An active metabolite of abiraterone; androgen receptor antagonist (IC50 = 5.3 nM); reduces expression of androgen receptor target genes in LNCaP, LAPC4, and C4-2 cell lines; inhibits CYP17A1 at 1 nM; blocks SRD5A conversion of Δ4-androstenedione to 5α-androstanedione in LAPC4 cells at 10 μM; delays tumor progression in a VCaP mouse xenograft model; increases progression-free survival in a C4-2 mouse xenograft model