In vitro activity: Abiraterone binds and inhibits wild-type and mutant androgen receptor (AR). Abiraterone inhibits in vitro proliferation and androgen receptor-regulated gene expression of androgen receptor-positive prostate cancer cells, which could be explained by androgen receptor antagonism in addition to inhibition of steroidogenesis. In fact, activation of mutant androgen receptor by eplerenone is inhibited by greater concentrations of Abiraterone. Abiraterone displaces ligand from both WT-AR and T877A with EC50 of 13.4 μM and 7.9 μM, respectively. Abiraterone inhibits lyase activity with an IC50 of 5.8 nM in rat testis microsomes. Abiraterone acetate significantly inhibits T secretion (–48%) and in turn increased LH concentration (192%).

Kinase Assay: Microsomes are diluted to a final protein concentration of 50 μg/mL in the reaction mixture which contained 0.25 M sucrose, 20 mM Tris–HCl (pH 7.4), 10 mM G6P and 1.2 IU/mL G6PDH. After equilibration at 37 °C for 10 minutes, the reaction is initiated by addition of βNADP to obtain a final concentration of 0.6 mM. Prior to the distribution of 600 μL of the reaction mixture in each tube, Abiraterone is evaporated to dryness under a stream of nitrogen and then are incubated at 37 °C for 10 minutes. After incubation with Abiraterone, 500 μL of the reaction mixture is transferred to tubes containing 1 μM of the enzyme substrate, 17OHP. After a further 10 minutes incubation, tubes are placed on ice and the reaction is stopped by addition of 0.1 ml NaOH 1N. Tubes are deep-frozen and stored at –20 °C until assayed for Δ4A levels. A Δ4A RIA is developed and automated on a microplate format using a specific antibody against Δ4A. The separation of free and bound antigen is achieved with a dextran-coated charcoal suspension. After centrifugation, aliquots of the clear supernatant are counted in duplicates in a 1450 MicrobetaPlus liquid scintillation counter. The Δ4A concentrations of unknown samples are determined from the standard curve. The detection limit is 0.5 ng/mL and the within and between assay coefficients of variation are 10.7 and 17.6%, respectively at an assay value of 13 ng/mL. The rate of enzymatic reaction is expressed as pmol of Δ4A formed per 10 minutes and per mg of protein. The value of maximum activity without inhibitor (control) is set at 100%. The IC50 values are calculated using non-linear analysis from the plot of enzyme activity (%) against log of inhibitor concentration.

Cell Assay: LNCaP and VCaP cells are seeded in 96-well plates and grown in CSS-supplemented phenol red-free or FBS-supplemented media for 7 days. Cells are treated with Abiraterone at 24 hours and 96 hours after plating and cell viability is determined on day 7 by adding CellTiter Glo and measuring luminescence.

J Clin Oncol. 2008 Oct 1;26(28):4563-71; J Med Chem. 2010 Aug 12;53(15):5749-58; Steroids. 2011 Nov;76(12):1268-79.