Inhibitors of the enzyme 17alpha-hydroxylase/17,20 lyase are a new class of anti-prostate cancer agents currently undergoing preclinical and clinical development. We have previously reported the superior anticancer activity of our novel 17alpha-hydroxylase/17,20 lyase inhibitor, VN/124-1, against androgen-dependent cancer models. Here, we examined the effect of VN/124-1 on the growth of the androgen-independent cell lines PC-3 and DU-145 and found that the compound inhibits their growth in a dose-dependent manner in vitro (GI50, 7.82 micromol/L and 7.55 micromol/L, respectively). We explored the mechanism of action of VN/124-1 in PC-3 cells through microarray analysis and found that VN/124-1 up-regulated genes involved in stress response and protein metabolism, as well as down-regulated genes involved in cell cycle progression. Follow-up real-time PCR and Western blot analyses revealed that VN/124-1 induces the endoplasmic reticulum stress response resulting in down-regulation of cyclin D1 protein expression and cyclin E2 mRNA. Cell cycle analysis confirmed G1-G0 phase arrest. Measurements of intracellular calcium levels ([Ca2+]i) showed that 20 micromol/L VN/124-1 caused a release of Ca2+ from endoplasmic reticulum stores resulting in a sustained increase in [Ca2+]i. Finally, cotreatment of PC-3 cells with 5, 10, and 20 micromol/L VN/124-1 with 10 nmol/L thapsigargin revealed a synergistic relationship between the compounds in inhibiting PC-3 cell growth. Taken together, these findings show VN/124-1 is endowed with multiple anticancer properties that may contribute to its utility as a prostate cancer therapeutic.