5PSQ-074 Activity of enzalutamide after abiraterone in castration-resistant prostate cancer

BackgroundThere is only limited information about the sequential use of abiraterone acetate (AA) and enzalutamide (ENZ) in metastatic castration-resistant prostate cancer (mCRPC) patients. Patients who receive AA or ENZ as first-line therapy and subsequently become resistant have only a response rate of 15% to 30% to the alternative agent as second-line treatment. That finding clearly shows that cross-resistance occurs between ENZ and AA.PurposeTo evaluate the effectiveness of ENZ after failure of AA in patients with mCRPC.Material and methodsRetrospective study including all patients with mCRPC having sequential therapy with AA and ENZ from May 2012 to October 2017. Posttreatment changes in prostate-specific antigen (PSA) and differences in the median duration treatment (MDT) with AA and ENZ were used to determine the effectiveness of ENZ. A PSA reduction 30% and three (20%) did not achieve any decline in PSA. Subsequent MDT-ENZ was 4 months (range: 1–12), showing a MDT ratio of 0.27. Three patients did not have PSA levels after taking enzalutamide. None of the CRPC patients who were or not initially AA-sensitive showed a>30% PSA decline while taking ENZ. The medium PSA decline after abiraterone and enzalutamide were 37% and 17.8% respectively. Of the 15 patients, 7 (46.6%) were primarily ENZ-resistant and showed a rising PSA as the best response. Median time to progression was 7 months (range: 2–12) for five of 15 patients with at least one declining PSA value while taking enzalutamide (33.3%).ConclusionAlthough the number of patients included in this study is small, ENZ therapy after AA failure shows a low activity in terms of PSA response and/or medium duration of treatment. Results would be compatible with qualifying the use of ENZ after failure to AA as ineffective. Further properly designed studies to this aim are needed.Reference and/or Acknowledgements1. J Clin Oncol2014;32:125.No conflict of interest