-treated mCRPC patients treated sequentially with both cabazitaxel and abiraterone was studied. Data were collected retrospectively from mCRPC patients at 12 hospitals across the Netherlands who initiated cabazitaxel and/or abiraterone before December 2012. Primary outcome measure was overall survival (OS); secondary measures were progression-free survival (PFS), biochemical PFS, and best clinical and PSA response. Hospital admission data during treatment were collected, as well as toxicities resulting in treatment discontinuation or patient death. Sixty-three and 69 patients received Cab→Abi (cabazitaxel prior to abiraterone) and Abi→Cab before July 10th, 2013, respectively. Median OS was 19.1 months and 17.0 months in Cab→Abi and Abi→Cab treated patients, respectively (p = 0.369). Median PFS and biochemical PFS were significantly longer in Cab→Abi treated patients: 8.1 versus 6.5 (p = 0.050) and 9.5 versus 7.7 months (p = 0.024), respectively. Although partial responses to cabazitaxel occurred in both groups, Abi→Cab treated patients had a significantly decreased antitumor response from cabazitaxel than Cab→Abi treated patients (median PFS 5.0 versus 2.6 months, p < 0.001). Minor differences in toxicities were observed based on therapy sequence; generally, toxicity from cabazitaxel could be severe, while abiraterone toxicity was milder. This retrospective analysis indicates that primary progression on cabazitaxel or abiraterone did not preclude a response to the other agent in mCRPC patients. However, tumor response of both agents, particularly cabazitaxel, was lower when administered as higher-line therapy in the selected study population.