Is there a withdrawal syndrome with abiraterone acetate (AA)?

Abstract

89 Background: The withdrawal syndrome phenomenon in prostate cancer is defined as a subset of patients (pts) who experience decreasing prostate-specific antigen (PSA) values with or without clinical improvement from the withdrawal of androgen receptor (AR) inhibitors or steroids used as endocrine therapies in castration-resistant prostate cancer (CRPC). In view of its steroidal backbone and structural similarity to progesterone, abiraterone acetate (AA) might act as an AR agonist in certain subsets of CRPC. So far, this phenomenon has not been reported with AA. The aim of this study was to assess the incidence and clinical meaning of such a PSA decrease in pts after AA discontinuation Methods: A retrospective review of 218 consecutive pts treated with AA for metastatic CRPC received after docetaxel between 01/2009 and 08/2012 was conducted in two French centers. PSA values were evaluated before, during and at the end of AA treatment. We retrospectively collected PSA values performed after AA discontinuation (at 4 weeks) and until further treatment or death. Pts were typically kept on prednisone at AA discontinuation. Results: Amongst 218 consecutive patients, 66 pts (30%) were assessed for PSA follow-up after AA discontinuation; reasons for no assessment were mostly: immediate chemotherapy (13%), death (21%); ongoing AA (23%), non evaluated (14%). Amongst the 66 pts, AA therapy was associated with a PSA decrease in 65% pts. Median AA treatment duration was 5.1 months[1.5-39.9]. Twenty-one out of 66 pts (32%) had a PSA decrease when measured at AA disruption and 4 weeks afterwards. The median PSA decrease was 19% [3-68%]. A PSA decrease of greater than 50% was observed in 4 pts (19%). The median duration before progression was 5.5 weeks [1.7-54]. Two of the 4 pts with a PSA decrease>50% exhibited clinical benefit. PSA decrease duration in the 4 pts was7.1 wks [1.1-11.5]. Conclusions: A serum PSA decrease occurred in 32% of pts after AA discontinuation. PSA decrease greater than 50% was observed in 6% of pts with a potential clinical benefit. This event was usually of minor magnitude and short-term. Physiopathological explanations for this observation remain unclear and may be related to the direct activity of AA on the AR.