Abiraterone with discontinuation of gonadotropin-releasing hormone (GnRH) analogues in patients (pts) with metastatic prostate cancer (PC): A single arm, phase II study.

5086 Background: Abiraterone acetate (AA) is approved in metastatic PC. AA inhibits CYP17, required for androgen biosynthesis. A first in human trial suggested that AA monotherapy was inadequate to maintain castrate testosterone (T) in non-castrate men and AA development required continued GnRH analogues. However, T recovery is often delayed post GnRH analogue cessation; the need for their ongoing use to maintain castrate T with AA use is unclear and studies are limited. We present a single arm, phase II trial of pts with metastatic PC treated with AA+Prednisone (AAP) after GnRH discontinuation. Methods: We conducted a single arm, phase II study for pts with metastatic PC treated with AAP with GnRH analogue discontinuation. Entry criteria required metastatic PC treated with AAP and GnRH analogue 27 were evaluable at 6 months. Median age was 69 years. Race/Ethnicity included 21 (68%) non-Hispanic black (68%) and 10 (32%) Hispanic pts. Sixteen (52%) had castration-resistant disease as AAP indication. Mean duration of ADT and AAP prior to enrollment were 29.1 (3-204) and 9.2 (1-38) months, respectively. Median follow up was 28 months. At 6 months, 25/27 (92%) patients had castrate T as did 22/26 (85%) at 12 months, 18/23 (78%) at 18 months, 16/22 (73%) at 24 months and 11/17 (65%) at 30 months. There were no treatment discontinuations due to toxicity Mean T, LH and PSA values at timepoints are reported. rPFS and OS will be reported later. Conclusions: Pts with metastatic PC treated with AAP after GnRH analogue discontinuation often remain castrate with AAP monotherapy even 30 months after GnRH analogue cessation. Phase III trials showing benefit of AAP required continued GnRH analogue; this small phase II study does not change standard of care but does suggest that further evaluation of AAP monotherapy may be warranted; AAP without use of GnRH inhibitors successfully suppressed T levels. Further randomized clinical trials are needed to assess the role of AAP monotherapy without GnRH analogues in metastatic PC. Clinical trial information: NCT03565835 . Table: see text