The Association Of D-4, 3-Keto-Abi (D4a) And Response To Abiraterone In Castration-Resistant Prostate Cancer (Crpc).

245 Background: Abiraterone (Abi), a potent inhibitor of 17α-hydroxylase/17,20-lyase (CYP17A1), is a standard treatment for men with metastatic CRPC. Abi is converted to D4A by 3β-hydroxysteroid dehydrogenase (3βHSD). D4A inhibits CYP17A1, 3βHSD, and steroid-5α-reductase (SRD5A) and has direct androgen receptor antagonist activity, which together make it a more potent agent than Abi in xenograft models. It is not known if conversion to D4A in patients (pts) correlates with response or resistance to Abi. Methods: Blood was collected (single time point on Abi) from CRPC pts who started Abi during 2011-2015. Abi and D4A were extracted from serum and analyzed by mass spectrometry. The purpose of this ongoing study is to assess the potential correlation between D4A and response to treatment. Results: 32 patients with CRPC had blood collected. 4 pts (12.5%) received ketoconazole and 6 (18.8%) chemotherapy prior to Abi. Median pre-Abi prostate-specific antigen (PSA) was 14.3 ng/ml (0.6-646.1). Median time from i...