Δ4-Abiraterone (D4A) is an active metabolite of the CYP17A1 inhibitor abiraterone (Item No. 9002768).1 D4A is an androgen receptor antagonist (IC50 = 5.3 nM) that reduces expression of the androgen receptor target genes PSA, TMPRSS2, and FKBP5 in LNCaP, LAPC4, and C4-2 cell lines. It inhibits the cytochrome P450 (CYP) isoform CYP17A1 by 66.5% when used at a concentration of 1 nM. D4A (10 μM) also blocks SRD5A conversion of Δ4-androstenedione to 5α-androstanedione in LAPC4 cells. In vivo, D4A delays tumor progression in a VCaP mouse xenograft model and increases progression-free survival in a C4-2 mouse xenograft model.